Investigation of Flow Separation around the Front Corners of a Truck using CFD

Abstract

Fuel consumption becomes more and more important for heavy duty vehicles. The increased fuel prices and the more restrictive legislations have pushed the development towards fuel efficient vehicles. The aerodynamic drag is a large contributor to the overall losses. The flow around the front corners of a truck is crucial since flow separation occurs there, which aeffects the overall flow and drag. It is difficult to predict the flow separation point at a rounded corner. Therefore, the purpose of this thesis work is to provide Volvo Group Truck Technology with knowledge about what influences the flow separation on the front corners of the truck. Focus is also put on investigating how the separation is aeffected by the manner of how the flow is simulated. In this investigation details are removed from the lower front of a Volvo FH16 truck. Removing details will affect the flow, but the assumption is that the behavior of the flow and the flow separation will be more apparent. A simplified model of the lower front was developed in a manner such that one geometrical feature of the front corner could be changed without affecting any other geometry. The geometrical features evaluated are the front corner radius between 80{300 mm and the following draft angle of either 0 or 12. There were two main requirements when designing the simplified model. The first one was that the geometry and flow situation should be close to reality and the second requirement was that the front corner radius should be adjustable between at least 100{300 mm. In this thesis CFD has been used to solve the flow with a mesh distribution mainly consistent of hexahedrals. The simulations have been carried out in STAR-CCM+. To model the turbulent flow, standard k{" model with realizable coefficient and two-layer all y+ wall treatment has been used. The hypothesis was that there would be no significant difference in the simulation results for values of y+ between 30{100. This has been shown not to be true for small radii where flow separation occurs. For a front corner radius smaller than 150 mm it was shown that the results deviated significant for meshes of y+ 30 compared to meshes of y+ 60, given that flow separation occurred. Therefore, it is recommended to have y+ 1 for areas consisting of small radii. It has also been found that for a three-dimensional flow the flow separation point is not connected to a maximum value of the local streamwise pressure gradient, but it seems to be connected to the positive second derivative. Using the simplified model with a draft angle of 12 resulted in an overall longer distance between the end of the corner arc and the gap just before the door. To vary the front corner radius has less impact on the drag with a draft angle of 12. Both due to the smaller turn around the front corner which prohibits flow separation and the longer distance.