Hi All,

I want to construct a CEL equation for velocity in CFX11.0.

Currently, I have the following working expressions: C1+2.6*sin(2*pi*t/Tp) [m*s^-1] C1 is 4.3 [m*s^-1] Tp is .345 [s]

I want to convert the speed into millilitres per second, anyone know the syntax?

Thanks

Hi,

millilitres per second isnt the same thing like speed. For getting speed you have to divide it through the area the fluid passes... The sin-function you use does not depend on an length. (maybe this is hidden in the factor 2.6 which has to be updated after conversion)

To convert this into ml/s you need to multiply the velocity times an area. Forget the expression for a moment. What are you trying to do?

-CycLone

Hi Cyclone,

I am using this expression as a volumetric inflow for a blood flow simulation. Previously, I had mistaken the units to be m/s. It is actually 4.3 +/- 2.6 ml, I don't know how to convert that into a velocity.

Cheers! CC.

http://en.wikipedia.org/wiki/Volume_flow_rate

Hi CC,

Volume flow rate is given as the integral of VndA (Normal velocity times area), thus if you have a volume flow rate, just divide by the area to get the velocity. Alternately you can multiply by the density to get the mass flow rate.

You can do this directly in your boundary condition. Say you boundary condition is called "inflow", then for velocity normal to the face, enter the following expression instead of a value:

4.3 [ml/s]/area()@inflow

The "area()@inflow" function will return the area of the inlet and give you the correct velocity. You can do the same thing to get an appropriate mass flow rate, in that case, put the mass flow rate in as:

4.3 [ml/s]*areaAve(Density)@inflow

If you like, you can enter an expression in the expression editor for your flow rate, which may make it easier to change from one simulation to the next (instead of revisiting the boundary condition each time). To do this create an expression named Flow Rate and then use "Flow Rate" in your boundary condition expression:

Flow Rate = 4.3 [ml/s]

boundary condition: Flow Rate/area()@inflow

You can also declare "Flow Rate" as a DesignXplorer variable, allowing you to use DX to run a sequence of runs with different flow rates automatically.

-CycLone