Simulation of the flow and the study of the effects of the surface roughness in isothermal gas flows of micro scale using Lattice Boltzmann method

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.In this paper, a lattice Boltzmann method used in the simulation of the fluid flow in micro/nano scale is introduced and studied. The method can be employed instead of NS equations in cases where the continuum assumption is no longer valid. In the present study the aim is to investigate the effects of surface roughness on flow characteristics of micro/nano gas flows. In order to compare the final results, two flow geometries were chosen for which the numerical and experimental results were available. Surface roughness was increased in each stage (from completely smooth to 12% roughness) and its influences on the flow regime, pressure and velocity distribution, slip velocity and mass flow rate were studied. It is shown that surface roughness results in a decrease in the mass flow rate as well as slip velocity. Increasing the amount of roughness causes the mass flow rate to continually decrease, however this trend is inverted for the slip velocity

Computation of Developing Turbulent Flow through a Straight Asymmetric Diffuser with Moderate Adverse Pressure Gradient

In this paper, numerical investigation of three-dimensional, developing turbulent flow, subjected to a moderate adverse pressure gradient, has been investigated using various turbulence models, namely: the low-Re , the SST , the and a variant of Reynolds stress model. The results are compared with the detailed velocity and pressure measurements. Since the inlet condition is uncertain, a study was first performed to investigate the sensitivity of the results to the inlet boundary condition. The results showed the importance of including the contraction effects. It is seen that the developing flow inside the straight duct, is highly sensitive to the inlet boundary condition. The comparisons indicate that all turbulence models are able to predict a correct trend for the centerline velocity and pressure recovery inside the straight duct and diffuser but the low-Re and RSM turbulence models yield more realistic results. The SST model largely overpredicts the centerline velocity and boundary layer thickness in the straight duct. The comparisons of the numerical results also revealed that the RSM model, due to its anisotropic formulation, is able to reproduce the secondary flows. As expected, the RSM model demonstrates the best performance in prediction of the flow field and pressure recovery in the asymmetric diffuser

Numerical study of turbulent flow around a square cylinder using low-Reynolds-number k-e model

This paper discusses the abilities of two different low-Reynolds-number eddy-viscosity models in resolving the complex physical features that arises in turbulent flows around a square cylinder at Re=22000. For the modeling of turbulence, the Launder & Sharma (LS) [1] and Kawamura & Kawashima (KK) [2] low-Re models have been employed. The present numerical results were obtained using a two-dimensional finite-volume code. The pressure field is obtained with the well-known SIMPLE algorithm. Advective volume-face fluxes are approximated using a bounded version of the upstream quadratic interpolation scheme, QUICK. Comparisons of the numerical results with the experimental data indicate that the steady computations, as expected, cannot produce reliable flow field predictions in the wake region downstream of the square cylinder. Consequently, the time derivatives of dependent variables are included in the transport equations and are approximated using the second-order Crank-Nicholson scheme. The unsteady computations significantly improve the predictions and the results of unsteady simulations with both turbulence models are in closer agreement with measurements compare to the steady predictions. The predicted value for St number is 0.126 and 0.123 using the LS and KK turbulence models respectively which are in good agreement with the measured value of 0.132. The unsteady predictions using the LS model are in better agreement with the measured data than those obtained with the KK model. Both turbulence models fail to produce reliable turbulence field predictions and, thus, it is necessary to apply more advance turbulence models, such differential stress models, for more accurate predictions of such flows

Computation of developing turbulent flow through a straight asymmetric diffuser with moderate adverse pressure gradient

In this paper, numerical investigation of three-dimensional, developing turbulent flow, subjected to a moderate adverse pressure gradient, has been investigated using various turbulence models, namely: the lowRe k , the SST k , the 2 v f and a variant of Reynolds stress model. The results are compared with the detailed velocity and pressure measurements. Since the inlet condition is uncertain, a study was first performed to investigate the sensitivity of the results to the inlet boundary condition. The results showed the importance of including the contraction effects. It is seen that the developing flow inside the straight duct, is highly sensitive to the inlet boundary condition. The comparisons indicate that all turbulence models are able to predict a correct trend for the centerline velocity and pressure recovery inside the straight duct and diffuser but the low-Re k and RSM turbulence models yield more realistic results. The SST k model largely overpredicts the centerline velocity and boundary layer thickness in the straight duct. The comparisons of the numerical results also revealed that the RSM model, due to its anisotropic formulation, is able to reproduce the secondary flows. As expected, the RSM model demonstrates the best performance in prediction of the flow field and pressure recovery in the asymmetric diffuser