No description available.

An investigation is conducted into the effect of cooling and suction on the stability of subsonic flows over two-dimensional roughness elements and supersonic flows over flat plates. First, the effect of wall cooling on the two-dimensional linear stability of subsonic flows over two-dimensional surface imperfections is investigated. Results are presented for flows over smooth humps and backward-facing steps with Mach numbers up to 0.8. The results show that, whereas cooling decreases the viscous instability, it increases the shear-layer instability and hence it increases the growth rates in the separation region. The coexistence of more than one instability mechanism makes a certain degree of wall cooling most effective. For the Mach numbers 0.5 and 0.8, the optimum wall temperatures are about 80% and 60% of the adiabatic wall temperature, respectively. Increasing the Mach number decreases the effectiveness of cooling slightly and reduces the optimum wall temperature. Second, the effect of suction on the stability of compressible flows over backward-facing steps is investigated. Mach numbers up to 0.8 are considered. As expected, suction considerably reduces the separation region. The results show that continuous suction stabilizes the flow outside the separation bubble, as expected, but it destabilizes the flow inside it. Nevertheless, the overall N factor decreases as the suction level increases. This is due to the considerable reduction in the separation bubble. For the same suction flow rate, properly distributed suction strips are more effective in stabilizing the flow than continuous-suction distributions. Furthermore, the size of the separation bubble, and hence its effect on the instability, can be considerably reduced by placing strips with high suction velocities in the separation region Third, the effect of suction on the stability of supersonic and hypersonic boundary layers is investigated. Calculations are performed for non-similar and self-similar boundary layers. The variation of the maximum growth rate with Mach number at low levels of suction is different from that at high levels of suction. This is due to the coexistence of viscous and inviscid instability mechanisms in supersonic and hypersonic boundary layers. Suction is more effective in stabilizing the viscous instability, and hence it is more effective at low Mach numbers. Although suction decreases the maximum growth rate of second-mode waves, small levels of suction increase the growth rates of disturbances having certain frequencies. On the other hand, first-mode waves are stabilized by suction at all frequencies. Constant-suction distributions considerably move the critical Reynolds numbers of second-mode waves to higher values while the critical Reynolds numbers of first-mode waves are not sensitive to suction. / Ph. D.

Mean flow measurements and short-duration Schlieren and Shadowgraph photographs of heated and unheated supersonic slot injection of air into a supersonic air stream are presented for the purpose of observing the mixing which occurs in the resulting shear layer. The heated injected jet (M_j, P_tj = 10.7 psia, and T_tj = 760°R) passes through a slot of height 0.475 inches (1.2 cm) tangent to a free stream with M_∞ = 3.0, P_∞= 95 psia and T_t_∞ = 540°R. The resulting density ratio is P_j / P_∞ = 0.3. The free stream Re/ft = 6.1 x 10⁷ (Re/cm = 2 x 10⁶). The thickness of the plate which separates the primary and secondary flows is 0.021 inches (0.052 cm). Pitot pressure, cone static pressure and stagnation temperature profiles are obtained at four axial stations downstream of the slot (x/H = 0.25, 4, 10, 20). An additional set of measurements is obtained at Station 4 for the case of a weak shock (P₂/P₁ = 1.8) interacting with the shear layer just upstream of Station 4. From the pressure and temperature measurements, Mach number, velocity, density, mass flux and static pressure profiles are generated. An unheated injection study is also performed at the same flow conditions for comparison to the heated case. The heated and unheated cases are very similar except in the slot where the temperature difference creates changes inp, U, and pU. This study is thus concerned with the effects of changes in P_j / P_∞ and U_j / U_∞, especially (U_j - U_∞), which are created by heating the injected flow. The heated slot flow did not create a marked difference in the location of the merging of the free stream boundary layer with the slot flow when compared to the unheated slot flow. Indeed, the appearance of the two flows on Schlieren photographs is similar even though the injected mass flow in the heated case is about 15% less than that in the unheated case. The pressure adjustments in the slot lip region are different for the two different cases. The flowfields are documented for both the heated and unheated cases with the added measurements and photographs for the shock impingement case. / Master of Science

Current emphasis on sustained supersonic and hypersonic cruise has sparked interest in more efficient power plants for this flight regime. Cycle studies have shown that the turbofan engine equipped with a supersonic through-flow fan, capable of accepting supersonic axial flow from the inlet, has the potential to be very efficient at the supersonic cruise condition. / Master of Science

An experimental study of the ignition of liquid fuels injected transverse to a hot supersonic (M=1.65) air stream was conducted. The liquids considered were kerosene, CS₂ and water as an inert control. The major variables were: air stagnation temperature in the range 1500 to 2300ºF, injectant flow rate and injection angles from 90º to 45º upstream. The experimental observations were: temperature measurements on the wall near the injector and in the flow downstream of injection self-luminosity photographs and infrared photographs taken with a Thermographic camera. Special attention was directed at the behavior of the liquid layer that had previously been found to form near the injector. No unequivocal evidence of ignition of either fuel was found for normal injection at these conditions. However, clear evidence of ignition of CS₂ was found for the upstream injection angle for T_o ≥ 2030°F and 80 ≤ P_j ≤ 135 psi. Higher injection pressures and thus high flow rates failed to produce ignition at any temperature tested. Evidence of CS₂ ignition was found in the infrared photographs and wall and in-stream temperature measurements simultaneously. The infrared photograph indicated possible ignition of the kerosene for upstream injection, but this could not be corroborated with the temperature measurements. / Master of Science

An investigation was conducted to design instruments to directly measure skin friction along the chamber walls of supersonic combustor models. Measurements were made in a combustor at the General Applied Science Laboratory (GASL) and in the Direct Connect Arcjet Facility (DCAF) supersonic combustor at the NASA AMES Research Center. Flow conditions in the high enthalpy combustor models ranged from total pressures of 275-800 psia (1900-5550 kPa) and total temperatures from 5800-8400 R (3222-4667 K). This gives enthalpies in the range of 1700-3300 BTU/Ib_m (3950-7660 KJ/kg) and simulated flight Mach number from 9 to 13. A direct force measurement device was used to measure the small tangential shear force resulting from the flow passing over a non-intrusive floating element. The floating head is mounted to a stiff cantilever beam arrangement with deflection due to the shear force on the order of 0.0005 in (0.0125 mm). This small deflection allows the balance to be a non-nulling type. Several measurements were conducted in cold supersonic flows to verify the concept and establish accuracy and repeatability. This balance design includes actively controlled cooling of the floating sensor head temperature through an internal cooling system to eliminate nonuniform temperature effects between the head and the surrounding chamber wall. This enabled the device to be suitable for shear force measurement in very hot flows. The key to this device is the use of a quartz tube cantilever with strain gages bonded at orthogonal positions directly on the surface at the base. A symmetric fluid flow was developed inside the quartz tube to provide cooling to the backside of the floating head. Bench tests showed that this did not influence the force measurement. Numerical heat transfer calculations were conducted for design feasibility and analysis, and to determine the effectiveness of the active cooling of the floating head. Analysis of the measurement uncertainty in cold supersonic flow tests show that uncertainty under 8% is achievable, but variations in the balance cooling during a particular test raised uncertainty up to 20% in these very hot flows during the early tests. Improvements to the strain gages and balance cooling reduced uncertainty for the later tests to under 15%. / Ph. D.

Experiments were performed with two different helium injector models at different injector transverse and yaw angles in order to determine the mixing rate and core penetration of the injectant and the flow field total pressure losses. when gaseous injection occurs into a supersonic freestream. Tested in the Virginia Tech supersonic tunnel. with a freestream Mach number of 3.0 and conditions corresponding to a freestream Reynolds number of 5.0 x 107 1m. was a single. sonic. 5X underexpanded, helium jet at a downstream angle of 30° relative to the freestream. This injector was rotated from 0° to _28° to test the effects of injector yaw. The second model was an array of three supersonic, 5X underexpanded helium injectors with an exit Mach number of 1.7 and a transverse angle of 15°. This model was tested in the NASA Langley Mach 6.0, High Reynolds number tunnel, with freestream conditions corresponding to a Reynolds number of 5.4 x 10⁷ /m. The injector array as tested at yaw angles of 0° and -15°. Surface flow visualization showed that significant flow asymmetries were produced by injector yaw. Nanosecond exposure shadowgraph pictures were taken, showing the gaseous injection plume to be unsteady, and further studies demonstrated this unsteadiness was related to shock waves orthogonal to the injectant bow shock, that were generated at a frequency of 30 kHz. The primary data technique used, was a concentration probe which measured the molar concentration of helium in the flow field. Concentration data and other meanflow data was taken at several downstream axial stations and yielded contours of helium concentration, total pressure, Mach number, velocity, and mass flux, as well as the static properties. From these contour plots, the various mixing rates for each case were determined. The injectant mixing rates, expressed as the maximum concentration decay, and mixing distances were found to be unaffected by injector yaw, in the Mach 3.0 experiments, but were adversely affected by injector yaw in the Mach 6.0 experiments. One promising aspect of injector yaw was the that as the yaw angle was increased, lateral motion of the injectant plume became significant, and the turbulent mixing region area increased by approximately 34%. Comparisons of the 15° transverse angled injection into a Mach 6.0 flow to previous experiments with 15° injection into a Mach 3.0 freestream, demonstrated that there is a significant decrease in initial mixing, at Mach 6.0, resulting in a much longer mixing distance. From a parametric computational study of the Mach 6.0 experiments, the effects of adjacent injectors was found to decrease lateral spreading while increasing the vertical penetration of the injectant plume, and marginally increasing the injectant core decay rate. Matching of the computational results to the experimental results was best achieved when using the Baldwin-Lomax turbulence model without the Degani-Schiff modification. / Ph. D.

Two aspirating hot-film probes are developed to make measurements in supersonic air/helium shear layers. The first probe is designed to measure local mean gas composition and is referred to as the mean concentration probe. This probe consists of a constant temperature hot-film sensor operating in a channel with a choked exit. The flow over the hot-fifm is influenced only by total temperature, total pressure, and gas composition. The mean probe is easily calibrated and shows acceptable sensitivity to flow angularity. The second probe is based on an improved design of the mean concentration probe. In addition to measuring mean composition, this second probe also measures turbulence intensities of Reynolds number and thermal conductivity using a multiple overheat method. This probe is referred to as the turbulent probe. Both probes are used in the study of supersonic air/helium mixing layers in the VPI&SU 23 cmx23 cm Supersonic Wind Tunnel. Profiles of mean and turbulent quantities are presented. / Master of Science

The panel flutter analysis presented herein has been restricted to the problem of an isolated simply supported plate of uniform thickness. The same type of analysis can be applied, however, to other panel configurations. Clamped panels, integrally stiffened panels, arrays of panels, end others should be amenable to treatment by the model approach based on the static aerodynamic approximation. / Master of Science

Tangential supersonic slot injection (M = 1.7) of air into a supersonic air stream (M = 2.91) was studied with a dual hot wire probe. This probe was used to simultaneously determine total temperature and mass flux. Mean profiles across the entire flow field at three axial stations (x/h = 4, 10, 20) were obtained, as well as the temperature and mass flux turbulence intensities. The probe can be used anywhere except at locations where features of the flow are smaller than the separation between the wires (0.18 mm). The calibration of the probe can be repeated with less than a three percent change in calculated Nusselt number. The hot wire probe can also be tuned to obtain a high frequency response. The interaction of a shock wave, caused by a wedge placed on the upper tunnel wall, with the mixing region was investigated. Little change in the mean profiles was observed, but there was a significant increase in turbulence levels. / Master of Science