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An Experimental Investigation of Turbulent Boundary Layer Flow over Surface-Mounted Circular Cavities

An Experimental Investigation of Turbulent Boundary Layer Flow over Surface-Mounted Circular Cavities. Jesse Dybenko Eric Savory Department of Mechanical and Materials Engineering University of Western Ontario, London, ON May 24, 2006. Flow Geometry. Motivation.

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An Experimental Investigation of Turbulent Boundary Layer Flow over Surface-Mounted Circular Cavities

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  1. An Experimental Investigation of TurbulentBoundaryLayerFlow over Surface-Mounted Circular Cavities Jesse Dybenko Eric Savory Department of Mechanical and Materials Engineering University of Western Ontario, London, ON May 24, 2006

  2. Flow Geometry

  3. Motivation • Cavities are found on aircraft and automobiles • Landing gear wheel wells • Recessed windows • Sun roofs • Symmetric geometry, asymmetric mean flow • Not well researched • A better understanding of these flows could lead to drag and noise reduction for airframes

  4. Background • Peak in cavity drag at h/D 0.5

  5. Background • Cavity Feedback Resonance (Rossiter, 1964)

  6. Background • Vortices shed from upstream cavity lip

  7. Background • Vortices convected downstream

  8. Background • Vortex impinges on downstream lip

  9. Background • Acoustic pulse radiates upstream

  10. Background • Pulse disturbs shear layer – causes vortex to be shed. Feedback loop is closed.

  11. Background • Frequency associated with this mechanism can be estimated using Rossiter’s Formula (Rossiter, 1964): • f is predicted oscillation frequency, m is integer mode number, is vortex-sound pulse lag-time factor, M is free stream Mach number, is ratio of vortex convection velocity to free stream velocity

  12. Background • Oscillation can also occur according to the depth scale of the cavity: depth-mode resonance • Can also estimate frequency due to this mechanism: • f is predicted oscillation frequency, N is odd-integer mode number, c is speed of sound in air, h is cavity depth

  13. Major Objectives • To understand the causes of abnormal flow in cavity and in its wake for h/D 0.5 • What causes this flow to differ from flow at other depth configurations? • To investigate the fluctuating nature of the flows at various cavity depths and their relationship with resulting cavity drag

  14. Experimental Setup

  15. Experimental Techniques • Three cavity depth ratios were used for measurements: • h/D = 0.20, 0.47 and 0.70 • Cavity depth was only variable • Three systems were used for measurements: • Pressure transducers • Surface pressure distribution • Microphones • Acoustic response of cavity • Two-component hot-wire anemometry • Mean Velocity and Turbulence Profiles in wake

  16. Experimental Variables • U0 = 27.0 m/s, δ = 55 mm (δ/D = 0.72), ReD = 1.3 x 105

  17. Results and Discussion • Mean pressure distributions on sidewall

  18. Results and Discussion • Mean surface pressure distributions on cavity base

  19. Results and Discussion • Vortex Skeleton Diagrams – h/D = 0.2

  20. Results and Discussion • Vortex Skeleton Diagrams – h/D = 0.47

  21. Results and Discussion • Vortex Skeleton Diagrams – h/D = 0.70

  22. Results and Discussion • RMS pressure distributions on sidewall

  23. Results and Discussion • RMS pressure distributions on cavity base

  24. Results and Discussion • Drag coefficient comparison

  25. Results and Discussion • Wake velocity profiles – Stream-wise velocity

  26. Results and Discussion • Wake velocity profiles – Stream-wise turbulence

  27. Results and Discussion • Frequency analysis • Estimate Frequencies • Cavity Feedback Resonance: • Predicted first-mode f = 145.5 Hz

  28. Results and Discussion • Frequency analysis • Estimate Frequencies • Depth Mode Resonance: • Predicted first-mode frequencies are depth-dependent, for three depths tested: • h/D = 0.20  f = 2329 Hz • h/D = 0.47  f = 1512 Hz • h/D = 0.70  f = 1164 Hz

  29. Results and Discussion • Frequency analysis – Microphone in base

  30. Results and Discussion • Frequency analysis – Microphone in base

  31. Conclusions • Pressure Measurements • RMS pressure patterns show maxima at shear layer reattachment points and vortex centres • Mean pressure patterns agree well with those done by previous investigators • Integrated drag coefficients also match well with previous data

  32. Conclusions • Wake Flow Analysis • Symmetric velocity and turbulence profiles for h/D = 0.20 • Asymmetric for h/D = 0.47, showing clear, circular trailing vortex feature, which can be disturbed to switch sides • In this feature, mean streamwise velocity is at a minimum, turbulence at a maximum

  33. Conclusions • Frequency analysis • Possible link between cavity feedback resonance and abnormal flow behaviour at h/D = 0.47 • Depth mode oscillations occur for h/D = 0.47 and 0.70; 0.20 not deep enough

  34. Recommendations • Aerodynamic Design • If circular cavity required on vehicle frame, shallow holes are best (h/D = 0.20 or less) • low drag, low noise in high frequency band, no resonances

  35. Acknowledgements • Technicians at BLWTL • Prof. Gregory Kopp • University Machine Shop • Advanced Fluid Mechanics Research Group • Tom Hering and Rita Patel

  36. Questions? For additional information on research done by the AFM Research Group, try our website: http://www.eng.uwo.ca/research/afm

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