On the relation between fluid flow over bluff bodies and accompanying acoustic radiation.
Date
2008
Authors
Blazewicz, Antoni Michal
Editors
Advisors
Bull, M. K.
Kelso, Richard Malcolm
Pickles, J.M.
Kelso, Richard Malcolm
Pickles, J.M.
Journal Title
Journal ISSN
Volume Title
Type:
Thesis
Citation
Statement of Responsibility
Conference Name
Abstract
The relationship between distinctive characteristic fluid-flow regimes and the sound radiation generated by them has been investigated, over a range of Reynolds numbers, for various single plates and two-plate arrays in nominally two-dimensional flows. In preliminary experiments, the characteristics of flow over single plates with rectangular cross-section and faired leading edges and over tandem arrays of an upstream plate with rectangular cross-section and faired leading edges and a downstream plate of rectangular cross-section were investigated, together with the sound radiation produced. However, the main investigation has been concentrated on single plates of rectangular cross-section with various chord-to-thickness ratios C and on arrays of two plates of rectangular cross-section in tandem having various chord-to-thickness ratios C₁ and C₂ and a range of gaps (with gap-to-thickness ratios G) between them. The range of Reynolds number based on plate thickness t and free-stream velocity U, Re[subscript]t = Ut/ν (where ν is the kinematic viscosity of fluid) covered in the measurements is 3.2 x 10³ ≤ Re[subscript]t 53 x 10³. Spectra of velocity fluctuations in the flow and radiated sound have been measured and their characteristic frequencies related. An attempt has been made to measure force fluctuations on surfaces of the plates in order to relate them to flow characteristics and radiated sound power. Mean and fluctuating pressures associated with the force fluctuations on the plates have also been obtained. The lengths of separation bubbles on long rectangular plates have also been determined. In most cases, the measurements have been complemented by flow-visualisation in a water tunnel to provide additional detailed insight into the flow patterns.
Three flow regimes have been identified for single plates of rectangular cross-section. In the first regime (1 ≤ C ≤ 3.13), shear layers separated from the leading edges form a vortex street downstream of the plate without reattachment to it. Associated force fluctuations on the plate are the main source of acoustic radiation. In the second regime (3.05 ≤ C ≤ 9.65), the separated shear layers reattach intermittently to the streamwise plate surfaces. Vortex formation in the shear layer is the dominant cause of sound radiation but the effect becomes weaker as C increases. In the third regime (6.52 ≤ C ≤ 68), the separated shear layers form closed leading-edge separation bubbles. Weak vortex shedding, with only a small contribution to the sound radiation, occurs only at the trailing edges of the plate. Bistable behaviour of the flow over a plate, with random switching between the regimes, occurs for C ≈ 3 and 6.52 ≤ C ≤ 9.65.
A proposed classification of possible flow regimes for the flow around two plates of rectangular cross-section in tandem has been confirmed experimentally. For small G, the flow in the gap between the plates is isolated from the external flow. When the gap G between the plates is increased to or beyond a critical value (between 2 and 3.5), the shear layers separated from the upstream plate form a von Karman vortex street in the gap before interacting with the downstream plate. Flow and acoustic measurements indicate that this transition is associated with dramatic changes in the flow character.
School/Discipline
School of Mechanical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2008
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.