Effect of Leading-Edge Geometry on Boundary-Layer Receptivity to Freestream Sound

Lin, Nay; Reed, Helen L.; Saric, William S.

doi:10.1007/978-1-4612-2956-8_42

Cited by 78 publications

(65 citation statements)

References 9 publications

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“…Numerical work by Murdock (1981) for a parabola in a flow with a parallel acoustic wave also showed a decrease in receptivity as the nose radius was increased. The computations of Lin et al (1992) were for parallel acoustic waves incident on half-ellipse leading edges connected to a flat plate and for super-ellipse leading edges (which avoid the discontinuity in curvature), geometries chosen to match the experiments of Saric et al (1994). For both these geometries there are regions of adverse pressure gradient near the leading edge, as well as the possibility of additional localised receptivity mechanisms (Goldstein, 1985); hence no direct comparisons can be made with the present results.…”

Section: Resultsmentioning

confidence: 99%

Boundary-layer receptivity for a parabolic leading edge

Hammerton

Kerschen

1996

J. Fluid Mech.

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The effect of the nose radius of a body on boundary-layer receptivity is analyzed for the case of a symmetric mean flow past a body with a parabolic leading edge. Asymptotic methods based on large Reynolds number are used, supplemented by numerical results. The Mach number is assumed small, and acoustic free-stream disturbances are considered. The case of free-stream acoustic waves, propagating obliquely to the symmetric mean flow is considered. The body nose radius, r n , enters the theory through a Strouhal number, S = ωr n /U , where ω is the frequency of the acoustic wave and U is the mean flow speed. The finite nose radius dramatically reduces the receptivity level compared to that for a flat plate, the amplitude of the instability waves in the boundary layer being decreased by an order of magnitude when S = 0.3. Oblique acoustic waves produce much higher receptivity levels than acoustic waves propagating parallel to the body chord.

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Section: Resultsmentioning

confidence: 99%

Boundary-layer receptivity for a parabolic leading edge

Hammerton

Kerschen

1996

J. Fluid Mech.

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“…The body diameter, D, is 196.5 mm and the length-to-diameter ratio, L/D, is 6.48. The nose employs a modified super ellipse profile (Lin et al 1992) with an aspect ratio of 2.5. The boundary layer is conditioned by a 2 mm wide strip of 120 grit emery paper located at z/L = −0.884 (approximately the point of minimum pressure) followed by a 25 mm wide strip at z/L = −0.784.…”

Section: Methodsmentioning

confidence: 99%

Low-dimensional dynamics of a turbulent axisymmetric wake

et al. 2014

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The coherent structures of a turbulent wake generated behind a bluff three-dimensional axisymmetric body are investigated experimentally at a diameter based Reynolds number ∼ 2×10 5 . Proper orthogonal decomposition of base pressure measurements indicates that the most energetic coherent structures retain the structure of the symmetry-breaking laminar instabilities and manifest as unsteady vortex shedding with azimuthal wavenumber m = ±1. In a rotating reference frame, the shedding preserves the reflectional symmetry and is linked with a reflectionally symmetric mean pressure distribution on the base. Due to a slow rotation of symmetry plane of the turbulent wake around the axis of the body, statistical axisymmetry is recovered in the time average. The ratio of the timescales associated with the slow rotation of the symmetry plane and the vortex shedding is of order 100.

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“…A 20mm thick, 500mm wide and 1000mm long Plexiglas R flat plate is inserted in the test section off-centre at a 30% distance from the floor for alleviating wind tunnel corner effects (Saric & White 1998). The flat plate leading edge is a modified super ellipse (Lin et al 1992), enabling seamless curvature change and ensuring development of laminar boundary layer. A movable flap is positioned at the trailing edge of the flat plate and is deflected such as to localise the leading edge stagnation point on the upper side of the flat plate, eliminating possible unsteady separation effects.…”

Section: Model Set-upmentioning

confidence: 99%

Response of a laminar separation bubble to impulsive forcing

2017

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The spatial and temporal response characteristics of a laminar separation bubble to impulsive forcing are investigated by means of time-resolved particle image velocimetry and linear stability theory. A two-dimensional impulsive disturbance is introduced with an AC dielectric barrier discharge plasma actuator, exciting pertinent instability modes and ensuring flow development under environmental disturbances. Phase-averaged velocity measurements are employed to analyse the effect of imposed disturbances at different amplitudes on the laminar separation bubble. The impulsive disturbance develops into a wave packet that causes rapid shrinkage of the bubble in both upstream and downstream directions. This is followed by bubble bursting, during which the bubble elongates significantly, while vortex shedding in the aft part ceases. Duration of recovery of the bubble to its unforced state is independent of the forcing amplitude. Quasi-steady linear stability analysis is performed at each individual phase, demonstrating reduction of growth rate and frequency of the most unstable modes with increasing forcing amplitude. Throughout the recovery, amplification rates are directly proportional to the shape factor. This indicates that bursting and flapping mechanisms are driven by altered stability characteristics due to variations in incoming disturbances. The emerging wave packet is characterised in terms of frequency, convective speed and growth rate, with remarkable agreement between linear stability theory predictions and measurements. The wave packet assumes a frequency close to the natural shedding frequency, while its convective speed remains invariant for all forcing amplitudes. The stability of the flow changes only when disturbances interact with the shear layer breakdown and reattachment processes, supporting the notion of a closed feedback loop. The results of this study shed light on the response of laminar separation bubbles to impulsive forcing, providing insight into the attendant changes of flow dynamics and the underlying stability mechanisms.

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Effect of Leading-Edge Geometry on Boundary-Layer Receptivity to Freestream Sound

Cited by 78 publications

References 9 publications

Boundary-layer receptivity for a parabolic leading edge

Boundary-layer receptivity for a parabolic leading edge

Low-dimensional dynamics of a turbulent axisymmetric wake

Response of a laminar separation bubble to impulsive forcing

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