Finding Angular Momentum Flux Through a Surface using Fluent6

This is a quantity that determines the strength of the swirl. It is usually used in in-cylinder engine applications. The axis of the swirl is along the piston direction of motion. The higher this number the better the mixing. Other quantity is called Tumble, where the axis of rotation is perpendicular to piston direction of motion. This method which about to be discussed can be used to find both.

The angular momentum flux , Lt, is defined as:

Lt = Integral Over Area [r * Vt * den * V . dA]

where r is the radial distance to the center of rotation, den is the air density, V is velocity vector, Vt is the angular velocity at element dA. It is possible to get Lt using Custom Field Function and Surface Integrals utility in Fluent.
1. First we need to settup Rotation-Axis Origin and Rotation-Axis Direction on any cell zone for Fluent to determine Vt.

-select the cell zone (any) under Zone
-click on Set
-specity Rotation-Axis Origin and Rotation-Axis Direction
-click on OK

To learn more, go to: <a target=_blank href=""></a>

2. Assign the cell zone, which you specified its Rotation-Axis Origin and Rotation-Axis Direction, as a reference zone.

-Report->Reference Values
-Select the cell zone under Reference Zone
-click on OK

3. Define a custom field function (CFF) for r * Vt

-Define->Custom Field Function
-select Grid under Field Functions
-select Radial Coordinate
-click on Select
-click on X calculator botton for multiplication
-select Velocity under Field Functions
-Tangential Velocity
-click on Select
-enter "r*Vt" for New Function Name
-click on Define

To learn about CFF, go to: <a target=_blank href=""></a>

4. Perform Integration
-Report->Surface Integrals
-select Custom Field Functions under Field Variables
-select r*Vt
-select the surface under Surfaces
-select Flow Rate under Report Type
-click on Compute

Flow Rate = integrals [phi * V . dA). Where phi is r * Vt, which we defined in CFF.

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