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At least 559 records · Page 31

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NASA NTRS1990

Breaking down the delta wing vortex: The role of vorticity in the breakdown process

Experimental x-wire measurements of the flowfield above a 70 and 75 deg flat plate delta wing were performed at a Reynolds number of 250,000. Grids were taken normal to the wing at various chordwise locations for angles of attack of 20 and 30 deg. Axial and azimuthal vorticity distributions were derived from the velocity fields. The dependence of circulation on distance from the vortex core and on chordwise location was also examined. The effects of nondimensionalization in comparison with other experimental data is made. The results indicate that the circulation distribution scales with the local semispan and grows in a nearly linear fashion in the chordwise direction. The spanwise distribution of axial vorticity is severely altered through the breakdown. The axial vorticity components with a negative sense, such as that found in the secondary vortex, seem to remain unaffected by changes in wind sweep or angle of attack, in direct contrast to the positive components. In addition, the inclusion of the local wing geometry into a previously derived correlation parameter allows the circulation of growing leading edge vortex flows to be reduced into a single curve.

Nelson, Robert C.↗

NASA NTRS1991

Drag reduction in nature

Recent studies on the drag-reducing shapes, structures, and behaviors of swimming and flying animals are reviewed, with an emphasis on potential analogs in vehicle design. Consideration is given to form drag reduction (turbulent flow, vortex generation, mass transfer, and adaptations for body-intersection regions), skin-friction drag reduction (polymers, surfactants, and bubbles as surface 'additives'), reduction of the drag due to lift, drag-reduction studies on porpoises, and drag-reducing animal behavior (e.g., leaping out of the water by porpoises). The need for further research is stressed.

Bushnell, D. M.↗

NASA NTRS1990

Finite-difference solutions of three-dimensional rotor blade-vortex interactions

A numerical finite-difference procedure has been developed for the modeling of rotor self-generated blade-vortex interactions (BVI). The interaction velocity field is obtained through a nonlinear superposition of the rotor and the vortex wake flow fields. Vortex effects are simulated using the velocity 'transpiration' approach. Potential blade-vortex encounters are identified and tracked in time at equal increments of rotor azimuth using the lifting-line helicopter/rotor trim code CAMRAD. Accuracy of the numerical results were found to rely heavily on the CAMRAD-predicted vortex orientation, vortex strength, and user specified vortex core radius. Results for the subcritical self-generated BVI for the model OLS rotor are presented. They show good agreement with the experimental wind tunnel data.

Hassan, A. A.↗

NASA NTRS1991

Two-dimensional particle displacement tracking in particle imaging velocimetry

A new particle imaging velocimetry data acquisition and analysis system, which is an order of magnitude faster than any previously proposed system, has been constructed and tested. The new particle displacement tracking (PDT) system is an all electronic technique employing a video camera and a large memory buffer frame-grabber board. Using a simple encoding scheme, a time sequence of single exposure images is time-coded into a single image and then processed to track particle displacements and determine two-dimensional velocity vectors. Use of the PDT technique in a counterrotating vortex flow produced over 1100 velocity vectors in 110 s when processed on an 80386 PC.

Wernet, Mark P.↗

NASA NTRS1991

Three-dimensional Navier-Stokes asymmetric solutions for cones and cone-cylinder configurations

The unsteady, compressible, thin-layer Navier-Stokes equations are used to obtain three-dimensional, asymmetric, vortex-flow solutions around cones and conecylinder configurations. The equations are solved using an implicit, upwind, flux-difference splitting, finite-volume scheme. The computational applications cover asymmetric flows around a 5 deg semiapex angle cone of unit length at various Reynolds number. Next, a cylindrical afterbody of various length is added to the conical forebody to study the effect of the length of cylindrical afterbody on the flow asymmetry. One of the computational solutions has been validated by comparing the computed surface pressure with those of the experimental data. All the asymmetric flow solutions have been obtained by using a short-duration side-slip disturbance.

Kandil, Osama A.↗

NASA NTRS1991

Wind tunnel investigation of aerodynamic and tail buffet characteristics of leading-edge extension modifications to the F/A-18

The impact of leading-edge extension (LEX) modifications on aerodynamic and vertical tail buffet characteristics of a 16-percent scale F/A-18 model has been investigated in the NASA Langley 30-foot by 60-foot tunnel. Modifications under consideration include variations in LEX chord and span, addition of upper surface fences, and removal of the LEX. Both buffeting and high-angle-of-attack aerodynamics are found to be strongly dependent upon the LEX geometry, which directly influences the strength, position, and breakdown characteristics of the vortex flow field. Concepts aimed at influencing the development of vortical flow field are considered to have much greater potential in design application than those geared toward altering already established flow fields. It is recommended that configuration effects on structural and aerodynamic characteristics be evaluated in parallel, so that trade-off studies can be conducted to ensure adequate structural fatigue life and desired high-angle-of-attack stability and control characteristics in the design of future high performance aircraft.

Shah, Gautam H.↗

NASA NTRS1991

Uranium droplet core nuclear rocket

Uranium droplet nuclear rocket is conceptually designed to utilize the broad temperature range ofthe liquid phase of metallic uranium in droplet configuration which maximizes the energy transfer area per unit fuel volume. In a baseline system dissociated hydrogen at 100 bar is heated to 6000 K, providing 2000 second of Isp. Fission fragments and intense radian field enhance the dissociation of molecular hydrogen beyond the equilibrium thermodynamic level. Uranium droplets in the core are confined and separated by an axisymmetric vortex flow generated by high velocity tangential injection of hydrogen in the mid-core regions. Droplet uranium flow to the core is controlled and adjusted by a twin flow nozzle injection system.

Anghaie, Samim↗

NASA NTRS1991

Turbomachinery noise

Summarized here are key advances in experimental techniques and theoretical applications which point the way to a broad understanding and control of turbomachinery noise. On the experimental side, the development of effective inflow control techniques makes it possible to conduct, in ground based facilities, definitive experiments in internally controlled blade row interactions. Results can now be valid indicators of flight behavior and can provide a firm base for comparison with analytical results. Inflow control coupled with detailed diagnostic tools such as blade pressure measurements can be used to uncover the more subtle mechanisms such as rotor strut interaction, which can set tone levels for some engine configurations. Initial mappings of rotor wake-vortex flow fields have provided a data base for a first generation semiempirical flow disturbance model. Laser velocimetry offers a nonintrusive method for validating and improving the model. Digital data systems and signal processing algorithms are bringing mode measurement closer to a working tool that can be frequently applied to a real machine such as a turbofan engine. On the analytical side, models of most of the links in the chain from turbomachine blade source to far field observation point have been formulated. Three dimensional lifting surface theory for blade rows, including source noncompactness and cascade effects, blade row transmission models incorporating mode and frequency scattering, and modal radiation calculations, including hybrid numerical-analytical approaches, are tools which await further application.

Groeneweg, John F.↗

NASA NTRS1991

Breaking down the delta wing vortex: The role of vorticity in the breakdown process

Experimental x-wire measurements of the flowfield above a 70 degree and 75 degree flat plate delta wing were performed at a Reynolds number of 250,000. Grids were taken normal to the wing at various chordwise locations for angles of attack of 20 degrees and 30 degrees. Axial and azimuthal vorticity distributions were derived from the velocity fields. The dependence of circulation on distance from the vortex core and on chordwise location was also examined. The effects of nondimensionalization in comparison with other experimental data is made. The results indicate that the circulation distribution scales with the local semispan and grows in a nearly linear fashion in the chordwise direction. The spanwise distribution of axial vorticity is severely altered through the breakdown region and the amount of vorticity present appears to reach a maximum immediately preceding breakdown. The axial velocity components with a negative sense, such as that found in the secondary vortex, seem to remain unaffected by changes in wing sweep or angle of attack, in direct contrast to the visible components. In addition, the inclusion of the local wing geometry into a previously derived correlation parameter allows the circulation of growing leading edge vortex flows to be reduced to a single curve.

Nelson, R. C.↗

NASA NTRS1992

Sensitivity analysis of hydrodynamic stability operators

The eigenvalue sensitivity for hydrodynamic stability operators is investigated. Classical matrix perturbation techniques as well as the concept of epsilon-pseudoeigenvalues are applied to show that parts of the spectrum are highly sensitive to small perturbations. Applications are drawn from incompressible plane Couette, trailing line vortex flow and compressible Blasius boundary layer flow. Parametric studies indicate a monotonically increasing effect of the Reynolds number on the sensitivity. The phenomenon of eigenvalue sensitivity is due to the non-normality of the operators and their discrete matrix analogs and may be associated with large transient growth of the corresponding initial value problem.

Schmid, Peter J.↗

NASA NTRS1991

Review of the physics of enhancing vortex lift by unsteady excitation

A review aimed at providing a physical understanding of the crucial mechanisms for obtaining super lift by means of unsteady excitations is presented. Particular attention is given to physical problems, including rolled-up vortex layer instability and receptivity, wave-vortex interaction and resonance, nonlinear streaming, instability of vortices behind bluff bodies and their shedding, and vortex breakdown. A general theoretical framework suitable for handling the unsteady vortex flows is introduced. It is suggested that wings with swept and sharp leading edges, equipped with devices for unsteady excitations, could yield the first breakthrough of the unsteady separation barrier and provide super lift at post-stall angle of attack.

Wu, J. Z.↗

NASA NTRS1992

Features of wavy vortices in a curved channel from experimental and numerical studies

Results are reported from an experimental study obtaining evidence of time-dependent, wavy vortex motions associated with undulating and twisting Dean vortices in a curved channel with 40-to-1 aspect ratio, and mild curvature (radius ratio = 0.979). The results are compared with direct numerical simulations of time-dependent 3D Navier-Stokes equations using boundary conditions in the spanwise and streamwise directions. When viewed in cross section, experimental visualizations of undulating and twisting vortex flows show rocking motion and changes in the direction of the flow between vortices that are like those observed in the simulations. Experimental spectra show that undulating vortices are replaced by the higher-frequency, shorter streamwise wavelength twisting vortices at higher Reynolds numbers. When undulating vortices are present, experimental power spectra and visualizations give frequencies that are somewhat lower than the most unstable frequencies predicted by linear stability analysis.

Ligrani, P. M.↗

NASA NTRS1991

Experiments with phase change thermal energy storage canisters for Space Station Freedom

The solar dynamic power module proposed for the Space Station Freedom (SSF) uses the heat of fusion of a phase change material (PCM) to efficiently store thermal energy for use during eclipse periods. The PCM, a LiF-20CaF2 salt, is contained in annular, metal canisters located in a heat receiver at the focus of a solar concentrator. PCM canister ground-based experiments and analytical heat transfer studies are discussed. The hardware, test procedures, and test results from these experiments are discussed. After more than 900 simulated SSF orbital cycles, no canister cracks or leaks were observed and all data were successfully collected. The effect of 1-g test orientation on canister wall temperatures was generally small while void position was strongly dependent on test orientation and canister cooling. In one test orientation, alternating wall temperature data were measured that supports an earlier theory of oscillating vortex flow in the PCM melt. Analytical canister wall temperatures compared very favorably with experimental temperature data. This illustrates that ground-based canister thermal performance can be predicted well by analyses that employ straight-forward, engineering models of void behavior and liquid PCM free convection.

Kerslake, Thomas W.↗

NASA NTRS1991

Adaptive domain decomposition for Monte Carlo simulations on parallel processors

A method is described for performing direct simulation Monte Carlo (DSMC) calculations on parallel processors using adaptive domain decomposition to distribute the computational work load. The method has been implemented on a commercially available hypercube and benchmark results are presented which show the performance of the method relative to current supercomputers. The problems studied were simulations of equilibrium conditions in a closed, stationary box, a two-dimensional vortex flow, and the hypersonic, rarefied flow in a two-dimensional channel. For these problems, the parallel DSMC method ran 5 to 13 times faster than on a single processor of a Cray-2. The adaptive decomposition method worked well in uniformly distributing the computational work over an arbitrary number of processors and reduced the average computational time by over a factor of two in certain cases.

Wilmoth, Richard G.↗

NASA NTRS1991

A general approach for the prediction of localized instability generation in boundary layer flows

The present approach to the prediction of instability generation that is due to the interaction of freestream disturbances with regions of subscale variations in surface boundary conditions can account for the finite Reynolds number effects, while furnishing a framework for the study of receptivity in compressible flow and in 3D boundary layers. The approach is illustrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer, due to the interaction of a freestream acoustic wave with a localized wall inhomogeneity. Results are presented for the generation of viscous and inviscid instabilities in adverse pressure-gradient boundary layers, supersonic boundary layer instabilities, and cross-flow vortex instabilities.

Choudhari, Meelan↗

NASA NTRS1993

Forebody vortex control on an F/A-18 using small, rotatable 'tip-strakes'

A mechanical scheme for manipulating the forebody vortices of an F/A-18, therefore creating controlled yawing moments at moderate and high angles of attack, was investigated in a wind tunnel. The technique consists of rotating miniature strakes (single or dual) about the radome centerline very close to the tip of the model. Forces, moments, and pressures were measured for angles of attack up to 60 deg and sideslip angles up to -10 deg. Results indicate that single and dual strakes can produce changes in side force and yawing moment, with magnitudes comparable to, or in some cases higher than, the directional changes produced by a 30-deg rudder deflection at 0-deg angle of attack. According to its circumferential position, the strake alters the separation location, inducing different degrees of asymmetry in the forebody vortex flow field that are translated into changes in side force and yawing moment. In comparison, the dual strakes appear to provide more gradual and better behaved changes than the single strake. Excellent correlation was found between this test and water tunnel tests performed on a similar configuration.

Suarez, Carlos J.↗

NASA NTRS1947

Analysis of Performance of Jet Engine from Characteristics of Components I : Aerodynamic and Matching Characteristics of Turbine Component Determined with Cold Air

The performance of the turbine component of an NACA research jet engine was investigated with cold air. The interaction and the matching of the turbine with the NACA eight-stage compressor were computed with the combination considered as a jet engine. The over-all performance of the engine was then determined. The internal aerodynamics were studied to the extent of investigating the performance of the first stator ring and its influence on the turbine performance. For this ring, the stream-filament method for computing velocity distribution permitted efficient sections to be designed, but the design condition of free-vortex flow with uniform axial velocities was not obtained.

Goldstein, Arthur W↗

NASA NTRS1994

Wind tunnel measurements on a full-scale F/A-18 with a tangentially blowing slot

A full-scale F/A-18 was tested in the 80 by 120-Foot Wind Tunnel at NASA Ames Research Center to measure the effectiveness of a tangentially blowing slot in generating significant yawing moments while minimizing coupling in the pitch and roll axes. Various slot configurations were tested to determine the optimum configuration. The test was conducted for angles of attack from 25 to 50 deg, angles of sideslip from -15 to +15 deg, and freestream velocities from 67 ft/sec to 168 ft/sec. By altering the forebody vortex flow, yaw control was maintained for angles of attack up to 50 deg. Of particular interest was the result that blowing very close to the radome apex was not as effective as blowing slightly farther aft on the radome, that a 16-inch slot was more efficient, and that yawing moments were generated without inducing significant rolling or pitching moments.

Lanser, Wendy R.↗