Roughness, in various distributions and on various scales, is widely encountered in turbine modules and has a complex modulating effect on the distribution of film cooling effectiveness. In this study, the fast-response pressure-sensitive paint technique was utilized to analyze the steady and unsteady behaviors of film cooling effectiveness under various roughness conditions. Two roughness conditions (upstream roughness and the combination of upstream and downstream roughness) were examined at three roughness scales (ks/D = 0.016, 0.129, and 0.782) and three blowing ratios (M = 0.5, 1.0, and 1.5). Comparisons of the steady behaviors of cooling effectiveness revealed that upstream roughness was the primary factor influencing cooling effectiveness for the adequate film attachment (M = 0.5 and 1.0) and that the cooling effectiveness deteriorated with rising ks/D. Downstream roughness improved the cooling effectiveness in cases with poor film attachment (M = 1.5). Considering that the distribution of film cooling effectiveness is modulated by the vortical structures of jet in crossflow, the unsteady behaviors of roughness-affected cooling effectiveness were analyzed through proper orthogonal decomposition. Upstream roughness influenced the counter-rotating vortex pair and horseshoe vortex (HV) signatures by separately widening and shortening the patterns in the lateral and streamwise directions, a trend that became more pronounced with increasing ks/D values. Moreover, larger roughness scales at lower blowing ratios caused asymmetry in the signatures. Downstream roughness primarily affected the signatures through local morphological variations, inducing oscillations in modal patterns. For attached films, downstream roughness had a nonsignificant impact, while for detached films, disturbances caused by downstream roughness resulted in vague and asymmetric modal patterns. Evaluation of the reconstructed cooling effectiveness revealed that roughness tended to affect the unsteady behavior of HV-modulated cooling effectiveness at high frequencies. These clarified steady and unsteady behaviors across various roughness conditions provide references for improving film-cooling structures to accommodate diversely roughened turbine modules.
