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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Wake Geometry Effects on Rotor Blade-Vortex Interaction Noise Directivity

Acoustic measurements from a model rotor wind tunnel test are presented which show that the directionality of rotor blade vortex interaction (BVI) noise is strongly dependent on the rotor advance ratio and disk attitude. A rotor free wake analysis is used to show that the general locus of interactions on the rotor disk is also strongly dependent on advance ratio and disk attitude. A comparison of the changing directionality of the BVI noise with changes in the interaction locations shows that the strongest noise radiation occurs in the direction of motion normal to the blade span at the time of interaction, for both advancing and retreating side BVI. For advancing side interactions, the BVI radiation angle down from the tip-path plane appears relatively insensitive to rotor operating condition and is typically between 40 and 55 deg below the disk. However, the azimuthal radiation direction shows a clear trend with descent speed, moving towards the right of the flight path with increasing descent speed. The movement of the strongest radiation direction is attributed to the movement of the interaction locations on the rotor disk with increasing descent speed.

Martin, R. M.↗
NASA NTRS1996

Blunt body near wake flow field at Mach 6

Tests were conducted in a Mach 6 flow to examine the reattachment process of an axisymmetric free shear layer associated with the near wake of a 70 deg. half angle, spherically blunted cone with a cylindrical after body. Model angle of incidence was fixed at 0 deg. and free-stream Reynolds numbers based on body diameter ranged from 0.5 x 10(exp 6) to 4 x 10(exp 6). The sensitivity of wake shear layer transition on reattachment heating was investigated. The present perfect gas study was designed to compliment results obtained previously in facilities capable of producing real gas effects. The instrumented blunted cone model was designed primarily for testing in high enthalpy hypervelocity shock tunnels in both this country and abroad but was amenable for testing in conventional hypersonic blowdown wind tunnels as well. Surface heating rates were inferred from temperature - time histories from coaxial surface thermocouples on the model forebody and thin film resistance gages along the model base and cylindrical after body. General flow feature (bow shock, wake shear layer, and recompression shock) locations were visually identified by schlieren photography. Mean shear layer position and growth were determined from intrusive pitot pressure surveys. In addition, wake surveys with a constant temperature hot-wire anemometer were utilized to qualitatively characterize the state of the shear layer prior to reattachment. Experimental results were compared to laminar perfect gas predictions provided by a 3-D Navier Stokes code (NSHYP). Shear layer impingement on the instrumented cylindrical after body resulted in a localized heating maximum that was 21 to 29 percent of the forebody stagnation point heating. Peak heating resulting from the reattaching shear layer was found to be a factor of 2 higher than laminar predictions, which suggested a transitional shear layer. Schlieren flow visualization and fluctuating voltage time histories and spectra from the hot wire surveys across the shear layer substantiate this observation. The sensitivity of surface heating to forebody roughness was characterized for a reattaching shear layer. For example, at R(sub infinity), d = 4 x 10(exp 6), when the shear layer was transitional, the magnitude of peak heating from shear layer impingement was reduced by approximately 24 percent when transition grit was applied to the forebody. The spatial location of the local peak, however, remained unchanged.

Horvath, Thomas J.↗
NASA NTRS1998

Direct Numerical Simulations of Boundary Layer Transition on a Flat Plate

In recent years the techniques of computational fluid dynamics (CFD) have been used to compute flows associated with geometrically complex configurations. However, success in terms of accuracy and reliability has been limited to cases where the effects of turbulence and transition could be modeled in a straightforward manner. Even in simple flows, the accurate computation of skin friction and heat transfer using existing turbulence models has proved to be a difficult task, one that has required extensive fine-tuning of the turbulence models used. In more complex flows (for example, in turbomachinery flows in which vortices and wakes impinge on airfoil surfaces causing periodic transitions from laminar to turbulent flow) the development of a model that accounts for all scales of turbulence and predicts the onset of transition is an extremely difficult task. Fortunately, current trends in computing suggest that it may be possible to perform direct simulations of turbulence and transition at moderate Reynolds numbers in some complex cases in the near future. This presentation will focus on direct simulations of transition and turbulence using high-order accurate finite-difference methods. The advantage of the finite-difference approach over spectral methods is that complex geometries can be treated in a straightforward manner. Additionally, finite-difference techniques are the prevailing methods in existing application codes. An application of high-order-accurate finite-difference methods to direct simulations of transition and turbulence in a spatially evolving boundary layer subjected to high levels of freestream turbulence will be presented.

Rai, Man Mohan↗
NASA NTRS1997

Direct Simulations of Transition and Turbulence Using High-Order Accurate Finite-Difference Schemes

In recent years the techniques of computational fluid dynamics (CFD) have been used to compute flows associated with geometrically complex configurations. However, success in terms of accuracy and reliability has been limited to cases where the effects of turbulence and transition could be modeled in a straightforward manner. Even in simple flows, the accurate computation of skin friction and heat transfer using existing turbulence models has proved to be a difficult task, one that has required extensive fine-tuning of the turbulence models used. In more complex flows (for example, in turbomachinery flows in which vortices and wakes impinge on airfoil surfaces causing periodic transitions from laminar to turbulent flow) the development of a model that accounts for all scales of turbulence and predicts the onset of transition may prove to be impractical. Fortunately, current trends in computing suggest that it may be possible to perform direct simulations of turbulence and transition at moderate Reynolds numbers in some complex cases in the near future. This seminar will focus on direct simulations of transition and turbulence using high-order accurate finite-difference methods. The advantage of the finite-difference approach over spectral methods is that complex geometries can be treated in a straightforward manner. Additionally, finite-difference techniques are the prevailing methods in existing application codes. In this seminar high-order-accurate finite-difference methods for the compressible and incompressible formulations of the unsteady Navier-Stokes equations and their applications to direct simulations of turbulence and transition will be presented.

Rai, Man Mohan↗
NASA NTRS1997

Direct Numerical Simulations of Boundary Layer Transition on a Flat Plate

In recent years the techniques of computational fluid dynamics (CFD) have been used to compute flows associated with geometrically complex configurations. However, success in terms of accuracy and reliability has been limited to cases where the effects of turbulence and transition could be modeled in a straightforward manner. Even in simple flows, the accurate computation of skin friction and heat transfer using existing turbulence models has proved to be a difficult task, one that has required extensive fine-tuning of the turbulence models used. In more complex flows (for example, in turbomachinery flows in which vortices and wakes impinge on airfoil surfaces causing periodic transitions from laminar to turbulent flow) the development of a model that accounts for all scales of turbulence and predicts the onset of transition is an extremely difficult task. Fortunately, current trends in computing suggest that it may be possible to perform direct simulations of turbulence and transition at moderate Reynolds numbers in some complex cases in the near future. This presentation will focus on direct simulations of transition and turbulence using high-order accurate finite-difference methods. The advantage of the finite-difference approach over spectral methods is that complex geometries can be treated in a straightforward manner. Additionally, finite-difference techniques are the prevailing methods in existing application codes. An application of accurate finite-difference methods to direct simulations of transition and turbulence in a spatially evolving boundary layer subjected to high levels of freestream turbulence will be presented.

Rai, Man Mohan↗
NASA NTRS1984

Martian crater dark streak lengths - Explanation from wind tunnel experiments

The mechanism determining the length of dark erosional (type-Id) streaks near craters and hills on the surface of Mars is investigated using Mariner-9 images and the results of wind-tunnel studies (Greeley et al., 1974; Iversen et al., 1975 and 1976). An empirical model of time-dependent growth of crater-wake streaks in the downwind direction involving deflation by sand-grain saltation is developed and shown to give relatively good agreement with streak growth rates observed over a 38-day period of Mariner observations and with the streak-length/crater-diameter and streak-length/crater-height ratios analyzed statistically by Lee (1984). This time-dependent mechanism is thus considered a reasonable alternative (for dark streaks) to the blocking model proposed by Lee for both bright and dark streaks. Preliminary analysis of further wind-tunnel experiments (Iversen et al., 1982) supports the blocking model of bright-streak formation.

Iversen, J. D.↗
NASA NTRS1972

Prediction of far flow field in trailing vortices.

A finite-difference machine code is brought to bear on the wake-vortex problem in the quasi-cylindrical boundary-layer approximation. A turbulent-energy model containing new features is developed. Parameters of the model are evaluated by comparison of calculated velocities and turbulent intensities with measurements in an axisymmetric wake. Comparisons are made with a previous calculation of the decay of an isolated vortex and with wind tunnel and flight measurements in trailing vortices. A self-similar solution develops at large axial distance that decays with the square root of distance. A slower decay occurs in the preceding transition region.

Baldwin, B. S.↗
NASA NTRS1976

Prediction of longitudinal aerodynamic characteristics of STOL configurations with externally blown high lift devices

A theoretical method has been developed to predict the longitudinal aerodynamic characteristics of engine-wing-flap combinations with externally blown flaps (EBF) and upper surface blowing (USB) high lift devices. Potential flow models of the lifting surfaces and the jet wake are combined to calculate the induced interference of the engine wakes on the lifting surfaces. The engine wakes may be circular, elliptic, or rectangular cross-sectional jets, and the lifting surfaces are comprised of a wing with multiple-slotted trailing-edge flaps or a deflected trailing-edge Coanda surface. Results are presented showing comparisons of measured and predicted forces, pitching moments, span-load distributions, and flow fields.

Mendenhall, M. R.↗
NASA NTRS1982

Calculation of pressure statistics in turbulent free shear flows by direct numerical simulation

Accurate numerical simulations of turbulent free shear flows were performed in order to study the behavior of certain critical turbulence quantities which have been difficult to measure in the laboratory. The three-dimensional time-dependent nonlinear Navier-Stokes equations are solved by the direct numerical simulation technique without turbulence modeling or ad hoc closure assumptions. The pressure interaction terms were factored into diffusion and return-to-isotropy components according to two models. Simulations of the evolution of a turbulent wake with and without swirl present were performed at sufficiently high Reynolds numbers to insure that the flows were strongly nonlinear and turbulent, but low enough so that all scales of motion containing significant energy are adequately resolved. It is noted that the presence of swirl tends to enhance the amplitude of the rms pressure levels.

Metcalfe, R. W.↗
NASA NTRS1985

Viscous flows in rotating pipes with internal cavities and related vortex breakdowns

The problem of predicting vortex breakdown, which is characterized by a drastic change in core size and trajectory, is still basically unsolved. In the present paper, viscous flow in a long rotating pipe is analyzed, as a model for the type of breakdown observed in wake vortices. The full Navier-Stokes equations are shown to simplify to a set of unlinked partial differential equations. These equations allow for two separate solutions, for both full pipe flows (the classic rotating Hagen-Poiseuille flow) and cavity flows, in which a central area, with no net mass flux or viscous dissipation, is produced. Conditions permitting switching from one type of flow to the other, with no change in the mass flux or velocities at the pipe surface, are found. It is then argued that these conditions indicate when vortex breakdown will occur.

Weihs, D.↗
NASA NTRS1986

Gust response of hingeless rotors

The gust response of a coupled hingeless rotor-fuselage system is studied in both hover and forward flight. Each rotor blade undergoes flap bending, lag bending, and torsional deflections. The blades are discretized into beam elements, each with fifteen nodal degrees of freedom. The fuselage is permitted three translational (vertical, longitudinal, and lateral) and two rotational (pitch and roll) degrees of freedom. The formulation considers a three-dimensional gust field wherein each gust velocity component can have an arbitrary variation in space and time. Aerodynamic loads are obtained using quasisteady strip theory. Wake-induced effects are introduced through dynamic inflow modeling. Dynamic stall and reverse flow effects are also included. Equations governing the rotor-fuselage gust response are linearized about the vehicle propulsive trim state and the blade steady-state defected positions, and solved by time integration. The effects of several parameters on the helicopter gust response are illustrated, including dynamic inflow, lag stiffness, forward speed, gust profile, gust penetration rate, and gust velocity direction.

Bir, G. S.↗
NASA NTRS1987

Far-infrared and submillimeter observations of interstellar clouds

Research in the use of FIR and sub-mm lines to provide data relevant to astrochemistry is examined. Spectroscopic features in the FIR and sub-mm are dominated by transitions from species of atoms and atomic ions, metal hydrides, and heavy molecules. The data of Lester et al.(1983) and Dinerstein et al. (1984) on the N/O abundance ratio are discussed. The N/O abundance ratio is analyzed as a function of the galactic radius, and the procedure for determining the N/O abundance ratio from the FIR and sub-mm lines is given. The conditions at the surface of molecular clouds and the chemical profile of molecular clouds are studied in the forbidden lines O I 63.18 and 145.53 microns and the forbidden line C II 157.74 microns. Results of recent FIR/sub-mm observations of post-shocked gas which relate to modeling the chemical processing that occurs in the wake of shock waves are considered.

Melnick, Gary↗
NASA NTRS1987

Helicopter rotor induced velocities theory and experiment

An investigation has been performed to assess methods used for rotor inflow modeling. A key element of this assessment has been the recent acquisition of high quality experimental measurements of inflow velocities taken in the proximity of a lifting rotor in forward flight. Widely used rotor performance predictive methods are based on blade element strip theory coupled with an inflow model. The inflow prediction models assessed in this paper include the uniform inflow based on momentum, a skewed disk model, and two methods based on a vortex wake structure.

Berry, John D.↗
NASA NTRS1992

Vortex cloud model for body vortex shedding and tracking

The present engineering prediction method addresses the aerodynamic characteristics and associated flowfields of circular and noncircular bodies at large incidence angles and arbitrary roll angles. 3D attached-flow models are used to represent the body, and 2D, incompressible, separated flow models are used to represent the separated vortex wake. The predicted pressure distribution of the body, under the influence of both the freestream and the separation vortex wake, is used to calculate aerodynamic loads. The separation vortex wake is represented by clouds of discrete vortices in crossflow planes normal to the body axis.

Mendenhall, Michael R.↗
NASA NTRS1993

Wind tunnel testing and research

The topics covered include the following: a test of a Space Shuttle model with various base enhancements intended to help reduce the drag of the Shuttle in the landing configuration; further tests conducted using a Shuttle model to explore base drag improvement techniques; testing of parafoils having different airfoil shapes and line lengths; a study of ground effects on a Shuttle model; and three dimensional velocity profiles in the wake aft of the Space Shuttle were determined to study the effects of wake velocities on the Orbiter drag chute.

Nicks, Oran W.↗
NASA NTRS1999

Study of the Transition Flow Regime using Monte Carlo Methods

This NASA Cooperative Agreement presents a study of the Transition Flow Regime Using Monte Carlo Methods. The topics included in this final report are: 1) New Direct Simulation Monte Carlo (DSMC) procedures; 2) The DS3W and DS2A Programs; 3) Papers presented; 4) Miscellaneous Applications and Program Modifications; 5) Solution of Transitional Wake Flows at Mach 10; and 6) Turbulence Modeling of Shock-Dominated Fows with a k-Enstrophy Formulation.

Hassan, H. A.↗
NASA NTRS2017

Anticipated Electrical Environment at Phobos: Nominal and Solar Storm Conditions

A passing coronal mass ejection (CME) will manifest a different response at an airless body compared to a magnetized planet. Specifically,because the regolith-rich surfaces of airless bodies are directly exposed to the variations in the plasma flow, the surfaces are found to undergo anomalous surface charging during the passing of CME fast plasma events. In this study, we model the surface charging expected at Phobos for nominal solar wind conditions and also those associated with disturbed solar wind conditions during the passage of a CME similar to that observed by MAVEN at Mars in early March 2015. We use an ambipolar diffusion model to examine the development of the trailing wake void in the plasma flow behind Phobos and the formation of mini-wakes within obstruction regions like Stickney Crater. We also consider the roving of an astronaut in Stickney Crater for Phobos positioned near 10 hours Local Time relative to Mars. We examine the plasma dissipation of the collected astronaut charge from contact electrification with the regolith.

Phobos; Surface chargin↗
NASA NTRSDate not supplied

High-Fidelity Analysis of Lift+Cruise VTOL Urban Air Mobility Concept Aircraft

This paper presents a high-fidelity multidisciplinary analysis for the NASA lift+cruise vertical takeoff and landing urban air mobility concept aircraft. The reported simulations couple comprehensive rotorcraft aeromechanics and high-fidelity computational fluid dynamics (CFD). Aerodynamic solutions are computed on dynamic, deforming, unstructured, overset grid systems by an unsteady CFD solver, FUN3D, developed at the NASA Langley Research Center. An integrated overset-grid assembler, Yoga, is used for communications between component grids. Two turbulence models are compared for loose-coupling simulations in hover and low-speed forward flight conditions. One model is the negative variant of the one-equation Spalart-Allmaras model (SA-neg), and the other model is the SA-neg-R model adding a simple rotation correction term to the SA-neg model. It demonstrated that the rotation correction in the SA-neg-R turbulence model significantly improves resolution of secondary vortices and wake interactions for rotorcraft simulations.

CFD↗