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Artykuły w czasopismach na temat "Stream Convecting Vortex"
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Garmann, D. J., i M. R. Visbal. "Interactions of a streamwise-oriented vortex with a finite wing". Journal of Fluid Mechanics 767 (24.02.2015): 782–810. http://dx.doi.org/10.1017/jfm.2015.51.
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AbstractA canonical study is developed to investigate the unsteady interactions of a streamwise-oriented vortex impinging upon a finite surface using high-fidelity simulation. As a model problem, an analytically defined vortex superimposed on a free stream is convected towards an aspect-ratio-six ($\mathit{AR}=6$) plate oriented at an angle of ${\it\alpha}=4^{\circ }$ and Reynolds number of $\mathit{Re}=20\,000$ in order to characterize the unsteady modes of interaction resulting from different spanwise positions of the incoming vortex. Outboard, tip-aligned and inboard positioning are shown to produce three distinct flow regimes: when the vortex is positioned outboard of, but in close proximity to, the wingtip, it pairs with the tip vortex to form a dipole that propels itself away from the plate through mutual induction, and also leads to an enhancement of the tip vortex. When the incoming vortex is aligned with the wingtip, the tip vortex is initially strengthened by the proximity of the incident vortex, but both structures attenuate into the wake as instabilities arise in the pair’s feeding sheets from the entrainment of opposite-signed vorticity into either structure. Finally, when the incident vortex is positioned inboard of the wingtip, the vortex bifurcates in the time-mean sense with portions convecting above and below the wing, and the tip vortex is mostly suppressed. The time-mean bifurcation is actually a result of an unsteady spiralling instability in the vortex core that reorients the vortex as it impacts the leading edge, pinches off, and alternately attaches to either side of the wing. The increased effective angle of attack inboard of impingement enhances the three-dimensional recirculation region created by the separated boundary layer off the leading edge which draws fluid from the incident vortex inboard and diminishes its impact on the outboard section of the wing. The slight but remaining downwash present outboard of impingement reduces the effective angle of attack in that region, resulting in a small separation bubble on either side of the wing in the time-mean solution, effectively unloading the tip outboard of impingement and suppressing the tip vortex. All incident vortex positions provide substantial increases in the wing’s lift-to-drag ratio; however, significant sustained rolling moments also result. As the vortex is brought inboard, the rolling moment diminishes and eventually switches sign as the reduced outboard loading balances the augmented sectional lift inboard of impingement.
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Kislitsyn, S. A., i V. S. Berdnikov. "Numerical studies of the advective flow of heptadecane in a horizontal layer with a longitudinal temperature gradient at the lower boundary". Journal of Physics: Conference Series 2119, nr 1 (1.12.2021): 012085. http://dx.doi.org/10.1088/1742-6596/2119/1/012085.
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Abstract Numerical studies of the convective flow of heptadecane in a horizontal layer with a suddenly applied longitudinal temperature gradient at the lower high-thermal conductivity boundary have been carried out by the finite element method. A system of nonstationary dimensionless equations of free convection containing stream function, velocity vortex, and temperature as variables was solved. The calculations were carried out with a free upper boundary with and without taking into account the influence of the thermocapillary effect.
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Radomsky, R. W., i K. A. Thole. "High Free-Steam Turbulence Effects on Endwall Heat Transfer for a Gas Turbine Stator Vane". Journal of Turbomachinery 122, nr 4 (1.02.2000): 699–708. http://dx.doi.org/10.1115/1.1312807.
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High free-stream turbulence along a gas turbine airfoil and strong secondary flows along the endwall have both been reported to increase convective heat transfer significantly. This study superimposes high free-stream turbulence on the naturally occurring secondary flow vortices to determine the effects on the flowfield and the endwall convective heat transfer. Measured flowfield and heat transfer data were compared between low free-stream turbulence levels (0.6 percent) and combustor simulated turbulence levels (19.5 percent) that were generated using an active grid. These experiments were conducted using a scaled-up, first-stage stator vane geometry. Infrared thermography was used to measure surface temperatures on a constant heat flux plate placed on the endwall surface. Laser-Doppler Velocimetry (LDV) measurements were performed of all three components of the mean and fluctuating velocities of the leading edge horseshoe vortex. The results indicate that the mean flowfields for the leading edge horseshoe vortex were similar between the low and high free-stream turbulence cases. High turbulence levels in the leading edge–endwall juncture were attributed to a vortex unsteadiness for both the low and high free-stream turbulence cases. While, in general, the high free-stream turbulence increased the endwall heat transfer, low augmentations were found to coincide with the regions having the most intense vortex motions. [S0889-504X(00)00704-2]
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Stechman, Daniel M., Robert M. Rauber, Greg M. McFarquhar, Brian F. Jewett i David P. Jorgensen. "Interaction of an Upper-Tropospheric Jet with a Squall Line Originating along a Cold Frontal Boundary". Monthly Weather Review 144, nr 11 (10.10.2016): 4197–219. http://dx.doi.org/10.1175/mwr-d-16-0044.1.
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Abstract On 8 June 2003, an expansive squall line along a surface cold frontal boundary was sampled during the Bow Echo and Mesoscale Convective Vortex Experiment. The Naval Research Laboratory P-3 aircraft and the National Oceanic and Atmospheric Administration P-3 aircraft simultaneously sampled the leading and trailing edge of this squall line, respectively, with X-band Doppler radars. Data from these two airborne radar systems have been synthesized to produce a pseudo-quad-Doppler analysis of the squall line, yielding a detailed three-dimensional kinematic analysis of its structure. A simulation of the squall line was carried out using the Weather Research and Forecasting Model to complement the pseudo-quad-Doppler analysis. The simulation employed a 3-km, convection-allowing, nested domain centered over the pseudo-quad-Doppler domain, along with a 9-km parent domain to capture the larger synoptic-scale cyclone. The pseudo-quad-Doppler analysis reveals that the convective line was embedded within the upper-tropospheric jet stream, causing local decelerations and deviations in the jet-level flow. The vertical transport of low momentum air from the boundary layer via convective updrafts is shown to significantly decelerate jet-level flow. Pressure perturbations associated with the intrusion of low momentum air into the jet stream–level flow led to deviation of the jet stream flow around the squall line that resulted in counter-rotating ribbons of vertical vorticity parallel to the squall line. Model results indicate that disturbances in the jet stream structure persisted downwind of the squall line for several hours.
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Kumar, Bhaskar, i Sanjay Mittal. "On the origin of the secondary vortex street". Journal of Fluid Mechanics 711 (24.09.2012): 641–66. http://dx.doi.org/10.1017/jfm.2012.421.
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AbstractThe origin of the secondary vortex street, observed in the far wake in the flow past a circular cylinder, is investigated. The Reynolds number, based on the diameter of the cylinder, is 150. The von Kármán vortex street, which originates in the near wake, decays exponentially downstream of the cylinder. Beyond the region of decay, a broad band of frequencies are selectively amplified, leading to the formation of a secondary vortex street consisting of packets of large-scale vortex structures. The streamwise location of the onset of the instability, frequency of the generation of packets and their convection speed are estimated via direct numerical simulation (DNS). Global linear stability analysis of the time-averaged flow reveals the presence of unstable convective modes that travel at almost the same speed and have a structure similar to the packet-like disturbances as observed in the DNS. Sensitivity analysis of the global convective modes to structural perturbations is carried out to locate the region of the wake that is most significant in generating the modes responsible for the appearance of the secondary vortex street. This information is utilized to control the flow. By placing a ‘slip’ splitter plate along the wake centre line, in the overlap region of the direct and the adjoint modes, the oscillations in the far wake are significantly reduced, though the oscillations related to the primary vortex shedding in the near wake are not. It is also found that suppression of the primary vortex shedding leads to annihilation of the secondary vortex street as well. Linear stability analysis of the steady-state flow does not yield any modes that can explain the appearance of the secondary vortex street. The steady and time-averaged wake profiles, for the $\mathit{Re}= 150$ flow, are compared to bring out the differences in the two. The effect of free-stream oscillations on the evolution of the secondary vortex street is investigated. By reducing the amplitude of inlet excitation, a gradual transition from ordered shedding in the far wake to the appearance of a broad-band spectrum of frequencies, as in the unforced wake, is observed. All the computations have been carried out using a stabilized finite element method.
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Mandal, A. C., i J. Dey. "An experimental study of boundary layer transition induced by a cylinder wake". Journal of Fluid Mechanics 684 (1.09.2011): 60–84. http://dx.doi.org/10.1017/jfm.2011.270.
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AbstractBoundary layer transition induced by the wake of a circular cylinder in the free stream has been investigated using the particle image velocimetry technique. Some differences between simulation and experimental studies have been reported in the literature, and these have motivated the present study. The appearance of spanwise vortices in the early stage is further confirmed here. A spanwise vortex appears to evolve into a $ \mrm{\Lambda} $/hairpin vortex; the flow statistics also confirm such vortices. With increasing Reynolds number, based on the cylinder diameter, and with decreasing cylinder height from the plate, the physical size of these hairpin-like structures is found to decrease. Some mean flow characteristics, including the streamwise growth of the disturbance energy, in a wake-induced transition resemble those in bypass transition induced by free stream turbulence. Streamwise velocity streaks that are eventually generated in the late stage often undergo sinuous-type oscillations. Similar to other transitional flows, an inclined shear layer in the wall-normal plane is often seen to oscillate and shed vortices. The normalized shedding frequency of these vortices, estimated from the spatial spacing and the convection velocity of these vortices, is found to be independent of the Reynolds number, similar to that in ribbon-induced transition. Although the nature of free stream disturbance in a wake-induced transition and that in a bypass transition are different, the late-stage features including the flow breakdown characteristics of these two transitions appear to be similar.
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Chang, Keun-Shik, i Jong-Youb Sa. "The effect of buoyancy on vortex shedding in the near wake of a circular cylinder". Journal of Fluid Mechanics 220 (listopad 1990): 253–66. http://dx.doi.org/10.1017/s002211209000324x.
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The phenomenon of vortex shedding from a heated/cooled circular cylinder has been investigated numerically in the mixed natural and forced convection regimes. Accuracy of the computation was achieved by the fourth-order Hermitian relation applied to the contravariant velocity components in the convection terms of the vorticity transport equation, and by the far-boundary stream-function condition of an integral-series form developed by the authors. Purely periodic flows at Re = 100, efficiently established through the use of a direct elliptic solver called the SEVP, was found to degenerate into a steady twin-vortex pattern at the critical Grashof number 1500, confirming an earlier experimental observation identified as ‘breakdown of the Kármán vortex street’. Various other buoyancy effects about the heated/cooled cylinder are discussed by means of the flow patterns, the Nusselt number and the drag coefficient curves.
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Moharreri, S. S., B. F. Armaly i T. S. Chen. "Measurements in the Transition Vortex Flow Regime of Mixed Convection Above a Horizontal Heated Plate". Journal of Heat Transfer 110, nr 2 (1.05.1988): 358–65. http://dx.doi.org/10.1115/1.3250492.
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Experimental results covering the transition vortex flow regime of mixed convection over a heated, horizontal flat plate are presented. A criterion for the onset of vortex instability as a function of critical Reynolds and Grashof numbers was established with the aid of a flow visualization technique. The three-dimensional nature of this flow regime was documented through both velocity and temperature measurements using laser-Doppler and hot/cold-wire anemometers, respectively. A higher buoyancy force, through a higher plate temperature or a larger downstream distance, and/or a lower free-stream velocity, intensifies the strength of the vortices. Velocity and temperature profiles through vortex peaks and valleys are reported to quantify the behavior of these vortices. It has been found from these measurements that the two-dimensional laminar mixed convection flow changes into a transitional three-dimensional vortex flow in a relatively short distance from the leading edge of the plate. The vortex three-dimensional flow continues to intensify as the buoyancy force increases and then develops into a two-dimensional fully turbulent flow at the end of the transition regime. These findings place an upper limit on the applicability of the two-dimensional, laminar boundary layer flow analysis for mixed convection over a heated horizontal flat plate.
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Heidarzadeh, Habibollah, Mousa Farhadi i Kurosh Sedighi. "Convective heat transfer over a wall mounted cube at different angle of attack using large eddy simulation". Thermal Science 18, suppl.2 (2014): 301–15. http://dx.doi.org/10.2298/tsci110614088h.
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Turbulent fluid flow and convective heat transfer over the wall mounted cube in different flow angle of attack have been studied numerically using Large Eddy Simulation. Cube faces and plate have a constant heat flux. Dynamic Smagorinsky (DS) subgrid scale model were used in this study. Angles were in the range 0???45 and Reynolds number based on the cube height and free stream velocity was 4200. The numerical simulation results were compared with the experimental data of Nakamura et al [6, 7]. Characteristics of fluid flow field and heat transfer compared for four angles of attack. Flow around the cube was classified to four regimes. Results was represented in the form of time averaged normalized streamwise velocity and Reynolds stress in different positions, temperature contours, local and average Nusselt number over the faces of cube. Local convective heat transfer on cube faces was affected by flow pattern around the cube. The local convective heat transfer from the faces of the cube and plate are directly related to the complex phenomena such as horse shoe vortex, arch vortexes in behind the cube, separation and reattachment. Results show that overall convective heat transfer of cube and mean drag coefficient have maximum and minimum value at ?=0 deg and ?=25 deg respectively.
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Hall, Philip. "Vortex–wave interactions: long-wavelength streaks and spatial localization in natural convection". Journal of Fluid Mechanics 703 (12.06.2012): 99–110. http://dx.doi.org/10.1017/jfm.2012.196.
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AbstractThe unidirectional shear flow driven by buoyancy effects in a vertical channel when a temperature difference is maintained between the walls of the channel is considered. The flow is unstable to waves which interact to reinforce the original flow and make it ‘streaky’. Such ‘vortex–wave’ interactions have been the subject of much recent research but little is yet known about what happens when the wavelength of the roll/streak flow becomes large. An asymptotic structure for long-wavelength interactions is derived and the tendency of the fluid to resist this state and the flow to become localized is revealed. Here the high-Grashof-number limit is considered and it is shown how a self-sustained process can occur with vortices interacting with a wave system in a manner similar to that discussed by Hall & Smith (J. Fluid Mech., vol. 227, 1991, pp. 641–666) and Hall & Sherwin (J. Fluid Mech., vol. 661, 2010, pp. 178–205). The work is closely related to numerical simulations of self-sustained processes in for example Couette flow but the fact that the basic flow here is generated by buoyancy effects enables us to make analytical progress. It is shown that the wave part of the interaction process has a flat critical layer and its wavelength is twice that of the streaky flow which supports it. Such subharmonic vortex–wave/self-sustained process interactions have not been previously identified.
Rozprawy doktorskie na temat "Stream Convecting Vortex"
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Olley, Peter, Timothy D. Gough, R. Spares i Philip D. Coates. "An experimental and simulation comparison of a 3-D abrupt contraction flow using the Molecular Stress Function constitutive model". Maney Publishing, 2020. http://hdl.handle.net/10454/18042.
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YesThe Molecular Stress Function (MSF) constitutive model with convective constraint release mechanism has been shown to accurately fit a large range of viscometric data, and also shown to give strong vortex growth in flows of LDPE through planar and axisymmetric contractions. This work compares simulation and experimental results for 3-D flows of Lupolen 1840H LDPE through a contraction slit; 3-D effects are introduced by using a slit with a low upstream aspect ratio of 5:3. Comparisons are made with vortex opening angles obtained from streak photography, and also with stress birefringence measurements. The comparisons are made with two versions of the convective constraint release (CCR) mechanism. The simulated vortex angles for one version of the CCR mechanism are found to approach what is seen experimentally. The best-fit value for the stress optical coefficient was found to vary between CCRs and to decrease with flow rate. This is partially explained by different centreline elongational rates with the two CCRs, which in turn is related to different opening angles. A 3-D simulation is compared to the corresponding 2-D simulation. It is shown that both velocity vectors and birefringence show only small changes to around 60% of the distance to the side wall.
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Dumouchel, Fabien. "Etude expérimentale des champs dynamiques et thermiques de l'écoulement de Benard-Von Karman en aval d'un obstacle chauffé dans l'air et dans l'eau". Rouen, 1997. http://www.theses.fr/1997ROUES067.
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Cette thèse a pour objet l'étude expérimentale de l'écoulement en aval d'un obstacle chauffé ou non (cylindre circulaire ou ruban) dans une gamme de nombres de Reynolds où le sillage 2D-périodique est caractérisé par l'existence d'une allée de Benard-Von Karman. Cette étude a été entreprise dans l'air et dans l'eau, en utilisant l'ADL pour la mesure des vitesses et la thermométrie à fil froid ou à thermocouple à fil fin pour la mesure des températures. Des études antérieures ont montré que ce type d'écoulement était très sensible au chauffage de l'obstacle, conduisant à une stabilisation dans le cas des gaz et à une déstabilisation dans le cas des liquides. L'objectif principal de cette étude a été de prendre en compte l'influence du chauffage sur le régime de l'écoulement quand ce contrôle thermique est uniquement dû aux modifications des propriétés physiques du fluide. Après une étude du champ de vitesse en aval d'un obstacle non chauffé, la même étude est entreprise dans le cas où l'obstacle est chauffé. Pour appréhender l'effet du chauffage sur le régime d'écoulement, les notions de température effective et de nombre de Reynolds effectif re#e#f#f ont été développées. L'ensemble des résultats obtenus dans l'air et dans l'eau montre que les caractéristiques de l'écoulement sont similaires dans des situations ou les re#e#f#f sont identiques, pour des valeurs différentes des vitesses amonts et du chauffage. L'utilisation de re#e#f#f permet également de comparer les champs thermiques dans l'air et dans l'eau. L'effet du nombre de Prandtl se traduit par des largeurs initiales des sillages thermiques beaucoup plus faibles dans l'eau que dans l'air et des écarts de température moyenne et des intensités des fluctuations de température beaucoup plus élevées dans l'eau que dans l'air. En revanche le nombre de Nusselt de l'obstacle chauffé dépend d'un nombre de Reynolds intermédiaire entre re#f#i#l#m et re#e#f#f.
Książki na temat "Stream Convecting Vortex"
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Kolmičkovs, Antons. Electric Field Effect on Combustion of Pelletized Biomass in Swirling Flow. RTU Press, 2022. http://dx.doi.org/10.7250/9789934227257.
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The Doctoral Thesis examines the control of the swirling flame flow dynamics with an external static electric field by firing the gaseous products of thermal decomposition of pelletized straw, woody biomass, and peat with the aim of more efficient heat production with a decrease of flue gas emissions. The intensification of the downward vortex in the electric field has been determined, ensuring improved mixing of the air vortex with the biomass thermal decomposition gas flow, intensifying the convective mass transfer towards the heating surfaces, and increasing the amount of heat energy produced in the biomass thermochemical conversion process.
Części książek na temat "Stream Convecting Vortex"
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Kennel, Charles F. "The Viscous Magnetosphere". W Convection and Substorms. Oxford University Press, 1996. http://dx.doi.org/10.1093/oso/9780195085297.003.0007.
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This chapter describes how the magnetosphere is shaped by the tangential shear stress exerted at the magnetopause by collisionless viscosity. In Section 4.2, we discuss the low-latitude boundary layer (LLBL), which contains plasma of solar wind origin that has been transported across the magnetopause current layer. The velocity shear in the LLBL drives field-aligned currents into the ionosphere on the morning side and out of the ionosphere on the evening side (Section 4.3). These currents are of the appropriate sense to drive two-cell convection in the highlatitude ionosphere. The footprint of the LLBL in the ionosphere to which the field aligned currents connect is clearly identifiable by its characteristic particle precipitation (Section 4.4). The shear in the LLBL also generates 1-20 mHz PC 4- 5 micropulsations whose polarizations, tailward propagation, and phase speeds are consistent with the Kelvin-Helmholtz (K-H) instability (Section 4.5). The K-H vortices may couple to “vortex auroras” in the local afternoon sector of the auroral oval (Section 4.6). Vortex auroral dissipation may be responsible for a morningevening asymmetry in the viscous interaction and its manifestations. Organized vortical flows have been observed not only next to the magnetopause, but also near the center of the plasma sheet, accompanied by local quasiperiodic magnetic field oscillations and PC 5 micropulsations on the ground (Section 4.7). In Section 4.8, we discuss observations of a thick boundary layer flow on closed field lines next to the magnetopause 220 RE downstream. This puts us in a position to estimate the rates of particle and energy injection into the magnetosphere due to the viscous interaction (Section 4.9). Spacecraft crossings of the magnetopause last from a few seconds to a few minutes and are characterized by a rapid, distinct rotation of the magnetic field and striking changes in plasma density, pressure, flow velocity, composition, and energetic particle distribution (Williams, 1979a; 1980; Williams et al., 1979). A broader boundary layer lies just inside the magnetopause. The so-called low-latitude boundary layer was first identified at 18 RE radial distance in the magnetotail using Vela 4B (Hones et al., 1972) and Vela 5 and 6 (Akasofu et al., 1973b) low-energy plasma measurements.
Streszczenia konferencji na temat "Stream Convecting Vortex"
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Baranyi, La´szlo´. "Forced Convection From a Stationary Cylinder Placed in a Uniform Stream". W ASME/JSME 2003 4th Joint Fluids Summer Engineering Conference. ASMEDC, 2003. http://dx.doi.org/10.1115/fedsm2003-45620.
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A finite difference solution is presented for 2D laminar unsteady flow around and forced convection from a stationary cylinder placed in an otherwise uniform flow. The fluid is assumed to be incompressible and of constant property. The temperature of the cylinder wall is kept constant, and the viscous energy dissipation term is neglected in the energy equation. The computed Strouhal numbers, time-mean drag and base pressure coefficients, as well as the average Nusselt numbers compare well with existing experimental results. The distribution of the local Nusselt number on the cylinder was investigated over a complete vortex shedding cycle, and shifts in Nu belonging to different phases were identified.
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Martynenko, Alex, Ivanna Bashkir i Tadeusz Kudra. "Electrically enhanced drying of white champignons". W 21st International Drying Symposium. Valencia: Universitat Politècnica València, 2018. http://dx.doi.org/10.4995/ids2018.2018.7318.
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Effects of convective cross-flow of air in electrohydrodynamic (EHD) drying on drying rate of 5 mm slices of champignons have been investigated. Electro-convection issued from discharge electrode (42 needles arranged into 6×7 rows with 2×2 cm spacing, 18 kV DC voltage and 3.5 cm gap) provided average ionic wind velocity of 1.0 m/s flowing perpendicularly to the surface of champignons slices, while forced air stream at atmospheric pressure 1000 kPa, superficial velocity 1.0 m/s, temperature 22-24°C, and relative humidity 25-40%, was blown parallel to the surface of champignons slices. To study interactions between forced air cross-flow and electro-convection, the experimental protocol was designed, exploring three cases in various combinations: (1) Sole EHD, (2) air cross-flow, and (3) EHD with simultaneous air cross-flow. The case # 3 was found to be the most efficient, resulting in 10.2 g/h of water evaporation whereas drying rate was 6.6 g/h (# 1) and 3.6 g/h for (# 2). Such numbers imply that these effects are additive. In some combinations the effect of air cross-flow was the same (3.6 g/g), but electro-convection was significantly suppressed to 3.2 g/h likely because air stream removed surface water, which reduced charge transfer and electro-diffusion.In trials with different initial moisture content it was found that drying kinetics followed exponential decay in the wide range of initial moisture contents from 4.9 to 12.0 g/g (db). Drying rate due to forced air convection was found to be independent of moisture content, whereas drying rate due to electro-convection significantly depended on the moisture content. For example, the EHD drying rate of fresh-cut champignons slices with initial moisture content 10.74 g/g was 0.237 g/h, while the slices after two days in the cooler (initial moisture content dropped to 4.92 g/g) it was 0.418 g/h. Also, it was found that electro-convective drying could not remove all residual water. At the end of drying the equilibrium moisture content attained 0.2 - 0.3 g/g (aw~0.3).It appears that performance of EHD drying depends also on the product porosity as water can exist as free in open pores or be trapped in closed pores. In some experiments we observed rotation of champignon slices in the plane perpendicular to ionic wind. It happened at the end of drying when slices were light enough to be lifted by electrostatic force and dragged by the vortex. This phenomenon could be attributed either to the effect of DC electric field on polarized water molecules trapped in closed pores, or it could be electrostatic effect of ionic wind on charged porous body. Also, the hypothesis that EHD has both linear and rotational (vortex) components require further investigation.
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Juniper, Matthew P. "Absolute and Convective Instability in Gas Turbine Fuel Injectors". W ASME Turbo Expo 2012: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2012. http://dx.doi.org/10.1115/gt2012-68253.
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Hydrodynamic instabilities in gas turbine fuel injectors help to mix the fuel and air but can sometimes lock into acoustic oscillations and contribute to thermoacoustic instability. This paper describes a linear stability analysis that predicts the frequencies and strengths of hydrodynamic instabilities and identifies the regions of the flow that cause them. It distinguishes between convective instabilities, which grow in time but are convected away by the flow, and absolute instabilities, which grow in time without being convected away. Convectively unstable flows amplify external perturbations, while absolutely unstable flows also oscillate at intrinsic frequencies. As an input, this analysis requires velocity and density fields, either from a steady but unstable solution to the Navier–Stokes equations, or from time-averaged numerical simulations. In the former case, the analysis is a predictive tool. In the latter case, it is a diagnostic tool. This technique is applied to three flows: a swirling wake at Re = 400, a single stream swirling fuel injector at Re ∼ 106, and a lean premixed gas turbine injector with five swirling streams at Re ∼ 106. Its application to the swirling wake demonstrates that this technique can correctly predict the frequency, growth rate and dominant wavemaker region of the flow. It also shows that the zone of absolute instability found from the spatio-temporal analysis is a good approximation to the wavemaker region, which is found by overlapping the direct and adjoint global modes. This approximation is used in the other two flows because it is difficult to calculate their adjoint global modes. Its application to the single stream fuel injector demonstrates that it can identify the regions of the flow that are responsible for generating the hydrodynamic oscillations seen in LES and experimental data. The frequencies predicted by this technique are within a few percent of the measured frequencies. The technique also explains why these oscillations become weaker when a central jet is injected along the centreline. This is because the absolutely unstable region that causes the oscillations becomes convectively unstable. Its application to the lean premixed gas turbine injector reveals that several regions of the flow are hydrodynamically unstable, each with a different frequency and a different strength. For example, it reveals that the central region of confined swirling flow is strongly absolutely unstable and sets up a precessing vortex core, which is likely to aid mixing throughout the injector. It also reveals that the region between the second and third streams is slightly absolutely unstable at a frequency that is likely to coincide with acoustic modes within the combustion chamber. This technique, coupled with knowledge of the acoustic modes in a combustion chamber, is likely to be a useful design tool for the passive control of mixing and combustion instability.
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Manca, O., S. Nardini, D. Ricci i S. Tamburrino. "A Numerical Investigation on Nanofluid Laminar Mixed Convection in Confined Impinging Jets". W ASME 2013 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2013. http://dx.doi.org/10.1115/imece2013-65915.
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In this paper a numerical investigation on confined impinging slot jets working with a mixture of water and Al2O3 nanoparticles is described. The flow is laminar and a constant temperature is applied on the target surface. A single-phase model approach has been adopted because the particle concentrations are low. Different geometric ratios and nanoparticle volume concentrations have been considered at different Richardson numbers in order to take into account also the buoyancy effects. The aim consists into study the behaviour of the system by means of average and local Nusselt number, convective heat transfer coefficient and required pumping power profiles, temperature fields and stream function contours. The dimensionless stream function contours showed that the intensity and size of the vortex structures depend on the confining effects, given by H/W ratio, Richardson number and particle concentrations. Furthermore, for increasing concentrations, nanofluids realize increasing fluid bulk temperature, as a result of the elevated thermal conductivity of mixtures. The local Nusselt number profiles show the highest values at the stagnation point and they depend on Richardson number and particle concentrations.
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Molki, Majid. "Heat Transfer From a Short Cylinder Situated Parallel to an Air Stream". W ASME 2021 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2021. http://dx.doi.org/10.1115/imece2021-73293.
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Abstract This work presents heat transfer for isothermal cylinders situated parallel to a uniform flow, and it compares the local and average heat transfer coefficients to several experimental data. The computations consider air as the working fluid, but the paper proposes an extension of correlations to other fluids. In this study, the cylinder length-to-diameter ratio is equal to one, and the flow Reynolds number ranges from 5000 to 42,000. The results show that the convective heat transfer coefficient for the cylinder’s front face increases radially outward in agreement with the available experimental data. The theoretical stagnation point Nusselt number agrees with the present results. The flow separation and recirculation on the cylinder side influence the local heat transfer. The wake region behind the cylinder fluctuates, but no vortex shedding is visible like a cylinder in cross flow. The heat flux on the cylinder lateral surface shows slight azimuthal variations possibly caused by the wake region oscillations. Local and average Nusselt number and heat flux contours are among the results presented in this paper.
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Dalton, Charles, i Wu Zheng. "Numerical Solutions of a Viscous Uniform Approach Flow Past Square and Diamond Cylinders". W ASME 2002 International Mechanical Engineering Congress and Exposition. ASMEDC, 2002. http://dx.doi.org/10.1115/imece2002-32287.
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Numerical results are presented for a uniform approach flow past square and diamond cylinders, with and without rounded corners, at Reynolds numbers of 250 and 1000. This unsteady viscous flow problem is formulated by the 2-D Navier-Stokes equations in vorticity and stream-function form on body-fitted coordinates and solved by a finite-difference method. Second-order Adams-Bashforth and central-difference schemes are used to discretize the vorticity transport equation while a third-order upwinding scheme is incorporated to represent the nonlinear convective terms. A grid generation technique is applied to provide an efficient mesh system for the flow. The elliptic partial differential equation for stream-function and vorticity in the transformed plane is solved by the multigrid iteration method. The Strouhal number and the average in-line force coefficients agree very well with the experimental and previous numerical values. The vortex structures and the evolution of vortex shedding are illustrated by vorticity contours. Rounding the corners of the square and diamond cylinders produced a noticeable decrease on the calculated drag and lift coefficients.
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Chen, Chin-Lung, i Chin-Hsiang Cheng. "Natural Convection Heat Transfer and Flow Pattern in an Inclined Arc-Shape Enclosure". W ASME 2001 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2001. http://dx.doi.org/10.1115/imece2001/htd-24122.
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Abstract Numerical and experimental investigations have been conducted to study the flow and heat transfer characteristics for the buoyancy-induced flow inside an inclined arc-shape enclosure. Mathematical model in form of a stream function-vorticity formulation representing the laws of conservation in mass, momentum, and energy is expressed in a curvilinear coordinate frame and solved by a finite-volume discretization method. Heat transfer and flow pattern are predicted at various Grashof numbers and inclination angles. Meanwhile, an experimental system is developed and a flow-visualization technique using smoke is employed to observe the flow pattern. Results show that when the Grashof number is higher than 105, the increase in natural convection heat transfer becomes appreciable. The vortex strength and pattern are found to be greatly dependent on the inclination angle. The range of the Grashof number considered in this study is up to 107 and the inclination angle is varied from 0 to π.
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Saeidi, S. M., i J. M. Khodadadi. "Flow Field and Heat Transfer in a Cavity With Inlet and Outlet Ports Due to Incoming Flow Oscillation". W ASME 2005 Summer Heat Transfer Conference collocated with the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems. ASMEDC, 2005. http://dx.doi.org/10.1115/ht2005-72677.
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A finite-volume-based computational study of transient laminar flow and heat transfer (neglecting natural convection) within a square cavity with inlet and outlet ports due to an oscillating velocity at the inlet port is presented. The inlet port is placed at the top of the left wall and the outlet port is positioned at the bottom of the right wall of the cavity. The inlet velocity varies sinusoidally with time for a range of dimensionless frequencies (St = 0.1, 0.5, 1, 2 and 10). The instantaneous Reynolds number also varies sinusoidally between 100 and 500 and Pr = 5. It takes more cycles for the temperature field to reach its periodic state in comparison to the corresponding flow field. For cases with greater Strouhal numbers, it takes more cycles to reach a periodic state. The throughflow stream undergoes cyclic growth and decay. The throughflow is in constant contact with the CW rotating primary vortex, which in turn interacts with two rotating vortices on the left wall. A CCW rotating vortex at the top right corner also experiences periodic growth and decay. Minimal heat transfer is consistently observed on the left wall. In contrast, certain segments of the other three walls and the boundary of the throughflow are zones of active heat exchange. For St = 0.1, the mean Nusselt numbers on the four walls clearly exhibit large amplitudes of oscillation and periodicity. With St = 10, the amplitudes of oscillation are degraded. These behaviors are directly linked to the relation between the period of oscillation and the convection time scale. Regardless of the Strouhal number, heat transfer enhancement in comparison to the steady state case is consistently observed. The best heat transfer rate is realized when the Strouhal number is close to unity. This is the case when the period of the incoming stream resonates with the convection time scale.
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Arik, Mehmet, i Yogen Utturkar. "Vortex Dynamics of Synthetic Jets: A Computational and Experimental Investigation". W 2010 14th International Heat Transfer Conference. ASMEDC, 2010. http://dx.doi.org/10.1115/ihtc14-23099.
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Seamless advancements in electronics industry resulted in high performance computing. These innovations lead to smaller electronics systems with higher heat fluxes than ever. However, shrinking nature of real estate for thermal management has created a need for more effective and compact cooling solutions. Novel cooling techniques have been of interest to solve the demand. One such technology that functions with the principle of creating vortex rings is called synthetic jets. The jets are meso-scale devices operating as zero-net-mass-flux principle by ingesting and ejection of high velocity working fluid from a single opening. These devices produce periodic jet streams, which may have peak velocities over 20 times greater than conventional, comparable size fan velocities. Those jets enhance heat transfer in both natural and forced convection significantly over bare and extended surfaces. Recognizing the heat transfer physics over surfaces require a fundamental understanding of the flow physics caused by micro fluid motion. A comprehensive computational and experimental study has been performed to understand the flow physics of a synthetic jet. Computational study has been performed via Fluent commercial software, while the experimental study has been performed by using Laser Doppler Anemometry. Since synthetic jets are typical sine-wave excited between 20 and 60 V range, they have an orifice peak velocity of over 60 m/s, resulting in a Reynolds number of 2000. CFD predictions on the vortex dipole location fall within 10% of the experimental measurement uncertainty band.
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Bencs, Pe´ter, Szila´rd Szabo´, Ro´bert Borda´s, Katharina Za¨hringer i Dominique The´venin. "Simultaneous Measurement of Velocity and Temperature Downstream of a Heated Cylinder". W ASME 2011 Pressure Vessels and Piping Conference. ASMEDC, 2011. http://dx.doi.org/10.1115/pvp2011-57789.
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Bluff bodies placed in a flow, such as electrical transmission lines, cartridge heaters, pipes of heat exchangers, factory chimneys and so on, often have a different temperature compared to that of the surroundings. The structure of the flow developing around bluff bodies has been investigated for a long time (Adrian, 1991; Williamson, 1996). The Ka´rma´n vortex street was and is examined by numerous researchers, both experimentally and numerically. Nevertheless, the question arises as to how this vortex street is modified by a heated cylindrical bluff body. What is the influence of heating on the frequency of the detaching vortices, the structure of the vortices and the location of the detachment? Many of these questions have already been answered by the help of numerical simulations and of measured velocity distribution using Particle Image Velocimetry (PIV) and the vortex distributions obtained from this (Venkatakrishnan and Meier, 2004). A further question is the heat loss caused by the vortex structure and the forced convection. To tackle this question, the Background Oriented Schlieren (BOS) method is applied here. At the same time, first steps have been taken towards determining temperature and vortex distributions simultaneously, which are introduced in this paper. Main objective and novelty of this work is the solution for the mentioned measurement problem with a single camera.