Interesting pressure and velocity contours

windywires · May 5, 2016, 10:09

I've been analysing the 2D flow over a not-quite-circular cylinder in ANSYS Fluent. The analysis has been transient and run long enough to produce this lift coefficient plot. It oscillates at the vortex shedding frequency as would be expected and its time-average gives a lift coefficient of -0.307.

With this downwards lift in mind, I don't understand these total pressure and velocity magnitude plots. There seems to be higher pressure AND higher velocity on the underside of the shape, which would be impossible according to Bernoulli. Also, wouldn't the higher pressure underneath suggest upwards lift and give a positive lift coefficient? And what can can we interpret from the fact that the shed vortexes take a path at an angle to the flow?

I'd really appreciate your thoughts

davidwilcox · May 5, 2016, 11:37

Let's have a look at your dynamic and static pressure distribution *wink*

windywires · May 5, 2016, 12:06

Thanks very much for the reply, David. I don't actually have access to the data right now (it's saved on a lab computer) but I'm assuming you mean that you'd expect there to be an area of high dynamic pressure and low static pressure on the underside of the shape? That makes sense!

Any ideas why the vortices move upwards? Is this because the shape is being 'lifted' downwards?

FMDenaro · May 5, 2016, 13:02

Quote:

Originally Posted by windywires

I've been analysing the 2D flow over a not-quite-circular cylinder in ANSYS Fluent. The analysis has been transient and run long enough to produce this lift coefficient plot. It oscillates at the vortex shedding frequency as would be expected and its time-average gives a lift coefficient of -0.307.

With this downwards lift in mind, I don't understand these total pressure and velocity magnitude plots. There seems to be higher pressure AND higher velocity on the underside of the shape, which would be impossible according to Bernoulli. Also, wouldn't the higher pressure underneath suggest upwards lift and give a positive lift coefficient? And what can can we interpret from the fact that the shed vortexes take a path at an angle to the flow?

I'd really appreciate your thoughts

could you better define "non-quite-circular"?

windywires · May 5, 2016, 13:10

It's got two notches in it - the cross-section can be found here

windywires · May 5, 2016, 13:20

Filoppo, it's a circle with notches along both sides on the upper half, as shown here. I hope that helps

May 5, 2016, 10:09	Interesting pressure and velocity contours	#1
windywires New Member Join Date: Apr 2016 Posts: 4 Rep Power: 9	I've been analysing the 2D flow over a not-quite-circular cylinder in ANSYS Fluent. The analysis has been transient and run long enough to produce this lift coefficient plot. It oscillates at the vortex shedding frequency as would be expected and its time-average gives a lift coefficient of -0.307. With this downwards lift in mind, I don't understand these total pressure and velocity magnitude plots. There seems to be higher pressure AND higher velocity on the underside of the shape, which would be impossible according to Bernoulli. Also, wouldn't the higher pressure underneath suggest upwards lift and give a positive lift coefficient? And what can can we interpret from the fact that the shed vortexes take a path at an angle to the flow? I'd really appreciate your thoughts

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May 5, 2016, 11:37		#2
davidwilcox Senior Member david Join Date: Oct 2012 Posts: 142 Rep Power: 13	Let's have a look at your dynamic and static pressure distribution wink

May 5, 2016, 12:06		#3
windywires New Member Join Date: Apr 2016 Posts: 4 Rep Power: 9	Thanks very much for the reply, David. I don't actually have access to the data right now (it's saved on a lab computer) but I'm assuming you mean that you'd expect there to be an area of high dynamic pressure and low static pressure on the underside of the shape? That makes sense! Any ideas why the vortices move upwards? Is this because the shape is being 'lifted' downwards?

May 5, 2016, 13:10		#5
windywires New Member Join Date: Apr 2016 Posts: 4 Rep Power: 9	It's got two notches in it - the cross-section can be found here

May 5, 2016, 13:20		#6
windywires New Member Join Date: Apr 2016 Posts: 4 Rep Power: 9	Filoppo, it's a circle with notches along both sides on the upper half, as shown here. I hope that helps