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

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Inverted U manometer

I have some doubts on inverted U manometer. In the picture there is one U inverted manometer, the space above the liquid in the manometer is filled with air which can be admitted or expelled through ...
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55 views

Tank draining derivation: is it correct to use Bernoulli and continuity equations?

I'm confused about a classical exercise on tank draining, which asks to find the time requested for empty the tank. I'll put here the derivation I found on textbook and on the web, which I'm not ok ...
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157 views

'Slapping' sound when trains pass each other

I have noticed that when riding in a train travelling at over 100 kmh$^{-1}$, a loud 'slap' can be heard when another train travelling at a similar speed passes in the opposite direction, followed by '...
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3answers
68 views

Space Rocket Ionizing Its Path To Decrease Fuel Consumption [closed]

A beam(maybe multiple beams from ground crossing at, or just some very high alternating potential, something like focused haarp) on tip of rocket targeting at path of it, ionizing just the closest ...
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1answer
114 views

Why does the parallel component of the velocity gradient affect viscous force in a fluid?

The force density due to viscosity in an incompressible fluid is $-\mu \nabla^2 \mathbf{u}$ where $\mathbf{u}$ is the velocity and $\mu$ is the dynamic viscosity. Let's suppose some particular small ...
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59 views

Conversion of pressure energy into kinetic energy

So we have Bernoulli's equation which tells you about the change in different types of energy when fluid flows from one point to another. I can understand the conversion between kinetic and potential ...
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Why is oil a better lubricant than water?

How come mineral oil is a better lubricant than water, even though water has a lower viscosity? When two surfaces slide over each other with a gap filled with a fluid, the different layers of the ...
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2answers
78 views

Non-dimensionalizing incompressible Navier-Stokes

I have a question about non-dimensionalization of the incompressible Navier-Stokes (NS) equations. My understanding is that the purpose of non-dimensionalization is to "collapse" solutions onto one ...
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35 views

Ultra-cold gas giant atmospheres

In the troposphere, which has the bulk of the atmospheric mass, it's colder at higher altitudes due to adiabatic lapse. Gas giants have several different chemicals that condense into cloud decks at ...
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Implementing fixed-temperature, solid-wall boundary conditions

I wish to simulate the behaviour of a fluid along one dimension, where the right boundary is transmissive and the left boundary is a solid wall at a fixed temperature. Temperature is not one of my ...
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43 views

On the boundary condition of solid boundaries for an enclosed, non moving fluid

First and foremost, I might say horribly wrong things. Feel free to correct any inconsistencies in the following post. Let's assume an incompressible, viscous, newtonian fluid that is not moving and ...
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33 views

How to estimate a drag of flow reversing cylinder?

I am assuming a turbulent flow and a full reversal of the flow (in the picture the arrows are at non-straight angle but please consider the skew as negligible). Assuming we are talking about regular ...
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66 views

Meaning of a Dirac delta speed in 1d shallow-water equations

A 1d thin layer of a fluid of constant density in hydrostatic balance can be modeled by shallow-water equations: $$\begin{align*}\partial_tu+\partial_x\left(v+\frac{u^2}{2}\right)&=0,\\ \...
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40 views

The relationship between height of water and the rotational speed inside a cup

When I stir the water inside a cup with a speed $v$ ( using a stick), I find that a vortex will be formed inside the cup, with the middle part of the water at the lowest point ( with the height $h_0$),...
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26 views

What are the dynamics of the “break the surface tension with a drop of soap” experiment?

There's a common experiment exemplified in the first few seconds of this video: https://www.youtube.com/watch?v=WsksFbFZeeU You put some food coloring in the center of a plate of milk, then put a ...
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3answers
62 views

Why does Bernoulli's equation only apply to flow along a streamline that is in viscid, incompressible, steady, irrotational?

I am learning about hydrofoil on this website. In a later video I watched, I learned that in the process of deriving Bernoulli's equation, $$constant=P/d+gh+1/2v^2$$ has to multiplied through by ...
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29 views

Why is the velocity of fluid on top a hydrofoil higher than that on the bottom? What keeps enlarging the difference in these 2 velocities?

I am trying to learn how hydrofoil works. A big part of it is Bernoulli's principles. I found this article on MIT website. There are two explanation for how it works: The conservation of linear ...
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1answer
48 views

Predict liquid film thickness outside a cylinder, under the influence of a) external gas flow and b) heating of the cylinder

I tried to post this question on Computational Science SE, but I was really unsure about the most appropriate SE site. The question didn't attract a lot of attention, and since Physics SE was my ...
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64 views

Are tides just very very very weak spaghettification?

I was wondering as to why there are two tidal bulges instead of just one on Earth, and then I saw this diagram: The oval shape that the Earth's water is transformed into in this image reminded me of ...
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83 views

The Madelung Equations are Euler Equations for Quantum Mechanics. What is the Lagrangian description of Quantum Hydrodynamics?

The Madelung Equations of Quantum Mechanics suggest the Hydrodynamic Model of Quantum Mechanics (Quantum Mechanics is described as a fluid of universes in a multiverse, which, in the non-relativistic ...
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1answer
181 views

Gas permeation through solid metal?

if i make a box from aluminum ( 0,5 mm thickness ) and put air in it ( about 2 bar or 3 bar). would the box lose the air over time (10 years period) or not ? assuming the temperature will change ...
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36 views

Is it possible that propagation of acustic waves leads to emission of radiation?

Question: Consider large cloud of gas. Assume it is electrically neutral (but as always, matter is composed of smaller things which are actually charged). Is it possible that propagation of sound ...
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27 views

How can I determine the height in this fluid dynamics problem? [closed]

Here's the problem: I used Bernoulli's Equation and tried to make the height the subject: The tricky thing about the problem is that the initial height is not given. Checking by dimensional analysis ...
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1answer
31 views

As hot air rises is the cold air that rushes in pushed in or sucked in?

I seem to remember from high school, the state of equilibrium. Volumes of more molecules wanting to get to a volume of less molecules and causing a state of equilibrium. From this I believe cold air ...
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47 views

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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51 views

Physical reason of adding salt when we're cooking rice (osmosis)

Why do we add salt when we're cooking rice? I know one reason is related to the boiling point of water but someone said it is also related to "Osmosis". What is the relation between adding salt and ...
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1answer
30 views

Systematic+Random Uncertainty for velocity measurement

I have a question regarding systematic and random uncertainties. I have to measure the mean value of a velocity measurement in flow field at a point. I've recorded say 1000 samples in time, which I ...
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32 views

How is zonal flow defined and computed?

The transition to turbulence in pipe flow was recently observed to be in the same universality class as directed percolation. This was done by reinterpreting the turbulence and laminar flow in terms ...
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7answers
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Supergun Launching of Satellites

I should say first that I don't believe this is a feasible launch method, otherwise NASA and other space agencies would be using it by now. It's based on this BBC news story Saddam Hussein's Supergun ...
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63 views

Mass of star from Lane-Emden equation

Suppose we have equation of state $p=K\rho^{1+\frac{1}{n}},$ where $\gamma=1+\frac{1}{n}$ for some star. Then by standard calculations we obtain equation for enthalpy $h$: $$\Delta h+4\pi G\left(\frac{...
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1answer
64 views

Power generation with a long, horizontal pipe, using atmospheric pressure differences

Straight to the point. If I float a pipe starting from a point in the ocean that has the atmospheric pressure of 101 kPa (Call it Point A) all the way to another geographic position on the ocean with ...
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22 views

What is vortex row image system in fluid mechanics?

What is vortex row image system in fluid mechanics? I have lot search but I did not find any best answer about this question. Please explain about this topic or give me best link of book or site.
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Analysis of a system consisting of a leaking tank of water

The departure point is this problem: A water tank on wheels is moving over an horizontal trail with negligible friction. There is a small opening in one of the walls, at a depth of $h$ below the tank'...
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54 views

Application of Bernoulli's equation to siphon

I'm trying to understand how to use Bernoulli's equation in practice. So I will make an example of where is my confusion. I found this great explanation on Wikipedia about the siphon, but there is a ...
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1answer
79 views

Raising a cut on your arm above your heart

I often read that when you have a deep cut on your arm, you should raise your arm to the point where the cut is above your heart. This should reduce the amount of blood loss because less blood reaches ...
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1answer
52 views

How could water pour out from upside-down bottle, unless its pressure is smaller than atmosphere pressure?

Just a simple question : when we remove a stopper of an upside-down bottle, water flows out. But the pressure caused by the mass of water's column is usually smaller than atmosphere pressure unless we ...
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How do we explain the existence of liquids, from a mathematical or computational perspective? [closed]

This post asks why matter exists in three phases. Most of its answers explain the existence of liquids with some variant of the following: liquids happen when thermodynamic conditions, temperature ...
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1answer
97 views

If a high speed car or train enters a tunnel, will the aerodynamics effect slow it down and hence requiring more energy to maintain speed?

Speed and distance are two main determinants in this question. A slow moving vehicle or train entering a tunnel is unlikely to be affected by the presence of solid stationery object covering them. But ...
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1answer
55 views

Why does stagnation pressure reduce across a normal shock?

I am seeking an explanation for this graph where the subscript "1" refers to the supersonic region and the subscript "2" refers to the subsonic region present beyond a normal shock. The static ...
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What is the variation of pressure in rotating fluids(in accordance to the question in the image)? [closed]

This is an interesting question that I came across. Please see attachment. It is a multiple answer type question. My answer was B and D. I have attached my derivation for this problem. I want to know ...
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1answer
40 views

When does $ S\cdot v=\mathrm{cost}$ hold?

In fluids dynamics what are the limitations for the use of the volume flow rate conservation law? $$ S\cdot v=\mathrm{cost}$$ Does it still hold when there are external forces acting on the system, e....
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1answer
56 views

What is the direction of buoyancy in the bulk of liquid on earth?

I have a trouble when considering the direction of buoyancy in the bulk of liquid subjected to vertical temperature gradient. The liquid is heated from below or above that induces a natural convection ...
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93 views

Why do I spill lesser water if it's hotter?

If I pour water in a glass to make a cup of tea, I noticed that if the water that comes out of the kettle is very hot, almost no water is spilled. If the water is cold though, much more water is ...
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1answer
31 views

Inertial Waves - Why neglecting the advecting term?

I'm trying to derive the dispersion relation for Inertial waves. In Cartesian coordinates: Inviscid and incompressible fluid is rotating uniformly with Angular Velocity: $\Omega = (0, 0, \Omega)$ ...
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1answer
45 views

The convective term in Reynolds Transport theorem

Reynolds Transport theorem states that $$\frac{D B_{sys}}{Dt}=\frac{\partial B_{CV}}{\partial t}-\dot{B_{in}}+\dot{B_{out}}$$ where $B_{in,t+\Delta t}=b_1m_{1,t+\Delta t}=b_1\rho_{1}\forall_{in,t+\...
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27 views

What is the relative velocity term in drag and lift force in the following case?

When a stationary particle is introduced in a fluid moving with horizontal velocity $V_fx$, what is the relative velocity terms in the drag force ($F_D$) and lift force ($F_L$) equations. Am I correct ...
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25 views

Strain Flow in Fluid Mechanics

I am working on hydrodynamic instabilities and was looking for non axisymmetric vortex models that are not Stuart or Taylor green vortices. What I came across was multipolar vortices where a uniform ...
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When beer tapping, why does only one of the beers foam up?

In the paper Physics of beer tapping the author go through an explanation of the phenomenon of beer tapping, i.e. the foaming up of beer in glass bottle after it has been hit at the top of the bottle ...
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Can a droplet of water bounce back when it hits a water surface?

Is it possible that a droplet of water can bounce from the surface of a volume of water?
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17 views

Lengthening in-flight evaporation time of water droplet in air

How one can increase the in-flight evaporation time of sprayed water droplets in air without changing the thermodynamic conditions of the ambient air ( i.e. RH, T ). For instance mixing water with ...