Lift Coefficient behaving strangely

Nikolaj1990 · December 9, 2014, 11:06

Hi guys.

I'm having a trouble with my simulations. I'm simulating a flat plate rotating counterclockwise from an angle of 0 deg to 90 deg with constant angular velocity.
I'm interested in watching what happens to the stall point, but something strange seems to happen. The lift coefficient have a weird offset and the lift coefficient seems to "converge" in a strange way too with an extreme initial value. This is shown in the two attachments.
Has anyone has the same experience or does anyone have an idea of what is causing this behaviour?

Thanks in advance.

agd · December 9, 2014, 12:42

My first thought would be that if you are starting the motion instantaneously from an otherwise static solution that you are generating a vortex due to the impulsive start. Your "problem" seems to get more pronounced as you increase the angular velocity, which would seem to support that conjecture. Have you looked at any visualizations to see what is happening at the onset of the rotational motion? Those might help to clarify what is happening.

Nikolaj1990 · December 9, 2014, 16:01

From the visualisations it seems that a vortex is created almost immidiately at the trailing edge of the plate, which swirls under the plate.. The center of rotation is close to the leading edge of the plate. But it seems peculiar to me that an offset is created. In my mind it should start at 0 lift at 0 deg and then the vortex swirling under the plate would contribute with positive lift. When that sheds from the plate the leading edge vortex should stabilize at the bottom of the plate and contribute to negative lift. When this leading edge vortex then sheds the lift coefficient is at its peak value.

In my mind it just seems like the entire graph somehow has a constant offset.

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December 9, 2014, 12:42		#2
agd Senior Member Join Date: Jul 2009 Posts: 351 Rep Power: 18	My first thought would be that if you are starting the motion instantaneously from an otherwise static solution that you are generating a vortex due to the impulsive start. Your "problem" seems to get more pronounced as you increase the angular velocity, which would seem to support that conjecture. Have you looked at any visualizations to see what is happening at the onset of the rotational motion? Those might help to clarify what is happening.

December 9, 2014, 16:01		#3
Nikolaj1990 New Member Nikolaj Andersen Join Date: Nov 2014 Posts: 2 Rep Power: 0	From the visualisations it seems that a vortex is created almost immidiately at the trailing edge of the plate, which swirls under the plate.. The center of rotation is close to the leading edge of the plate. But it seems peculiar to me that an offset is created. In my mind it should start at 0 lift at 0 deg and then the vortex swirling under the plate would contribute with positive lift. When that sheds from the plate the leading edge vortex should stabilize at the bottom of the plate and contribute to negative lift. When this leading edge vortex then sheds the lift coefficient is at its peak value. In my mind it just seems like the entire graph somehow has a constant offset.