Aerodynamics study of Automobile car Ahmed body using Ansys Fluent

Title: Grid dependancy test & simulation of Ahmed body 

Objective:

1. To perforam grid dependancy test

2. T plot Velocity and pressure contours. 

3. The plot velocity vector plot clearly showing the wake region. 

4. To obtain coefficient Drag & Lift

Ahmed body

                      Ahmed Body was one such generic bluff body proposed to study the external aerodynamics of the vehicles. The flow over the geometrically simple Ahmed reference body is used for the validation case for numerical simulations and it still continues a challenge numerical algorithms and turbulence modeling due to its complex three dimensional wake vortex interaction. The Ahmed body is a simplified car-shaped made-up of a parallelepiped with rounded edges at the front and a slanted face at the rear. The slant at the rear side of the vehicle causes the separation of flow and also a significant factor for drag forces of Ahmed body.The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.

The flow for this model is turbulent, which is based on the Reynolds number determined by the body length and inlet velocity. The simulation solves for the turbulent kinetic energy in addition to the velocity field. For this simulation, we need a larger mesh size than what is usually common to resolve the turbulent flow. More specifically, we use a finer mesh downstream of the model to capture the wake zone.The simulation solves for the turbulent kinetic energy and dissipation in addition to the velocity and pressure fields.

Importance of Ahmed body 

              The three-dimensional flow around a vehicle has become a subject of significant importance in the automotive industry. One apparent technique of improving the fuel efficiency of a vehicles is to reduce aerodynamic drag force by optimizing the body shape. Execution of fine aerodynamic design under stylistic constraints requires an immense understanding of the flow pattern phenomena and, especially, how the aerodynamics are impacted by changes in body shape. The flow area which is presents the major contribution to a cars drag is the wake flow backing the vehicle. The point at which the flow separates directs the size of the separation zone, and as a result the drag force. Clearly, a more accurate simulation of the wake flow and of the separation process is required for the accuracy of drag forecast

Negative pressure in Wake Region

           When the air moving over the vehicle is separated at the rear end, it leaves a large low pressure turbulent region behind the vehicle known as the wake. This wake contributes to the formation of pressure drag, which is eventually reduces the vehicle performance.

           The region of recirculating flow immediately behind a moving or stationary blunt body, caused by viscosity, which may be accompanied by flow sepration and turbulence flow, or the wave pattern on the water surface downstream of an object in a flow, or produced by a moving object (e.g. a ship), caused by density differences of the fluids above and below the free surface and gravity.

               When the vehicle is moving at a certain velocity, the viscous effects in the fluid are
restricted to a thin layer called boundary layer. Outside the boundary layer is the inviscid flow. This fluid flow imposes pressure force on the boundary layer. When the air reaches
the rear part of the vehicle, the fluid gets detached. Within the boundary layer, the movement of the fluid is totally governed by the viscous effects of the fluid.The Reynolds number is dependent on the characteristic length of the vehicle, the kinematic viscosity and the speed of the vehicle. The fluid moving around the vehicle is dependent on the shape of the vehicle and the Reynolds number. There is another important phenomenon, which affects the flow of the car and the performance of the vehicle. This phenomenon is commonly known as ‘Wake’ of the vehicle. When the air moving over the vehicle is separated at the rear end, it leaves a large low-pressure turbulent region behind the vehicle known as the wake. This wake contributes to the formation of pressure drag, which is eventually reduces the vehicle performance.

 

Singnificance of the point of Sepration

                 Separation of flow is said to occur when the direction of the flow velocity near the surface is opposed to the direction of the free stream velocity, which means (du/dy) ≤ 0. Such a situation arises when there is a negative pressure gradient near the wall (Refer Fig.). An example is subsonic diffuser or rear portion of the car.

                                   

                  In the direction of flow the pressure increases which causes decreases in velocity and creates condition for separation to occur. It is important to note that, the skin-friction coefficient reaches the value zero. At this point the wall shear stress vanishes is called the separation point. Reverse flow takes place beyond point of separation. Due to reverse flow, irregular eddies are formed and lot of energy is dissipated. This region is called „Wake‟. The pressure distribution in the wake is quite different from that on the remaining boundary and this gives rise to an additional drag force on the body. Wake also leads to lateral vibration to the object which may be harmful. Also large amount of K.E. is lost.

             In the turbulent boundary layer, some of the energy is dissipated in friction, slowing airflow velocity, resulting in a pressure increase. If the increase in pressure is gradual, the process of turbulent mixing will cause a transfer of energy from the fast moving eddies to slower ones in the turbulent boundary layer. If the rate of change in pressure is too great, for example in sharp corners, the mixing process will be too slow to push the slower air molecules moving. When this happens, the boundary layer flow stops following the contours of the surface, resulting in separation. Air particles downstream of the separation region will then move towards the lower pressure region in the reverse direction to the main
flow, the separation region will reattach. In the region between separation and reattachment points, air flow is circulating and this is called the „separation bubble Separation will normally occur if the resultant flow encounters a sharp edge and that is why it is always important
for ground vehicles to have smoothly rounded edges everywhere.

Steps to run the simuation

1. Model is importated in to spaceclaim window

        1. Drawn the bigger & smaller enclosure for better simulation on ahmed body 

        2. Check the interferance region

        3. Use of option share topology for transfer information to each other

2. Open the Mesh window, name given to domain like(Inlet, outlet, symmtery, bottom wall,                   car wall )

        1. Give element size & body sizing to bigger, small enclosure, car wall, car stand for                  coarse to finer mesh

        2. Addition of inflatio layer to get accurate the solution near wall.

3. Open the Setup & solution window

      1. Choose the Turbulence model like K-epsilon & k-omega model & also turn on enrgy eq.

      2. Give the boudry condition like inlet velocity 25m/s, outlet  zero guage pressure.

      3. Prepare report Coefficient Drag, lift. Check the Y+ criteria

 4. Open Result window

      1. Plot the velocity & Pressure contour

      2. Plot the velocity vector plot

5. Analysis of Coefficient Drag & lift , Wake region , Pressure distribution

Ahmed Body

                                   

Domain in Spaceclaim

Name slection in Mesh

Input Data for Mesh & Boundry condition

Refrence value 

                                 

Case I: Baseline Mesh with Use of K-epsilon Method

      In case I there is use of default size for run the simulation , there is use k-epsilon turbulence model. & it ran for 100o iteration , but solution is converged at 698 iteration

Residual Plot

                  we can see that from above plot the solution is converged at 698 iteration.

Coefficient Drag

         Coefficien drag obtain 0.45

Coefficient Lift

Coefficien lift  obtain 0.27

Velocity Vector

                from above plot we can see that solution is not correct as we expect, there is should be generation of wake region behind the car, its happen due to the coarse mesh of car & enclosure.

Pressure contour

From above plot we can see that pressure is higher at frontal portion of car, where there is stagnation point.

Velocity vector plot

               In the vector plot we can see that still , there is not generate recirculation zone behind the car. 

Case-II:Medium Mesh with Use of K-epsilon Method

      In case II  tried to refine the mesh , so that there is made in changes in element size of bigger enclosure 100mm, small encloure 50 mm, car wall size 20mm , car stand size 5mm. and also there is use of inflation layer 5 for correct capture the solution near wall.

Residual plot

 the solution is converged at 808 iteration

Coefficent Drag

         coefficient drag obtain is 0.40

Velocity contour

   from the above contour we can see that, the velocity at frontal section is lower due to stagnation point & also there is stat the generation of wake region behind the car, which is occurs due to the sepration of boundry layer & pressure reduction in rear side.

Pressure contour

From above plot we can see that pressure is higher at frontal portion of car, where there is stagnation point.

velocity vector plot

from above plot we can see that , on rear side of car there is decreases the pressure & velocity which leads to generation of  the recirculation zone on rear side 

         

Y+Wall function

  Y+ wall function is imporatant to decide the to use of  coarse or fine mesh near the wall & also it affect on coefficient drag value

Case-II:Medium Mesh with Use of K-omega Method

In case II , there is experimentation done on turbuent model, here there is use of k-omega model & same element size are uses.  after run the simulation it s found that k-omega model gives better result than the k-epsilon model.

solution is converged at 994 iteration 

Coefficient Drag

coefficient drag got 0.37

Coefficient Lift

coefficient lift got 0.27

Velocity Vector

       from the above contour we can see that, the velocity at frontal section is lower due to stagnation point & also there is stat the generation of wake region behind the car, which is occurs due to the sepration of boundry layer & pressure reduction in rear side.

Pressure contour

From above plot we can see that pressure is higher at frontal portion of car, where there is stagnation point.

Velocity Vector

from above plot we can see that , on rear side of car there is decreases the pressure & velocity which leads to generation of  the recirculation zone on rear side

Y+Wall function

Y+ wall function is imporatant to decide the to use of  coarse or fine mesh near the wall & also it affect on coefficient drag value. so to get better result Y + value is restricted between 30-100 , so that we can get good result.

Case III: Fine Mesh with use of K-omega Method

   In case III  there is fully refine the mesh , so that there is made in changes in element size of bigger enclosure 80mm, small encloure 50 mm, car wall size 12mm , car stand size 5mm. and also there is use of inflation layer 6 for correct capture the solution near wall.

Residual plot

from above plot we can see that after the 1350 iteartion , same repetative pattern generated which tell us that solution is converged

Coefficient Drag

coefficient drag got it 0.34

Coefficient Lift

coefficient lift got it 0.306

Velocity Contour

 from the above contour we can see that, the velocity at frontal section is lower due to stagnation point & also there is  generated  wake region behind the car, which is occurs due to the sepration of boundry layer & pressure reduction in rear side.

Pressure contour

From above plot we can see that pressure is higher at frontal portion of car, due to there is stagnation point.

Velocity Vector

Recirculation & Wake Region

From above plot we can see that there is generation of recirculation & wake region , its happen due to the sepration of boundry layer ,pressure , velocity reduction in this zone

Y+ Wall function

Y+ wall function is imporatant to decide the to use of  coarse or fine mesh near the wall & also it affect on coefficient drag value. so to get better result ,Y + value is restricted between 5-10 where there is viscous sub layer occur, so that we can get good result.

Output Results

Overall conclusion

1.Grid dependancy test is important to make the mesh from coarse to fine, so that we can get good result.

2. One imporatant result found that, use of K-omega model give better result than the k-epsilon model, also it is found that k-omega take more iteration for solution convergence but it gives good result.

3. As we refine the setup ,the reduction of coefficient drag & increase of coefficien lift .

4. Y+wall function is very imporant to get the correct result of coefficient drag &  lift.

5. Negative pressure in wake region due to 

           1.It is due to the boundry layer sepration, vaccum region is genearated & in that                       region there is presence of negative pressure

           2. Due to recirculation zone also there is negative pressure generate

6. Boundry layer sepration causes & its importance

          1. In the direction of flow the pressure increases which causes decreases in velocity                  and creates condition for separation to occur.

          2. It is important to note that, the skin-friction coefficient reaches the value zero. At                   this point the wall shear stress vanishes is called the separation point.

          3. If the boundry layer is seprate out, it affect on coefficient drag & lift result.