Particle image velocimetry study of the impinging height effect on the overexpanded supersonic impinging free jet of Ma = 1.754. Part 2: detailed velocity distribution

Abstract

In this article, the flowfield of an overexpanded axisymmetric supersonic impinging jet with exit Mach number 1.754 is studied using particle image velocimetry. In particular, the influence of the impinging height on the detailed flowfield velocity distributions has been investigated. The velocity distribution precisely indicates where the Mach disk and the plate shock are generated. With the increasing impinging height, the plate shock position is found to move away from the impinging wall, until it reaches the maximum of 0.767 D for the dimensionless impinging height of Z/ D = 2.5. With a further increasing of the impinging height, the plate shock moves back to the impinging wall. Furthermore, the shape, position, as well as size of the stagnation bubble zone can also be deduced from the velocity distribution. According to the time-averaged flowfield result, the velocity coefficient of the radial wall jet is found to be as high as 0.8146, which is critical to the ground erosion problem in the application. It indicated that although the radial velocity appeared to be supersonic in the instantaneous result in Part I [Xu J, Lin C and Sha J. Particle image velocimetry study of the impinging height effect on the overexpanded supersonic impinging free jet of Ma = 1.754. Part I: global coherent structure. Proc IMechE, Part G: J Aerospace Engineering 2012; 226(1): 64–73.], it is high subsonic in the mean result in this part. This disagreement is due to the intrinsic highly unsteady nature of the flow under investigation. The vorticity isopleths demonstrate that the stagnation bubble and the recirculating flow are formed due to the impingement of the nozzle inner shear layer of the main jet.