Abstract
Typical engineering rough surfaces show only limited resemblance to the artificially constructed rough surfaces that have been the basis of most previous fundamental research on turbulent flow over rough walls. In this article flow past an irregular rough surface is investigated, based on a scan of a rough graphite surface that serves as a typical example for an irregular rough surface found in engineering applications. The scanned map of surface height versus lateral coordinates is filtered in Fourier space to remove features on very small scales and to create a smoothly varying periodic representation of the surface. The surface is used as a no-slip boundary in direct numerical simulations of turbulent channel flow. For the resolution of the irregular boundary an iterative embedded boundary method is employed. The effects of the surface filtering on the turbulent flow are investigated by studying a series of surfaces with decreasing level of filtering. Mean flow, Reynolds stress and dispersive stress profiles show good agreement once a sufficiently large number of Fourier modes are retained. However, significant differences are observed if only the largest surface features are resolved. Strongly filtered surfaces give rise to a higher mean-flow velocity and to a higher variation of the streamwise velocity in the roughness layer compared with weakly filtered surfaces. In contrast, for the weakly filtered surfaces the mean flow is reversed over most of the lower part of the roughness sublayer and higher levels of dispersive shear stress are found.
Highlights
Rough surfaces affect the flow in many engineering systems, since many processes can cause surface roughness
This study enables the investigation of the effects of small-scale surface features on turbulent flow past an irregular rough wall
Before the surface height map can be used as a boundary condition for direct numerical simulations the surface data needs to be processed
Summary
Rough surfaces affect the flow in many engineering systems, since many processes can cause surface roughness. A number of experimental studies have investigated flow over irregular roughness; the best known example being the study of Nikuradse [42] on turbulent flow over rough surfaces made from sieved sand grains This approach was adopted by others, e.g. rough surfaces made up from sand and round pebbles have been used in flow channel experiments (see [18]), while packed gravel chips (see [11]), abrasive sheets (see [46,5]) and scratched surfaces (see [50]) have been used to produce rough surfaces for wind tunnel experiments. In this paper a different type of surface filtering, low-pass Fourier filtering, is investigated The effect of this type of surface filtering on turbulent rough-wall channel flow is studied systematically using a range of surfaces based on a scan of a graphite surface.
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