High value of vorticity

Divyaprakash · February 13, 2015, 14:25

I am doing a simulation of a spoon stirring water in a cup. I Wanted to observe the vortices formed. When I plot the vorticity contours, they range from 0 to 5000 /s (the high values are present on the edge of the spoon), even though the rotation rate i am giving to the spoon is only 15 rad/s. Shouldn't the vorticity be comparable to the spoon rotation rate? What am i missing here. The simulation seems to be right. I have got all the residuals to converge. Please help me out here.

FMDenaro · February 13, 2015, 15:03

Quote:

Originally Posted by Divyaprakash

I am doing a simulation of a spoon stirring water in a cup. I Wanted to observe the vortices formed. When I plot the vorticity contours, they range from 0 to 5000 /s (the high values are present on the edge of the spoon), even though the rotation rate i am giving to the spoon is only 15 rad/s. Shouldn't the vorticity be comparable to the spoon rotation rate? What am i missing here. The simulation seems to be right. I have got all the residuals to converge. Please help me out here.

vorticity generated at walls is the normal derivative of the streamwise velocity which is generally high.

Divyaprakash · February 13, 2015, 15:15

uh.. Can you please explain a bit more in detail. I guess my fundamentals aren't very strong.

lovecraft22 · February 14, 2015, 05:30

You can find the vorticity definition here:
http://en.wikipedia.org/wiki/Vorticity

For a 2D case, you have:

$\vec{\omega} = \left( \frac{\partial v_{y}}{\partial x} - \frac{\partial v_{x}}{\partial y} \right) \vec{z}$

In the simple case of a flat plate (i.e. velocity aligned with the plate) this becomes:

$\vec{\omega} = \left( - \frac{\partial v_{x}}{\partial y} \right) \vec{z}$

Now, assuming your boundary layer is 2mm thick and your free stream velocity os 5m/s, you get a vorticity value of 2500 1/s

Divyaprakash · February 15, 2015, 14:19

Thanks a lot!!

February 13, 2015, 14:25	High value of vorticity	#1
Divyaprakash Member Divyaprakash Join Date: Jun 2014 Posts: 74 Rep Power: 12	I am doing a simulation of a spoon stirring water in a cup. I Wanted to observe the vortices formed. When I plot the vorticity contours, they range from 0 to 5000 /s (the high values are present on the edge of the spoon), even though the rotation rate i am giving to the spoon is only 15 rad/s. Shouldn't the vorticity be comparable to the spoon rotation rate? What am i missing here. The simulation seems to be right. I have got all the residuals to converge. Please help me out here.

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February 13, 2015, 15:15		#3
Divyaprakash Member Divyaprakash Join Date: Jun 2014 Posts: 74 Rep Power: 12	uh.. Can you please explain a bit more in detail. I guess my fundamentals aren't very strong.

February 14, 2015, 05:30		#4
lovecraft22 Senior Member lore Join Date: Mar 2010 Location: Italy Posts: 460 Rep Power: 18	You can find the vorticity definition here: http://en.wikipedia.org/wiki/Vorticity For a 2D case, you have: $\vec{\omega} = \left( \frac{\partial v_{y}}{\partial x} - \frac{\partial v_{x}}{\partial y} \right) \vec{z}$ In the simple case of a flat plate (i.e. velocity aligned with the plate) this becomes: $\vec{\omega} = \left( - \frac{\partial v_{x}}{\partial y} \right) \vec{z}$ Now, assuming your boundary layer is 2mm thick and your free stream velocity os 5m/s, you get a vorticity value of 2500 1/s

February 15, 2015, 14:19		#5
Divyaprakash Member Divyaprakash Join Date: Jun 2014 Posts: 74 Rep Power: 12	Thanks a lot!!