Week 3 - External flow simulation over an Ahmed body.

Q1. Describe Ahmed's body and its importance.

The Ahmed body is a simplified, ground vehicle geometry of a bluff body type. It allows accurate flow simulation but retains some important practical features relevant to automobile bodies. Ahmed body gives direction about how to calculate the turbulent flow field around a car-like geometry using the Turbulent Flow, k-epsilon model interfaces.

The Ahmed Body was first developed by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Ahmed body is the benchmark model for aerodynamics simulation tools. 

Some of the key characteristics which can be obtained from simulating Ahmed's Body are Velocity Magnitude, Pressure Magnitude, Wake region, Drag Force, Lift Force.

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

In fluid dynamics, the wake region is the area behind an object that is moving through a fluid, such as air or water. When an object moves through a fluid, it disturbs the fluid flow, creating a region of lower pressure immediately behind it. This phenomenon is known as the "wake" or "wake turbulence."

The reason for the negative pressure in the wake region can be understood through Bernoulli's principle, which states that in a fluid flow, an increase in the fluid's speed (velocity) corresponds to a decrease in its pressure. As the fluid flows around an object, it accelerates to fill the void left by the object's motion, and this increased velocity in the wake region results in lower pressure compared to the surrounding fluid.

To illustrate this, consider the flow of air around a moving object, such as an aircraft or a vehicle. As the object moves forward, the air in front of it is pushed aside and forced to flow around the object's shape. As the air moves around the object, its speed increases due to the need to fill the space left behind by the object. According to Bernoulli's principle, this increase in velocity leads to a decrease in pressure in the wake region.

The negative pressure in the wake region can have various effects depending on the context:

1. Aerodynamics: In the field of aerodynamics, understanding the wake region and its negative pressure is crucial for designing efficient and stable aircraft and other vehicles. The presence of low pressure in the wake can lead to drag, which opposes the motion of the object and affects fuel consumption and performance.

2. Vortex Shedding: In certain situations, the wake region may also be associated with the shedding of vortices. These vortices can create additional forces on the object, leading to unsteady behavior and potential vibration-related issues.

3. Water Flow: In hydrodynamics, similar phenomena occur in water flow around submerged objects, such as ships or underwater structures. Understanding the wake region's behavior is important for optimizing designs and preventing unwanted effects like turbulence and wake interference.

Overall, the negative pressure in the wake region is a natural consequence of fluid dynamics and is an essential consideration in various engineering applications to ensure efficiency, stability, and safety.

Q3. Explain the significance of the point of separation.

The point of separation refers to the location where the fluid flow detaches or separates from the surface of an object moving through the fluid. This phenomenon occurs when the fluid's velocity decreases to the point where it can no longer overcome the pressure gradient, leading to the fluid detaching from the surface and forming a separated flow region.

The significance of the point of separation lies in its influence on various fluid flow phenomena and engineering applications. Some key aspects of its significance are as follows:

1. Aerodynamics: In aerodynamics, the point of separation plays a crucial role in determining the lift and drag forces experienced by an aircraft or any other object moving through a fluid. When the fluid separates from the surface, it creates regions of low pressure (in the wake) and high pressure (on the object's surface). This pressure distribution affects the lift and drag characteristics of the object, which are essential in determining the object's stability, control, and overall aerodynamic performance.

2. Drag Reduction: Understanding and controlling the point of separation is vital for reducing drag in various engineering applications, such as in vehicles, aircraft, and ships. By optimizing the flow pattern and delaying separation, engineers can reduce the drag forces acting on the object, leading to improved fuel efficiency and performance.

3. Lift Enhancement: In some cases, the point of separation can be strategically controlled to enhance lift generation, such as in the design of airfoils used in aircraft wings. By manipulating the flow separation, engineers can enhance lift without sacrificing stability, leading to more efficient and maneuverable aircraft

Simulation Steps

1. Baseline Simulation 

Ahmed Body Model is loaded in Ansys Fluent - Spaceclaim

Unit set to Meters

Using Enclosure Tool to create a wind tunnel

Unchecking Symmetric Dimensions 

For a Wind Tunnel ,

Standard Dimesnions are taken.

Distance from Front Face - 2m 

Distance from Back Face - 5m

Distance from Bottom Face - 0m

Distance from Top Fcae - 1m 

Distance from Side Faces - 0.5 m each 

Wind Tunnel is created 

We do not want mesh inside the Ahmed body, hence we are using Supress for physics on Ahmed Body 

Selecting Plane at Origin , Cross Section View 

2. Meshing

Click on Generate Mesh , make sure whether everything is captured correctly using 

Creating Named Selection 

3. Setup Physics

Pressure Based Solver is selected, as the inlet velocity here is 25 m/sec , the mach no becomes 0.07, which is very less than o.3.

Boundry Condition - Inlet Velocity - 25 m/sec

Turbulence Model k-epsilon , Energy Equation - on

Changing reference values as per geometry of Ahmed Body 

Area and Length are calculated.

Hybrid Initialization is done.

Post Processing 

Creating a Cut Section , Plane Normal to z-axis

Create-Plane- Point and Normal 

Creating Velocity Contours at cut-plane-z

Solution Animaition Velocity Animation 

Running simulation for 500 Iterations 

Residuals

Coefficient of Drag

Computed Value of Cd - 0.37010368

Coefficient of Lift 

coefficient-of-lift = 0.19881537

Velocity Contour 

Post Processing 

Car Wall - Variable - Pressure

Pressure is shown with the help of various colours on the car wall

Inserting a cut-plane, Colour variable velocity 

Pressure Countor

Inserting Vectors using black constant colour, Variable - Velocity

we must create lines after, before and above of ahmed body to measure the velocities.

Creating two lines 

Insert - Chart - Series - Location- Line1&2

Extracting Data 

Improving Mesh Quality 

Case -2 Refined Mesh

Baselined Setup is duplicated and renamed.
We need to add a region of refinement in wake region, which is called as local refinement.

Another enclosure is created 

 Looking at the cross section view 

If we look the cross section, from the pattern of hatches we can identify that there is an overlapping element present.

Tool - Interference

 Share Topology - Share

Mesh - Clear Generated Data - Reset

Generate Mesh 

Insert Method - Multi Zone - Outer Side

Mesh Element and Max. Size = 100 mm

Sizing for Inner enclosure -  50mm 

Insering Sizing for Ahmed Body 

Further Mesh element Size is reduced to 5mm for smaller bodies

Named Selection

Adding a boundry layer mesh 

Creating Inflation Layer 

To calculate Total Thickness , 

a1 = 5 , r = 0.2 , n = 6 

Corrected Formula 

( Here r is greater than 1 hence another formula for sum of GP is used)

\frac{5\cdot\left(1.2^{6}-1\right)}{1.2-1}

Correction 

Sn = 49.64 mm = Total Thickness

Boundry Layers are created

 Creating Named Selction 

Fluent 

Result

Scaled Residuals 

Coefficient of Drag

coefficient-of-drag = 0.35267681

Coefficient of Lift

coefficient-of-lift = 0.28333815

Velocity Contour 

Pressure Contour 

Vector Plot 

Refined Case -2 

Body Sizing - 40 mm 

   

Scaled Residuals 

Coefficient of Drag

coefficient-of-drag = 0.34555084

Coefficient of Lift

coefficient-of-lift 0.27684529

Velocity Contour 

Pressure Contour 

Vector Plot 

Refined Case -3

Body Sizing - 30 mm 

Scaled Residuals 

Coefficient of Drag

coefficient-of-drag =  0.33292887

Coefficient of Lift 

coefficient-of-lift = 0.2669546

Velocity Contour 

Pressure Contour 

Vector Plot 

Refined Case - 4 

Half Geometry

Mesh Size 

Body Sizing - 33 mm 

Face Sizing - 7 mm 

Scaled Residuals

Coefficient of Drag

coefficient-of-drag =  0.15489402

Coefficient of Lift 

coefficient-of-lift = 0.11692308

Velocity Contour 

Pressure Contour 

Vector Plot 

 The Grid Independency Test 

Case No	Mesh Details	No. of Elements	Coefficient of Drag	Coefficient of Lift
1	Baseline Setup		0.37010368	0.19881537
2	Refined Mesh - 50 mm	177,007	0.35267681	0.28333815
3	Refined Mesh - 40 mm	262834	0.34555084	0.27684529
4	Refined Mesh - 30 mm	507875	0.33292887	0.2669546
5	Half Geometry	508824	0.15489402	0.11692308

Conclusion 

1.  Coefficient of Drag tends to decrease as the mesh size of the elements is lowered. Higher the number of elements in the body,lesser   will be the value of Coefficient of Drag.
2.  As we increase the no.of elements in the body, more prominent wake region can be seen.
3.  Coefficient of Lift behaves as same manner as Cd.