On the propagation of two types of disturbances through a laminar boundary layer subjected to separation and transition

Irps, T J B and Kanjirakkad, V (2019) On the propagation of two types of disturbances through a laminar boundary layer subjected to separation and transition. In: IUTAM symposium on Laminar-Turbulent Transition 2019, 02-06 Sept 2019, Imperial College, London.

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Abstract

In this paper we aim to discuss the propagation characteristics of two types of disturbances over a boundary layer subjected to laminar separation and subsequent transition to turbulent flow. The base boundary layer is formed over a flat plate subjected to a compressor blade like pressure field (over the suction side) using symmetrically contoured top and bottom walls within a wind tunnel test section. The boundary layer is initially laminar in nature in the accelerating part of the flow before undergoing deceleration due to the adverse pressure gradient (APG) resulting in separation and reattachment as turbulent boundary layer further downstream. The first of the disturbances that is studied is a synthetic puff introduced into the upstream laminar boundary layer using a loudspeaker. Such puffs were used in the past by various researchers to generate turbulent spots within boundary layers (Vasudevan et al., 2001, Experiments in Fluids). The second type of disturbance studied here is in the form of a propagating wake originating from an upstream cylindrical bar. The wake essentially is a disturbance introduced in the freestream that then interacts with the base boundary layer. In the past, researchers have looked at the effect of APG on propagation of turbulent spot produced using triggered loudspeaker puffs (Van Hest, 1995, Doctoral thesis, TU Delft). It was shown that the APG accelerated the growth rate of the turbulent spot compared to when no pressure gradient was present. However, the interaction of the disturbance with the separation bubble is not well understood. The effect of turbine like wakes on laminar separation bubble and subsequent transition was also studied (Coull and Hodson, 2011, Journal of Fluid Mechanics). It was shown that the wake induces amplified Klebanoff streaks prior to the separation point and further destabilises the Kelvin-Helmholtz (K-H) instability over the separated shear layer to produce short span K-H structures. Both these phenomena have been reported to encourage early transition. It is however unclear what happens in the presence of a compressor like wake and pressure field, which is the subject of the present investigation. The influence of both the disturbance types, i.e. wakes and puffs, within the plate boundary layer and their development and interaction with the separation bubble and the transition process is studied experimentally using surface pressure measurements, hotwire traverses and instantaneous PIV (particle image velocimetry data) measurements. The results are compared to aid further understanding of the instabilities leading to the transition process.

Item Type: Conference or Workshop Item (Poster)
Keywords: Boundary Layer, Separation Bubble. Flow Transition
Schools and Departments: School of Engineering and Informatics > Engineering and Design
Research Centres and Groups: Thermo-Fluid Mechanics Research Centre
Subjects: T Technology > TJ Mechanical engineering and machinery > TJ0163.13 Power resources
T Technology > TJ Mechanical engineering and machinery > TJ0255 Heat engines
T Technology > TJ Mechanical engineering and machinery > TJ0266 Turbines. Turbomachines (General)
T Technology > TJ Mechanical engineering and machinery > TJ0807 Renewable energy sources
Depositing User: Vasudevan Kanjirakkad
Date Deposited: 12 Mar 2020 09:33
Last Modified: 12 Mar 2020 09:33
URI: http://sro.sussex.ac.uk/id/eprint/90318
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