(320a) Studies of Realistic Roughness Effects in Wall Turbulence

This keynote lecture will highlight on-going studies of highly-irregular surface roughness commonly encountered on practical flow surfaces, such as turbine blades, hulls of ships and within transport  pipelines, for example.   Such realistic roughness is typically due to cumulative damage to a flow surface that was likely aerodynamically or hydrodynamically smooth prior to deployment but has become significantly roughened via cumulative damage and/or deposition.  Previous efforts have shown such realistic roughness patterns to generate bulk flow and heat-transfer characteristics quite different than that predicted from correlations developed from studies of much more idealized roughness topographies (often single scale arranged in an ordered manner).  Thus, the present effort is devoted toward identifying the impact of highly-irregular roughness replicated from a turbine blade damaged by deposition of foreign materials on wall turbulence.  To this end, several particle-image velocimetry (PIV) experiments were conducted for flow over the aforementioned surface, both within and outside the roughness sublayer, to explore its impact on the canonical behavior of smooth-wall turbulence.  The impact of this roughness on various statistical and structural aspects of wall turbulence will be highlighted.  In addition, the possibility of devising low-order topographical representations of this complex surface using singular value decomposition (SVD) is considered.  Truncation of this decomposition so that only the largest topographical scales are included in a given model surface allows one to isolate the impact of these scales from the intermediate and finer-scale details of the roughness.  The fidelity of two different low-order reconstructions in generating a similar impact on the flow as the original surface is evaluated using 2D and stereo PIV measurements within and outside the roughness sublayer.