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 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

Background Peak in cavity drag at h/D 0.5

Background Cavity Feedback Resonance (Rossiter, 1964)

Background Vortices shed from upstream cavity lip

Background Vortices convected downstream

Background Vortex impinges on downstream lip

Background Acoustic pulse radiates upstream

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

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

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

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

Experimental Setup

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

Experimental Variables U 0 = 27.0 m/s, δ = 55 mm (δ/D = 0.72), Re D = 1.3 x 10 5

Results and Discussion Mean pressure distributions on sidewall

Results and Discussion Mean surface pressure distributions on cavity base

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

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

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

Results and Discussion RMS pressure distributions on sidewall

Results and Discussion RMS pressure distributions on cavity base

Results and Discussion Drag coefficient comparison

Results and Discussion Wake velocity profiles – Stream-wise velocity

Results and Discussion Wake velocity profiles – Stream-wise turbulence

Results and Discussion Frequency analysis – Estimate Frequencies Cavity Feedback Resonance: Predicted first-mode f = Hz

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

Results and Discussion Frequency analysis – Microphone in base

Results and Discussion Frequency analysis – Microphone in base

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

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

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

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

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

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