DNS Study of Transient Disturbance Growth and Bypass Transition due to Realistic Roughness

Abstract

Direct numerical simulation was used to investigate the detailed flow past a periodic array of cylindrical roughness elements. A spectral DNS code was used to model flow over a flat plate surface, and roughness elements were formed using an immersed boundary technique. Solutions were obtained for two roughness heights corresponding to Reynolds numbers (Rek) of 202 and 334, and results are presented for both cases. Cylindrical roughness elements with Rek=202 produced minimal disturbances and the flow appeared generally laminar in the wake downstream of the roughness elements. Flow past cylindrical roughness elements corresponding to Rek=334 was found to transition as soon as 2-3 cylinder diameters downstream and had developed into fully turbulent flow by the end of the domain. Results were found to compare reasonably well with a similar set of DNS computations by Rizzetta and Visbal 1 using a sixth-order-accurate centered compact finite difference scheme as well as experimental results obtained by Ergin and White 2 using timeaveraged hotwire measurements of the velocity components. Nomenclature