Unsteady force on a spherical bubble at finite Reynolds number with small fluctuations in the free‐stream velocity

1992 ◽  
Vol 4 (1) ◽  
pp. 63-70 ◽  
Author(s):  
Renwei Mei ◽  
James F. Klausner
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Spherical Bubble ◽  
Unsteady Force

Unsteady drag on a sphere at finite Reynolds number with small fluctuations in the free-stream velocity

1991 ◽  
Vol 233 ◽  
pp. 613-631 ◽  
Author(s):  
Renwei Mei ◽  
Christopher J. Lawrence ◽  
Ronald J. Adrian
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Low Frequency ◽  
Reynolds Numbers ◽  
Free Stream Velocity ◽  
Creeping Flow ◽  
Stream Velocity ◽  
Velocity Difference ◽  
Long Time ◽  
The Time Domain

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 force 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. Effects of Reynolds number on the unsteady drag due to the fluctuating free-stream velocity are presented. The implications for particle motion in turbulence are discussed.

Experiments on Leading-Edge Induced Separated Shear Layer Under Various Imposed Pressure Gradients

2014 ◽  
Author(s):  
K. Anand ◽  
S. Sarkar ◽  
N. Thilakan
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Free Stream ◽  
Separation Bubble ◽  
Leading Edge ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Pressure Gradients ◽  
Mean Flow ◽  
Separated Shear Layer

The behaviour of a separated shear layer past a semi-circular leading edge flat plate, its transition and reattachment downstream to separation are investigated for different imposed pressure gradients. The experiments are carried out in a blowing tunnel for a Reynolds number of 2.44×105 (based on chord and free-stream velocity). The mean flow characteristics and the instantaneous vector field are documented using a two-component LDA and a planar PIV, whereas, surface pressures are measured with Electronically scanned pressure (ESP). The onset of separation occurs near the blend point for all values of β (flap angle deflection), however, a considerable shift is noticed in the point of reattachment. The dimensions of the separation bubble is highly susceptible to β and plays an important role in the activity of the outer shear layer. Instantaneous results from PIV show a significant unsteadiness in the shear layer at about 30% of the bubble length, which is further amplified in the second half of the bubble leading to three-dimensional motions. The reverse flow velocity is higher for a favourable pressure gradient (β = +30°) and is found to be 21% of the free stream velocity. The Reynolds number calculated based on ll (laminar shear layer length), falls in the range of 0.9×104 to 1.4×104. The numerical values concerning the criterion for separation and reattachment agree well with the available literature.

On the Resistance of Screens

1953 ◽  
Vol 4 (2) ◽  
pp. 186-192 ◽  
Author(s):  
K. E. G. Wieghardt
Keyword(s):  
Reynolds Number ◽  
Average Velocity ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Single Piece ◽  
Experimental Function ◽  
Almost All

Summary The resistance of various screens in a tube is reduced to the drag of a single piece of wire by assuming that the average velocity in the screen itself rather than the free stream velocity is the significant factor. Almost all the available tests on screens or gauzes are brought into line with a single experimental function similar to the drag of a cylinder plotted against Reynolds number.

On the asymptotic similarity of the zero-pressure-gradient turbulent boundary layer

2008 ◽  
Vol 616 ◽  
pp. 195-203 ◽  
Author(s):  
M. B. JONES ◽  
T. B. NICKELS ◽  
IVAN MARUSIC
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Similarity Solution ◽  
Friction Velocity ◽  
Free Stream ◽  
Outer Region ◽  
Free Stream Velocity ◽  
Stream Velocity

We investigate similarity solutions for the outer part of a zero-pressure-gradient turbulent boundary layer in the limit of infinite Reynolds number. Previous work by George (Phil. Trans. R. Soc. vol. 365, 2007 p. 789) has suggested that the only appropriate velocity scale for the outer region is U1, the free-stream velocity. This is based on the fact that scaling with U1 leads to a mathematically valid similarity solution of the momentum equation for the outer region in the asymptotic limit of infinite Reynolds number. Here we show that the classical scaling using the friction velocity also leads to a valid similarity solution for the outer flow in this limit. Therefore on this basis it is not possible to dismiss the friction velocity as a possible scaling as has been suggested by George (2007) and others. We show that both the free-stream velocity and the friction velocity are potentially valid scalings according to this theoretical criterion.

Effect of Free-Stream Velocity Vector and Aspect Ratio on the Output of a Free-Standing Circular Disk Heat Flux Gage

1984 ◽  
Vol 106 (1) ◽  
pp. 229-233 ◽  
Author(s):  
M. F. Young
Keyword(s):  
Heat Flux ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Aspect Ratio ◽  
Free Stream ◽  
Angle Of Attack ◽  
Circular Disk ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Free Standing

The effects of free-stream velocity, angle of attack, and aspect ratio on the output of a free-standing circular disk heat flux gage subjected to a combined radiative and convective heat flux are reported. The Reynolds number range investigated extends from 6000 to 25,000, while the gage angle of attack was varied from 0 to 90 deg. Results for three gage aspect ratios, 5.85, 8.77, and 11.76, are presented. The Nusselt number is used to represent the effects of convection on the gage output. The Nusselt number was found to increase with increasing Reynolds number and angles of attack. At an angle of attack of about 90 deg, however, a significant reduction in the Nusselt number was noted. A correlation relating the Nusselt number (based on the disk diameter) to the Reynolds number (based on the gage outside diameter) and the angle of attack is reported. This correlation represents the data to within ±5 percent.

scholarly journals Boundary-Layer Type Solutions for Initial Platelet Activation and Deposition

2002 ◽  
Vol 4 (2) ◽  
pp. 95-108 ◽  
Author(s):  
T. David ◽  
P. G. de Groot ◽  
P. G. Walker
Keyword(s):  
Platelet Activation ◽  
Peclet Number ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Bulk Fluid ◽  
Péclet Number ◽  
Activated Platelets ◽  
Activation Rate ◽  
Initial Adhesion

This paper presents, on the basis of high Peclet number, a mathematical model for the activation and initial adhesion of flowing platelets onto a surface. In contrast to past work, the model is applicable to general 2D and axi-symmetric flows where the wall shear stress is knowna priori. Results indicate that for high activation reaction rates there exist two layers, one containing only activated platelets and the other both activated and non-activated platelets. Fundamental relationships are proposed between the adhesion rate of platelets to the surface and the characteristic parameters of Peclet number and Reynolds number. Activation in the bulk fluid (blood) is characterised by the Damkohler number, which is a function of activation rate and the free-stream velocity. It is shown that, as the free-stream velocity varies, there exists a maximum of activated platelet flux to the wall for particular values of the velocity. These values, at which the maximum occur, are themselves functions of the platelet activation rate. As the free-stream velocity increases the activation of platelets ceases altogether and adhesion is reduced to a very small value strengthening the hypothesis of the correlation between atherogenesis/thrombogenesis and areas of low shear.

Free Convective Couette flow1of1a1Dusty1Fluid1through1a Porous Medium1with1Periodic1Permeability1in1the1Absence1of Electrically Magneto Hydro Dynamic Model

2021 ◽  
Vol 58 (2) ◽  
pp. 6072-6083
Author(s):  
K. Rajesh, A. Govindarajan, M. Vidhya
Keyword(s):  
Flow Field ◽  
Dynamic Model ◽  
Analytical Solutions ◽  
Free Stream ◽  
Porous Plate ◽  
Free Stream Velocity ◽  
Dust Particles ◽  
Stream Velocity

“The purpose of this investigation stands to discuss the effects of periodic permeability on1the; free1convective flow of a dusty viscous; incompressible1fluid through a1highly1porous1channel. The porous1medium is confined by an infinite perpendicular porous plate supercilious the free stream velocity to be uniform. Analytical solutions are gained for the dusty flow field, the1temperature field, the1skin1friction and the rate1of heat1transfer. when there is an increase in mass concentration1of dust1particles, it is found that the1velocity profile of fluid and dust particles reduces.”

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