Abstract
This study reports an experimental investigation of the effects of upstream roughness on a low Reynolds number turbulent boundary layer over a forward-facing step. Two types of upstream roughness were investigated, including a transitionally rough 16-grit sandpaper (ks+ ≈ 69) and fully rough staggered cubes (ks+ ≈ 500). A two-dimensional two-component time-resolved particle image velocimetry (2D-2C TR-PIV) method was used to measure the time-averaged mean velocities, Reynolds stresses, temporal auto-correlations and frequency spectra of the flow field to quantify the influence of upstream roughness on the downstream evolution of the turbulence over the step. The results indicate that upstream roughness decreased the vortex shedding frequency. Roughness also decreased the reattachment length by enhancing the streamwise turbulence intensity level, reducing the magnitude of backflow and suppressing the vortex shedding frequency in comparison to the smooth wall. In the recirculation region, upstream roughness reduced the mean streamwise velocity only in the outer layer. The Reynolds stresses remained relatively unchanged by the sandpaper roughness but were significantly modified by the cube roughness. Downstream of the leading edge, the staggered cubes increased the streamwise Reynolds stress both near the wall and outside the shear layer but decreased the wall-normal Reynolds stress and Reynolds shear stress within the shear layer. These modifications are inversely proportional to distance in the recirculation region. The life times of the streamwise and wall-normal velocity fluctuations increase with streamwise distance and are much longer in the redevelopment region than in the recirculation region. Quadrant decomposition and joint probability density functions of the velocity fluctuations were also measured to characterize upstream roughness effects on the downstream evolution of the dominant motions producing the Reynolds shear stress.
Highlights
Separated and reattached turbulent flows are of enduring interest in fluids engineering as they play a key role in the transport and mixing of fluids
The low pass filtering effect of particle image velocimetry (PIV) is a well-known source of bias error in PIV measurements which arises from the integration of the instantaneous velocities by the correlation algorithm over the interrogation area
A planar time-resolved particle image velocimetry method was used to investigate the effects of upstream roughness on the flow characteristics of a low Reynolds number turbulent boundary layer over a forward-facing step
Summary
Closure models that capture the essential features of separation, reattachment and redevelopment of the boundary layer still remains a formidable task. The roughness elements (ks+ = 84) were placed on the top surface of the step, while the upstream wall was kept smooth at an approach flow Reynolds number of Reh = 3450 and relative boundary layer thickness, δ/h = 8 Their results indicate that wall roughness reduced the mean velocities as well as the Reynolds stresses in the primary recirculation region, while the upstream flow remained relatively invariant with roughness. It was found that the reattachment length increased with increasing Reynolds number over the smooth wall but decreased with wall roughness at similar Reynolds number They reported that the profiles of the mean velocity and Reynolds stresses on the FFS did not attain self-similarity even at 60 step heights downstream of the leading edge. The 2D-2C TR-PIV was used to measure the instantaneous vector fields, probability density functions and the distributions of the time-averaged mean velocities, Reynolds stresses, and frequency spectra to quantify the impact of upstream roughness on the flow structure
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