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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Atmospheric turbulence parameters for modeling wind turbine dynamics

A model which can be used to predict the response of wind turbines to atmospheric turbulence is given. The model was developed using linearized aerodynamics for a three-bladed rotor and accounts for three turbulent velocity components as well as velocity gradients across the rotor disk. Typical response power spectral densities are shown. The system response depends critically on three wind and turbulence parameters, and models are presented to predict desired response statistics. An equation error method, which can be used to estimate the required parameters from field data, is also presented.

Holley, W. E.↗
NASA NTRS1985

Diverter Lip for Seal

Proposed one-piece design change for cooling turbine rotor disks eliminates temperature differential that normally causes thermal stresses and cracks.

Jackson, E. D., III↗
NASA NTRS1983

Helicopter rotor wake geometry and its influence in forward flight. Volume 1: Generalized wake geometry and wake effect on rotor airloads and performance

An analytic investigation to generalize wake geometry of a helicopter rotor in steady level forward flight and to demonstrate the influence of wake deformation in the prediction of rotor airloads and performance is described. Volume 1 presents a first level generalized wake model based on theoretically predicted tip vortex geometries for a selected representative blade design. The tip vortex distortions are generalized in equation form as displacements from the classical undistorted tip vortex geometry in terms of vortex age, blade azimuth, rotor advance ratio, thrust coefficient, and number of blades. These equations were programmed to provide distorted wake coordinates at very low cost for use in rotor airflow and airloads prediction analyses. The sensitivity of predicted rotor airloads, performance, and blade bending moments to the modeling of the tip vortex distortion are demonstrated for low to moderately high advance ratios for a representative rotor and the H-34 rotor. Comparisons with H-34 rotor test data demonstrate the effects of the classical, predicted distorted, and the newly developed generalized wake models on airloads and blade bending moments. Use of distorted wake models results in the occurrence of numerous blade-vortex interactions on the forward and lateral sides of the rotor disk. The significance of these interactions is related to the number and degree of proximity to the blades of the tip vortices. The correlation obtained with the distorted wake models (generalized and predicted) is encouraging.

Egolf, T. A.↗
NASA NTRS1986

Toward comparing experiment and theory for corroborative research on hingeless rotor stability in forward flight (an experimental and analytical investigation of isolated rotor-flap-lag stability in forward flight)

For flap-lag stability of isolated rotors, experimental and analytical investigations are conducted in hover and forward flight on the adequacy of a linear quasisteady aerodynamics theory with dynamic inflow. Forward flight effects on lag regressing mode are emphasized. Accordingly, a soft inplane hingeless rotor with three blades is tested at advance ratios as high as 0.55 and at shaft angles as high as 20 degrees. The 1.62 m model rotor is untrimmed with an essentially unrestricted tilt of the tip path plane. In combination with lag natural frequencies, collective pitch settings and flap-lag coupling parameters, the data base comprises nearly 1200 test points (damping and frequency) in forward flight and 200 test points in hover. By computerized symbolic manipulation, a linear analytical model is developed in substall to predict stability margins with mode identificaton. To help explain the correlation between theory and data it also predicts substall and stall regions of the rotor disk from equilibrium values. The correlation shows both the strengthts and weaknesses of the theory in substall.

Gaonkar, G.↗
NASA NTRS1987

Toward comparing experiment and theory for corroborative research on hingeless rotor stability in forward flight

For flap lag stability of isolated rotors, experimental and analytical investigations were conducted in hover and forward flight on the adequacy of a linear quasisteady aerodynamics theory with dynamic flow. Forward flight effects on lag regressing mode were emphasized. A soft inplane hingeless rotor with three blades was tested at advance ratios as high as 0.55 and at shaft angles as high as 20 deg. The 1.62 m model rotor was untrimmed with an essentially unrestricted tilt of the tip path plane. In combination with lag natural frequencies, collective pitch settings and flap lag coupling parameters, the data base comprises nearly 1200 test points (damping and frequency) in forward flight and 200 test points in hover. By computerized symbolic manipulation, a linear model was developed in substall to predict stability margins with mode identification. To help explain the correlation between theory and data it also predicted substall and stall regions of the rotor disk from equilibrium values. The correlation showed both the strengths and weaknesses of the theory in substall ((angle of attack) equal to or less than 12 deg).

Gaonkar, G.↗
NASA NTRS1987

The prediction of helicopter tail rotor noise on the flight path

Experimental measurements on a model scale helicopter in a wind tunnel have shown that the levels radiated in the direction of flight are strongly dependent on the fuselage angle to the mean flow. Here a theoretical model is derived to explain these measurements using unsteady thickness noise as the principle source mechanism. The model includes the effect of fuselage wake turbulence which is convected through only the lower sector of the rotor disk. This causes spectral peaks which do not occur at blade passing frequencies, but rather at frequencies associated with convected flow inhomogeneities. The results are compared with measurements and show good agreement over a range of fuselage angles and mean flow speeds.

Glegg, Stewart A. L.↗
NASA NTRS1989

Environmental noise characteristics of the MOD5-B (3.2 MW) wind turbine generator

Both narrow band and broad band acoustic data were obtained for the MOD5-B wind turbine for a range of wind speeds from 5.8 to 14.3 m/s; for a range of power outputs from 300 to 3100 kW; and for various azimuth angles and distances. Comparisons are made with those of other large machines and with predictions by available methods. The highest levels occur at the lower frequencies and generally decrease as the frequency increases. Low frequency rotational noise components were more intense than expected for an upwind machine and are believed to result from localized wind gradients across the rotor disk due to upwind terrain features. Predicted broad band spectra follow the general trends of the data but tend to underestimate the levels in the frequency range where the turbulent boundary layer-trailing edge interaction noise is expected to be significant.

Shepherd, Kevin P.↗
NASA NTRS1989

Computational and experimental evaluation of helicopter rotor tips for high speed forward flight

A computational and experimental method for the evaluation of helicopter rotor tips in high-speed forward flight is presented which uses an unsteady full-potential solver on the advancing side of the rotor disk. Forces and moments measured during angle-of-attack sweeps reveal the soft-stall phenomenon of the double-swept planforms as well as the delayed stall of the single-swept hyperbolic tip. Double-swept planforms are shown to exhibit the most favorable performance, creating counter-rotating vortices that augment the lifting capabilities of the blade at high angles of attack and delay the onset of stall.

Sigl, Dave↗
NASA NTRS1990

The validation and application of a rotor acoustic prediction computer program

An essential prerequisite to reducing the acoustic detectability of military rotorcraft is a better understanding of main rotor noise which is the major contributor to the overall noise. A simple, yet accurate, Rotor Acoustic Prediction Program (RAPP) was developed to advance the understanding of main rotor noise. This prediction program uses the Ffowcs Williams and Hawkings (FW-H) equation. The particular form of the FW-H equation used is well suited for the coupling of the measured blade surface pressure to the prediction of acoustic pressure. The FW-H equation is an inhomogeneous wave equation that is valid in all space and governs acoustic pressure generated by thin moving bodies. The nonhomogeneous terms describe mass displacement due to surface motion and forces due to local surface stresses, such as viscous stress and pressure distribution on the surface. This paper examines two of the four types of main rotor noise: BVI noise and low-frequency noise. Blade-vortex interaction noise occurs when a tip vortex, previously shed by a rotor blade, passes close enough to a rotor blade to cause large variations in the blade surface pressures. This event is most disturbing when it happens on the advancing side of the rotor disk. Low-frequency noise includes hover and low to moderate speed forward flight. For these flight conditions, the low frequency components of the acoustic signal dominate.

Gallman, Judith M.↗
NASA NTRS1989

Calculation of the rotor induced download on airfoils

Interactions between the rotors and wing of a rotary wing aircraft in hover have a significant detrimental effect on its payload performance. The reduction of payload results from the wake of lifting rotors impinging on the wing, which is at 90 deg angle of attack in hover. This vertical drag, often referred as download, can be as large as 15 percent of the total rotor thrust in hover. The rotor wake is a three-dimensional, unsteady flow with concentrated tip vortices. With the rotor tip vortices impinging on the upper surface of the wing, the flow over the wing is not only three-dimensional and unsteady, but also separated from the leading and trailing edges. A simplified two-dimensional model was developed to demonstrate the stability of the methodology. The flow model combines a panel method to represent the rotor and the wing, and a vortex method to track the wing wake. A parametric study of the download on a 20 percent thick elliptical airfoil below a rotor disk of uniform inflow was performed. Comparisons with experimental data are made where the data are available. This approach is now being extended to three-dimensional flows. Preliminary results on a wing at 90 deg angle of attack in free stream is presented.

Lee, C. S.↗
NASA NTRS1991

General flow field analysis methods for helicopter rotor aeroacoustics

Previous work in the analysis of rotor flow fields for aeroacoustic applications involved the preliminary development of an efficient and accurate Lagrangian simulation of the unsteady vorticity field in the vicinity of helicopter main rotor that could analyze a limited class of rotor/wake interactions. The capabilities of this analysis have subsequently been considerably enhanced to allow it to serve as the foundation for a general analysis of the rotor/wake interaction noise. This paper presents the details of these enhancements, which focus on the expansion of the reconstruction approach developed previously to handle arbitrary vortex wake interactions within three-dimensional regions located near or within the rotor disk. Also, the development of nearfield velocity corrections appropriate for the analysis of such interactions is described, as is a preliminary study of methods for using the new high-resolution flow field analysis for noise predictions. The results show that by employing this novel flow field reconstruction technique it is possible to employ full-span free wake analyses with temporal and spatial resolution suitable for acoustic applications while reducing the computation time required by one to two orders of magnitude relative to traditional methods.

Quackenbush, Todd R.↗
NASA NTRS1991

An improved three-dimensional aerodynamics model for helicopter airloads prediction

The importance of unsteady aerodynamic effects due to rotor blade motion is demonstrated for a helicopter in high-speed forward flight. These unsteady effects are modeled as surface boundary conditions to a three-dimensional, unsteady Computational Fluid Dynamics (CFD) code called the Full-Potential Rotor Code (FPR). These boundary conditions cause significant changes in the computed lift and pitching moment at the front and rear of the rotor disk. Airloads from the modified FPR code are then iteratively coupled with the helicopter comprehensive code, CAMRAD/JA. Computed airloads show good agreement with flight-test data when lift values from the FPR code are used in the coupled calculation. However, the computed airloads from CAMRAD/JA along also show good agreement with the experimental data. Thus for this case one cannot demonstrate a significant improvement in computed airloads with the hybrid coupled scheme. The addition of the pitching moment values from the FPR code into the CAMRAD/JA calculation slows down the overall iterative convergence and does not yield any improvement in the final results.

Strawn, Roger C.↗
NASA NTRS1992

Stiffer Circumferential Dovetail Mount For Turbine Blades

Multihook circumferential dovetail configuration for mounting blades around circumference of rotor disk in turbomachine reduces bending stresses in disk. Provides substantial section of post where most needed to resist maximum bending moment near lower hook.

Ward, Steven D.↗
NASA NTRS1991

A comparative analysis of XV-15 tiltrotor hover test data and WOPWOP predictions incorporating the fountain effect

Acoustic measurements from a hovering full scale XV-15 tilt rotor with the advanced technology blades are presented which show the directionality of fountain effect noise. Predicted acoustic directivity results are also presented which show agreement with the measured data. The aeroacoustic code, WOPWOP, was used in conjunction with a mathematical model which simulated the fountain recirculation aerodynamic effect on the rotor blade surface pressures. The predictions were used to identify the spike character in the measured data as fountain effect associated noise. The directivity of the fountain effect noise was observed to be dominant at the rear of the aircraft with increased intensities 45 degrees below the rotor disk planes.

Rutledge, Charles K.↗
NASA NTRS1991

Nonlinear rotordynamics analysis

Effective analysis tools were developed for predicting the nonlinear rotordynamic behavior of the Space Shuttle Main Engine (SSME) turbopumps under steady and transient operating conditions. Using these methods, preliminary parametric studies were conducted on both generic and actual HPOTP (high pressure oxygen turbopump) models. In particular, a novel modified harmonic balance/alternating Fourier transform (HB/AFT) method was developed and used to conduct a preliminary study of the effects of fluid, bearing and seal forces on the unbalanced response of a multi-disk rotor in the presence of bearing clearances. The method makes it possible to determine periodic, sub-, super-synchronous and chaotic responses of a rotor system. The method also yields information about the stability of the obtained response, thus allowing bifurcation analyses. This provides a more effective capability for predicting the response under transient conditions by searching in proximity of resonance peaks. Preliminary results were also obtained for the nonlinear transient response of an actual HPOTP model using an efficient, newly developed numerical method based on convolution integration. Currently, the HB/AFT is being extended for determining the aperiodic response of nonlinear systems. Initial results show the method to be promising.

Noah, Sherif T.↗
NASA NTRS1992

Adequacy of modeling turbulence and related effects on helicopter response

'Blade-fixed' atmospheric turbulence encountered by a helicopter substantially differs from body-fixed (i.e., fuselage) turbulence, because the rotational velocity moves the blade station fore and aft through the turbulence. The present closed-form solution of a frequency-time spectrum for the dominant vertical turbulence velocity at arbitrary blade station defines in what way and in what degree the rotational velocity affects rotor-disk turbulence frequency and temporal characteristics. Blade-flapping response to blade- and body-fixed turbulence is also presented over a range of turbulence-scale length and advance ratio.

George, V. V.↗
NASA NTRS1991

Aerodynamic characteristics of a rotorcraft airfoil designed for the tip region of a main rotor blade

A wind tunnel investigation was conducted to determine the 2-D aerodynamic characteristics of a new rotorcraft airfoil designed for application to the tip region (stations outboard of 85 pct. radius) of a helicopter main rotor blade. The new airfoil, the RC(6)-08, and a baseline airfoil, the RC(3)-08, were investigated in the Langley 6- by 28-inch transonic tunnel at Mach numbers from 0.37 to 0.90. The Reynolds number varied from 5.2 x 10(exp 6) at the lowest Mach number to 9.6 x 10(exp 6) at the highest Mach number. Some comparisons were made of the experimental data for the new airfoil and the predictions of a transonic, viscous analysis code. The results of the investigation indicate that the RC(6)-08 airfoil met the design goals of attaining higher maximum lift coefficients than the baseline airfoil while maintaining drag divergence characteristics at low lift and pitching moment characteristics nearly the same as those of the baseline airfoil. The maximum lift coefficients of the RC(6)-08 varied from 1.07 at M=0.37 to 0.94 at M=0.52 while those of the RC(3)-08 varied from 0.91 to 0.85 over the same Mach number range. At lift coefficients of -0.1 and 0, the drag divergence Mach number of both the RC(6)-08 and the RC(3)-08 was 0.86. The pitching moment coefficients of the RC(6)-08 were less negative than those of the RC(3)-08 for Mach numbers and lift coefficients typical of those that would occur on a main rotor blade tip at high forward speeds on the advancing side of the rotor disk.

Noonan, Kevin W.↗
NASA NTRS1991

A comparative analysis of XV-15 tiltrotor hover test data and WOPWOP predictions incorporating the fountain effect

Acoustic measurements from a hovering full-scale XV-15 tiltrotor with the advanced technology blades are presented which show the directionality of fountain effect noise. Predicted activity directivity results are also presented which show agreement with the measured data. The aeroacoustic code, WOPWOP, was used in conjunction with a mathematical model which simulated the fountain/recirculation aerodynamic effect on the rotors' blade surface pressures. The predictions were used to identify the spike character in the measured data as fountain effect associated noise. The directivity of the fountain effect noise was observed to be dominant at the rear of the aircraft with increased intensities 45 degrees below the rotor disk planes.

Rutledge, Charles K.↗