Computational study of the hydrodynamic stability of gaps and cavities in a subsonic compressible boundary layer

• Ano de defesa: 2021
• Autor(a) principal: Mathias, Marlon Sproesser
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Direct Numerical Simulation; Cavidade aberta; Cavidade pequena; Compressible flow; Escoamento compressível; Estabilidade global; Gap; Global stability; Open Cavity; Pacotes de onda; Wave packets
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/18/18161/tde-29032022-085318/

We study the effect of small cavities, or other discontinuities, on the laminar to turbulent flow transition over a flat plate. Understanding the transition mechanism is of significant importance for the reduction of drag in an aerodynamic project, which impacts the fuel consumption of aircrafts, for example. Predicting the correct region in which the transition to turbulence occurs is still a challenge, especially when accounting for parameters such as surface imperfections or manufacturing tolerances. Ideally, aircraft designers would balance the costs of manufacturing smoother surfaces with their aerodynamic benefits. We use two different computational tools in this work. First, a Direct Numerical Simulation (DNS) code is used to simulate the flow, either just by itself or in conjunction with controlled disturbances designed to model a natural transition. The second tool is a Linear Stability Theory (LST) code which can compute the linear stability modes of a given base flow, this algorithm is of the time-stepping type, for which we analyze the sources of errors and develop guidelines for obtaining optimum results. Both tools were designed to capture a wide spectrum of oscillations, even at very low amplitudes, which is essential for such modeling, as the natural transition is triggered by the interaction of multiple modes of different frequencies that are several orders of magnitude smaller than the base flow. There are many previous works that study either the natural transition on a flat plate or the flow disturbances caused by discontinuities on the surface, but the interaction between those two phenomena is still open for investigation. We study two different scenarios: when the cavity is much larger than the incoming boundary layer; and when they are of comparable magnitudes. Similar conclusions are reached in both cases. Increasing the Mach number has a mostly destabilizing effect on the 2D modes of the cavity. Greater ratios between cavity size and the incoming boundary layer thickness also increase the instability. We then compare the linear stability results to non-linear simulations. We also conclude that, even though 2D modes are usually more linearly unstable, the presence of 3D modes is essential for an accurate simulation of the flow. Moreover, we compare our results to experimental works and conclude that when both 2D and 3D modes are unstable in a small open cavity, it may cause a bypass transition in the boundary layer.