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Journal articles on the topic 'Turbulent flow'
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Oluwadare, Benjamin Segun, Paul Chukwulozie Okolie, David Ojo Akindele, Oluwafemi Festus Olaiyapo, Ayobami Phillip Akinsipe, and Oku Ekpenyong Nyong. "Transition to Turbulence of a Laminar Flow Accelerated to a Statistically Steady Turbulent Flow." European Journal of Theoretical and Applied Sciences 2, no. 3 (May 1, 2024): 430–45. http://dx.doi.org/10.59324/ejtas.2024.2(3).34.
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This current study investigates the turbulence response in a flow accelerated from laminar to a statistically steady turbulent flow utilising Particle Image Velocimetry (PIV) and Constant Temperature Anemometry (CTA). The dimensions of the rectangular flow facility are 8 m in length, 0.35 m in width, and 0.05 m in height. The flow is increased via the pneumatic control valve from a laminar to a statistically steady turbulent flow, and the laminar-turbulent transition is examined. As the flow accelerates to turbulent from laminar, the friction coefficient increases quickly and approaches its maximum value within a short period. As a result, a boundary layer forms extremely near to the wall, increasing the velocity gradient and viscous force. The friction coefficient and viscous force decrease with increasing boundary layer thickness, and transition occurs as a result of instability of the boundary layer. The friction coefficient is used to specify the beginning and end of the transition. The transition starts when the friction coefficient reaches its minimal value. It increases again, and its maximum value marks the end of the transition to turbulence. The study shows that three stages lead to turbulence near the wall when the flow is accelerated from laminar to turbulent. These phases are similar to the transient turbulent flow reported. The reaction of mean velocity as laminar flow is accelerated to turbulent flow is investigated. The mean velocity behaves like a "plug flow" when the flow accelerates from laminar to turbulent, meaning that everywhere in the flow zone, except for the position extremely near the wall, the flow behaves like a solid body. The changes in the channel flow that accelerates from a laminar to a turbulent condition are presented, together with the turbulence statistics, wall shear stress, bulk velocity, and friction coefficient. Like the boundary layer bypass transition and transient turbulent flows, the transition to turbulence follows a similar process.
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Oluwadare, Benjamin Segun, Paul Chukwulozie Okolie, David Ojo Akindele, Oluwafemi Festus Olaiyapo, Ayobami Phillip Akinsipe, and Oku Ekpenyong Nyong. "Transition to Turbulence of a Laminar Flow Accelerated to a Statistically Steady Turbulent Flow." European Journal of Theoretical and Applied Sciences 2, no. 2 (March 1, 2024): 928–43. http://dx.doi.org/10.59324/ejtas.2024.2(2).82.
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This current study investigates the turbulence response in a flow accelerated from laminar to a statistically steady turbulent flow utilising Particle Image Velocimetry (PIV) and Constant Temperature Anemometry (CTA). The dimensions of the rectangular flow facility are 8 m in length, 0.35 m in width, and 0.05 m in height. The flow is increased via the pneumatic control valve from a laminar to a statistically steady turbulent flow, and the laminar-turbulent transition is examined. As the flow accelerates to turbulent from laminar, the friction coefficient increases quickly and approaches its maximum value within a short period. As a result, a boundary layer forms extremely near to the wall, increasing the velocity gradient and viscous force. The friction coefficient and viscous force decrease with increasing boundary layer thickness, and transition occurs as a result of instability of the boundary layer. The friction coefficient is used to specify the beginning and end of the transition. The transition starts when the friction coefficient reaches its minimal value. It increases again, and its maximum value marks the end of the transition to turbulence. The study shows that three stages lead to turbulence near the wall when the flow is accelerated from laminar to turbulent. These phases are similar to the transient turbulent flow reported. The reaction of mean velocity as laminar flow is accelerated to turbulent flow is investigated. The mean velocity behaves like a "plug flow" when the flow accelerates from laminar to turbulent, meaning that everywhere in the flow zone, except for the position extremely near the wall, the flow behaves like a solid body. The changes in the channel flow that accelerates from a laminar to a turbulent condition are presented, together with the turbulence statistics, wall shear stress, bulk velocity, and friction coefficient. Like the boundary layer bypass transition and transient turbulent flows, the transition to turbulence follows a similar process.
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A., Gorin. "1111 UNIVERSAL TRENDS OF FORCED CONVECTION IN COMPLEX TURBULENT FLOWS CLASSIFIED UNDER TURBULENT SEPARATED FLOW." Proceedings of the International Conference on Jets, Wakes and Separated Flows (ICJWSF) 2013.4 (2013): _1111–1_—_1111–6_. http://dx.doi.org/10.1299/jsmeicjwsf.2013.4._1111-1_.
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Neuhaus, Lars, Daniel Ribnitzky, Michael Hölling, Matthias Wächter, Kerstin Avila, Martin Kühn, and Joachim Peinke. "Model wind turbine performance in turbulent–non-turbulent boundary layer flow." Journal of Physics: Conference Series 2767, no. 4 (June 1, 2024): 042018. http://dx.doi.org/10.1088/1742-6596/2767/4/042018.
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Abstract With increasing distance from the coast and greater hub heights, wind turbines expand into unknown, hardly researched environmental conditions. As height increases, laminar flow conditions become more likely. With the simultaneous increase in rotor diameter, very different flow conditions, from laminar to turbulent, occur over the rotor area. It is crucial to understand the effects of these different flow conditions on wind turbines. We approach this through wind tunnel experiments, presenting a setup with two different active grids. This setup enables the generation of four different flows – homogeneous, shear, turbulent–non-turbulent, and turbulent–non-turbulent shear flow – each with four different turbulence levels. The turbulent–non-turbulent flows exhibit a turbulence intensity gradient between the quasi-laminar flow at the upper and turbulent flow at the lower rotor half, establishing a turbulent–non-turbulent interface between the two rotor halves. In a second step, we investigate the Model Wind Turbine Oldenburg with a rotor diameter of 1.8 m (MoWiTO 1.8) under these conditions and analyze their effects on power output and blade loads. While the power fluctuations depend directly on the turbulence intensity, an additional turbulence intensity gradient shows no significant effect. A stronger effect can be observed for the blade root bending moments, the fluctuations of which increase with shear and also in turbulent–non-turbulent flow.
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Stamenkovic, Zivojin, Milos Kocic, and Jelena Petrovic. "The CFD modeling of two-dimensional turbulent MHD channel flow." Thermal Science 21, suppl. 3 (2017): 837–50. http://dx.doi.org/10.2298/tsci160822093s.
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In this paper, influence of magnetic field on turbulence characteristics of twodimensional flow is investigated. The present study has been undertaken to understand the effects of a magnetic field on mean velocities and turbulence parameters in turbulent 2-D channel flow. Several cases are considered. First laminar flow in a channel and MHD laminar channel flow are analyzed in order to validate model of magnetic field influence on electrically conducting fluid flow. Main part of the paper is focused on MHD turbulence suppression for 2-D turbulent flow in a channel and around the flat plate. The simulations are performed using ANSYS CFX software. Simulations results are obtained with BSL Reynolds stress model for turbulent and MHD turbulent flow around flat plate. The nature of the flow has been examined through distribution of mean velocities, turbulent fluctuations, vorticity, Reynolds stresses and turbulent kinetic energy.
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Gorin, Alexander V. "HEAT TRANSFER IN TURBULENT SEPARATED FLOWS(Flow around Cylinder 1)." Proceedings of the International Conference on Jets, Wakes and Separated Flows (ICJWSF) 2005 (2005): 445–50. http://dx.doi.org/10.1299/jsmeicjwsf.2005.445.
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Kashyap, Pavan, Yohann Duguet, and Olivier Dauchot. "Flow Statistics in the Transitional Regime of Plane Channel Flow." Entropy 22, no. 9 (September 8, 2020): 1001. http://dx.doi.org/10.3390/e22091001.
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The transitional regime of plane channel flow is investigated above the transitional point below which turbulence is not sustained, using direct numerical simulation in large domains. Statistics of laminar-turbulent spatio-temporal intermittency are reported. The geometry of the pattern is first characterized, including statistics for the angles of the laminar-turbulent stripes observed in this regime, with a comparison to experiments. High-order statistics of the local and instantaneous bulk velocity, wall shear stress and turbulent kinetic energy are then provided. The distributions of the two former quantities have non-trivial shapes, characterized by a large kurtosis and/or skewness. Interestingly, we observe a strong linear correlation between their kurtosis and their skewness squared, which is usually reported at much higher Reynolds number in the fully turbulent regime.
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Ansorge, Cedrick, and Juan Pedro Mellado. "Analyses of external and global intermittency in the logarithmic layer of Ekman flow." Journal of Fluid Mechanics 805 (September 23, 2016): 611–35. http://dx.doi.org/10.1017/jfm.2016.534.
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Existence of non-turbulent flow patches in the vicinity of the wall of a turbulent flow is known as global intermittency. Global intermittency challenges the conventional statistics approach when describing turbulence in the inner layer and calls for the use of conditional statistics. We extend the vorticity-based conditioning of a flow to turbulent and non-turbulent sub-volumes by a high-pass filter operation. This modified method consistently detects non-turbulent flow patches in the outer and inner layers for stratifications ranging from the neutral limit to extreme stability, where the flow is close to a complete laminarization. When applying this conditioning method to direct numerical simulation data of stably stratified Ekman flow, we find the following. First, external intermittency has a strong effect on the logarithmic law for the mean velocity in Ekman flow under neutral stratification. If instead of the full field, only turbulent sub-volumes are considered, the data fit an idealized logarithmic velocity profile much better; in particular, a problematic dip in the von Kármán measure$\unicode[STIX]{x1D705}$in the surface layer is decreased by approximately 50 % and our data only support the reduced range$0.41\lesssim \unicode[STIX]{x1D705}\lesssim 0.43$. Second, order-one changes in turbulent quantities under strong stratification can be explained by a modulation of the turbulent volume fraction rather than by a structural change of individual turbulence events; within the turbulent fraction of the flow, the character of individual turbulence events measured in terms of turbulence dissipation rate or variance of velocity fluctuations is similar to that under neutral stratification.
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Bech, Knut H., and Helge I. Andersson. "Secondary flow in weakly rotating turbulent plane Couette flow." Journal of Fluid Mechanics 317 (June 25, 1996): 195–214. http://dx.doi.org/10.1017/s0022112096000729.
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As in the laminar case, the turbulent plane Couette flow is unstable (stable) with respect to roll cell instabilities when the weak background angular velocity Ωk is antiparallel (parallel) to the spanwise mean flow vorticity (-dU/dy)k. The critical value of the rotation number Ro, based on 2Ω and dU/dy of the corresponding laminar flow, was estimated as 0.0002 at a low Reynolds number with fully developed turbulence. Direct numerical simulations were performed for Ro = ±0.01 and compared with earlier results for non-rotating Couette flow. At the low rotation rates considered, both senses of rotation damped the turbulence and the number of near-wall turbulence-generating events was reduced. The destabilized flow was more energetic, but less three-dimensional, than the non-rotating flow. In the destabilized case, the two-dimensional roll cells extracted a comparable amount of kinetic energy from the mean flow as did the turbulence, thereby decreasing the turbulent kinetic energy. The turbulence anisotropy was practically unaffected by weak spanwise rotation, while the secondary flow was highly anisotropic due to its inability to contract and expand in the streamwise direction.
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Zhao, Hanqing, Jing Yan, Saiyu Yuan, Jiefu Liu, and Jinyu Zheng. "Effects of Submerged Vegetation Density on Turbulent Flow Characteristics in an Open Channel." Water 11, no. 10 (October 16, 2019): 2154. http://dx.doi.org/10.3390/w11102154.
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The vegetation density λ affects turbulent flow type in the submerged vegetated river. This laboratory study investigates different types of vegetated turbulent flow, especially the flow at 0.04 < λ < 0.1 and λ = 1.44 by setting the experimental λ within a large range. Vertical distributions of turbulent statistics (velocity, shear stress and skewness coefficients), turbulence kinetic generation rate and turbulence spectra in different λ conditions have been presented and compared. Results indicate that for flow at 0.04 < λ < 0.1, the profiles of turbulent statistics manifest characteristics that are similar to those of both the bed-shear flow and the free-shear flow, and the turbulence spectral curves are characterized with some slight humps within the low-frequency range. For λ = 1.44, the turbulent statistics above the vegetation top demonstrate the characteristics of boundary-shear flow. The spectral curves fluctuate intensely within the low-frequency range, and the spectra of low-frequency eddies above vegetation top are significantly larger than the values below. The change of turbulent flow type induced by an increase of λ would increase the maximum value of turbulence kinetic generation rate GS and change the point where GS is vertically maximum upwards to the vegetation top.
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Buice, C. U., and J. K. Eaton. "Turbulent Heat Transport in a Perturbed Channel Flow." Journal of Heat Transfer 121, no. 2 (May 1, 1999): 322–25. http://dx.doi.org/10.1115/1.2825983.
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The recovering boundary layer downstream of a separation bubble is known to have a highly perturbed turbulence structure which creates difficulty for turbulence models. The present experiment addressed the effect of this perturbed structure on turbulent heat transport. The turbulent diffusion of heat downstream of a heated wire was measured in a perturbed channel flow and compared to that in a simple, fully developed channel flow. The turbulent diffusivity of heat was found to be more than 20 times larger in the perturbed flow. The turbulent Prandtl number increased to 1.7, showing that the turbulent eddy viscosity was affected even more strongly than the eddy thermal diffusivity. This result corroborates previous work showing that boundary layer disturbances generally have a stronger effect on the eddy viscosity, rendering prescribed turbulent Prandtl number models ineffective in perturbed flows.
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TAGUCHI, Y. H. "TURBULENT FLOW IN VIBRATED BED OF POWDER: NEW TARGET TO INVESTIGATE TURBULENT FLOW." Fractals 01, no. 04 (December 1993): 1080–85. http://dx.doi.org/10.1142/s0218348x93001192.
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The flow patterns in the vibrated bed of powder are investigated numerically in two dimensions. They turn out to exhibit turbulent flow like that observed in fully developed turbulence. The power spectrum of their flow lines has k−5/3 power, where k is the wave number. Since the numerical investigation of powder requires very few computational resources, it provides us with a new tool to study turbulence easily.
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Barkley, D. "Taming turbulent fronts by bending pipes." Journal of Fluid Mechanics 872 (June 4, 2019): 1–4. http://dx.doi.org/10.1017/jfm.2019.340.
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The flow of fluid through a pipe has been instrumental in illuminating the subcritical route to turbulence typical of many wall-bounded shear flows. Especially important in this process are the turbulent–laminar fronts that separate the turbulent and laminar flow. Four years ago Michael Graham (Nature, vol. 526, 2015, p. 508) wrote a commentary entitled ‘Turbulence spreads like wildfire’, which is a picturesque but also accurate characterisation of the way turbulence spreads through laminar flow in a straight pipe. In this spirit, the recent article by Rinaldi et al. (J. Fluid Mech., vol. 866, 2019, pp. 487–502) shows that turbulent wildfires are substantially tamed in bent pipes. These authors find that even at modest pipe curvature, the characteristic strong turbulent–laminar fronts of straight pipe flow vanish. As a result, the propagation of turbulent structures is modified and there are hints that the route to turbulence is fundamentally altered.
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Guerra, Viviane Da Silva, Otávio Costa Azevedo, Felipe Denardin Costa, and Pablo Eli Soares de Oliveira. "Análise do escoamento horizontal de movimento não turbulento na camada limite noturna sob influência de obstruções." Ciência e Natura 42 (August 28, 2020): e8. http://dx.doi.org/10.5902/2179460x45316.
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When turbulence is well developed, the diffusivity tends to quickly destroy other flow variability modes, so that the turbulent processes become dominant. However, in cases of weak or intermittent turbulence the turbulence scales are restricted to small values, both spatially and temporally. Non-turbulent processes can become important in such cases. This is particularly possible in the Stable Boundary Layer, some studies have focused on non-turbulent flow modes such as submeso, for example. Non-turbulent motions occur simultaneously on other scales and may to dominate the fluctuations of the horizontal flow and vertical flux The physical forcing of submeso flow is still poorly understood, but it is believed to depend significantly on local conditions such as topography and vegetation. The hypothesis assumed in this paper is that obstacles of different nature and dimensions, such as trees, buildings and topography elements affect different flow scales and analyze how turbulent and submeso processes are affected differently.
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Teixeira, M. A. C., and C. B. da Silva. "Turbulence dynamics near a turbulent/non-turbulent interface." Journal of Fluid Mechanics 695 (February 13, 2012): 257–87. http://dx.doi.org/10.1017/jfm.2012.17.
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AbstractThe characteristics of the boundary layer separating a turbulence region from an irrotational (or non-turbulent) flow region are investigated using rapid distortion theory (RDT). The turbulence region is approximated as homogeneous and isotropic far away from the bounding turbulent/non-turbulent (T/NT) interface, which is assumed to remain approximately flat. Inviscid effects resulting from the continuity of the normal velocity and pressure at the interface, in addition to viscous effects resulting from the continuity of the tangential velocity and shear stress, are taken into account by considering a sudden insertion of the T/NT interface, in the absence of mean shear. Profiles of the velocity variances, turbulent kinetic energy (TKE), viscous dissipation rate ($\varepsilon $), turbulence length scales, and pressure statistics are derived, showing an excellent agreement with results from direct numerical simulations (DNS). Interestingly, the normalized inviscid flow statistics at the T/NT interface do not depend on the form of the assumed TKE spectrum. Outside the turbulent region, where the flow is irrotational (except inside a thin viscous boundary layer),$\varepsilon $decays as${z}^{\ensuremath{-} 6} $, where$z$is the distance from the T/NT interface. The mean pressure distribution is calculated using RDT, and exhibits a decrease towards the turbulence region due to the associated velocity fluctuations, consistent with the generation of a mean entrainment velocity. The vorticity variance and$\varepsilon $display large maxima at the T/NT interface due to the inviscid discontinuities of the tangential velocity variances existing there, and these maxima are quantitatively related to the thickness$\delta $of the viscous boundary layer (VBL). For an equilibrium VBL, the RDT analysis suggests that$\delta \ensuremath{\sim} \eta $(where$\eta $is the Kolmogorov microscale), which is consistent with the scaling law identified in a very recent DNS study for shear-free T/NT interfaces.
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Mohmmed Ahmed, Osman Abu Bakr, and Mark Ovinis. "EVALUATION OF K-EPSILON MODEL FOR TURBULENT BUOYANT JET." Platform : A Journal of Engineering 3, no. 2 (October 31, 2019): 55. http://dx.doi.org/10.61762/pajevol3iss2art5085.
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The modelling of a turbulent buoyant jet is challenging due to the complex nature of such flow, which consists of two fluids with different densities, as well as the multi-scale flow phenomena associated in both space and time. In this paper, the k-epsilon turbulence model is applied to model a turbulent buoyant jet at different flow regimes including laminar and turbulent. The velocity field and centerline velocity are in good agreement with the experiments, as well as the expected results based on jet theory. Moreover, the distribution of the radial velocity matches with Gaussian distribution. The k-epsilon model is an appropriate turbulent model that can be applied for larger Reynolds number flow simulation.
Keywords: k-epsilon, turbulence model, CFD, turbulent buoyant jet.
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Righi, Marcello. "A Modified Gas-Kinetic Scheme for Turbulent Flow." Communications in Computational Physics 16, no. 1 (July 2014): 239–63. http://dx.doi.org/10.4208/cicp.140813.021213a.
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AbstractThe implementation of a turbulent gas-kinetic scheme into a finite-volume RANS solver is put forward, with two turbulent quantities, kinetic energy and dissipation, supplied by an allied turbulence model. This paper shows a number of numerical simulations of flow cases including an interaction between a shock wave and a turbulent boundary layer, where the shock-turbulent boundary layer is captured in a much more convincing way than it normally is by conventional schemes based on the Navier-Stokes equations. In the gas-kinetic scheme, the modeling of turbulence is part of the numerical scheme, which adjusts as a function of the ratio of resolved to unresolved scales of motion. In so doing, the turbulent stress tensor is not constrained into a linear relation with the strain rate. Instead it is modeled on the basis of the analogy between particles and eddies, without any assumptions on the type of turbulence or flow class. Conventional schemes lack multiscale mechanisms: the ratio of unresolved to resolved scales – very much like a degree of rarefaction – is not taken into account even if it may grow to non-negligible values in flow regions such as shocklayers. It is precisely in these flow regions, that the turbulent gas-kinetic scheme seems to provide more accurate predictions than conventional schemes.
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Brethouwer, Geert. "Passive scalar transport in rotating turbulent channel flow." Journal of Fluid Mechanics 844 (April 4, 2018): 297–322. http://dx.doi.org/10.1017/jfm.2018.198.
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Passive scalar transport in turbulent channel flow subject to spanwise system rotation is studied by direct numerical simulations. The Reynolds number $Re=U_{b}h/\unicode[STIX]{x1D708}$ is fixed at 20 000 and the rotation number $Ro=2\unicode[STIX]{x1D6FA}h/U_{b}$ is varied from 0 to 1.2, where $U_{b}$ is the bulk mean velocity, $h$ the half channel gap width and $\unicode[STIX]{x1D6FA}$ the rotation rate. The scalar is constant but different at the two walls, leading to steady scalar transport across the channel. The rotation causes an unstable channel side with relatively strong turbulence and turbulent scalar transport, and a stable channel side with relatively weak turbulence or laminar-like flow, weak turbulent scalar transport but large scalar fluctuations and steep mean scalar gradients. The distinct turbulent–laminar patterns observed at certain $Ro$ on the stable channel side induce similar patterns in the scalar field. The main conclusions of the study are that rotation reduces the similarity between the scalar and velocity field and that the Reynolds analogy for scalar-momentum transport does not hold for rotating turbulent channel flow. This is shown by a reduced correlation between velocity and scalar fluctuations, and a strongly reduced turbulent Prandtl number of less than 0.2 on the unstable channel side away from the wall at higher $Ro$. On the unstable channel side, scalar scales become larger than turbulence scales according to spectra and the turbulent scalar flux vector becomes more aligned with the mean scalar gradient owing to rotation. Budgets in the governing equations of the scalar energy and scalar fluxes are presented and discussed as well as other statistics relevant for turbulence modelling.
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Radomsky, R. W., and K. A. Thole. "Measurements and Predictions of a Highly Turbulent Flowfield in a Turbine Vane Passage." Journal of Fluids Engineering 122, no. 4 (July 10, 2000): 666–76. http://dx.doi.org/10.1115/1.1313244.
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As highly turbulent flow passes through downstream airfoil passages in a gas turbine engine, it is subjected to a complex geometry designed to accelerate and turn the flow. This acceleration and streamline curvature subject the turbulent flow to high mean flow strains. This paper presents both experimental measurements and computational predictions for highly turbulent flow as it progresses through a passage of a gas turbine stator vane. Three-component velocity fields at the vane midspan were measured for inlet turbulence levels of 0.6%, 10%, and 19.5%. The turbulent kinetic energy increased through the passage by 130% for the 10% inlet turbulence and, because the dissipation rate was higher for the 19.5% inlet turbulence, the turbulent kinetic energy increased by only 31%. With a mean flow acceleration of five through the passage, the exiting local turbulence levels were 3% and 6% for the respective 10% and 19.5% inlet turbulence levels. Computational RANS predictions were compared with the measurements using four different turbulence models including the k-ε, Renormalization Group (RNG) k-ε, realizable k-ε, and Reynolds stress model. The results indicate that the predictions using the Reynolds stress model most closely agreed with the measurements as compared with the other turbulence models with better agreement for the 10% case than the 19.5% case. [S0098-2202(00)00804-X]
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Elsinga, G. E., and C. B. da Silva. "How the turbulent/non-turbulent interface is different from internal turbulence." Journal of Fluid Mechanics 866 (March 5, 2019): 216–38. http://dx.doi.org/10.1017/jfm.2019.85.
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The average patterns of the velocity and scalar fields near turbulent/non-turbulent interfaces (TNTI), obtained from direct numerical simulations (DNS) of planar turbulent jets and shear free turbulence, are assessed in the strain eigenframe. These flow patterns help to clarify many aspects of the flow dynamics, including a passive scalar, near a TNTI layer, that are otherwise not easily and clearly assessed. The averaged flow field near the TNTI layer exhibits a saddle-node flow topology associated with a vortex in one half of the interface, while the other half of the interface consists of a shear layer. This observed flow pattern is thus very different from the shear-layer structure consisting of two aligned vortical motions bounded by two large-scale regions of uniform flow, that typically characterizes the average strain field in the fully developed turbulent regions. Moreover, strain dominates over vorticity near the TNTI layer, in contrast to internal turbulence. Consequently, the most compressive principal straining direction is perpendicular to the TNTI layer, and the characteristic 45-degree angle displayed in internal shear layers is not observed at the TNTI layer. The particular flow pattern observed near the TNTI layer has important consequences for the dynamics of a passive scalar field, and explains why regions of particularly high scalar gradient (magnitude) are typically found at TNTIs separating fluid with different levels of scalar concentration. Finally, it is demonstrated that, within the fully developed internal turbulent region, the scalar gradient exhibits an angle with the most compressive straining direction with a peak probability at around 20$^{\text{o}}$. The scalar gradient and the most compressive strain are not preferentially aligned, as has been considered for many years. The misconception originated from an ambiguous definition of the positive directions of the strain eigenvectors.
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Farano, Mirko, Stefania Cherubini, Jean-Christophe Robinet, and Pietro De Palma. "Optimal bursts in turbulent channel flow." Journal of Fluid Mechanics 817 (March 15, 2017): 35–60. http://dx.doi.org/10.1017/jfm.2017.107.
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Bursts are recurrent, transient, highly energetic events characterized by localized variations of velocity and vorticity in turbulent wall-bounded flows. In this work, a nonlinear energy optimization strategy is employed to investigate whether the origin of such bursting events in a turbulent channel flow can be related to the presence of high-amplitude coherent structures. The results show that bursting events correspond to optimal energy flow structures embedded in the fully turbulent flow. In particular, optimal structures inducing energy peaks at short time are initially composed of highly oscillating vortices and streaks near the wall. At moderate friction Reynolds numbers, through the bursts, energy is exchanged between the streaks and packets of hairpin vortices of different sizes reaching the outer scale. Such an optimal flow configuration reproduces well the spatial spectra as well as the probability density function typical of turbulent flows, recovering the mechanism of direct-inverse energy cascade. These results represent an important step towards understanding the dynamics of turbulence at moderate Reynolds numbers and pave the way to new nonlinear techniques to manipulate and control the self-sustained turbulence dynamics.
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Long, Hoyt, and Richard Jean So. "Turbulent Flow." Modern Language Quarterly 77, no. 3 (August 15, 2016): 345–67. http://dx.doi.org/10.1215/00267929-3570656.
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Liu, Zhenchen, Peiqing Liu, Hao Guo, and Tianxiang Hu. "Experimental investigations of turbulent decaying behaviors in the core-flow region of a propeller wake." Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering 234, no. 2 (August 1, 2019): 319–29. http://dx.doi.org/10.1177/0954410019865702.
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This work investigates the turbulent decaying behaviors downstream of a propeller in the core-flow region. Both axial and tangential velocity fluctuations behind a two-bladed propeller were measured using a stationary hot-wire probe. Unexpectedly, the complex near-wake core-flow of the propeller is found to show a similar decay characteristic of homogeneous turbulence, such as grid turbulence. The decay of turbulence intensity is found to be dominated by the level of periodic velocity fluctuations, showing a similar behavior of the homogenous and isotropic turbulence. This turbulent decaying behavior of the core-flow can be adopted for future turbulent modeling techniques.
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Blair, M. F. "Boundary-Layer Transition in Accelerating Flows With Intense Freestream Turbulence: Part 2—The Zone of Intermittent Turbulence." Journal of Fluids Engineering 114, no. 3 (September 1, 1992): 322–32. http://dx.doi.org/10.1115/1.2910033.
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Hot-wire anemometry was employed to examine the laminar-to-turbulent transition of low-speed, two-dimensional boundary layers for two (moderate) levels of flow acceleration and various levels of grid-generated freestream turbulence. Flows with an adiabatic wall and with uniform-flux heat transfer were explored. Conditional discrimination techniques were employed to examine the zones of flow within the transitional region. This analysis demonstrated that as much as one-half of the streamwise-component unsteadiness, and much of the apparent anisotropy, observed near the wall was produced, not by turbulence, but by the steps in velocity between the turbulent and inter-turbulent zones of flow. Within the turbulent zones u′/v′ ratios were about equal to those expected for equilibrium boundary-layer turbulence. Near transition onset, however, the turbulence kinetic energy within the turbulent zones exceeded fully turbulent boundary-layer levels. Turbulent-zone power-spectral-density measurements indicate that the ratio of dissipation to production increased through transition. This suggests that the generation of the full equilibrium turbulent boundary-layer energy cascade required some time (distance) and may explain the very high TKE levels near onset.
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Abramov, Rafail V. "Turbulence via Intermolecular Potential: Viscosity and Transition Range of the Reynolds Number." Fluids 8, no. 3 (March 18, 2023): 101. http://dx.doi.org/10.3390/fluids8030101.
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Turbulence in fluids is an ubiquitous phenomenon, characterized by spontaneous transition of a smooth, laminar flow to rapidly changing, chaotic dynamics. In 1883, Reynolds experimentally demonstrated that, in an initially laminar flow of water, turbulent motions emerge without any measurable external disturbance. To this day, turbulence remains a major unresolved phenomenon in fluid mechanics; in particular, there is a lack of a mathematical model where turbulent dynamics emerge naturally from a laminar flow. Recently, we proposed a new theory of turbulence in gases, according to which turbulent motions are created in an inertial gas flow by the mean field effect of the intermolecular potential. In the current work, we investigate the effect of viscosity in our turbulence model by numerically simulating the air flow at normal conditions in a straight pipe for different values of the Reynolds number. We find that the transition between laminar and turbulent flow in our model occurs, without any deliberate perturbations, as the Reynolds number increases from 2000 to 4000. As the simulated flow becomes turbulent, the decay rate of the time averaged Fourier spectrum of the kinetic energy in our model approaches Kolmogorov’s inverse five-thirds law. Both results are consistent with experiments and observations.
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Humphrey, Luke J., Benjamin Emerson, and Tim C. Lieuwen. "Premixed turbulent flame speed in an oscillating disturbance field." Journal of Fluid Mechanics 835 (November 27, 2017): 102–30. http://dx.doi.org/10.1017/jfm.2017.728.
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This paper considers the manner in which turbulent premixed flames respond to a superposition of turbulent and narrowband disturbances. This is an important fundamental problem that arises in most combustion applications, as turbulent flames exist in hydrodynamically unstable flow fields and/or in confined systems with narrowband acoustic waves. This paper presents the first measurements of the sensitivity of the turbulent displacement speed to harmonically oscillating flame wrinkles. The flame is attached to a transversely oscillating, heated wire, resulting in the introduction of coherent, convecting wrinkles on the flame. The approach flow turbulence is varied systematically using a variable turbulence generator, enabling quantification of the effect of turbulent flow disturbances on the harmonic wrinkles. Mie scattering measurements are used to quantify the flame edge dynamics, while high speed particle image velocimetry is used to measure the flow field characteristics. By ensemble averaging the results, the ensemble-averaged flame edge and flow characteristics are recovered. For low turbulence intensities, sharp cusps are present in the negative curvature regions of the ensemble-averaged flame position, similar to laminar flames. These cusps are smoothed out at high turbulence intensities. The coherent, ensemble-averaged flame wrinkle amplitude decays with increasing turbulence intensity and with downstream distance. In addition, the ensemble-averaged turbulent flame speed is modulated in space and time. The most significant result of these measurements is the clear demonstration of the correlation between the ensemble-averaged turbulent flame speed and ensemble-averaged flame curvature, with the phase-dependent flame speed increasing in regions of negative curvature. These results have important implications on turbulent combustion physics and modelling, since quasi-coherent velocity disturbances are nearly ubiquitous in shear driven, high turbulent flows and/or confined systems with acoustic feedback. Specifically, these data clearly show that nonlinear interactions occur between the multi-scale turbulent disturbances and the more narrowband disturbances associated with coherent structures. In other words, conceptual models of the controlling physics in combustors with shear driven turbulence must account for the fundamentally different effects of spectrally distributed turbulent disturbances and more narrowband, quasi-coherent disturbances.
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Avramenko, Andriy A., Andrii I. Tyrinov, and Igor V. Shevchuk. "Analytical simulation of normal shock waves in turbulent flow." Physics of Fluids 34, no. 5 (May 2022): 056101. http://dx.doi.org/10.1063/5.0093205.
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The focus of the work is on analytical modeling of normal shock wave propagation in a turbulent adiabatic gas flow. For this, a modified Rankine–Hugoniot model was developed. A solution is obtained for the Rankine–Hugoniot conditions in a turbulent gas flow with different turbulence intensity. Variation of the velocity of an adiabatic turbulent gas flow during its passage through a normal shock wave is elucidated depending on the turbulence intensity. The equation of the modified Hugoniot adiabat is also obtained.
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VREMAN, A. W. "Turbulence characteristics of particle-laden pipe flow." Journal of Fluid Mechanics 584 (July 25, 2007): 235–79. http://dx.doi.org/10.1017/s0022112007006556.
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Turbulence characteristics of vertical air–solid pipe flow are investigated in this paper. Direct numerical simulations of the gas phase have been performed, while the solid particles have been simulated by a Lagrangian approach, including particle collisions. The modelling of wall roughness is shown to be important to obtain agreement with experimental data. Reynolds stresses and Reynolds stress budgets are given for both phases and for a wide range of solid–air mass load ratios (mass loads), varying from 0.11 to 30. Air turbulence intensities, Reynolds shear stress, and turbulence production reduce with increasing mass load. The mean air profile does not alter for low mass loads. In this regime, a simple theory predicts that the reduction of air turbulent production relative to unladen turbulent production is approximately equal to the mass load ratio. The insight that the solids Reynolds shear stress can be significant, even for low mass loads, is essential for this explanation. It is shown that at least two mechanisms cause the turbulence reduction. In addition to the classically recognized mechanism of dissipation of turbulent fluctuations by particles, there is another suppressing mechanism in inhomogeneous flows: the non-uniform relative velocity of the phases, created because particles slip at the wall, collide, and slowly react with the continuous phase. Investigation of the air turbulent kinetic energy equation demonstrates that the relative reduction of air pressure strain is larger than the reduction of turbulent production and dissipation, and pressure strain may therefore be a cause of the reduction of the other quantities. The fluctuational dissipation induced by the drag forces from particles is small compared to the other terms, but not negligible. For intermediate and high mass loads the air turbulence remains low. The relatively small turbulence intensities are not generated by the standard turbulent mechanisms any more, but directly caused by the particle motions. The particle–fluid interaction term in the turbulent kinetic energy equation is no longer dissipative, but productive instead. On increasing the mass load, the radial and azimuthal fluctuations of the particles grow. The corresponding reduction of solids anisotropy is an effect of the inter-particle collisions, which act as a solids pressure strain term. For intermediate and high mass loads, fluctuational drag force and particle collisions appear to be the relevant dissipation mechanisms in the solids fluctuational energy equation. In contrast to the air turbulent production, the solids ‘turbulent’ production term has the same level for low and high mass loads, while it attains a clear local minimum between. With increasing mass load, large-scale coherent turbulent fluid structures weaken, and eventually disappear. Simultaneously, the fluid fluctuations at relatively small length scales are enhanced by the motion of the particles. The highest particle concentration occurs near the wall for low mass loads, but on increasing the mass load, the concentration profile becomes uniform, while for the highest mass load particles accumulate in the centre of the pipe. Two-point correlation functions indicate that the addition of a small number of small solid particles to a clean pipe flow increases the streamwise length scale of the turbulence.
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Sofiadis, G., and I. Sarris. "Reynolds number effect of the turbulent micropolar channel flow." Physics of Fluids 34, no. 7 (July 2022): 075126. http://dx.doi.org/10.1063/5.0098453.
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The turbulent regime of non-Newtonian flows presents a particular interest as flow behavior is directly affected by the internal microstructure type of the fluid. Differences in the dispersed phase of a particle laden flow can either lead to drag reduction and turbulence attenuation or to drag and turbulence enhancement in polymer flows and dense suspensions, respectively. A general concept of non-Newtonian fluid flow may be considered in a continuous manner through the micropolar theory, recognizing the limitations that bound this theory. In recent articles [Sofiadis and Sarris, “Microrotation viscosity effect on turbulent micropolar fluid channel flow,” Phys. Fluids 33, 095126 (2021); Sofiadis and Sarris, “Turbulence intensity modulation by micropolar fluids,” Fluids 6, 195 (2021)], the micropolar viscosity effect of the turbulent channel flow under constant Reynolds number and its turbulent modulation were investigated. The present study focuses on the investigation of the turbulent micropolar regime as the Reynolds number increases in a channel flow. Findings support that the micropolar stress, which was found to assist turbulence enhancement in the present model, attenuates as Re increases. Effects on the friction behavior of the flow, as Reynolds number increases, become more important for cases of higher micropolar viscosity, where a reverse drag behavior is observed as compared to lower micropolar viscosity ones. Finally, turbulence intensification for these cases declines close to the wall in contrast to lower micropolar viscosity flows, which manage to sustain high turbulence and increase drag in the near-wall region along with Re.
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Schuster, Jean Jonathan, Cristiano Henrique Schuster, Eduardo Stüker, Áttila Leães Rodrigues, Luiz Eduardo Medeiros, and Felipe Denardin Costa. "OCORRÊNCIA DE INTERMITÊNCIA NA TRANSIÇÃO LAMINAR-TURBULENTA EM UM ESCOAMENTO DE COUETTE PLANO." Ciência e Natura 38 (July 20, 2016): 354. http://dx.doi.org/10.5902/2179460x20258.
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The transition from laminar-turbulent flow regime is important in most of the fluid mechanics application areas. In the planetary boundary layer (PBL), the flow is predominantly turbulent. However, shortly after sunset, the incidence of solar radiation ceases and the surface begins to lose heat through the emission of long-wave, yielding in a thermical stratified stable boundary layer (SBL), where turbulence can be suppressed in almost all scales. Under these conditions the production of turbulence is predominantly mechanical, and at nights with strong stratification, the turbulent activity is reduced by several orders of magnitude and can rise abruptly in unpredictable ways, giving origin to a phenomenon known as global intermittency. The globla intermittency is a phenomenon that occurs in the transition flow in the PBL, similarly to intermittency which occurs in the laminar-turbulent transition. Thus, this work aims to develop a numerical experiment to reproduce the laminar-turbulent transition in a thermally stratified Couette flow, using a large eddy simulation model. The simulations show that for a certain range of parameters during the transition laminar-turbulent, turbulence appeared intermittently in the flow.
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Tuckerman, Laurette S., Matthew Chantry, and Dwight Barkley. "Patterns in Wall-Bounded Shear Flows." Annual Review of Fluid Mechanics 52, no. 1 (January 5, 2020): 343–67. http://dx.doi.org/10.1146/annurev-fluid-010719-060221.
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Experiments and numerical simulations have shown that turbulence in transitional wall-bounded shear flows frequently takes the form of long oblique bands if the domains are sufficiently large to accommodate them. These turbulent bands have been observed in plane Couette flow, plane Poiseuille flow, counter-rotating Taylor–Couette flow, torsional Couette flow, and annular pipe flow. At their upper Reynolds number threshold, laminar regions carve out gaps in otherwise uniform turbulence, ultimately forming regular turbulent–laminar patterns with a large spatial wavelength. At the lower threshold, isolated turbulent bands sparsely populate otherwise laminar domains, and complete laminarization takes place via their disappearance. We review results for plane Couette flow, plane Poiseuille flow, and free-slip Waleffe flow, focusing on thresholds, wavelengths, and mean flows, with many of the results coming from numerical simulations in tilted rectangular domains that form the minimal flow unit for the turbulent–laminar bands.
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He, S., C. Ariyaratne, and A. E. Vardy. "Wall shear stress in accelerating turbulent pipe flow." Journal of Fluid Mechanics 685 (September 21, 2011): 440–60. http://dx.doi.org/10.1017/jfm.2011.328.
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AbstractAn experimental study of wall shear stress in an accelerating flow of water in a pipe ramping between two steady turbulent flows has been undertaken in a large-scale experimental facility. Ensemble averaged mean and r.m.s. of the turbulent fluctuations of wall shear stresses have been derived from hot-film measurements from many repeated runs. The initial Reynolds number and the acceleration rate were varied systematically to give values of a non-dimensional acceleration parameter $k$ ranging from 0.16 to 14. The wall shear stress has been shown to follow a three-stage development. Stage 1 is associated with a period of minimal turbulence response; the measured turbulent wall shear stress remains largely unchanged except for a very slow increase which is readily associated with the stretching of existing turbulent eddies as a result of flow acceleration. In this condition of nearly ‘frozen’ turbulence, the unsteady wall shear stress is driven primarily by flow inertia, initially increasing rapidly and overshooting the pseudo-steady value, but then increasing more slowly and eventually falling below the pseudo-steady value. This variation is predicted by an analytical expression derived from a laminar flow formulation. The start of Stage 2 is marked by the generation of new turbulence causing both the mean and turbulent wall shear stress to increase rapidly, although there is a clear offset between the responses of these two quantities. The turbulent wall shear, reflecting local turbulent activities near the wall, responds first and the mean wall shear, reflecting conditions across the entire flow field, responds somewhat later. In Stage 3, the wall shear stress exhibits a quasi-steady variation. The duration of the initial period of nearly frozen turbulence response close to the wall increases with decreasing initial Reynolds number and with increasing acceleration. The latter is in contrast to the response of turbulence in the core of the flow, which previous measurements have shown to be independent of the rate of acceleration.
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Sumiadi, Sumiadi. "DISTRIBUSI INTESITAS TURBULEN PADA BELOKAN SALURAN DENGAN DASAR TERGERUS." Gorontalo Journal of Infrastructure and Science Engineering 4, no. 1 (May 10, 2021): 1. http://dx.doi.org/10.32662/gojise.v4i1.1356.
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The flow characteristics on the channel bend are very complex. The Centrifugal force causes an increase in secondary flow. Increased secondary flow triggers changes in the channel bed topography. This phenomenon is very possible to change the flow structure including turbulent intensity. This research aims to determine the distribution of turbulent intensity at eroded-bed channel bend and to compare whether the equations of the turbulent intensity distribution by Nezu (1977) are still valid. This research was conducted at the Hydraulic Laboratory using 180° curved open channels. The bed material is sand with a median diameter, 1 mm and a density of 2.67 gr/cm3. The flow in the approach section is uniform flow and changes to non-uniform flow in channel bend. Measurements were carried out on equilibrium conditions of bed material. 3D instantaneous velocity measurements are carried out at 8 cross sections using Acoustic Doppler Velocimeter (ADV). The results showed that due to the presence of secondary flow in the channel bend caused a change in turbulent intensity distribution. In general, the horizontal turbulent intensity is greater than the vertical turbulent intensity. While the turbulent intensity in the outer bank tends to be greater than the inner bank. Furthermore, the vertical distribution of turbulent intensity in the eroded channel bend does not follow the exponential equation proposed by Nezu (1977).Karakteristik aliran di saluran menikung sangat kompleks. Gaya sentrifugal menyebabkan peningkatan aliran sekunder. Peningkatan aliran sekunder memicu perubahan topografi dasar saluran. Fenomena ini sangat berpotensi mengubah struktur aliran termasuk intensitas turbulen. Penelitian ini bertujuan untuk mengetahui distribusi intensitas turbulen pada belokan saluran dengan dasar tergerus dan membandingkan apakah persamaan distribusi intensitas turbulen oleh Nezu (1977) masih berlaku. Penelitian ini dilakukan di Laboratorium Hidrolika menggunakan flume dengan sudut belokan 1800. Material dasar yang digunakan adalah pasir dengan diameter median 1 mm dan rapat massa 2.67 gr/cm3.Kondisi aliran pada saluran pengarah di upstream adalah seragam dan berubah menjadi aliran tidak seragam pada bagian belokan. Pengukuran dilakukan pada kondisi material dasar equilibrium. Pengukuran kecepatan sesaat 3D dilakukan pada 8 cross section menggunakan Acoustic Doppler Velocimeter (ADV). Hasil penelitian menunjukkan akibat adanya aliran sekunder pada belokan saluran menyebabkan terjadinya perubahan distribusi intensitas turbulen. Secara umum, intensitas turbulen horizontal lebih besar dibandingkan dengan intensitas turbulen vertikal. Sedangkan intensitas turbulen di sisi luar belokan cenderung lebih besar dari sisi dalam belokan. Selanjutnya, distribusi vertikal intensitas turbulen pada belokan saluran dengan dasar tergerus tidak mengikuti persamaan eksponensial yang diusulkan oleh Nezu (1977).
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Alhumairi, Mohammed, and Özgür Ertunç. "Active-grid turbulence effect on the topology and the flame location of a lean premixed combustion." Thermal Science 22, no. 6 Part A (2018): 2425–38. http://dx.doi.org/10.2298/tsci170503100a.
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Lean premixed combustion under the influence of active-grid turbulence was computationally investigated, and the results were compared with experimental data. The experiments were carried out to generate a premixed flame at a thermal load of 9 kW from a single jet flow combustor. Turbulent combustion models, such as the coherent flame model and turbulent flame speed closure model were implemented for the simulations performed under different turbulent flow conditions, which were specified by the Reynolds number based on Taylor?s microscale, the dissipation rate of turbulence, and turbulent kinetic energy. This study shows that the applied turbulent combustion models differently predict the flame topology and location. However, similar to the experiments, simulations with both models revealed that the flame moves toward the inlet when turbulence becomes strong at the inlet, that is, when Re? at the inlet increases. The results indicated that the flame topology and location in the coherent flame model were more sensitive to turbulence than those in the turbulent flame speed closure model. The flame location behavior on the jet flow combustor significantly changed with the increase of Re?.
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Gan, X., M. Kilic, and J. M. Owen. "Flow Between Contrarotating Disks." Journal of Turbomachinery 117, no. 2 (April 1, 1995): 298–305. http://dx.doi.org/10.1115/1.2835659.
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The paper describes a combined experimental and computational study of laminar and turbulent flow between contrarotating disks. Laminar computations produce Batchelor-type flow: Radial outflow occurs in boundary layers on the disks and inflow is confined to a thin shear layer in the midplane; between the boundary layers and the shear layer, two contrarotating cores of fluid are formed. Turbulent computations (using a low-Reynolds-number k–ε turbulence model) and LDA measurements provide no evidence for Batchelor-type flow, even for rotational Reynolds numbers as low as 2.2 × 104. While separate boundary layers are formed on the disks, radial inflow occurs in a single interior core that extends between the two boundary layers; in the core, rotational effects are weak. Although the flow in the core was always found to be turbulent, the flow in the boundary layers could remain laminar for rotational Reynolds numbers up to 1.2 × 105. For the case of a superposed outflow, there is a source region in which the radial component of velocity is everywhere positive; radially outward of this region, the flow is similar to that described above. Although the turbulence model exhibited premature transition from laminar to turbulent flow in the boundary layers, agreement between the computed and measured radial and tangential components of velocity was mainly good over a wide range of nondimensional flow rates and rotational Reynolds numbers.
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Lin, Jianzhong, Suhua Shen, and Xiaoke Ku. "Characteristics of Fiber Suspension Flow in a Turbulent Boundary Layer." Journal of Engineered Fibers and Fabrics 8, no. 1 (March 2013): 155892501300800. http://dx.doi.org/10.1177/155892501300800103.
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The equations of averaged momentum, turbulence kinetic energy, turbulence dissipation rate with the additional term of the fibers, and the equation of probability distribution function for mean fiber orientation are derived and solved numerically for fiber suspension flowing in a turbulent boundary layer. The mathematical model and numerical code are verified by comparing the numerical results with the experimental ones in a turbulent channel flow. The effects of Reynolds number, fiber concentration and fiber aspect-ratio on the mean velocity profile, turbulent kinetic energy, Reynolds stress, turbulent dissipation rate and eddy viscosity coefficient are analyzed. The results show that the velocity profiles become full, and the turbulent kinetic energy, Reynolds stress and eddy viscosity coefficient increase, while turbulent dissipation rate decreases, as the Reynolds number, fiber concentration and fiber aspect-ratio increase. The effect of the fiber aspect-ratio on the turbulent properties is larger than that of the Reynolds number, but smaller than that of the fiber concentration in the range of parameters considered in this paper.
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Čantrak, Đorđe S., and Novica Z. Janković. "High speed stereoscopic PIV investigation of the statistical characteristics of the axially restricted turbulent swirl flow behind the axial fan in pipe." Advances in Mechanical Engineering 14, no. 11 (November 2022): 168781322211305. http://dx.doi.org/10.1177/16878132221130563.
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Investigation of the turbulent swirl flow in the piping system is one of the most complex investigations in the field of energetics and turbulence. Axial fans in a pipe, without guide vanes, are widely used in practice and the problem of their duty point and energy efficiency is still extensively discussed. Analysis of the interaction between axial fans energy and construction parameters is one of the main topics in defining the fans energy efficiency potential. On one side, there is a three-dimensional velocity field in the wall-bounded flow with regions of great turbulence intensity. On the other side, there is a complex blade geometry, which generates the turbulent swirl flow. This paper presents research on the turbulent swirl flow, Rankine type, in an axially restricted system, using high-speed stereo particle image velocimetry (HSS PIV). Axial fan impeller, with outer diameter 0.399 m and nine twisted blades is the flow generator. The Reynolds number Re = 176,529 is achieved in the pipe. Reynolds stresses, statistical moments of higher order, and invariant maps are calculated based on the three component velocity fields. Here, intensive changes of all statistical parameters occur in radial and axial direction. In the flow region, four flow regions can be identified. Interaction of all these four flow regions produces extremely complex turbulent swirl flow, which is generated behind the axial fans. Determined invariant maps reveal turbulence structure. It is shown that the state of turbulence on the pipe axis is three-component isotropic, which is contrary to the case of axially unrestricted turbulent swirl flows. In the rest of the space, in the region up to r/ R = 0.52, the states of turbulence occur in the area in between the boundaries which designate axis-symmetric turbulence (contraction) and axis-symmetric turbulence (expansion), in the vicinity of the state of three-component isotropic turbulence.
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LESCHZINER, M. A., G. M. FISHPOOL, and S. LARDEAU. "TURBULENT SHEAR FLOW: A PARADIGMATIC MULTISCALE PHENOMENON." Journal of Multiscale Modelling 01, no. 02 (April 2009): 197–222. http://dx.doi.org/10.1142/s1756973709000104.
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The paper provides a broad discussion of multiscale and structural features of sheared turbulent flows. Basic phenomenological aspects of turbulence are first introduced, largely in descriptive terms with particular emphasis placed on the range of scales encountered in turbulent flows and in the identification of characteristic scale ranges. There follows a discussion of essential aspects of computational modeling and simulation of turbulence. Finally, the results of simulations for two groups of flows are discussed. These combine shear, separation, and periodicity, the last feature provoked by either a natural instability or by unsteady external forcing. The particular choice of examples is intended to illustrate the capabilities of such simulations to resolve the multiscale nature of complex turbulent flows, as well as the challenges encountered.
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Kaller, Thomas, Vito Pasquariello, Stefan Hickel, and Nikolaus A. Adams. "Turbulent flow through a high aspect ratio cooling duct with asymmetric wall heating." Journal of Fluid Mechanics 860 (December 4, 2018): 258–99. http://dx.doi.org/10.1017/jfm.2018.836.
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We present well-resolved large-eddy simulations of turbulent flow through a straight, high aspect ratio cooling duct operated with water at a bulk Reynolds number of $Re_{b}=110\times 10^{3}$ and an average Nusselt number of $Nu_{xz}=371$. The geometry and boundary conditions follow an experimental reference case and good agreement with the experimental results is achieved. The current investigation focuses on the influence of asymmetric wall heating on the duct flow field, specifically on the interaction of turbulence-induced secondary flow and turbulent heat transfer, and the associated spatial development of the thermal boundary layer and the inferred viscosity variation. The viscosity reduction towards the heated wall causes a decrease in turbulent mixing, turbulent length scales and turbulence anisotropy as well as a weakening of turbulent ejections. Overall, the secondary flow strength becomes increasingly less intense along the length of the spatially resolved heated duct as compared to an adiabatic duct. Furthermore, we show that the assumption of a constant turbulent Prandtl number is invalid for turbulent heat transfer in an asymmetrically heated duct.
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He, S., and M. Seddighi. "Turbulence in transient channel flow." Journal of Fluid Mechanics 715 (January 9, 2013): 60–102. http://dx.doi.org/10.1017/jfm.2012.498.
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AbstractDirect numerical simulations (DNS) are performed of a transient channel flow following a rapid increase of flow rate from an initially turbulent flow. It is shown that a low-Reynolds-number turbulent flow can undergo a process of transition that resembles the laminar–turbulent transition. In response to the rapid increase of flow rate, the flow does not progressively evolve from the initial turbulent structure to a new one, but undergoes a process involving three distinct phases (pre-transition, transition and fully turbulent) that are equivalent to the three regions of the boundary layer bypass transition, namely, the buffeted laminar flow, the intermittent flow and the fully turbulent flow regions. This transient channel flow represents an alternative bypass transition scenario to the free-stream-turbulence (FST) induced transition, whereby the initial flow serving as the disturbance is a low-Reynolds-number turbulent wall shear flow with pre-existing streaky structures. The flow nevertheless undergoes a ‘receptivity’ process during which the initial structures are modulated by a time-developing boundary layer, forming streaks of apparently specific favourable spacing (of about double the new boundary layer thickness) which are elongated streamwise during the pre-transitional period. The structures are stable and the flow is laminar-like initially; but later in the transitional phase, localized turbulent spots are generated which grow spatially, merge with each other and eventually occupy the entire wall surfaces when the flow becomes fully turbulent. It appears that the presence of the initial turbulent structures does not promote early transition when compared with boundary layer transition of similar FST intensity. New turbulent structures first appear at high wavenumbers extending into a lower-wavenumber spectrum later as turbulent spots grow and join together. In line with the transient energy growth theory, the maximum turbulent kinetic energy in the pre-transitional phase grows linearly but only in terms of ${u}^{\ensuremath{\prime} } $, whilst ${v}^{\ensuremath{\prime} } $ and ${w}^{\ensuremath{\prime} } $ remain essentially unchanged. The energy production and dissipation rates are very low at this stage despite the high level of ${u}^{\ensuremath{\prime} } $. The pressure–strain term remains unchanged at that time, but increases rapidly later during transition along with the generation of turbulent spots, hence providing an unambiguous measure for the onset of transition.
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Obi, S., K. Hishida, and M. Maeda. "Heat Transfer Characteristics From a Flat Plate to a Gas–Solid Two-Phase Flow Downstream of a Slit Injection." Journal of Heat Transfer 110, no. 3 (August 1, 1988): 687–94. http://dx.doi.org/10.1115/1.3250546.
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The behavior of fine glass particles and their influence on fluid motion are investigated in a flow over a flat plate downstream of a two-dimensional slit injection. Heat transfer characteristics are examined in terms of the mass loading ratio of the particles ranging up to 0.8 and particle size varying from 68.6 to 148 μm in mean diameter. The particles promote turbulence of the fluid in a weakly turbulent flow, but suppress turbulence in a strongly turbulent flow. The heat transfer characteristics along the wall are well correlated to the variation of turbulent flow field due to the effect of the particles.
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Melamed, L. E., and G. A. Filippov. "The concept of turbulent «vortex backfill» - models and methods. Power engineering: research, equipment, technology." Power engineering: research, equipment, technology 21, no. 5 (December 17, 2019): 97–109. http://dx.doi.org/10.30724/1998-9903-2019-21-5-97-109.
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Models and methods for studying turbulence based on the concept of turbulent "vortex backfill" are presented. The essence of this concept is that the turbulent flow is considered as laminar, flowing through a "vortex backfill ", which creates internal resistance. This resistance can be considered either as distributed, or as locally concentrated. Based on the first representation, a modified Navier-Stokes equation, its approximate analytical and numerical solutions are obtained. Based on the second concept and the local fluctuation method developed for these purposes, a computer model of the turbulent flow in the pipes is obtained. Using simulation, it is shown that, when a certain system of local viscosity fluctuations is specified, the calculated flow profile corresponds to the profile of the turbulent flow velocity. The magnitude and profile of the turbulent viscosity of the flow are completely determined by the structure and properties of the "vortex backfill ". The results of the work confirm the possibility and efficiency of considering turbulence based on this concept.
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Siswantara, Ahmad Indra, Budiarso, and Steven Darmawan. "Investigation of Inverse-Turbulent-Prandtl Number with Four RNG k-ε Turbulence Models on Compressor Discharge Pipe of Bioenergy Micro Gas Turbine." Applied Mechanics and Materials 819 (January 2016): 392–400. http://dx.doi.org/10.4028/www.scientific.net/amm.819.392.
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Inverse-Turbulent Prandtl number (α) is an important parameter in RNG k-ε turbulence models since it affects the ratio of molecular viscosity and turbulent viscosity. In curved pipe, this highly affects the model prediction to a large range eddy-scale flow. According to Yakhot & Orzag, the α range from 1-1.3929 has not been investigated in detail in curved pipe flow (Yakhot & Orszag, 1986) and specific Re. This paper varied inverse-turbulent Prandtl number α to 1-1.3 in RNG k-ε turbulence model on cylindrical curved pipe in order to obtain the optimum value of α to predict unfully-developed flow in the curve with curve ratio R/D of 1.607. Analysis was conducted numericaly with inlet specified Re of 40900 which was generated from the experiment at α 1, 1.1, 1.2, 1.3. Wall surface roughness is not considered in this paper. With assumption that thermal diffusivity is always dominant to turbulent viscosity, higher Inverse-turbulent Prandtl number represent domination of turbulent viscosity to molecular viscosity of the flow and predict to have more interaction between large scale eddy to small scale eddy as well. The results show the use of α = 1.3 has increased the turbulent kinetic energy by 7% and the turbulent dissipation by 5% compared to general inverse-turbulent Prandtl number of 1. The value difference shows that the use of higher α on RNG turbulence model described more interaction between eddies in secondary and swirling flow at pipe curve at Re = 40900.
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Owolabi, Bayode E., Robert J. Poole, and David J. C. Dennis. "Experiments on low-Reynolds-number turbulent flow through a square duct." Journal of Fluid Mechanics 798 (June 3, 2016): 398–410. http://dx.doi.org/10.1017/jfm.2016.314.
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Previous experimental studies on turbulent square duct flow have focused mainly on high Reynolds numbers for which a turbulence-induced eight-vortex secondary flow pattern exists in the cross-sectional plane. More recently, direct numerical simulations (DNS) have revealed that the flow field at Reynolds numbers close to transition can be very different; the flow in this ‘marginally turbulent’ regime alternating between two states characterised by four vortices. In this study, we experimentally investigate the onset criteria for transition to turbulence in square ducts. In so doing, we highlight the potential importance of Coriolis effects on this process for low-Ekman-number flows. We also present experimental data on the mean flow properties and turbulence statistics in both marginally and fully turbulent flow at relatively low Reynolds numbers using laser Doppler velocimetry. Results for both flow categories show good agreement with DNS. The switching of the flow field between two flow states at marginally turbulent Reynolds numbers is confirmed by bimodal probability density functions of streamwise velocity at certain distances from the wall as well as joint probability density functions of streamwise and wall normal velocities which feature two peaks highlighting the two states.
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Yao, Jianfeng, Wenjuan Lou, Guohui Shen, Yong Guo, and Yuelong Xing. "Influence of Inflow Turbulence on the Flow Characteristics around a Circular Cylinder." Applied Sciences 9, no. 17 (September 2, 2019): 3595. http://dx.doi.org/10.3390/app9173595.
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To study the influence of turbulence on the wind pressure and aerodynamic behavior of smooth circular cylinders, wind tunnel tests of a circular cylinder based on wind pressure testing were conducted for different wind speeds and turbulent flows. The tests obtained the characteristic parameters of mean wind pressure coefficient distribution, drag coefficient, lift coefficient and correlation of wind pressure for different turbulence intensities and of Reynolds numbers. These results were also compared with those obtained by previous researchers. The results show that the minimum drag coefficient in the turbulent flow is basically constant at approximate 0.4 and is not affected by the turbulence intensity. When the Reynolds number is in the critical regime, the lift coefficient increased sharply to 0.76 in the smooth flow, indicating that flow separation has an asymmetry; however, the asymmetry does not appear in the turbulent flow. Drag coefficient decreases sharply at a lower critical Reynolds number in the turbulent flow than in the smooth flow. In the smooth flow, the separation point is about 80° in the subcritical regime; it suddenly moves backwards in the critical regime and remains almost unchanged at about 140° in the supercritical regime. However, the angular position of the separation point will always be about 140° for turbulent flow for the Reynolds number in these three regimes. Turbulence intensity and Reynolds number have a significant effect on the correlation of wind pressures around the circular cylinder. Turbulence will weaken the positive correlation of the same side and also reduce the negative correlation between the two sides of the circular cylinder.
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Kwon, Y. S., J. Philip, C. M. de Silva, N. Hutchins, and J. P. Monty. "The quiescent core of turbulent channel flow." Journal of Fluid Mechanics 751 (June 18, 2014): 228–54. http://dx.doi.org/10.1017/jfm.2014.295.
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AbstractThe identification of uniform momentum zones in wall-turbulence, introduced by Adrian, Meinhart & Tomkins (J. Fluid Mech., vol. 422, 2000, pp. 1–54) has been applied to turbulent channel flow, revealing a large ‘core’ region having high and uniform velocity magnitude. Examination of the core reveals that it is a region of relatively weak turbulence levels. For channel flow in the range $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}Re_{\tau } = 1000\text {--}4000$, it was found that the ‘core’ is identifiable by regions bounded by the continuous isocontour lines of the streamwise velocity at $0.95U_{CL}$ (95 % of the centreline velocity). A detailed investigation into the properties of the core has revealed it has a large-scale oscillation which is predominantly anti-symmetric with respect to the channel centreline as it moves through the channel, and there is a distinct jump in turbulence statistics as the core boundary is crossed. It is concluded that the edge of the core demarcates a shear layer of relatively intense vorticity such that the interior of the core contains weakly varying, very low-level turbulence (relative to the flow closer to the wall). Although channel flows are generally referred to as ‘fully turbulent’, these findings suggest there exists a relatively large and ‘quiescent’ core region with a boundary qualitatively similar to the turbulent/non-turbulent interface of boundary layers, jets and wakes.
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NIKITIN, NIKOLAY. "Four-dimensional turbulence in a plane channel." Journal of Fluid Mechanics 680 (May 4, 2011): 67–79. http://dx.doi.org/10.1017/jfm.2011.148.
Full textAbstract:
The four-dimensional (4D) incompressible Navier–Stokes equations are solved numerically for the plane channel geometry. The fourth spatial coordinate is introduced formally to be homogeneous and mathematically orthogonal to the others, similar to the spanwise coordinate. Exponential growth of small 4D perturbations superimposed onto 3D turbulent solutions was observed in the Reynolds number range from Re = 4000 to Re = 10000. The growth rate of small 4D perturbations expressed in wall units was found to be λ+4D = 0.016 independent of Reynolds number. Nonlinear evolution of 4D perturbations leads either to attenuation of turbulence and relaminarization or to establishment of a self-sustained 4D turbulent solution (4D turbulent flow). Both results on flow evolution were obtained at the lowest Reynolds number, depending on the grid resolution, pointing to the proximity of Re = 4000 as the critical Reynolds number for 4D turbulence. Self-sustained 4D turbulence appeared to be less intense compared with 3D turbulence in terms of mean wall friction, which is about 55% of that predicted by the empirical Dean law for turbulent channel flow at all Reynolds numbers considered. Thus, the law of resistance of 4D turbulent channel flow can be expressed as Cf = 0.04Re−0.25.
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Sarkar, S. "The stabilizing effect of compressibility in turbulent shear flow." Journal of Fluid Mechanics 282 (January 10, 1995): 163–86. http://dx.doi.org/10.1017/s0022112095000085.
Full textAbstract:
Direct numerical simulation of turbulent homogeneous shear flow is performed in order to clarify compressibility effects on the turbulence growth in the flow. The two Mach numbers relevant to homogeneous shear flow are the turbulent Mach number Mt and the gradient Mach number Mg. Two series of simulations are performed where the initial values of Mg and Mt are increased separately. The growth rate of turbulent kinetic energy is observed to decrease in both series of simulations. This ‘stabilizing’ effect of compressibility on the turbulent energy growth rate is observed to be substantially larger in the DNS series where the initial value of Mg is changed. A systematic comparison of the different DNS cases shows that the compressibility effect of reduced turbulent energy growth rate is primarily due to the reduced level of turbulence production and not due to explicit dilatational effects. The reduced turbulence production is not a mean density effect since the mean density remains constant in compressible homogeneous shear flow. The stabilizing effect of compressibility on the turbulence growth is observed to increase with the gradient Mach number Mg in the homogeneous shear flow DNS. Estimates of Mg for the mixing layer and the boundary layer are obtained. These estimates show that the parameter Mg becomes much larger in the high-speed mixing layer relative to the high-speed boundary layer even though the mean flow Mach numbers are the same in the two flows. Therefore, the inhibition of turbulent energy production and consequent ‘stabilizing’ effect of compressibility on the turbulence (over and above that due to any mean density variation) is expected to be larger in the mixing layer relative to the boundary layer, in agreement with experimental observations.
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Ikeda, Yuji. "The Interaction between In-Cylinder Turbulent Flow and Flame Front Propagation in an Optical SI Engine Measured by High-Speed PIV." Energies 15, no. 8 (April 11, 2022): 2783. http://dx.doi.org/10.3390/en15082783.
Full textAbstract:
The relationship between the flow field and flame propagation is essential in determining the dynamics and effects of turbulent flow in an optical SI engine. In this study, high turbulence flow at stable operations was achieved using 12,000 rpm engine speed, 60 kPa absolute intake pressure, 14.7 A/F, and 15 deg. BTDC spark timing. The turbulent flow field and flame propagation interplay were analyzed through the simultaneous high-speed PIV measurements of the in-cylinder flow and flame front propagation under firing conditions. The intensity of the seeder used was optimized by changing the crank angle. Successful simultaneous detection of the flame front and turbulent flow was demonstrated. Strong turbulence was produced at the flame front simultaneously with the flame movement. After ignition timing, the flame accelerated in the unburned region, and a vital turbulence region occurred.
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Wisler, D. C., R. C. Bauer, and T. H. Okiishi. "Secondary Flow, Turbulent Diffusion, and Mixing in Axial-Flow Compressors." Journal of Turbomachinery 109, no. 4 (October 1, 1987): 455–69. http://dx.doi.org/10.1115/1.3262127.
Full textAbstract:
The relative importance of convection by secondary flows and diffusion by turbulence as mechanisms responsible for mixing in multistage, axial-flow compressors has been investigated by using the ethylene tracer-gas technique and hot-wire anemometry. The tests were conducted at two loading levels in a large, low-speed, four-stage compressor. The experimental results show that considerable cross-passage and spanwise fluid motion can occur and that both secondary flow and turbulent diffusion can play important roles in the mixing process, depending upon location in the compressor and loading level. In the so-called freestream region, turbulent diffusion appeared to be the dominant mixing mechanism. However, near the endwalls and along airfoil surfaces at both loading levels, the convective effects from secondary flow were of the same order of magnitude as, and in some cases greater than, the diffusive effects from turbulence. Calculations of the secondary flowfield and mixing coefficients support the experimental findings.