Effect of Richardson number on entropy generation over backward facing step

The flow over a backward facing step （BFS） has been taken as a useful proto- type to investigate intrinsic mechanisms of separated flow with heat transfer. However, to date, the open literature on the effect of Richardson number on entropy generation over the BFS is absent yet, although the flow pattern and heat transfer characteristic both will receive significant influence caused by the variation of Richardson number in many prac- tical applications, such as in microelectromechanical systems and aerocrafts. The effect of Richardson number on entropy generation in the BFS flow is reported in this paper for the first time. The entropy generation analysis is conducted through numerically solving the entropy generation equation. The velocity and temperature, which are the inputs of the entropy generation equation, are evaluated by the lattice Boltzmann method. It is found that the distributions of local entropy generation number and Bejan number are significantly influenced by the variation of Richardson number. The total entropy gen- eration number is a monotonic decreasing function of Richardson number, whereas the average Bejan number is a monotonic increasing function of Richardson number.

Analysis of flow over backward facing step with transition

The present study deals with the study of the velocity distribution and the separation phenomenon of flow of air over a two dimensional backward facing step. The flow of air over a backward facing step has been investigated numerically using FLUENT. Flow simulation has been carried out in a backward facing step having an expansion ratio （ratio of the height before and after the step） of 1：1.94 and the results obtained are compared with the published experimental results. Comparison of flow characteristics between steps with three different transitions is made. The variation ofreattachment length for all the three cases are analyzed for wide range of Reynolds number ranging from 100 to 7000 which covers the laminar, transition and turbulent flow of air. Simulation of the flow over steps with expansion ratios of 1：1.24, 1：1.38, 1：1.47, 1：1.53, 1：1.94, 1：2.20 are also carried out to examine the effect of different expansion ratios on the reattachment length. It is found that the primary reattachment length increases with increase in the expansion ratio. The primary reattachment length at the bottom wall downstream of the step is minimum for the step with round edged transition and maximum for the step with a vertical drop transition.

EXPERIMENTAL INVESTIGATION ON THE TURBULENT COHERENT STRUCTURES OF LAMINAR SEPARATION FLOW OVER A BACKWARD FACING STEP

The experimental investigation is conducted with LDV and hydrogen bubble technique in water flow. The shear layer thickness. the vorticity thickness. the maximulll value of turbulence intensities. the turbulent coherent structure. the variations of wall shear stress and the boundary layer shape factor are obtained. In the redevelopment region. the detailed analysis is first made for the streak structures in the near wall region and the turbulent boundary layer is formed at (x-xr) / h = 20.

Error Estimates for Finite-Element Navier-Stokes Solvers without Standard Inf-Sup Conditions

The authors establish error estimates for recently developed finite-element methods for incompressible viscous flow in domains with no-slip boundary conditions.The methods arise by discretization of a well-posed extended Navier-Stokes dynamics for which pressure is determined from current velocity and force fields.The methods use C1 elements for velocity and C0 elements for pressure.A stability estimate is proved for a related finite-element projection method close to classical time-splitting methods of Orszag,Israeli,DeVille and Karniadakis.

Experimental study of the effects of a transversal air-flow deflector in electronics air-cooling

This paper presents an experimental investigation of the influence of a transversal flow deflector on the cooling of a heated block mounted on a flat plate.The deflector is inclined and therefore it guides the air flow to the upper surface of the block.This configuration is simulating the air-cooling of a rectangular integrated circuit or a current converter mounted on an electronic card.The electronic component is assumed dissipating low heat power,as such,air forced convection is still a sufficient cooling way even without fan or heat sink on the component.The measurements are given by hot and cold wires anemometers and by an InfraRed camera.The results give details of the effects of the deflection on the hydrodynamic and the thermal fields on and over the block for different inclination angles.They show that the deviation caused by the deflector may significantly enhance the heat transfer from the component.Deflection is also able to avoid local overheating of the electronic component.Optimum heat transfer rate and homogenised temperature are shown to be obtained with an inclination angle =30°.

Numerical Simulation of Separated Flows in Channels

The work deals with numerical modelling of turbulent flows in channels with an expansion of the cross-section where flow separation and reattachment occur. The performance of several eddy viscosity models and an explicit algebraic Reynolds stress model (EARSM) is studied. The used test cases are flows in channels with various backward facing steps where the step is perpendicular or inclined and the top wall is parallel or deflected. Furthermore, a channel with the circular ramp is considered. The numerical solution is achieved by the finite volume method or by the finite element method. The results of both numerical approaches are compared.