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Aerodynamic Studies of Flows Around Bluff Bodies. By: Eng. Jaber, Ala H. Tutor at El-Quds College February, 8 th , 2006. Aerodynamic Studies of Flows Around Bluff Bodies (Ahmed Body). Presentation Outline. Abstract Introduction Examples of Ahmed Body Utilization

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aerodynamic studies of flows around bluff bodies

Aerodynamic Studies of Flows Around Bluff Bodies

By: Eng. Jaber, Ala H.

Tutor at El-Quds College

February, 8th, 2006

presentation outline
Presentation Outline
  • Abstract
  • Introduction
  • Examples of Ahmed Body Utilization
  • Large Eddy Simulation (LES) Governing Equations
  • Solving Unsteady Turbulent Flows over Ahmed Body by FlowLab software
  • Conclusions
  • References

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abstract
Abstract
  • Aerodynamic studies of flow around bluff bodies, especially Ahmed body, are explored.
  • The wake flow behind a vehicle is investigated, so the separation zone and consequently the drag coefficient are to be determined.
  • Several slant angles were used in the experimental studies, especially (25° - 35°) since they are surrounding the critical angle of 30°, where the flow changes its character.
  • Numerical works e.g. large eddy simulations and softwares e.g. CFD-FlowLab were used to solve unsteady turbulent flows over Ahmed body as well as experimental works

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introduction
Introduction
  • Ahmed body model is a car type bluff body (see Fig. 1) and has been used in several experimental and numerical studies (Krajnovic and Davidson, 2004)

Fig. 1. Ahmed body shape and dimensions in mm.

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introduction cont
Introduction (Cont.)
  • Ahmed Body has been selected in numerous studies.
  • Because of its geometric simplicity and availability of the experimental results (Ahmed et al., 1984).
  • A real-life automobile is a very complex shape to model or to study experimentally.

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introduction cont7
Introduction (Cont.)
  • Therefore, the MOVA group partners (TU Delft, University of Manchester, LSTM, Electricite de France, AVL List, and PSA Peugeot Citroen) agreed to study the vehicle shape by Ahmed body (Ahmed and Ramm, 1984).

Fig. 2. Predicted velocity vectors by k- ε model. Peugeot 405.

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introduction cont8
Introduction (Cont.)

Fig. 3. Predicted velocity vectors by k- ε model. Volkswagen

.

Fig. 4. Predicted velocity vectors by k- ε model. Citroen ZX

.

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introduction cont9
Introduction (Cont.)
  • It should be noted that Ahmed model is used for the purpose of numerical simulation of the external flow analysis and drag calculation (Kapadia and Roy, 2003).

Fig. 5. External flows around bluff bodies.

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introduction cont10
Introduction (Cont.)
  • The flow region which represents:
    • The major contribution to a vehicle’s drag.
    • And which causes harsh problems to numerical predictions and experimental studies, is the wake flow behind the vehicle (Lienhart et al., 2003).

Fig. 6. The wake flow behind a vehicle.

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introduction cont11
Introduction (Cont.)
  • The location at which the flow separates determines the size of the separation zone, and consequently the drag force.
  • Several slant angles were used in the experimental studies, but two angles, 25° and 35° are overrepresented. Since they are surrounding the critical angle of 30°, where the flow changes its character (Ahmed et al., 1984).

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introduction cont12
Introduction (Cont.)

Fig. 7. Drag Coefficient vs. different slant angles values.

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introduction cont13
Introduction (Cont.)
  • The phenomenon of flow separation has undesirable effects. i.e. the energy losses associated with separating flows.
  • Turbulent flows are adequately described by the unsteady Navier-Stokes equations, but they are rarely solved due to high computational cost.
  • Instead, the simplified Reynolds Averaged Navier-Stokes (RANS) equations are solved, but these equations are time-averaged and thus they provide the mean information of the flow and the unsteady information is lost (Krajnovic and Davidson, 2004).

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introduction cont14
Introduction (Cont.)
  • Reynolds stresses in RANS are modeled with a turbulence model.
  • Unfortunately it is difficult to define a model that can accurately represent the Reynolds stresses in the regions of the separated flow such as a wake behind a car.
  • I.e. it is very difficult to construct turbulence model for automobile applications (Spohn. and Gillieron, 2002).

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introduction cont15
Introduction (Cont.)
  • The increase in the computer power has made time-dependent simulations possible where large flow structures are directly computed and only the influence of the structures smaller than the computational cells are modeled.
  • This is done in a technique called large eddy simulation (LES).
  • Moreover, FlowLab software is strongly recommended to solve unsteady turbulent flows over Ahmed body (2D)

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examples of ahmed body utilization
Examples of Ahmed Body Utilization
  • In Aeroacoustic study: Study of noise generated by bluff bodies. Application: External mirror on a car (LADA s 29th WWW, 2006).

Fig. 8. Modified Ahmed body with external mirror.

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examples of ahmed body utilization17
Examples of Ahmed Body Utilization
  • In Aerodynamics for High-Speed Trains: Aerodynamic forces when high-speed trains enter asymmetric tunnels (LADA s 29th WWW, 2006).

Fig. 9. Modified Ahmed body utilized for high speed train.

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examples of ahmed body utilization18
Examples of Ahmed Body Utilization
  • In Side-wind stability: Side-wind stability for cars and trucks (LADA s 29th WWW, 2006).

Fig. 10. Modified Ahmed body utilized for side-wind stability.

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les governing equations
LES Governing Equations
  • The governing LES equations are the incompressible Navier-Stokes & the continuity equations filtered with the implicit spatial filter of a characteristic width:

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solving unsteady turbulent flows over ahmed body by flowlab software
Solving Unsteady Turbulent Flows over Ahmed Body by FlowLab software
  • The problem is to solve unsteady turbulent flows over the Ahmed body (2D).

The Program Implication:

  • Reynolds number is around 768,000 based on inlet velocity and vehicle height (h).
  • Uniform velocity specified at the inlet and constant pressure specified at the outlet.
  • The top boundary of the domain is regarded as “Symmetry” and there is a distance between the car body and road.

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flowlab software cont
FlowLab software (Cont.)

Step 1: Geometry

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flowlab software cont22
FlowLab software (Cont.)

Step 2: Physics (Material properties, viscous models, boundary and initial conditions.

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flowlab software cont23
FlowLab software (Cont.)

Step 2: (Cont.)

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flowlab software cont24
FlowLab software (Cont.)

Step 2: (Cont.)

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flowlab software cont25
FlowLab software (Cont.)

Step 3: Mesh

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flowlab software cont26
FlowLab software (Cont.)

Step 4: Solve

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flowlab software cont27
FlowLab software (Cont.)

Step 4: (Cont.)

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flowlab software cont28
FlowLab software (Cont.)

Step 4: (Cont.)

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flowlab software cont29
FlowLab software (Cont.)

Step 4: (Cont.)

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conclusions
Conclusions
  • Ahmed Body is selected in numerous studies because of its simplicity.
  • Thus, MOVA group partners agreed to study the vehicle shape by Ahmed body.
  • Ahmed model is used for the external flow analysis and drag calculation.
  • At critical angle of 30°, the flow changes its character.
  • The phenomenon of flow separation has undesirable effects. i.e. the energy losses

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references
References
  • Ahmed, S. R., Faltin, G., and Ramm, G. (1984). Some Salient Features of the Time-Averaged Ground Vehicle Wake. SAE Technical Paper, 840300.
  • Bendat, J. S., and Piersol, A. G. (1986). Random Data Analysis and Measurement Procedures. John Wiley & Sons, New York.
  • Kapadia, S., and Roy, S. (2003). Detached Eddy Simulation over a Reference Ahmed Car Model. AIAA, Paper No. 0857

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references cont
References (Cont.)
  • Krajnovic, S., and Davidson, L. (2004). Large-Eddy Simulation of the Flow around Simplified Car Model. SAE,Paper No. 2004-01-0227, Detroit, USA.
  • LADA s 29th WWW. January 29, 2006, from http://www.tfd.chalmers.se/lada.
  • Spohn, A. and Gillieron, P. (2002). Flow separations generated by a simplified geometry of an automotive vehicle. IUTAM Symposium: Unsteady SeparatedFlows, 8-12, Toulouse, France.

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