LES-VMS SIMULATIONS OF THERMALLY STRATIFIED TURBULENT WAKES BEHIND TOWED AND AUTO-PROPELLED AXISYMMETRICAL BODY

Abstract

Numerical simulations of close and far wakes behind an axisymmetrical body in a stratified medium are carried out using a Large Eddy Simulation Variational Multi Scale approach to model turbulence. Towed and auto-propelled flow regimes are scrutinized and compared. The characteristic parameters of the flow are Pr = 7, Re = 10000 based on the diameter of the cylinder and F = 25. Realistic results are obtained for the towed case where the so-called three-dimensional (3D), non-equilibrium (NEQ) and quasi two-dimensional (Q2D) regimes are exhibited with very good agreement with experimental and theoretical findings of the litterature. In addition, the effect of auto-propulsion on the flow dynamics is reproduced in a satisfying manner. INTRODUCTION Context The understanding of far turbulent wakes in thermally stratified water is of prime interest for submarine engineers concerned with both hydrodynamic and acoustic stealths. Indeed, the massive separation and the resulting turbulent flow downstream of the device generate both velocity and temperature fluctuations which to turbulent kinetic energy / dissipation and thermal dissipation. Such characteristics of ∗Address all correspondence to this author. the flowfield can be measured by detection devices. In addition, the variations of the fluid’s density can be identified in the reflected signal captured by a sonar. Hence, the objective of submarine engineers is to understand the time-evolution of such quantities in order to establish the influence of parameters such as Reynolds and Prandtl numbers, angle of attack, propulsion, asymmetry, appendages... Their goal is to find a relation between the variations of the measured quantities and the properties of the device (nature of the object, distance, cruising regime, size...) in order to qualify the signals obtained for detection purposes. However, such flows are tedious to accurately analyse on simple theoretical grounds because of non linear phenomena such as turbulence and also because of the complexity of the geometries considered. Moreover, reproducing such wakes at a reasonable scale is very demanding experimentally and the subsequent measurements only give access to a limited number of data too scarce to allow an accurate description of the flow. On the other hand, thanks to the increasing reliability of Computational Fluid Dynamic approaches and the growth of High Performance Computing, numerical simulations of such phenomena are now utilized to tackle such stringent issues. These methods give the engineers an affordable and trustworthy alternative to costly and time-consuming experimental campaigns. Thus, the understanding of turbulent far wakes is made possible thanks to a joint effort between experimental, theoretical and numerical approaches.