Near field development of artificially generated high Reynolds number turbulent boundary layers

Abstract

Particle image velocimetry (PIV) is conducted in the near field of two distinct wall-mounted trips for the artificial generation of a high Reynolds number turbulent boundary layer. The first of these trips consists of high aspect ratio obstacles which are supposed to minimize the influence of their wakes on the near-wall region, contrasting with low aspect ratio trips which would enhance this influence.A comprehensive study involving flow description, turbulent/non-turbulent interface detection, a low order model description of the flow and an exploration of the influence of the wake in the near-wall region is conducted and two different mechanisms are clearly identified and described. First, high aspect ratio trips generate a wall-driven mechanism whose characteristics are: a thinner, sharper and less tortuous turbulent/non-turbulent interface and a reduced influence of the trips' wake in the near-wall region. Second, low aspect ratio trips generate a wake-driven mechanisms in which their turbulent/nonturbulent interface is thicker, less sharply defined and with a higher tortuosity and the detached wake of the obstacles presents a significant influence on the near-wall region. Study of the low order modelling of the flow field suggests that these two mechanisms may not be exclusive to the particular geometries tested in the present study but, on the contrary, can be explained based on the predominant flow features. In particular, the distinction of these two mechanisms can explain some of the trends that have appeared in the literature in the past decades.

Experimental measurement of wall shear stress in strongly disrupted flows

“…A possible explanation could be related to the large number of wakes generated by the numerous bars of different sizes that this geometry presents close to the wall (ŷ < 0.3, figure 2.3). These wakes generate a highly turbulent fluid which would undoubtedly improve the mixing of the outer and inner regions thus potentially increasing the fluctuating level in the inner region; a similar effect has been reported for over-tripped boundary layers downstream of blunt obstacles generating large recirculation regions (Rodríguez-López et al, 2016b). Another possible explanation could be that the outer, highly energetic, fluid is seen as a different boundary condition for the inner region which, contrary to the natural case, enhances the level of the fluctuations of the near-wall region without changing its characteristic frequency.…”

“…These facts make it difficult to extrapolate any clear trend in the vicinity of the grids. Moreover, different grids may generate distinct turbulent structures which may influence the near wall region differently (Rodríguez-López et al, 2016b). Further downstream, the boundary layer growth appears to be uniform and more solid conclusions can be drawn.…”

Influence of strong perturbations on wall-bounded flows

“…In order to explore the causes of the two different boundary layer thickening mechanisms downstream of the cylinder and sawtooth trips Rodríguez-López et al (28) conducted a particle image velocimetry (PIV) study immediately downstream of the trips. They reported the importance of the dynamics of the turbulent/nonturbulent interface (TNTI) to determining which mechanism, wall-driven or wake-driven, prevailed.…”

“…These motions transport fluid across the entire wall-normal extent of the flow and thus disrupt the near-wall motions from their canonical state. Rodríguez-López et al (28) concluded by postulating three distinct geometrical parameters that would govern whether the wall-driven or wake-driven mechanism would prevail. These were the aspect ratio of the trips, the overall blockage ratio and the blockage at the wall.…”

“…Rodríguez-López et al (28) then revisited some existing literature on artificially thickened TBLs to try to view the reported adaptation regions through the prism of either the wall-driven or wake-driven mechanisms. For example, Klebanoff and Diehl (16) reported that the rods that they tested were rapidly discarded due to their extraordinarily long adaptation region.…”