Two dimensional backward facing single step flow preceding an automotive air-filter

Scope and method of study. The step flow is a basic separating flow that resembles the flow characteristics found in automotive air filter housings. The two dimensional backward facing single step flow has been studied computationally and experimentally, and serves as an ideal test case for filter effects on separation. A second order accurate, two-step time-split finite difference method was used to solve the Navier-Stokes equations. The air filter is modeled as a homogeneous porous medium governed by an extended Darcy's law. The effects of the filter on the separated flow field were studied for step height Reynolds numbers of 100 to 800. The stream-wise velocity fields were measured with a Laser Doppler Anemometer system for a step flow in a nominally two-dimensional duct of aspect ration 10:1. The filter was placed either at 4.25 or 6.7 5 step heights from the step, with the non-filter flow reattachment point at about 6 step heights. At four Reynolds numbers between 2000 to 10000, velocity profiles were measured upstream of the filter. Two-dimensional Large Eddy Simulation model with wall damping was used to compute the turbulent flow at Re=10000.; Findings and conclusions. The computation at Reynolds numbers between 100 and 800 indicates that the air filter has no apparent effects on the separated flow when it is far downstream. However the velocity profiles close to the upstream surface of the filter are altered significantly. The velocity in the center is reduced and the velocity close to the wall is increased due to the filter. When the filter is placed close to the step, the reversing flow area at the step side is reduced dramatically in most cases due to the acceleration in that area because of the presence of the filter. In some cases the flow at the non-step side is separated due to the deceleration caused by the filter. The LDA measurements at the Reynolds numbers between 2000 and 10000 provide the same findings as the computations. LES computations yielded similar results with the measured flow fields, however quantitatively do not match well with the measured profiles.