Showing papers in "AIAA Journal in 1984"

Vortex stability and breakdown - Survey and extension

Abstract: Analyse theorique et experimentale des conditions de stabilite et d'eclatement de tourbillons isoles incompressibles pour des valeurs elevees du nombre de Reynolds. On rappelle des methodes d'experimentation en laboratoire qui ont ete utilisees. On decrit les deux types principaux d'eclatements de tourbillons. On discute de l'importance de la symetrie par rapport a l'axe. On donne un bref apercu de la theorie de l'onde transcritique

Structure of particle-laden jets - Measurements and predictions

Abstract: Measurements of mean and fluctuating velocities of both phases as well as particle mass fluxes were completed in turbulent, particle-laden jets containing monodisperse particles with well-defined initial and boundary conditions. The new measurements were used to evaluate a stochastic separated flow model of the process which treated effects of interphase slip and turbulent dispersion using random-walk computations for particle motion. The continuous phase was treated using a modified k-epsilon model allowing for direct contributions of interphase transport to both mean and turbulence properties. The model performed reasonably well over the new data base, with all empirical parameters fixed from earlier work. In contrast, simplified models ignoring either interphase slip or turbulent dispersion yielded poor agreement with the measurements.

A comparison of vaporization models in spray calculations

Abstract: : The effects of different gas- and liquid-phase models on the vaporization behavior of a single-component isolated droplet are studied for both stagnant and convection situations in a high-temperature gas environment. In conjunction with four different liquid-phase models, namely, d2 law, infinite conductivity, diffusion limit, and internal vortex circulation, the different gas-phase models include a spherically symmetric model in the stagnant case and Ranz-Marshall correlation plus two other axisymmetric models in the convective case. A critical comparison of all these models is made. The use of these models in a spray situation is examined. A transient one-dimensional flow of an air- fuel droplet mixture is considered. It is shown that the fuel vapor mass fraction can be very sensitive to the particular liquid- and gas-phase models. The spherically symmetric conduction or diffusion limit model is recommended when the droplet Reynolds number is negligible compared to unity, while the simplified vortex model accounting for internal circulation is suggested when the droplet Reynolds number is large compared to unity. Keywords include: Spray; Droplet; Evaporation; Combustion; Modeling.

Grid Generation Techniques in Computational Fluid Dynamics

Abstract: Revue des differentes methodes. On montre que les regions tridimensionnelles sont mieux traitees par fragmentation du champ en sous-regions avec generation de grilles dans chaque sous-region. Interet des grilles adaptatives couplees a des solutions physiques

Aluminum Agglomeration in Solid-Propellant Combustion

Abstract: -... Results of a series of tests on AP/aluminum/PBAN propellants, with particular attention to bimodal AP particle size distribution, show the effect of particle size of fine AP and the concentration of fine AP on aluminum agglomeration and ignition. Results are interpreted in terms of the distribution of aluminum in propellant microstructure and the proximity of AP-binder flamelets to precipitate ignition of the accumulating aluminum.

Natural convection-radiation interaction in boundary-layer flow over horizontal surfaces

Abstract: Analyse mathematique de l'interaction entre la convection naturelle et le rayonnement thermique dans un ecoulement de couche limite laminaire sur une plaque horizontale isotherme. Presentation de resultats numeriques pour des gaz avec une valeur du nombre de Prandtl de 0,7

An efficient feasible directions algorithm for design synthesis

Abstract: A nonlinear optimization algorithm is described that combines the best features of the method of feasible directions and the generalized reduced gradient method. The algorithm uses the direction-finding subproblem from the method of feasible directions to find a search direction that is equivalent to that of the generalized reduced gradient method, but without the need to add a large number of slack variables associated with inequality constraints. This leads to a core-efficient algorithm for the solution of optimization problems with a large number of inequality constraints. Also, during the one-dimensional search, it is not necessary to separate the design space into dependent and independent variables using the present method. The concept of infrequent gradient calculations is introduced as a means of gaining further optimization efficiency. Finally, it is shown that, using the basic direction-finding algorithm contained in this method, the sensitivity of the optimum design with respect to problem parameters can be obtained without the need for second derivatives or Lagrange multipliers. The optimization algorithm and sensitivity analysis is demonstrated by numerical example.

Toward a consistent beam theory

Abstract: It is well known that the Euler-Bernoulli theory of the bending of beams makes use of a contradicting assumption of zero shear strains and nonzero shear stresses. Sometimes, this type oJ assumption is also carried over to more refined shear deformation theories. This paper outlines a theory thai avoids this assumption. With the aid of the specific example of a tip loaded cantilever beam, it is shown that the present theory gives Euler Bernoulli solutions in that part of the beam where shear deformation is unimportant and a shear deformation type of solution in the pari of the beam where shear deformation is important, with transition stress patterns between the two. Numerical studies, with a shear modulus representative of sandwich beams, bring out the usefulness of the present theory for the analysis of such soft-cored beams.

Acoustic nonlinearities and power losses at orifices

Abstract: Acoustic power losses occur when high-amplitude sound waves impinge on an orifice in the absence of mean flow. Described is a simple theoretical model of these losses, in which a harmonic, linear acoustic field is coupled to a nonlinear hydrodynamic flowfield at the orifice. Experimental and theoretical data on power losses at orifices with both pc and flanged acoustic terminations are compared, and fairly good agreement is noted. The structure of the flowfield in the neighborhood of the orifice is broadly described and some quantitative comparisons are made between the measured translational velocity of the ring vortices shed from both sides of the orifice and theoretical predictions. Again, the theory is seen to give generally reasonable results.

A Contribution to the Numerical Prediction of Unsteady Flows

Abstract: Methods based on "flux-differe nce splitting" are investigated with reference to the prediction of unsteady compressible flows. They combine the feature of taking into account the wake-like nature of these flows and the capability of correctly capturing shock waves. Two recently proposed approaches and a third one suggested here are compared for one-dimensional flow, with particular reference to the means of operating the splitting. They are extended to the cases of variable-area ducts and a general transformation of the independent variables. Firstand second-order accurate schemes are presented and procedures for the computation at the boundaries are shown. Finally, numerical experiments are reported and discussed.

The Baldwin-Lomax Turbulence Model for Two-Dimensional Shock-Wave/Boundary-Layer Interactions

Abstract: The algebraic turbulent eddy viscosity model of Baldwin and Lomax has been critically examined for the case of two-dimension al (2-D) supersonic compression corner interactions. The flowfields are computed using the Navier-Stokes equations together with three different versions of the Baldwin-Lomax model, including the incorporation of a relaxation technique. The turbulence models are evaluated by a detailed comparison with available experimental data for compression ramp flows over a range of corner angle and Reynolds number. The Baldwin-Lomax outer formulation is found to be unsuitable for separated 2-D supersonic interactions due to the unphysical streamwise variation of the computed length scale in the vicinity of separation. Minor modifications are proposed to partially remedy this difficulty. The use of relaxation provides significant improvement in the flowfield prediction upstream of the corner. However, the relaxation length required is one-tenth of that employed in a previous computational study. AH of the turbulence models tested here fail to simulate the rapid recovery of the boundary layer downstream of reattachment.

Numerical Computations of Turbulence Amplification in Shock-Wave Interactions

Abstract: Numerical computations are presented which illustrate and test various effects pertinent to the amplification and generation of turbulence in shock wave-turbulent boundary layer interactions. Several fundamental physical mechanisms are identified. Idealizations of these processes are examined by nonlinear numerical calculations. The results enable some limits to be placed on the range of validity of existing linear theories. Additional results are given which are of a fundamentally nonlinear nature.

Computation of strongly swirling axisymmetric free jets

Abstract: Calcul par la methode des differences finies de jets libres fortement tourbillonnaires. Une recirculation apparait a proximite de la tuyere. Etude de la reponse du champ calcule a l'introduction de modifications liees au tourbillonnement dans le modele de turbulence k-e et aux changements dans les conditions aux limites pour la vitesse de dissipation e et la vitesse radiale au plan d'entree du jet

Numerical Simulation of Near-Critical and Unsteady, Subcritical Inlet Flow

Abstract: The unsteady, Reynolds-averaged Navier-Stokes equations were solved for the flow about an experimentally tested external compression axisymmetric inlet operating in the near-critical and unsteady subcritical flow regimes. The near-critical solution reached a stable steady state, while the subcritical solution attained an unstable, bounded oscillatory state characterized by large-amplitude pressure oscillations and traveling shock waves. The self-sustained oscillations, known as inlet buzz, result from a shear layer instability combined with a closed-loop feedback of reflected disturbances. Solutions are compared with experimental results.

Multiobjective optimization in structural design with uncertain parameters and stochastic processes

Abstract: The application of multiobjective optimization techniques to structural design problems involving uncertain parameters and random processes is studied. The design of a cantilever beam with a tip mass subjected to a stochastic base excitation is considered for illustration. Several of the problem parameters are assumed to be random variables and the structural mass, fatigue damage, and negative of natural frequency of vibration are considered for minimization. The solution of this three-criteria design problem is found by using global criterion, utility function, game theory, goal programming, goal attainment, bounded objective function, and lexicographic methods. It is observed that the game theory approach is superior in finding a better optimum solution, assuming the proper balance of the various objective functions. The procedures used in the present investigation are expected to be useful in the design of general dynamic systems involving uncertain parameters, stochastic process, and multiple objectives.

The Structure of Turbulence in a Supersonic Shock-Wave/Boundary-Layer Interaction

Abstract: This paper reports the experimental results from a detailed investigation of the turbulent flowfields in supersonic shock-wave/boundary-layer interactions at Af=2.25. The interactions were produced by twodimensional compression corners having angles of 8, 13, and 18 deg, the flows being respectively attached, incipiently separated, and separated. Turbulence data from a laser velocimeter and a constant-temperature, hotwire anemometer are presented along with a mean flow survey from wall and pitot-static pressures. Qualitative aspects of the turbulence deduced from spectral analysis and high-speed schlieren pictures are also discussed. It is shown that a large amount of the turbulent energy is contained in large-scale structures that are still observed downstream of the interacting region, despite the severe pressure gradient. The lateral extent of these structures is of the order of one boundary-layer thickness and is roughly half of their longitudinal scale. A low-frequency unsteadiness is associated with the existence of a separation bubble, but does not affect the rest of the flow. The Reynolds shear stress is reduced in the vicinity of the separation bubble where the spatial derivatives of the normal stresses become significant.

Acoustic near-field properties associated with broadband shock noise

Abstract: Shock noise associated with unheated supersonic jets was investigated using a near-field microphone array and a single-sensor wedge-shaped hot-film probe. Both over- and underexpanded cases were investigated using Mach 1.45 and 1.99 convergent-divergent nozzles. Correlation measurements through each shock cell of a single underexpanded case with the Mach 1.45 nozzle were obtained between the hot-film probe and the microphone array. The results of the hot-film/near-field microphone correlations show general agreement with certain theoretical models as to the location for shock noise production, and provide evidence for the existence of some large-scale flow structure that collectively interacts and phases the motion of the downstream shocks. The nearfield microphone correlations demonstrate that downstream shocks dominate shock noise production and suggest the existence of a Doppler effect in the near field of the sources. In addition, broadband shock noise is found to propagate at small angles to the jet axis.

Forced oscillation experiments in supercritical diffuser flows

Abstract: Low-frequency oscillations induced in ramjet inlets by combustion instabilities were simulated by mechanically modulating the exit area of a two-dimensional, supercritical diffuser at frequencies up to 330 Hz. Boundary layers were attached below a terminal shock Mach number of 1.27, and shock-induced separation occurred above this value up to the experimental limit of 1.35. Shock position histories were obtained and streamwise distributions of static/total dynamic pressures were determined both on the wall and within the flow for various shock strengths and frequencies. Excitation at the natural frequencies of the shock motion produced no obvious resonance effects. For weak shocks, the perturbations and their reflections from the shock are reasonably approximated by one-dimension al, acoustic considerations, but this description fails for strong shocks.

Combined function specification-regularization procedure for solution of inverse heat conduction problem

Abstract: Methode combinee basee sur 2 methodes differentes: la methode de specification sequentielle proposee par Beck et la methode de regularisation utilisee par Tikhonov

Cylindrical and conical flow regimes of three-dimensional shock/boundary-layer interactions

Abstract: Etude parametrique des interactions couche limite turbulente onde de choc tridimensionnelle. On montre l'existence de deux regimes d'ecoulement caracteristique: cylindrique et conique

Application of the Godunov method and its second-order extension to cascade flow modeling

Abstract: The Godunov method and a new second-order accurate extension of the method are used for the solution of two-dimensional Euler equations. Both numerical schemes are described in detail. Their performances in the subsonic, transonic, and supersonic flow regimes are first tested on the problem of flow in a channel with a circular arc bump. The niethods are then applied to calculate the transonic flow through a supercritical com­ pressor cascade designed by J. Sanzo For this case, the solution with the second-order extension of the Godunov method gives verygood agreement with the design distribution of parameters given by Sanzo

Grid adaption for problems in fluid dynamics

Abstract: Analyse et application d'une technique de grille adaptative aux problemes fortement non lineaires en mecanique des fluides et transfert de chaleur. Resolution de problemes d'ecoulement externe avec decollement, de propagation de flamme et d'interaction choc-couche limite

LDV measurements of drop velocity in diesel-type sprays

Abstract: Drop velocity measurements were made within nonvaporizing, steady, diesel-type, fast dense sprays from single-hole nozzles into compressed nitrogen. The spray gas was n-hexane and velocity data were taken with laser Doppler anemometry. All data were gathered in steady spray conditions. The effects of variations in the gas/liquid density ratios, injection pressures and nozzle geometries were examined in terms of axial and radial drop velocity components. The entrained gas structure was dominant at about 300 nozzle diameters downstream. The injection momentum and nozzle diameter were found to be sufficient data for calculating the steady-state centerline velocity and the tip penetration rate. Further studies of the nonequilibrium developing region up to the 300 diameters distance are necessary for employing the results for engine calculations. 29 references.

Time Finite Element Discretization of Hamilton's Law of Varying Action

Abstract: Hamilton's Law of Varying Action is used as a variational source for the derivation of finite element discretization procedure in the time domain. Three different versions of the proposed algorithms are presented and verified for accuracy and stability. [The first one is the high-precision, finite time element, analogous to the standard finite elements, with cutoff frequency; the second version is the step by step, one-time element from which the unconditionally stable, with slightly altered accuracy, third algorithm is derived. ] The new operator, connected with the proposed algorithms, bears attractive properties of much greater accuracy than other existing stable methods, and easy computer implementation. Thus, the work herein shows that the reservations expressed against the use of finite elements in time domain seem unjustified.

Injection-induced turbulence in stagnation-point boundary layers

Abstract: A theory is developed for the stagnation point boundary layer with injection under the hypothesis that turbulence is produced at the wall by injection. From the existing experimental heat transfer rate data obtained in wind tunnels, the wall mixing length is deduced to be a product of a time constant and an injection velocity. The theory reproduces the observed increase in heat transfer rates at high injection rates. For graphite and carbon-carbon composite, the time constant is determined to be 0.0002 sec from the existing ablation data taken in an arc-jet tunnel and a balistic range.

Proper definition of curvature in nonlinear beam kinematics

Abstract: Nevertheless, the present results are useful from a comparative rather than absolute standpoint. Tests were also conducted to determine those locations of the tube mouth relative to the nozzle that produced maximum end-wall temperatures. Results for the logarithmic spiral profile at the jet pressure ratio of 3.77 show that the maximum temperature varies periodically with the nozzletube separation h due to the periodic structure of the exciting jet. The highest temperatures were recorded at h/d= 1.7. This means that the peak corresponds to a location where the tube mouth is situated downstream of the first pressure cell. This result differs from the findings of Przirembel and Fletcher.8 However this difference might be explained by the fact that in their case, the open-end walls were rounded in contrast to the 30-deg chamfered entrances of the present tests. Conclusion Thermocouple measurements have demonstrated that a Hartmann-Sprenger tube with a logarithmic spiral profile achieves a significantly greater base temperature than a 10deg tapered tube or a rectangular tube of comparable length. This improved performance is interpreted to result from a higher degree of shock amplification which produces a greater entropy rise in the shock-heated indigenous gas that remains trapped inside the tube. Therefore, it appears that an axisymmetric logarithmic spiral tube could be used to improve the performance further.

Aeroacoustic Computation of Cylinder Wake Flow.

Abstract: Sound generation by uniform flow over a cylinder at the Reynolds number of 200 is calculated. The incompressible, time dependent flow field is first computed by standard computational fluid dynamics techniques using a stream function/vorticity formulation. The sound radiation is then obtained by integrating over the flow field with the Coriolis acceleration as the source term and a low frequency Green's function technique. Time histories, spectra, and directivity of the radiated sound are obtained. Comparisons with experimental data, where possible, indicate the feasibility of this computational aeroacoustics approach.