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At least 1,027 records · Page 57

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NASA NTRS1996

Aerodynamic Requirements for BVI Noise Control

As a rotor blade moves through the air, it sheds vortices. These vortices shed along the length of the blade over time form the wake. The strongest vortices of the wake are those trailing from the tip of the blade. When a rotating blade system moves under certain operating conditions, each blade will impinge on the tip vortices shed by itself or other blades. This impingement is called a blade-vortex interaction, or BVI. Although the blade and trailing tip vortices interact with many different orientations, one of the two extremes, either parallel or perpendicular interaction, is usually modelled. In a perpendicular interaction, the portion of the blade that is actually interacting with the travelling vortex at any given time is very small. A parallel interaction, however, has the largest concurrent interaction with the blade, as a result this case is given the most attention. One of the most commonly studied occurrences of blade-vortex interactions is associated with low-speed descending rotorcraft flight. BVI occur when the tip vortices shed by the blades intersect the plane of the rotor. BVI cause local pressure changes over the blades which are responsible, in part, for the acoustic signature of the rotorcraft. The local pressure changes also cause vibrations which lead to fatigue of both the blades and the mechanical components driving the blades.

Wells, Valana L.↗

NASA NTRS2001

Calculation of the Aerodynamic Behavior of the Tilt Rotor Aeroacoustic Model (TRAM) in the DNW

Comparisons of measured and calculated aerodynamic behavior of a tiltrotor model are presented. The test of the Tilt Rotor Aeroacoustic Model (TRAM) with a single, 1/4-scale V- 22 rotor in the German-Dutch Wind Tunnel (DNW) provides an extensive set of aeroacoustic, performance, and structural loads data. The calculations were performed using the rotorcraft comprehensive analysis CAMRAD II. Presented are comparisons of measured and calculated performance and airloads for helicopter mode operation, as well as calculated induced and profile power. An aerodynamic and wake model and calculation procedure that reflects the unique geometry and phenomena of tiltrotors has been developed. There are major differences between this model and the corresponding aerodynamic and wake model that has been established for helicopter rotors. In general, good correlation between measured and calculated performance and airloads behavior has been shown. Two aspects of the analysis that clearly need improvement are the stall delay model and the trailed vortex formation model.

Johnson, Wayne↗

NASA NTRS2001

Airloads and Wake Geometry Calculations for an Isolated Tiltrotor Model in a Wind Tunnel

Comparisons of measured and calculated aerodynamic behavior of a tiltrotor model are presented. The test of the Tilt Rotor Aeroacoustic Model (TRAM) with a single, 0.25-scale V-22 rotor in the German-Dutch Wind Tunnel (DNW) provides an extensive set of aeroacoustic, performance, and structural loads data. The calculations were performed using the rotorcraft comprehensive analysis CAMRAD II. Presented are comparisons of measured and calculated performance for hover and helicopter mode operation, and airloads for helicopter mode. Calculated induced power, profile power, and wake geometry provide additional information about the aerodynamic behavior. An aerodynamic and wake model and calculation procedure that reflects the unique geometry and phenomena of tiltrotors has been developed. There are major differences between this model and the corresponding aerodynamic and wake model that has been established for helicopter rotors. In general, good correlation between measured and calculated performance and airloads behavior has been shown. Two aspects of the analysis that clearly need improvement are the stall delay model and the trailed vortex formation model.

Johnson, Wayne↗

NASA NTRS1985

Steady and unsteady incompressible free-wake analysis

The flows around highly sweptback wings and bodies of revolution at high angle of attack are described, and inviscid model approximations and mathematical formulation of the problem are given to steady and unsteady incompressible flows. A general presentation of the methods of solution is given, with emphasis on current computational techniques. Detailed descriptions of the nonlinear vortex-lattice and vortex-panel techniques are presented to show how the boundary conditions are enforced using iteration. Typical numerical results are compared with the available experimental data.

Kandil, O. A.↗

NASA NTRS1981

A vortex-lattice method for calculating lifting-surface interference

The interference effect of closely coupled lifting surfaces was investigated. A nonsteady vortex-lattice method was applied to calculate the steady-state and nonsteady lift characteristics of configurations having interference effects. The configurations consist of low-aspect-ratio delta wings, at low and high angles of attack. Leading-edge separation and wake roll-up are simulated by a time-dependent wake-shedding procedure. For steady-state flow conditions, this numerical procedure saves a considerable amount of computer time, compared with iterative methods, and yields the same results. A better understanding of the interference effect can be gained by using the method to study the transient behavior. The nonsteady approach also offers the capability of calculating various nonsteady motions, as is demonstrated in the calculation of the longitudinal damping.

Levin, D.↗

NASA NTRS1986

Transient induced drag

In the present treatment of the calculation of forces on a wing that is suddenly brought into motion at a constant speed, attention is given to the unsteady potential's contribution to the force balance. Total bound vorticity is produced at the initial impulse. The results obtained are independent of wing aspect ratio; as time increases, this effect on the drag force becomes smaller as the vortex emanating from the trailing edge is left behind. The second contributor to induced drag is the spanwise vorticity shedding that results from the spanwise load distribution of three-dimensional wings. This contribution grows with time as the length of the wake grows.

Weihs, D.↗

NASA NTRS2012

NASA Trapezoidal Wing Computations Including Transition and Advanced Turbulence Modeling

Flow about the NASA Trapezoidal Wing is computed with several turbulence models by using grids from the first High Lift Prediction Workshop in an effort to advance understanding of computational fluid dynamics modeling for this type of flowfield. Transition is accounted for in many of the computations. In particular, a recently-developed 4-equation transition model is utilized and works well overall. Accounting for transition tends to increase lift and decrease moment, which improves the agreement with experiment. Upper surface flap separation is reduced, and agreement with experimental surface pressures and velocity profiles is improved. The predicted shape of wakes from upstream elements is strongly influenced by grid resolution in regions above the main and flap elements. Turbulence model enhancements to account for rotation and curvature have the general effect of increasing lift and improving the resolution of the wing tip vortex as it convects downstream. However, none of the models improve the prediction of surface pressures near the wing tip, where more grid resolution is needed.

Rumsey, C. L.↗

NASA NTRS1982

Three-dimensional stability of vortex arrays

The stability to three-dimensional disturbances of three classical steady vortex configurations in an incompressible inviscid fluid is studied in the limit of small vortex cross-sectional area and long axial disturbance wavelength. The configurations examined are the single infinite vortex row, the Karman vortex street of staggered vortices and the symmetric vortex street. It is shown that the single row is most unstable to a two-dimensional disturbance, while the Karman vortex street is most unstable to a three-dimensional disturbance over a significant range of street spacing ratios. The symmetric vortex street is found to be most unstable to three-dimensional or two-dimensional symmetric disturbances depending on the spacing ratio of the street. Short remarks are made concerning the relevance of the calculations to the observed instabilities in free shear layer, wake and boundary-layer type flows.

Robinson, A. C.↗

NASA NTRS1992

Direct numerical simulations of a spatially developing plane wake

In the present paper, direct numerical methods by which to simulate the spatially developing free shear flows in the transitional region are described and the numerical results of a spatially developing plane wake are presented. The incompressible time-dependent Navier-Stokes equations were solved using Pade finite difference approximations in the streamwise direction, a mapped pseudospectral Fourier method in the cross-stream direction, and a third-order compact Runge-Kutta scheme for time advancement. The unstable modes of the Orr-Sommerfeld equations were used to perturb the inlet of the wake. Statistical analyses were performed and some numerical results were compared with experimental measurements. When only the fundamental mode is forced, the energy spectra show amplification of the fundamental and its higher harmonics. In this case, unperturbed alternate vortices develop in the saturation region of the wake. The phase jitter around the fundamental frequency plays a critical role in generating vortices of random shape and spacing. Large- and small-scale distortions of the fundamental structure are observed. Pairing of vortices of the same sign is observed, as well as vortex coupling of vortices of the opposite sign.

Maekawa, Hiroshi↗

NASA NTRS1986

Visualization and flow surveys of the leading edge vortex structure on delta wing planforms

In the present experimental investigation of thin delta wing vortex breakdown, for the cases of sweep angles of 70, 75, 80, and 85 deg, and smoke flow visualization/laser light sheet technique is used to obtain cross sectional views of the leading edge vortices as they break down. A combination of lateral and longitudinal cross sectional views furnishes data on the three-dimensional character of the vortex before, during, and after breakdown. Velocity measurements conducted with a laser Doppler anemometer on the 70 deg sweep delta, at 30 deg angle-of-attack, indicate that when breakdown occurs the core flow is transformed from a jet-like to a wake-like flow.

Payne, F. M.↗

NASA NTRS1990

Parametric analysis of swept-wing geometry with sheared wing tips

A computational parameter study is presented of potential reductions in induced drag and increases in lateral-directional stability due to sheared wing tips attached to an untwisted wing of moderate sweep and aspect ratio. Sheared tips are swept and tapered wing-tip devices mounted in the plane of the wing. The induced-drag results are obtained using an inviscid, incompressible surface-panel method that models the nonlinear effects due to the deflected and rolled-up wake behind the lifting surface. The induced-drag results with planar sheared tips are compared to straight-tapered tip extensions and nonplanar winglet geometries. The lateral-directional static-stability characteristics of the wing with sheared tips are estimated using a quasi-vortex-lattice method. For certain combinations of sheared-tip sweep and taper, both the induced efficiency of the wing and the relevant static-stability derivatives are predicted to increase compared to the wing with a straight-tapered tip modification.

Fremaux, C. M.↗

NASA NTRS1973

Rotating-blade vortex noise

An experimental investigation has been made of the Langley full-scale tunnel and outdoors to investigate some of the characteristics of vortex noise generated on a rotating-blade system. Acoustic measurements were made at several microphone positions for two different blade sections with several tip shapes and spoiler configurations. The blades were operated only at zero lift at each radial station, both for operating in their own wake and for operating with the wake blown downstream. Rotors with cylindrical blades generally created more noise throughout the noise spectrum than the rotor with NACA 0012 blades. Blowing the shed wake from the rotor with cylindrical blades did not have an appreciable effect on the frequency-amplitude spectrum. The tip shape changes had very little effect on the frequency-amplitude spectrum of the noise. Spoilers applied to the rotor with NACA 0012 blades increased the amplitude of the spectrum and decreased the number of harmonics of blade passage frequency.

Scheiman, J.↗

NASA NTRS1987

Vortex/boundary layer interaction

The second test case (longitudinal vortex above, but not merging with, a turbulent boundary layer) was investigated with flow visualization studies, and photographs selected. The results of quantitative data acquisition for the non-merging test case are presented. Other work in progress includes further flow visualization of the delta-wing wake. Considerable effort is being devoted to the development of graphical output routines.

Bradshaw, P.↗

DOE OSTI2024

A generalized wind turbine cross section as a reduced-order model to gain insights in blade aeroelastic challenges

In this work, we present an approach to study the aeroelastic stability of a wind turbine by focusing on the dynamics of a blade cross section. We present a methodology to obtain a reduced-order model of the blade dynamics in the form of generalized cross-sectional quantities that approximates the aerodynamic and structural properties of the full blade. The motivation for the work is to gain a physical understanding of the influence of aerodynamic models such as dynamic wake and dynamic stall on the frequency and damping of the structure using a reduced-order model with low computational cost. The model may be coupled to two-dimensional computational fluid dynamics softwares or engineering unsteady airfoil aerodynamics models accounting for dynamic wake and dynamic stall. In the latter case, we can obtain monolithic state-space forms of the aeroelastic system of equations, which simplifies the determination of the modal parameters and therefore the study of stability. The work investigates wind turbines in operation or at standstill, where vortex-induced vibrations and stall-induced vibrations, respectively, might be an issue. The implementation is made available as part of the open-source Python package WELIB and as part of the open-source unsteady aerodynamic driver of OpenFAST.

17 WIND ENERGY↗

NASA NTRS2021

The Causes of Propeller Pitching Moment and the Conditions for its Significance

Recent development of vertical takeoff and landing (VTOL) aircraft has renewed interest in the study of propellers. One metric in particular, the propeller pitching moment, has been observed to be important to VTOL aircraft stability and control in the past. Propellers at angles of attack could not be accurately modeled in generations past due to a lack of computational power, but even with advances in computer technology, modern designers seem to possess insufficient knowledge in this area. In this dissertation, we study the physics behind propeller pitching moment in the context of an isolated propeller and a propeller upstream of a wing. An unsteady 3D vortex lattice method is developed specifically to model propellers at angles of attack and is validated by comparing to high-fidelity CFD analyses. We then use the model to isolate velocity influences to show that the propeller pitching moment is largely caused by two effects: a skewed wake and the presence of wing circulation. Generated maps of propeller pitching moment over a range of operational parameters corresponding to VTOL transition show that the low flight speeds and high angles of attack encountered during transition lead to significant magnitudes of propeller pitching moment that would be difficult to trim using passive methods. Also, derivation of a generalizable metric of significance shows that the peak contribution of propeller pitching moment to aircraft stability is comparable to a longitudinal displacement of the center of gravity by several percent of the wing chord. Finally, we give a concluding discussion on the impact of propeller pitching moment on VTOL aircraft design.

Xiaofan Fei↗

NASA NTRSDate not supplied

Quantitative Approach for the Accurate CFD Simulation of Hover in Turbulent Flow

Time-dependent Navier-Stokes simulations have been carried out for a V22 rotor in hover using an improved, low-diffusion, HLLE++ upwind algorithm in the OVERFLOW code. Emphasis is placed on lessons learned over the past decade regarding the effects of high-order spatial accuracy, grid resolution, and the use of detached eddy simulation in predicting the rotor performance, i.e., figure-of-merit (FM). A quick-start procedure is described together with a statistical measure of FM convergence that reduces hover computations by 5 times, similar in computational work for forward flight. Furthermore, Cartesian adaptive mesh refinement in the rotor wake revealed a complex turbulent flow with worm-like structures of various scales. These turbulent worms, found computationally more than a decade ago, have recently been observed in a separate German Aerospace Center (DLR) experiment. Moreover, adaptive mesh refinement has been used to resolve the tip-vortex to its correct physical size.

Neal M. Chaderjian↗

DOE OSTI2024

Vortex bursting and associated twist dynamics on helical vortex tubes and vortex rings

The interaction of opposite-signed twist waves on vortex tubes can lead to vortex bursting, a process where the core expands into a double ring-like structure with strong swirling flows. Previous works have studied vortex bursting on rectilinear vortices by axially perturbing the initial core size to generate the twist waves, and observed largely axisymmetric bursting dynamics. In this work, we numerically study bursting on vortical structures with curved centrelines, analysing the interaction between the centreline dynamics, twist wave generation and propagation, and vortex bursting. We focus on axially perturbed helical vortex tubes with small radius-to-pitch ratios up to$0.0625$, as well as vortex rings with a large radius-to-core size ratio$10$, both at a circulation-based Reynolds number$5000$. The results show that though the initial twist wave propagation speeds are relatively unaffected by the curvature and torsion of the centreline, the bursting process is altered significantly compared with rectilinear vortices. The self-induced rotation of the centreline of the helical tube induces a non-axisymmetric distortion of the bursting structure, which rapidly breaks up the vortex core into small-scale helical structures. A similar destabilization of the bursting structure also occurs on vortex rings. The enstrophy increase and accelerated energy decay associated with bursting are predominantly determined by the twist wave strength, rather than the curvature and torsion of the centreline. Combined, our findings imply that bursting could play an important role in transferring and dissipating energy of vortical structures in wakes, and turbulent flows in general.

Mechanics↗

NASA NTRS1988

Research investigation of helicopter main rotor/tail rotor interaction noise

Acoustic measurements were obtained in a Langley 14 x 22 foot Subsonic Wind Tunnel to study the aeroacoustic interaction of 1/5th scale main rotor, tail rotor, and fuselage models. An extensive aeroacoustic data base was acquired for main rotor, tail rotor, fuselage aerodynamic interaction for moderate forward speed flight conditions. The details of the rotor models, experimental design and procedure, aerodynamic and acoustic data acquisition and reduction are presented. The model was initially operated in trim for selected fuselage angle of attack, main rotor tip-path-plane angle, and main rotor thrust combinations. The effects of repositioning the tail rotor in the main rotor wake and the corresponding tail rotor countertorque requirements were determined. Each rotor was subsequently tested in isolation at the thrust and angle of attack combinations for trim. The acoustic data indicated that the noise was primarily dominated by the main rotor, especially for moderate speed main rotor blade-vortex interaction conditions. The tail rotor noise increased when the main rotor was removed indicating that tail rotor inflow was improved with the main rotor present.

Fitzgerald, J.↗