title: Numerical simulation of unsteady separated flows
Creators: name: Petrocchi, Andrea
Publisher: name: University of Glasgow
Year: 2024

description: Transonic buffet is a phenomenon occurring at particular flight conditions and has an adverse effect on structural integrity, controllability, and passenger safety. The research carried out so far evidenced a lack of accuracy in the prediction of this phenomenon and high costs associated with numerical simulations. Moreover, a variety of flow control strategies were proposed in recent years, all presenting disadvantages in terms of aerodynamic performance, ease of implementation, and reduction of engine efficiency. This thesis studies transonic buffet by means of computational fluid dynamics and aims at expanding the current state of the art by filling gaps in the literature. In particular, this work focuses on buffet prediction by means of time marching simulations and, in a second part, engineeristic criteria are investigated. In the last part, the problem of flow control is addressed.
A detailed survey of past works was carried out with the aim of determining the optimal computational strategy and a detailed test case for this study, and understanding the current research status on transonic buffet and its control. The conclusion of the survey motivated the development of the rest of the thesis.
A Partially-Averaged Navier-Stokes approach was then used, as a good compromise between computational costs and accuracy. The method is first validated on a circular cylinder in the subcritical regime by comparison against available experiments. Several approaches targeting an adequate reduction of eddy viscosity around the body were proposed in the literature, and the most promising were validated here. The use of the method was then extended to transonic buffet. A comparison between two-dimensional and three-dimensional buffet on unswept wings confirms the substantial differences between 2D and 3D buffet, and detailed descriptions of the buffet dynamics, spectral content, and flow topology are given.
To avoid the costs associated with time marching simulations, engineeristic criteria, based on steady simulations, were used for 2D aerofoil sections. Their accuracy was validated against available datasets for buffet onset. An algorithm combining the aforementioned criteria with an adjoint method was formulated and validated on the same test cases, to obtain a reduction of the computer costs without loss of accuracy.
A second algorithm was then proposed. It uses the harmonic balance method to estimate the load oscillations at several flow conditions and detect buffet. The harmonic balance gives additional information on dynamic quantities and allows for the description of buffet in its phases over a period of oscillations. The advantages and limitations of this method over the engineeristic criteria are pointed out.
Finally, two flow control devices, i.e. spoilers and upper trailing edge flaps, were modelled on 2D and 3D configurations. Their ability to alleviate buffet was investigated and differences between infinite and finite-extent devices were described. The role of piloting in coupling with automatic flow control was briefly investigated.