The Interaction of Homogeneous Isotropic Turbulence with Solid and Sediment Boundaries: An Experimental Investigation

The uprush phase of the swash zone is characterized by rapidly advected strong turbulence generated by offshore wave-breaking. Levels of turbulence observed in this flow significantly exceed levels of turbulence generated by benthic shear stress (Cowen et. al, 2003). These enhanced turbulence levels cause significant sediment suspension and bed deformation that present equilibrium stress models do not fully capture. Motivated by the uprush, we choose to isolate the turbulence from the mean flow to understand this fundamental boundary layer in a laboratory facility designed to generate homogeneous isotropic turbulence in the absence of mean shear. We investigate both impermeable glass and mobile and porous sediment boundaries (D₅₀ = 260 mm). Acoustic Doppler velocimetry (ADV) and particle image velocimetry (PIV) measurements are used to characterize the near-boundary flows with statistical metrics such as turbulence intensities, turbulent kinetic energy, turbulent dissipation, temporal and spatial spectra, and integral length scales, all of which we are able to control by altering the jet firing parameters or jet spacing of our randomly actuated synthetic jet array (RASJA). Interestingly, we observe the formation of sediment ripple patterns, a result that points to a link between pure turbulence and ripples. Our investigations examine the effect of altering the integral length scale of the turbulence on the resulting sediment dynamics, and we conclude with thoughts on a parameterization of the turbulent bed stress requisite for sediment motion in the absence of mean shear.