Week 11- Broadband Noise modelling over Ahmed body

Aim: To perform Broadband Noise modeling over Ahmed's body and also plot for the drag and lift coefficient.

Introduction:

Aero-Acoustics deals with the sound or noise generated from the aerodynamic forces and motion of turbulent flow in the domain. Some of the common areas of noise are car mirrors, wheels, airplane landing gear. When an object vibrates, it causes slight changes in air pressure. These air pressure changes travel as waves through the air and produce sound. Noise is unwanted or irritable sound. Sound waves ate longitudinal waves that vibrate along with the direction of the propagation of the wave. These are pressure waves.

Need For the study of the Broadband Noise modeling and dynamics of fluid over a Body

Aerodynamic forces and motion of turbulent flow generate sound/noise which left without treated can cause damage to the exterior parts of the car and can be significantly disturbing for the passengers in the car due to the noise generated. So Acoustic modeling needs to be performed to locate the noise generating locations and make certain design changes.

Anybody moving in any kind of fluid experiences different resistances depending on the body's speed, shape, surface smoothness, and velocity. Automobiles, Aircraft, etc move through an ocean of fluid called air. Thus experiences resistance to move. One of the major resistance offered by a fluid is Drag. The amount of energy an automobile utilizes to overcome these resistances are significant(about 10-15%). Because of this, fuel consumption will be increase. And the lift is also generated over a body moving in through the air, it is an unnecessary force in a moving car and can cause severe accidents if not controlled. The significant lift force can make the F1 car fly away and downforce need to be generated to avoid it.

Thus, engineers need to study the dynamics of fluid around the body and try To reduce lift, drag forces to the maximum extent.

Ahmed's body and its importance.

Ahmed's body is very much useful in predicting the most aerodynamic features that act on the automobile. The Ahmed body is a model very useful in the automotive industry for validating simulation tools. The Ahmed body shape is a simple model while maintaining car-like geometry features. This is a bluff body though we can't be able to predict the flow separation effects in a useful way. Many factors affect the Drag forces Viscosity, velocity, and shape of the body. So to study and perform experiments on the behavior of air on Automobiles Ahmed's body is developed. The real-world simulations are performed on the Ahmed body it is a similar replica of the cars. As it is difficult to perform simulations on Realworld cars In the late 80's Ahmed body is developed. Advancements in CFD led to Acoustic modeling. It is a generic car model to validate wind tunnel tests.

Geometry:

Ahmed body Dimensions:

H = 0.297m

length of Ahmed body= 3.7H

Height of Ahmed body = H

Width of the Ahmed body = 1.18H

Inclination of the rear = 25 deg

Domain

the rear of the Ahmed body = 5L

Front of the Ahmed body = 3L

height of the Domain = 3L

Width of half the domain = 1.5L

The inner domain is also created(To refine the mesh) with more length at the downstream

Meshing:

• Global element size: 100mm
• Sizing- Inner enclosure 27mm
• Inflation layer 1: Road and Ahmed body, Growth rate 1.2, smooth transition, 5layers
• Inflation layer 2: First layer thickness- 0.01mm, number of layers 28, Growth rate 1.2

Solver Setup:

Solver settings for Broadband Noise modeling over Ahmed body
Type:	Pressure-based
Velocity formulation:	Absolute
Time setting:	Transient: Steady
Space:	3D
Energy:	ON
Model:	k-epsilon-Realizable
Operating Condition:	1 atm
Acoustics(Broadband Noise sources)	ON
CellZone Domain	Fluid
Boundary conditions:	Inlet:Velocity Inlet : 31m/s Road:(Wall) No-slip condition Moving wall -31m/s Outlet: pressure outlet
Solution methods:	COUPLED
Pressure interpolation scheme:	Second-Order Upwind
Momentum:	Second-Order Upwind
Turbulence:	Second-Order Upwind
Flow courant number	10
Initialization:	Standard, From the inlet
The material used, Fluid	Air – Ideal gas
Monitor :	Lift Coefficient Drag Coefficient Surface Acoustic Power Level (dB) Acoustic Power Level (dB) Lilley's Total Noise Source

Boundary Inlet

Boundary road

Initialization:

Procedure:

• Domains are created over the Ahmed body. Two enclosures are created over the Ahmed body to vary the mesh size near and away from the Ahmed body.
• Simulation is performed on half of the geometry as the Ahmed body is symmetric.
• Meshing is done with sizing and inflation layers as per the data mentioned above.
• The fluent is opened and solver settings are Given properly.
• Required contours and plots are saved.

Plots and contours:

Residuals:

It can be seen from the residuals that the solution converged

Lift Coefficient:

Drag Coefficient:

The lift and drag coefficients are not changing hence the solution is converged.

Velocity Contour:

The wake region is created behind the body. The velocity on the front of the body is zero. and the velocity on the surface of the body is also zero as per the no-slip condition.

Pressure contour:

high pressure is observed at the front of the body and in the wake region.

Acoustic level Decibels:

• It can be seen that the major contributors to the noise are the boundary layer formation and formation of the wake region as there is more turbulence.

Surface acoustics:

On the surface, the noise generation is due to the edges so any protrusions from the cat body should be avoided.

Lillys Total noise:

Noise is more where there is a profile change.

Conclusion:

• As most of the noise is generated due to boundary layer formation and the wake region. Desing of the surface and body should be made to reduce the thickness of the boundary layer and the wake region. As it is impossible to completely eliminate the boundary layer.
• Sharp changes in the profile structures should be avoided.