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At least 469 records · Page 26

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NASA NTRS1973

Behavior of nitric oxide formed by the space shuttle in the mesosphere

A numerical model of eddy diffusive transport of emitted gases from the space shuttle wake, including chemical reactions between the emitted constituents and the ambient atmosphere, has been constructed for 75 km altitude. The numerical methods involve explicit solution of the diffusion equation and Runge-Kutta method for the chemical reactions. The time required to reach background levels of nitric oxide concentration of 7 x 10 to the 7th power molecules/cc has been calculated. This relaxation time depends strongly on atmospheric conditions.

Eberstein, I. J.↗

NASA NTRS1973

Swirling jet turbulent mixing and combustion computations

Computations are presented describing the mixing and combustion of swirling jets in a coaxial stream. It is demonstrated that the boundary layer equations represent the flow reasonably well until reversed flow is imminent. For the range of parameters investigated indications are that the edge velocity has little effect on the behavior of the flow. Furthermore, confining the flow with a constant pressure wall, or impressing a favorable pressure gradient on the coaxial flow, acts to reduce the severity of the centerline adverse pressure gradient created by the swirl decay. A simple scalar eddy viscosity model, including a potential core formulation, is shown to described the behavior of weak swirling flow in the far region but is only in fair agreement with observations in the near region. The effects of swirl on a burning hydrocarbon jet exhausting into a cold coaxial stream are shown to be intensified by the reduction of the density due to combustion. The enhanced mixing properties of high swirl flow produce rapid diffusion of the burning gases into the cold edge flow causing early cessation of the NO producing reactions. Computations show that doubling the initial jet swirl could reduce the NO production by 25 percent.

Rubel, A.↗

NASA NTRS1973

Analysis of free turbulent shear flows by numerical methods

Studies are described in which the effort was essentially directed to classes of problems where the phenomenologically interpreted effective transport coefficients could be absorbed by, and subsequently extracted from (by comparison with experimental data), appropriate coordinate transformations. The transformed system of differential equations could then be solved without further specifications or assumptions by numerical integration procedures. An attempt was made to delineate different regimes for which specific eddy viscosity models could be formulated. In particular, this would account for the carryover of turbulence from attached boundary layers, the transitory adjustment, and the asymptotic behavior of initially disturbed mixing regions. Such models were subsequently used in seeking solutions for the prescribed two-dimensional test cases, yielding a better insight into overall aspects of the exchange mechanisms.

Korst, H. H.↗

NASA NTRS1973

Turbulent jet mixing at high supersonic speeds.

The results of three related studies of turbulent jet mixing are presented and discussed. The major effort was the collection of mean-flow data at a much higher free stream Mach number than was previously available. The effects of initial swirl in the injectant were found to be minimal. Numerical predictions of the non-swirling experimental case obtained using an eddy viscosity approximation are found to be in reasonable agreement with the data. Lastly, an approximate analysis of the decay of the swirling velocity is developed. This analysis is based upon an eddy viscosity model for transport in the circumferential direction adapted from wing-tip vortex work.

Schetz, J. A.↗

NASA NTRS1973

Supersonic turbulent boundary-layer flows with tangential slot injection.

An implicit finite-difference method has been developed for the solution of the compressible boundary-layer equations. This method is applied to tangential slot injection into supersonic turbulent boundary-layer flows. In addition, the effects induced by the interaction between the boundary-layer displacement thickness and the external pressure field are considered. Three different eddy viscosity models have been used to specify the turbulent momentum exchange. One model depends on the species concentration profile, and the species conservation equation has been included in the system of governing partial differential equations. For air injected into air, the freestream and injected gases are treated as separate species which have common fluid properties. Calculations were made and results were compared with experimental data at freestream Mach numbers of 2.4 and 6.0 and with results of another finite-difference method. Good agreement was obtained for the reduction of wall skin friction with slot injection.

Miner, E. W.↗

NASA NTRS1973

Non-reacting and equilibrium chemically reacting turbulent boundary-layer flows.

Turbulent boundary-layer flows of nonreacting gases are predicted for both internal (nozzle) and external flows. Effects of favorable pressure gradients on two eddy viscosity models were studied in rocket and hypervelocity wind tunnel flows. Nozzle flows of equilibrium air with stagnation temperatures up to 10,000 K were computed. Predictions of equilibrium nitrogen flows through hypervelocity nozzles were compared with experimental data. A slender spherically blunted cone was studied at 70,000 ft altitude and 19,000 ft/sec in the earth's atmosphere. Comparisons with available experimental data showed good agreement. A computer program was developed and fully documented during this investigation for use by interested individuals.

Anderson, E. C.↗

NASA NTRS1974

Effect of interjet spacing on mixing of multiple coaxial jets in supersonic flow

An experimental and analytical investigation of strut-mounted, four-nozzle, downstream facing, gaseous fuel injector assembly was conducted in a Mach 4 air-stream with P = 154 psia and T sub o 520 R. Helium was used as the injectant, and the interjet spacing (S/D) was the main parameter varied. The principal data are in the form of Helium concentration profiles at seven axial stations, Mach number distributions at three axial stations and schlieren photographs of the flow field at different interjet spacings. An approximate analysis was developed based upon a linearization in the Von Mises plane, Crocco integrals for the temperature and concentrations fields and an eddy viscosity model. Good agreement with the data was achieved. It was found that, at these conditions, interjet spacing has an effect on mixing only up to S/D of approximately 2.5, with concentration of injectant becoming smaller as S/D is increased, and the Mach number downstream of the injector tends to decrease with decreasing S/D.

Lorber, A. K.↗

NASA NTRS1975

Analytical comparison of hypersonic flight and wind tunnel viscous/inviscid flow fields

Flow fields were computed about blunted, 0.524 and 0.698 radians, cone configurations to assess the effects of nonequilibrium chemistry on the flow field geometry, boundary layer edge conditions, boundary layer profiles, and heat transfer and skin friction. Analyses were conducted at typical space shuttle entry conditions for both laminar and turbulent boundary layer flow. In these calculations, a wall temperature of 1365 K (2000 F) was assumed. The viscous computer program used in this investigation was a modification of the Blottner non-similar viscous code which incorporated a turbulent eddy viscosity model after Cebeci. The results were compared with equivalent calculations for similar (scaled) configurations at typical wind tunnel conditions. Wind tunnel test gases included air, nitrogen, CF4 and helium. The viscous computer program used for wind tunnel conditions was the Cebeci turbulent non-similar computer code.

Fivel, H. J.↗

NASA NTRS1975

A study of reacting free and ducted hydrogen/air jets

The mixing and reaction of a supersonic jet of hydrogen in coaxial free and ducted high temperature test gases were investigated. The importance of chemical kinetics on computed results, and the utilization of free-jet theoretical approaches to compute enclosed flow fields were studied. Measured pitot pressure profiles were correlated by use of a parabolic mixing analysis employing an eddy viscosity model. All computations, including free, ducted, reacting, and nonreacting cases, use the same value of the empirical constant in the viscosity model. Equilibrium and finite rate chemistry models were utilized. The finite rate assumption allowed prediction of observed ignition delay, but the equilibrium model gave the best correlations downstream from the ignition location. Ducted calculations were made with finite rate chemistry; correlations were, in general, as good as the free-jet results until problems with the boundary conditions were encountered.

Beach, H. L., Jr.↗

NASA NTRS1975

Supersonic turbulent boundary-layer flows with mass injection through slots and/or porous walls

An implicit finite-difference method was used to solve the compressible boundary-layer equations, and to study the effects of mass transfer through porous plates, slots, and a combination of the two. The effects of the external pressure field were also included by using a global pressure interaction scheme. Two different eddy viscosity models were used for the slot and slot-porous combination cases: one was a two-layer model with inner and outer laws, and the other was a multi-layer model with as many as five separate layers. Results of the present method were compared with experimental data at a Mach number of 2.8. Comparisons of the skin friction reduction and Mach number profiles gave good to excellent agreement. Pressure interaction had little effect on the slot injection skin friction but increased the skin friction of the porous and slot-porous combination markedly.

Murray, A. L.↗

NASA NTRS1974

Turbulent mixing of multiple co-axial gaseous fuel jets in a supersonic airstream

An experimental and analytical investigation of a strut-mounted, four-nozzle, downstream-facing, gaseous fuel-injector assembly was conducted in a Mach 4 airstream at 154 psia and 520 R. Helium was used as the injectant in order to simulate a low molecular weight fuel, and the interjet spacing (S/D) was the main parameter varied. The principal data are in the form of helium concentration profiles at seven axial stations, Mach number distributions at three axial stations, and Schlieren photographs of the flow field at different interjet spacings. An approximate analysis was developed based upon a linearization in the Von Mises plane, Crocco integrals for the temperature and concentrations fields, and an eddy viscosity model. Good agreement with the data was achieved. It was found that, at these conditions, interjet spacing has a significant effect on mixing only up to S/D of approximately 2.5, with the concentration of the injectant at a given point becoming smaller as S/D is increased.

Lorber, A. K.↗

NASA NTRS1975

Computation of separated transonic turbulent flows

The two-dimensional Reynolds-averaged compressible Navier-Stokes equations are solved using MacCormack's second-order-accurate explicit finite difference method to simulate the separated transonic turbulent flow field over an airfoil. Four different algebraic eddy viscosity models are tested for viability to achieve turbulence closure for the class of flows considered. These models range from an unmodified boundary-layer mixing-length model to a relaxation model incorporating special considerations for the separation bubble region. Results of this study indicate the necessity for special attention to the separated flow region and suggest limits of applicability of algebraic turbulence models to these separated flow fields.

Deiwert, G. S.↗

NASA NTRS1975

Inviscid to turbulent transition of trailing vortices

The plateau region in which trailing vortices initially exhibit slow decay of maximum tangential speed is demonstrated with the aid of constant and variable eddy viscosity models. The existence of the plateau is explained by the presence of the viscous core and by the necessity for transition of the initial, nearly inviscid profile to similarity far downstream. Span loading is shown to affect the initial maximum tangential speed as well as the duration of the plateau region. Triangular span loading is shown to present less hazard to trailing aircraft, at least in the near field, than elliptic loading because the inviscid profile for triangular loading exhibits a finite tangential speed at the vortex centerline and a large inviscid core.

Iversen, J. D.↗

NASA NTRS1975

Interaction between a shock wave and a turbulent boundary layer in transonic flow

Interaction between a shock wave and an unseparated turbulent boundary layer is considered. The method of matched asymptotic expansions is used, with solutions valid in the double limit as Reynolds number tends to infinity and Mach number tends to unity. The shock is weak enough that interaction effects can be considered as perturbations to the undisturbed flow; the case considered is that where the sonic line is near the outer edge of the boundary layer. It is shown that, with order estimates for Reynolds stress perturbations, the induced wall pressure distribution can be calculated using only the two outer interaction regions, independent of a specific closure condition and that this solution is in fact a turbulent free interaction solution. A detailed analysis of the inner regions, for which an eddy viscosity model for the Reynolds shear stress is used, provides a description of the variations in velocity, temperature and density near and at the wall.

Adamson, T. C., Jr.↗

NASA NTRS1974

The Ames two-dimensional stratosphere-mesospheric model

A two-dimensional model of the stratosphere and mesosphere has recently been developed at Ames Research Center. The model contains chemistry based on 18 species that are solved for at each step and a seasonally-varying transport model based on both winds and eddy transport. The model is described and a preliminary assessment of the impact of supersonic aircraft flights on the ozone layer is given.

Whitten, R. C.↗

NASA NTRS1975

Solutions for incompressible separated boundary layers including viscous-inviscid interaction

Numerical solutions are presented for the laminar and turbulent boundary-layer equations for incompressible flows with separation and reattachment. The separation angularity is avoided by using an inverse technique in which the displacement thickness is prescribed and the pressure is deduced from the resulting solution. The turbulent results appear qualitatively correct despite the use of a two-layer eddy-viscosity model which is generally assumed appropriate only for mild-pressure-gradient flows. A new viscous-inviscid interaction technique is presented in which the inviscid flow is solved inversely by prescribing the pressure from the boundary-layer solution and deducing the new displacement thickness from the solution of a Cauchy integral. Calculations are presented using this interaction procedure for a laminar flow in which separation and reattachment occur on a solid surface.

Carter, J. E.↗

NASA NTRS1976

A normal shock-wave turbulent boundary-layer interaction at transonic speeds

Experimental results, including surveys of the mean and fluctuating flow, and measurements of surface pressure, skin friction, and separation length, are compared with solutions to the Navier-Stokes equations utilizing various algebraic eddy viscosity models to describe the Reynolds shear stresses. The experimental data, obtained at a free-stream Mach number of 1.5 and Reynolds numbers between 10 million and 80 million, show that a separated zone forms near the foot of the shock and that its length is proportional to the initial boundary-layer thickness; that a supersonic region forms downstream of the shock; and that the shear stress increases significantly through the interaction and subsequently decays downstream. The computations adequately represent the qualitative features of the flow field throughout the interaction but quantitatively underpredict the extent of separation and the downstream level of skin friction.

Mateer, G. G.↗

NASA NTRS1976

Second-order closure analysis of turbulent reacting flows

A reacting shear layer program for a complete second-order closure of turbulent reacting flows is developed. The program uses a typical eddy box model and many of the fluid mechanical turbulence models developed in earlier studies. The computer program actually solves a total of 30 equations, where the numerical integration of the equations is performed by a forward-time-centered space quasi-implicit upwind finite-difference scheme. Results for nonreacting and simple isothermal reacting flows are presented along with preliminary results for propane-air flames. It is shown that the program predictions for simple nonreacting flows are in good agreement with available data. The calculations for isothermal reacting flows demonstrate the importance of including a mixedness correlation in finite rate chemistry calculations, where significant errors in the flowfield properties can result from neglect of the interaction between turbulence and the chemistry. Results for propane-air diffusion flames further support this conclusion.

Varma, A. K.↗