An Experimental Study of a Turbulent Wall Jet on Smooth and Transitionally Rough Surfaces

Abstract

The effect of surface roughness on the mean velocity and skin friction characteristics of a plane turbulent wall jet was experimentally investigated using laser Doppler anemometry. The Reynolds number based on the slot height and exit velocity of the jet was approximately Re = 7500. A 36-grit sheet was used to create a transitionally rough flow (44 < ks+ < 70). Measurements were carried out at downstream distances from the jet exit ranging from 20 to 80 slot heights. Both conventional and momentum-viscosity scaling were used to analyze the streamwise evolution of the flow on smooth and rough walls. Three different methods were employed to estimate the friction velocity in the fully developed region of the wall jet, which was then used to calculate the skin friction coefficient. This paper provides new experimental data for the case of a plane wall jet on a transitionally rough surface and uses it to quantify the effects of roughness on the momentum field. The present results indicate that the skin friction coefficient for the rough-wall case compared to a smooth wall increases by as much as 140%. Overall, the study suggests that for the transitionally rough regime considered in the present study, roughness effects are significant but mostly confined to the inner region of the wall jet.

Incomplete similarity of a plane turbulent wall jet on smooth and transitionally rough surfaces

“…They investigated the effects of roughness on both the mean and fluctuating velocity fields, and concluded that the effects of roughness are mostly confined to the inner region of the wall jet. Finally, Tang et al [18] assessed the similarity behaviour in both the inner and outer regions of the wall jet based on the rough surface data of Rostamy and co-workers, [19,20] and observed agreement with the incomplete similarity theory of Barenblatt et al [10]. A general conclusion based on the above studies is that roughness effects are mostly confined to the inner layer and do not appear to affect the coupling of the inner and outer regions.…”

“…x/H ≤ 70, the mean flow was essentially two-dimensional; no three-dimensional effects were observed to be significant in this region. Further discussion of the experiments can be found in references [19,20].…”

“…Note that two previous papers, i.e. Rostamy et al [19] and Rostamy et al [20], have documented extensive assessments of the mean and turbulence velocity fields, respectively, based on the same set of experimental measurements. This paper focuses specifically on scaling issues, and integrates and further develops some preliminary results previously presented at two conference venues.…”

Interaction of inner and outer layers in plane and radial wall jets

“…The numerical model for the plane wall jet was previously validated [10] using the experiment of Rostamy et al [51,68] at Re = 7500. To further validate the model for cylindrical coordinates, the simulation of a radial wall jet was performed and compared with the experiment of Fairweather and Hargrave.…”

“…Some of the experiments indicate that the wall jet in a stagnant surrounding is self-similar when the local u m and y 1/2 are considered as the similarity parameters, [5,47,51,52] while others considered u m and y m as the similarity parameters. [40] Glauert, [3] who was the first who used the term 'wall jet', derived an analytical solution for both laminar and turbulent cases, suggested a gradual change of the similarity profile with the Reynolds number; his theory, however, makes the incorrect assumption of zero Reynolds shear stress at the point of maximum velocity.…”