Part A: experimental investigation of unsteady supercavitating flows

Abstract

The present experimental study is the first part of an investigation of unsteady cavitating flows. In the second part, the proper numerical modeling of such flow fields is introduced in order to remove the deficiencies of customary computational models. This study focuses on supercavitation initiation and its transitional process to partial cavitation and, finally, noncavitating flow as the body bears acceleration from rest and deceleration in a water test tank. Using a high speed camera, cavitation bubble status and motion characteristics of the body displacement after being launched in the prepared test tank are recorded and extracted. After processing the recorded images, body motion parameters including, speed, acceleration and consequently the total drag force in addition to cavitation bubble length and diameter were derived. Due to the high acceleration imposed on the body (7000g), during the short interval of launching time, the model design is a complex process compromising low weight and high strength. Some aspects of the test set up and model modification, as well as the proper data processing are explained in this paper. From the experiments, it is observed that in the accelerating phase, as the velocity increases it is equivalent to lowering the cavitation number, the length of cavitation bubble is also increased, which is consistent with the expectations. However, in the decelerating phase this trend is reversed, which is totally unexpected. Finally, the experimental results are compared with the results of steady state flow field solutions and the difference between the two sets of results is huge. This deficiency is related to the dynamics of unsteady cavitating flow that is discussed in the second part of this paper.