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

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Conceptualising the switch from discrete molecules to the continuum (fluid dynamics)

I understand that $\rho(x,t)$ represents the density of the small volume surrounding the point $x$. However what have I done with the discrete molecules in the process of averaging?
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47 views

Deriving an Euler Equation

In this derivation is it necessary to write the triple integral, as I thought that if we are dealing with one fluid particle it only contains one "point" and hence we do not have to take a sum?
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40 views

What does the continuum hypothesis of fluid mechanics mean?

I'm a bit confused by the continuum hypothesis stating that fluid are continuous objects rather than made out of discrete objects. Say for $\rho (x,t)$ (density) is there more than one fluid ...
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26 views

Pressure as the magnitude of the reaction force

Here $\vec{F}$ is the reaction force caused by the interaction of the fluid particle on the surface and another. I don't think the two parts I have highlighted in green tally with one another. If ...
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51 views

Mass flux across a volume

Here mass flux means the mass flow rate, units: kilograms/second This is for a one-dimensional flow Firstly is the reason why the mass flux/ mass flow rate $=$ $\rho(x_1,t)A(x_1,t)u(x_1,t)$ ...
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60 views

Continuity equation in fluid mechanics

The continuity equation in fluid mechanics states that $$ \frac{\partial\rho}{\partial t} + \nabla\cdot(ρ\mathbf u)=0 $$ Can you explain to me what is the physical meaning of each term of the ...
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24 views

One dimensional fluid flow with slowly changing cross sectional area [on hold]

I am rather confused by what's written in the green box. If $\frac{\partial A}{\partial x}$ was not $<<1$ would this mean that the velocity now has a vertical component and is hence not ...
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1answer
17 views

Is the Mass flow rate (Mass flux) a scalar quantity?

Wikipedia states that mass flow rate is a scalar quantity, however Mass Flow Rate= Density x Cross Sectional Area x Velocity and velocity is a vector quantity, so this would imply Mass Flow Rate is ...
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29 views

Rate of Dripping of tube

My original problem was to discover the rate at which leaking taps dripped, and its relation with the tap's size, radius, etc. However, I have no clue how taps work, so I simplified the problem. ...
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40 views

Creating a realistic simulation [on hold]

I've recently read an article from I believe it was Cambridge or something, where they created an extremely detailed simulation of bubble interaction. I've come here to ask what are some common ...
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8 views

What to measure for cylinder stiffness/compressibility?

I have 2 cylinders made from different polymers. Both cylinders are made to 'accept' the same volume of water. When I fill cylinder 1 it is subjectively stiff while cylinder 2 is subjectively floppy. ...
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27 views

How to calculate liquid pressure regardless of the gravity?

Assume we have a balloon in a tank filled with a liquid , without any gravitational forces: We just know the Temperature, V0 and V1 I want to calculate the pressure exerted from the liquid on the ...
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Where can I find a table for NIST standards on temperature and pressure?

The Wikipedia article Standard conditions for temperature and pressure notes that IUPAC and NIST are two of the most common standards for temperature and pressure in use. Even if I could have gotten ...
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48 views

Why is mass per second constant in the Equation of continuity?

Why should the mass per second be constant in the equation of continuity? Why can't the mass accumulate inside a tube?
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65 views

Walking on water

I was wondering about walking on water. I wonder if we can use surface tension to do this. Lets say I make a pair of shoes in shape which has infinite perimeter (e.g a koch snowflake) with glass. Now ...
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35 views

What will happen to this fan in this situation?

Lets say I have a pipe with a fan at the end of it and it is drawing ambient air from the other end of the pipe. Once I account for some blockages and looking at the fan curve, it was found the ...
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2answers
50 views

By what mechanism is lift produced on a rotating cylinder in an inviscid flow?

I am taking some introductory fluid dynamic classes, and have become very confused by the Kutta-Joukowski theorem. One of the conclusions that can be derived by applying Kutta-Joukowski is that a ...
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1answer
38 views

Why does a propeller suck in air from the front?

A rotating but fixed propeller sucks in air from the front. What is the cause for this acceleration of air particles? Since propeller blades act like rotating wings, my best guess is that the blades ...
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1answer
59 views

Is Magnus effect a corollary of Bernoulli principle?

Magnus effect is commonly explained using Bernoulli principle. However, taking the lift on a rotating cylinder as an example, the velocity difference is caused by the extra work done by the rotating ...
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36 views

Superfluids (meaning, properties)

I am curious as to what a superfluid is? I have seen videos that say that they have zero viscosity, and I know that it means that there is no friction between the layers of the fluid. But what is its ...
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20 views

Microscopic definition of dynamic pressure of fluids

So we have a moving fluid and we know from Bernouli's equation that there is a term called dynamic pressure(not to be confused with the hydrostatic pressure of the fluid). So,what exactly is it and ...
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24 views

What is the difference between high speed water supply and high pressure water supply coming out of a hose?

According to Bernoulli's principle, $P + \frac{1}{2} d v^2 + dgh = constant$ where, P is pressure, d is density of water, v is velocity and h is height. Assuming that height of a plastic pipe is ...
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1answer
26 views

Tsunami effects of a nuclear explosion

Sea surface nuclear explosion arouses a tidal wave of water. How does this kind of wave compare to naturally occurring tsunamis? E.g. what would be the potential of a 100KT bomb 100km away the from ...
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Why does this capillary action generator not work?

So I was doing a bit of reading. Apparently the the obstacle to generating energy from the forces driving capillary action is breaking the surface tension at the top of a capillary tube. It is just ...
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Practical nozzle question / mass of finger?

A spray bottle i own broke off its compressor tip so i fashioned another one from a rubber tube, a pen lid, an ear plug and a 1 mm radius tube from a computer cleaning duster. The opening of the ...
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1answer
57 views

How to obtain this equation for skipping stones over water?

I think I have a decent conceptual understanding of the forces at work when stones are skipped over water. My question pertains to the equation found in this [source][1]. Reproduced in Latex this ...
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15 views

boundary layer pipe

What is the shape of fluid(say water) on exiting a pipe due to boundary layer formation?Since the velocity decreases as we move from centerline of pipe to boundary along the radius(say fully developed ...
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35 views

Boundary Layer in aerofoil

I want to know how the top and bottom boundary layer interact at the trailing edge of an aerofoil (zero angle of attack) and what happens to the boundary layer after a small distance from the trailing ...
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Effect Pressure In Main tube due to outlets [closed]

We are trying to measure water pressure at various points in the city in order to detect the leakage in the main pipeline,but i cannot really predict the impact of the outlets in between,on the ...
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35 views

What happens to the d/dr term of the continuity equation in cylindrical coordinates?

The derivation of the continuity equation in cyclindrical coordinates should look like this, $$ \dfrac{dm_{cv}}{dt}=\sum \dot{m}_{in} - \sum \dot{m}_{out} $$ where $$ m_{cv} = \rho r ...
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23 views

Liquid Flow Over Object

When there is a large object in the bottom of the ocean of height h, the surface of the ocean rises by a height less than h. In fact, if the ocean depth >> h, then the surface will rise negligibly. ...
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52 views

2 different density liquids in space

Lets say you are in deep space with basically zero gravity. You have a ball of water floating in front of you. If you carefully injected that ball with a syringe filled with vegetable oil, what ...
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2answers
72 views

Are standing waves in water exact sine waves?

In many physics demonstrations waves in water are used to illustrate principles of standing waves and wave propagation. After such demonstrations, classes tend to move on to basic problems ...
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32 views

Complex potentials in plane polar coordinates - stream function

Determine the stream function and the potential in plane polar coordinates and sketching streamlines. We need to take the value of m=1. I know how to find the stream function and velocity potential, ...
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87 views

Water Waves in the Wake of a Boat

As a boy I noticed that the waves from the wake of my model boat would fan out. If I looked at the end furthest from the boat the front had turned so that it was almost running in the same direction ...
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Air flow through a hole [closed]

If I drill a hole of 1 cm diameter in each of two opposite window frames of my house, assume that the temperature outside is 0 deg C, the atmospheric pressure is 1020 millibar, wind speed is 10 meter ...
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41 views

The relation between the velocity and the static head

I would like to ask about the Bernoulli's principle: as the principle states, increasing the velocity causes a decrease in the pressure. I made an experiment using a duct and 6 manometers and what I ...
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3answers
88 views

Is viscosity simply “slowing down time”?

The Pitch drop experiment has pitch, a fluid with extremely high viscosity, flowing through a funnel. In 85 years, only 9 drops have fallen. Photographs of the drops, however, show an appearance just ...
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If I accelerate a mass quickly enough through a short distance (i.e. unit step function), would I create wind, sound, and heat?

As far as I know, the movement of air molecules can result in what we call wind, sound, or heat (and maybe some other things I'm not thinking of), depending on the assumptions about its motion. Wind ...
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2answers
25 views

Hydro-electric gravity problem - Is there a air-resistance-like force affecting water flow down a mountain making placing hydro-electric ?

Say I have a theoretical mountain with a river starting exactly at the peak. Gravity would act on the water, accelerating it down the mountain. There would also be a force limiting the water's ...
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2answers
41 views

How do you calculate/estimate hypersonic leading edge and skin temperatures?

At lower speeds (below Mach 5-ish), stagnation temperature (TAT) is a very accurate proxy for skin temperature. But at mid/high hypersonic speeds (especially in the thin upper atmosphere where mass ...
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How do i correctly go from a two variable function to a function of difference? [migrated]

I would like to know how I can go from a two argument function $g(x_1,x_2)$ formally correct to a function of the difference of the parameters $g(x_1-x_2)=g(x)$ this seems to involve integration over ...
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1answer
42 views

How can Bernoullis be solved without Vectors [duplicate]

Some time ago I asked a question why Dynamic pressure is considered scalar. Why is the dynamic pressure not a vector quantity? This still puzzles me so I hope to give a scenario that doesn't make ...
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3answers
61 views

What is the exact difference between diffusion, convection and advection?

I have tried to explore the information but still not very clear on the exact difference between diffusion, convection and advection. Can anyone help me out to clear my concept?
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26 views

Mass flow rate and force on moving disk

If I understood correctly, the force $F$ related to a given mass flow rate $\dot{m}$ through orifice area $A$ (or, say, on a disk of area $A$) is given by Newton's 2nd law of motion - assuming a ...
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2answers
51 views

How does the radius of a pipe affect the rate of flow of fluid?

Poiseuille's law states that the rate of flow of water is proportionate to $r^4$ where $r$ is the radius of the pipe. I don't see why. Intuitively I would expect rate of flow of fluid to vary with ...
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1answer
16 views

Water in an evacuated tube travels 10.3 meters. How does less water in the tube not violate some equilibrium condition?

At atmospheric pressure, water in an evacuated tube travels 10.3 meters. Let's say I don't have 10.3 meters worth of water but I only have enough for 1 meter, like in the image. The atmosphere still ...
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2answers
32 views

Pressure-Velocity relation at a point in a flow

How can we relate pressure and velocity at the same point in a fluid flow? Why does pressure decrease with increase in velocity?
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34 views

Experiment of plume of smoke

If a burning candle is covered by a transparent glass, the flame extinguishes and a steady upward stream of smoke is produced. Which factors will infulence the figure and properties of the plume of ...
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Influence of a defect on the flow field

I have a long microchannel where flows some water. The reynolds number is much smaller than one. Within the structure of this microchannel there is a big defect. It looks like a bump of size ...