[jmj99]

What is the formula to calculate the Reynolds number for parallel plates in 2D? If the distance between the plates is h, would it turn out to be Re = u x h / nu?

Would it be a case where a rectangular duct has no width and therefore the hydraulic diameter D = 4ab/2(a+b) would become D = 2a or 2h in this case?

Does the length of the plates matter when calculating the Reynolds number or would it only affect the development or entrance length?

[LuckyTran]

Parallel plates classically uses the gap height.

Yes the hydraulic diameter tends towards double the gap height and the two cases therefore have different Reynolds numbers definitions.

Classically you study the fully developed flow between parallel plates and fully developed flow in pipes where the solution does not depend on streamwise location. There, the length of the plate doesn't matter.

Length of the plate matters when it matters. If you have a developing bonundary layer then the length matters. You can calculate additional Reynolds numbers to characterize this. There's more than 1 Reynolds number.

[FMDenaro]

Originally Posted by jmj99

What is the formula to calculate the Reynolds number for parallel plates in 2D? If the distance between the plates is h, would it turn out to be Re = u x h / nu?

Would it be a case where a rectangular duct has no width and therefore the hydraulic diameter D = 4ab/2(a+b) would become D = 2a or 2h in this case?

Does the length of the plates matter when calculating the Reynolds number or would it only affect the development or entrance length?

In such a case, just consider the theory of the boundary layer along one flat plate. It is the local Re number to be considered, at least until the two BLs will merge.