Showing papers on "Freestream published in 1988"

Nonequilibrium thermal radiation for an aeroassist flight experiment vehicle

Abstract: The direct-simulation Monte Carlo method incorporating a dissociating and ionizing gas model for air with thermal radiation is used to characterize the hypersonic flow about an axisymmetric representation of an aeroassist flight experiment (AFE) vehicle, whose freestream conditions correspond to selected points along the entry, aerobraking, and exit phases of the trajectory. Calculations for two trajectory conditions indicate that the radiative heating of the AFE forebody is lower than the convective heating, but becomes significant as the maximum convective heating rate condition is approached.

Experimental Study of Bypass Transition in a Boundary Layer

Abstract: A detailed investigation to compare the boundary layer transition process in a low intensity disturbance environment to that in an environment in which the disturbances are initially non-linear in amplitude was conducted using a flat plate model. The transition mechanism based on linear growth of Tollmien Schlichting (T-S) waves was associated with a freestream turbulence level of 0.3 percent; however, for a freestream turbulence intensity of 0.65 percent and higher, the bypass transition mechanism prevailed. The results of detailed measurements acquired to study and compare the two transition mechanisms indicate that there exists a critical value for the peak rms of the velocity fluctuations within the boundary layer of approximately 3 to 3.5 percent of the freestream velocity. Once the unsteadiness within the boundary layer reached this critical value, turbulent bursting initiated, regardless of the transition mechanism. The two point correlations and simultaneous time traces within the transition region illustrate the features of a turbulent burst and its effect on the surrounding flowfield.

Unsteady nature of shock-wave/turbulent boundary-layer interaction

Abstract: This study addresses the unsteady aspect of shock-wave/turbulent boundary-layer interaction. The interaction was generated using a two-dimensional compression ramp at a freestream Mach number of 2.9. Three models were tested, with flow conditions ranging from fully separated to incipient separation. An array of flushmounted, miniature, high-frequency pressure transducers was used to make multichannel measurements of the fluctuating wall pressure within the interaction. The present results show that, upstream of the mean separation line, the flow is dominated by the large-scale "flapping" motions of a single sharp shock wave. Instantaneously, this shock front is considerably spanwise nonuniform, giving it a "rippling" appearance. In the separated region, however, convective turbulence phenomena play a much larger role. The energy containing turbulent eddies above the recirculation zone and traveling in the downstream direction appear to be the major contributors to the wall pressure fluctuations. Here, the time scale of the pressure signals is found to decrease significantly as the shock strength is increased.

Unique, clean-air, continuous-flow, high-stagnation-temperature facility for supersonic combustion research

Abstract: Accurate, spatially-resolved measurements can be conducted of a model supersonic combustor in a clean air/continuous flow supersonic combustion facility whose long run times will allow not only the point-by-point mapping of flow field variables with laser diagnostics but facilitate the simulation of steady-state combustor conditions. The facility will provide a Mach 2 freestream with static pressures in the 1 to 1/6 atm range, and stagnation temperatures of up to 2000 K.

Unsteady viscous calculations of supersonic flows past deep and shallow three-dimensional cavities

Abstract: Computational simulations were performed for supersonic, turbulent flows over deep and shallow three-dimensional cavities. The width and the depth of these cavities were fixed at 2.5 in. and 0.5 in., respectively. Length-to-depth ratio of the deep cavity was 6 and that of the shallow cavity was 16. Freestream values of Mach number and Reynolds number were 1.50 and 2.0 x 10 to the 6th/ft., respectively, at a total temperature of 585 R. The thickness of the turbulent boundary layer at the front lip of the cavity was 0.2 in. Simulations of these oscillatory flows were generated through time-accurate solutions of Reynolds-averaged full Navier-Stokes equations using the explicit MacCormack scheme. The solutions are validated through comparisons with experimental data. The features of open and closed cavity flows and effects of the third dimension are illustrated through computational graphics.

Unsteady shock-induced turbulent separation in Mach 5 cylinder interactions

Abstract: Wall pressure fluctuations have been determined under the unsteady separation shock in interactions generated by unswept circular cylinders, using a nominal freestream Mach number of 5 and a freestream unit Reynolds number of 53 x 10 to the 6th/m. The distributions of shock frequency and period were calculated using a conditional sampling algorithm. The shock frequency distributions were all found to be broadband, with frequencies being typically 1-2 kHz. The results support the previous suggestion that pressure fluctuations in the separated flow drive the shock motion.

Direct simulation of AFE forebody and wake flow with thermal radiation

Abstract: Calculated results for the flowfield structure and surface quantities are presented for an axisymmetric representation of an aeroassist flight experiment vehicle. The direct simulation Monte Carlo (DSMC) method is used to perform the calculations, since the flow is highly nonequilibrium about the vehicle during both the compression and expansion phases. The body configuration is an elliptically blunt nose followed by a skirt with a circular radius and an afterbody. Freestream conditions correspond to a single point along the entry trajectory at an altitude of 90 km and a velocity of 9.9 km/s. The calculations account for nonequilibrium in the translational and internal modes, dissociation, ionization, and thermal radiation. The degree of dissociation is large, but the maximum ionization is only about 2 percent by mole fraction. The blunt forebody flow experiences a high degree of thermal nonequilibrium in which the translational temperature is generally greater than the internal temperature. However, as the flow expands about the aerobrake skirt and afterbody, the internal temperature is generally greater than the translational temperature.

Particle Deposition Onto a Flat Surface from a Point Particle Source

Abstract: A series of two-dimensional calculations are performed to determine particle fluxes to wafers in a stagnation flow configuration. Mechanisms that influenced particle deposition included convection, diffusion, sedimentation, and thermophoresis. Particle deposition patterns resulting from a uniform freestream concentration are compared with deposition patterns from a narrow particle beam.

Joint measurements of velocity and scalars in a turbulent diffusion flame with moderate swirl

Abstract: Simultaneous measurements by two-component LDA and Mie scattering methods have been made in a moderately swirling, turbulent hydrogen diffusion flame in a horizontal co-flowing stream. Mean temperature is obtained separately by thermocouple. The three velocity components are measured in pairs so that all components of the stress tensor are obtained. The Mie signal is used to obtain scalar time traces. The flame expands into the freestream, which does not act as a confinement, and is shortened one third and widened around one quarter by the swirl. This contrasts with results in the literature for confined swirling flames which lengthen with increasing swirl. The difference is thought to be caused mainly by the action of the velocity pressure-gradient correlations: in a confined flame the radial pressure gradient is maintained high downstream, so these terms appear to suppress the u-v stress. In the present work, the upstream decay of streamwise velocity is increased, as is the turbulence intensity, whilst far downstream the normalized data look similar to those of unswirled flames. There is less penetration of external fluid towards the flame axis than in non-swirling flames. The v-w and u-w stress components have similar magnitudes, and it appears that the v-w stress may be dealt with by gradient transport modelling where the radial pressure gradient is not too high.

Resonant flow in small cavities submerged in a boundary layer

Abstract: Experiments were performed to determine the conditions under which a narrow-frequency-band flow excitement could be sustained in a row of small transverse square cavities while the boundary layer in which they were submerged was already turbulent. This is shown to happen when the depth of the Stokes layer generated by monochromatic freestream disturbances is nearly the same as the cavity depth. This indicates that viscous waves can exist in thin layers adjacent to the surface under a turbulent boundary layer.

Measurements of droplet drag coefficients in a polydispersed turbulent flow field

Abstract: Measurements of drag coefficient versus Reynolds number were obtained in a polydispersed turbulent flow field for three turbulence velocities and for freestream turbulence intensities between 3 and 12 percent. Results are presented for a single nozzle with low number density and a twin nozzle with higher number density. The results for drops smaller than 30 microns showed strong differences from existing low turbulence results. The drag relation was found to be weakly affected by the freestream velocity and to be strongly affected by the velocity fluctuation levels of individual size classes. A drag effect with the drop-drop interaction may also be possible.

Three-dimensional viscous flow computations of a circular jet in subsonic and supersonic cross flow

Abstract: Three-dimensional viscous flow computations are presented for 90 deg injection angle jets in subsonic and supersonic cross flow Comparisons with experimental data include jet centerline and vortex trajectories for the subsonic cross flow, and surface pressure measurement for the supersonic crossflow case The vortices induced in the jet/freestream interaction are computed and illustrated The vortices persist in subsonic flow and die out quickly in supersonic flow The structure of the shocks in the unconfined supersonic flow is illustrated

Three-dimensional viscous flow computations of a circular jet in subsonic and supersonic cross flow

Abstract: Three-dimensional viscous flow computations are presented for 90 deg injection angle jets in subsonic and supersonic cross flow. Comparisons with experimental data include jet centerline and vortex trajectories for the subsonic cross flow, and surface pressure measurement for the supersonic crossflow case. The vortices induced in the jet/freestream interaction are computed and illustrated. The vortices persist in subsonic flow and die out quickly in supersonic flow. The structure of the shocks in the unconfined supersonic flow is illustrated.

Unsteady Influences in Droplet Dynamics and Combustion

Abstract: This research investigation addresses the problem of the unsteady dynamics associated with the vaporization. ignition and hurning of hydrocarhon fuel droplets in laminar and turhulent flowfields. The method of approach is numerical. and detailed solutians of the Navier-Stokes equations have heen carried out to determine the important physical processes which occur around a burning droplet. For the relatively high pressure conditions of the present problem the most unsteady feature of the flow was the ignition and flame formation process. The ignition process causes major changes in the drag and heat transfer compared to the influences of eddy turhulence or the natural decay of the frecstream velocity between the droplet and the surroundings. However, both eddy turbulence and the natural decay of the freestream velocity play a very important role in the history over the droplet lifetime. All of the flow processes comhine to cause the trajectory or the history of the droplet to he much different th...

Mixing and Reaction at Low Heat Release in the Non-Homogeneous Shear Layer

Abstract: The effects of freestream density ratio on the mixing and combustion in a high Reynolds number, subsonic, gas-phase, non-buoyant, two-dimensional turbulent mixing layer, have been investigated. Measurements of temperature rise (heat release) have been made which enable us to examine the effect of freestream density ratio on several aspects of the mixed fluid state within the turbulent combustion region. In experiments with very high and very low stoichiometric mixture ratios ("flip" experiments), the heat release from an exothermic reaction serves as a quantitative label for the lean reactant freestream fluid that becomes molecularly mixed. Properly normalized, the sum of the mean temperature rise profiles of the two flip experiments represent the probability of fluid molecularly mixed at any composition. The mole fraction distribution and number density profile of the mixed fluid can also be inferred from such measurements. Although the density ratio in these experiments was varied by a factor of thirty, profiles of these quantities show little variation, with integrals varying by less than 10%. This insensitivity differs from that of the composition of molecularly mixed fluid, which is very sensitive to the density ratio. While the profiles of composition exhibit some similarity of shape, the average composition of mixed fluid in the layer varies from nearly 1:2 to over 2:l as the density ratio is increased. A comparison of data and available theory for this offset or average composition is discussed.

Measurements of the Turbulent Transport of Heat and Momentum in Convexly Curved Boundary Layers: Effects of Curvature, Recovery and Free-Stream Turbulence

Abstract: The effects of streamwise convex curvature, recovery, and freestream turbulence intensity on the turbulent transport of heat and momentum in a mature boundary layer are studied using a specially designed three-wire hot-wire probe. Increased freestream turbulence is found to increase the profiles throughout the boundary layer on the flat developing wall. Curvature effects were found to dominate turbulence intensity effects for the present cases considered. For the higher TI (turbulence intensity) case, negative values of the turbulent Prandtl number are found in the outer half of the boundary layer, indicating a breakdown in Reynolds analogy.

The Effect of Pressure Gradient and Freestream Turbulence Intensity on the Length of Transitional Boundary Layers

Abstract: The paper presents the results of an experimental investigation aimed at an elucidation of the individual and collective influence of freestream turbulence intensity and pressure gradient on the length of the transitional boundary layer. A correlation for the transition length Reynolds number is developed and used, in an intermittency-based computational scheme, to predict the transitional boundary layer behaviour in a number of practical flows representative of the suction surface of gas turbine blading.

Simple Multilayer Panel Method for Partially Separated Flows Around Two-Dimensional Masts and Sails

Abstract: A multilayer panel method has been developed using superimposed combinations of vortex doublet and source/ sink elements to predict the static pressure distributions found over two-dimensional mast/sail geometries. The method takes into account all the partially separated regions present, and all the computations are performed without any need for iteration. The only inputs required are: mast/sail geometry, aerodynamic incidence angle, freestream Reynolds number, and empirically determined separation and reattachment locations. All of the base pressures involved are obtained as part of the solution. Comparison between experimental and theoretical results showed excellent agreement.

Navier-Stokes calculations of scramjet-afterbody flowfields

Abstract: Scramjet simulant-gas exhaust flows are computed using two-dimensional Navier-Stokes (NS) equations. The NS equations are solved using an implicit, upwind, finite-volume scheme, and the Reynolds stresses are modeled algebraically. The solutions are compared with experimental data where the freestream and the nozzle exhaust fluids are air. The NS equations for the flow, where the scramjet exhaust simulant gas is a mixture of Freon-12 and argon, are solved by an explicit, finite difference scheme. The results are compared with experimental data, and certain discrepancies are explained.

Modeling of surface blowing as an anti-icing technique for aircraft surfaces

Abstract: An analysis of a proposed possible anti-icing technique applicable to aircraft surfaces has been studied and is described herein. Air injection at the leading edge of an ice-accreting surface is used to reduce and/or eliminate ice collection by preventing the supercooled water droplets in the atmosphere from impinging on the surface. In this envisioned technique, a discrete stream of fluid (air) is injected into the mainstream through slots located oh the cylinder surface. The modified flow around the surface produces modified droplet trajectories, deflecting the droplets away from the surface. Exact mathematical expressions for the velocities are obtained from potential flow theory. Droplet trajectories are obtained for a variety of surface blowing conditions. It was found that for a given cylinder diameter, freestream velocity, droplet size, and injection, there is an optimum slot location for which the injection has its maximum effect, i.e., minimum water collection and subsequent ice accretion. The effect of injection rate as well as the number of slots on the collection efficiency are also investigated.

Experimental study of unsteady separating turbulent boundary layers

Abstract: : A detailed experimental study of two large amplitude unsteady freestream flows and a third unsteady flow produced by an oscillating roof damper is reported. Detailed ensemble-averaged velocity and turbulence measurements were made using hot-wire and laser anemometers. While the primary function of this report is to present and document these data, some discussion of the results and a number of data plots are presented. The reduced data are available on computer diskettes. Upstream of detachment at the phases with no flow reversal, the flows are quasi-steady. For the large amplitude flows, at low velocity phases some flow reversal occurs upstream of detachment, which was not observed in the moderate amplitude flow studied by Simpson et al. (1983). After the beginning of detachment, large amplitude and phase variations develop through each flow. Huge backflow velocities, as large as free-stream, were measured. Keywords: Unsteady flows, Laser anemometry, Turbulence, Separation.

The Effects of Freestream Turbulence on Airfoil Boundary Layer Behavior at Low Reynolds Numbers

Abstract: : An experimental study was conducted to determine the effects of freestream turbulence on airfoil boundary layer behavior. Freestream turbulence intensity levels up to approximately 4% and length scales up to approximately two inches were generated using turbulence-generating grids. Data were collected using a single-wire hot-wire probe in conjunction with a three-dimensional traversing system. Increased levels of freestream turbulence were found to cause correspondingly earlier transition to a turbulent boundary layer. Boundary layer growth was found to be unaffected by freestream turbulence levels up to 4% at length scales an order of magnitude greater than the boundary layer thickness. For length scales on the order of boundary layer thickness, a 12% increase in the turbulent boundary layer thickness was found with an increase in turbulence intensity from 0.23% to 0.5%.

Aerodynamics of complex bodies of revolution

Abstract: The aerodynamics of five complex bodies of revolution are investigated experimentally and theoretically at a low Mach number (M f ^ 01) and over the range of angle of attack from 0 to 35 deg The geometrical forms of the bodies are generally complex with the discontinuities in the slope of the body surface The surface-flow visualization is performed by using the oil method The balance measurements were made and the results compared with the potential theory and the method based on the crossflow analogy It was observed that the discontinuities in the slope of the body surface make the flow separation and consequently the flowfield very complicated It was also found that the method of crossflow analogy is applicable not only to simple-type bodies of revolution but also the complex ones Nomenclature CA — axial force coefficient CD = drag coefficient CDO — drag coefficient at a = 0 deg Cd = crossflow drag coefficient cdc = crossflow drag coefficient of circular cylinder section CL = lift coefficient CM = pitching moment coefficient d — maximum body diameter ds = diameter of corresponding cylinder to body of revolution ( = S p /l) /(X) = normal force distribution / = total body length lr = reference length M^ = freestream Mach number MCoo = crossflow Mach number ( = M^ sina) q^ = freestream dynamic pressure (q^ = ^p R = local body radius Red = Reynolds number based on the maximum body diameter d (Re d — V^d/v) Recds = crossflow Reynolds number based on ds ( = F^ sina ds/v) Ret - Reynolds number based on / S = local cross-sectional area ( = nR2) Sb = body base area Sp = body planform area ( = Jo 2R d*) Sr = reference area S, = body flat-nose area V^ = freestream velocity W = body volume ( = &nR2 d*) x = body axis (axial distance from body nose) xc = axial distance from body nose to centroid of body planform area xcp = axial distance from body nose to center of pressure xm = pitching moment center (axial distance from body nose to pitching moment center) a = angle of attack p = density of air D = kinematic viscosity of air r\ — correction factor for influence of fineness ratio

Asymptotic analysis of local disturbances in a periodic boundary layer

Abstract: fluid is considered in the present study. The boundary layer of such type is typically present in the flow past a semiinfinite flat plate, with the freestream velocity U~(I + k cos t), where t is the nondimensional time, normalized by T~(2~) -I with T= being the period of oscillation. We will assume that the constant t satisfies the condition 0 < k < I, so that the freestream velocity does not change its direction over the entire period of oscillation. The system of Cartesian coordinates has the origin at the leading edge of the plate with the abscissa pointing downstream along the plate (see Fig. I). The coordinates of the points in the plane of the flow are denoted by (2~)-IU=T~(x, y) and the corresponding components of the velocity vector by (u, v)U~. The relative change of pressure with respect to the freestream pressure is pU~, where the density p as well as the kinematic viscosity u are considered as constants. The Navier-Stokes equations in nondimensional variables take the form

Rarefied flow past a flat plate at incidence

Abstract: Results of a numerical study using the direct simulation Monte Carlo (DSMC) method are presented for the transitional flow about a flat plate at 40 deg incidence. The plate has zero thickness and a length of 1.0 m. The flow conditions simulated are those experienced by the Shuttle Orbiter during reentry at 7.5 km/s. The range of freestream conditions are such that the freestream Knudsen number values are between 0.02 and 8.4, i.e., conditions that encompass most of the transitional flow regime. The DSMC simulations show that transitional effects are evident when compared with free molecule results for all cases considered. The calculated results demonstrate clearly the necessity of having a means of identifying the effects of transitional flow when making aerodynamic flight measurements as are currently being made with the Space Shuttle Orbiter vehicles. Previous flight data analyses have relied exclusively on adjustments in the gas-surface interaction models without accounting for the transitional effect which can be comparable in magnitude. The present calculations show that the transitional effect at 175 km would increase the Space Shuttle Orbiter lift-drag ratio by 90 percent over the free molecule value.