SCHOOL OF MECHANICAL ENGINEERING SEMINAR
WEDNESDAY 22.11.2023 at 14:00
Numerical study of stratified turbulent/non-turbulent interfaces
Marco Boetti
Ph.D. student of Prof. A. LiberzonSchool of Mechanical Engineering, Tel Aviv University, Tel Aviv, Israel

A numerical study of turbulent/non-turbulent interfaces in stratified flows (abbreviated as STNTI) is
presented. STNTIs appear in flows like plumes or convective layers, which are relevant in many fluid
mechanics problems. Fluid dynamics of STNTI play an essential role in ocean dynamics and influence the
spread of contamination in the atmosphere.
First, STNTI between two layers of stratified fluids is modeled in direct numerical simulations (DNS). Two
cases with distinct types of zero-mean shear forcing were modeled, and the results were used to study
inertial particles. We discovered that the typically used vorticity ω cannot uniquely identify STNTI, and the
Kolmogorov lengthscale ηK is not representative of the small-scale dynamics of the interface. Instead, the
potential enstrophy π2 and the new lengthscale ηπ=(ν3/π)1/6 are shown to be the most appropriate variables
to describe the flow dynamics. Conditionally averaged profiles of potential enstrophy π2, enstrophy ω2, and
dissipation ε of the two simulations collapse on a single curve when scaled by ηπ. . This implies not only the
self-similarity of the small-scale statistics of the TNTI in either of the two cases but also the similarity
between the statistics of the two different turbulent flows in the proximity of the STNTI.
Second, the peculiar motion of inertial solid particles across STNTIs is studied using the two DNS simulation
results. Previous experiments in quiescent stratified layers demonstrated that inertial particles slow down
substantially due to an additional force term related to stratification. We advanced the study and explored
similar effects on inertial particles moving across STNTI. The numerical analysis results are compared with
the experimental data. The inertial spheres are tracked numerically across STNTI using a modified Basset-
Boussinesq-Oseen equation with a new stratification-induced force term model FS. The stratification force is
modeled as an additional buoyancy effect of a caudal wake with varying density. This algorithm creates
Lagrangian trajectories that resemble the motion of inertial particles across stratified interfaces in quiescent
and turbulent experiments. Furthermore, numerical results for the STNTI cases help distinguish the
essential features observed in the experiments caused by stratification from those relating to turbulenceparticle
interactions.
This study revealed the new lengthscales of STNTI and explored in detail the contributions of various effects
on the motion of inertial particles, contributing to the understanding of industrial and natural processes.
https://tau-ac-il.zoom.us/j/86497933118