AHMED BODY

1.) The Ahmed body is a generic bluff body (a simplified vehicle model). It is used in the automotive field to study the impact of the flow pattern on the drag. The simplified topology of this model allows easy modeling, meshing, and comparisons between experimental and numerical results while retaining important practical features pertaining to the automobile body.

2.)Reasons for negative pressure could be:

• Increase in velocity when it flows over the Ahmed body. In order to maintain the flow, the pressure becomes negative (law of conservation of energy).
• Flow separation takes place when the fluid hits the body which leads to the formation of air bubbles which are at very low pressure.
• In order to overcome the shear stress.
• Due to flow recirculation.

3.) The flow around the Ahmed body has several flow separations from the front to the rear of the vehicle. The flow recirculation caused by these flow detachments contributes to the vehicle’s drag. The location point at which the flow separates determines the size of the separation zone, and accordingly the drag force.

The stream of air flowing over a car’s body tends to follow closely to the contour of the body unless there is a sudden change in shape. The front bonnet (hood) is usually slightly curved and slopes up towards the front windscreen. From here there is an upward windscreen tilt (rake) which is, followed by a curved but horizontal roof. The rear windscreen then tilts downwards where it either merges with the boot (trunk) or continues to slope gently downwards until it reaches the rear end of the car.

The sudden geometry change causes the fluid to separate from the surface. This separation comes with a pressure drop that eventually pulls the fluid back towards the surface. The space between the detachment and reattachment point converts into a recirculation area. Flow detachment is generally identified by a low-pressure zone. The places where fluid separation occurs are accompanied by steep pressure gradients that generate vortices and eddies, which in turn augment drag on the body. Other factors affecting the aerodynamic drag, and which are partially responsible for the pressure drag a body experiences, are the high-pressure bubbles in the front of the geometry and low-pressure bubbles in the back.

The vortex is generated by the interaction between these three streams, as the high-speed stream passes the end of the sidewall it flows towards the midplane (pushed by the low-pressure zone found in the back), crashing into the slow speed stream, decelerating before colliding against the main flow closer to the midplane (where the pressure has a higher value). This second collision changes the mixed stream direction giving it a twirling motion, creating the longitudinal vortex.

BASIC TERMS EXPLAINED:

Boundary layer: A boundary layer is the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant.

Laminar boundary layer flow: The laminar boundary is a very smooth flow, while the turbulent boundary layer contains swirls or \"eddies.\" The laminar flow creates less skin friction drag than the turbulent flow but is less stable. Boundary layer flow over a wing surface begins as a smooth laminar flow. As the flow continues back from the leading edge, the laminar boundary layer increases in thickness.

Turbulent boundary layer flow: At some distance back from the leading edge, the smooth laminar flow breaks down and transitions to a turbulent flow. From a drag standpoint, it is advisable to have the transition from laminar to turbulent flow as far aft on the wing as possible or have a large amount of the wing surface within the laminar portion of the boundary layer. The low energy laminar flow, however, tends to break down more suddenly than the turbulent layer.

K epsilon model: It is a two-equation model, which means, it includes two extra transport equations to represent the turbulent properties of the flow. This allows a two-equation model to account for history effects like convection and diffusion of turbulent energy.

The first transported variable is turbulent kinetic energy, k. The second transported variable, in this case, is the turbulent dissipation, epsilon. It is the variable that determines the scale of the turbulence, whereas the first variable, k, determines the energy in the turbulence.

The standard k- epsilon model is the most common model used in Computational Fluid Dynamics (CFD) to simulate mean flow characteristics for turbulent flow conditions. It is a two-equation model that gives a general description of turbulence by means of two transport equations (PDEs).

• Widely used despite the known limitations of the model. Performs poorly for complex flows involving severe pressure gradient, separation, strong streamline curvature. Suitable for initial iterations, initial screening of alternative designs, and parametric studies.

Re-Normalisation Group (RNG): To renormalize the Navier-Stokes equations, a modified form of the epsilon equation which attempts to account for the different scales of motion through changes to the production term.

Realizable: The term ``realizable\'\' means that the model satisfies certain mathematical constraints on the normal stresses, consistent with the physics of turbulent flows.

• K epsilon realizable is suitable for complex shear flows involving rapid strain, moderate swirl, vortices and locally transitional flow (e.g. boundary layer, vortex shedding behind bluff bodies, etc)

The Prandtl Number is a dimensionless number approximating the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity - and can be expressed as

Pr = v / α

where

Pr = Prandtl\'s number

v = momentum diffusivity (m²/s)

α = thermal diffusivity (m²/s)

The Prandtl number can alternatively be expressed as

Pr = μ c_p / k

where

μ = absolute or dynamic viscosity (kg/ms)

c_p = specific heat (J/kg K)

k = thermal conductivity (W/m K)

The Prandtl Number is often used in heat transfer and free and forced convection calculations. It depends on the fluid properties.

• TKE PRANDTL numbers: (only for the standard or realizable k epsilon model, the standard or SST k omega model, the k-kl-omega Transition model, or the RSM) is the effective \"Prandtl\'\' number for the transport of turbulence kinetic energy. This effective Prandtl number defines the ratio of the momentum diffusivity to the diffusivity of turbulence kinetic energy via turbulent transport.
• TDR PRANDTL Number Is the effective \"Prandtl\'\' number for the transport of the turbulent dissipation rate, for the standard or realizable k epsilon model or the RSM. This effective Prandtl number defines the ratio of the momentum diffusivity to the diffusivity of turbulence dissipation via turbulent transport.
• Near wall treatment: Appears if k-epsilon or Reynolds Stress is selected as the Model.

Standard wall functions: Give reasonable accuracy for a majority of high-Reynolds-number, wall-bounded flows but reach their limitation when the flow condition differ too much from the ideal conditions used to define the functions.

When the near-wall flows are subjected to severe pressure gradients, and when the flows are in

strong non-equilibrium, which means that the turbulence production term and the dissipation term

are not equals, the results given by the standard functions are not satisfactory enough and the Nonequilibrium wall function allows for better calculations.

Enhanced wall functions: The enhanced function in FLUENT are used to achieve near-wall modeling approach having the accuracy of the standard two-layer approach for fine meshes and at the same time not degrading the results for the wall function meshes. In order to do so the enhanced wall functions are combined with the two-layer model.

The two-layer model is defined as a near-wall model that resolves the entire boundary layer until the viscous sub-layer. All domain is devised in two regions, a viscosity-affected region and a fully turbulent region and the separation is determined by a wall distance defined from a Reynolds number based on y.

GEOMETRY

half geometry is done by using split command and cutting the body into two halves from the front plane.

BASELINE ANALYSIS:

MESH

adjusting the reference value according to the given problem and start the setup

turbulence model: K epsilon with standard wall functions

inlet velocity as 25m/s

after hybrid initialize and making suitable contours we run the simulation for 500 iterations.

RESIDUALS:

DRAG PLOT

drag coefficient=0.4182

VELOCITY CONTOUR:

PRESSURE CONTOUR

POST PROCESSING:

REFINED CASE :

GEOMETRY:

MESH

using the same reference value from the baseline setup and then running the simulation

RESIDUALS:

SOLUTION GOT CONVERGED AT ITER=179

DRAG PLOT:

VELOCITY CONTOUR:

PRESSURE CONTOUR:

POST PROCESSING:

CONCLUSIONS :

• the solution has been converged at 179 iterations and by residual plots the repeating pattern signifies it.
• in the total pressure plot, we can see that a low-pressure region i.e. wake is present on the rear end of the body.
• recirculation of fluid can cause lift and negative force.
• the drag coefficient is near to 0.35.

the simulation was used at near the maximum capacity of an academic license, so more the number of elements more time was consumed in solving.