1. Although the supersonic jet impingement phenomena has been capably examined by many researchers for almost three decades, with notable exceptions, most of the quantitative data obtained in these studies was limited to mean surface pressure distributions. Very few quantitative measurements have been obtained in the flowtield above the surface, especially in the critical earsurface'regions. Using a simple configuration, an extensive study of the supersonic impinging jet flowfield has been in progress at the Fluid Mechanics Research Laboratory located at the Florida State University, Tallahassee. The goal of this research program is to provide a better overall understanding of the physics governing this flow by obtaining detailed measurements of the surface flow and more importantly of the flowfield above the surface. As a result, we would like to identify some of the important parameters contributing to the problems cited above such as hover lift loss, noise and ground erosion. Results of a companion study examining the aeroacoustic properties of the global impinging jet flowfield, especially its influence on lift loss and jet noise were presented in an earlier publication by Krothapalli et al. (1999). In this paper, emphasis will be placed on investigating the near-surface flowfield in hopes of obtaining some insight into the ground erosion problem. 1.1 Background - Impinging Jet Flowfield
2. In addition to the studies referred to earlier, Gubanova et al. (1973), Ginzberg et al. (1973) and Gummer and Hunt (1974) have also reported investigations of the flow structure, particularly in the impingement zone, for a range of conditions. These investigators discuss reasons for the formation (or lack thereof) of a stagnation bubble. In a very comprehensive study, Kalghatgi and Hunt (1976) further investigated this phenomenon. Hence, only a brief discussion of the flow structure is provided herein, the interested reader is referred to the previous work, especially the paper by Kalghatgi and Hunt. As supersonic flow in the primary jet approaches the ground plane, it decelerates through the formation of a plate shock. If the jet is not ideally expanded, oblique shocks in the jet plume (indicated as et shocks'; in Fig. 1) interact with the plate shock resulting in the well-known triple-shock structure where the third shock is generally referred to asthe tail shock. It is the nature of this interaction, which appears to determine the flowfield in the impingement zone. A shear layer, known as the slip line, emanates from the triple-point and impinges on the surface. The slip line is due to the velocity difference between the flow processed through the jet and tail shocks and the flow that which goes through the strong plate shock. The stagnation pressure in the slipline flow may be higher than the pressure in the fluid processed through the plate shock. The impingement of the slip line on the surface raises the local surface pressure. If the pressure is sufficiently high, the outwardly flowing radial boundary layer separates from the surface and forms a stagnation bubble as shown in Fig. la.
3. When present, the stagnation bubble is hypothesized to enclose a region of recirculating fluid with relatively low velocities. The nature of the flow in the stagnation bubble has primarily been understood through interpretation of surface pressure distributions, surface streakline patterns (Donaldson and Snedeker, 1971a; Carling and Hunt, 1976) and schlieren'shadowgraph flow visualization techniques. However, pitot probe measurements by Gubanova et al. (1973) have confirmed the presence of reversed flow in this region. One of the most commonly used indicators of the appearance of the stagnation bubble is the surface pressure distribution shown in Fig. la, where the relatively low, constant pressure region in the center is thought to coincide with the presence of a stagnation bubble and the annular high pressure peak is expected to be in the vicinity of the slip line impingement. The subsequent secondary pressure maxima and minima shown in the pressure distributions have been attributed to the presence of a series of shock and expansion waves; a good discussion of which is provided by Carling and Hunt (1974). More recently, Messersmith (1995) and Messersmith and Mm-thy (1997) have also obtained surface pressure and temperature distributions for normally impinging jets. Their surface temperature measurements confirm the pressuredistributions in that similar annular temperature peaks are observed in the temperature distributions. In closing, we note that even though the presence of the stagnation bubble is fairly well-accepted, most of the evidence for its existence is indirect. Furthermore, in the authors' opinion, the conditions that lead to its formation are still not entirely clear. For underexpanded jets, the strength of the interaction between thejet and plate shocks and the location of the intersection of these two, play a primary role in determining if stagnation bubble flow will occur. Kalghatgi and Hunt (1976) which was later revised by Lamont and Hunt (1980) originally proposed a critical parameter for this occurrence. However, as these authors themselves note, the universal applicability of this parameter still needs to be verified over a broader range of conditions. 1.2 Present Study