Week 3 - External flow simulation over an Ahmed body.

#) Objective of the project- Run the simulation for the flow of velocity = 25 m/sec with the default air properties in fluent.

Importance of the Ahmed body- 

The Ahmed body is a generic car body it is a bluff body (a simplified vehicle model) to study the external aerodynamics. The airflow around the Ahmed body captures the essential flow features around an automobile and it is used for the validation study of the numerical CFD results and turbulence modeling because of the complex three-dimensional wake vortex interaction. was first defined and characterized in the experimental work of Ahmed. Although it has a very simple shape, Ahmed's body allows us to capture characteristic features that are relevant to bodies in the automobile industry. The Ahmed body is a simplified parallelopiped with round edges at the front and slanted edges at the back. 

The simple geometrical shape has a length of 1.04 m, height of 0.28m and with a height of 0.389m . It also has 0.5 m long cylindrical leg of 0.5 m and the surface has a slant of 40-degree angle.

This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model is validated, it is used to design new models of the car.

#) picture of the Ahmed body-

#) The flow for this model is turbulent which is determined by the Reynolds number determined by the characteristics length and inlet velocity. The simulation solves for the turbulent kinetic energy in addition to the velocity field. The simulation solves for the turbulent kinetic energy in addition to velocity field. For this reason we need finer mesh downstream of the model to resolve the turbulent flow . More specifically we need a finer mesh downstream of the model to capture the wake zones . The simulation solves the turbulent kinetic energy and dissipation in addition to the velocity and pressure fields.

Q2. Explain the reason for the negative pressure in the wake region. 

The flow behind an object separates from the surface and creates a highly turbulent region behind the object, called the wake. The pressure inside the wake region remains low as the flow separates and a net pressure force (pressure drag) is produced. In other words, due to the production of pressure drag being produced from flow separation, the pressure in the wake region is low. This is what is causing the vacuum.

 When the vehicle is moving bwith a certain speed the boundary layer the viscous effect creates one boundary layer over the body . The negative pressure is due to velocity difference applied at a certain Reynolds number when this difference in pressure occurs the wake obtained increases the drag due to the pressure drop behind the Ahmed body which at a high Reynolds number due to vortices generated it produces a small amount of pressure thrusting the body. 

The flow region which presents the major contribution to a car’s drag, and which poses severe
problems to numerical predictions and experimental studies as well, in the wake flow behind
the car. The location at which the flow separates determines the size of the separation zone,
and consequently the drag force. Clearly, a more exact simulation of the wake flow and of the
separation process is essential for the correctness of drag predictions. However, a real-life
the automobile is a very complex shape to model or study experimentally.

Question3 answer-

According to the point of separation defined by the Ahmed body and the flow the alpha angle depending on which the as it  causes chnage in the flow direction forces required like the lift and lift induced drag due to tip vortices at a critical value flow in slant stalls and reduced lift reducing as well drag all depending on the value of alpha nd Reynolds Number. 

The Reynolds Number is the dimensionless number it is the ratio of inertial force to the viscous force .

Re = inertial force/ viscous force Re = (rho*v*l)/mu.   mu is the dynamic viscosity.

#) The law of the wall - y+ according to fluent turbulence model

1) The flow demands solving viscous sublayers.

These situations include those cases where wall-bounded effects are important common examples are heat transfer , aerodynamic drag and pressure drop flows with adverse pressure gradients.

The flow surrounding a bluff body there are cases where the wall-bounded effects are secondary or flow undergoes geometry induced separation.

To solve these case the y+ value for the k-omega turbulence model should have a y+ value less than 5 and the k-epsilon model y+ value should be between 30 to 300.

The classification of different turbulence models and its characteristics-

 Features of the turbulent flow-

1)Unsteady, irregular (aperiodic) motion in which transported quantities (mass, momentum, scalar species) fluctuate in time and space.

2) Identifiable swirling patterns characterize turbulent eddies.Enhanced mixing (matter, momentum, energy, etc.) resultsFluid properties and velocity exhibit random variations.

3)Statistical averaging results in accountable, turbulence related transport mechanisms.This characteristic allows for turbulence modeling.

#) Solution setup-

To perform simulation setup Ansys/Fluent it's desirable to have knowledge in the type of fluid model to adjust the condition of laminar and turbulent flows according to Reynolds number and the Navier Stokes equation and mesh characteristics are also made very fine to capture the actual velocity distribution and to capture the wake region that develops due to the turbulence at the back of the Ahmed body. For this project many simulations will be performed to take into account the grid dependence study by which we would be refining the mesh. 

The geometry that is being created in the spaceclaim to incorporate the modifications done -

 In this spaceclaim geometry we are using the enclosure option to create two different domain one inside the other . The second domain is being created to make the region inside that domain much finer so that it can capture the solutions of the governing equation in a much correct way. Since the mesh size is much will be finer inside the second domain than in the first domain.

#) The mesh created in the Ansys mesher is -

Since we are putting two body sizing one is for the outer domain and the second body sizing is for the second smaller domain.

The element size of 100 mm is used for the body sizing.

The third face sizing is used inside the half-cut portion of the Ahmed body by using face sizing option on the mesh option so that the walls of the body are made finer by giving 50 mm size.

#) Finally the inflation layer is added around the Ahmed body so that the mesh makes the smooth transition from the surface of the body to the first domain of the body . This inflation layer helps to capture the boundary conditions around the wall of the Ahmed body.

The initial spacing of the first layer or the y+ calculation is done using the following input parameters nin an online calculator-

delta s value is the spacing value of 0.7162 mm and 6 inflation layers were added.

Thenn in the solver the setup was made 

3)The contours of the velocity contour magnitude- 

 The residual plots it is constantly decreasing beyond the 1e-3 so we can say the iterative process have converged and thjis is an acceptable solution-

#) Thr drag coefficient plot obtained-

#) For the k- omega turbulence model used for the same simulation mesh-

#) Now the mesh independency test was done by refining the mesh further frrom thne in itial case to take into account the physics that would develop due to finer mesh sizes as  a result the mesh size in the second body or the second rectangular domain was refined further .

Mesh containg the element size of 40 mm inside of the smaller rectangular domain the mesh is of-

The results obtained are-

#) For the second one we are again decreasing the element  size to 35 mm and then it is meshe d again now the much more refined mesh is used to calculate the new drag and lift coefficient-

#)  The same procedure is folowed a density based solver is used with an incoming velocity v = 25 m/ sec and using the k -epsilon turbulence model-

#) The streamline representation of the velocity fields is obtained as- 

#) Conclusins-

 element size                   number of nodes                    drag coefficient              lift coefficient   

312972                              81362                                   0.88                            0.54

431784                              92737                                  0.51                           0.56

493522                              129607                                 0.44                          0.80

   So the mesh independence study was carried out and as the element size increases the drag and lift coefficient are captured more accurately.