A vortex that's on the ground (tornado) can rip tiles off a roof of a house and basically suck in the roof tiles. On an aircraft that develops vortex lift the vortex sucks in air molecules and reduces the pressure on the wing. When there is a dust devil, it sucks in a lot of dust and throws it. So why does a vortex have so much suction power?
[Physics] Why does a vortex have so much suction power
vortex
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So the vortex street appears as the stream goes faster and faster: the stream obeys Newton's laws, "objects in motion tend to stay in motion unless acted on by a net force."
The force that keeps the stream laminar around the cylinder is fluid pressure formed by the viscous forces of the fluid, which means there is a pressure gradient and a region of low pressure is being formed behind the cylinder. (This also doubles as one explanation of why, when you let go of the cylinder, it starts to flow downstream. Inverting that explanation, the fact that the rod has to be held in place means that these pressure gradients must exist.)
But as you increase the speed of the fluid and hence its momentum, if you do not alter other parameters to keep the Reynolds number constant, then the fluid flow lines must separate from the cylinder. You're increasing the inertial forces in the fluid but the viscous forces do not rise to compensate them and keep the flow laminar. This causes the fluid boundary to detach from the cylinder.
The fluid that's stuck inside the wake then receives shear forces from these two boundaries flowing around it on either side. For a certain parameter regime the cylinder just keeps two vortices in its wake, fed by the shear forces from the boundary layer.
The vortex street happens when these shear forces get to be so large that they actually push the vortices harder than the pressure gradients are pushing them back. If you could theoretically get the symmtery exactly right, both vortices would be shed together by symmetry: but it's like trying to balance a pencil on its point, the opposite rotation of the vortices forms effectively a force which repels them. Some asymmetry is inevitable, and they start to shed alternately. This creates the vortex street.
Does that help?
Do this at the equator, so you can forget about the angular momentum of things on the earth. Is the container circular, with no obstructions on the bottom? Is the drain in the center? Is the water in the container absolutely still? Like if you sprinkled some dust on it and came back 24 hours later it would not have moved? If so, when you start the drain, the water will flow straight into the drain, with no vortex.
Anything that gives it the slightest angular momentum, in other words, any motion that is not toward or away from the drain, will be magnified as the water approaches the drain. It's the same as a spinning figure skater pulling in his or her arms and legs. Any weight that's pulled toward the center obeys Kepler's equal-area rule for orbits, so reduced radius results in increased angular velocity.
Best Answer
If we consider the incompressible, 1-dimensional Euler equations (basically $F=ma$ but for a fluid without viscosity), the equation looks like:
$$ \frac{\partial u}{\partial t} + u\frac{\partial u}{\partial x} = -\frac{1}{\rho}\frac{\partial p}{\partial x}$$
The left-hand side is the acceleration of a small lump of fluid and the right hand side is the force divided by the mass. Since we said this is inviscid, there is only the pressure gradient.
The negative sign in front of the pressure gradient means the acceleration vector is in the opposite direction of the pressure gradient vector. Gradient vectors go from low to high, and so the acceleration is in the direction of high pressure to low pressure.
The core of a vortex is, in general, a pressure minimum (although viscosity changes this slightly, not enough to talk about here). Therefore, there is a force from the outside of the vortex to the inside of the vortex; this is the suction force you are talking about. It is based entirely on how low the pressure can get inside the vortex relative to outside.
To get more suction, you need to make the pressure lower.