Book Description

The recent development of fifth generation fighter aircraft has been designed to takeoff in short distances, accelerate to supersonic flight, and land vertically. For short takeoff and vertical landing operations, STOVL, aircraft use thrust vectoring to direct high-speed, high temperature jet exhaust from the primary nozzle towards the ground. This redirection of flow provides approximately half of the thrust required for lift. The remaining thrust is generated by a lift fan which produces a sonic jet plume. The complex nature of the flow-field generated by these aircraft severely impacts the surrounding operational environment. The need to understand the high temperature, high velocity flow-field associated with these aircraft in close proximity to the ground is extremely important. These flow conditions give rise to hazardous conditions for personnel and equipment in the nearby proximity of the landing aircraft.To understand the effects of high temperature impinging flows and the subsequent heat transfer into the impingement plane, the existing High Speed Aeroacoustics Laboratory was redesigned to achieve high temperatures flows while in impinging configurations. High temperature flows are achieved through the use of two electric heaters in a parallel configuration. The parallel heater configuration allows the jet exhaust on an impinging jet model to reach a total temperature ratio of 2.0 with a jet Mach number of 1.34. More importantly, the use of electric heaters and PID controllers produces stable flow conditions within 4 K of the desired jet temperature.Once facility development was completed, a series of experiments were conducted to determine the flow-field characteristics of a heated, supersonic rear jet impinging on a ground plane. A dual impinging jet model (using a cold, sonic front jet) was also studied. This model is representative of a generic military-style STOVL aircraft in a hover configuration. The operational conditions were limited to jet stand-off distances between 4 and 15 nozzle diameters, and jet Mach numbers between 1.16 and 1.56. The total temperature of the jet was also varied between 1.2 and 2.0.Schlieren flow visualization was used to qualify the average flow-field behavior exhibited in both single and dual jet configurations. Single jet configurations showed a strong relationship between jet stand-off distances and thickness of the thermal outwash. For dual jet configurations, the interaction between the two jets, and the subsequent mixing, can be seen. The location of this mixing region is strongly dependent on the Mach number of the rear jet.Steady-state and transient surface temperature measurements were also a focus in this study. Steady-state measurements were compared to adiabatic wall temperatures calculated from CFD. For the limited CFD cases, experimental results compared well with the calculations, with less than 10% difference at all locations. For single impinging jets, the normalized temperature distribution in the impingement region is heavily dependent upon the jet separation distance. For large separation distances, increases in the total temperature ratio can reduce the peak normalized temperature at the stagnation point by as much as 10%. For dual impinging jet operations, the region between the two jets experience an increase in normalized temperature when compared to the single jet operations. A slight increase in the stagnation temperature is also seen.Transient thermal distributions obtained for a rear jet Mach number of 1.16 at a total temperature of 1.5 and a jet separation distance of 6 nozzle diameters are compared to a transient conduction computation model. The experimental model compared poorly to the experimental model for all time increments. However the differences can be attributed to boundary condition assumptions, and accuracy can be improved with model refinement.