Structure of turbulent boundary layers on smooth and rough walls

Abstract

The structure of turbulent boundary layers which develop with zero pressure gradient on a smooth wall and a k-type rough wall was examined using arrays of X-wires. Although the data were obtained only on two orthogonal planes, the technique provides some information on the three-dimensionality of the large-scale structures. The major effect of the roughness is to tilt the inclination of the structures towards the wall-normal direction. This is caused by the reduced damping of the wall-normal velocity fluctuations close to the rough surface and the break-up of structures whose scales are comparable to the size of the roughness elements. Both effects cause a reduction in the streamwise lengthscales, as suggested by all the measured two-point correlations. The correlations also show that the roughness tends to reduce the overall anisotropy of the large-scale motion. There is evidence to suggest that the magnitude of the vorticity field is larger over the rough wall.

On the Effects of Surface Morphology on the Structure of Wall-Turbulence

“…The current results are in line with previous findings on both smooth and rough walls investigations, which suggested similar values for the characteristic inclination of the packets [17,18,8,6,9,5,1,15,12]. On the contrary, the current results do not seem to suggest that the surface morphology can have a significant influence on the vortex packets inclination, certainly not to the extend some researchers have previously documented [11]. Also reported in figure 1.1 (b) are correlations of the wall-normal fluctuations, R v v .…”

Turbulent flow in a ribbed channel: Flow structures in the vicinity of a rib

“…9a, the elongated contour of R uu is nearly aligned in the x-direction, while the contour of R vv is aligned in the y-direction. The yalignment of R vv was also noticed by previous studies in turbulent channel flow [12] and turbulent boundary layer flow over smooth and rough surface [13]. Further inspection of Fig.…”

Similarity of the streamwise velocity component in very-rough-wall channel flow

“…The authors attributed the apparent discrepancy to fundamental differences between channel and boundary layer flows. This conclusion was also reached by Bakken et al (2005) for similarly scaled roughness over a range of Re τ , as well as by Volino, Schultz & Flack (2007), whose correlations showed no evidence of structural differences between the boundary layers over a smooth wall and a mesh-roughened wall geometrically similar to that of Krogstad & Antonia (1994) with k/δ = 1.4 %. Flack, Schultz & Shapiro (2005) also showed that outer scaling was satisfied in a boundary layer for velocity statistics up to the third order for k/δ 6 2.2 %.…”

“…Krogstad, Antonia & Browne (1992) compared the boundary layer developed over a smooth wall with that developed over a wall roughened by a wire mesh with k/δ ≈ 2.1 %: they found a significant increase in the wall-normal turbulence intensity, v 2 , through the outer layer, as well as an increase in the frequency and magnitude of sweep and ejection events. Krogstad & Antonia (1994) later demonstrated that these observations could be attributed to a rotation of the large-scale structure towards the wall-normal axis relative to the smooth-wall case. These observations were corroborated by the particleimage velocimetry results of Keirsbulck et al (2002) and the direct numerical channel simulations of Bhaganagar, Kim & Coleman (2004), with k/δ = 3.8 and 5.4 %, respectively.…”