Carnegie Mellon

Mechanical Engineering

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FEM/ANSYS

## Test F3: Flow Inside a Computer Case

Fluid Test #3: 3D Flow Inside a Computer USING FLOTRAN

Introduction: In this example you will model the flow of air inside a computer case due to its cooling fans.

Physical Problem: Compute and plot the velocity distribution in the case shown in the figure.

Problem Description:

Objective:
 To plot the velocity profile within the case.

You are required to hand in print outs for the above.

Figure:

Important Dimensions: (all dimensions are in meters)

Height   = 0.414 m

Width    = 0.1905 m

Depth    = 0.4064 m

Inlet Fan Position:

The X and Y position of the corner of the block (to create the fan) is (0.07,0.04).

Width = 0.1 m

Height = 0.1 m

The air entering the computer is traveling 0.1m/s.

Outlet Position: (From a plane parallel to the inlet fan at the corner farthest from the origin)

The X and Y position of the corner of the block (to create the outlet) is (-0.17,-0.14).

Width = 0.1 m

Height = 0.1 m

·         To best model this system, model the volume defining the case first.

·         Then, delete only the volume (leaving the areas, lines, and keypoints) and create the areas defining the inlet and the outlet of the air flow.

·         Overlap the areas to their respective faces of the computer case. (front and back)

·         Once the areas have been overlapped, they should then be married back into an “arbitrary” volume. (defined “by areas”)

·         At this point, define the Element Properties as a 3D Air Element

·         Define the Material Properties of the Air Element (Density and Viscosity are the important qualities)

 Mesh the volume with a mesh size of 0.02 on all lines. Apply Boundary Conditions (No Slip along the areas of the case that do not function as an inlet or outlet, velocity into the inlet area, and Atmospheric Pressure (P=0 in ANSYS) on the outlet area) Iterate 25 times and solve. (Ideally the iteration count would be at least several thousand times to make sure that the solution converges… but computational time dictates that in order to be able to solve the problem in a reasonable amount of time, the iteration number should be trimmed down to 25) Plot the Velocity distribution in the X and Y directions.  At this point only the outermost region of the case will be evident, so make the workplane the “cutting plane” and show the velocity distribution along the Z axis in the middle of the case) Plot this with both a Contour Plot and a Vector Plot. This is the answer you should obtain with 25 iterations:

(Contour Plot)

(Vector Plot)