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

Abstract

A micro-cavity array has been shown to locally reduce sweep and turbulence intensities by 13% and 14% respectively. This paper aims to identify the extent of the effect a micro-cavity array has on a turbulent boundary layer, and to observe the effects on the streamwise domain located significantly downstream of the installed cavity array. A fixed geometry array was investigated across two Reynolds numbers, (Reθ= 2,028 and 3,075) at four downstream locations (800 – 7,500 viscous wall units from the last row of orifices). All velocity data were obtained through single-component hot-wire anemometry and were compared against a canonical flat plate boundary layer at identical locations. At Reθ= 2,028 a reduction in time-averaged turbulence intensity was observed, and by approximately 5,000 wall units downstream the intensity was returned to a canonical flow’s intensity. This trend was not observed at Reθ= 3,075, as the turbulence intensity at 7,500 wall units downstream was 2.4% greater than the canonical counterpart. Sweep event intensities showed initial reductions just downstream of the array, though data on sweep events further downstream of the array are largely inconclusive due to the nature of their variance. The pre-multiplied energy spectra were reduced immediately after the last row of orifices at y+=50 by a maximum of 2% at Reθ=2,028 and 1.8% at Reθ= 3,075. Further downstream, the energy spectra of flow affected by the cavity array showed no signs of recovering to canonically expected values of streamwise turbulent kinetic energy, though these changes were observed in greater proportions in large-scale structures (λ⁺>4,000).