Turbulent ﬂow and transition are some of the most important phenomena of ﬂuid mechanics and aerodynamics and represent a challenging engineering problem for aircraft manufacturers looking to improve aerodynamic eﬃciency. Laminar ﬂow technology has the potential to provide a significant reduction to aircraft drag by manipulating the instabilities within the laminar boundary layer to achieve a delay in transition to turbulence.
Currently prediction and simulation of laminar-turbulent transition is con- ducted using either a low-ﬁdelity approach involving the stability equations or via a full Direct Numerical Simulation (DNS). The work in this thesis uses an alternative high-ﬁdelity simulation method that aims to bridge the gap between the two simulation streams. The methodology uses an LES approach with a low-computational cost sub-grid scale model (WALE) that has inherent ability to reduce its turbulent viscosity contribution to zero in laminar regions. With careful grid spacing the laminar regions can be explicitly modelled as an unsteady Navier-Stokes simulation while the turbulent and transitional regions are simulated using LES. The methodology has been labelled as an unsteady Navier-Stokes/Large Eddy Simulation (UNS/LES) approach.
Two test cases were developed to test the applicability of the method to simulate and control the crossﬂow instability. The ﬁrst test case replicated the setup from an experiment that ran at a chord-based Reynolds number of 390, 000. Two methods were used to generate the initial disturbance for the crossﬂow vortices, ﬁrstly using a continuous suction hole and secondly an isolated roughness element. The results for this test case showed that the approach was capable of modelling the full transition process, from explicitly modelling the growth of the initial amplitude of the disturbances to ﬁnal breakdown to turbulence. Results matched well with the available experimental data. The second test case replicated an experimental setup using a custom- designed aerofoil run at a chord-based Reynolds number of 2.4 million. The test case used Distributed Roughness Elements (DRE) to induce crossﬂow vortices at both a critical and a control wavelength. By forcing the crossﬂow vortices at a stable (control) wavelength a delay in laminar-turbulent transition can be achieved. The results showed that the UNS/LES approach was capable of capturing the initial disturbance amplitudes due to the roughness elements and their growth rates matched well with experimental data. Finally, downstream a transitional region was assessed with low-freestream turbulence provided using a modiﬁed Synthetic Eddy Method (SEM). The full laminar-turbulent transition pro- cess was simulated and results showed signiﬁcant promise.
In conclusion, the method employed in this thesis showed promising results and demonstrated a possible route to high-ﬁdelity transition simulation run at more realistic ﬂow conditions and geometries than DNS. Further work and validation is required to test the secondary instability region and the ﬁnal breakdown to turbulence.
A dissertation thesis submitted in partial fulfilment of the requirements for the award of the Engineering Doctorate (EngD) degree at Loughborough University.