Unsteady Numerical Simulations and Reduced-Order Modelling of Flows around Vehicles

Doctoral thesis, 2014

In this thesis several simulations have been carried out aimed at improving the knowledge of vehicle aerodynamics and improving the flow around vehicles with respect to the aerodynamic drag. Five flows around different simplified vehicle models have been considered in five appended publications. One tractor-trailer model, two passenger car models, one freight wagon model and one regional train model. In the simulations of the flow around the tractor-trailer model, the focus is on the flow in the gap between the tractor and the trailer and how this flow affects the global drag of the model. Large Eddy Simulation is used to simulate the flow around four variants of the model with different geometrical configurations. The behavior of the drag coefficient of the tractor-trailer model when varying the gap width and the shape of the front edges on the tractor is explained by identifying and analysing the large vortices around and in the the gap. The focus in the study on one of the passenger vehicle models is on the so-called A-pillar vortex. This is a swirling longitudinal vortex formed along the side windows on passenger vehicles due to the separation of the flow from the side edges of the front window and the engine hood. Flow control, both steady blowing and suction, from actuators located on the side of the front of the model, is applied in the LES simulations. Steady blowing into the vortex causes expedited breakdown of the vortex, which in turn influence the pressure distribution on the side windows and the overall drag of the model. Steady suction prevents the formation of the vortex, thereby removing the vortex entirely. Simulations aimed at improving the knowledge of the flow around a generic freight wagon model using LES is also reported. The model is smoothed in comparison to a real container wagon, but the overall geometrical features such as wheels and underhood are included. The simulations of the flow around the regional train model is done using Partially Averaged Navier Stokes (PANS). PANS is a recently proposed hybrid turbulence model for engineering types of flow. The regional train model consists of a bluff body with a length to height/width ration of 7:1. The flow around this model poses several challenging flow situations to simulate such as separation from the leading curved front edges, an attached boundary layer flow and separation from the curved rear edges at the moderate Reynolds number of 400 000 based on the models’ width. An open cavity is placed on the model at the base and the drag is thereby decreased by some 10%. The second passenger vehicle model that has been investigated is a square back Ahmed body. This flow is simulated using LES, and the temporally and spatially resolved flow field is used as input to a Reduced-Order Model (ROM). The ROM is constructed by a decomposition of the flow field into spatially stationary modes with corresponding time-varying amplitudes, using the Proper Orthogonal Decomposition (POD). The introduction of the POD into the governing equations reduces these equations to a linear-quadratic dynamical system with known coefficient. This system is then truncated, so that the remaining system contains a low number of modes. It is investigated how to best model the influence of the truncated high-frequency modes on the remaining system.