PIV Measurements Of Unsteady Characteristics Of Separated And Reattaching Flow On Finite Blunt Plate

Flow around the finite blunt plate has been widely found in many engineering domains, such as chemistry, architecture, aerospace, electronics, power machinery and fuel, which contains complex flow mechanics, such as separated and reattaching flow, separation bubble and the oscillation. The complex flow mechanics significantly affect the oscillation and stability of the plate, flow-induced vibration, aerodynamic noise and heat transfer. Therefore, it is of important theoretical significance and application value to carry out the experimental study of the flow around the finite plate.This paper is focused on the complex unsteady characteristics of the flow around the blunt plate by applying testing technique of particle image velocimetry and dynamic pressure sensor. Based on the advanced decomposed methods proper orthogonal decomposition(POD) and dynamic mode decomposition(DMD), the dominant structures were extracted from the huge amounts of data in the experiment, revealing the influence of the chord-to-thickness ratio on the complex unsteady characteristics of the flow around the blunt plate.The dynamic mode decomposition(DMD), which is an effective method to extract the coherent structures in turbulent flow, has been developed to established a complete set of modal analysis method. Through a series of fabricated patterns and the flow around a circular cylinder and side-by-side cylinders, systematic and comparative analysis has been made between the DMD method and the proper orthogonal decomposition(POD) from the aspects of modes, mode coefficients, convergence and the identification of high order harmonic and multi-scale structures.To reveal the convecting characteristics of the large-scale structures around the blunt plate and the corresponding influence on the global characteristics, time-resolved particle image velocimetry(TR-PIV) was applied to measure the flow region around the blunt plate with different chord-to-thickness ratio in a low speed water channel(ReD = 1,000). Then the dynamic mode decomposition was applied to analyze the velocity fields of the flow around the blunt plate in a water channel, identifying the large-scale structures over the blunt plate, Karman vortex street in the wake. The results reveal: for case with small chord-to-thickness ratio, the separated shear layers interacts with each other in the wake region and the flow over the blunt plate is dominated by the wake flow; for case with large chord-to-thickness ratio, the separated shear layers always reteach onto surface of the plate and the flow over the blunt plate is dominated by the impinging shear layer instability; for case with medium chord-to-thickness ratio, both of the wake flow and the large-scale structures play significant role. The phase-angle was determined to do phase analysis with the help of coefficients of DMD modes, and the contour plot of swirling strength clearly shows the growth(or decay) trend of the large-scale structures in the convection process and the interactions between the large-scale structures and flow in the wake flow.To reveal the temporal evolvement of large-scale structures and the corresponding relationship with the wall-pressure fluctuations, the combined measurements of particle image velocimetry and dynamic pressure sensor were applied to measure the velocity fields around the plate and the wall-pressure fluctuations on the surface of the plate in the wind tunnel(ReD = 15,800). The joint POD analysis of PIV velocity field and wall-pressure fluctuation was applied to extract the dominant structures around the plate and figure out the concern relation between the large-scale structures and wall-pressure fluctuations by reconstructing the velocity field and wall-pressure fluctuations and removing the background noise and the small-scale structures.Finally, with measurement of particle image velocimetry of the flow around the blunt plate in the low speed water channel, this paper exploringly study the influence of the incident vortex street on the flow around the blunt plate. The results reveal that the separated shear layers were restrained in a small region at the leading edge. Meanwhile the incident vortex street was split by the blunt plate, reforming two small vortices with the same rotating direction. It was also found that the chord-to-thickness ratio has no influence on the flow around the blunt plate.