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INFLUENCES OF LARGE-SCALE STRUCTURES ON SKIN FRICTION IN AN ADVERSE PRESSURE GRADIENT TURBULENT BOUNDARY LAYER

Min Yoon
Dept. of Mechanical Engineering KAIST Daejeon 305-701, Korea

Jinyul Hwang
Department of Mechanical Engineering Korea Advanced Institute of Science and Technology (KAIST) Daejeon 34141, Korea

Hyung Jin Sung
Department of Mechanical Engineering, KAIST 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea

Sinopsis

Direct numerical simulation (DNS) of a turbulent boundary layer (TBL) subjected to adverse pressure gradient (APG) at Reτ = 834 is performed to investigate large-scale influences on vortical motions. For comparison, DNS data of a zero pressure gradient (ZPG) TBL at Reτ = 837 is analyzed. The spanwise energy spectra of the streamwise velocity fluctuations show that the large-scale energy above λz+~400 (λz/δ ~ 0,5) is significantly enhanced in the APG TBL. Large-scale streamwise velocity fluctuations (uL) is extracted by employing a long-wavelengthpass filter with a cut-off wavelength of λz+ ~ 400. Two velocity-vorticity correlations (〈vωz and 〈−wωy) which represent the advective vorticity transport and vortex stretching, respectively, are conditionally averaged with respect to uL to explore the extension of large-scale influences on the vortical motions. The velocity-vorticity correlations are directly related to the skin friction coefficient (Cƒ). The total Cƒ in the APG TBL is reduced by 28% from that in the ZPG TBL. The skin friction induced by 〈vωz and 〈−wωy contribute negatively and positively to the total Cƒ respectively. In the APG TBL, the negative contribution of 〈vωz decreases 29.6%, while the positive contribution of 〈−wωy slightly increases about 7.0%. Under the intense negative and positive uL (uL+ ≤ −2 and uL+ ≥+2), the contribution of 〈wωz in the APG TBL is enhanced 8.33 and 2.72 times compared to the ZPG TBL. The skin friction induced by 〈−wωy increases 1.8 times only under uL+ ≥ +2 in the APG TBL. The enhanced largescale motions in the outer region strongly modulate the vortical motions. In particular, the low-speed structures augment the contribution of the advective vorticity transport and the contribution of the vortex stretching is enhanced under the influence of the high-speed structures in the APG TBL.