Showing papers on "Freestream published in 1986"

Modification of vortex interactions in a reattaching separated flow

Abstract: Recent experimental observations have shown that large-scale organized vortices are produced in reattaching separated flows. Interactions between these vortices are important in the development of these flows downstream. Experimental studies from a downstream-fac ing step flow are presented to demonstrate that substantial changes in a reattaching flow can be produced by controlled forcing techniques. The forcing apparently works by affecting the vortex merging process in a fashion similar to that observed in forced mixinglayer experiments. The separated mean flow spreading rate could be increased most effectively by forcing at a nondimensional frequency (based on step height and freestream velocity) between 0.2 and 0.4. This result was found to be relatively independent of step Reynolds numbers over the range (26,000-76,000) studied. A significant decrease in the reattachment length accompanied the increased growth of the separated shear layer. Considerable changes in the turbulence energy and the Reynolds stress levels were also observed for the forced flows.

Comparison of computations and experimental data for leading edge vortices - Effects of yaw and vortex flaps

Abstract: Computations are presented using the conical Euler equations for swept delta wings with leading edge vortices. All the wings have sharp leading edges swept at 75 degrees to the freestream. In addition to an idealized flat plate model, geometrical features also included are thickness, centerbody, and two vortex flaps. Freestream Mach numbers of 1.7 to 2.8, angles of attack of 10 and 12 degrees, and angles of yaw of 0 and 8 degrees are considered. The computations are compared with pitot pressure traverses for one case. Other calculations are compared with pitot pressure traverses for one case. Other calculations are compared with surface pressure data and vapor screen pictures recently obtained at NASA Langley Research Center. The comparisons indicate that the dominant features of these flows are adequately modeled by the Euler equations, but viscous models are needed for the surface boundary layer and secondary separations.

Circulation control airfoils as applied to rotary-wing aircraft

Abstract: Nomenclature c — airfoil chord cd = corrected section drag coefficient cdg = effective drag coefficient rake = d drag coefficient from wake survey ct = section lift coefficient cm = pitching moment about the half-chord C^ = blowing momentum coefficient, =m-Vj/(qco'S) h = slot height m =jet mass flow A^ = freestream Mach number qx = freestream dynamic pressure s = chordwise slot location S =wing reference area V = velocity a. = angle of attack

Influence of numerical dissipation in computing supersonic vortex-dominated flows

Abstract: Steady supersonic vortex-dominated flows are solved using the unsteady Euler equations for conical and three-dimensional flows around sharp- and round-edged delta wings. The computational method is a finite-volume scheme which uses a four-stage Runge-Kutta time stepping with explicit second- and fourth-order dissipation terms. The grid is generated by a modified Joukowski transformation. The steady flow solution is obtained through time-stepping with initial conditions corresponding to the freestream conditions, and the bow shock is captured as a part of the solution. The scheme is applied to flat-plate and elliptic-section wings with a leading edge sweep of 70 deg at an angle of attack of 10 deg and a freestream Mach number of 2.0. Three grid sizes of 29 x 39, 65 x 65 and 100 x 100 have been used. The results for sharp-edged wings show that they are consistent with all grid sizes and variation of the artificial viscosity coefficients. The results for round-edged wings show that separated and attached flow solutions can be obtained by varying the artificial viscosity coefficients. They also show that the solutions are independent of the way time stepping is done. Local time-stepping and global minimum time-steeping produce same solutions.

The role of wave-induced pressure fluctuations in the transfer processes across an air–water interface

Abstract: The structure of the pressure and velocity fields in the air above mechanically generated water waves was investigated in order to evaluate their contribution to the transfer of momentum and energy from wind to water waves. The measurements were taken in a transformed Eulerian wave-following frame of reference, in a wind-wave research facility at Stanford University.The organized component of the fluctuating static pressure at the channel roof was found to contain contributions from both the sound field and the reflected water wave. The acoustic contributions were accounted for by appropriately correcting the pressure amplitude and phase (relative to the wave) and its contribution to the momentum and energy exchange. The wave-induced pressure coefficient at the fundamental mode shows in general an exponential decay behaviour with height, but the rate of decay is different from that predicted by potential-flow theory. The wave-induced pressure phase relative to the wave remains fairly constant throughout the boundary layer, except when the ratio of the wave speed to the freestream velocity, c/Uδ0 = 0.78 and 0.68. This phase difference was found to be about 130° during active wave generation, with the pressure lagging the wave. The momentum and energy transfer rates supported by the waves were found to be dominated by the wave-induced pressure, but the transfer of the corresponding total quantities to both waves and currents may or may not be so dominated, depending on the ratio c/Uδ0. The direct contribution of the wave-induced Reynolds stresses to the transfer processes is negligible.

Nonlinear Aerodynamics of a Two-Dimensional Membrane Airfoil with Separation

Abstract: A chain model is proposed for solving the nonlinear problem of a single-membrane sail wing, eventually in the presence of elasticity and porosity. The presented results are compared with existing data. The model deals with extended ranges of slack and angle of attack effectively and evaluates the corresponding lift and tension coefficients. The model is also extended to partially separated flows. Nomenclature b = length of the element in the chain over chord b0 = initial length of the element over chord c = chord length CL = lift coefficient ACp = pressure difference coefficient, = (pv -pL)/q CT = tension coefficient, = T/qc k = stretch coefficient, = (

Modeling of transition and surface roughness effects in boundary-layer flows

Abstract: Experiments were carried out to examine the influence of three-dimensional, stochastic roughness on the growth of incompressible turbulent boundary layers, as well as the effect of streamwise pressure gradients and freestream turbulence intensity on smooth-wall boundary-layer transition. The modeling of these effects in a two-dimensional boundary-layer computation program was examined with the help of the experiments. A model for surface roughness was developed that relates directly measurable statistical parameters quantifying the roughness geometry to the aerodynamic effects. This model should be valid for a limited class of surfaces found on turbomachinery blading and in other engineering applications. Commonly used criteria for the transition onset performed poorly and presumably need to be modified to account for other factors influencing the process.

Vortex cloud model for body vortex shedding and tracking

Abstract: The discrete vortex cloud approach models a missile airframe's vortex wake by means that are capable of treating a variety of configurations over a range of flow conditions. Attention is given to the sheets of vorticity formed on the lee side of a missile at moderate angles-of-attack. While three-dimensional attached flow models are used to represent the missile body, two-dimensional, incompressible, separated flow models are used to represent the separated vortex wake. The predicted pressure distribution of the body under the influence of the freestream and the separation vortex wake are used to calculate aerodynamic loads on the body. The separation vortex wake is represented by clouds of discrete vortices in cross flow planes normal to the body axis.

Hypersonic-finite rate chemically reacting viscous flows over an ablating carbon surface

Abstract: Hypersonic finite-rate chemically reacting viscous flows over an ablating carbon surface have been analyzed using the viscous shock-layer method to study the effects of ablating carbon on the surface-measurable quantities and on the electron density, temperature, and species profiles across the shock layer. The results for nonequilibrium air injection and the ablating carbon surface mass transfer were compared. Of all the test cases considered, the ablating carbon case shows an approximately 10°7o higher stagnation heat-transfer rate than the equivalent nonequilibrium air injection case. The effects of fully catalytic, noncatalytic, and equilibrium catalytic wall boundary conditions were also compared to analyze the effects of catalytic wall boundary conditions on the heat-transfer rate. At low wall temperatures, such as 1000 K, the noncatalytic wall condition showed the lowest heat-transfer rate. However, at high wall temperatures (such as 3333 K) due to the positive wall diffusion heat transfer, the equilibrium catalytic wall condition showed the lowest heat-transfer rate at an altitude of 84 km and a freestream velocity of 7.6 km/s. A method for analyzing nonequilibrium finite-rate chemically reacting flows over multiconic geometries with ablating carbon surface has been developed.

Direct simulation of hypersonic flows over blunt slender bodies

Abstract: Results of a numerical study of low-density hypersonic flow about cylindrically blunted wedges and spherically blunted cones with body half angles of 0, 5, and 10 deg are presented. Most of the transitional flow regime encountered during entry between the free molecule and continuum regimes is simulated for a reentry velocity of 7.5 km/s by including freestream conditions of 70 to 100 km. The bodies are at zero angle of incidence and have diffuse and finite catalytic surfaces. Translational, thermodynamic, and chemical nonequilibrium effects are considered in the numerical simulation by utilizing the direct simulation Monte Carlo (DSMC) method. The numerical simulations show that noncontinuum effects such as surface temperature jump, and velocity slip are evident for all cases considered. The onset of chemical dissociation occurs at a simulated altitude of 96 km for the two-dimensional configurations. Comparisons between the DSMC and continuum viscous shock-layer calculations highlight the significant difference in flowfield structure predicted by the two methods.

A Navier-Stokes Solution for a Bulbous Payload Shroud

Abstract: The unsteady, compressible Navier-Stokes equations in mass averaged variables are solved for flow past a bulbous shroud. For the freestream Mach number 0.8 and Reynolds number 3.37xl0/m, a time-dependent computation is performed, using MacC or mack's explicit finite-difference scheme with 64 X 30 grid points. The entire flowfield along the shroud is analyzed to capture a terminal shock and the separation zones. A comparison with available experimental results is reported and evaluation of the unsteady data for surface pressure fluctuations is presented.

Experimental and computational study of a swept compression corner interaction flowfield

Abstract: The results of an experimental and computational study of the three-dimensional shock-wave/turbulent boundary-layer interaction generated by a swept compression corner are presented. This corner had a streamwise compression angle of 24 deg and a sweepback angle of 40 deg. Two equilibrium incoming turbulent boundary layers with thicknesses varying by 3:1 were used. In both cases the freestream Mach number was 2.95, the freestream Reynolds number was 63xl0/m, and the wall temperature was close to adiabatic. This study was undertaken to test the applicability to flowfield features of a Reynolds number similarity law originally based on surface features only. A second goal of the study was to test the ability of a state-of-the-art numerical solution of the Navier-Stokes equations to predict the subject flow. The experimental results, composed primarily of flowfield pitot pressure and yaw angle surveys, confirmed the general applicability of the similarity law. The comparison of experiment and computation revealed that the latter captured may of the qualitative features of the flow. Further, only very limited reasons were found to prefer a two-equation eddy-viscosity turbulence model rather than an algebraic model for predictions of this flow.

Integral Calculations of Transitional Boundary Layers

Abstract: A method is presented for the calculation of two-dimensional, incompressible, transitional boundary layers under small pressure gradient and moderate freestream turbulence conditions. Established integral techniques are used in conjunction with an intermittency weighted model of the transitional boundary layer, and empirical correlations are used to predict the onset and length of the transition region. The only input data required to compute the entire unseparated boundary layer are the ambient pressure and temperature, the freestream turbulence level and the freestream velocity distribution in a power law, or a polynomial form. Alternatively, the freestream velocity can be input in tabular form as a function of x.The computed integral parameters and mean velocity profiles are seen to compare favourably with present and other published experimental data.

Rectangular Cylinders in Flows with Harmonic Perturbations

Abstract: The effects of small amplitude harmonic perturbations on the pressure coefficients and wake development behind rectangular cylinders are examined. Two body configurations with slenderness ratios (streamwise body dimension/transverse body dimension) of B/D=0.5 and 2.0 are used. For the B/D=0.5 body, with the pulsation frequency equal to twice or four times the natural shedding frequency, and with amplitudes of the order of 1% of the freestream velocity, the mean base pressure shows a decrease of up to 18% when compared with its value in steady flow. Flow visualization shows this decrease in mean base pressure to be associated with increased vortex strength, decreased vortex formation length, and the corresponding increase in shear layer curvature. When the perturbation amplitude exceeds an undetermined threshold value at a frequency corresponding to four times the natural shedding frequency, two vortices are shed simultaneously and symmetrically with respect to the body centerline. For the B/D=2.0 body, th...

Ignition of a Fuel Spray by a Hot Surface

Abstract: An experimental study of the ignition of Jet-A fuel sprays by an isothermal hot surface was conducted in a vertical axisymmetric duct. The ranges of flow conditions under which ignition was investigated were: 1) freestream velocity, 1-5 m/s; 2) boundary-laye r momentum thickness, 3-20 mm; 3) freestream air temperature, 40-250°C; 4) fuel concentration, ignitability limits; and 5) droplet size (SMD), 20-200 /im. In addition to measurements of the wall temperature necessary for ignition under the above conditions, local measurements of velocity, "turbulence" intensity, fuel concentration, and the fraction of fuel vaporized were measured in the boundary layer at surface temperatures just below that required for ignition. The results exhibited vapor ignition trends for most of the flow conditions, with some exceptions where single-droplet ignition appeared to be present. The experimental data are compared with existing vapor ignition theory.

Coupling conditions for integrating boundary layer and rotational inviscid flow

Abstract: The matching of a boundary layer and a rotational inviscid flow is reexamined by extending the Johnson and Sockol (1979) coupling conditions to include the case where the boundary layer solution includes the second-order effects of the freestream vorticity and the total temperature gradient. It is pointed out that two of the three conditions are not independent. If the boundary layer solution satisfies the appropriate momentum and energy integral equations, it follows that the imposition of the normal mass flux condition insures that the conditions on a normal flux of streamwise momentum and total enthalpy will also be satisfied.

Aerodynamic Force Calculations of an Elliptical Circulation Control Airfoil

Abstract: A method is developed to predict the aerodynamic forces on a circulation control elliptical airfoil in a twodimensional flow environment. By distributing source panels on the airfoil surface in the separation region and using conformal mapping techniques, a simple solution for the potential flow including effects of separated wake is obtained. The development of boundary layers and wall-jet is calculated by a finite difference method. The potential flow with separated wake effect calculations and boundary layer and wall-jet calculations are combined in an iterative process to determine the aerodynamic forces under given jet momentum coefficient and freestream condition. The effect of separated wake is found significant for a cylinder. The correlation of the calculation results with the available experimental data appears reasonable.

Vortex unsteadiness on slender bodies at high incidence

Abstract: Existing experimental evidence of vortex unsteadiness on slender bodies at high incidence is examined. It is found that, for the high laminar Reynolds numbers at which the tests have been performed, a likely source of the transient vortex behavior is the transition-promoting effect of freestream turbulence. By considering the similar effects on transition of Reynolds number, freestream turbulence, and surface roughness, the transitionpromoting flow phenomenon provides a consistent explanation also for the observed large effects of roll angle on the vortex-induced asymmetric loads. The experimental results of vortex unsteadiness reinforce earlier established evidence of the existing coupling between body motion and asymmetric vortex shedding, raising concern about rigid and elastic response of long slender bodies.

Cryogenic tunnel measurement of total temperature and pressure

Abstract: A newly developed, 3-mm-diam, dual hot-wire aspirating probe was used to measure the time-resolved stagnation temperature and pressure in a transonic cryogenic wind tunnel. Measurements were taken in the freestream of the settling chamber and test section. Data were also obtained in the unsteady wake shed from an airfoil oscillating at 5 Hz. The investigation revealed the presence of large fluctuations in the settling chamber occuring at the blade passing frequency of the driving fan of the tunnel. These fluctuations decrease at the test section. The rms value of the fluctuating stagnation pressure decreased from 17.5 percent in the settling chamber to 3.7 percent in the test section. Fluctuating stagnation temperature decreased from 12.3 percent to 8.4 percent. Measurements in the wake of the oscillating airfoil showed a fluctuating stagnation temperature of as much as 42 K in rms value.

Boundary-layer flow past a cylinder with massive blowing

Abstract: The effect of massive blowing on the unsteady laminar compressible boundary layer flow along the stagnation line of an infinite swept cylinder, when the freestream velocity and blowing rate vary with time, is studied. An implicit finite-difference scheme is used, together with the quasilinearization technique, to numerically solve the governing partial differential equations. Blowing is found to strongly affect skin friction, heat transfer, velocity, and total enthalpy profiles.

The separation of flow past a circular cylinder on a β-plane

Abstract: Effects of the Ekman friction on the prograde (eastward) flows past a cylinder on aΒ-plane are investigated when\(\hat \beta \) (=ΒR2/U, whereR is the cylinder radius andU the freestream speed)∼O(1) andα(=2Ek1/2/R0·δ∼O(1) where\(\hat \beta \) is the non-dimensional beta parameter andα the ratio of the square root of the Ekman numberEk multiplied by 2 to the Rossby numberRo multiplied by the aspect ratioδ(=H/R, whereH is the fluid depth). Previous studies without the Ekman friction have shown that theΒ-effect inhibits flow separation for pragrade flows through the asymptotic boundary condition by shifting the region of the adverse pressure gradient toward the rear stagnation point. It is found that the Ekman friction alleviates thisΒ-effect on the exterior flow. In the Ek1/4-boundary layer, on the other hand, Ekman friction suppresses the vorticity advection along the wall, which tends to make the boundary layer thickness thin and delay the flow separation. The Ekman friction thus affects flow separation in a complicated manner. Details of the boundary layer structures and the separation angles are described for 0.3<\(\hat \beta \)<4.0 and 0.1<α<1.5.

Thrust augmentor inlet optimization

Abstract: A zonal computational method is developed to optimize inlet shape and primary nozzle location for twodimensional thrust augmenting ejectors. An inviscid zone comprising the irrotational flow about the device is patched together with a viscous zone containing the turbulent mixing flow. The inviscid region is computed by a higher order panel method, while an integral method is used for the description of the viscous part. A nonlinear, constrained optimization study is undertaken for the design of the inlet region. In this study, boundary layer separation on the walls of the inlet is taken into account through the use of a penalty function. Two nondimensional groups, a thrust based Reynolds number, and the ratio formed by the freestream velocity and a characteristic velocity of the jet, are found to be important parameters in the design of an optimal inlet.

Modification of the Karman-vortex street in the freestream

Abstract: An experimental study conducted in the NASA Langley 15-in Low-Turbulence Wind Tunnel demonstrated that a relatively short-chord flow-aligned plate placed in the proximity of a cylindar could produce nearly one-signed oscillatory transverse control vortices. Typical results showed the screen with the 40 mesh to have a solidity of 35 percent, and the screen with 57 mesh to have a solidity of 35 percent, and the screen with 57 mesh to have a solidity of 72 percent. In all cases, the turbulence level of the incoming freestream was seen to have little effect on the suppression of the shed vorticity within the range considered.

Effects of cone surface waviness and freestream noise on transition in supersonic flow

Abstract: A comparison of transition on wavy-wall and smooth-wall cones in a Mach 3.5 wind tunnel is made under conditions of either low freestream noise (quiet flow) or high freestream noise (noisy flow). The noisy flow compares to that found in conventional wind tunnels while the quiet flow gives transitional Reynolds numbers on smooth sharp cones comparable to those found in flight. The waves were found to have a much smaller effect on transition than similar sized trip wires. A satisfatory correlating parameter for the effect of waves on transition was simply the wave height-to-length ratio. A given value of this ratio was found to cause the same percentage change in transition location in quiet and noisy flows.

Active Control of Asymmetric Vortex Effects in a Circular Cylinder

Abstract: The low speed wind tunnel investigations for the anomalous side force acting on a slender body at high angles of attack are conducted at Reynolds numbers 1.1 x IO -4.8 x IO. The effects of the geometric configurations of the model and the freestream Reynolds number are studied by measuring the aerodynamic force. The anomalous side force that appears on the simple circular cylinder is reduced by the attachment of a cone on the side end of the cylinder, although attachment of another cone on the other side increases the anomalous side force up to the level of the simple cylinder. The anomalous side force can be alleviated by the use of the antennas around the cylinder. The antennas behave as surface roughness and promote the early transition of the boundary layer to the turbulent one.

Navier-Stokes and Experimental Modeling of Blunt-Base Rocket Nozzle Flow

Abstract: The influence of grid placement and grid density on the ability to compute an underexpanded jet in a supersonic afterbody flowfield is studied using a conservation-based body-fitted computational technique. The thin-shear-layer formulation of the compressible, Reynolds-averaged, Navier-Stokes equations together with mass and energy conservation equations are modeled using an artificial time-dependent, explicit numerical algorithm with the turbulence approximated by a two-layer algebraic model with wall functions for the solid boundaries and whose properties are tailored to the three physically distinct mixing zones. Solutions are obtained for supersonic flow over an axisymmetric conical afterbody with a blunt base, containing a centered propulsive jet where the freestream Mach number is 2.0 and the jet exit Mach number is 2.5. Exhaust exit-plane static pressures are considered in the range of one to nine times the freestream static pressure. The conical nozzle-exit half-angles are zero and twenty degrees. Comparisons are made between computed and experimental results for base pressure, separation length, afterbody pressure distribution, and flowfield structure. The numerical solutions are found to be sensitive to the computational grid structure and the mixing (turbulence) model. Error norms are applied to aid the detection of inappropriate grid choices. The best results are obtained with adaptive grids that track both free shear layers and a mixing model which is germain to the local flow features.