A General Methodology Of The Project Work Is Displayed Below:

A general methodology of the project work is displayed below:

Geometrical Modeling The 3D solid model of car body with proportionate dimensions was generated in IDEAS. For accurate modeling, bidirectional tracing of real time photographs on graph paper was done. Only two photographs i.e. of side and back views as shown below were used:

Fig 3.1 Side view of Maruti 800

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Fig 3.2 Back view of Maruti 800

Figs 3.3 and 3.4 below show the outlines of the car body traced on the graph paper with different points through which the curves pass.

Fig. 3.3 Side view outline curves traced on graph paper

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Fig. 3.4 Back view outline curves traced on graph paper

Then from graphs, coordinates of different points through which the curves passed were obtained. Then a scale factor between the photographical dimensions and the actual dimensions was obtained by comparing wheel base obtained from the photograph and actual wheelbase obtained from company catalogue as shown in Fig 3.5.

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Fig 3.5 Company catalogue of Maruti 800 (on World Wide Web)

After that, coordinates of each point were multiplied by scale factor. Then using those coordinates, the 3D model was generated in I-DEAS using the Variable Section Sweep Tool. Fig 3.6 shows the wireframe model in I-DEAS.

Fig 3.6 3-D Solid model in I-DEAS

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The I-DEAS file was then converted into an IGES file and was then imported into ICEM CFD 5.1. Fig 3.7 shows the IGES file in ICEM CFD 5.1.

Fig 3.7 IGES file in ICEM CFD 5.1

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After minor geometry repairing, a wind tunnel was created around the model. Figs 3.8 and 3.9 show the model with two types of wind tunnel which were used.

Fig 3.8 Cuboidal Wind tunnel (used for maximum tests)

Fig 3.9 Semi-cylindrical Wind Tunnel

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Meshing

To generate a mesh within ICEM CFD 5.1 the following steps are required: 1. Import a geometry file. 2. Select the curves requiring fine mesh sizes and define the curve mesh size. 3. Select the surfaces requiring fine mesh sizes and define the surface mesh size. 4. Assign a global mesh size for the overall domain. 5. Generate the mesh by applying automatic tetrahedral meshing. 6. Write output files to the desired solvers. Figs 3.10, 3.11, 3.12, 3.13, 3.14, 3.15 show the meshed geometry in ICEM CFD 5.1.

Fig 3.10 Surface mesh on car body (tetra size 25mm)

Fig 3.11 Surface mesh on car body (tetra size 25mm)

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Fig 3.12 Surface mesh on car body associated with global mesh of wind tunnel

Fig 3.13 Completely meshed geometry (surface tetra 25mm, global tetra 1000 mm)

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Fig 3.14 Completely meshed geometry (surface tetra 25mm, global tetra 500 mm)

Fig 3.15 Completely meshed geometry (surface tetra 25mm, global tetra 500 mm)

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After meshing, the mesh file was send into CFX-Pre and there boundary conditions were applied on the geometry.

Boundary Conditions The equations relating to fluid (air) flow can be closed (numerically) by the specifications of conditions on the external boundary of the fluid domain. Hence boundary conditions determine to a large extent the characteristics of the solution obtained. Before giving the boundary conditions the fluid domain has to be defined. Air ideal gas at a temperature of 288 K with a reference pressure of 1 atm was used to represent the air flow in the wind tunnel. The boundary conditions used for aerodynamic analysis are as follows: Inlet An inlet boundary condition is used where it is known that the flow is directed into the domain. The boundary conditions can be set in a number of ways depending on how you want to specify the conditions, and what particular physical models you are using for the simulation. For preliminary tests which were conducted to find a feasible mesh size combination from the point of view of time and hardware limitations, an inlet air velocity of 50 m/s was used. After standardizing the mesh sizes, tests were carried out at different inlet air velocities. For side wind simulations, left side tunnel wall was specified as second inlet and two side wind tests were conducted at two different side wind speeds keeping the straight wind speed as 50m/s. As it is evident that the flow around a car body is always turbulent, turbulent models (k- and SST) were used for flow modeling. Outlet An outlet boundary condition can be used where it is known that flow is directed out of the domain. The hydrodynamic boundary condition specification (i.e. those for mass and momentum) for a subsonic outlet involves some constraint on the boundary: static pressure, velocity or mass flow. For the wind tunnel test, a static pressure of reference value 0 Pa was used as the outlet boundary condition as prescribed by the aerodynamic analysis tutorial in CFX 5.7.1 and also recommended by several researchers in their research papers. For side wind tests, the right side tunnel wall was given an “opening” type boundary condition where recirculation of the fluid flow is allowed. Car Surface The car surface was given no-slip boundary condition as prescribed in the aerodynamic analysis tutorial in CFX 5.7.1.

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Tunnel surface The tunnel surface (including side walls, roof and floor) was given free-slip boundary condition as prescribed in the aerodynamic analysis tutorial in CFX 5.7.1. Figs 3.16 and 3.17 show the model in CFX-Pre after applying boundary conditions.

Fig 3.16 Boundary conditions imposed on geometry

Fig 3.17 Boundary conditions imposed on geometry

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Numerical Solution After defining the boundary conditions the model was then send into CFX-Solver which computes different variables like pressure, force, air density etc. at different points of the geometry. CFX-Solver carries out iterations depending on the number specified, and the convergence limit specified in CFX-Pre. Nearly 100 iterations were carried out for each test. More number of iterations was not possible because of time limitations and hardware requirements.

Post-Processing Results The result file from the solver was then opened into the post processor CFX-Post where different types of results were obtained e.g. pressure distribution around the car body, air density around the car body, total force on car body in the direction of air flow etc. From the results obtained, the drag and lift coefficients were computed. A total number of about sixteen tests were performed. Different tests were conducted for: 1. Different mesh sizes to check the accuracy of mesh size combinations. 2. Different straight wind speeds and combined straight and side winds for a particular mesh size combination 3. Stationary and rotating tyres 4. Modified car body for drag reduction 5. Modified car body for lift reduction 6. Comparing the results of k- and SST models

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