The effect of surface roughness on the turbulence structure of a plane wall jet

Abstract

In this paper, an experimental investigation of the turbulence characteristics of a plane wall jet over smooth and rough surfaces, using laser Doppler anemometry (LDA), is reported. The Reynolds number based on the slot height and exit velocity of the jet was approximately Re = 7500. A 36-grit sheet was used as the rough surface, creating a transitionally rough flow regime (44<ks+<70). Both inner and outer scales were used to analyze the effects of surface roughness on the Reynolds stress profiles. Comparisons between the present results and other LDA and hot-wire anemometry studies for a smooth surface indicate a similar behavior for the Reynolds stress profiles. However, the magnitudes of the peak values of the Reynolds stress were higher than in most previous studies due to the lower slot Reynolds number. The present results indicate that surface roughness does not appear to significantly modify the Reynolds stress profiles in the outer region of the jet except for a reduction in the level. In contrast, surface roughness modifies both the shape and magnitudes of the Reynolds stress profiles in the inner layer. Due to the much higher friction velocity for a rough surface, the magnitudes of both the streamwise and wall-normal Reynolds stress decrease in the inner region when normalized using inner scales compared to the smooth-wall values.

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.…”

“…As explained in Rostamy et al [20], the variable ε is the distance that the wall normal coordinate y, measured from the nominal top of the roughness elements, was adjusted downward to account for the virtual origin of the mean velocity profile. For the rough surface, all parameters related to the wall normal distance, i.e.…”

“…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

“…[22] It should be noted that the data reported in all the well-controlled experimental studies show that in the fully developed region of wall jets, profiles of first-order and second-order statistics normalised by the local outer length and velocity scales, u m and y 1/2 , are all within 20% of a mean profile. [5,6,29,46,68] However, in the experiment of Fairweather and Hargrave, [22] the Reynolds shear-stress profiles normalised by local u m and y 1/2 have a much larger scatter. Thus, the agreement between the simulated Reynolds shear-stress profile and the experimental data is reasonable, considering the experimental uncertainty.…”

“…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.…”