Tangential slot injection of supersonic air (M_j = 1.70) into a supersonic air freestream (M₁ = 2.93) was investigated. The model used had a slot height of 1.27 cm, which was also approximately the boundary layer thickness in the primary stream, and a thin splitter plate with a thickness of 0.052 cm. Computer controlled measurement systems were used at four streamwise stations, (x/a) = 0.25, 4, 10, 20, to determine cone static pressure, pitot pressure, and stagnation temperature. From this information profiles of M, U, and ρ̅ were also generated. Spark Schlieren and Nano-Shadowgraph pictures were taken of the flowfield. The results showed excellent repeatability and documented the development of the injected flow to beyond the region where the outer shear layer and wall boundary layer had merged. The effect of a weak shock interaction upon the shear layer was also investigated. An oblique shock, generated on the upper wall with a wedge, impinged on the shear layer. The shock wave had a nominal pressure ratio of 1.82, which did not result in separation of the wall boundary layer. The profile distortion caused by this shock/viscous layer interaction was documented. / M.S.

Experiments were performed with single, sonic, helium jets at downstream angles of 15° and 30° relative to the free stream to determine their mixing, penetration and total pressure loss when injected into a supersonic air cross flow. From this information, the performance of these jets as fuel injectors in a supersonic combustion ramjet (scramjet) combustion chamber was estimated. Both injection angle jets were made flush to the wind tunnel wall. The jets were injected into a Mach 3 free stream with a total pressure of 6.5 atm, a total temperature of 283 K and a Reynolds Number of 52.5x10⁶ /m. The flow field of each injection angle was documented at jet expansion ratios of one and five. Spark schlieren and nanoshadowgraph methods were used to visualize each flowfield. At axial stations 20, 40, and 90 jet diameters downstream of each jet, continuous vertical profiles of flow quantities were made. Profiles were taken at seven lateral stations including the jet centerline at each axial station. Spacing between the lateral stations was one jet diameter. This data yielded profiles of helium concentration, Mach number, static temperature, static pressure, density, flow speed, mass flux, total pressure, and total temperature. The different injection schemes were then compared on the basis of helium mass fraction decay, the distance required to reach the stochiometric H₂-air concentration and total pressure loss. For all cases except the 15° jet with an expansion ratio of one, large eddies were observed to penetrate into the free stream. These eddies were believed to significantly enhance large scale mixing. The jet cores of the underexpanded jets had bifurcated 20 jet diameters downstream of the injection point, but had re-united by the 40 diameter station. Wandering of the jet core about the geometric centerline was observed for all cases. The decay rates increased rapidly with the jet to free stream dynamic pressure ratio until about 1.5 where the decay rate leveled off. This indicated that there was no significant increase in mixing from increasing the dynamic pressure ratio of the present jets past 1.5. The decay rate of the present 30°, matched pressure case was about 16 percent greater than that of a normal jet at similar dynamic pressure and expansion ratios. These results were reflected in the distances required to reach the stochiometric H₂-air concentration. The 15° jet with an expansion ratio of one had the lowest total pressure loss. It was concluded that injection at low downstream angle shows promise for application to scramjet fuel injection and merits further study. / Master of Science

The design of the VPI plasma torch, which has been used as an ignitor and flameholder in supersonic combustion studies, has been modified in order to decrease the electrode wear and to increase stability. The plasma torch can be used as a source of hydrogen or nitrogen radicals which initiate and stabilize combustion. During previous testing of the unmodified torch, electrode erosion limited operation of the torch to about two hours. The improved torch features a flow swirler in the gas inlet, which adds vortex stabilization to the arc. The vortex stabilization causes the anode attachment point of the arc to be anchored in the low pressure region, downstream of the constrictor. This lowers the heat flux to the anode, so that erosion is decreased. The torch body was redesigned with an emphasis on the alignment of the electrodes. Also, the electrode gap in the improved torch was made continuously adjustable, allowing fine adjustment of the electrode gap during operation of the torch. The operational characteristics of the improved torch were monitored by a microcomputer-based data acquisition system. Stable operation of the improved torch with pure nitrogen was demonstrated, thus eliminating the requirement for argon to stabilize the arc. Operational characteristics of the improved torch running on argon, nitrogen, argon/hydrogen and argon/nitrogen mixtures as feedstocks, are reported. The electrode wear was studied between tests by observation with a microscope, and by measuring the mass change of the electrodes. The electrode erosion of the improved torch was reduced significantly. Anode lifetimes of greater than 20 hours have been demonstrated with operation on mixtures of nitrogen and argon. / Master of Science

Experiments were performed to determine the effects of impinging oscillating shocks of different frequencies on a 15° downstream angled, underexpanded, sonic helium jet injected into a supersonic airflow. Information on mixing, penetration, total pressure loss and turbulence structure from these experiments was used to estimate mixing control achieved by adding an oscillating shock to the helium injection flow field. Tests were conducted at Mach 3.0, with a total pressure of 6.5 atm, a total temperature of 290 K and a Reynolds number of 51.0 x 10⁶ per meter. Oscillating shocks of three different frequencies were studied. The frequencies selected were designed to allow tuning of the shock frequency to the estimated frequency, about 100 - 150 kHz, of the largest eddies in the approach boundary layer. Visualization using nanoshadowgraph photography showed large turbulent structures in all cases. In addition, there were clear changes in eddy size with changing shock frequency visible on the nanoshadowgraphs. The primary measurement made for the mixing studies was the molar concentration of helium. Concentration data, as well as mean flow data, was collected at nine lateral positions at each of three axial stations downstream of the helium injector. The resulting data produced contours of helium concentration, total pressure, Mach number, velocity, mass flux and static flow properties. Additional tests were conducted to determine the shock oscillation frequency, the correlation between the oscillating shock and the turbulence in the shear layer and the angle of large-scale structures in the flow. Mixing and penetration rates were determined from the helium concentration data. The major result of this study was that impingement of an oscillating shock on a high-speed shear layer can be used to control the rate of mixing. Depending on the shock oscillation frequency, mixing enhancement or inhibition can be produced. It was found that increasing shock oscillation frequency resulted in more rapid injectant concentration decay and increased freestream air entrainment leading to a stoichiometric H₂-air mixture ratio while also reducing penetration of the helium injectant. A strong correlation was found between the highest frequency shock and changes in the mixing flow field. The maximum oscillation frequency was approximately 140 kHz, which was consistent with numerical estimates for the frequency necessary for mixing augmentation under these test conditions. It was concluded that oscillating shock impingement has promise as a means of controlling gaseous mixing in a high-speed cross-flow. / Ph. D.

With the renewed interest in the supersonic and hypersonic flight vehicles, new inverse Euler methods are developed in these flow regimes where a space marching numerical technique is valid. In order to get a general understanding for the specification of target pressure distributions, a study of minimum drag body shapes was conducted over a Mach number range from 3 to 12. Numerical results show that the power law bodies result in low drag shapes, where the n=.69 (l/d = 3) or n=.70 (l/d = 5) shapes have lower drag than the previous theoretical results (n=.75 or n=.66 depending on the particular form of the theory). To validate the results, a numerical analysis was made including viscous effects and the effect of gas model. From a detailed numerical examination for the nose regions of the minimum drag bodies, aerodynamic bluntness and sharpness are newly defined. Numerous surface pressure-body geometry rules are examined to obtain an inverse procedure which is robust, yet demonstrates fast convergence. Each rule is analyzed and examined numerically within the inverse calculation routine for supersonic (M_∞= 3) and hypersonic (M_∞ = 6.28) speeds. Based on this analysis, an inverse method for fully three dimensional supersonic and hypersonic bodies is developed using the Euler equations. The method is designed to be easily incorporated into existing analysis codes, and provides the aerodynamic designer with a powerful tool for design of aerodynamic shapes of arbitrary cross section. These shapes can correspond to either "wing like" pressure distributions or to "body like" pressure distributions. Examples are presented illustrating the method for a non-axisymmetric fuselage type pressure distribution and a cambered wing type application. The method performs equally well for both nonlifting and lifting cases. For the three dimensional inverse procedure, the inverse solution existence and uniqueness problem are discussed. Sample calculations demonstrating this problem are also presented. / Ph. D.

This investigation studies the mixing in a shear layer developed from helium slot injection into a parallel supersonic airstream and compares the results to those of previous slot-injection tests. The objectives of this study include documenting the helium slot-injection flowfield; providing a baseline for use as a reference for future work; contributing representative and consistent data to the general database; and increasing understanding of shear layer dynamics, especially as a result of foreign-gas injection. The helium injectant exits the slot at y = 1.67, M₋₁ = 1.78, P_u = 0.892 atm, and T_u = 287° K tangentially to an airstream at y = 1.4, M_∞= 3, P_t∞= 6.5 atm, and T_t∞= 282° K. The freestream has Re/cm = 5.4x10⁵ and a boundary-layer thickness of (δ_au/H) = 0.58. The pertinent ratios are (P₁/P_∞) = 0.838, (U₁/U_∞) = 2, and (P₁/P_∞) = 0.1. The slot height H is 1.21 cm. Along with short-duration Schlieren and Shadowgraph photography, concentration, Pitot, cone-static, and stagnation-temperature measurements are taken at each of four streamwise stations (x H = 0.3, 4.2, 10.5, 21.1) to document the development of the mixing layer. ln light of the binary-gas mixture, local concentration information is required to reduce the data to pertinent mean-flow variables (M, p, U, pU, P, and T). As expected, slot injection in general shows poor initial penetration of the injectant into the freestream, and, thus, poor initial mixing. Nevertheless, the helium case shows better mixing than a similar air injection case of a previous experiment, as the mixing shear layer grows 25 percent larger than that in the air case by the last station. Also, about 30 percent more freestream air is entrained into the shear layer in the helium case and is confined mainly to the top third of the mixing layer. The higher mixing rate stems from larger gradients in velocity and density and lower pU values which result in more active transport mechanisms in the helium injection test / Master of Science