Flow Simulation analysis of an automobile to improve the aerodynamics .| FINAL INDEPENDENT PROJECT

AIM:

Flow Simulation analysis of an automobile to improve the aerodynamics.

ABSTRACT-

An Engineer is always focused on the challenges of bringing ideas and concepts to life. Therefore, Software and Computer Techniques must be constantly developed and implemented for Virtual and Economic Analysis. At the same time, we must take care that there is no compromise with the Accuracy of the Techniques used. In the Age of Speed, Automobiles have become an integral part of the human routine. By designing Aerodynamically Sound Automobiles, the overall efficiency of an Automobile can be increased, by a reduction in the Drag Force (Losses) and too by the reduction in the Fuel Consumption, eventually developing an Environment-Friendly Vehicle. The Engineer is constantly conformed to the challenges of bringing ideas and design into reality. The Computational Fluid Dynamics is a step towards bringing numerous Hypothetical Concepts to reality, as the Analysis done gives better results compared to the Physical Analysis (Wind Tunnel Testing), without any costly setup and space requirement. In this Project, Modelling of Automobile (Car) was done using the Software-SOLIDWORKS. The Lift and the Drag of the car were determined by the Analysis of Fluid (Air) Flow around it using Software-SOLIDWORKS- Flow Simulation.

INTRODUCTION:

Speed is the need of today’s world. The ironic saying that ‘Distance must be measured in minutes, rather than in mile’ is turning out to be a reality in the current world of pace. The ambitious construction of the Autobahns in Germany which began in the 1920s and which was carried forward by the Nazi Supremo - Adolph Hitler, encouraged the people in that region to shift from using the conventional carts to mechanically powered vehicles. Gradually the trend spread around the world and reached our country, which resulted in the construction of Flyovers, Expressways, Sea Links, etc. which connected the vastly spread India. But along with the availability of good infrastructure, Automobile Manufacturers started feeling the need to develop faster vehicles, which they have managed to do well in the past two decades. But challenges never end. Lower efficiencies of the vehicles have troubled the Automobile world for years, as to produce good results, huge power inputs are required. Hence, an effective way known, to better the outputs without regulating the inputs is – ‘Aerodynamics’.

OBJECTIVE:

The goal is to simulate the airflow around the vehicle and obtain an accurate value of its drag and lift coefficient. The next step would be to make modifications to vehicle geometry which could improve its lift and drag characteristics making the vehicle handle better at cruise speeds and also improve its fuel efficiency. The major objectives are – Getting optimal solution with a maximum number of iterations, reducing the lift, working out with the coefficient of Presentation drag in a way that it facilitates stability and better handling of the car, developing pressure and velocity contours and gaining proficiency in the modeling and analysis in SOLIDWORKS.

THEORY:

CFD or Computational fluid dynamics is a branch of fluid mechanics that, with the help of computers, uses numerical methods to solve and analyses problems involving fluid flows. Computers are used to carry out calculations using an iterative procedure wherein the solution accuracy improves with every iteration. The underlying equations that are solved in CFD problems are the Naiver-Stokes equations. In the laminar regime, the flow of the fluid can be completely predicted by solving the steady-state Naiver-Stokes equations, which predict the velocity and the pressure fields. As the flow begins its transition to turbulence, it is no longer possible to assume that the flow is invariant with time. In this case, it is necessary to solve the problem in the time domain. As the Reynolds number increases, the flow field exhibits small eddies, and the timescales of the oscillations become so short that it is computationally infeasible to solve the Naiver-Stokes equations, so in such flow regime, Reynold’s Average Naiver Stokes Formulation is used to cope up with the issue.

Reynolds Averaged Naiver-Stokes equations (RANS): Equation the Reynolds Averaged Naiver-Stokes equations (also known as RANS equations) are equations used to predict the fluid flow using a time-averaged formulation. The primary concept applied is Reynolds decomposition which involves decomposing an instantaneous quantity into its time-averaged and fluctuating quantities. The time-averaged nature of its equations makes it an attractive choice while simulating turbulent flows. Considering certain approximations based on the knowledge of properties of turbulent flows, these equations can be used to give time-averaged solutions to the Naiver– Stokes equations.

Non-equilibrium wall function (NWF): For high Reynolds number flows, such as in external flow around vehicles, resolving the near-wall region down to the wall is not practical. To overcome this, wall functions are used. NWF takes into account the effects of local variation in the thickness of the viscous sublayer when computing the turbulent kinetic energy budget in wall adjacent cells. Besides this, NWF is also sensitized to adverse pressure gradients which are common inflow around vehicles. Compared to traditional wall functions, NWF provides more realistic predictions of the behavior of the turbulent boundary layers, including flow separation, and they do so without a significant increase in either CPU time or dynamic memory.

Aerodynamic Devices – Rear Spoiler: A spoiler is an automotive aerodynamic device whose intended design function is to 'spoil' unfavorable air movement across a body of a vehicle in motion, usually described as turbulence or drag. Spoilers are often fitted to race and high-performance sports cars, although they have become common on passenger vehicles as well. Some spoilers are added to cars primarily for styling purposes and have some Aerodynamic benefits. The term "spoiler" is often mistakenly used interchangeably with "Rear Wing". An automotive wing is a device whose intended design is to generate downforce as air passes around it, not simply disrupt existing airflow patterns. As such, rather than decreasing drag, automotive wings actually increase drag.

PROCEDURE:

• Vehicle Geometry Model For modeling the geometry, 3D modeling software SOLIDWORKS was used. The modeling process involved importing the vehicle blueprints into Solid works with the help of which, 3D curves were projected. These curves then acted as boundaries to generate surfaces.
• Creating Fluid Enclosure In order to simulate the airflow around the vehicle, a fluid volume needs to be created which will encompass the vehicle. This was done by creating an enclosure around the vehicle and subtracting the vehicle's body. This enclosure acts as the air domain. To reduce the overall computational cost and time, the vehicle was considered symmetric laterally.
• Mesh Generation While generating the mesh, sizing functions were used wherever necessary in order to obtain accurate lift/drag parameters. Two bodies of refinements were added to properly capture the flow in the region closest to the vehicle and also capture the flow in the wake. Since boundary layer separation has a significant effect on drag, five layers of inflation were added to the vehicle surface to properly resolve the boundary layer and local mesh is generated over the surface of the car and is refined up to the level of 4.
• Following conditions were considered for the Analysis:

Analysis type	External
Gases	Air
Type of flow	Laminar and Turbulent
Velocity in X dir.	1. 27.77 m/s (100 kmph). 2. 41.67 m/s (150 kmph). 3. 55.55 m/s (200 kmph).

3D CAD MODEL USED FOR SIMULATION:

SOLVER RESULTS:

CUTPLOTS:

Velocity at 100 Km/h:

Velocity at 150 Km/h:

Velocity at 200 Km/h:

Drag and Lift Force:

At 100 Km/h:

PLOTS:

Drag Force:

Lift Force:

At 150 Km/h:

PLOTS:

Drag Force:

Lift Force:

At 200 Km/h:

PLOTS:

Drag Force:

Lift Force:

CONCLUSION:

CFD analysis was successfully carried out on the production vehicle. The results of the simulation (drag coefficient) were found to be in close agreement with the officially provided values. Once the validity of the simulation was achieved, the next step was to make modifications in the geometry of the original model which could positively affect performance characteristics (lift and drag). A diffuser was added to the rear end of the vehicle and further simulations were performed.