Effects of wall suction on a 2D rough wall turbulent boundary layer

Two-dimensional local wall suction is applied to a fully developed turbulent boundary layer such that near-wall turbulence structures are completely sucked out, but most of the turbulent vortices in the original outer layer can survive the suction and cause the resulting laminar flow to undergo re-transition. This enables us to observe and clarify the whole process by which the suction-surviving strong vortical motions give rise to near-wall low-speed streaks and eventually generate wall turbulence. Hot-wire and particle image velocimetry (PIV) measurements show that low-frequency velocity fluctuations, which are markedly suppressed near the wall by the local wall suction, soon start to grow downstream of the suction. The growth of low-frequency fluctuations is algebraic. This characterizes the streak growth caused by the suction-surviving turbulent vortices. The low-speed streaks obtain almost the same spanwise spacing as that of the original turbulent boundary layer without the suction even in the initial stage of the streak development. This indicates that the suction-surviving turbulent vortices are efficient in exciting the necessary ingredients for the wall turbulence, namely, low-speed streaks of the correct scale. After attaining near-saturation, the low-speed streaks soon undergo sinuous instability to lead to re-transition. Flow visualization shows that the streak instability and its subsequent breakdown occur at random in space and time in spite of the spanwise arrangement of streaks being almost periodic. Even under the high-intensity turbulence conditions, the sinuous instability amplifies disturbances of almost the same wavelength as predicted from the linear stability theory, though the actual growth is in the form of a wave packet with not more than two waves. It should be emphasized that the mean velocity develops the log-law profile as the streak breakdown proceeds. The transient growth and eventual breakdown of low-speed streaks are also discussed in connection with the critical condition for the wall-turbulence generation.

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Small-scale characteristics of a turbulent boundary layer over a rough wall

Journal of Fluid Mechanics ◽

10.1017/s0022112097005612 ◽

1997 ◽

Vol 342 ◽

pp. 263-293 ◽

Cited By ~ 22

Author(s):

H. S. SHAFI ◽

R. A. ANTONIA

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Outer Layer ◽

Large Scale ◽

Energy Dissipation Rate ◽

Wall Layer ◽

Rough Wall ◽

Small Scale ◽

Smooth Wall ◽

Velocity Derivative

Measurements of the spanwise and wall-normal components of vorticity and their constituent velocity derivative fluctuations have been made in a turbulent boundary layer over a mesh-screen rough wall using a four-hot-wire vorticity probe. The measured spectra and variances of vorticity and velocity derivatives have been corrected for the effect of spatial resolution. The high-wavenumber behaviour of the spectra conforms closely with isotropy. Over most of the outer layer, the normalized magnitudes of the velocity derivative variances differ significantly from those over a smooth wall layer. The differences are such that the variances are much more nearly isotropic over the rough wall than on the smooth wall. This behaviour is consistent with earlier observations that the large-scale structure in this rough wall layer is more isotropic than that in a smooth wall layer. Isotropy-based approximations for the mean energy dissipation rate and mean enstrophy are consequently more reliable in this rough wall layer than in a smooth wall layer. In the outer layer, the vorticity variances are slightly larger than those over a smooth wall; reflecting structural differences between the two flows.

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