Supersonic flow over circular protuberances on a flat plate

The nanotracer planar laser scattering (NPLS) technique is used to test the flow field of Mach 3.0 supersonic flow over circular protuberances. The protuberance heights (H) are 1, 2, and 4 mm, and the diameters (D) are all 4 mm with the Reynolds number of 7.7 × 10⁶/m, while the thickness of the laminar boundary layer (δ) is about 1.2 mm at the mounting position, corresponding to H/δ values of 0.83, 1.67 and 3.33. The fine structures in the symmetrical plane of the flow field around the cylinders are imaged. In the NPLS images, expansion wave, three-dimensional bow-detached shock wave, reattachment shock wave and separation region ahead of the cylinders are clearly visualized. The spatial and temporal evolution characteristics of the vortex structures are analyzed. Special attention is paid to the study of the wake flow and the influence of the height of the protuberances on the wake flow, showing that vortices in the wake flow of the 1-mm high cylinder are isolated and simple, while to the 4-mm high cylinder are complex and mixed-up with less periodicity. After reattachment, the lower cylinder has a relatively longer laminar section as well as transition section. Fractal analysis is done on the wake flow, indicating that higher protuberances tend to have a larger fractal dimension and increasing slope. Based on the average fractal dimension of 50 NPLS images, the 2- and 4-mm cylinders have a close value while the 1-mm cylinder which is submerged in the boundary layer has a much smaller fractal dimension. Streamwise pressure distributions are measured both upstream and downstream, and the results show that the influence extent tends to depend little on the height of the cylinder. While the H/D values only influence on the size of the plateau region and the pressure increasing slope.