Characterizing turbulent flow over 3-D idealized and irregular rough surfaces at low Reynolds number

The focus of this work is to demonstrate that the response of flow over a rough wall is significantly different for idealized and irregular roughness when the spacing between the roughness elements (λ) is changed for a given height of the roughness (h). For this purpose, turbulence flow in a channel with walls covered with idealized and irregular roughness has been simulated where the λ/h has been systematically varied in the range of 3–35. A detailed study of the flow statistics such as mean flow, root mean square of velocity and vorticity fluctuations was performed. For the idealized roughness both the inner and outer layer of the turbulent boundary layer are altered due to surface roughness, whereas, only the inner layer is altered due to an irregular roughness with similar geometrical parameters. Two distinct regimes that are evident for flow over idealized roughness based on streamwise spacing (λ/h) can be unified using solidity ratio (measure of degree of sparseness) parameter. Irregular roughness exhibits no sensitivity to λ/h parameter, suggesting that instead of solidity ratio, statistical parameters such as skewness and kurtosis of roughness distribution are more important to classify flow over irregular roughness.