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I have trouble understand how Vorticity occurs in a Rotational and not in a Irrotational vortex when I consider it in terms of shear forces.

A Rotational vortex has a velocity profile where

$v∝r$

whereas a Irrotational vortex has a velocity profile

$v = \frac{1}{r}$

enter image description here

So between two different radii $r+$ $r-$ with a clockwise velocity and flow -

Rotational - the velocity is proportional to radius so between a larger $r+$ and smaller radius$r-$ there will be no velocity profile so no shear forces

.enter image description here

Irrotational - the velocity is inversely related to radius so between a larger $r+$ and smaller radius $r-$ there will be a velocity profile and this will cause shear forces ,resulting in rotation towards the larger radius $r+$

enter image description here So if we only considering shear forces to this point, then only the Irrotational has any rotation- vorticity.

Other forces ?

In the Rotational example

We have a centripetal force caused by changing the bulk velocity vector around a solid curve This centripetal reaction force has no counter-clockwise action of the shear to oppose it. So as its acted on by the centripetal force the parcel is free to rotate with the flow. This means that the rotational parcel changes vector as it goes around the bend.

In the Irrotational example

We have a Bernoulli pressure gradient which causes acceleration of the fluid radially this changes the vector clockwise but this is counteracted by the counterclockwise shear forces - so the combination of these two actions (shear and pressure gradient) somehow results in no net rotation ( I guess because both shear and pressure are both proportional to the difference in velocity

Allowing the Irrotational parcel to maintain the same vector as it goes around the bend and the rotational to change heading.

Rotional .................................... Irrotational

enter image description here

My question Isn't it more appropriate (considering all forces shear, pressure gradients and resultant forces-centripetal) to say that an Irrotational flow has no net vorticity and a Rotational flow has a net vorticity ?

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  • $\begingroup$ Rotation and shearing are two independent kinds of motion that a fluid element can have. Why are you trying to understand one in terms of another? $\endgroup$
    – Deep
    Oct 16, 2016 at 7:40
  • $\begingroup$ hi @Zero thanks for the interest, because I don't understand how they are independent ? The variables in both are a velocity gradient and viscosity without both present you cant have shear or vorticity $\endgroup$
    – Quentin Chester
    Oct 16, 2016 at 7:59
  • $\begingroup$ @zero sorry I should also add pressure gradient . Hence the term forced vortex for rotational . As it requires this externally to sustain the motion. Where as a Irrotational creates its own internal Bernoullis $\endgroup$
    – Quentin Chester
    Oct 16, 2016 at 8:07
  • $\begingroup$ I assume you know tensors. Shear and rotation are respectively symmetric and anti-symmetric parts of velocity gradient tensor. One can be zero while the other non-zero. It is just kinematics. Pressure gradient, viscous forces etc. have nothing to do with it. $\endgroup$
    – Deep
    Oct 17, 2016 at 4:31
  • $\begingroup$ @zero again thanks for your interest . Im really sorry but I don't enough to understand. Is there a simpler way to explain it. Are you saying that the two types of motion don't work to counteract each other somehow if they are both non zero? $\endgroup$
    – Quentin Chester
    Oct 17, 2016 at 7:04

1 Answer 1

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Your calculation of vorticity is wrong. Vorticity is not zero if fluid element rotates, even it does not deform, which is the case in the rotational case. Just use your finger pointing from one side to another side of a fluid element and follow this element for a while will convince you it is rotating. On the other hand, in the irrotational case if you follow two perpendicular sides of an element, you may find they rotate in opposite directions and the sum of the angular velocity is zero.

You are recommended to watch video #6 of the fluid mechanics films. Just google national committee fluid mechanics films.

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