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danbence April 6, 2014 19:07

Interpreting streamlines of a rotating fan.
I have modelled a rotating fan inside a cylinder. The fan is simulated as stationary in a rotating reference frame. When I view streamlines in the rotating reference frame, I get the familiar flow lines going above and below the airfoil sections as we have seen in text books.

My problem is trying to interpret the streamlines when viewed from the absolute reference frame. I wanted to see how much rotation the fan had imparted to the flow downstream. This is not possible when viewing the rotating reference frame streamlines.

In the absolute reference frame, the streamlines look odd. When I start them from the inlet, they travel down and terminate on the surface of the fan. When I do a reverse streamline from the outlet, they terminate on the underside of the fan.

The absolute vector field looks correct and the streamlines reflect those vectors correctly, but the streamlines are implying that a parcel of air is vanishing at the top surface of the fan blade and appearing on the underside, which is clearly nonsense.

I am questioning whether it is actually valid to visualize streamlines in the absolute reference frame ? The absolute vectors show the velocity of parcels of air at a frozen instant of time with the fan also frozen at that time. Streamlines are a method of viewing the passage of time, seeing how a spec of dust will travel through the time it takes to go from the inlet to the outlet. By viewing the streamlines in an absolute reference frame, I am combining a fan that is frozen and the movement of air over a time period. Is this the reason for the odd looking behaviour?

If it is not sensible to view the streamlines in an absolute reference frame, what should I do ? Create an animation where the fan is rotating ?

Any comments would be appreciated.

danbence April 8, 2014 10:13

2 Attachment(s)
I have attached two images of my simulation to illustrate my description . It is one fan blade with a rotational periodic boundary condition. As you can see, the rotating reference frame streamline is easy to interpret, but the streamlines in the absolute reference frame terminates at the upper surface of the fan blade.

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