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High and low frequency dominated boundary layer transition through widely spaced discrete suction perforations

Crowley, B. J. (2020). High and low frequency dominated boundary layer transition through widely spaced discrete suction perforations. (Unpublished Doctoral thesis, City, University of London)

Abstract

Boundary layer or wall-suction is a drag reduction technology with application to civil aircraft wings. Drag is reduced by increasing the extent of laminar flow across the boundary layer. Laminar flow has lower friction and form drag compared to turbulent flow. Wall-suction works by accelerating the air towards the surface of the wing, which has a stabilising effect on disturbances in the boundary layer; it also reduces the boundary layer thickness and thus the Reynolds number. In practice, this accelerated air must permeate through discrete pores in the surface. This creates local three-dimensional flow structures, which have been known to destabilise the flow and reduce the extent of laminar flow over wings, typically when the suction mass flow-rate is high. When this occurs the wall suction is said to be in a state of ‘over-suction’.

In this work a set of experimental studies were performed on the problem of ‘over-suction’ for large arrays of suction perforations (in a widely spaced configuration), and for an isolated single suction perforation. It was found that the suction introduced a low frequency disturbance, this disturbance was found to grow (in amplitude) proportional to the suction rate. Furthermore, it was found that this low frequency was attenuated down-stream of the suction perforations, unless a pre-established N-type transition front was located close to the suction array. In this case the low-frequency was seen to interact with the N-type transition front, causing it to move upstream when at high suction rates.The amplitude of the low frequency also appeared to be Reynolds number independent, suggesting it may be an inflectional instability.

The low frequency was correlated with inflection points in the stream-wise velocity‘u’profile across the span ‘z’. It has also been correlated with inflection points in the span-wise velocity ‘w’ profile across the wall-normal ‘y’.

In the absence of any pre-established transition front, a high frequency disturbance was found to dominate the ‘over-suction’ process. This high frequency was found to only appear when the suction mass flow rate was sufficiently high, whereas the low frequency was found at all suction rates, and grew proportionally the non-dimensional suction mass flow-rate.

This high frequency was also correlated with inflection points in the stream-wise velocity ‘u’ profile across the span‘z’. It has also been correlated with inflection points in the stream-wise velocity ‘u’ profile across the wall-normal ‘y’.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering > Engineering > Mechanical Engineering & Aeronautics
Date Deposited: 12 Aug 2020 14:21
URI: https://openaccess.city.ac.uk/id/eprint/24744
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