Mechanism of control of the near-wall turbulence using a micro-cavity array

Abstract

Coherent structures in a turbulent boundary layer have been shown to have an influence on the skin-friction drag acting on surfaces beneath the boundary layer. The use of micro-cavities on a flat surface has recently shown the potential to passively control a turbulent boundary layer by attenuating the sweep events. Previous experiments have determined the design parameters of the cavity array for the optimal boundary-layer control by reducing the sweep events. However, investigating the flow physics behind the interaction of the boundary-layer flow with the cavities is challenging. High near-wall velocity gradients and very small scales and sizes of the cavity holes limit the experiments from investigating the flow characteristics very close to the wall and inside the holes. Therefore, in the present work, direct numerical simulations have been utilized to model the boundary layer flow over a flat surface with a micro-cavity array in order to understand the flow interactions. Detection of coherent structures in the boundary layer shows a reduction in the number of events over the cavity array. Reynolds stresses have been analyzed to determine the effect of micro-cavities. The reduction in the Reynolds shear stress results in a lower skin-friction drag. The flow fluctuations through the holes in the streamwise sequence have been found to be highly correlated using cross correlation. These flow fluctuations interact with the boundary layer to suppress the coherent structures. Overall, the use of the micro-cavity array has resulted in a reduced wall shear stress and approximately 5.6% lower local skin-friction drag.

Characterization of a Turbulent Boundary Layer Downstream of a Micro-Cavity Array

“…For both 𝑅𝑒 𝜃 cases at Location 1, a reduction in TI + is observed, particularly in the buffer and logarithmic region of the boundary profile. This trend in reduction is in agreement with previously investigated experiments from Silvestri et al (2017a;2017b;2018b) and direct numerical simulations from Bhat et al (2021). It should be noted that the magnitude of the reductions for both Reynolds cases are less than that of previous investigations (a maximum of 13% TI + reduction from Silvestri et al (2018b)).…”

“…From the statistics investigated, characterizing the downstream behavior of the flow effected by a micro-cavity array remains complex, and interpretation of the statistics presented in this paper must be made with caution. Timeaveraged streamwise turbulence intensity plots indicate that the array is capable of achieving a reduction in intensity immediately downstream of the last row of orifices by a maximum of 5.7%, which is comparable to data from (Bhat et al 2021). Both 𝑅𝑒 𝜃 cases investigated showed this initial reduction in intensity diminish by approximately 2,500 wall units downstream of the array itself.…”

Frequency-tuned surfaces for passive control of wall-bounded turbulent flow – a resolvent analysis study

“…2017; Bhat et al. 2021; Jafari, Cazzolato & Arjomandi 2022), permeable (Breugem, Boersma & Uittenbogaard 2006; Kuwata & Suga 2017; Suga et al. 2018; Chavarin et al.…”

“…Hence, due to the significant economic and environmental benefits, reducing skin friction has motivated considerable effort for the control of wall-bounded turbulence, including active and passive control techniques. A particularly attractive passive control concept, requiring no energy input and no complex control algorithms, is taking benefit of the wall material properties through the interaction between the wall and the turbulent flow, examples of which include perforated (Silvestri et al 2017;Bhat et al 2021;Jafari, Cazzolato & Arjomandi 2022), permeable (Breugem, Boersma & Uittenbogaard 2006;Kuwata & Suga 2017;Suga et al 2018;Chavarin et al 2020Chavarin et al , 2021 and compliant walls (Lee, Fisher & Schwarz 1993;Xu, Rempfer & Lumley 2003;Kim & Choi 2014;Xia, Huang & Xu 2017). Based on the surface properties, these walls may suppress and/or energise specific frequency bandwidths within the turbulent flow.…”