Large eddy simulation for automotive vortical flows in ground effect
thesisposted on 21.06.2013, 12:46 by Lara Schembri-Puglisevich
Large Eddy Simulation (LES) is carried out using the Rolls-Royce Hydra CFD code in order to investigate and give further insight into highly turbulent, unsteady flow structures for automotive applications. LES resolves time dependent eddies that are modelled in the steady-state by Reynolds-Averaged Navier-Stokes (RANS) turbulence models. A standard Smagorinsky subgrid scale model is used to model the energy transfer between large and subgrid scales. Since Hydra is an unstructured algorithm, a variety of unstructured hexahedral, tetrahedral and hybrid grids are used for the different cases investigated. Due to the computational requirements of LES, the cases in this study replicate and analyse generic flow problems through simplified geometry, rather than modelling accurate race car geometry which would lead to infeasible calculations. The first case investigates the flow around a diffuser-equipped bluff body at an experimental Reynolds number of 1.01 times 10 to the power 6 based on model height and inlet velocity. LES is carried out on unstructured hexahedral grids of 10 million and 20 million nodes, with the latter showing improved surface pressure when compared to the experiments. Comparisons of velocity and vorticity between the LES and experiments at the diffuser exit plane show a good level of agreement. Flow visualisation of the vortices in the diffuser region and behind the model from the mean and instantaneous flow attempts to explain the relation or otherwise between the two. The main weakness of the simulation was the late laminar to turbulent transition in the underbody region. The size of the domain and high experimental Reynolds number make this case very challenging. After the challenges faced by the diffuser-equipped bluff body, the underbody region is isolated so that increased grid refinement can be achieved in this region and the calculation is run at a Reynolds number of 220, 000, reducing the computational requirement from the previous case. A vortex generator mounted onto a flat underbody at an onset angle to the flow is modelled to generate vortices that extend along the length of the underbody and its interaction with the ground is analysed. Since the vortex generator resembles a slender wing with an incidence to the flow, a delta wing study is presented as a preliminary step since literature on automotive vortex generators in ground effect is scarce. Results from the delta wing study which is run at an experimental Reynolds number of 1.56 times 10 to the power 6 are in very good agreement with previous experiments and Detached Eddy Simulation (DES) studies, giving improved detail and understanding. Axial velocity and vorticity contours at several chordwise stations show that the leading edge vortices are predicted very well by a 20 million node tetrahedral grid. Sub-structures that originate from the leading edge of the wing and form around the core of the leading edge vortex are also captured. Large Eddy Simulation for the flow around an underbody vortex generator over a smooth ground and a rough ground is presented. A hexahedral grid of 40 million nodes is used for the smooth ground case, whilst a 48 million node hybrid grid was generated for the rough ground case so that the detailed geometry near the ground could be captured by tetrahedral cells. The geometry for the rough surface is modelled by scanning a tarmac surface to capture the cavities and protrusions in the ground. This is the first time that a rough surface representing a tarmac road has been computed in a CFD simulation, so that its effect on vortex decay can be studied. Flow visualisation of the instantaneous flow has shown strong interaction with the ground and the results from this study have given an initial understanding in this area.
- Aeronautical, Automotive, Chemical and Materials Engineering
- Aeronautical and Automotive Engineering