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  • 1
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    Effect of acoustic excitation on the flow over a low- Re airfoil (1987)
    Cambridge University Press
    Details
    Publication Date: 1987-09-01
    Description: Wind-tunnel measurements of lift, drag and wake velocity spectra were carried out under (tonal) acoustic excitation for a smooth airfoil in the chord-Reynolds-number (Rec) range of 4 x 104-1.4 x 105The data are supported by smoke-wire flow-visualization pictures. Small-amplitude excitation in a wide, low-frequency range is found to eliminate laminar separation that otherwise degrades the airfoil performance at low Recnear the design angle of attack. Excitation at high frequencies, scaling as U3/2∞eliminates a pre-stall, periodic shedding of large-scale vorticesU∞is the free-stream velocity. Significant improvement in lift is also achieved during post-stall, but with large-amplitude excitation. Wind-tunnel resonances strongly influence the results, especially in cases requiring large amplitudes. It is shown that large transverse velocity fluctuations, induced near the airfoil by specific cross-resonance modes, lead to the most effective separation control; resonances inducing only large-amplitude pressure fluctuations are demonstrated to be less effective. © 1987, Cambridge University Press. All rights reserved.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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    PAPER CURRENT
  • 2
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    Boundary layer transition (1986)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The boundary layer stability, its active control by sound and surface heating and the effect of curvature are studied numerically and experimentally for subsonic flow. In addition, the experimental and flight test data are correlated using the stability theory for supersonic Mach numbers. Active transition fixing and feedback control of boundary layer by sound interactions are experimentally investigated at low speed over an airfoil. Numerical simulation of active control by surface heating and cooling in air shows that by appropriate phase adjustment a reduction in the level of perturbation can be obtained. This simulation is based on the solution of two-dimensional compressible Navier-Stokes equations for a flat plate. Goertler vortices are studied experimentally on an airfoil in the Low Turbulence Pressure Tunnel (LTPT). The flow pattern was visualized using the sublimating chemical technique and data were obtained using a three component laser velocimeter. The effect of curvature on swept leading-edge stability on a cylinder was numerically studied. The results suggest that transition is dominated by traveling disturbance waves and that the waves with the greatest total amplification has an amplitude ratio of e sup 11. Experimental data from the quiet supersonic tunnel and flight tests are analyzed using linear compressible stability theory.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Langley Symposium on Aerodynamics, Volume 1; p 333-345
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    A study of laminar separation bubble in the concave region of an airfoil using laser velocimetry (1985)
    In:  Other Sources
    Details
    Publication Date: 2018-12-01
    Description: Laser velocimetry (LV) was used to study the nature of laminar separation bubbles in the concave region of a 1.83-meter airfoil model which was tested in the NASA Langley Low Turbulence Pressure Tunnel. Three component, coincident data from LV measurements including histograms of particle velocity, mean velocity profiles, turbulence intensity, and Reynolds stresses within the shear layer were used to determine the locations of laminar separation, transition, and turbulent reattachment. boundary-layer parameters determined from velocity profiles were used to compare the results with existing empirical relations for describing the laminar separation bubble.
    Keywords: AERODYNAMICS
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    Disturbance functions of the Goertler instability on an airfoil (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Goertler vortices arise in boundary layers along concave surfaces due to centrifugal effects. This paper presents some results of an experiment conducted to study the development of these vortices on an airfoil with a pressure gradient in the concave region where an attached laminar boundary layer was insured with suction through a perforated panel. A sublimating chemical technique was used to visualize Goertler vortices and the velocity field was measured by laser velocimetry. Experimental disturbance functions are compared with those predicted by the linear stability theory. The trend of vortex amplification in the concave zone and damping in the following convex region is shown to essentially follow the theoretical predictions.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 86-1048
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    In-flight detection of flow separation, stagnation, and transition (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Flight tests were conducted to demonstrate the feasibility of simultaneous determination of: (1) boundary-layer instability and transition characteristics; (2) flow separation region; and (3) the location of the leading-edge stagnation point. Surface mounted, multi-element, micro-thin, hot-film sensors were used in conjunction with a bank of constant-temperature anemometers and a PC-based 8-channel simultaneous data acquisition system. Conventional techniques were used to determine the boundary-layer instability and transition characteristics while the flow separation and the stagnation points were determined through the presence of phase reversal signatures.
    Keywords: AIRCRAFT INSTRUMENTATION
    Type: In: ICAS, Congress, 18th, Beijing, China, Sept. 20-25, 1992, Proceedings. Vol. 2 (A93-14151 03-01); p. 1382-1391.
    Format: text
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  • 6
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    A new approach to high-speed flow measurements using constant voltage anemometry (1992)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The paper addresses the basic features of conventional instrumentation, such as the constant temperature (CTA) and the constant current (CCA) anemometers, their limitations, and describes a totally new approach to high-speed dynamic measurements using a constant voltage anemometer (CVA). The paper describes the design features of a newly developed CVA and compares preliminary results obtained with CVA and conventional anemometry in low- and high-speed flows.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: AIAA PAPER 92-3957
    Format: text
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    NASA TECHNICAL REPORTS
  • 7
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    The Goertler instability on an airfoil (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Goertler vortices arise in boundary layers along concave surfaces due to centrifugal effects. This paper presents some results of an experiment conducted to study the development of these vortices on an airfoil with a pressure gradient in the concave region where an attached laminar boundary layer was insured with suction through a perforated panel. The sublimating chemical technique was used to visualize Goertler vortices and the velocity field was measured by laser velocimetry. The vortex wavelength clearly varied with Goertler number as predicted by linear theory. Both flow visualization and velocity measurements indicated vortex damping in the convex zone. Secondary instability was observed at the higher Goertler numbers.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 85-0491
    Format: text
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    NASA TECHNICAL REPORTS
  • 8
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    Development of wing and tail configurations for low altitude unmanned research aircraft (LAURA) (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The Low Altitude/Airspeed Unmanned Research Aircraft (LAURA) is being developed by the U.S. Navy for flight test research using low-Reynolds number airfoils. This vehicle consists of a standard modular fuselage designed to accept the installation of several wings/tails having low Reynolds number airfoils, and various planform shapes. Design constraints include shipboard storage, long flight endurance at very low airspeeds and sea-skimming cruise altitude. The stringent design constraints require the development of high-performance low Reynolds number (LRN) airfoils, suitable lifting surface configuration, and advanced airframe-propulsion systems. The present paper describes ongoing efforts to develop wing and tail configurations for LAURA using airfoils designed at NASA Langley Research Center.
    Keywords: AIRCRAFT DESIGN, TESTING AND PERFORMANCE
    Type: SAE PAPER 871882
    Format: text
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    NASA TECHNICAL REPORTS
  • 9
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    The phase reversal phenomenon at flow separation and reattachment (1988)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Tests were conducted on two different airfoils, one of them in a low-turbulence tunnel, to detect laminar separation and turbulent reattachment locations. A 'nonintrusive' multielement sensor consisting of a large number of closely spaced individual nickel films was vacuum deposited on a thin substrate and bonded to the airfoil model surface. Each sensor element was a part of an independent constant temperature anemometer system. Time history as well as spectral analysis of signals from surface film gauges were used to determine the surface shear flow characteristics. A major breakthrough was achieved with the discovery of phase reversal in low-frequency dynamic shear stress signals across regions of flow separation and reattachment.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 88-0408
    Format: text
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    NASA TECHNICAL REPORTS
  • 10
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    Measurement of crossflow vortices, attachment-line flow, and transition using microthin hot films (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A flow diagnostic experiment was conducted on a 45-deg swept-wing model using surface-mounted, multielement, microthin, hot-film sensors. The cross-flow vortex spacing, the attachment-line flow characteristics, and the transition region were all determined using an advanced data acquisition and instrumentation system. In addition to the frequencies of traveling waves predicted by linear stability theory, amplified disturbances at much higher frequencies were observed. Simultaneous measurements from sensors located at a number of chord and span locations highlighted the strong three-dimensionality of the boundary-layer flow in the presence of cross-flow vortices. The state of the attachment-line boundary layer was determined using a multielement sensor wrapped around the wing leading edge. The transition region flow characteristics were also identified.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: AIAA PAPER 90-1636
    Format: text
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