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Week 3 - External flow simulation over an Ahmed body.

SSudheerkumar Segu26 Dec 2022
CFD

EXTERNAL FLOW ANALYSIS OVER AHMED BODY

In the study of aerodynamics on an automobile, we are using a shape called the Ahmed body with a rear slant angle of 20 degrees to investigate certain aspects of the flow, such as the drag and lift forces, the separation of flow, and the forces in the wake region. We are also using this shape to validate the measured data.

AHMED BODY:

INTRODUCTION:

The Ahmed body is a geometrical shape that is often used in CFD (Computational Fluid Dynamics) simulations to study the flow of fluids around a vehicle. It was first proposed by Ahmed and his colleagues in the 1980s as a simple and reliable benchmark geometry for evaluating the accuracy of CFD codes.

The Ahmed body has a streamlined shape with a flat base and a sloping rear, which makes it a good approximation of a simplified vehicle. It is commonly used to study the flow around cars, trucks, and other vehicles, as well as to investigate the effect of various design parameters on the aerodynamic performance of the vehicle.

The Ahmed body has several features that make it a popular choice for CFD simulations:

  1. Simple geometry: The Ahmed body has a simple geometry with only a few flat surfaces, which makes it easy to model and mesh.

  2. Symmetry: The Ahmed body has a symmetry plane running through its center, which allows for reduced-order simulations using only one-half of the geometry.

  3. Realistic flow conditions: The Ahmed body generates realistic flow conditions that are similar to those found in real-world vehicles, such as separation and reattachment of the flow on the sloping rear.

  4. Widely available experimental data: There is a large body of experimental data available for the Ahmed body, which allows for validation of CFD simulations.

Overall, the Ahmed body is a useful tool for studying the flow of fluids around a vehicle and for evaluating the performance of CFD codes.

"Our main Intention is to study the flow of fluid and the effectscaused by it around the vehicle."

AIM: To perform flow simulation around the ahmed body with rear slant angle of 20 degrees and validate the results with experimental data.
PROCEDURE:
For flow simulation we need to assume certain boundary conditions and flow parameters. Below are the assumptions made.
Assumptions:
  • In this simulation, we are only considering the external flow around the Ahmed body and excluding the internal part of the body, as it is not necessary to perform calculations there. The simulation will be performed within an enclosure that surrounds the body, which will act as the control volume for the flow.
Physics Setup:
  • The Ahmed body that we are using for studying has a rear slant angle of 20 degrees.
  • Assuming the flow medium is AIR with Inlet flow velocity (25 m/s) . The density of air at standard temperature and pressure (STP) is 1.225 kg/m^3, and the dynamic viscosity of air at STP is 1.81x10^-5 Ns/m^2. The thermal conductivity of air is 0.026 W/mK and no pressure gradient at the outlet.
  • Assuming the body and the tunnel for measuring are both insulated, there is no heat transfer between them, and no heat exchange occurs.
  • NoSlip condition over the Ahmed body .

  • To calculate the Mach number for the flow over the Ahmed body, you will need to know the flow velocity and the properties of the flow medium.

    Assuming that the flow velocity is 25 m/s and the flow medium is air at standard temperature and pressure (STP), the Mach number can be calculated using the following equation:

    Mach = V/C

    where V is the flow velocity, and C is the speed of sound in the flow medium. The speed of sound in air at STP is approximately 343 m/s.

    The values for V and C, we get:

    Mach = 25/343 = 0.0726

    This means that the flow over the Ahmed body has a Mach number of approximately 0.0726, which indicates that the flow is Subsonic (i.e. the flow velocity is less than the speed of sound. This means that the flow is not fast enough to generate a shock wave or compressibility effects, and the flow can be treated as incompressible.)

    If Mach Number is >0.33 we may need to consider the compressibility effects of the fluid. Here it is less.So, we are not considering the compressibility effects of fluids.
I am using ANSYS software which is commercially available CFD code widely used in this industry. In this the whole CFD procedures can be done with the help of it.
CFD of this problem has to be approached in stepwise.
1)Geometry Creation/Cleaning up of geometry,
2)Meshing,
3)Solving,
4)Post Processing.
1)Geometry Creation/Cleaning up of geometry:
By importing the Ahmed Body model which is in .STEP file format,using Spaceclaim tool it is modified according to the given problem statement. As the fluid flow simulation is on externally we are creating an Enclosure around the Ahmed body with following dimensions.
Ahmed Body Dimensions:
Since the given model is a symmetry model, we can divide it into equal halves and perform the simulation calculations on only one half, knowing that the same results will be obtained on the other half. This allows us to reduce the simulation time and calculation time, as we only need to solve the governing equations for one half of the model.
Control Volume Creation:
To create control volume around the model, by using PREPARE>ENCLOSURE option in Spaceclaim created a box like structure with Ahmed Body as stationary object.
Boundaries of the box in the view of Ahmed body Top, Bottom, Front, Back & Rear as follows in -X to +X direction.
TOP BOTTOM LEFT HAND SIDE RIGHT HAND SIDE REAR FRONT
1000 0 500 0 5000 2000
Since the enclosure around the body is very large, meshing the whole domain would not result in good enough volume transition to capture the flow accurately. To improve the volume transition, we are creating a sub-zone enclosure around the body, which helps to capture and transition the volume in an acceptable manner.
MESHING:
Due to the large volume, I decided to conduct a grid independence test to determine whether the results vary with the mesh size. To accurately capture the flow around the body, I also created an inflation layer by calculating the first layer thickness for the flow velocity using a Y+ calculator.

The Y+ value is a dimensionless parameter that is used to calculate the first layer thickness in computational fluid dynamics (CFD) simulations. It is defined as the ratio of the first layer thickness to the viscous length scale, which is a measure of the distance over which the viscous forces become dominant.

The Y+ value is typically in the range of 30-300 for fully turbulent flows, with a value of around 100 being commonly used as a reference value. The Y+ value is dependent on the flow velocity, the flow medium, the viscosity of the flow medium, and the roughness of the surface.

To decide on the appropriate Y+ value for a given flow, you can consider the following factors:

  1. Convergence of the solution: A higher Y+ value may result in a more accurate solution, but it may also require a finer mesh and more computational resources.

  2. Accuracy requirements: The Y+ value should be chosen based on the accuracy requirements of the simulation. If a higher accuracy is required, a higher Y+ value may be needed.

  3. Computational resources: The Y+ value should also be chosen based on the available computational resources. If the simulation is computationally intensive, a lower Y+ value may be needed to reduce the computational burden.

  4. Comparison with experimental data: If experimental data is available for the flow, the Y+ value should be chosen such that the simulated results are in good agreement with the experimental data.

By using Y+ calculator I took first layer thickness 0.4mm with Y+ value as 30.(Y+=30 is taken based on considering the meshing and computational parameters into consideration).
CASE 1:
Meshing method = Multizone
Mesh Length=110mm
Y+=30
First Layer Thickness=0.4mm
Number of layers =10
Obtained results:
Number of Mesh Elements=330880
Quality of the Mesh Elements =Satisfied with the results.
SETUP:
Since the Mach number is 0.0726, indicating that there are no compressibility effects, I used a pressure-based solver with the k-epsilon method to simulate the flow. This means that the density of the flow will not change during the simulation. Created reports for observatin of lift and drag Forces.

The k-epsilon model is a widely used turbulence model in computational fluid dynamics (CFD) simulations. It is a two-equation model that is used to describe the turbulent flow of a fluid, and it is based on the Reynolds-averaged Navier-Stokes (RANS) equations.

The k-epsilon model consists of two equations: one for the turbulent kinetic energy (k) and one for the turbulent dissipation rate (epsilon). The turbulent kinetic energy equation describes the rate of change of k due to production, dissipation, and transport, while the turbulent dissipation equation describes the rate of change of epsilon due to production and dissipation.

The k-epsilon model is widely used in CFD simulations because it is relatively simple and easy to implement, and it provides good results for a wide range of flow conditions. It is particularly useful for simulating flows with moderate to high levels of turbulence, such as flows around aerodynamic bodies or in turbulent boundary layers.

SOLUTION:
By using Hybrid method for Initialization flow is simulated for 300 Iterations. Below were the results obtained.
Velocity contour :
Pressure Contour:
Velocity Sream Line Contour:
Case 2: This time reduced only the mesh element size as 100mm and repeated the same solution procedure.
Obtained results:
Number of Mesh Elements=363613
Quality of the Mesh Elements =Satisfied with the results.
Velocity Contour:
Pressure Contour:
RESULTS:
Mesh Element Size Number of mesh Elements Obtined Type of solver used Model Used Coefficient of Drag Obtained Experimental Data For Validation Coefficient of Lift Obtained Experimental Data For Validation
Case1:110mm 330880 Pressure Based k-epsilon 0.2867

0.3 to 0.5

depend on flow type

 0.2459

-0.2 to 0.29

depend upon flow type.
Case2:100mm 363613 Pressure Based k-epsilon 0.2879 0.2389

We got the results very near to the experimental results.

Flow Seperation :

Flow separation is the phenomenon in which the flow separates from the surface of a body and forms a separation bubble. Flow separation can occur at different locations on a body depending on the shape of the body and the flow conditions.

When flow separation occurs before the desired location on a body, it can have negative consequences on the aerodynamic performance of the body. For example, it may lead to increased drag, reduced lift, and reduced stability of the body. Flow separation can also lead to unsteady flow and vortex shedding, which can cause vibrations and noise.

To avoid or minimize flow separation, it is often necessary to design the body with appropriate geometry and surface roughness, or to apply some form of flow control to the body. This can be done using passive methods, such as using vortex generators or spoilers, or using active methods, such as blowing or suction.

Flow separation on the Ahmed body is observed on the backside of the body through velocity contours, which form a low pressure region behind the body and create a wake region. The flow separation also causes the formation of Vortices and results in increasing the drag and lift forces on the body. Observed two vortices were formed in backside using velocity contours.

Vortices are swirling, circular patterns of fluid flow that can occur in a variety of flow situations. In the context of the Ahmed body, vortices may be formed as a result of flow separation around the body.

Vortices can have a number of effects on the flow around the Ahmed body, including:

  1. Drag force: Vortices can cause an increase in the drag force on the body, as they create a wake region with low pressure that can pull on the body.

  2. Lift force: Vortices can also affect the lift force on the body, as they can cause the flow to separate at different locations on the body.

  3. Flow separation: Vortices can cause the flow to separate from the surface of the body, leading to flow separation and the formation of a separation bubble.

  4. Unsteady flow: Vortices can cause unsteady flow, which can lead to vibrations and noise.

  5. Vortex shedding: Vortices can also shed from the body, leading to vortex shedding and the formation of a von Kármán vortex street.

It is important to note that the effects of vortices on the Ahmed body will depend on the specific flow conditions and the shape of the body.

Wake Region:

A wake region is a region of fluid flow that is influenced by the presence of a body, such as the Ahmed body. The wake region is typically located behind the body and is characterized by low pressure and the presence of vortices.

The size and shape of the wake region behind the Ahmed body will depend on the flow conditions and the shape of the body. The wake may be elongated or more compact, and it may contain a variety of vortical structures.

The wake region can have a number of effects on the flow around the Ahmed body, including:

  1. Drag force: The low pressure in the wake region can cause an increase in the drag force on the body.

  2. Lift force: The wake region can also affect the lift force on the body, as it can cause the flow to separate at different locations on the body.

  3. Flow separation: The wake region can cause the flow to separate from the surface of the body, leading to flow separation and the formation of a separation bubble.

  4. Unsteady flow: The wake region can cause unsteady flow, which can lead to vibrations and noise.

  5. Vortex shedding: The wake region can also lead to vortex shedding and the formation of a von Kármán vortex street.

It is important to note that the effects of the wake region on the Ahmed body will depend on the specific flow conditions and the shape of the body.

CONCLUSION:
Based on the observations, it can be concluded that the results of the simulations were in good agreement with the experimental data. The drag and lift forces on the Ahmed body are largely influenced by the streamline of the body, with a greater emphasis on the rear side of the body, which is the main cause of flow separation, wake region formation, and vortex generation.
REFERENCES:
https://www.researchgate.net/publication/266883948_Flow_and_Turbulence_Structures_in_the_Wake_of_a_Simplified_Car_Model_Ahmed_Model
Book on FLUID MECHANICS Fundamentals And Applications written byYUNUS A CENGEL and JOHN M CIMBALA
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