How to determine if vortex shedding will occur?

[kentigens]

With given object dimensions and speed moving inside a fluid. How do i decide if vortex shedding will occur? It is sometimes obvious just to look at the speed and dimension and say it will not occur as its going too slow or vice versa. Is there formula or some mathematical criterion to predict vortex shedding?

Thank you, Kent

 

[Cyrus]

Hi Kent,

I believe the parameter you are interested in is the strouhal number. However, there isn't going to be a number where you can automatically say 'shedding occurs'. Just like you cannot say above a certain Reynolds number flow is turbulent (a common misconception many students have when posting here, because textbooks say turbulent flow is at about a Re = 15k for pipe flow). What you can say, though, is that if your model and your prototype have the same Strohaul number, then they will both encounter shedding, at a particular frequency. You would have to find the prototype shedding frequency by solving for the equality of dynamic similitude. It should just go as the ratios of the L/V, as per the definition.

 

[kentigens]

Thank you cyrus.
I know strouhal number and it only tells information providing that vortex shedding does occur. But sometimes when making an engineering design, hmmm... don't we need to take vortex shedding into consideration? instead of making a prototype and see if there's presence of vortex shedding?

and by the way, any suggested materials i can have a read on??

Thank you, Kent

 

[Cyrus]

Of course you do, but that does not mean you can calculate when and where it will occur. That's why we put things in the wind tunnel, or run CFD analysis. Typically, this is done on a model (experimentally), or at prototype Reynolds number (computational).

If you are concerned about vortex shedding, my recommendation is to look for papers of similar designs and use that as a historical guideline.