The Experts below are selected from a list of 11418 Experts worldwide ranked by ideXlab platform
Renwei Mei - One of the best experts on this subject based on the ideXlab platform.
-
History force on a sphere due to a step change in the Free-Stream Velocity
International Journal of Multiphase Flow, 1993Co-Authors: Renwei MeiAbstract:Abstract Finite-difference solutions for unsteady flows over a stationary sphere due to a step change in the Free-Stream Velocity from U 1 to U 2 (0 U 1 UZ ) are obtained, from which the unsteady drag is evaluated, for Reynolds numbers, Re (based on the diameter of the sphere and the Free-Stream Velocity U 2 ), ranging from 0.1 to 100 over a large range of time. The history force on the sphere is determined by subtracting the quasi-steady drag from the computed total drag. The numerical result shows a complicated behavior of the history force at finite Re for both U 1 = 0 and U 1 > 0. It decays as t − 1 2 for small time; it then decays as t − n ( n ⩾ 2 with n = 2 for small Re) for an intermediate range of time; and it decays exponentially at large time. The numerical results are used to assess a recently developed expression for the history force for finite Re. Good overall agreement is observed for the history force between the analytical prediction and the finite-difference solution for small and intermediate time for the Re values tested.
-
Flow past a sphere with an oscillation in the Free-Stream Velocity and unsteady drag at finite Reynolds number
Journal of Fluid Mechanics, 1992Co-Authors: Renwei Mei, Ronald J. AdrianAbstract:Unsteady flow over a stationary sphere with a small fluctuation in the Free-Stream Velocity is considered at small Reynolds number, Re. A matched asymptotic solution is obtained for the frequency-dependent (or the acceleration-dependent) part of the unsteady flow at very small frequency, w, under the restriction St % Re 4 1, where St is the Strouhal number. The acceleration-dependent part of the unsteady drag is found to be proportional to St - w instead of the wi dependence predicted by Stokes’ solution. Consequently, the expression for the Basset history force is incorrect for large time even for very small Reynolds numbers. Present results compare well with the previous numerical results of Mei, Lawrence & Adrian (1991) using a finitedifference method for the same unsteady flow at small Reynolds number. Using the principle of causality, the present analytical results at small Re, the numerical results at finite Re for low frequency, and Stokes’ results for high frequency, a modified expression for the history force is proposed in the time domain. It is confirmed by comparing with the finite-difference results at arbitrary frequency through Fourier transformation. The modified history force has an integration kernel that decays as t-*, instead of t-i, at large time for both small and finite Reynolds numbers.
-
Unsteady force on a spherical bubble at finite Reynolds number with small fluctuations in the Free‐Stream Velocity
Physics of Fluids A: Fluid Dynamics, 1992Co-Authors: Renwei Mei, James F KlausnerAbstract:Unsteady flow over a stationary spherical bubble with small fluctuations in the Free‐Stream Velocity is considered for Reynolds number ranging from 0.1 to 200. Solutions to the Navier–Stokes equations of both steady and unsteady components are obtained using a finite‐difference method and a regular perturbation scheme based on the amplitude of the fluctuations being small. The dependence of the unsteady drag on the frequency of the fluctuations is examined at finite Reynolds number. It is shown that the quasisteady drag can be represented by using the steady‐state drag coefficient and the instantaneous Velocity. Numerical results indicate that the unsteady force at low frequency, ω, increases linearly with ω rather than increasing linearly with ω1/2, which results from the creeping flow solution of the Stokes equation. The added‐mass force at finite Reynolds number is found to be the same as in creeping flow and potential flow. The history force at finite Re is identified and carefully evaluated. The imaginary component of the history force increases linearly with ω when ω is small and decays as ω−1/2 as ω becomes large. The implication is that the history force has a much shorter memory in the time domain than predicted by the solution of the unsteady Stokes equation. Numerical results suggest that the history force, which is due to the combination of the viscous diffusion of the vorticity and the acceleration of the flow field, at low frequency is finite even at large Reynolds number.
-
UNSTEADY DRAG ON A SPHERE AT FINITE REYNOLDS NUMBER WITH SMALL FLUCTUATIONS IN THE Free-Stream Velocity
Journal of Fluid Mechanics, 1991Co-Authors: Renwei Mei, Christopher J. Lawrence, Ronald J. AdrianAbstract:Unsteady flow over a stationary sphere with small fluctuations in the Free-Stream Velocity is considered at finite Reynolds number using a finite-difference method. The dependence of the unsteady drag on the frequency of the fluctuations is examined at various Reynolds numbers. It is found that the classical Stokes solution of the unsteady stokes equation does not correctly describe the behaviour of the unsteady drag at low frequency. Numerical results indicate that the force increases linearly with frequency when the frequency is very small instead of increasing linearly with the square root of the frequency as the classical Stokes solution predicts. This implies that the forces has a much shorter memory in the time domain. The incorrect behaviour of the Basset force at large times may explain the unphysical results found by Reeks & McKee (1984) wherein for a particle introduced to a turbulent flow the initial Velocity difference between the particle and fluid has a finite contribution to the long-time particle diffusivity. The added mass component of the force at finite Reynolds number is found to be the same as predicted by creeping flow and potential theories. Effect of Reynolds number of the unsteady drag due to the fluctuating Free-Stream Velocity are presented. The implications for particle motion in turbulence are discussed.
Ronald J. Adrian - One of the best experts on this subject based on the ideXlab platform.
-
Flow past a sphere with an oscillation in the Free-Stream Velocity and unsteady drag at finite Reynolds number
Journal of Fluid Mechanics, 1992Co-Authors: Renwei Mei, Ronald J. AdrianAbstract:Unsteady flow over a stationary sphere with a small fluctuation in the Free-Stream Velocity is considered at small Reynolds number, Re. A matched asymptotic solution is obtained for the frequency-dependent (or the acceleration-dependent) part of the unsteady flow at very small frequency, w, under the restriction St % Re 4 1, where St is the Strouhal number. The acceleration-dependent part of the unsteady drag is found to be proportional to St - w instead of the wi dependence predicted by Stokes’ solution. Consequently, the expression for the Basset history force is incorrect for large time even for very small Reynolds numbers. Present results compare well with the previous numerical results of Mei, Lawrence & Adrian (1991) using a finitedifference method for the same unsteady flow at small Reynolds number. Using the principle of causality, the present analytical results at small Re, the numerical results at finite Re for low frequency, and Stokes’ results for high frequency, a modified expression for the history force is proposed in the time domain. It is confirmed by comparing with the finite-difference results at arbitrary frequency through Fourier transformation. The modified history force has an integration kernel that decays as t-*, instead of t-i, at large time for both small and finite Reynolds numbers.
-
UNSTEADY DRAG ON A SPHERE AT FINITE REYNOLDS NUMBER WITH SMALL FLUCTUATIONS IN THE Free-Stream Velocity
Journal of Fluid Mechanics, 1991Co-Authors: Renwei Mei, Christopher J. Lawrence, Ronald J. AdrianAbstract:Unsteady flow over a stationary sphere with small fluctuations in the Free-Stream Velocity is considered at finite Reynolds number using a finite-difference method. The dependence of the unsteady drag on the frequency of the fluctuations is examined at various Reynolds numbers. It is found that the classical Stokes solution of the unsteady stokes equation does not correctly describe the behaviour of the unsteady drag at low frequency. Numerical results indicate that the force increases linearly with frequency when the frequency is very small instead of increasing linearly with the square root of the frequency as the classical Stokes solution predicts. This implies that the forces has a much shorter memory in the time domain. The incorrect behaviour of the Basset force at large times may explain the unphysical results found by Reeks & McKee (1984) wherein for a particle introduced to a turbulent flow the initial Velocity difference between the particle and fluid has a finite contribution to the long-time particle diffusivity. The added mass component of the force at finite Reynolds number is found to be the same as predicted by creeping flow and potential theories. Effect of Reynolds number of the unsteady drag due to the fluctuating Free-Stream Velocity are presented. The implications for particle motion in turbulence are discussed.
James F Klausner - One of the best experts on this subject based on the ideXlab platform.
-
unsteady force on a spherical bubble at finite reynolds number with small fluctuations in the Free Stream Velocity
Physics of Fluids, 1992Co-Authors: James F KlausnerAbstract:Unsteady flow over a stationary spherical bubble with small fluctuations in the Free‐Stream Velocity is considered for Reynolds number ranging from 0.1 to 200. Solutions to the Navier–Stokes equations of both steady and unsteady components are obtained using a finite‐difference method and a regular perturbation scheme based on the amplitude of the fluctuations being small. The dependence of the unsteady drag on the frequency of the fluctuations is examined at finite Reynolds number. It is shown that the quasisteady drag can be represented by using the steady‐state drag coefficient and the instantaneous Velocity. Numerical results indicate that the unsteady force at low frequency, ω, increases linearly with ω rather than increasing linearly with ω1/2, which results from the creeping flow solution of the Stokes equation. The added‐mass force at finite Reynolds number is found to be the same as in creeping flow and potential flow. The history force at finite Re is identified and carefully evaluated. The imaginary component of the history force increases linearly with ω when ω is small and decays as ω−1/2 as ω becomes large. The implication is that the history force has a much shorter memory in the time domain than predicted by the solution of the unsteady Stokes equation. Numerical results suggest that the history force, which is due to the combination of the viscous diffusion of the vorticity and the acceleration of the flow field, at low frequency is finite even at large Reynolds number.
-
Unsteady force on a spherical bubble at finite Reynolds number with small fluctuations in the Free‐Stream Velocity
Physics of Fluids A: Fluid Dynamics, 1992Co-Authors: Renwei Mei, James F KlausnerAbstract:Unsteady flow over a stationary spherical bubble with small fluctuations in the Free‐Stream Velocity is considered for Reynolds number ranging from 0.1 to 200. Solutions to the Navier–Stokes equations of both steady and unsteady components are obtained using a finite‐difference method and a regular perturbation scheme based on the amplitude of the fluctuations being small. The dependence of the unsteady drag on the frequency of the fluctuations is examined at finite Reynolds number. It is shown that the quasisteady drag can be represented by using the steady‐state drag coefficient and the instantaneous Velocity. Numerical results indicate that the unsteady force at low frequency, ω, increases linearly with ω rather than increasing linearly with ω1/2, which results from the creeping flow solution of the Stokes equation. The added‐mass force at finite Reynolds number is found to be the same as in creeping flow and potential flow. The history force at finite Re is identified and carefully evaluated. The imaginary component of the history force increases linearly with ω when ω is small and decays as ω−1/2 as ω becomes large. The implication is that the history force has a much shorter memory in the time domain than predicted by the solution of the unsteady Stokes equation. Numerical results suggest that the history force, which is due to the combination of the viscous diffusion of the vorticity and the acceleration of the flow field, at low frequency is finite even at large Reynolds number.
S. Dholey - One of the best experts on this subject based on the ideXlab platform.
-
Unsteady separated stagnation-point flow and heat transfer of a viscous fluid over a moving flat surface
Physics of Fluids, 2018Co-Authors: S. DholeyAbstract:In this paper, we have investigated numerically the laminar unsteady separated stagnation-point flow and heat transfer of a viscous fluid over a moving flat surface in the presence of a time dependent Free Stream Velocity which causes the unsteadiness of this flow problem. The plate is assumed to move in the same or opposite direction of the Free Stream Velocity. The flow is therefore governed by the Velocity ratio parameter λ (ratio of the plate Velocity to the Free Stream Velocity) and the unsteadiness parameter β. When the plate surface moves in the same direction of the Free Stream Velocity (i.e., when λ > 0), the solution of this flow problem continues for any given value of β. On the other hand, when they move in opposite directions (i.e., when λ < 0), the solution does not exist after a certain value of λ depending upon the values of β. In this case, separation appears inside the layer only for a negative value of β, and for a positive value of β, the boundary layer solution is terminated after a c...
Swati Mukhopadhyay - One of the best experts on this subject based on the ideXlab platform.
-
Effects of Partial Slip on Chemically Reactive Solute Distribution in MHD Boundary Layer Stagnation Point Flow Past a Stretching Permeable Sheet
International Journal of Chemical Reactor Engineering, 2015Co-Authors: Swati MukhopadhyayAbstract:Abstract This paper presents the magnetohydrodynamic (MHD) boundary layer stagnation point flow with diffusion of chemically reactive species undergoing first-order chemical reaction over a permeable stretching sheet in presence of partial slip. With the help of similarity transformations, the partial differential equations corresponding to momentum and the concentration equations are transformed into non-linear ordinary differential equations. Numerical solutions of these equations are obtained by shooting method. It is found that the horizontal Velocity increases with the increasing value of the ratio of the Free Stream Velocity and the stretching Velocity. Velocity decreases with the increasing magnetic parameter when the Free-Stream Velocity is less than the stretching Velocity but the opposite behavior is noted when the Free-Stream Velocity is greater than the stretching Velocity. Due to suction, fluid Velocity decreases at a particular point of the surface. With increasing Velocity slip parameter, Velocity increases when the Free-Stream Velocity is greater than the stretching Velocity. But the concentration decreases in this case. Concentration decreases with increasing mass slip parameter.
-
Effects of thermal radiation and variable fluid viscosity on stagnation point flow past a porous stretching sheet
Meccanica, 2013Co-Authors: Swati MukhopadhyayAbstract:Similarity analysis is performed to investigate the structure of the boundary layer stagnation-point flow and heat transfer over a stretching sheet subject to suction. Fluid viscosity is assumed to vary as a linear function of temperature. Thermal radiation term is considered in the energy equation. The symmetry groups admitted by the corresponding boundary value problem are obtained by using a special form of Lie group transformations viz. scaling group of transformations. With the help of them the partial differential equations corresponding to momentum and energy equations are transformed into highly non-linear ordinary differential equations. Numerical solutions of these equations are obtained by shooting method. It is found that the horizontal Velocity increases with the increasing values of the ratio of the Free Stream Velocity to the stretching Velocity. Velocity increases with the increasing temperature dependent fluid viscosity parameter when the Free-Stream Velocity is less than the stretching Velocity but opposite behavior is noted when the Free-Stream Velocity is greater than the stretching Velocity. Due to suction, fluid Velocity decreases at a particular point of the surface. Temperature at a point of the surface is found to decrease with increasing thermal radiation.
-
heat and mass transfer analysis for boundary layer stagnation point flow towards a heated porous stretching sheet with heat absorption generation and suction blowing
International Communications in Heat and Mass Transfer, 2007Co-Authors: G C Layek, Swati Mukhopadhyay, Sk A SamadAbstract:A similarity analysis is performed to investigate the structure of the boundary layer stagnation-point flow and heat transfer over a stretching sheet in a porous medium subject to suction/blowing and in the presence of internal heat generation/absorption. A scaling group of transformations is applied to get the invariants. Using the invariants, a third and a second order ordinary differential equations corresponding to the momentum and energy equations are obtained respectively. Boundary layer Velocity and temperature profiles are determined numerically for various values of the ratio of Free Stream Velocity and stretching Velocity, the permeability parameter, suction/blowing parameter, heat source/sink parameter, Prandtl number. It is found that the horizontal Velocity increases with the increasing value of the ratio of the Free Stream Velocity (ax) and the stretching Velocity (cx). The temperature decreases in this case. At a particular point of the porous stretching sheet, the non-dimensional fluid Velocity decreases with the increase of the permeability of the porous medium and also with the increasing suction parameter when the Free Stream Velocity is less than stretching Velocity whereas fluid Velocity increases with the increasing injection parameter. But when the Free Stream Velocity is greater than the stretching Velocity the opposite behaviour of horizontal Velocity is noticed. The dimensionless temperature at a point of the sheet decreases due to suction but increases due to injection. The temperature at a point is found to decrease with the increasing Prandtl number.
