On the generation of free-stream turbulence at low Reynolds number: A numerical study

We investigate the generation of free-stream perturbations at a relatively low characteristic Reynolds number of 1000 by means of direct numerical simulations using a synthetic turbulence generation method. This approach consists of generating turbulent fluctuations by means of digital filtering and a source term formulation in the Navier-Stokes equations. To assess its validity in the framework of decaying turbulence, we compare the results with those obtained with a physically-based, grid-induced turbulent flow in terms of spatial decay, evolution of characteristic length-scales and energy spectra. Also, we highlight relevant differences such as those in the streamwise development length and the anisotropy of the largest scales. Then, we characterize the generated perturbations when systematically varying the input parameters, namely the initial integral length-scale and turbulence intensity. Here, we notice differences in the streamwise decay of the turbulence intensity and the development length as we vary these parameters. By inspecting the evolution of the characteristic length-scales and the micro-scale Reynolds number, we also identify that the effective scale separation is highly sensitive to these variations.