Master thesis and internship[BR]- Master's thesis : Comparison of steady and unsteady viscous-inviscid coupling strategies in BLASTER[BR]- Internship

Abstract

Modern aircraft design relies on the usage of computational fluid dynamics for the prediction of aerodynamic performance. High fidelity methods such as the Reynolds-Averaged Navier-Stokes (RANS) equations are too computationally expensive for early design stages such that a simpler method known as viscous-inviscid interaction can be used instead. The inviscid flow is calculated and is corrected by the viscous flow in the boundary layer. The coupling between the two regions is complex and prone to numerical issues.

The present work aims to compare steady and unsteady coupling strategies to solve for steady-state problems within the BLASTER solver. The existing inviscid solver is replaced by an incompressible panel method in its steady and unsteady forms. The viscous solver is also adapted to allow for unsteady simulations; the pseudo time marching algorithm and transition treatment in BLASTER are modified accordingly. The missing elements for a complete unsteady model are identified and discussed.

The steady and unsteady coupling strategies are compared based on speed, accuracy and stability for different test cases in various flow regimes of interest. The unsteady coupling shows better stability and faster convergence especially for high incidence flows with separation. This advantage is diminished as the incidence decreases and the flow becomes simpler. For all cases, both strategies yield similar results with little to no difference. The low-Reynolds number flow proves to be challenging for the solver, and its divergence is not resolved by the unsteady coupling strategy.

The method is also tested on true unsteady pitching cases. Understanding the limitations of the model, simple conditions can be predicted with good accuracy compared to RANS simulations. Nonetheless, the solver lacks the ability to predict fast motion, and suffers from issues when refining the time step.