flow in the wake

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
Bengt Karlsson, Specialkarosser AB
bengt@specialkarosser.se
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%.

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. 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
  1. M. El-Alti, P. Kjellgren, L. Davidson
    On the Download Alleviation for the XV-15 Wing by Active Flow Control Using Large-Eddy Simulation DLES7: Direct and Large-Eddy Simulations 7, 8-10 Sept 2008, Trieste, 2008.
    View PDF file
     
  2. M. El-Alti, P. Kjellgren, L. Davidson
    "Experimental Investigation of a Simple Synthetic Jet Actuator for Active Flow Control Purpose", Report, Div. of Fluid Dynamics, Dept. of Applied Mechanics, Chalmers University of Technology, Göteborg, Sweden, 2008.
    View PDF file
     
  3. P. Kjellgren, M. El-Alti, L. Davidson
    "Download Alleviation of a Tilt-rotor Wing by Active Flow Control Strategies", KATnet II Conference: Key Aerodynamic Technologies, May 12-14, Bremen, Germany, 2009.
    View PDF file
     
  4. M. El-Alti, P. Kjellgren, L. Davidson
    "Drag Reduction for Trucks by Active Flow Control of the Wake Behind the Trailer Using Large-Eddy Simulation", Report, Div. of Fluid Dynamics, Dept. of Applied Mechanics, Chalmers University of Technology, Göteborg, Sweden, 2009.
    View PDF file
     
  5. M. El-Alti
    "Active Flow Control for Aircrafts and Heavy Vehicles", thesis of Lic. of Engng,, Division of Fluid Dynamics, Dept. of Applied Mechanics, Chalmers University of Technology, Göteborg, Sweden, 2009.
    Paper View PDF file of thesis
     
  6. M. El-Alti, P. Kjellgren and L. Davidson
    "Drag Reduction for Trucks by Active Flow Control of the Wake Behind the Trailer", Turbulence, Heat and Mass Transfer 6, K. Hanjalic, Y. Nagano and S. Jakirlic (Editors), 2009 Begell House, Inc., 2009.
    View PDF file
     
  7. M. El-Alti, P. Kjellgren and L. Davidson
    "A-LES for Drag Reduction of Truck-Trailers", A-LES: Workshop of Active FLow Control, Gothenburg, 13-14 Oct 2009.
    View PDF file
     
  8. M. El-Alti, P. Kjellgren, L. Davidson
    On the Download Alleviation for the XV-15 Wing by Active Flow Control Using Large-Eddy Simulation, DLES7: Direct and Large-Eddy Simulations 7, V. Armenio, B. Geurts and J. Fröhlich (eds.), Springer, ERCOFTAC Series, Vol. 13. pp. 509-514, 2010.
    View PDF file
     
  9. E. Andersson, E. Andersson, B. Granström, N. Löfgren, N. Nyberg, J. Svedman
    "Active flow control og the wake behind truck (in Swedish)", BSc thesis, Division of Fluid Dynamics, Dept. of Applied Mechanics, Chalmers University of Technology, Göteborg, Sweden, 2011.
    View PDF file
     
  10. M. El-Alti, V. Chernoray, M. Jahanmiri, L. Davidson. "Experimental and computational studies of active flow control on a model truck-trailer", European Physics Journal - Web of Conferences, volume 25, 2012
     
  11. M. El-Alti, V. Chernoray, P. Kjellgren, L. Hjelm and L. Davidson
    "Computations and full-scale tests of Active Flow Control Applied on a VOLVO Truck-Trailer (to appear)", Aerodynamics of Heavy Vehicles III: Trucks, Buses and Trains, Potsdam Germany, 12-17 September 2010
    View PDF file
     
  12. V. Chernoray, M. El-Alti, M. Jahanmiri
    "Active Flow Control: Experimtal investigation of a 1:10 Volvo Truck in Chalmers Windtunnel", April 2011.
    Visualization
     
  13. El-Alti, M.
    "Active Flow Control for Drag Reduction of Heavy Vehicles", PhD thesis, Dept. of Applied Mechanics, Chalmers University of Technology, Göteborg, Sweden, 2012.
    View thesis
     


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