There are several devices that do as you suggest. Google "stormwater vortex seperator" for both articles and products.

It would be a lot better for you to use a lamella plate clarifier made from oleophilic materials. Agitating the water fast enough will disperse the oil globules rather than allow them to coalesce together.

I would use an underflow weir rather than any type of a centrifugal seperator. Oil would float to the top and be held back by the weir and could easily be skimmed off. If the residence time is long enough the particulates would settle out. Multiple weirs could used in series. Returning this water to the ground would not be allowed in my area.

I agree with the previous posts, a vortex separator is not the what you want for this application. A vortex would be better suited for removing solids. For this application you are better of with a gravity separator as shown here:

http://www.suparator.com/volume4_e/gravity-separator.html

Here is a fact sheet for a classic oil-water separator we use them at all of the garages and equipment wash facilities where the vehicles are dirty from soils, oils, and grease.

I'm not sure where you live or what you plan on using it for but if you plan on using it in an enclosed area near regulated tanks that contain oily products in it the oil-water separator will have to be 110% the carrying capacity of your largest tank. Plus an SPCC and NPDES plans need to be designed and implement before installation. You should also check with your state environmental agency to see if additional permits are required

http://dnr.metrokc.gov/wlr/indwaste/OW_Fact.pdf

Hi Doc

I agree vortex / cyclone / gravity separators are for solids etc.

What you seem to have in mind is something like a cream separator powered by the turbine. I am not sure but i think the maximum efficiency is only 98% and may not be sufficient for recharging an aquifer.