A part of the current flow control research at Chalmers University is directed towards vehicle aerodynamics optimization. The focus is on drag reduction (and hence also CO2 reduction) using periodic excitation.

XV-15 Wing

As a first step in our research, we investigated the tilt-rotor XV-15 wing, which is a continuation of previous research. The wing has a deflected flap at the trailing edge. The optimal angle is 70 degrees, where the flow reattaches. If the deflection angle is increased, the flow separates and hence the download increases. With active flow control (AFC), the flow reattaches, the wake becomes narrower and the download is alleviated. In AFC the flow is controlled by supplying energy to the system. The energy input is provided in this study by an actuator that can blow in or suck out flow. The use of periodic excitation was shown to be more effective than steady blowing or suction Periodic excitation depends on many parameters that must be optimized. We have used the optimal values found in to analyze the download reduction process.

Without AFC, the (normalized) predicted drag is 0.99 and with AFC the drag is reduced down to 0.76; these values agree very well experiments. The AFC makes the flow reattach at the flap and creates strong vortices along the flap which break up the vortex shedding in the wake and reduce the size of the wake. As a result the wake is much less intensive with AFC than without AFC; the fluctuating pressure coefficient, C_p,RMS, on the downstream surface of the wing is reduced by a factor of five and the resolved turbulent kinetic energy in the wake is reduced by 50%.

A simplified truck

The larger part of the drag for trucks is created by the wake which gives a low pressure at the rear of the truck. An effective way to reduce the drag (and thus the fuel consumption and CO2 emissions) is to increase the pressure at the rear side of the truck. This pressure increase can be achieved by reducing the size of the wake behind the truck. In the present project we will achieve that with AFC.

We are currently working on a simplified truck. We have added flaps at the read end. At the upsteam part of the flap we introduce a slot where we apply an oscillating jet, i.e. AFC. We are now optimizing the angle of the flap, the location of the slot, the frequency of the jet etc. We are using two CFD codes: a fast finite element solver, FlowPhys (developed by Per Kjellgren) and a general purpose commerical CFD package, STAR-CD.

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Research Projects at Computational Fluid Dynamics