Ahmed Body Challenge

 1. Describe about Ahmed body and its importance

  •  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.
  • Ahmed Body had some practical features relevant to automobile bodies.

                    Fig.1   Basic Dimensions of the Ahmed Body

  • 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 0.05 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 of air around Ahmed Body captures the essential flow features around an atuomobile. 
  • This model describes how to calculate the turbulent flow field around a simple car-like geometry using the Turbulent flow, k-epsilon interface.
  • Numerical results obtained using Ahmed Body were compared with the Analytical results and its calculation time is less. 

Modeling Airflow Over an Ahmed Body:

  •   The verification Ahmed Body model has 25-degree slant and is          placed in the following domain, measuring 8.532 x 2.088 x 2.088      metres, to compute flow field.   

Fig.2 Computational domain and boundary conditions for the fluid flow simulation.

  • The front of the body is placed at a distance of 2 car lengths (2L) from the flow inlet. To reduce the computational cost, a symmetry plane is introduced to model half of the model.
  • 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 and dissipation in addition to the velocity and pressure fields. 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.

CASE:1

  • Element size = 300 mm
  • Number of Nodes = 29947
  • Number of Elements = 99667

Simulation Details for 3 cases:

Type of simulation: Steady-state

Type of Model: k-epsilon (Turbulence Model)

Type of Fluid : Air 

Inlet velocity : 50 m/s

Outlet pressure : 1 atm

Scaled Residuals

 

Drag coefficient

Lift coefficient

Moment coefficient 

contours of velocity at symmetry

contours of velocity with AhmedBody at symmetry

velocity pathlines

velocity vectors

contours of pressure at symmetry

Pressure Distribution on Ahmed Body

The velocity and pressure contours are less smoother than compared to other two cases, the computation time is less for this case and gives  less accurate nearby values. This kind of less redefined mesh can be used as a baseline mesh to check whether everything is working or not

CASE:2

  • Element size = 250 mm
  • Number of Nodes = 37239
  • Number of Elements = 136520

Scaled Residuals

Drag coefficient

Lift coefficient

contours of velocity at symmetry

contours of velocity with AhmedBody at symmetry

velocity pathlines

velocity vectors

contours of pressure at symmetry

Pressure Distribution on Ahmed Body

 

The velocity and pressure contours are less smoother as compared with case 3 but they are smoother than case 1.

CASE:3

  • Element size = 200 mm
  • Number of Nodes = 55828
  • Number of Elements = 227284

Scaled Residuals

Drag coefficient

Lift coefficient

contours of velocity at symmetry

contours of velocity with AhmedBody at symmetry

 

velocity pathlines

velocity vectors

contours of pressure at symmetry

Pressure Distribution on Ahmed Body

The computation time is more, but the results are more accurate and closer to the true values.

Grid Dependency Test:

  • Mesh is more refined from case 1 to case 3.
  • A smoother mesh provides a smoother and more accurate values.
  • The flow is more defined at lower element size i.e, at case 3
  • Mesh refinement increases the closeness of values to the true values. 

3. In the post processed results, explain the variation in pressure as to why it goes negative

  • The flow around a body has maximum velocity, where as the pressure becomes negative to maintain the flow, but at the front tip of the Ahmed body has least velocity which inturn increase of pressure takes place.
  • The pressure drag results depends on velocity, pressure and vorticity field, pressure becomes negative because of velocity and vortices.
  • The separation of flow from the curved surface interface of the Ahmed body results in negative pressure.
  • The flow gets accelerated at low pressure regions. 
  • The pressure turns negative to counter balance the effect of shear stress.

Projects by Shravankumar Nagapuri

Objectives: To perform a parametric study on the gate valve simulation. To obtain the mass flow rates at the outlet for 5 design points. To show the cut section view for different design points, and also to show the gate disc lift and fluid volume extraction. To s Read more

Objectives: To Write a Matlab program that can generate the computational blockMesh file  automatically for a wedge angle of 3 degrees and compare the results obtained for Symmetry BC and Wedge BC. To Write a Matlab program that takes an angle as input and gene Read more

1Q. Briefly explain about the possible types of combustion simulations in FLUENT. A:  Combustion or burning is a high temperature exothermic chemical reaction between a fuel and an oxidant accompanied by the production of heat, light and unburnt gases in the fo Read more

Cyclone Separator: Principle and Working: A high speed rotating (air)flow is established within a cylindrical or conical container called a cyclone. Air flows in a spiral pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end bef Read more

Objective: To analyse the flow pattern of the fluid inside the gearbox, for two different clearances of the same geometry and each geometry flow is analyzed by two different fluids (Water and Oil) as lubricants. Gearbox sloshing effect:  Slosh refers Read more

Objectives: To write a program in Matlab that can generate the computational mesh automatically for any wedge angle and Grading scheme. To calculate, length of the pipe using the entry length formula for laminar flow through a pipe To show that entry length is suf Read more

Objectives: To find out the maximum temperature attained by the processor. To Prove that the simulation has achieved convergence with appropriate images and plots. To Find out the heat transfer coefficient at appropriate areas of the model. To Identify potential h Read more

Finite volume Method: The finite volume method (FVM) is a method for representing and evaluating partial differential equations in the form of algebraic equations. Similar to the finite difference method (FDM) or finite element method (FEM), values are calculated at Read more

Q1. What are some practical CFD models that have been based on the mathematical analysis of Rayleigh Taylor waves? In your own words, explain how these mathematical models have been adapted for CFD calculations  Kelvin–Helmholtz instability: The Kelvin Read more

Objectives: To Generate the BlockMesh file for the given Geometry using OpenFOAM and use the icoFoam solver to simulate the flow through a backward-facing step. To create multiple meshes and will be comparing the results obtained from each mesh. To show the velocit Read more


Loading...

The End