Two scenarios for bypass transition in a boundary layer subjected to a localized patch of distributed surface roughness

Anand, A and Diwan, SS (2019) Two scenarios for bypass transition in a boundary layer subjected to a localized patch of distributed surface roughness. In: 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019, 30 July - 2 Aug 2019, Southampton, UK.

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Abstract

Here, we present two scenarios for bypass transition in a boundary layer downstream of a streamwise-localized patch of a distributed surface roughness. The two cases chosen for comparison (one at a relatively low speed and farther from the roughness, and the other at a higher speed but close to the roughness) are in the late stages of transition and have nominally similar Reynolds numbers and shape factors. However, the time-frequency behaviour, investigated using the Fourier and wavelet analysis, shows many contrasting features for the two cases. The pre-multiplied spectrum for the low-speed case shows a bimodal shape whereas that for the high- speed case is unimodal in shape. �Events� are detected in the time-series for the wavelet coefficients for various frequencies. It is observed that for the low-speed case the wavelet events become increasingly more localized with increase in frequency whereas in the high-speed case the event intermittency remains constant over the entire frequency range. Furthermore, there seems to be a clustering of the events for the low-speed case, which can be identified with the turbulent spots in the velocity signal whereas no such clustering is observed for the high-speed case. The �spotty� transition observed for the low-speed case shares similarities with the bypass transition induced by freestream turbulence as well as the �natural� transition process. On the other hand, the �non-spotty� transition for the high-speed case seems to take a somewhat different transition route (in the late stages), wherein it proceeds without appearance of distinct turbulent spots. Such a scenario could have implications towards turbomachinery flow situations. © 2019 International Symposium on Turbulence and Shear Flow Phenomena, TSFP. All rights reserved.

Item Type:	Conference Paper
Publication:	11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019
Publisher:	International Symposium on Turbulence and Shear Flow Phenomena, TSFP
Additional Information:	cited By 1; Conference of 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019 ; Conference Date: 30 July 2019 Through 2 August 2019; Conference Code:151732
Keywords:	Atmospheric thermodynamics; Boundary layers; Reynolds number; Shear flow; Speed; Turbulence, Bypass transition; Freestream turbulence; Frequency ranges; Transition process; Turbomachinery flows; Turbulent spots; Velocity signals; Wavelet coefficients, Surface roughness
Department/Centre:	Division of Mechanical Sciences > Aerospace Engineering(Formerly Aeronautical Engineering)
Date Deposited:	23 Nov 2020 07:06
Last Modified:	28 Aug 2022 09:06
URI:	https://eprints.iisc.ac.in/id/eprint/65392

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