Development of turbulent boundary layers past a step change in wall roughness

Abstract

In this study, the development of a boundary layer past a change in surface roughness (from rough to smooth, R→S) is examined. Measurements of the flow were made by hotwires, whereas the friction velocity was estimated by Preston tube measurements. By means of a diagnostic plot of the turbulence intensity, it is shown that above the internal layer the flow exhibits characteristics of a rough wall-bounded flow, whereas near the wall the turbulence intensity is similar to that of an isolated smooth wall. Similarly, viscous scaling of the mean streamwise velocity shows an excessive wake region downstream of the R→S wall surface change that diminishes with the fetch from the surface change. Above the internal layer a second peak in the streamwise Reynolds stress was associated with the upstream rough wall flow. Examination of the turbulent spectra revealed the presence of large scale motions within this region that gradually diminishes in strength with increasing distance from the change in surface roughness. The magnitude of the near-wall peak failed to collapse to that of a comparable smooth wall boundary layer under viscous scaling, however, the wall-normal location of the peak appears to be at y + ≈ 15 at all downstream distances. A new mixed scaling is proposed for the nearwall peak based on the corrected wake deficit and the friction velocity. This shows the importance of outer region to the growth of near-wall peak and suggests the presence of amplitude modulation of the near-wall region by the outer region in this non-equilibrium boundary layer.

Simulations of rib-roughened rough-to-smooth turbulent channel flows

“…The expression for z s is derived by equating the log-law in terms of viscous length-scale, δ ν = ν/u τ , to an equivalent log-law in terms of a hydrodynamic roughness length-scale, z s . Values of M given in table 4 are comparable to M = 3.4 and 5.1, reported by Hanson & Ganapathisubramani (2016). Hosni & Coleman (1993) reported M = 3.15 for their RTS boundary layer experiments.…”

“…By the second streamwise station, at x = 2.08 in figure 11a, U + has attained a qualitative shape that remains essentially unchanged and gradually shifts upwards with downstream distance. This slow upward shift is contrary to the trend observed by Hanson & Ganapathisubramani (2016). With downstream fetch, their profiles were initially pushed up, well above the smooth wall level (a consequence of low u τ ) and then settled down toward it.…”

Flow characteristics and scaling past highly porous wall-mounted fences

“…This is shown in figure 5 where the streamwise evolution of the mean profiles 15 is studied. Whilst for the sake of brevity, the linear fit in the shear layer is only shown for the Fra grid; no 16 qualitative changes are observed for the other grids. As expected, the thickness of the shear layer increases 17 with the streamwise coordinate and, similarly, the intensity of the shear (dū/dy) decreases with x. Forx > 24…”

“…This may be caused 16 by two different reasons: firstly, increasing δ I implies that the highly intermittent region of the TBL appears 1 at a similar height as the shear layer. This interaction may increase the fluctuations of the shear-layer peak.…”

“…The latter scaling was first suggested to represent the point at 16 which the spectral contours are narrowest (i.e. the point which separates the wall and shear layer contributions).…”