7
$\begingroup$

So this Youtube video has been around for a while now Veritasium Hydrodynamic levitation! (2017).

Also these questions Fluid Mechanics explanation of an object levitated next to an air jet and Explain hydrodynamic Levitation may be related.

Description

To avoid rot links and people that do not want to see the video, I'll try to explain the phenomena.

You have a sphere (radius of about various centimetres) and a water jet. The water jet is vertical (jet velocity goes against gravity), and the jet touches the sphere almost tangentially. The ball not only rotates due to the water jet but also stays at constant height from the ground. The angular velocity can be defined in the direction of the cross product between the position of the contact point between the jet and the sphere (taken the origin in the center of the sphere) and the direction of the jet velocity. Some water drops fly away at the top of the sphere away from the jet. The whole system seems stationary, as long as the water flow is constant, the height and frequency seem to also stay constant.

Suggestions

Derek clearly states it is not about Bernoulli principle, as in this case the ball rotation is important.

There seems to be two explanations "water adhere to the sphere and is ejected conserving horizontal momentum" (Newtonian effect?) and the Magnus effect (ME). The later is not so obvious to me as the system is at rest with respect to still air and there are two fluids, water and air, (does ME mean air pushes the ball horizontally?).

Note that another explanation is Coandă effect (CE) is the typical hairdryer ping-pong ball levitation demo which is explained by Bernoulli's principle and works for small balls as air can surround the ball. So I am not sure if CE works as a satisfactory answer.

Question

Has any recent paper/videos discussed what happens if you do the same experiment with hydrophobic spheres? Can it be predicted with laminar flow theories or does it need turbulence? Would this phenomena occur without air for example?

Too many questions have been posed, but not so many responses on the internet yet.

I just wanted to know if any standard hydrodynamic formalism has been developed to model this phenomena or at least if there has been some experimental characterisation of the phenomena (any study of the link between friction, density, frequency, height, radius, and so on).

Improve this question
$\endgroup$
7
  • $\begingroup$ It's currently unclear what exactly this question is asking without clicking on the link you provided. To make questions more accessible and guard against link rot, please include all relevant information, such as the explanation of notation or specific terminology used, in your question. $\endgroup$
    – ACuriousMind ♦
    Commented Nov 22, 2017 at 15:23
  • $\begingroup$ @ACuriousMind edited. $\endgroup$
    – Mauricio
    Commented Nov 22, 2017 at 15:41
  • $\begingroup$ Since you are revisiting this old question, consider this guidance. $\endgroup$
    – rob ♦
    Commented Jul 24, 2021 at 13:34
  • $\begingroup$ @rob thanks, would merging all edits into a coherent post with the same question be ok? $\endgroup$
    – Mauricio
    Commented Jul 24, 2021 at 14:07
  • $\begingroup$ That's the idea: just make your post say what it ought to say. People who are curious about previous versions can click the "edited 5 hours ago" link. $\endgroup$
    – rob ♦
    Commented Jul 24, 2021 at 14:35

2 Answers 2

Reset to default
1
$\begingroup$

This is because of Friction, Coanda effect followed by Magnus effect.

In the middle, he also showed how the water followed the curve of the ball. This is mainly because of 'Coanda effect'.

From Wikipedia,

The tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops.

Here, the water is trying to entrain fluid from surroundings. When it comes near to a surface, it cannot pull air from surroundings which makes a low-pressure region between surface and jet. This is because, 'when something is emptied from its place, then something should fill the void to keep the balance.'

Firstly, the ball starts to rotate because of the friction between water and ball surface which is just like a Tesla turbine. As the ball is being hit by the water on one side (not centre), it will push the ball to the other side because it comes in the way of the water. Once the ball starts rotating, the fluid following the surface don't adhere to the surface much longer and drift apart tangentially.

This is where the 'Magnus effect' kicks in. The magnus effect creates a force perpendicular to the jet direction. This force pushes back the ball to remain in contact with the jet on one side. So, the weight of the rotating ball is born by the jet completely.

The magnus force is proportional to the speed of rotation of the ball which is proportional to the velocity of the jet of water.

So, the ball levitates in air as long as the jet discharge is kept constant.

Improve this answer
$\endgroup$
6
  • $\begingroup$ Well, in a boundary layer, due to no-slip condition, fluid adheres to the wall. and viscosity between the layers of fluid tend to slow down the further layers nearby due to viscous shearing thus a boundary layer forms. Here, by friction, I mean the no-slip condition at the surface. $\endgroup$
    – VL Srinivas
    Commented Nov 25, 2017 at 6:47
  • $\begingroup$ Magnus force doesn't seem right. Do yo know an example of Magnus force between two fluids? My problem with Magnus effect is that the ball is at rest with respect to air. $\endgroup$
    – Mauricio
    Commented Nov 26, 2017 at 11:43
  • $\begingroup$ Magnus force is a phenomenon and Bernoulli's principle is merely a scheme to calculate the total amount of pressure; it is just a way to get the approximate amount of pressure, velocity trade-offs. Moreover, Bernoulli's theorem is only valid for incompressible, inviscid and irrorational flows across a streamline. In this case, the entire lateral force to keep it in contact with the liquid jet is provided by the liquid rotated across the ball as a result of Magnus effect. Forget about the air, think about a rotating ball or cylinder in a fluid, it experiences Magnus force. $\endgroup$
    – VL Srinivas
    Commented Nov 27, 2017 at 13:30
  • $\begingroup$ Yeah, the problem is that it gets too descriptive and from principles you cannot get very predictive. Do you know any papers with similar experiments? $\endgroup$
    – Mauricio
    Commented Nov 27, 2017 at 13:33
  • 1
    $\begingroup$ I tried searching for some but couldn't find any. If you come across any of that kind, kindly post it in the comments. $\endgroup$
    – VL Srinivas
    Commented Nov 27, 2017 at 13:43
0
$\begingroup$

I am unconvinced by his (simplistic) use of Bernoulli's law to try to explain this effect. Clearly, the water adheres to the ball due to viscous effects and surface tension, where Bernoulli cannot be used because it assumes that viscous forces are negligible.

Instead this is an application of the Magnus effect which explains how a spinning ball curves away from its principal flight path as observed e.g. in ballsports such as baseball. Due to the rotation of the ball, it drags air faster around one side, creating a difference in pressure that moves it in the direction of the lower-pressure side.

Improve this answer
$\endgroup$
5
  • $\begingroup$ Actually tour link to NASA directly states that the lift is due to Bernoullis law. Last bullet point on that page.(what is a myth though.is the equaltime hypothesis) $\endgroup$
    – lalala
    Commented Sep 9, 2017 at 10:21
  • $\begingroup$ his? Actually nobody seems to be considering this experiment as a simple Bernoulli's law application. But using hydrodynamics equations (continuity-Euler eqs, Navier-Stokes) could you predict this phenomenon? Has anyone given a clear demonstration that it is adhesion based? $\endgroup$
    – Mauricio
    Commented Sep 9, 2017 at 10:52
  • $\begingroup$ @lalala - I was alluding to the longer path hypothesis, i realize now it is not so relevant so i have removed the statement. $\endgroup$
    – nluigi
    Commented Sep 9, 2017 at 11:16
  • $\begingroup$ @Mauricio - The presenter in the video (which is whom i mean by 'his') clearly mentions Bernoulli multiple times. The low-pressure region is caused by drag due to the spinning motion of the ball, this can be capture just fine using hydrodynamic equations in some rotational reference frame. I am not aware of any such demonstration for this particular case, but maybe this is of interest? $\endgroup$
    – nluigi
    Commented Sep 9, 2017 at 11:21
  • $\begingroup$ @nluigi Derek the presenter states clearly that is not a demonstration about the Bernoulli principle, also stated in the description of the video. He says thought the Coănda effect is explained using Bernoulli, but this experiment with water doesn't. $\endgroup$
    – Mauricio
    Commented Sep 9, 2017 at 12:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.