Eventos Anais de eventos
COBEM 2021
26th International Congress of Mechanical Engineering
DIRECT NUMERICAL SIMULATION OF THE FLOW PAST SMALL CAVITY INSIDE LAMINAR BOUNDARY LAYER
Submission Author:
Victor Barcelos Victorino , SP
Co-Authors:
Victor Barcelos Victorino, Marlon Sproesser Mathias, Marcello Augusto Faraco de Medeiros
Presenter: Victor Barcelos Victorino
doi://10.26678/ABCM.COBEM2021.COB2021-1509
Abstract
The boundary layer over an aircraft surface causes friction forces that account for a great portion of the vehicle’s total drag. This is intensified when the laminar boundary layer becomes turbulent. Thus, pushing the turbulence transition as downstream as possible may result in significantly increased aircraft performance, for instance, fuel efficiency. In the two-dimensional boundary layer, which is found in several parts of an aircraft, e.g. wings and nacelles, the transition is dominated by the Tollmien-Schlichting (T-S) mode. This mode oscillates and, depending on the flow conditions, certain frequencies become unstable, leading to laminar breakdown. Several irregularities are found in a real aircraft surface, such as roughness, excrescences, steps, and gaps; the latter is the main scope of this study. Typically, the presence of the gap alters the T-S growth, anticipating the transition location. However, some recent studies carried out a parametric search and identified a combination of geometric and flow configurations where the transition location changes abruptly to a region near the downstream of the cavity, the so-called bypass transition. Therefore, the aim of the current study is to perform a direct numeric simulation (DNS) of the flow past a rectangular cavity positioned inside a Blasius boundary layer, close to the previously cited parametric region, in order to investigate better the physical aspects involved in this threshold of transition forms. An in-house code is employed to solve the compressible Navier-Stokes equations, without turbulence modeling. The code uses a high order spectral-like compact differentiation method for spatial derivatives, and a fourth-order Runge-Kutta for time marching. An anti-aliasing filter is employed to attenuate high-frequency numeric noise. The computational domain is represented by a structured mesh with refinement controlled by attractors in particular regions. Buffer zones are employed close to open boundaries to avoid reflection and to provide a suitable far-field condition, increasing substantially the spacing between grid points and reducing the order of spatial derivatives. Selective frequency damping is also employed on the buffer zones and may be switched on inside the domain to generate the unsteady solution, also known as the base flow. Once reached the base flow, a time-stepping Bi-Global instability analysis routine computes the flow’s eigenvalues and eigenfunctions. Preliminary runs showed that, in the bypass transition case, the oscillating frequency of the cavity flow is close to the frequency associated with transition reported by results from the literature.
Keywords
transition, small cavity, DNS, Flow instability
