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  • 1
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    Effect of Wavy Trailing Edge on 100meter Flatback Wind Turbine Blade (2016)
    Institute of Physics
    Details
    Publication Date: 2016-09-01
    Print ISSN: 1742-6588
    Electronic ISSN: 1742-6596
    Topics: Physics
    Published by Institute of Physics
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    PAPER CURRENT
  • 2
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    Acoustic propagation using computational fluid dynamics (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The propagation characteristics of several helicopter airfoil profiles have been investigated using the transonic small disturbance equation. A test case was performed to generate a moving shock that propagated off the airfoil. Various grids were then examined to determine their ability to accurately capture these propagating shock waves. Finally, the case of airfoil-vortex interactions was thoroughly studied over a wide range of Mach numbers and airfoil shapes with particular emphasis on the transonic regime; this results in a highly conplicated fluctuation of lift, drag, and pitching moment. The calculated acoustic intensity levels, along with the details of the computational flow field, provide new insights into the understanding of transonic airfoil-vortex interactions.
    Keywords: AERODYNAMICS
    Format: text
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    NASA TECHNICAL REPORTS
  • 3
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    Calculation of helicopter airfoil characteristics for high tip-speed applications (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: In this paper, we have applied a new aerodynamic tool to the study of helicopter airfoil characteristics. We have shown that the computed airloads reproduce completely the experimental behavior of representative airfoils across the transonic regime. In addition, the computational details of the flow fields, the surface pressure distributions, and the viscous-layer characteristics enable us to trace the evolution of the physical changes that occur as m infinity or Re increases. Descriptions of the complicated development of shock waves, shock-induced separation supplement the information that has been obtained heretofore in wind tunnels. In validating our calculations and assessing the accuracy of the results, including extensive grid-refinement studies and comparisons with data from numerous wind tunnels, we have defined the capabilities and limitations of the code ARC2D more precisely. This important aspect of the investigations can complement wind-tunnel tests, by providing flow-field details that are difficult to measure and by extending the range of low parameters beyond the capabilities of existing wind tunnels. The code has now progressed from a purely research stage to almost a production stage, where it can be run by specialists in the helicopter industry.
    Keywords: AERODYNAMICS
    Type: American Helicopter Society, Journal (ISSN 0002-8711); 31; 3-9
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    Numerical simulation of tip vortices of wings in subsonic and transonic flows (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A multi block zonal algorithm which solves the thin-layer Navier-Stokes and the Euler equations is used to numerically simulate the formation and roll-up of the tip vortex in both subsonic and transonic flows. Four test cases which used small and large aspect ratio wings have been considered to examine the influence of the tip-cap shape, the tip planform and the free-stream Mach number. It appears that both the tip-planform and the tip-cap shape have some influence on the formation of the tip vortex, but its subsequent roll-up seems to be more influenced by the tip-planform shape. In general, a good definition of the formation and the roll-up of the tip vortex has been observed for all the cases considered here. Comparions of the numerical results with the limited, available experimental data show good agreement with both the surface pressures and the tip-vortex strength.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 86-1095
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    Calculations of the flow past bluff bodies, including tilt-rotor wing sections at alpha = 90 deg (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: An attempt was made to model in two dimensions the effects of rotor downwash on the wing of the tilt-rotor aircraft and to compute the drag force on airfoils at - 90 deg angle of attack, using a well-established Navier-Stokes code. However, neither laminar nor turbulent calculations agreed well with drag and base-pressure measurements at high Reynolds numbers. Therefore, further efforts were concentrated on bluff-body flows past various shapes at low Reynolds numbers, where a strong vortex shedding is observed. Good results were obtained for a circular cylinder, but the calculated drag of a slender ellipse at right angles to the freestream was significantly higher than experimental values reported in the literature for flat plates. Similar anomalous results were obtained on the tilt-rotor airfoils, although the qualitative effects of flap deflection agreed with the wind tunnel data. The ensemble of results suggest that there may be fundamental differences in the vortical wakes of circular cylinders and noncircular bluff bodies.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 90-0032
    Format: text
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    NASA TECHNICAL REPORTS
  • 6
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    Computation of non-linear acoustics in two-dimensional blade-vortex interactions (1987)
    In:  Other Sources
    Details
    Publication Date: 2018-12-01
    Description: The propagation characteristics of the interaction between a vortex and a helicopter airfoil are investigated by a variety of methods, and a comparison is made between solutions to the linearized transonic small disturbance equation, transonic small disturbance equation, Euler equations, and Navier-Stokes equations. Although the first two methods are able to accurately predict the propagation of acoustic waves, they are unable to accurately describe the initial formation of acoustic waves. The Euler and Navier-Stokes equations are shown to be well suited to the investigation of acoustic waves and give approximately the same results.
    Keywords: AERODYNAMICS
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    NASA TECHNICAL REPORTS
  • 7
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    Some recent advances in computational aerodynamics for helicopter applications (1985)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The growing application of computational aerodynamics to nonlinear helicopter problems is outlined, with particular emphasis on several recent quasi-two-dimensional examples that used the thin-layer Navier-Stokes equations and an eddy-viscosity model to approximate turbulence. Rotor blade section characteristics can now be calculated accurately over a wide range of transonic flow conditions. However, a finite-difference simulation of the complete flow field about a helicopter in forward flight is not currently feasible, despite the impressive progress that is being made in both two and three dimensions. The principal limitations are today's computer speeds and memories, algorithm and solution methods, grid generation, vortex modeling, structural and aerodynamic coupling, and a shortage of engineers who are skilled in both computational fluid dynamics and helicopter aerodynamics and dynamics.
    Keywords: AERODYNAMICS
    Type: NASA-TM-86777 , REPT-85345 , NAS 1.15:86777 , USAVSCOM-TR-85-A-06 , AD-A160722
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 8
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    Flowfield of a lifting rotor in hover - A Navier-Stokes simulation (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The viscous, three-dimensional flowfield of a lifting helicopter rotor in hover is calculated by using an upwind, implicit, finite-difference numerical method for solving the thin layer Navier-Stokes equations. The induced effects of the wake, including the interaction of tip vortices with successive blades, are calculated as part off the overall flowfield solution without using any ad hoc wake models. Comparison of the numerical results for the subsonic and transonic conditions show good agreement with the experimental data and with the previously published Navier-Stokes calculations using a simple wake model. Some comparisons with Euler calculations are also presented, along with some discussions of the grid refinement studies.
    Keywords: AERODYNAMICS
    Type: AIAA Journal (ISSN 0001-1452); 30; 10; 2371-237
    Format: text
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    NASA TECHNICAL REPORTS
  • 9
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    Numerical simulation of tip vortices of wings in subsonic and transonic flows (1988)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Keywords: AERODYNAMICS
    Type: AD-A204525 , AIAA Journal (ISSN 0001-1452); 26; 1153-116
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    NASA TECHNICAL REPORTS
  • 10
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    Euler solutions to nonlinear acoustics of non-lifting rotor blades (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: For the first time a computational fluid dynamics (CFD) method is used to calculate directly the high-speed impulsive (HSI) noise of a non-lifting hovering rotor blade out to a distance of over three rotor radii. In order to accurately propagate the acoustic wave in a stable and efficient manner, an implicit upwind-biased Euler method is solved on a grid with points clustered along the line of propagation. A detailed validation of the code is performed for a rectangular rotor blade at tip Mach numbers ranging from 0.88 to 0.92. The agreement with experiment is excellent at both the sonic cylinder and at 2.18 rotor radii. The agreement at 3.09 rotor radii is still very good, showing improvements over the results from the best previous method. Grid sensitivity studies indicate that with special attention to the location of the boundaries a grid with approximately 60,000 points is adequate. This results in a computational time of approximately 40 minutes on a Cray-XMP. The practicality of the method to calculate HSI noise is demonstrated by expanding the scope of the investigation to examine the rectangular blade as well as a highly swept and tapered blade over a tip Mach number range of 0.80 to 0.95. Comparisons with experimental data are excellent and the advantages of planform modifications are clearly evident. New insight is gained into the mechanisms of nonlinear propagation and the minimum distance at which a valid comparison of different rotors can be made: approximately two rotor radii from the center of rotation.
    Keywords: ACOUSTICS
    Type: In: AHS and Royal Aeronautical Society, Technical Specialists' Meeting on Rotorcraft Acoustics(Fluid Dynamics, Philadelphia, PA, Oct. 15-17, 1991, Proceedings (A93-29401 10-71); 16 p.
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