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Howard, F. G.

Publications and source records attributed to Howard, F. G..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Control of low-speed turbulent separated flow using jet vortex generators

A parametric study has been performed with jet vortex generators to determine their effectiveness in controlling flow separation associated with low-speed turbulent flow over a two-dimensional rearward-facing ramp. Results indicate that flow-separation control can be accomplished, with the level of control achieved being a function of jet speed, jet orientation (with respect to the free-stream direction, and jet location (distance from the separation region in the free-stream direction). Compared to slot blowing, jet vortex generators can provide an equivalent level of flow control over a larger spanwise region (for constant jet flow area and speed).

Selby, G. V.↗
NASA NTRS1991

Exploratory study of vortex-generating devices for turbulent flow separation control

Flow phenomena and the relative performance associated with several devices for controlling turbulent separated flow were investigated at low speeds. Vortex-generating devices examined included: submerged vortex generators (Wheeler doublet and wishbone types), spanwise cylinders, large-eddy breakup devices (LEBU) at small angle of attack (alpha), and vortex-generator jets (VGJ). Dye flow visualization tests in a water tunnel indicated that wishbone vortex generators in the forward orientation shed horseshoe vortices; wishbone vortex generators oriented in the reverse direction and doublet vortex generators shed streamwise counterrotating vortices; a spanwise cylinder located near the wall and LEBUs at alpha = -10 deg produced eddies which rotated with the same sign as the mean vorticity in a turbulent boundary layer; and the most effective VGJs produced streamwise corotating vortices. Comparative wind tunnel tests conducted on a curved backward-facing ramp indicated that transferring momentum from the outer region of a turbulent boundary layer by embedded streamwise vortices is more effective than by transverse vortices for separation control applications.

Lin, J. C.↗
NASA NTRS1990

Investigation of several passive and active methods for turbulent flow separation control

Relative performance of several passive and active methods for controlling two-dimensional turbulent separated flow associated with a curved backward-facing ramp were investigated at low speeds. Surface static pressure measurement and oil flow visualization results indicate that submerged vortex generators, vortex generator jets, elongated arches at +-alpha, and large-eddy breakup devices at +-alpha placed near the baseline separation location reduce flow separation and increase pressure recovery. Spanwise cylinders reduce flow separation but decrease pressure recovery downstream. Arches with alpha = 0 deg, Helmholtz resonators, and Viets' fluidic flappers examined so far have no significant effect in reducing separation. Wall cooling computation indicates that separation delay on a partially cooled ramp is nearly the same as on a fully-cooled ramp while minimizing the frictional drag increase associated with the wall cooling process.

Lin, J. C.↗
NASA NTRS1990

Turbulent flow separation control over a backward-facing ramp via transverse and swept grooves

Separation control experiments were conducted in a 51-71-cm shear-flow control tunnel using a backward-facing ramp to investigate the performance of transverse and swept grooves for controlling a two-dimensional turbulent separated flow at low speeds and moderate Reynolds numbers. In the experiments, transverse grooves, located in the maximum +dP/dx region, with a height-to-width ratio greater than 2.5, reduced the reattachment distance by 20 percent over the baseline configuration. Unlike transverse and longitudinal grooves of equivalent size, the 45-degree swept-groove configurations tested enhanced separation.

Selby, G. V.↗
NASA NTRS1989

Wind-tunnel investigations of wings with serrated sharp trailing edges

Exploratory wind-tunnel force measurements are presented for two wing geometries with small-scale planar and nonplanar serrated trailing-edge devices (chord-Reynolds numbers ranged from 1.0 to 3.7 million). The planar serrated trailing-edge extensions reduced the drag at conditions when trailing-edge separation occurred at low angles of attack. The introduction of serrations reduced or eliminated the drag penalty, due to the small (1-2 percent of the chord length) nonplanar trailing-edge flaps, while maintaining the effects of increase in camber. The presence of streamwise vortices immediately downstream of the serrated trailing edges is believed to have favorably affected the boundary-layer flow approaching the trailing edge and the near-wake development, resulting in reduced pressure (form) drag.

Vijgen, P. M. H. W.↗
NASA NTRS1989

Turbulent flow separation control through passive techniques

Several passive separation control techniques for controlling moderate two-dimensional turbulent flow separation over a backward-facing ramp are studied. Small transverse and swept grooves, passive porous surfaces, large longitudinal grooves, and vortex generators were among the techniques used. It was found that, unlike the transverse and longitudinal grooves of an equivalent size, the 45-deg swept-groove configurations tested tended to enhance separation.

Lin, J. C.↗
NASA NTRS1989

Wind-tunnel investigations of wings with serrated sharp trailing edges

Exploratory wind-tunnel force measurements are presented for two wing geometries with small-scale planar and nonplanar serrated trailing-edge devices (chord-Reynolds numbers ranged from 1.0 - 3.7 x 10 to the 6th). The planar serrated trailing-edge extensions reduced the drag at conditions when trailing-edge separation occurred at low angles of attack. The introduction of serrations reduced or eliminated the drag penalty due to the small (1-2 percent of the chord length) nonplanar trailing-edge flaps, while maintaining the effects of increase in camber. The presence of streamwise vortices immediately downstream of the serrated trailing edges is believed to have favorably affected the boundary-layer flow approaching the trailing edge and the near-wake development, resulting in reduced pressure drag.

Vijgen, P. M. H. W.↗
NASA NTRS1986

Separated flow

A brief overview of flow separation phenomena is provided. Langley has many active research programs in flow separation related areas. Three cases are presented which describe specific examples of flow separation research. In each example, a description of the fundamental fluid physics and the complexity of the flow field is presented along with a method of either reducing or controlling the extent of separation. The following examples are discussed: flow over a smooth surface with an adverse pressure gradient; flow over a surface with a geometric discontinuity; and flow with shock-boundary layer interactions. These results will show that improvements are being made in the understanding of flow separation and its control.

Sellers, W. L., III↗
NASA NTRS1985

Axisymmetric bluff-body drag reduction through geometrical modification

The effect of shoulder radiusing and grooving (longitudinally and circumferentially) the afterbodies of bluff bodies to reduce the base drag at low speeds is investigated experimentally. Shoulder radii as large as 2.75 body diameters are examined. Reynolds number (ReD) based on body diameter varied from 20,000 to 200,000. Results indicate that increasing the shoulder radius to 2.00 body diameters can reduce the drag levels to those of a streamline body having 67 percent greater fineness ratio. For the relatively sharp shoulder case, body drag reductions as large as 50 and 33 percent are obtained using circumferential or longitudinal grooves, respectively.

Howard, F. G.↗
NASA NTRS1984

Axisymmetric bluff-body drag reduction through geometrical modifications

The effect of shoulder radiusing and grooving (longitudinally or circumferentially) the afterbodies of bluff bodies of reduce the base drag at low speeds is discussed. Shoulder radii as large as 2.75 body diameters are examined. Reynolds number based on body diameter varied from 20,000 to 200,000. Results indicate that increasing the shoulder radius to 2.75 body diameters can reduce the drag levels to those of a streamline body having 67 percent greater fineness ratio. For the relatively sharp shoulder case, body drag reductions as large as 50% are obtained using circumferential or longitudinal grooves.

Howard, F. G.↗
NASA NTRS1983

Axisymmetric bluff-body drag reduction using circumferential grooves

The effect of shoulder radiusing and circumferentially grooving the afterbodies of bluff bodies to reduce the base drag at low speeds is discussed. Shoulder radii as large as 2.75 body diameters are examined. Reynolds number based on body diameter varied from 20,000 to 200,000. Results indicate that increasing the shoulder radius to 2.75 body diameters can reduce the drag levels to those of a streamline body having 67 percent greater fineness ratio. For zero shoulder radius, circumferential grooves were found to be effective in reducing body drag for zero shoulder radius in both laminar and tripped flow. Circumferential grooves on the afterbody with a shoulder radius of one-half the body diameter were only effective in reducing drag for laminar flow.

Howard, F. G.↗
NASA NTRS1981

Longitudinal afterbody grooves and shoulder radiusing for low-speed bluff body drag reduction

A new low-speed drag reduction approach is proposed which employs longitudinal surface V-shaped grooves cutting through the afterbody shoulder region. The test Reynolds number range was from 20,000 to 200,000 based on undisturbed free-stream flow and a body diameter of 6.08 cm. The V-grooves are shown to be most effective in reducing drag when the afterbody shoulder radius is zero. Reductions in drag of up to 33% have been measured for this condition. For large shoulder radius, the grooves are only effective at the lower Reynolds numbers of the test.

Howard, F. G.↗
NASA NTRS1978

Direct measurements and analysis of skin friction and cooling downstream of multiple flush-slot injection into a turbulent Mach 6 boundary layer

Experiments were conducted to determine the reduction in surface skin friction and the effectiveness of surface cooling downstream of one to four successive flush slots injecting cold air at an angle of 10 deg into a turbulent Mach 6 boundary layer. Data were obtained by direct measurement of surface shear and equilibrium temperatures, respectively. Increasing the number of slots decreased the skin friction, but the incremental improvement in skin-friction reduction decreased as the number of slots was increased. Cooling effectiveness was found to improve, for a given total mass injection, as the number of slots was increased from one to four. Comparison with previously reported step-slot data, however, indicated that step slots with tangential injection are more effective for both reducing skin friction and cooling than the present flush-slot configuration. Finite-difference predictions are in reasonable agreement with skin-friction data and with boundary-layer profile data.

Howard, F. G.↗
NASA NTRS1977

Cooling effectiveness of slot injection into a turbulent boundary layer

The cooling effectiveness of injection through multiple flush slots at an angle of 10 deg was studied experimentally in a wind tunnel. Air was injected from one to four slots into a turbulent Mach 6 boundary layer. The slot mass flow ratio is defined, and data which describe the dependence of the cooling effectiveness on the slot mass flow ratio are presented. Experimental values are indicated graphically for various cases of single and multiple slot injection, where the total mass injection (i.e., the sum of flow rates from each slot) is the same for each case. The results show that, for a given coolant mass flow rate, thermal protection over the maximum surface area can be accomplished best by injecting the coolant flow through multiple slots.

Howard, F. G.↗
NASA NTRS1976

Direct measurements at Mach 6 of turbulent skin-friction reduction by injection from single and multiple flush slots

The surface skin-friction reduction downstream of one to four successive flush slots injecting at an angle of 10 deg into a turbulent Mach 6 boundary layer has been determined from direct measurements of surface shear. Increasing the number of succeeding slots increases the skin-friction reduction, but this trend reverses at high injection rates. The incremental improvement in skin-friction reduction decreases with increasing number of slots. Comparison with previously reported step slot data indicates that step slots with tangential injection are more effective in reducing skin friction than the present flush slot configuration. Finite-difference predictions are in reasonable agreement with skin-friction and boundary-layer profile data.

Srokowski, A. J.↗
NASA NTRS1975

Evaluation of viscous drag reduction schemes for subsonic transports

The results are described of a theoretical study of viscous drag reduction schemes for potential application to the fuselage of a long-haul subsonic transport aircraft. The schemes which were examined included tangential slot injection on the fuselage and various synergetic combinations of tangential slot injection and distributed suction applied to wing and fuselage surfaces. Both passive and mechanical (utilizing turbo-machinery) systems were examined. Overall performance of the selected systems was determined at a fixed subsonic cruise condition corresponding to a flight Mach number of free stream M = 0.8 and an altitude of 11,000 m. The nominal aircraft to which most of the performance data was referenced was a wide-body transport of the Boeing 747 category. Some of the performance results obtained with wing suction are referenced to a Lockheed C-141 Star Lifter wing section. Alternate designs investigated involved combinations of boundary layer suction on the wing surfaces and injection on the fuselage, and suction and injection combinations applied to the fuselage only.

Marino, A.↗