vortex cause pressure gradient or pressure gradient induce vortex?

fruitkiwi · June 7, 2012, 23:01

Hi,Dear all,

The more i go through into CFD, i sometimes get lost.
Is the vortex cause pressure different in a chamber or is because the pressure gradient then it cause vortex formation?

I read some sentence like below:
"The vortex induces an adverse pressure gradient at the wall"
"The evolution of a vortex in an adverse pressure gradient"
"This adverse pressure gradient will cause the vortex core to expand"
"subjected to the favorable pressure gradient the vortex showed an increase"

we all know in the center of vortex there is always low pressure region, erm, back to topic,
vortex cause pressure gradient or pressure gradient induce vortex?

thanks.

Martin Hegedus · June 11, 2012, 15:29

A vortex is a byproduct. The various elements which make up the forcing functions for the momentum equation (pressure, viscous forces, etc.) are the driver for the creation, and destruction, of vorticity. Another way to look at it is to take the curl of the momentum equation.

fruitkiwi · June 11, 2012, 20:48

Quote:

Originally Posted by Martin Hegedus

A vortex is a byproduct. The various elements which make up the forcing functions for the momentum equation (pressure, viscous forces, etc.) are the driver for the creation, and destruction, of vorticity. Another way to look at it is to take the curl of the momentum equation.

Hi, Martin Hegedus,
Thanks for your reply.
i agree it is true that pressure drive the flow.Then let us consider the vortex ring which interact with solid boundary. What is reason where" the vortex induce an adverse pressure gradient on the wall"?
I see this kind of sentence very often appears in the article. It is written in such a way that the vortex is the one which drive the adverse pressure gradient.

Or Do i miss anything?

Martin Hegedus · June 11, 2012, 22:13

You'll need to explain the geometry or show a picture. I gather you have a specific question and I don't understand it.

Of course the interaction of a preexisting vortex and a solid wall modifies the pressure field due to the need of the flow to change direction at the wall surface. However, it is something else which created that original vortex. In other words, the vortex is an energy/momentum source and it can be used to do things or it can change form but something (a wall, viscosity, etc) must be introduced to change the form of that energy and/or momentum. A vortex can not change on it's own. It can not, for example, just split into two or burst. Conservation of momentum and energy do not allow for it.

IMO, you should not view it as the vortex creating the adverse pressure gradient, you should view it as the interaction of the wall and vortex which creates the pressure gradient.

fruitkiwi · June 12, 2012, 02:12

Dear Martin Hegedus,

Yup, you are right. When i look back at that particular article, it stated that in the upper site, there is interaction of vortex ring and the rigid wall. The vortex is induced when it enters the chamber. Then it flows in and interact with nozzle tip, then it cause the adverse pressure gradient.

Thanks for thorough explanation.
i should study more details.

June 7, 2012, 23:01	vortex cause pressure gradient or pressure gradient induce vortex?	#1
fruitkiwi Member kiwi Join Date: Mar 2012 Location: South East Asia Posts: 58 Rep Power: 14	Hi,Dear all, The more i go through into CFD, i sometimes get lost. Is the vortex cause pressure different in a chamber or is because the pressure gradient then it cause vortex formation? I read some sentence like below: "The vortex induces an adverse pressure gradient at the wall" "The evolution of a vortex in an adverse pressure gradient" "This adverse pressure gradient will cause the vortex core to expand" "subjected to the favorable pressure gradient the vortex showed an increase" we all know in the center of vortex there is always low pressure region, erm, back to topic, vortex cause pressure gradient or pressure gradient induce vortex? thanks.

June 11, 2012, 22:13		#4
Martin Hegedus Senior Member Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19	You'll need to explain the geometry or show a picture. I gather you have a specific question and I don't understand it. Of course the interaction of a preexisting vortex and a solid wall modifies the pressure field due to the need of the flow to change direction at the wall surface. However, it is something else which created that original vortex. In other words, the vortex is an energy/momentum source and it can be used to do things or it can change form but something (a wall, viscosity, etc) must be introduced to change the form of that energy and/or momentum. A vortex can not change on it's own. It can not, for example, just split into two or burst. Conservation of momentum and energy do not allow for it. IMO, you should not view it as the vortex creating the adverse pressure gradient, you should view it as the interaction of the wall and vortex which creates the pressure gradient. fruitkiwi likes this.

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June 11, 2012, 15:29		#2
Martin Hegedus Senior Member Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19	A vortex is a byproduct. The various elements which make up the forcing functions for the momentum equation (pressure, viscous forces, etc.) are the driver for the creation, and destruction, of vorticity. Another way to look at it is to take the curl of the momentum equation.

June 12, 2012, 02:12		#5
fruitkiwi Member kiwi Join Date: Mar 2012 Location: South East Asia Posts: 58 Rep Power: 14	Dear Martin Hegedus, Yup, you are right. When i look back at that particular article, it stated that in the upper site, there is interaction of vortex ring and the rigid wall. The vortex is induced when it enters the chamber. Then it flows in and interact with nozzle tip, then it cause the adverse pressure gradient. Thanks for thorough explanation. i should study more details.