Assessment of Small-Scale Anisotropy in Stably Stratified Turbulent Flows Using Direct Numerical Simulations

In this study, the validity of the widely used Kolmogorov hypothesis of local isotropy of dissipative scales in stably stratified turbulent flows is investigated using high-resolution direct numerical simulations. Departure from isotropy is investigated for the first time in terms of the turbulent Froude number defined as Fr = epsilon/(NEk), where N is the buoyancy frequency, E-k is the turbulent kinetic energy, and epsilon is the rate of dissipation of turbulent kinetic energy. It is shown that for both epsilon and the rate of scalar (density) dissipation, epsilon(rho), the small-scale statistics are nearly isotropic in the weakly stratified turbulent regime (Fr > 1). For strongly stable flows (Fr < O(1)), the vertical gradients of the fluctuating velocity field as well as the scalar fluctuations overestimate, while horizontal gradients underestimate the true rates of dissipation computed directly using the three-dimensional fluctuating velocity and scalar fields. However, in practice, microstructure instruments are typically limited to one-dimensional measurements of the fluctuating fields and hence very likely to yield erroneous estimates of dissipation rates of turbulent kinetic energy and scalar variance. Thus, a new parameteri-zation is proposed here that can be used to infer true dissipation rates from one-dimensional measured values in oceanic flows. Published by AIP Publishing.