Unsteady jet impingement: Heat transfer on smooth and non-smooth surfaces

Often studies that determine the influence of unsteadiness on flat plate impinging jet heat transfer implicitly assume that the effect of unsteadiness found on smooth impingement surfaces also holds on surfaces with certain obstacles on them. In order to test this assumption a single roughness element was added to an otherwise smooth surface, and it was found that the steady heat transfer was almost the same as that for a totally smooth surface. The effect of unsteadiness, however, can be fundamentally different when roughness elements are added to a smooth surface. Slight changes in the surface geometry thus can have strong impact with respect to the effect of unsteadiness on heat transfer under impinging jets and cannot be neglected a priori.