CODE VALIDATION AND NUMERICAL INVESTIGATION OF ACTUATION PRINCIPLES FOR TRIGGERING TRANSITION IN AN AXISYMMETRIC DIFFUSER USING LARGE EDDY SIMULATION

Abstract

Transitional flows including separation are of great importance to many applications, e.g. the flow around airfoils or in turbo-machines. Herein, many as yet unresolved questions concerning the laminar-turbulent transition over a laminar separation bubble are of special interest. In the model geometry, a long laminar separation bubble develops at the cross section expansion The bubble extends several inflow diameters downstream and the laminar-turbulent transition occurs far downstream of the diffuser. The triggering of transition over the separation bubble is realized by the introduction of perturbations up-stream of the diffuser. In the frame of this thesis, the transition of the flow in an axisymmetric diffuser is investigated by means of Large Eddy Simulation (LES). For the numerical calculations, the flow solver Elliptic Analysis of Navier-Stokes equations (ELAN) developed at the HFI, Germany, is used. Minor differences are attributed to smaller number of snapshots(25000) taken for statistical analysis of previous results. However code validation showed that revised version of ELAN3 gives results comparable to previous version and new computations performed for flow at higher Reynolds number of Re —7800 using thinner boundary layer thickness. The periodic perturbations were tested by .two different actuation methods, one using slit actuator and other using membrane actuator. Base and perturbed flow computations were analyzed and compared to experimental results Instantaneous snapshots of perturbed flow for pressure and velocity components, in addition, rms values were analyzed Finally vortex cores were visualized for both base and perturbed