Active control of wakes using LES |
PhD student: |
Mohammad El-Alti mohammad.el-alti@chalmers.se |
Supervisor: |
Lars Davidson lada@chalmers.se |
Co-supervisor: |
Per Kjellgren per_kjellgren@hotmail.com |
Co-supervisor: |
Sinisa Krajnovic sinisa@chalmers.se |
Cooperation: |
Linux Hjelm, Volvo Trucks linus.hjelm@volvo.com |
Johan Engström, Volvo Technology johan.je.engstrom@volvo.com |
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Bengt Karlsson, Specialkarosser AB bengt@specialkarosser.se |
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Sponsors: | Vinnova, Volvo Trucks, Specialkarosser AB |
Publications: | [1-13] |
Start of project: | Decmber 2006 |
PROJECT 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%. 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. Several parameters have to be considered in the development process. As a first step an investigation is done by carrying out a large number of LES and varying the governing variables of AFC. 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. Except from determining flap angle and length, the slot width, position, velocity amplitude, angle and frequency are the governing variables of AFC. We assumed the flap length to be fixed and investigated three flap angle configurations. Assuming also that the slot width and position being fixed and well resolved in each configuration, several velocity amplitudes, angles and frequencies of the slot are varied in order to achieve the largest drag reduction. As a second step, an optimisation of these parameters is to be done in order to maximize the drag reduction.
Several hundred of large-eddy simulations with AFC have been carried out. It is found that for certain configurations, a
reduction of drag by up to 30% is achieved
REFERENCES
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