Search

Fluctuating velocity fields have been determined from particle image velocimetry data acquired in the far field of the interaction generated by an overexpanded axisymmetric supersonic jet exhausting transversely from a flat plate into a subsonic compressible crossflow. Peak magnitudes of the turbulent stresses were found to be larger and located farther from the wall for greater values of the jet-to-freestream dynamic pressure ratio J while the crossflow Mach number M[infinity] remained constant. These stress magnitudes diminish with downstream distance as their peak location moves farther from the wall. The vertical positions of the peak normal stresses and shear stress inflection point coincide with the maximum mean streamwise velocity deficit induced by the jet. Similarity in the turbulent stress profiles was demonstrated for constant J at two transonic M [infinity], but not at two lower M [infinity]. Instantaneous realizations of the velocity fluctuation fields reveal large-scale eddies whose mean diameter is greater for larger J and decreases with downstream distance. The integral length scale calculated from profiles of the correlation coefficient instead shows an increasing downstream trend along with a larger magnitude; the discrepancy between the two length scales results from a low-pass filter effect of the correlation coefficient.

The transition of a boundary layer on a flat plate with an impinging shock wave is studied numerically by compressible large-eddy simulation using a hybrid compact/Roe scheme. The numerical code is verified by comparison with experimental observations of shock wave/turbulent boundary-layer interaction and then applied for the analysis of shock wave/transitional boundary-layer interaction. The simulation provides accurate results with respect to the location of reattachment and the boundary-layer properties downstream of reattachment. Large-scale coherent structures such as longitudinal vortex pairs, low-speed streaks, and hairpin vortices are identified in the transitional region, and it is revealed that these coherent structures play important roles in the transition. Thus, it is important to resolve these structures adequately to obtain an accurate prediction of the reattachment point. The subsequent breakdown of these coherent structures have smaller length scales, and resolving the breakdown requires much finer grid resolution. However, the influence of underresolution of breakdown on the downstream flowfield can be largely ignored under the conditions examined. Large-eddy simulation is, therefore, useful for the qualitative analysis of flowfields involving a supersonic transitional boundary layer.

A novel injector was developed as the result of numerical and experimental studies of the self-ignition of hydrogen and the flame stabilization in a supersonic flow (Ma = 2.1, [T.sub.0] = 900 K). The injector was realized as a serial system of a conventional supersonic diffuser and a nozzle dividing the flow into two parts. Most of the air flows into the combustion zone almost undisturbed. A small amount enters a shaped channel with a supersonic-subsonic-supersonic flow. Hydrogen is injected into both flows. The incomplete preburning in the subsonic area increases the total temperature and produces a large radical concentration, which acts as source for the ignition of the main flow. It is shown that the flame propagation is much faster under the presence of radicals. This allows the reduction of the required residence time of the fuel and the length of the chamber. The lower total temperature limit of proper ignition and flame stabilization is decreased to 900 K compared to 1200 K for the limit of the strut injection concept investigated previously. Furthermore, the formation of strong shock waves, inducing a large loss of total pressure, can be avoided. The new injector has the potential of a much higher combustion performance than standard flame stabilization methods and extends the operating range of scram jet engines to low flight Mach numbers.

Large-eddy simulation/Reynolds-averaged Navier--Stokes (LES/RANS) hybrid methodology is applied to a high-Reynolds-number supersonic axisymmetric base flow. Accurate predictions of the base flowfield and base pressure are successfully achieved by using the LES/RANS hybrid methodology with less computational cost than that of pure LES and monotone integrated large-eddy simulation (MILES) approaches. Both the efficiency and reliability of the present LES/RANS hybrid methodology for the prediction of massively separated high-Reynolds-number flows are identified by comparison with the results obtained by LES, MILES, RANS, and the experiments in detail. The LES/RANS hybrid simulation accurately resolves the physics of unsteady turbulent motions, such as shear-layer rollup, large-eddy motions in the downstream region, small-eddy motions inside the recirculating region, and formation of large mushroom-shaped patterns in the end view. Comparison of the results shows that it is necessary to resolve approaching boundary layers and free shear-layer velocity profiles from the base edge correctly for the accurate prediction of base flows. Given the required mesh resolution near solid surfaces for pure LES and MILES at high-Reynolds-number flows, reduction of the computational cost is considerable when using the LES/RANS hybrid methodology. The consideration of a Smagorinsky constant for a compressible flow analysis may suggest that the optimal value of Smagorinsky constant may be larger in the flows with strong compressibility than in incompressible flows.

Simulation of sub- and supersonic thermochemical equilibrium flows in plasmatrons is considered. A physico-chemical model, numerical method, and computation results for equilibrium inductive coupled plasma flows in a plasmatron are given. An effective preconditioning technique along with an implicit total-variation-diminishing scheme is used to solve the Navier-Stokes equations in both subsonic and supersonic regimes. The governing equations include source terms corresponding to the electromagnetic field influence: the Lorentz force components (so-called magnetic pressure) and Joule heat production. The necessary transport coefficients were calculated in advance for equilibrium air plasma as the functions of pressure and temperature. Transport properties were calculated by the precise formulas of the Chapman-Enskog method in the temperature range 300 [less than or equal to] T [less than or equal to] 15,000 K. Calculations of equilibrium air plasma flows for the IPG-4 (Institute for Problems in Mechanics, Russian Academy of Science) discharge channel geometry with the channel radius [R.sub.c] = 0.04 m and length [Z.sub.c] = 0.40 m were performed. Creation of both underexpanded and overexpanded jets exhausted from the plasmatron channel is considered. A comparison with experimental results is given.

Airbreathing supersonic-combustion ramjets or scramjets could prove to be viable alternatives to rockets in hypersonic space flights. NASA has designed the Hyper-X program to validate the strong potentials of scramjet technology in enhancing space missions' effectiveness, which could serve as basis for future vehicle developments. The program includes four autonomous research flights to test scramjets' performance in high-speed travel. The first airbreathing engine-powered hypersonic flight will take place in 1999, with the remaining three test flights scheduled at one year intervals.