Active Flow Control for Drag Reduction of Heavy Vehicles

Doktorsavhandling, 2012

This thesis considers the aerodynamics of trucks. The work focuses on the aerodynamic drag caused by the wake. Bluff-body flows are dominated by large dissipative vortex-shedding wakes with a low mean pressure compared to the high pressure on their frontal area. This induced pressure difference is the major contribution to the total aerodynamic drag. The flow separation at the aft of the trailer results in a large low pressure region that increases the pressure difference between the front and back. An effective way to decrease the size of the separation zone and increase the base pressure is to use active and passive flow control techniques (AFC and PFC). The objective of the present work is to apply active and passive flow control (AFC) strategies to reduce the typically large base pressure on bluff-bodies. The work is mainly based on numerical computations using large-eddy simulation of the turbulent flow on a simple semi-3D truckmodel. A thorough parameter study of a novel rear-end trailer geometry and the AFC parameters is carried out and an attempt is made to optimize the flap shape with AFC using Response Surface Methodology. The flow reattachment process, dynamics of the flow with and without AFC and corresponding wake structures are discussed. An experimental investigation of a typical synthetic-jet actuator is also carried out in order to evaluate the order of magnitude of the maximum possible momentum coefficient, Cμ. A semi-detailed 1/10th scale of a VOLVO cab with a trailer is experimentally and numerically evaluated with and without AFC. A full-scale road test employing AFC in a fullscale Volvo truck-trailer is also carried out.