(A) study of subgrid-scale models for large eddy simulation on the generation of body surface vortex

Abstract

The current PhD thesis proposes ample insight into highly unsteady flow induced by the impulsive motions of the blunt bodies and is aiming at developing a new subgrid-scale model for LES being able to more precisely predict it.

First, in order to scrutinize complex flow phenomena induced by impulsively accelerated/decelerated motions, Scale Resolving Simulation (SRS) such as Hybrid LES/RANS, LES is currently adopted. Thus the SRS in terms of time and scale-resolving is expected to more precisely trace highly transient behaviors of fluid flow caused by the blunt bodies. The secondary and primary vortices created by the impulsive motions of the blunt bodies are identified essential factors determining the characteristics of extremely unsteady flow fields. They are believed to be directly related to the inertial force and the transient flow separation according to the bluntness of configurations. The mobility of the flow separation point along the surface of the object with bluntness is able to make a contribution to the characteristics of the secondary vortex and its interaction with the primary one. Therefore, a distinctive phenomenon of drag plateau is manifested through the impulsively accelerated motion of the circular cylinder rather than other cylinders.

Secondly, a new SGS model for LES using two transportation equations for ksgs and ωsgs is presented. In the new SGS model, another transportation equation for ωsgs (specific SGS eddy dissipation rate) should be solved and the instantaneous value of calculated ωsgs is transferred to the equation ksgs and thus two transient values with time are updated with each other. Moreover, a new formula for the evaluation of the eddy viscosity is derived and an advanced method for the production term in the SGS energy equation is also adopted. For the purpose of the validation of the new SGS model, the lid-driven cavity flow and the flow around three dimensional square cylinder are employed and investigated. The new two-equation SGS model shows the results closer to the experimental data for turbulent fluctuating velocities in wall-bounded flow regions.