The quantitative study of how fluids (gases and liquids) move.

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Water pressure in free fall

The increasing water pressure as you go deeper is generally explained in terms of the weight of the water column above the observation point pressing down. The question, then, is what would happen if ...
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Why can, or can not, a perfectly incompressible fluid exist?

Water is normally assumed to be an incompressible fluid - for example in the context of calculations involving water pressure. I wondered whether that is strictly true, or an approximation? Later I ...
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water flow in a sink

When one turns on the tap in the kitchen, a circle is observable in the water flowing in the sink. The circle is the boundary between laminar and turbulent flow of the water (maybe this is the wrong ...
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Is it possible to accelerate air to supersonic speeds? What would it look like?

The speed of sound is the rate that disturbances in air propagate through it. Is it possible to have a wind that itself is moving at supersonic speeds relative to stationary winds around it? Or ...
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Why do liquids separate in space with no gravity?

I've seen videos of people in space (on ISS) who squeeze a bottle or something and liquid comes out, it then separates into smaller balls. Why is this surely it should stay pretty much together ...
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Why do stellar boundaries exist?

What theorems or insights are available to decide wether a star with some given interdependence between its density, pressure and temperature distributions ought to have a boundary at a finite ...
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Solving the Young-Laplace equation for arbitrary axisymmetric geometry

Say I have a non-ellipsoidal soap bubble and I want to numerically analyse the pressure in the inner lobe of this bubble here: The Young Laplace equation gives the pressure difference across a ...
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Why don't surfboards fly out the back of a utility truck when driving on the highway?

I have never had it happen myself and every time I throw my boards (shortboards not malibus) in the back of someone's ute, with no cover on the tray, they always assure me they'll be alright( the ...
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When does a pot not spill?

What shape for the edges of a pot and what surface properties are best for a tea pot not to spill? Apart from the cross-section from the side, how does the radius of curvature as seen from above play ...
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How does Earth's interior dynamo work?

I'm interested in getting a basic physical understanding of how Earth's magnetic field is generated. I understand that it's a "dynamo" type of effect, driven by convection currents in the molten outer ...
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Are coffee's properties different enough from water's to cause increased spillage while walking?

I recently found this article, which describes how... It just so happens that the human stride has almost exactly the right frequency to drive the natural oscillations of coffee, when the fluid is ...
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Stationary waves on a laminar liquid flow near collision with a surface [duplicate]

Look at the picture below I know about plateau rayleigh instability. And my opinion is that the reason of this phenomena is plateau rayleigh instability. But there's a question, in the picture ...
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What is Pressure Energy?

While deriving Bernoulli's Theorem, our teacher said that the sum of KE, PE and Pressure Energy per unit volume remains constant at any two points. $$P + \rho g h + \frac{\rho v^2}{2} = \text{...
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Dyson Air Blade as a propulsion system?

I've read from many sources that Dyson Air Multipliers are more efficient and quieter than normal fans. Now, with the proof of concept, is it possible to use its principles as a propulsion system for, ...
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What do bullet trails really look like?

In the movie The Matrix, during "bullet time" sequences, bullets are shown trailing evenly-spaced refracting blobs: http://2.bp.blogspot.com/_uypMqSrmflo/S18yZJPET_I/AAAAAAAAAGk/CDjoZaOLbwY/s400/...
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Fluid Mechanics from a variational principle

It is possible to define a good variational principle to describe Fluid Mechanics? if so, what is the correct treatment of the issue. I guess something like: $$I=\int d^4x \left(\frac{1}{2}\rho v^2-P-...
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How are boundary consitions implemented correctly in time dependent hydrodynamics?

I posted this question more than one year ago and got an answer recently. This answer looks good to me, but indicates that something is wrong in my original approach to the problem. Can someone tell ...
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assumptions about sound waves

When deriving the sound wave equation: $${1 \over c^2} {\partial^2 p' \over \partial t^2 }= \Delta^2 p' $$ by linearizing the Euler equation: $$\rho {d v \over dt }= - \nabla p $$ and the continuity ...
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Calculate water flow rate through orifice

I'm not very good with fluid physics, and need some help. Imagine the following setup with water contained in-front of a wall with an opening on the bottom: How do I calculate the water flow $Q$?. ...
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Why is the dynamic pressure not a vector quantity? [duplicate]

I understand that static pressure is a scalar quantity as it acts equally in all directions, then by the same reasoning dynamic pressure should be a vector quantity as it only can be measured by ...
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What's the difference between “Ohmic dissipation”, “Joule heating”, “ion drag” and “resistive heating”?

The following terms are sometimes used to refer to ... more or less ... the same thing by different people and in different contexts (electronic circuits vs. plasma physics, etc.): Ohmic ...
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What is the theoretical justification for a fluid flow's being irrotational?

I am not a fluid dynamicist, and I really just began thinking about this problem as my curiousity drew me into building an answer for the question What really allows airplanes to fly?. It is very ...
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How to compute the speed necessary for an airplane to fly?

I give some physics lessons to a friend. She asked me a question that I am unable to answer. Could you help me ? A plane has a weight of $2\times10^6$kg. The surface of the wing is $1200 \text{m}^2$. ...
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Why does a continuous water stream form ripples when colliding with a surface? [duplicate]

I was in the shower one day washing my hair, and noticed that the water falling off my hair formed perfect streams that ceased to break into droplets for quiet some distance (probably about 5-6 cm). ...
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How much water must flow trough canal to maintain a constant water deep?

In order to maintain a constant water deep in canal, how much water must flow trought the pipe ? As shown on picture, canal have a rectangular shape. I don't know if canal length have an influence. ...
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Landau & Lifshitz - Euler's equation for one-dimensional flow

One page 5 in Landau & Lifshitz Fluid Mechanics (2nd edition), the authors pose the following problem: Write down the equations for one-dimensional motion of an ideal fluid in terms > of the ...
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Physics behind Water drops during falling from a tap

what is physics behind Water drops during falling from a tap. water drop animation A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. Why Water ...
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Flow rate of a syringe

Suppose a syringe (placed horizontally) contains a liquid with the density of water, composed of a barrel and a needle component. The barrel of the syringe has a cross-sectional area of $\alpha~m^2$, ...
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Pressure coefficient on fluid mechanics

I have to solve numerically a fluid mechanics problem. I have an airfoil with a flap deployed downwards, under the airfoil I have set a mesh with some nodes. I have already computed stream function ...
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Capillary action in a slanted tube

Suppose there is a slanted capillary tube and a fluid rises in it. Why does the fluid rise to the same vertical height as when the tube is perfectly vertical? If I'm right surface tension force ...
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Derivation of Archimedes' principle

It is my understanding that upthrust from a liquid on a body is due to pressure difference on the top of the body and the bottom of the body. How, then, is this fact used in order to derive/work out ...
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Can a nozzle increase the thrust of a water rocket?

First of all, what is the correct equation to determine the thrust produced by a water rocket? $$ F_T = \dot{m} V_e \tag{1} $$ $$ F_T = \dot{m} V_e + (p_e - p_0)A_e \tag{2} $$ Using the first ...
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Fluid filled harmonic oscillator

A vessel (preferably circular) filled with water is accelerating unidirectionally such that the level of water is higher on one end than the other. What I want to know is that if the vessel is ...
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To calculate pressure at point in a vertical water pipe of given diameter

Is it possible to calculate pressure at a point in a pipe, where a solenoid valve shall be placed, given that the pipe carries water (utility supplied, regular water) from an overhead tank (at a ...
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What is the reason behind Laminar Flow and the ability to return to near-original state in Laminar fluids?

Laminar flow here refers to the smooth disruption of fluid in layers, in a medium. A peculiar trait in a few examples of laminar flow is the ability to return to near-original state, as demonstrated ...
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What is the pressure of falling water?

Imagine a breaking wave 10m high. The wave breaks onto a beach. What is the maximum pressure created by the wave on the beach, ignoring effects such as might be cause by air?
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How does a piezoelectric drop generator work?

In a piezoelectric drop generator used for generating a single droplet which is 0.5 mm in diameter, there is a piezo-actuated brass diaphragm to generate droplets (for example here or here). What is ...
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Could a fish in a sealed ball, move the ball?

If you had a glass ball filled with water, completely sealed and containing a fish, could the fish move the ball?
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What can wavy patterned sand tell about the Fluid that formed it?

Sand on the bottom of the ocean as well as sand on the low-tide beach often forms wavy patterns. Do the parameters of these wavy patterns have any relation to the water and waves that formed them? If ...
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Why water in the sink follow a curved path?

When you fill the sink with water and then allow the water to be drained, the water forms a vortex.. And then it starts to follow a curved path downwards by effects of gravity.. Why this phenomena ...
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How deep can a whirlpool descend?

My question is based around comparing the physics of a tornado against, what I imagine/assume to be the marine equivalent of a tornado, that is a whirlpool located either at sea, in a tidal region or ...
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Could some design of a propeller be used in both air and water?

Propellers in water are smaller in diameter. They also move more slowly. On the other hand, aircraft propellers are larger in diameter, have narrower blades and operate at very high speeds. An ...
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Could the phenomenon of vortex bursting be exploited to reduce wake turbulence?

One of the classic stories in the annals of aerospace engineering is the development (and subsequent redesign) of the F-18 and its Leading Edge Extensions (LEX) due to fatigue problems, problems that ...
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Why don't we talk about angular momentum at all in fluid mechanics?

People usually talk about similar (or maybe not?) things like vorticity or enstrophy in fluid mechanics, but no one talks about angular momentum, why?
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Sailing against the wind. Is this a fair model?

I was ruminating the explanations about how boats can sail against the wind (or "into the wind"), and wondered if one could devise a simple mechanical model without hydrodynamics involved. Imagine a ...
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Distance a curveball travels?

I've seen some discussions regarding the movement of a spinning object, say a curveball. However, all have been largely qualitative. I was wondering if anyone has seen or worked through a ...
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Is it possible for wind to break the sound barrier?

I understand that in nature wind would never get high enough, but I am just curious as to whether physics would allow this to occur or not.
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Non-Newtonian Fluid Stop a Bullet?

I just saw a YouTube video about Non-Newtonian fluids where people could actually walk on the surface of the fluid but if they stood still, they'd sink. Cool stuff. Now, I'm wondering: Could a pool ...
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A book sits on a table. What is the net force of air pressure?

An elementary problem asks, A book is at rest on a table top. In what direction is the net force of air pressure on the book? Is this a meaningful question, and if so, what's the answer? If ...
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Why does a stream of water behave like this when aimed against a spoon or into a glass?

I noticed while doing everything except the dishes that water behaves quite strange. These two things might be two different phenomenon, but the water acts in a similar way in both cases. Spoon and ...