Reynolds number

About: Reynolds number is a research topic. Over the lifetime, 68440 publications have been published within this topic receiving 1666116 citations.

Momentum Transport Mechanism for Water Flow over Porous Media

Abstract: The momentum transport phenomena at the interface of the porous medium and fluid have been numerically investigated. The single domain approach is used with matching boundary conditions; that is, the Brinkman-Forchheimer-extended Darcy equation is used for the present study. Five typical porous media found in natural and engineered systems are selected in order to cover a wide range of the Darcy number (6.25 × 10−4≤Da≤ 5.90 × 10−11). In addition, six different Reynolds numbers (10 ≤R≤ 1,000) are tested for each case. When Da> 10−7, the results showed the importance of viscous shear in the channel fluid. The viscous shear propagates across the interface into the porous medium and forms a transition region of disturbed flow in the porous medium. The depth of penetration is only dependent on the Darcy number of the porous medium rather than the Reynolds number and the shape of velocity profile. In the vicinity of the interface, it is clear that Darcy's law is inappropriate to describe flow in a permeable wal...

Experiments on Plane Couette Flow

Abstract: A simple method for generating plane Couette flow is described. In this technique the water surface of a covered open-channel flume represents the moving boundary, and measurements are performed in the overlying (sandwiched) air layer. Experimental results are presented which suggest transition from laminar to turbulent plane Couette flow at a Reynolds number (based on center-line velocity and channel half-depth) of about 280. Turbulent mean velocities from all available sources are correlated in terms of inner and outer law coordinates, and universal skin friction laws are derived therefrom for the case of hydrodynamically smooth boundaries. However, it is concluded that none of the presently available experimental data on turbulent plane Couette flow render support for von Karman's theory of homologous turbulence. Results of a cursory exploration of the effects of unilateral boundary roughness suggest that mean-flow characteristics on the smooth side of a Couette channel assume the features prevailing on the rough side.

Feedback Control Applied to the Bluff Body Wake

Abstract: In the present study the flow around a 2D bluff body with blunt stern is investigated experimentally and theoretically. The goal is to decrease and stabilize drag by active control. Low-dimensional vortex models are used to describe actuation effects on the coherent structures and the pressure field. Open-loop actuation as well as feedback control is applied using robust H ∞-controllers and slope-seeking feedback for a range of Reynolds numbers based on the height from 20 000 to 60 000. As expected, a decreased drag is observed to be related to delayed vortex shedding, i.e. an extended recirculation zone. Intriguingly, a control which mitigates the natural coupling between separating upper and lower shear-layer and the vortex street serves that purpose.

Laboratory Measurements of Differential Diffusion in a Diffusively Stable, Turbulent Flow

Abstract: Laboratory experiments were performed to determine the conditions under which differential diffusion occurs and to evaluate its effect on the mixing efficiency. Diffusively stable profiles of temperature and salinity were stirred steadily by horizontally oscillating vertical rods. The two-component stratification ensures that both scalars experience the same stratification and forcing, or Richardson and Reynolds numbers. The eddy diffusivities KT and KS, for temperature and salinity, were estimated by fitting theoretical solutions of diffusion equations to measured profiles, and the mixing efficiency was computed as the ratio of the potential energy change during a stirring interval to the work done in that interval. Differential diffusion occurred for «a/nN 2 , 300‐500, where «a is an average dissipation rate computed from an integrated energy budget. The diffusivity ratio d 5 KS/KT varied between 0.5 and 1 in the range 50 ,« a/nN 2 , 500. The experiments also show that differential diffusion can significantly affect the mixing efficiency. An important dimensionless parameter is the density ratio Rr, which is the ratio of the density change due to temperature to that due to salinity. Measurements in cases with low density ratio (Rr 0.25) and high density ratio (Rr 5) showed that the mixing efficiencies agreed well for weak stratification, or small Richardson number. For larger Richardson number, the efficiency for the highdensity-ratio case exceeded that for the low-density-ratio case by as much as a factor of 1.5.