Eventos Anais de eventos
COBEM 2021
26th International Congress of Mechanical Engineering
DNS simulation of compressible flows over gaps: Strategies for reaching a base flow under stable and unstable conditions
Submission Author:
Marlon Sproesser Mathias , SP
Co-Authors:
Marlon Sproesser Mathias, Marcello Augusto Faraco de Medeiros
Presenter: Marlon Sproesser Mathias
doi://10.26678/ABCM.COBEM2021.COB2021-0581
Abstract
An accurate Direct Numerical Simulation (DNS) of flows may lead to important insights on the origins of turbulence which, in turn, allows quieter, greener and more efficient aircraft to be designed. Often, it is important to obtain a base flow to be used as initial state for a DNS analysis; such flow is defined as an equilibrium point, in which the time derivatives of all variables in the system are null. For stable flows, it is possible to simply let the simulation run for long enough until all oscillations are damped out; nonetheless, it is often a time-consuming process due to near-critical modes, which may be of physical or numerical nature. Unstable flows, on the other hand, are defined by an unstable equilibrium at the base flow and, therefore, will not converge to it naturally. In this work, two techniques are combined to achieve rapid and consistent convergence to base flows. A time-averaging process is carefully done so that whole periods of the least stable modes are contained in the window, damping such modes by roughly an order of magnitude at each cycle. In the case of unstable flows, the well-documented Selective Frequency Damping (SFD) method artificially stabilizes all modes; however, we only employ this technique in selected portions of the domain, where we can maximize its benefits. Our test scenario is a small gap in a subsonic boundary layer, for which unstable Rossiter modes may appear depending on the Mach number and on the gap geometry. By activating the SFD near the gap's trailing edge, we were able to successfully damp unstable Rossiter modes by breaking their feedback mechanism. The time-averaging routine is used to remove lower frequency phenomena, such as a standing wave we have observed in our domain, which is of numerical origin and spans from the inflow to the outflow. By using both techniques in our DNS code, we can achieve residua at least twelve orders of magnitude lower than the free-flow, which allows us to have accurate measures of Tollmien-Schlichting waves well into the linear regime, as well as use a time-stepping method for obtaining the flow’s global stability modes.
Keywords
direct numerical simulation, Selective Frequency Damping, Boundary Layer, Open Cavities
