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Research Papers: Fundamental Issues and Canonical Flows

Effects of Freestream Turbulence on the Characteristics in the Boundary Layer Near the Transition Onset Location

[+] Author and Article Information
Norah Patten

e-mail: norah.patten@ul.ie

Trevor M. Young

Department of Mechanical,
Aeronautical and Biomedical Engineering,
University of Limerick,
Limerick, Ireland

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received January 9, 2012; final manuscript received March 6, 2013; published online May 17, 2013. Assoc. Editor: Michael G. Olsen.

J. Fluids Eng 135(7), 071203 (May 17, 2013) (9 pages) Paper No: FE-12-1005; doi: 10.1115/1.4023989 History: Received January 09, 2012; Revised March 06, 2013

Experimental data is presented for a flat plate test facility with augmented levels of freestream turbulence (FST). The turbulence decay downstream of two turbulence generating grids, in addition to the integral length scales, is provided and good comparison with established correlations is presented. Boundary layer measurements using a single normal hotwire probe were obtained at FST intensities of 7%, 6%, 5.5%, 1.55%, and 1.45%, and the results presented include the fifth and 95th percentile of the velocity fluctuations and the root mean squared (RMS) velocity profiles near the transition onset region. The transition onset Reynolds number for each of the turbulence levels considered is consistent with theoretical findings. In all cases analyzed, the maximum fifth and 95th percentile far exceeded the maximum RMS values, with the location of the maximum 95th percentile closer to the wall compared to the maximum fifth percentile. Using probability density function (PDF) analysis, it is demonstrated that there is a dominating positive velocity fluctuation in the near-wall region and a dominating negative velocity fluctuation further out in the boundary layer and that the fluctuations in the boundary layer are greater compared to the freestream. The effect of the FST on the boundary layer is discussed with comparison to the Blasius solution and the influence of the fluctuations on the deviation from the Blasius profile is presented and discussed. Through investigation of the energy spectrum of the fluctuating velocity component within the boundary layer, it is shown that there is a higher energy content at lower frequency in the boundary layer when compared to that of the freestream.

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Figures

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Fig. 1

Comparison between experimental results and theoretical Blasius solution (o = Reθ 158, • = Reθ 249, x = Reθ 275, Δ = Reθ 291, + = Reθ 373, —Blasius solution)

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Fig. 2

Nondimensional velocity profiles compared to linear law of the wall (— linear law of the wall) for the same conditions as Fig. 1

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Fig. 3

(a) Turbulence decay downstream of each grid compared to the Roach (1987) correlation. (— = 1.13(x/d)-5/7; - - - = 0.8(x/d)-5/7; + = SMR, Red = 1000; o = SMR, Red = 750; Δ = PP, Red = 2500; X = PP, Red = 1400. Red is the Reynolds number based on the thickness of the grid bar/wire). (b) Integral length scale growth downstream of the grids. (— = 0.2(x/d)0.5; + = Estimation using (Eq. (7)) for SMR, Red = 1000;  □= Estimation using (Eq. 7) for PP, Red = 1400; o Estimation using (Eq. (6)) for SMR, Red = 1000; X = Estimation using (Eq. (6)) for PP, Red = 1400).

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Fig. 4

(a) Nondimensional velocity profiles compared to linear law of the wall (— linear law of the wall). (b) Normalized RMS velocity profiles where δ1 is the displacement thickness. (+ = Tu 7%, Reθ = 121; x = Tu 6%, Reθ = 145; o = Tu 5.5%, Reθ = 147; ◊ = Tu 5.2%, Reθ = 160; Δ = Tu 1.55%, Reθ = 304; • = Tu 1.45% Reθ = 323)

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Fig. 5

Deviation in the velocity profiles due to the increase in FST compared to the Blasius profile. (+ = Tu 7%, Reθ = 121; x = Tu 6%, Reθ = 145; o = Tu 5.5%, Reθ = 147; ◊ = Tu 5.2%, Reθ = 160; Δ = Tu 1.55%, Reθ = 304; • = Tu 1.45% Reθ = 323; — Blasius profile).

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Fig. 6

Probability Density Function (PDF) at near-wall, mid-boundary layer and boundary layer edge locations. (a) Tu = 7%, Reθ = 121, o = y/δ 0.28, x = y/δ 0.5, + = y/δ 1; (b) Tu = 6%, Reθ = 145, o = y/δ 0.25, x = y/δ 0.49, + = y/δ 1; (c) Tu = 5.5%, Reθ = 147, o = y/δ 0.27, x = y/δ 0.5, + = y/δ 1; and (d) Tu = 1.55%, Reθ = 304, o = y/δ 0.34, x = y/δ 0.5, + = y/δ 0.98.

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Fig. 7

PDF analysis at different freestream turbulence and at the same wall normal locations. •: Tu = 1.55%, Reθ = 304; ▴: Tu = 7%, Reθ = 121; Open symbols represent y/δ=0.5 and closed symbols represent y/δ=1.1.

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Fig. 8

Fifth percentile (average 5% negative velocity), 95th percentile (average 5% positive velocity) and RMS velocity components normalized with the freestream velocity upstream of transition onset. (a) Tu = 7%, Reθ 121; (b) Tu = 6%, Reθ = 145; (c) Tu 5.5%, Reθ = 147; and (d) Tu 1.55%, Reθ = 304. (▴ = 5th percentile; ▪ = 95th percentile; • = RMS).

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Fig. 9

Spectral analysis (coresponding to traces provided in Fig. 8) at different freestream turbulence and at the same wall normal locations. (a) ▴: Tu = 7%, Reθ = 121. (b) •: Tu = 1.55%, Reθ = 304. Open symbols represent y/δ=0.5 and closed symbols represent y/δ=1.1.

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