Influence of Viscoelasticity on Turbulent Flow Over a Backward-Facing Step

2015 ◽  
Author(s):  
Akito Ikegami ◽  
Takahiro Tsukahara ◽  
Yasuo Kawaguchi
Keyword(s):  
Fluid Flow ◽  
Turbulent Flow ◽  
Newtonian Fluid ◽  
Expansion Ratio ◽  
Weissenberg Number ◽  
Recirculation Region ◽  
Viscoelastic Flows ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
Significant Difference

We studied viscoelastic turbulent flow over a backward-facing step of the expansion ratio ER = 1.5 using DNS (direct numerical simulation) at a friction Reynolds number Reτ0 of 100. We chose the Giesekus model as a viscoelastic constitutive equation, and the Weissenberg number is Wiτ0 = 10 and 20. Visualized instantaneous vortices revealing that a few vortices occur only above the recirculation regions in the viscoelastic fluid flow compared to those in the Newtonian flow. This phenomenon might be caused by the fluid viscoelasticity that would suppress the Kelvin-Helmholz vortex emanating from the step edge. The reattachment length from the step is 6.80h for the Newtonian fluid, 7.82h for Wiτ0 = 10, and 8.82h for Wiτ0 = 20, where h is the step height. In the mean velocity distributions normalized by maximum inlet velocity, we have observed no significant difference among the three fluids, except for region near the upper or bottom wall, i.e., the recirculation and recovery regions at the front and behind the reattachment point. The streamwise turbulent intensity u’rms is weaken in the recirculation region of the viscoelastic flows. In terms of v’rms, its magnitude in the recirculation region becomes largest in the case of Wiτ0 = 10, not for the Newtonian fluid flow or more viscoelastic case of Wiτ0 = 20.

The Mechanism of Size-Based Particle Separation by Dielectrophoresis in the Viscoelastic Flows

2018 ◽  
Vol 140 (9) ◽  
Author(s):  
Teng Zhou ◽  
Yongbo Deng ◽  
Hongwei Zhao ◽  
Xianman Zhang ◽  
Liuyong Shi ◽  
...  
Keyword(s):  
Electric Field ◽  
Newtonian Fluid ◽  
Finite Size ◽  
Particle Separation ◽  
Weissenberg Number ◽  
Viscoelastic Flow ◽  
Viscoelastic Flows ◽  
Arbitrary Lagrangian Eulerian ◽  
Particle Movement ◽  
Physical Insight

Viscoelastic solution is encountered extensively in microfluidics. In this work, the particle movement of the viscoelastic flow in the contraction–expansion channel is demonstrated. The fluid is described by the Oldroyd-B model, and the particle is driven by dielectrophoretic (DEP) forces induced by the applied electric field. A time-dependent multiphysics numerical model with the thin electric double layer (EDL) assumption was developed, in which the Oldroyd-B viscoelastic fluid flow field, the electric field, and the movement of finite-size particles are solved simultaneously by an arbitrary Lagrangian–Eulerian (ALE) numerical method. By the numerically validated ALE method, the trajectories of particle with different sizes were obtained for the fluid with the Weissenberg number (Wi) of 1 and 0, which can be regarded as the Newtonian fluid. The trajectory in the Oldroyd-B flow with Wi = 1 is compared with that in the Newtonian fluid. Also, trajectories for different particles with different particle sizes moving in the flow with Wi = 1 are compared, which proves that the contraction–expansion channel can also be used for particle separation in the viscoelastic flow. The above results for this work provide the physical insight into the particle movement in the flow of viscous and elastic features.

PIV Measurements in Square Backward-Facing Step

2007 ◽  
Vol 129 (8) ◽  
pp. 984-990 ◽  
Author(s):  
Mika Piirto ◽  
Aku Karvinen ◽  
Hannu Ahlstedt ◽  
Pentti Saarenrinne ◽  
Reijo Karvinen
Keyword(s):  
Shear Stress ◽  
Reynolds Stress ◽  
Reynolds Shear Stress ◽  
Experimental Tests ◽  
Expansion Ratio ◽  
Spanwise Direction ◽  
Duct Flow ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
Stress Profiles

Measurements with both two-dimensional (2D) two-component and three-component stereo particle image velocimetry (PIV) and computation in 2D and three-dimensional (3D) using Reynolds stress turbulence model with commercial code are carried out in a square duct backward-facing step (BFS) in a turbulent water flow at three Reynolds numbers of about 12,000, 21,000, and 55,000 based on the step height h and the inlet streamwise maximum mean velocity U0. The reattachment locations measured at a distance of Δy=0.0322h from the wall are 5.3h, 5.6h, and 5.7h, respectively. The inlet flow condition is fully developed duct flow before the step change with the expansion ratio of 1.2. PIV results show that the mean velocity, root mean square (rms) velocity profiles, and Reynolds shear stress profiles in all the experimental flow cases are almost identical in the separated shear-layer region when they are nondimensionalized by U0. The sidewall effect of the square BFS flow is analyzed by comparing the experimental statistics with direct numerical simulation (DNS) and Reynolds stress model (RSM) data. For this purpose, the simulation is carried out for both 2D BFS and for square BFS having the same geometry in the 3D case as the experimental case at the lowest Reynolds number. A clear difference is observed in rms and Reynolds shear stress profiles between square BFS experimental results and DNS results in 2D channel in the spanwise direction. The spanwise rms velocity difference is about 30%, with experimental tests showing higher values than DNS, while in contrast, turbulence intensities in streamwise and vertical directions show slightly lower values than DNS. However, with the modeling, the turbulence statistical differences between 2D and 3D RSM cases are very modest. The square BFS indicates 0.5h–1.5h smaller reattachment distances than the reattachment lengths of 2D flow cases.

scholarly journals Unsteady solute dispersion in non-Newtonian fluid flow in a tube with wall absorption

2016 ◽  
Vol 472 (2193) ◽  
pp. 20160294 ◽  
Author(s):  
Jyotirmoy Rana ◽  
P. V. S. N. Murthy
Keyword(s):  
Fluid Flow ◽  
Newtonian Fluid ◽  
Transport Coefficients ◽  
Blood Stream ◽  
Weissenberg Number ◽  
Dispersion Coefficients ◽  
Dispersion Process ◽  
Solute Dispersion ◽  
Link Type ◽  
Power Law Index

The theory of miscible dispersion in a straight circular pipe with interphase mass transfer that was investigated by Sankarasubramanian & Gill (1973Proc. R. Soc. Lond. A333, 115–132. (doi:10.1098/rspa.1973.0051); 1974Proc. R. Soc. Lond. A341, 407–408. (doi:10.1098/rspa.1974.0195)) in Newtonian fluid flow is extended by considering various non-Newtonian fluid models, such as the Casson (Rana & Murthy 2016J. Fluid Mech.793, 877–914. (doi:10.1017/jfm.2016.155)), Carreau and Carreau–Yasuda models. These models are useful to investigate the solute dispersion in blood flow. The three effective transport coefficients, i.e. exchange, convection and dispersion coefficients, are evaluated to analyse the dispersion process of solute. The convection and dispersion coefficients are determined asymptotically at large time which is sufficient to understand the nature of the solute dispersion process in a tube. The axial mean concentration is analysed, using the asymptotic expressions for these three coefficients. The effect of the wall absorption parameter, Weissenberg number, power-law index, Yasuda parameter and Peclet number on the dispersion process is discussed clearly in this study. A comparative study of the solute dispersion among the Newtonian and all other non-Newtonian models is presented. At low shear rate, it is observed that Carreau fluid behaves like Newtonian fluid, whereas the other fluids exhibit significant differences during the solute dispersion. This study may be applicable to understand the dispersion process of drugs in the blood stream.

Peristaltic transport of a MHD Carreau fluid in a tapered asymmetric channel with permeable walls

2015 ◽  
Vol 08 (04) ◽  
pp. 1550054 ◽  
Author(s):  
M. Kothandapani ◽  
J. Prakash ◽  
S. Srinivas
Keyword(s):  
Fluid Flow ◽  
Pressure Rise ◽  
Weissenberg Number ◽  
Physical Parameters ◽  
Asymmetric Channel ◽  
Carreau Fluid ◽  
Flow Equations ◽  
Axial Pressure Gradient ◽  
Significant Difference ◽  
Permeable Walls

The effect of permeable walls and magnetic field on the peristaltic flow of a Carreau fluid in a tapered asymmetric channel is studied. The tapered asymmetric channel is normally created due to the intra-uterine fluid flow induced by myometrial contractions and it was simulated by asymmetric peristaltic fluid flow in a two-dimensional infinite non-uniform channel. The analysis has been performed under long wavelength and low-Reynolds number assumptions to linearize the governing flow equations. A series solution in respect of a small Weissenberg number is obtained for the stream function, axial pressure gradient and shear stress. Time average of pressure rise and frictional force on the upper wall has also been computed using numerical integration. The results have been presented graphically for the various interested physical parameters. It is observed that for Carreau fluids the peristalsis works as a pump against a greater pressure rise compared with a Newtonian fluid, while there exists no significant difference in free pumping flux for Newtonian and Carreau fluids in the tapered asymmetric channel.

scholarly journals Modeling and Simulation of Newtonian Fluid Flow through Two-Dimensional Backward-Facing Step Channel with Finite Element�s Technique

2019 ◽  
Vol 12 (32) ◽  
pp. 1-6
Author(s):  
Abid Ali Memon ◽  
Hisam-uddin Shaikh ◽  
Baqir Ali Shah ◽  
Muhammad Afzal Soomro ◽  
Abdul Ghafoor Shaikh ◽  
...  
Keyword(s):  
Finite Element ◽  
Fluid Flow ◽  
Modeling And Simulation ◽  
Newtonian Fluid ◽  
Two Dimensional ◽  
Backward Facing Step ◽  
Flow Through

scholarly journals Direct numerical simulation of turbulent flow over a backward-facing step

1997 ◽  
Vol 330 ◽  
pp. 349-374 ◽  
Author(s):  
HUNG LE ◽  
PARVIZ MOIN ◽  
JOHN KIM
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Shear Layer ◽  
Stokes Equations ◽  
Recirculation Region ◽  
Wall Region ◽  
Free Shear Layer ◽  
Backward Facing Step ◽  
Temporal Behaviour ◽  
Log Law

Turbulent flow over a backward-facing step is studied by direct numerical solution of the Navier–Stokes equations. The simulation was conducted at a Reynolds number of 5100 based on the step height h and inlet free-stream velocity, and an expansion ratio of 1.20. Temporal behaviour of spanwise-averaged pressure fluctuation contours and reattachment length show evidence of an approximate periodic behaviour of the free shear layer with a Strouhal number of 0.06. The instantaneous velocity fields indicate that the reattachment location varies in the spanwise direction, and oscillates about a mean value of 6.28h. Statistical results show excellent agreement with experimental data by Jovic & Driver (1994). Of interest are two observations not previously reported for the backward-facing step flow: (a) at the relatively low Reynolds number considered, large negative skin friction is seen in the recirculation region; the peak |Cf| is about 2.5 times the value measured in experiments at high Reynolds numbers; (b) the velocity profiles in the recovery region fall below the universal log-law. The deviation of the velocity profile from the log-law indicates that the turbulent boundary layer is not fully recovered at 20 step heights behind the separation.The budgets of all Reynolds stress components have been computed. The turbulent kinetic energy budget in the recirculation region is similar to that of a turbulent mixing layer. The turbulent transport term makes a significant contribution to the budget and the peak dissipation is about 60% of the peak production. The velocity–pressure gradient correlation and viscous diffusion are negligible in the shear layer, but both are significant in the near-wall region. This trend is seen throughout the recirculation and reattachment region. In the recovery region, the budgets show that effects of the free shear layer are still present.

INVESTIGATION OF CHARACTERISTICS OF SHEAR LAYER: APPLICATION OF SYNTHETIC JET IN BACKWARD-FACING STEP FLOW FIELD

2016 ◽  
Vol 40 (5) ◽  
pp. 787-797 ◽  
Author(s):  
Chia-Wei Hsu ◽  
San-Yih Lin
Keyword(s):  
Flow Field ◽  
Expansion Ratio ◽  
Synthetic Jet ◽  
Recirculation Region ◽  
Jet Flows ◽  
Backward Facing Step ◽  
Synthetic Jet Actuator ◽  
Step Flow ◽  
Flow Through ◽  
Low Speed Wind Tunnel

The purpose of this research is to explore the two-dimensional backward-facing-step flow perturbed by a synthetic-jet actuator experimentally. The step was placed in a low-speed wind tunnel and the flow passage expansion ratio was 1.07. The Reynolds number was Reh = 1.18 × 104 based on the step height and the upstream velocity before the step edge. An electromagnetic loudspeaker was used to generate synthetic jet flows in the direction of the main flow through a slot of 1 mm in width. The peak value of the driven AC voltage was 8 V and the frequency varied in the range of 80–120 Hz. The velocity of the flow field was measured by hot wire anemometers. The distributions of averaged velocity, fluctuation intensity, and Reynolds stress were analyzed to understand the influences of the synthetic jet. The characteristic frequencies were also obtained. The size of the recirculation region was affected by the synthetic jet. The downstream reattached flow was more stable with the synthetic-jet perturbation.

The Calculation of Properties of the Flow over a Backward Facing Step With the k-ε Model of Turbulence

1984 ◽  
Vol 8 (3) ◽  
pp. 165-170
Author(s):  
L.P. Hackman ◽  
A.B. Strong ◽  
G.D. Raithby
Keyword(s):  
Experimental Data ◽  
Kinetic Energy ◽  
Turbulent Flow ◽  
Skin Friction ◽  
Turbulent Kinetic Energy ◽  
Reasonable Agreement ◽  
Shear Stresses ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
The Mean

This paper reports predictions of the mean velocity, the turbulent kinetic energy and the pressure and skin friction coefficients for turbulent flow over a backward facing step based on the standard k – ε closure for the turbulence shear stresses. In previous publications, errors due to the numerical algorithm as distinct from the turbulence model have been carefully assessed using different numerical schemes and finite volume geometries and it is argued that the current results are numerically accurate. Thus one can now assess the accuracy of the k – ε model of turbulence independently of numerical error. The results predicted herein were found to be in reasonable agreement with relevant experimental data.

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