Effect of free-stream turbulence on flow characteristics over a transversely-grooved surface

Abstract

An experimental investigation has been performed to study the free-stream turbulence influence on the flow over a transversely-grooved surface. Free-stream flow at 8.1m/s with turbulence intensities of 0.5% and 4.4% have been provided over smooth and grooved surfaces. Mean velocity profiles, fluctuating velocity moments and turbulence length scales are studied using data obtained from hot-wire anemometry measurements. Introducing quasi-isotropic free-stream turbulence of 4.4% to the flow resulted in fuller velocity profiles, increased boundary layer thickness and significantly augmented streamwise turbulence intensity throughout the boundary layer in flows over both smooth and grooved surfaces. Probability density functions of the velocity fluctuation show that interaction of quasi-isotropic free-stream turbulence with wall turbulence leads to nearly-isotropic turbulence in a small portion of the boundary layer in the near-wall region; while the remaining boundary layer experiences an anisotropic turbulence. In other words, with the introduction of additional turbulence generated by the grooves, the momentum of the fluid particles is increased, resulting in better flow separation resistance. Accordingly, the flow over a grooved surface showed an increase in turbulence intensity, and hence in energy dissipation rate. This is amplified under free-stream turbulence conditions as a result of the interaction of groove-generated turbulence with free-stream turbulence in the boundary layer. The energy cascade span as indicated by the integral to Kolmogorov ratio is also widened by the grooves.