Large-scale influences in near-wall turbulence

TLDR: Hot-wire data acquired in a high Reynolds number facility are used to illustrate the need for adequate scale separation when considering the coherent structure in wall-bounded turbulence and it is found that a large-scale motion in the log region becomes increasingly comparable to the near-wall cycle as the Reynolds number increases.

Abstract: Hot-wire data acquired in a high Reynolds number facility are used to illustrate the need for adequate scale separation when considering the coherent structure in wall-bounded turbulence. It is found that a large-scale motion in the log region becomes increasingly comparable in energy to the near-wall cycle as the Reynolds number increases. Through decomposition of fluctuating velocity signals, it is shown that this large-scale motion has a distinct modulating influence on the small-scale energy (akin to amplitude modulation). Reassessment of DNS data, in light of these results, shows similar trends, with the rate and intensity of production due to the near-wall cycle subject to a modulating influence from the largest-scale motions.

Figures

Figure 2. Pre-multiplied energy spectra of streamwise velocity fluctuation kxfuu=U 2 t for Retz1000. See the caption of figure 1 for details.

Figure 4. Example fluctuating u signals in the near-wall region, zCZ15 (Retz7300). (a) Raw fluctuating component, (b) large-scale fluctuation lCx O7300, (c) small-scale fluctuation l C x !7300 and (d ) smaller-scale fluctuation lCx !1000. Dashed vertical lines show region of negative largescale fluctuation.

Figure 7. Conditionally averaged fields on the occurrence of a low-speed u event at zCz150. (a) Conditional u field huj(u!0)iC. (b) Conditional swirl field hswirlj(u!0)iC normalized by the mean swirl at each height. Contours show: solid, uCZ0.1; dashed, uCZK0.1. Arrows show the vector due to hvj(u!0)i and hwj(u!0)iC.

Figure 1. Pre-multiplied energy spectra of streamwise velocity fluctuation kxfuu=U 2 t for a turbulent boundary layer at Retz7300. (a) In the viscous buffer region at z Cz15, (b) in the logarithmic region at z/dz0.06, (c) as a three-dimensional surface plot for all wall-normal locations and (d ) corresponding (open small circle) mean velocity and (filled circle) broadband turbulence intensity profiles where the dotted line shows linear behaviour in the viscous sublayer UCZzC and the solid line shows UCZ(1/k)ln(zC)CA (where kZ0.41 and AZ5.0). For all plots, the dashed lines denote locations of inner and outer spectral peaks.

Figure 5. Turbulent intensity of decomposed small-scale energy under (dashed) negative and (solid) positive large-scale fluctuations. Curves sum to the dotted line on figure 3b.

Figure 6. Example fluctuating velocity signals from the Retz950 channel of del Álamo et al. (2004) at zCZ15. (a) Fluctuating u component, thicker line shows large-scale component, (b) wall-normal w fluctuation, (c) spanwise v fluctuation, (d ) Reynolds shear-stress fluctuation. Dashed vertical lines show region of negative large-scale fluctuation.