What is vortex breakdown

[sashankhrao]

Can someone please explain to me the concept of vortex breakdown?

 

[Andy Resnick]

Other than viscosity? Is there a particular instability you are thinking about?

 

[sashankhrao]

Hi Andy,

Thanks for your reply.Actually I don't understand the concept of vortex breakdown and its relation to Swirl Number (SN) which is defined as the ratio of swirl to meridianol motion strengths. I have read that vortex breakdown diappearance occurs as the SN decreases.

 

[Andy Resnick]

Yikes... I've never heard of a swirl number. My only resource is "Vortex Dynamics" by Saffman.

I found a link here (http://www.enme.umd.edu/combustion/underwater_propulsion.htm) with some information that helped.

If I understand your context, you are referring to a "vortex with helicity", which I think means that the vortex lines form axial screw patterns. The swirl number seems to be related to the pitch angle of the vortex lines, but I can't tell for sure. A vortex with helicity has moving vortex lines, which are then subject to instabilities.

"Vortex breakdown" is a rapid change in the core radius of the vortex filament or tube- it was an experimentally-discovered phenomenon, and is characterized by a rapid increase in the core radius, which appears as a bubble or spiraling thread. Saffman claims there is no widely accepted theory for this phenomenon yet.

In practical terms, it means that the relationship between the two is established empirically- by varying the swirl number (and other relevant parameters, for example capillary number, Reynolds number, Rossby number...)- this is glaringly missing from the above webpage. There's probably a lot of opportunity for original contributions to the topic.

 

[sashankhrao]

Hi Andy,

Thank you for your interest in my doubt. If you could assist me with your email address i could send you the document that i am trying to understand. Its on Vortex Breakdown. A pdf file.

 

[Andy Resnick]

Sashankhrao,

I just sent you a personal message with my address. Hope I can help!