Статті в журналах з теми "Velocity on the wall"
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1
Mizuno, Yoshinori, and Javier Jiménez. "Wall turbulence without walls." Journal of Fluid Mechanics 723 (April 16, 2013): 429–55. http://dx.doi.org/10.1017/jfm.2013.137.
Повний текст джерелаАнотація:
AbstractWe perform direct numerical simulations of turbulent channels whose inner layer is replaced by an off-wall boundary condition synthesized from a rescaled interior flow plane. The boundary condition is applied within the logarithmic layer, and mimics the linear dependence of the length scales of the velocity fluctuations with respect to the distance to the wall. The logarithmic profile of the mean streamwise velocity is recovered, but only if the virtual wall is shifted to a position different from the location assumed by the boundary condition. In those shifted coordinates, most flow properties are within 5–10 % of full simulations, including the Kármán constant, the fluctuation intensities, the energy budgets and the velocity spectra and correlations. On the other hand, buffer-layer structures do not form, including the near-wall energy maximum, and the velocity fluctuation profiles are logarithmic, strongly suggesting that the logarithmic layer is essentially independent of the near-wall dynamics. The same agreement holds when the technique is applied to large-eddy simulations. The different errors are analysed, especially the reasons for the shifted origin, and remedies are proposed. It is also shown that the length rescaling is required for a stationary logarithmic-like layer. Otherwise, the flow evolves into a state resembling uniformly sheared turbulence.
2
Kind, R. J., F. M. Yowakim, and S. A. Sjolander. "The Law of the Wall for Swirling Flow in Annular Ducts." Journal of Fluids Engineering 111, no. 2 (June 1, 1989): 160–64. http://dx.doi.org/10.1115/1.3243617.
Повний текст джерелаАнотація:
Expressions for the logarithmic portion of the law of the wall are derived for the axial and tangential velocity components of swirling flow in annular ducts. These expressions involve new shear-velocity scales and curvature terms. They are shown to agree well with experiment over a substantial portion of the flow near both walls of an annulus. The resultant velocity data also agree with the law of the wall. The success of the proposed logarithmic expressions implies that the mixing-length model used in deriving them correctly describes flow-velocity behavior. This model indicates that the velocity gradient at any height y in the near-wall region is determined by the wall shear stress, not by the local shear stress. This suggests that the influence of wall shear stress is dominant and that it determines the near-wall wall flow even in flows with curvature and pressure gradient. A physical explanation is suggested for this.
3
Ryu, Jisu, and Hyun-Woo Lee. "Current-induced domain wall motion: Domain wall velocity fluctuations." Journal of Applied Physics 105, no. 9 (May 2009): 093929. http://dx.doi.org/10.1063/1.3125522.
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4
Laín, Santiago, and Andres D. Caballero. "Simulation of unsteady blood flow dynamics in the thoracic aorta." Ingeniería e Investigación 37, no. 3 (September 1, 2017): 92–101. http://dx.doi.org/10.15446/ing.investig.v37n3.59761.
Повний текст джерелаАнотація:
In this work, blood flow dynamics was analyzed in a realistic thoracic aorta (TA) model under unsteady-state conditions via velocity contours, secondary flow, pressure and wall shear stress (WSS) distributions. Our results demonstrated that the primary flow velocity is skewed towards the inner wall of the ascending aorta; but this skewness shifts towards the posterior wall in the aortic arch and then towards the anterior-outer wall in the descending aorta. Within the three arch branches, the flow velocity is skewed to the distal walls with flow reversal along the proximal walls. Strong secondary flow motion is observed in the TA, especially at the inlet of the arch branches. WSS is highly dynamic, but was found to be the lowest along the proximal walls of the arch branches. Finally, pressure was found to be low along the inner aortic wall and in the proximal walls of the arch branches, and high around the three stagnation regions distal to the arch branches and along the outer wall of the ascending aorta.
5
Seth, G. S., S. Sarkar, and O. D. Makinde. "Combined Free and Forced Convection Couette-Hartmann Flow in a Rotating Channel with Arbitrary Conducting Walls and Hall Effects." Journal of Mechanics 32, no. 5 (August 17, 2016): 613–29. http://dx.doi.org/10.1017/jmech.2016.70.
Повний текст джерелаАнотація:
AbstractCombined free and forced convection Couette-Hartmann flow of a viscous, incompressible and electrically conducting fluid in rotating channel with arbitrary conducting walls in the presence of Hall current is investigated. Boundary conditions for magnetic field and expressions for shear stresses at the walls and mass flow rate are derived. Asymptotic analysis of solution for large values of rotation and magnetic parameters is performed to highlight nature of modified Ekmann and Hartmann boundary layers. Numerical solution of non-linear energy equation and rate of heat transfer at the walls are computed with the help of MATHEMATICA. It is found that velocity depends on wall conductance ratio of moving wall and on the sum of wall conductance ratios of both the walls of channel. There arises reverse flow in the secondary flow direction near central region of the channel due to thermal buoyancy force. Thermal buoyancy force, rotation, Hall current and wall conductance ratios resist primary fluid velocity whereas thermal buoyancy force and Hall current favor secondary fluid velocity in the region near lower wall of the channel. Magnetic field favors both the primary and secondary fluid velocities in the region near lower wall of the channel.
6
Squire, D. T., N. Hutchins, C. Morrill-Winter, M. P. Schultz, J. C. Klewicki, and I. Marusic. "Applicability of Taylor’s hypothesis in rough- and smooth-wall boundary layers." Journal of Fluid Mechanics 812 (December 28, 2016): 398–417. http://dx.doi.org/10.1017/jfm.2016.832.
Повний текст джерелаАнотація:
The spatial structure of smooth- and rough-wall boundary layers is examined spectrally at approximately matched friction Reynolds number ($\unicode[STIX]{x1D6FF}^{+}\approx 12\,000$). For each wall condition, temporal and true spatial descriptions of the same flow are available from hot-wire anemometry and high-spatial-range particle image velocimetry, respectively. The results show that over the resolved flow domain, which is limited to a streamwise length of twice the boundary layer thickness, true spatial spectra of smooth-wall streamwise and wall-normal velocity fluctuations agree, to within experimental uncertainty, with those obtained from time series using Taylor’s frozen turbulence hypothesis (Proc. R. Soc. Lond. A, vol. 164, 1938, pp. 476–490). The same applies for the streamwise velocity spectra on rough walls. For the wall-normal velocity spectra, however, clear differences are observed between the true spatial and temporally convected spectra. For the rough-wall spectra, a correction is derived to enable accurate prediction of wall-normal velocity length scales from measurements of their time scales, and the implications of this correction are considered. Potential violations to Taylor’s hypothesis in flows above perturbed walls may help to explain conflicting conclusions in the literature regarding the effect of near-wall modifications on outer-region flow. In this regard, all true spatial and corrected spectra presented here indicate structural similarity in the outer region of smooth- and rough-wall flows, providing evidence for Townsend’s wall-similarity hypothesis (The Structure of Turbulent Shear Flow, vol. 1, 1956).
7
Papadopoulos, G., and M. V. O¨tu¨gen. "Separating and Reattaching Flow Structure in a Suddenly Expanding Rectangular Duct." Journal of Fluids Engineering 117, no. 1 (March 1, 1995): 17–23. http://dx.doi.org/10.1115/1.2816809.
Повний текст джерелаАнотація:
The incompressible turbulent flow over a backward-facing step in a rectangular duct was investigated experimentally. The side wall effects on the core flow were determined by varying the aspect ratio (defined as the step span-to-height ratio) from 1 to 28. The Reynolds number, based on the step height and the oncoming free-stream velocity, was 26,500. Detailed velocity measurements were made, including the turbulent stresses, in a region which extended past the flow reattachment zone. Wall static pressure was also measured on both the step and flat walls. In addition, surface visualizations were obtained on all four walls surrounding the separated flow to supplement near-wall velocity measurements. The results show that the aspect ratio has an influence on both the velocity and wall pressure even for relatively large aspect ratios. For example, in the redevelopment region downstream of reattachment, the recovery pressure decreases with smaller aspect ratios. The three-dimensional side wall effects tend to slow down the relaxation downstream of reattachment for smaller aspect ratios as evidenced by the evolution of the velocity field. For the two smallest aspect ratios investigated, higher centerplane streamwise and transverse velocities were obtained which indicate a three-dimensional mean flow structure along the full span of the duct.
8
Azatov, Aleksandr, and Miguel Vanvlasselaer. "Bubble wall velocity: heavy physics effects." Journal of Cosmology and Astroparticle Physics 2021, no. 01 (January 27, 2021): 058. http://dx.doi.org/10.1088/1475-7516/2021/01/058.
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9
Rojas, J., J. H. Whitelaw, and M. Yianneskis. "Forced Convective Heat Transfer in Curved Diffusers." Journal of Heat Transfer 109, no. 4 (November 1, 1987): 866–71. http://dx.doi.org/10.1115/1.3248196.
Повний текст джерелаАнотація:
Measurements of the velocity characteristics of the flows in two curved diffusers of rectangular cross section with C and S-shaped centerlines are presented and related to measurements of wall heat transfer coefficients along the heated flat walls of the ducts. The velocity results were obtained by laser-Doppler anemometry in a water tunnel and the heat transfer results by liquid crystal thermography in a wind tunnel. The thermographic technique allowed the rapid and inexpensive measurement of wall heat transfer coefficents along flat walls of arbitrary boundary shapes with an accuracy of about 5 percent. The results show that an increase in secondary flow velocities near the heated wall causes an increase in the local wall heat transfer coefficient, and quantify the variation for maximum secondary-flow velocities in a range from 1.5 to 17 percent of the bulk flow velocity.
10
Kim, W. J., S. M. Seo, T. D. Lee, and K. J. Lee. "Oscillatory domain wall velocity of current-induced domain wall motion." Journal of Magnetism and Magnetic Materials 310, no. 2 (March 2007): 2032–34. http://dx.doi.org/10.1016/j.jmmm.2006.10.943.
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11
Duncan, D. D., C. B. Bargeron, S. E. Borchardt, O. J. Deters, S. A. Gearhart, F. F. Mark, and M. H. Friedman. "The Effect of Compliance on Wall Shear in Casts of a Human Aortic Bifurcation." Journal of Biomechanical Engineering 112, no. 2 (May 1, 1990): 183–88. http://dx.doi.org/10.1115/1.2891170.
Повний текст джерелаАнотація:
Rigid and compliant casts of a human aortic bifurcation were subjected to physiologically realistic pulsatile fluid flows. At a number of sites near the wall in the approximate median plane of the bifurcation of these models, fluid velocity was measured with a laser Doppler velocimeter, and wall motion (in the case of the compliant cast) was determined with a Reticon linescan camera. The velocity and wall motion data were combined to estimate the instantaneous shear rates at the cast wall. Analysis showed that at the outer walls the cast compliance reduced shear rates, while at the walls of the flow divider the shear rate was increased.
12
Srinivas, S. S., and V. Kumaran. "Transitions to different kinds of turbulence in a channel with soft walls." Journal of Fluid Mechanics 822 (June 1, 2017): 267–306. http://dx.doi.org/10.1017/jfm.2017.270.
Повний текст джерелаАнотація:
The flow in a rectangular channel with walls made of polyacrylamide gel is experimentally studied to examine the effect of soft walls on transition and turbulence. The bottom wall is fixed to a substrate and the top wall is unrestrained. As the Reynolds number increases, two different flow regimes are observed. The first is the ‘soft-wall turbulence’ (Srinivas & Kumaran,J. Fluid Mech., vol. 780, 2015, pp. 649–686). There is a large increase in the magnitudes of the velocity fluctuations after transition and the fluid velocity fluctuations appear to be non-zero at the soft walls, although higher resolution measurements are required to establish the nature of the boundary dynamics. The fluid velocity fluctuations are symmetric about the centreline of the channel, and they show relatively little downstream variation. The wall displacement measurements indicate that there is no observable motion perpendicular to the surface to within the experimental resolution, but displacement fluctuations parallel to the surface are observed after transition. As the Reynolds number is further increased, there is a second ‘wall-flutter’ transition, which involves visible downstream travelling waves in the top (unrestrained) wall alone. Wall displacement fluctuations of frequency less than approximately$500~\text{rad}~\text{s}^{-1}$are observed both parallel and perpendicular to the wall. The mean velocity profiles and turbulence intensities are asymmetric, with much larger turbulence intensities near the top wall. The transitions are observed in sequence from a laminar flow at Reynolds number less than 1000 for a channel of height 0.6 mm and from a turbulent flow at a Reynolds number greater than 1000 for a channel of height 1.8 mm.
13
Avramenko, A. A., N. P. Dmitrenko, Yu Yu Kovetska, and E. A. Kondratieva. "FEATURES OF HEAT TRANSFER IN A FLAT POROUS MICROCHANNEL." Thermophysics and Thermal Power Engineering 42, no. 1 (April 12, 2020): 12–18. http://dx.doi.org/10.31472/ttpe.1.2020.1.
Повний текст джерелаАнотація:
A steady heat transfer process of mixed convection in a flat vertical porous microchannel is considered.
The results of simulation showed that Knudsen number effects are more significant in the neighborhood of the wall where growth of Knudsen numbers is accompanied with the velocity and temperature jumps on wall. With increasing parameter of porosity M (decreasing permeability), the flow velocity decreases and the velocity jump decrease as well.
For all combinations of the criteria Ra, Kn and M increasing Knudsen number reduces heat transfer intensity. This can be attributed to increasing temperature jump on wall which causes deterioration of thermal interaction between the fluid and the wall.
For low Rayleigh numbers increasing parameter M leads to increasing heat transfer since the temperature jump decrease on walls. For large Rayleigh numbers the trend becomes reversed, since for larger parameters M, the near-wall velocity decreases.
For low Rayleigh numbers increasing the Knudsen number leads to decreasing hydraulic resistance coefficient, but with increasing parameter M leads to increasing this coefficient. At high Ra numbers increasing Knudsen number leads to growth of hydraulic resistance, which is due to increasing velocity gradient on the wall.
14
Moore, James E., and David N. Ku. "Pulsatile Velocity Measurements in a Model of the Human Abdominal Aorta Under Resting Conditions." Journal of Biomechanical Engineering 116, no. 3 (August 1, 1994): 337–46. http://dx.doi.org/10.1115/1.2895740.
Повний текст джерелаАнотація:
Oscillations in near-wall velocity direction have been found to correlate with atherosclerotic plaque localization in the carotid sinus bifurcation. However, it remains unproven whether these conditions could account for the localization of the disease at other sites where atherosclerosis forms. The abdominal aorta is an important site of clinical disease in a relatively straight segment of artery. This study was initiated to quantify the velocity field in the abdominal aorta in order to determine if local differences in hemodynamic velocity directions could account for the localization of disease in this segment. Magnetic Resonance Imaging velocimetry was used to measure the pulsatile velocity profiles in an anatomically accurate in vitro model of the abdominal aorta. Velocities measured in the suprarenal aorta were laminar and reversed minimally, comparing well with theoretical solutions of Womersley flow (r = 0.96). The time-averaged velocity was +3.0 cm/s near-wall at a distance of 1 mm away from the wall. In the infrarenal aorta, the maximal velocities were skewed toward the anterior wall. At the posterior wall, velocity oscillated in direction and was retrograde for 82 percent of the cardiac cycle. The time-averaged velocity near the posterior wall was −12.5 cm/s as compared to +3.00 cm/s near the anterior wall. At the aortic bifurcation, the locations of maximal and minimal velocities in this slice were concentrated near the lateral posterior walls. This study quantifies the magnitude of low and oscillatory velocity that may exist in the abdominal aorta and suggests that there is a strong relationship between the velocities in the retrograde direction under resting conditions and the distribution of atherosclerotic plaque.
15
Chandra, Peeyush, and J. S. V. R. Krishna Prasad. "Pulsatile flow in circular tubes of varying cross-section with suction/injection." Journal of the Australian Mathematical Society. Series B. Applied Mathematics 35, no. 3 (January 1994): 366–81. http://dx.doi.org/10.1017/s0334270000009358.
Повний текст джерелаАнотація:
AbstractWe consider here pulsatile flow in circular tubes of varying cross-section with permeable walls. The fluid exchange across the wall is accounted for by prescribing the normal velocity of the fluid at the wall. A perturbation analysis has been carried out for low Reynolds number flows and for small amplitudes of oscillation. It has been observed that the magnitude of the wall shear stress and the pressure drop decrease as the suction velocity increases. Further, as the Reynolds number is increased, the magnitude of wall shear stress increases in the convergent portion and decreases in the divergent portion of a constricted tube.
16
DEY, SUBHASISH, TUSHAR K. NATH, and SUJIT K. BOSE. "Submerged wall jets subjected to injection and suction from the wall." Journal of Fluid Mechanics 653 (April 27, 2010): 57–97. http://dx.doi.org/10.1017/s0022112010000182.
Повний текст джерелаАнотація:
This paper presents an experimental study on turbulent flow characteristics in submerged plane wall jets subjected to injection (upward seepage) and suction (downward seepage) from the wall. The vertical distributions of time-averaged velocity components, turbulence intensity components and Reynolds shear stress at different horizontal distances are presented. The horizontal distributions of wall shear stress determined from the Reynolds shear stress profiles are also furnished. The flow field exhibits a decay of the jet velocity over a horizontal distance. The wall shear stress and the rate of decay of the jet velocity increase in the presence of injection and decrease with suction. Based on the two-dimensional Reynolds-averaged Navier–Stokes equations of a steady turbulent flow, the velocity and Reynolds shear stress distributions in the fully developed zone subjected to no seepage, injection and suction are theoretically computed. The response of the turbulent flow characteristics to injection and suction is analysed from the point of view of similarity characteristics, growth of the length scale and decay of the velocity and turbulence characteristics scales. The significant observation is that the velocity, Reynolds shear stress and turbulence intensities in the fully developed zone are reasonably similar under both injection and suction on applying the appropriate scaling laws. An analysis of the third-order moments of velocity fluctuations reveals that the inner layer of the jet is associated with the arrival of low-speed fluid streaks causing an effect of retardation. On the other hand, the upper layer of the jet is associated with the arrival of high-speed fluid streaks causing an effect of acceleration. Injection influences the near-wall distributions of the third-order moments by increasing the upward turbulent advection of the streamwise Reynolds normal stress. In contrast, suction influences the near-wall distributions of the third-order moments by increasing the downward turbulent advection of the streamwise Reynolds normal stress. Also, injection and suction change the vertical turbulent flux of the vertical Reynolds normal stress in a similar way. The streamwise turbulent energy flux travels towards the jet origin within the jet layer, while it travels away from the origin within the inner layer of the circulatory flow. The turbulent energy budget suggests that the turbulent and pressure energy diffusions oppose each other, and the turbulent dissipation lags the turbulent production. The quadrant analysis of velocity fluctuations reveals that the inward and outward interactions are the primary contributions to the Reynolds shear stress production in the inner and outer layers of the jet, respectively. However, injection induces feeble ejections in the vicinity of the wall.
17
Huang, Y. Q., Y. Han, J. Chen, L. C. Qiu, and X. L. He. "Effect of wall normal velocity on velocity distribution in unsteady flow." IOP Conference Series: Earth and Environmental Science 344 (November 1, 2019): 012086. http://dx.doi.org/10.1088/1755-1315/344/1/012086.
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Srinivas, S. S., and V. Kumaran. "After transition in a soft-walled microchannel." Journal of Fluid Mechanics 780 (September 7, 2015): 649–86. http://dx.doi.org/10.1017/jfm.2015.476.
Повний текст джерелаАнотація:
In comparison to the flow in a rigid channel, there is a multifold reduction in the transition Reynolds number for the flow in a microchannel when one of the walls is made sufficiently soft, due to a dynamical instability induced by the fluid–wall coupling, as shown by Verma & Kumaran (J. Fluid Mech., vol. 727, 2013, pp. 407–455). The flow after transition is characterised using particle image velocimetry in the $x{-}y$ plane, where $x$ is the streamwise direction and $y$ is the cross-stream coordinate along the small dimension of the channel of height 0.2–0.3 mm. The flow after transition is characterised by a mean velocity profile that is flatter at the centre and steeper at the walls in comparison to that for a laminar flow. The root mean square of the streamwise fluctuating velocity shows a characteristic sharp increase away from the wall and a maximum close to the wall, as observed in turbulent flows in rigid-walled channels. However, the profile is asymmetric, with a significantly higher maximum close to the soft wall in comparison to that close to the hard wall, and the Reynolds stress is found to be non-zero at the soft wall, indicating that there is a stress exerted by fluid velocity fluctuations on the wall. The maximum of the root mean square of the velocity fluctuations and the Reynolds stress (divided by the fluid density) in the soft-walled microchannel for Reynolds numbers in the range 250–400, when scaled by suitable powers of the maximum velocity, are comparable to those in a rigid channel at Reynolds numbers in the range 5000–20 000. The near-wall velocity profile shows no evidence of a viscous sublayer for $(yv_{\ast }/{\it\nu})$ as low as two, but there is a logarithmic layer for $(yv_{\ast }/{\it\nu})$ up to approximately 30, where the von Karman constants are very different from those for a rigid-walled channel. Here, $v_{\ast }$ is the friction velocity, ${\it\nu}$ is the kinematic viscosity and $y$ is the distance from the soft surface. The surface of the soft wall in contact with the fluid is marked with dye spots to monitor the deformation and motion along the fluid–wall interface. Low-frequency oscillations in the displacement of the surface are observed after transition in both the streamwise and spanwise directions, indicating that the velocity fluctuations are dynamically coupled to motion in the solid.
19
Yuan, J., and U. Piomelli. "Numerical simulation of a spatially developing accelerating boundary layer over roughness." Journal of Fluid Mechanics 780 (September 3, 2015): 192–214. http://dx.doi.org/10.1017/jfm.2015.437.
Повний текст джерелаАнотація:
The direct numerical simulation of an accelerating boundary layer over a rough wall has been carried out to investigate the coupling between the effects of roughness and strong free-stream acceleration. While the favourable pressure gradient is sufficient to achieve quasi-laminarization on a smooth wall, the flow reversion is prevented on a rough wall, and a higher friction coefficient, a faster increase of turbulence intensity compared to the free-stream velocity and more isotropic turbulence near the wall are observed. The logarithmic region of the mean-velocity profile presents an initial decrease in slope as in the smooth case, but soon recovers, as the fully rough regime is reached and a new overlap region is established. A strong coupling between the roughness and acceleration effects develops as roughness leads to more responsive turbulence and prevents the strong acceleration from stabilizing the turbulence, and the acceleration intensifies the velocity scale of the wake field (i.e. the near-wall spatial heterogeneity of the time-averaged velocity distribution). The combined effect is a ‘rougher’ surface as the flow accelerates. In addition, the link between the local values of the free stream and the near-wall velocity depends on the flow history; this explains the different flow responses observed in previous studies, in terms of friction coefficient, turbulent kinetic energy and Reynolds-stress anisotropy. This study elucidates the near-wall flow dynamics, which may be used to explain other non-canonical flows over rough walls.
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Manes, C., D. Poggi, and L. Ridolfi. "Turbulent boundary layers over permeable walls: scaling and near-wall structure." Journal of Fluid Mechanics 687 (October 10, 2011): 141–70. http://dx.doi.org/10.1017/jfm.2011.329.
Повний текст джерелаАнотація:
AbstractThis paper presents an experimental study devoted to investigating the effects of permeability on wall turbulence. Velocity measurements were performed by means of laser Doppler anemometry in open channel flows over walls characterized by a wide range of permeability. Previous studies proposed that the von Kármán coefficient associated with mean velocity profiles over permeable walls is significantly lower than the standard values reported for flows over smooth and rough walls. Furthermore, it was observed that turbulent flows over permeable walls do not fully respect the widely accepted paradigm of outer-layer similarity. Our data suggest that both anomalies can be explained as an effect of poor inner–outer scale separation if the depth of shear penetration within the permeable wall is considered as the representative length scale of the inner layer. We observed that with increasing permeability, the near-wall structure progressively evolves towards a more organized state until it reaches the condition of a perturbed mixing layer where the shear instability of the inflectional mean velocity profile dictates the scale of the dominant eddies. In our experiments such shear instability eddies were detected only over the wall with the highest permeability. In contrast attached eddies were present over all the other wall conditions. On the basis of these findings, we argue that the near-wall structure of turbulent flows over permeable walls is regulated by a competing mechanism between attached and shear instability eddies. We also argue that the ratio between the shear penetration depth and the boundary layer thickness quantifies the ratio between such eddy scales and, therefore, can be used as a diagnostic parameter to assess which eddy structure dominates the near-wall region for different wall permeability and flow conditions.
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Cohen, Caroline, Baptiste Darbois-Texier, Guillaume Dupeux, Eric Brunel, David Quéré, and Christophe Clanet. "The aerodynamic wall." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 470, no. 2161 (January 8, 2014): 20130497. http://dx.doi.org/10.1098/rspa.2013.0497.
Повний текст джерелаАнотація:
We study the trajectory of dense projectiles subjected to gravity and drag at large Reynolds number. We show that two types of trajectories can be observed: if the initial velocity is smaller than the terminal velocity of free fall, we observe the classical Galilean parabola: if it is larger, the projectile decelerates with an asymmetric trajectory first drawn by Tartaglia, which ends with a nearly vertical fall, as if a wall impeded the movement. This regime is often observed in sports, fireworks, watering, etc. and we study its main characteristics.
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Squire, D. T., C. Morrill-Winter, N. Hutchins, M. P. Schultz, J. C. Klewicki, and I. Marusic. "Comparison of turbulent boundary layers over smooth and rough surfaces up to high Reynolds numbers." Journal of Fluid Mechanics 795 (April 14, 2016): 210–40. http://dx.doi.org/10.1017/jfm.2016.196.
Повний текст джерелаАнотація:
Turbulent boundary layer measurements above a smooth wall and sandpaper roughness are presented across a wide range of friction Reynolds numbers, ${\it\delta}_{99}^{+}$, and equivalent sand grain roughness Reynolds numbers, $k_{s}^{+}$ (smooth wall: $2020\leqslant {\it\delta}_{99}^{+}\leqslant 21\,430$, rough wall: $2890\leqslant {\it\delta}_{99}^{+}\leqslant 29\,900$; $22\leqslant k_{s}^{+}\leqslant 155$; and $28\leqslant {\it\delta}_{99}^{+}/k_{s}^{+}\leqslant 199$). For the rough-wall measurements, the mean wall shear stress is determined using a floating element drag balance. All smooth- and rough-wall data exhibit, over an inertial sublayer, regions of logarithmic dependence in the mean velocity and streamwise velocity variance. These logarithmic slopes are apparently the same between smooth and rough walls, indicating similar dynamics are present in this region. The streamwise mean velocity defect and skewness profiles each show convincing collapse in the outer region of the flow, suggesting that Townsend’s (The Structure of Turbulent Shear Flow, vol. 1, 1956, Cambridge University Press.) wall-similarity hypothesis is a good approximation for these statistics even at these finite friction Reynolds numbers. Outer-layer collapse is also observed in the rough-wall streamwise velocity variance, but only for flows with ${\it\delta}_{99}^{+}\gtrsim 14\,000$. At Reynolds numbers lower than this, profile invariance is only apparent when the flow is fully rough. In transitionally rough flows at low ${\it\delta}_{99}^{+}$, the outer region of the inner-normalised streamwise velocity variance indicates a dependence on $k_{s}^{+}$ for the present rough surface.
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Rosti, Marco E., and Luca Brandt. "Numerical simulation of turbulent channel flow over a viscous hyper-elastic wall." Journal of Fluid Mechanics 830 (October 5, 2017): 708–35. http://dx.doi.org/10.1017/jfm.2017.617.
Повний текст джерелаАнотація:
We perform numerical simulations of a turbulent channel flow over an hyper-elastic wall. In the fluid region the flow is governed by the incompressible Navier–Stokes (NS) equations, while the solid is a neo-Hookean material satisfying the incompressible Mooney–Rivlin law. The multiphase flow is solved with a one-continuum formulation, using a monolithic velocity field for both the fluid and solid phase, which allows the use of a fully Eulerian formulation. The simulations are carried out at Reynolds bulk $Re=2800$ and examine the effect of different elasticity and viscosity of the deformable wall. We show that the skin friction increases monotonically with the material elastic modulus. The turbulent flow in the channel is affected by the moving wall even at low values of elasticity since non-zero fluctuations of vertical velocity at the interface influence the flow dynamics. The near-wall streaks and the associated quasi-streamwise vortices are strongly reduced near a highly elastic wall while the flow becomes more correlated in the spanwise direction, similarly to what happens for flows over rough and porous walls. As a consequence, the mean velocity profile in wall units is shifted downwards when shown in logarithmic scale, and the slope of the inertial range increases in comparison to that for the flow over a rigid wall. We propose a correlation between the downward shift of the inertial range, its slope and the wall-normal velocity fluctuations at the wall, extending results for the flow over rough walls. We finally show that the interface deformation is determined by the fluid fluctuations when the viscosity of the elastic layer is low, while when this is high the deformation is limited by the solid properties.
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Skarbalius, Gediminas, Algis Džiugys, Edgaras Misiulis, Robertas Navakas, Paulius Vilkinis, Justas Šereika, and Nerijus Pedišius. "Molecular Dynamics Study on Water Flow Behaviour inside Planar Nanochannel Using Different Temperature Control Strategies." Energies 14, no. 20 (October 19, 2021): 6843. http://dx.doi.org/10.3390/en14206843.
Повний текст джерелаАнотація:
In the present paper, molecular dynamics simulations were performed to study the influence of two temperature control strategies on water flow behaviour inside planar nanochannel. In the simulations, the flow was induced by the force acting on each water molecule in the channel. Two temperature control strategies were considered: (a) frozen wall simulations, in which the dynamics of confining wall atoms was not solved and the thermostat was applied to the water, and (b) dynamic wall simulations, in which the dynamics of confining wall atoms was solved, and the thermostat was applied to walls while water was simulated in the microcanonical ensemble. The simulation results show that the considered temperature control strategies has no effect on the shape of the water flow profile, and flow behaviour in the channel is well described by the Navier–Stokes equation solution with added slip velocity. Meanwhile, the slip velocity occurring at the boundaries of the channel is linearly dependent on the magnitude of the flow inducing force in both frozen wall and dynamic wall simulations. However, the slip velocity is considerably greater in simulations when the wall dynamics are solved in contrast to the frozen wall simulations.
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Baheri, Sima, Seyedeh Zobeydeh Falaki, and Reza Gharraei. "The effect of geometrical and flow parameters on mixed convection of developing flow in vertical and inclined microchannels." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 227, no. 9 (November 23, 2012): 1956–64. http://dx.doi.org/10.1177/0954406212468076.
Повний текст джерелаАнотація:
In this study, mixed convection in vertical and inclined parallel plate microchannels, has been investigated. The flow is steady, laminar, and incompressible and the walls are at constant temperature. A FORTRAN program has been written to solve the governing equations with wall velocity slip and temperature jump boundary conditions, and the results have been presented for Reynolds numbers of 25, 50, and 100, Knudsen numbers of 0, 0.005, 0.02, and 0.09, Richardson numbers of 0, 1, and 1.77, and microchannel inclination angles of 10°, 30°, 50°, and 90°. Results showed that wall velocity slip, friction coefficient, centerline velocity, and heat transfer rate decreases with increasing the Knudsen number. Increasing the Richardson number, for a given Reynolds number, decreases the microchannel centerline velocity because of the increasing of wall adjacent velocity, which is due to the promotion effect of natural convection. Also, the friction coefficient and Nusselt number increases with channel inclination angle.
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Castro, Ian P. "Turbulence intensity in wall-bounded and wall-free flows." Journal of Fluid Mechanics 770 (March 31, 2015): 289–304. http://dx.doi.org/10.1017/jfm.2015.168.
Повний текст джерелаАнотація:
Turbulence intensity variations in the outer region of turbulent shear flows are considered, in the context of the diagnostic plot first introduced by Alfredsson et al. (Phys. Fluids, vol. 23, 2011, 041702) and for both (smooth and rough) wall-bounded flows and classical free shear flows. With $U$ defined as the mean velocity within the flow, $U_{e}$ as a suitable reference velocity and $u^{\prime }$ as the root mean square of the fluctuating velocity, it is demonstrated that, for wall flows, the attached eddy hypothesis yields a closely linear diagnostic plot ($u^{\prime }/U$ versus $U/U_{e}$) over a certain Reynolds number range, explaining why the relation seems to work well for both boundary layers and channels despite its lack of any physical basis (Castro et al., J. Fluid Mech., vol. 727, 2013, pp. 119–131). It is shown that mixing layers, jets and wakes also exhibit linear variations of $u^{\prime }/U$ versus $U/U_{e}$ over much of the flows (starting roughly from where the turbulence production is a maximum), with slopes of these variations determined by the total mean strain rate, characterised by Townsend’s flow constant $R_{s}$. The diagnostic plot thus has a wider range of applicability than might have been anticipated.
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Óvári, Tibor-Adrian, Sorin Corodeanu, and Horia Chiriac. "Domain wall velocity in submicron amorphous wires." Journal of Applied Physics 109, no. 7 (April 2011): 07D502. http://dx.doi.org/10.1063/1.3536797.
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Kanjirakat, Anoop, and Reza Sadr. "Near-wall velocity profile measurement for nanofluids." AIP Advances 6, no. 1 (January 2016): 015308. http://dx.doi.org/10.1063/1.4939986.
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Khlebnikov, S. Yu. "Fluctuation-dissipation formula for bubble-wall velocity." Physical Review D 46, no. 8 (October 15, 1992): R3223—R3226. http://dx.doi.org/10.1103/physrevd.46.r3223.
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Kim, Sun-Chul. "Batchelor–Wood formula for negative wall velocity." Physics of Fluids 11, no. 6 (June 1999): 1685–87. http://dx.doi.org/10.1063/1.870031.
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Shi, Jun-Mei, Michael Breuer, Franz Durst, and Michael Scha¨fer. "An Improved Numerical Study of the Wall Effect on Hot-Wire Measurements." Journal of Heat Transfer 125, no. 4 (July 17, 2003): 595–603. http://dx.doi.org/10.1115/1.1571848.
Повний текст джерелаАнотація:
Numerical investigations of the heat transfer from hot wires in near-wall measurements were carried out. Special attention was paid to the effect of the wall thickness, the flow conditions below the wall and the shear velocity connected to different wall materials. Compared with previous studies, an improved physical model taking into account the flow region below the wall in the computational domain was applied. The results obtained agree well with experimental data in the literature for walls consisting of both highly and poorly conducting materials. The investigation showed that the shear velocity Uτ has a significant influence on hot-wire measurements in the vicinity of a wall. Nevertheless, discernible effects of the wall thickness and the flow condition below the wall were found only in the case of a poorly conducting wall. In addition, the results also suggest a weak effect of the overheat ratio for a wire with an infinitely large aspect ratio.
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Echouchene, Fraj, Thamraa Al-shahrani, and Hafedh Belmabrouk. "Simulation of the Slip Velocity Effect in an AC Electrothermal Micropump." Micromachines 11, no. 9 (August 31, 2020): 825. http://dx.doi.org/10.3390/mi11090825.
Повний текст джерелаАнотація:
The principal aim of this study was to analyze the effect of slip velocity at the microchannel wall on an alternating current electrothermal (ACET) flow micropump fitted with several pairs of electrodes. Using the finite element method (FEM), the coupled momentum, energy, and Poisson equations with and without slip boundary conditions have been solved to compute the velocity, temperature, and electrical field in the microchannel. The effects of the frequency and the voltage, and the electrical and thermal conductivities, respectively, of the electrolyte solution and the substrate material, have been minutely analyzed in the presence and absence of slip velocity. The slip velocity was simulated along the microchannel walls at different values of slip length. The results revealed that the slip velocity at the wall channel has a significant impact on the flow field. The existence of slip velocity at the wall increases the shear stress and therefore enhances the pumping efficiency. It was observed that higher average pumping velocity was achieved for larger slip length. When a glass substrate was used, the effect of the presence of the slip velocity was more manifest. This study shows also that the effect of slip velocity on the flow field is very important and must be taken into consideration in an ACET micropump.
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Azzouz, El Amin, and Samir Houat. "Asymmetrical Flow Driving in Two-Sided Lid-Driven Square Cavity with Antiparallel Wall Motion." MATEC Web of Conferences 330 (2020): 01009. http://dx.doi.org/10.1051/matecconf/202033001009.
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The two-dimensional flow in a two-sided lid-driven cavity is often handled numerically for the same imposed wall velocities (symmetrical driving) either for parallel or antiparallel wall motion. However, in this study, we present a finite volume method (FVM) based on the second scheme of accuracy to numerically explore the steady two-dimensional flow in a two-sided lid-driven square cavity for antiparallel wall motion with different imposed wall velocities (asymmetrical driving). The top and the bottom walls of the cavity slide in opposite directions simultaneously at different velocities related to various imposed velocity ratios, λ = -2, -6, and -10, while the two remaining vertical walls are stationary. The results show that varying the velocity ratio and consequently the Reynolds ratios have a significant effect on the flow structures and fluid properties inside the cavity.
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Liu, Zhao Cun, and Wei Jia Fan. "Approach to some Properties near the Wall of Parallel to Wall Shear Flow." Applied Mechanics and Materials 197 (September 2012): 396–400. http://dx.doi.org/10.4028/www.scientific.net/amm.197.396.
Повний текст джерелаАнотація:
From the viewpoint of vortex dynamics, the flowing properties and the characters of transition from laminar flow to turbulent flow were analyzed, the concept of the Reynolds number was reviewed and studied. The different form of the Reynolds number was presented to relate it with fluctuating and breaking down processes to show its dynamical mechanism. On the basis of phenomenological properties, the velocity distribution near the wall was studied to show that the profile connects with the flowing structure which is different from the linear form what usually considered to be, thus the velocity distribution near the wall remains open. Finally, the form of the velocity distribution should follow was probed.
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Rashid, Madhia, Sohail Nadeem, and Iqra Shahzadi. "Permeability impact on electromagnetohydrodynamic flow through corrugated walls of microchannel with variable viscosity." Advances in Mechanical Engineering 12, no. 7 (July 2020): 168781402094433. http://dx.doi.org/10.1177/1687814020944336.
Повний текст джерелаАнотація:
This investigation based on electromagnetohydrodynamic flow in microchannels through lightly corrugated walls effects is reported in the presence of variable liquid properties. In microparallel plates, we consider incompressible and electrically conducting viscous fluid. With small amplitudes, the wall corrugations are described by periodic sin waves. The governing equations are rendered dimensionless and solved with the help of the perturbation technique. The analytical solutions for velocity are obtained and analyzed graphically. A connection between flow rate and roughness is acquired by perturbation solutions of the stream function. By utilizing numerical computations, we analyzed the corrugation consequences on the velocity for electromagnetohydrodynamic flow. We graphically clarified the velocity and temperature profiles and their dependencies on all parameters. The three-dimensional velocity and contour distributions shown that the wall roughness can cause changes in the velocity distribution. For in phase the phase difference among the two corrugated walls is equals to 0°, and for out of phase the phase difference is equal to 180° between the two walls. The wave phenomenon of the flow shape becomes obvious with the expansion of the corrugation. The electromagnetohydrodynamic velocities first grow and then reduce. The electromagnetohydrodynamic velocity increases for Reynolds number, Hartmann number, and Darcy parameter. Velocity profile decreases for variable viscosity, velocity slip parameter.
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Jalali, Esmaeil, and Arash Karimipour. "Simulation the effects of cross-flow injection on the slip velocity and temperature domain of a nanofluid flow inside a microchannel." International Journal of Numerical Methods for Heat & Fluid Flow 29, no. 5 (May 7, 2019): 1546–62. http://dx.doi.org/10.1108/hff-04-2018-0149.
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Purpose In this paper, the forced convection heat transfer of the nanofluid composed of water and AL2O3 nanoparticles is simulated in a two-dimensional horizontal microchannel by injecting the lower wall. The upper wall of the microchannel is 303 K at temperature TH. On the lower wall of the microchannel, there are three holes for flow injection. Other parts of the wall are insulated. In this paper, the effect of parameters such as Reynolds number, slip coefficient and volume fraction of nanoparticles is investigated. Design/methodology/approach The boundary condition of the slip velocity is considered on the upper and lower walls of the microchannel. In this work, the flow of nanofluid in the microchannel is considered to be slow, permanent and Newtonian. In the present study, the effect of injection through the microchannel wall on the slip velocity is examined for the first time. Findings The results are also presented as velocity profiles and Nusselt number diagrams. It was found that the Nusselt number increases with increasing the amount of slip coefficient of velocity and the weight percentage of solid nanoparticles. The rate of this increase is higher in the high values of the Reynolds number. Originality/value A novel paper concerned the simulation of cross-flow injection effects on the slip velocity and temperature domain of a nanofluid flow inside a microchannel.
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Mbogba, Guy Leopold, Elisabeth Ngo Nyobe, Maurice Lamara, Yves Christian Mbono Samba, and Elkana Pemha. "Effects of an external constant pressure gradient on a steady incompressible laminar flow through a semi-porous annular pipe." Zeitschrift für Naturforschung A 77, no. 2 (November 11, 2021): 131–41. http://dx.doi.org/10.1515/zna-2021-0257.
Повний текст джерелаАнотація:
Abstract In this paper, we examine a steady laminar flow for an incompressible fluid located in a semi porous annular pipe and subjected to a favorable constant pressure gradient applied between the two borders of the pipe. The inner wall is impermeable and the fluid is sucked or injected at the outer wall at constant and uniform velocity, orthogonally to the wall. The problem under study depends on three parameters: the pipe gap ratio, the dimensionless external pressure gradient, and the Reynolds number defined from the sum of the suction or injection velocity and the maximum Hagen–Poiseuille velocity. The conservation of mass induces the zero-divergence velocity field which allows replacing the steady-flow Navier–Stokes equations with a single equation satisfied by the stream function and called the vorticity equation. Assuming the similarity-solution hypothesis, the problem under consideration is reduced to a fourth-order nonlinear ordinary differential equation with two boundary conditions at each wall. The numerical shooting technique including the Runge–Kutta algorithm and the Newton–Raphson optimization method is applied to obtain the solution for the steady flow. For various values of the dimensionless external pressure gradient, the profiles of the velocity components are found and investigations on the wall shear stress for both walls are performed. The results obtained are discussed and physical understandings for the problem studied are derived.
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Fatemi, Ray S., and Stanley E. Rittgers. "Derivation of Shear Rates From Near-Wall LDA Measurements Under Steady and Pulsatile Flow Conditions." Journal of Biomechanical Engineering 116, no. 3 (August 1, 1994): 361–68. http://dx.doi.org/10.1115/1.2895743.
Повний текст джерелаАнотація:
Atherosclerosis, thrombosis, and intimal hyperplasia are major forms of cardiovascular diseases in the United States. Previous studies indicate a significant correlation between hemodynamics, in particular, wall shear rate, and pathology of the arterial walls. While results of these studies implicate morphologic and functional changes related to wall shear rate magnitude, a standard technique for wall shear rate measurement has not been established. In this study, theoretical and in-vitro experimental fully developed steady and physiologic pulsatile flow waveforms have been used to obtain velocity profiles in the near-wall region. The estimated wall shear rates from these results are compared to the theoretical value to assess the accuracy of the approximating technique. Experimentally obtained results from LDA suggest that in order to minimize the error in velocity data, and subsequently, the wall shear rate, the first measured velocity has to be 500 μm away from the wall. While a linear approximation did not produce errors larger than 16.4 percent at peak systole, these errors substantially increased as the velocity magnitudes decreased during late systole and diastole. Overall, a third degree polynomial curve fit using four points produced the most accurate estimation of wall shear rate through out the cardiac cycle. Results of higher degree curve-fitting functions can be unpredictable due to potential oscillations of the function near the wall. Hence, based on the results of this study, use of a linear approximation is not recommended; a third degree curve-fitting polynomial, using four points provided the most accurate approximation for these flow waveforms.
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Tiwari, Ambrish K., and Ashok K. Singh. "Natural Convection in a Porous Medium Bounded by a Long Vertical WavyWall and a Parallel Flat Wall." Zeitschrift für Naturforschung A 65, no. 10 (October 1, 2010): 800–810. http://dx.doi.org/10.1515/zna-2010-1006.
Повний текст джерелаАнотація:
This paper presents natural convection in a porous medium bounded by a long vertical wavy wall and a parallel wall. The shape of the wavy wall is assumed to follow a profile of cosine curve. The wall is kept at a constant heat flux while the parallel wall is kept at a constant temperature. The governing systems of nonlinear partial differential equations in their non-dimensional form are linearised by using the perturbation method in terms of amplitude and the analytical solutions for velocity and temperature fields have been obtained in terms of various parameters occurring in the model. A numerical study of the analytical solution is performed with respect to the realistic fluid air in order to illustrate the interactive influences of governing parameters on the temperature and velocity fields as well as skin friction and Nusselt number. It is found that in the case of maximum waviness (positive and negative), the velocity component along the wall has a reverse trend near the flat wall. It is observed that the parallel flow through the channel at zero waviness is greater than at maximum waviness (positive and negative) while the same trend occurs for perpendicular flow in the opposite direction. Examination of the Nusselt number shows that in the presence and absence of a heat source, the heat flows from the porous region towards the walls but in the presence of a sink, the heat flows from the walls into the porous region.
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Deters, O. J., C. B. Bargeron, F. F. Mark, and M. H. Friedman. "Measurement of Wall Motion and Wall Shear in a Compliant Arterial Cast." Journal of Biomechanical Engineering 108, no. 4 (November 1, 1986): 355–58. http://dx.doi.org/10.1115/1.3138628.
Повний текст джерелаАнотація:
Initial measurements of the time-varying wall shear rate at two sites in a compliant cast of a human aortic bifurcation are presented. The shear rates were derived from flow velocities measured by laser Doppler velocimetry (LDV) near the moving walls of the cast. To derive these shear rate values, the distance from the velocimeter sampling volume to the cast wall must be known. The time variation of this distance was obtained from LDV measurements of the velocity of the wall itself.
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Oliveira, Carlos, Armando A. Soares, André Simões, Sílvia Gonzaga, and Abel Rouboa. "Numerical Study of Non-Newtonian Blood Behavior in the Abdominal Aortic Bifurcation of a Patient-Specific at Rest." Open Sports Sciences Journal 10, no. 1 (December 29, 2017): 279–85. http://dx.doi.org/10.2174/1875399x01710010279.
Повний текст джерелаАнотація:
Background:The interaction of blood flow with walls of blood vessels is central for the development and maintenance of cardiovascular health. The analysis of wall shear stress is, therefore, fundamental in hemodynamic studies.Objective:The aim of this work is to study numerically the influence of the shear thinning blood properties on the hemodynamics in the abdominal aortic bifurcation for a patient-specific at rest.Methods:Were tested two models for the blood dynamic viscosity, one Newtonian and other non-Newtonian, with dependence on hematocrit and total protein minus albumin.Results and Conclusion:The results show the shear thinning behavior influence on the velocity distribution and wall shear stress. Furthermore, wall shear stress values are globally lower for non-Newtonian blood model at high velocity values than those for the Newtonian blood model. However, for low velocity values this behavior is inverted.
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Roy, R. P., V. Velidandla, and S. P. Kalra. "Velocity Field in Turbulent Subcooled Boiling Flow." Journal of Heat Transfer 119, no. 4 (November 1, 1997): 754–66. http://dx.doi.org/10.1115/1.2824180.
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The velocity field was measured in turbulent subcooled boiling flow of Refrigerant-113 through a vertical annular channel whose inner wall was heated. A two-component laser Doppler velocimeter was used. Measurements are reported in the boiling layer adjacent to the inner wall as well as in the outer all-liquid layer for two fluid mass velocities and four wall heat fluxes. The turbulence was found to be inhomogeneous and anisotropic and the turbulent kinetic energy significantly higher than in single-phase liquid flow at the same mass velocity. A marked shift toward the inner wall was observed of the zero location of the axial Reynolds shear stress in the liquid phase, and the magnitude of the shear stress increased sharply close to the inner wall. The near-wall liquid velocity field was quite different from that in single-phase liquid flow at a similar Reynolds number. Comparison of the measurements with the predictions of a three-dimensional two-fluid model of turbulent subcooled boiling flow show reasonably good agreement for some quantities and a need for further development of certain aspects of the model.
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Mao, Zhi Hong, Fu Bing Bao, and Yuan Lin Huang. "Molecular Dynamics Simulation of Rarefied Gaseous Flows in Nano-Channels." Applied Mechanics and Materials 446-447 (November 2013): 12–17. http://dx.doi.org/10.4028/www.scientific.net/amm.446-447.12.
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Molecular dynamics simulation method was used to study the rarefied gaseous flows in nanochannels. A pressure-driven force was introduced to drive the gas to flow between two parallel walls. The effects of driven force magnitude and channel height were investigated. The results show that a single layer of gaseous molecules is adsorbed on the wall surface. The density of adsorption layer decreases with the increase of channel height, but doesnt vary with driven force. The velocity profile across the channel has the traditional parabolic shape. The average velocity and gas slip velocity on the wall increase linearly with the increase of pressure-driven force. The gas slip velocity decreases linearly with the increase of channel height. The ratio of slip to average velocity decreases linearly with the increase of channel height.
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Lenaers, Peter, Qiang Li, Geert Brethouwer, Philipp Schlatter, and Ramis Örlü. "Rare backflow and extreme wall-normal velocity fluctuations in near-wall turbulence." Physics of Fluids 24, no. 3 (March 2012): 035110. http://dx.doi.org/10.1063/1.3696304.
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Bao, Fubing, Zhihong Mao, and Limin Qiu. "Study of gaseous velocity slip in nano-channel using molecular dynamics simulation." International Journal of Numerical Methods for Heat & Fluid Flow 24, no. 6 (July 29, 2014): 1338–47. http://dx.doi.org/10.1108/hff-04-2013-0145.
Повний текст джерелаАнотація:
Purpose – The purpose of this paper is to investigate the gas flow characteristics in near wall region and the velocity slip phenomenon on the wall in nano-channels based on the molecular dynamics simulation. Design/methodology/approach – An external gravity force was employed to drive the flow. The density and velocity profiles across the channel, and the velocity slip on the wall were studied, considering different gas temperatures and gas-solid interaction strengths. Findings – The simulation results demonstrate that a single layer of gas molecules is adsorbed on wall surface. The density of adsorption layer increases with the decrease of gas temperature and with increase of interaction strength. The near wall region extents several molecular diameters away from the wall. The density profile is flatter at higher temperature and the velocity profile has the traditional parabolic shape. The velocity slip on the wall increases with the increase of temperature and with decrease of interaction strength linearly. The average velocity decreases with the increase of gas-solid interaction strength. Originality/value – This research presents gas flow characteristics in near wall region and the velocity slip phenomenon on the wall in nano-channels. Some interesting results in nano-scale channels are obtained.
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Walsh, M., T. McGloughlin, D. W. Liepsch, T. O'Brien, L. Morris, and A. R. Ansari. "On using experimentally estimated wall shear stresses to validate numerically predicted results." Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine 217, no. 2 (February 1, 2003): 77–90. http://dx.doi.org/10.1243/09544110360579286.
Повний текст джерелаАнотація:
The objective of this investigation was to assess the use of experimentally estimated wall shear stresses to validate numerically predicted results. The most commonly cited haemodynamic factor implicated in the disease initiation and proliferation processes at graft/artery junctions is wall shear stress (WSS). WSS can be determined from the product of the viscosity of the fluid and the wall shear rate. Numerically, the wall shear rate is predicted using velocity values stored in the computational cell near the wall and assuming zero velocity at the wall. Experimentally, the wall shear rate is estimated by applying a curve-fit to near-wall velocity measurements and evaluating the shear rate at a specific distance from the wall. When estimating the wall shear rate from the laser Doppler anemometry (LDA) point velocity measurements, large differences between the experimentally estimated and numerically predicted WSSs were introduced. It was found that the estimated WSS distributions from the experimental results are highly dependent on the curve-fitting method used to calculate the wall shear rate. However, the velocity profiles for both the experimental and numerical investigations show extremely good comparison. It is concluded that numerical models should be validated using unprocessed LDA point velocity measurement and not estimated WSS values.
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Song, Lijun, and John Abraham. "The Structure of Wall-Impinging Jets: Computed Versus Theoretical and Measured Results." Journal of Fluids Engineering 125, no. 6 (November 1, 2003): 997–1005. http://dx.doi.org/10.1115/1.1625686.
Повний текст джерелаАнотація:
In this work, the structure of computed wall-impinging gas jets is compared with theoretical and experimental results presented in the literature. The computational study employs the k-ε model to represent turbulence. Wall functions are employed to model momentum transfer at the walls. The computed penetration and growth rate of the jet agree with measured and analytical results within 10%. Computed radial velocities in the developed region of the wall jet are self-similar as found in prior experimental and analytical works. The computed radial velocity profile and quantitative values in the outer layer of the jet and the location of the maximum radial velocity agree within 5% with measurements and analytical results. Greater quantitative differences are found in the near-wall region. Mixing characteristics of a wall-impinging jet are compared with those of a round free jet. The wall-impinging jet mixes slower than the round free jet.
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Chetkin, Mikhail V., Yuliya N. Kurbatova, and Tatiana B. Shapaeva. "Peculiarity of Solitary Deflection Waves Dynamics on the Domain Walls of Yttrium Orthoferrite." Solid State Phenomena 233-234 (July 2015): 435–38. http://dx.doi.org/10.4028/www.scientific.net/ssp.233-234.435.
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The solitary deflection waves accompany moving antiferromagnetic vortices in the domain wall of yttrium orthoferrite. These waves allow to investigate generation and nonlinear dynamics of these vortices on the moving domain wall with the help of two-and three-fold digital high speed photography. As the domain walls and the antiferromagnetic vortices dynamics is quasi-relativistic with the limiting velocity c=20 km/s, which is equal to the spin-wave velocity. The dynamics of solitary deflection waves can be explained assuming existence of the gyroscopic force. This work is devoted to experimental results on the dynamics of the solitary deflection waves on the domain wall of yttrium orthoferrite.
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Umehara, Mika, and Ko Okumura. "How universal is the vibration-velocity controlled granular convection?" EPJ Web of Conferences 249 (2021): 03019. http://dx.doi.org/10.1051/epjconf/202124903019.
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Recently, a number of articles have reported that granular convection induced by continuous vibration is controlled by vibration velocity, in contrast with some previous studies. We have reported such an example for the Brazil nut effect when the vibration is given discontinuously, using a one-layer granular bed in a cell with down-facing side walls. Here, we report the effect of vibration phase and wall friction using the same experimental system, to confirm rising motion of an intruder induced by granular convection is again governed by vibration velocity. We compare two different cases of vibration phase for giving intermittent vibration cycles, and found one, in which granular packing is well established before grains start to lose contacts due to vibration, provides distinctly high reproducibility. We further control the side wall friction using a microfabrication technique, and found that significantly high reproducibility is attained in a cell with vertical side walls when a millimeter texture is introduced on the side walls. Our results indicate that the granular convection is universally controlled by vibration velocity. The present study opens a way to conduct highly reproducible experiments on granular dynamics, which is indispensable for deep physical understanding of granular flow and segregation.
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Rosti, Marco E., Luca Brandt, and Alfredo Pinelli. "Turbulent channel flow over an anisotropic porous wall – drag increase and reduction." Journal of Fluid Mechanics 842 (March 12, 2018): 381–94. http://dx.doi.org/10.1017/jfm.2018.152.
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The effect of the variations of the permeability tensor on the close-to-the-wall behaviour of a turbulent channel flow bounded by porous walls is explored using a set of direct numerical simulations. It is found that the total drag can be either reduced or increased by more than 20 % by adjusting the permeability directional properties. Drag reduction is achieved for the case of materials with permeability in the vertical direction lower than the one in the wall-parallel planes. This configuration limits the wall-normal velocity at the interface while promoting an increase of the tangential slip velocity leading to an almost ‘one-component’ turbulence where the low- and high-speed streak coherence is strongly enhanced. On the other hand, strong drag increase is found when high wall-normal and low wall-parallel permeabilities are prescribed. In this condition, the enhancement of the wall-normal fluctuations due to the reduced wall-blocking effect triggers the onset of structures which are strongly correlated in the spanwise direction, a phenomenon observed by other authors in flows over isotropic porous layers or over ribletted walls with large protrusion heights. The use of anisotropic porous walls for drag reduction is particularly attractive since equal gains can be achieved at different Reynolds numbers by rescaling the magnitude of the permeability only.