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Robust features of a turbulent boundary layer subjected to high-intensity free-stream turbulence

Robust features of a turbulent boundary layer subjected to high-intensity free-stream turbulence
Robust features of a turbulent boundary layer subjected to high-intensity free-stream turbulence

The influence of the large scale organisation of free-stream turbulence on a turbulent boundary layer is investigated experimentally in a wind tunnel through hot-wire measurements. An active grid is used to generate high-intensity free-stream turbulence with 7.2% ≤ u'_∞ / U_∞ ≤ 13.0% and 302 ≤ Re_λ ,∞ ≤ 760. In particular, several cases are produced with fixed u'_∞ / U_∞ and Re_λ ,∞ , but up to a 65% change in L_u,∞ / δ . It is shown that while qualitatively the spectra at various wall-normal positions in the boundary layer look similar, there are quantifiable differences at the large wavelengths all the way to the wall. Nonetheless, profiles of the longitudinal statistics up to fourth-order are well collapsed between cases at the same u'_∞ / U_∞ . It is argued that a larger separation of the integral scale would not yield a different result, nor would it be physically realisable. Comparing cases across the wide range of turbulence intensities and free-stream Reynolds numbers tested, it is demonstrated that the near-wall spectral peak is independent of the free-stream turbulence, and seemingly universal. The outer peak was also found to be described by a set of global scaling laws, and hence both the near-wall and outer spectral peaks can be predicted a priori with only knowledge of the free-stream spectrum, the boundary layer thickness (δ ) and the frictional velocity U_τ . Finally, a conceptual model is suggested that attributes the increase in U_τ as u'_∞ / U_∞ increases to the build-up of energy at large wavelengths near the wall because that energy cannot be transferred to the universal near-wall spectral peak.
0022-1120
416-435
Hearst, Robert
965708e6-ddf4-4cbb-af74-866bb4cdb4de
Dogan, Eda
deb7b97e-7985-4d5b-98a7-d68852713207
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Hearst, Robert
965708e6-ddf4-4cbb-af74-866bb4cdb4de
Dogan, Eda
deb7b97e-7985-4d5b-98a7-d68852713207
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052

Hearst, Robert, Dogan, Eda and Ganapathisubramani, Bharathram (2018) Robust features of a turbulent boundary layer subjected to high-intensity free-stream turbulence. Journal of Fluid Mechanics, 851, 416-435. (doi:10.1017/jfm.2018.511).

Record type: Article

Abstract


The influence of the large scale organisation of free-stream turbulence on a turbulent boundary layer is investigated experimentally in a wind tunnel through hot-wire measurements. An active grid is used to generate high-intensity free-stream turbulence with 7.2% ≤ u'_∞ / U_∞ ≤ 13.0% and 302 ≤ Re_λ ,∞ ≤ 760. In particular, several cases are produced with fixed u'_∞ / U_∞ and Re_λ ,∞ , but up to a 65% change in L_u,∞ / δ . It is shown that while qualitatively the spectra at various wall-normal positions in the boundary layer look similar, there are quantifiable differences at the large wavelengths all the way to the wall. Nonetheless, profiles of the longitudinal statistics up to fourth-order are well collapsed between cases at the same u'_∞ / U_∞ . It is argued that a larger separation of the integral scale would not yield a different result, nor would it be physically realisable. Comparing cases across the wide range of turbulence intensities and free-stream Reynolds numbers tested, it is demonstrated that the near-wall spectral peak is independent of the free-stream turbulence, and seemingly universal. The outer peak was also found to be described by a set of global scaling laws, and hence both the near-wall and outer spectral peaks can be predicted a priori with only knowledge of the free-stream spectrum, the boundary layer thickness (δ ) and the frictional velocity U_τ . Finally, a conceptual model is suggested that attributes the increase in U_τ as u'_∞ / U_∞ increases to the build-up of energy at large wavelengths near the wall because that energy cannot be transferred to the universal near-wall spectral peak.

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Accepted/In Press date: 20 June 2018
e-pub ahead of print date: 20 July 2018
Published date: 25 September 2018

Identifiers

Local EPrints ID: 421516
URI: http://eprints.soton.ac.uk/id/eprint/421516
ISSN: 0022-1120
PURE UUID: 92b39b3b-92e1-4088-9b47-74b042ff5f83
ORCID for Bharathram Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

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Date deposited: 14 Jun 2018 16:30
Last modified: 07 Oct 2020 05:35

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