The Sound Radiated by Tip Clearances Submerged in a Boundary Layer

The present study investigates the behaviour of the far-field sound radiated by low Mach number tip clearance flow induced by placing a stationary cambered airfoil adjacent to a stationary wall. The tip clearance heights ranged from 14% to 30% of the incoming, undisturbed boundary layer thickness an...

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Bibliographic Details
Published in:arXiv.org 2024-10
Main Authors: Awasthi, Manuj, Moreau, Danielle, Croaker, Paul, Dylejko, Paul
Format: Article
Language:English
Subjects:
Airfoils
Angle of attack
Boundary layer thickness
Cambering
Far fields
Flow velocity
Fluid dynamics
Fluid flow
Leading edges
LF noise
Mach number
Noise generation
Pressure
Reynolds number
Separation
Tip clearance
Trailing edges
Turbulence
Turbulent flow
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Summary:The present study investigates the behaviour of the far-field sound radiated by low Mach number tip clearance flow induced by placing a stationary cambered airfoil adjacent to a stationary wall. The tip clearance heights ranged from 14% to 30% of the incoming, undisturbed boundary layer thickness and the clearance heights based Reynolds numbers were between 2,600 and 16,000. The far-field sound measured using a microphone array was beamformed to reveal the dominant noise sources and how they behave when the flow Mach number, angle of attack and the clearance height were varied. The near-field behaviour was also examined through PIV measurements and surface pressure fluctuation measurements on the tip. The results show that the mid-to-high frequency noise generated by tip clearances is dominated by the leakage flow in the mid-chord and leading-edge regions, while a distinct low-frequency noise source with a different scaling behaviour exists close to the trailing-edge of the tip clearance. The origin of this low-frequency noise source is believed to be the tip separation vortex that resides close to the trailing-edge and induces significant turbulence levels in the region. The strength of this noise source decreases with clearance height which is consistent with a reduction in turbulence levels associated with the separation vortex. The magnitude of the mid-frequency clearance noise which scales with the sixth power of the Mach number, decreases with tip clearance height due to a reduction in the fluctuating pressure on the airfoil tip surface. The time-scale of this sound was independent of the flow velocity, implying that the source is non-compact. Smaller tip clearances were also found to generate louder high-frequency noise due to intense turbulence and pressure fluctuation levels concentrated near the leading-edge of the clearance.
ISSN:2331-8422