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

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9
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3answers
152 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 '...
4
votes
4answers
183 views

Is there any electric current in a continuous medium of moving charges?

Consider a continuous charged fluid (unlike in real life where charge is carried in discrete chunks like electrons) in a bottle. Suppose that the fluid is stirred in a circular manner and then left to ...
1
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1answer
65 views

Bernoulli equation and pumps [closed]

I have a doubt on the use of Bernoulli equation for pumps. Consider the situation in the picture. I marked different points: $1$ on the surface of first tank, $2$ in the exit from first tank, $3$ ...
1
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3answers
76 views

Pressure just before the hole in a draining tank

I get extremely confused in fluid dynamics problem where the pressure in some points is required. Consider the ideal fluid in the tank in the following picture. I would like to understand what is the ...
2
votes
3answers
64 views

Flame shape and size(length) depending on gravity

How would the shape and size of a flame(from a simple candle for example) depends on the gravity? Suppose all the relevant information is known, like candle dimensions and its chemical composition, ...
9
votes
4answers
11k views

What equation of state is needed for liquid states?

I'm familiar with the ideal gas law $$PV=nRT$$ but I don't think it applies to liquids like water. If I'm wrong, please correct me! If I'm right, then what equation of state applies to liquids such ...
0
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1answer
173 views

Equation of a water pump with a nozzle

If I have a small water pump with a flow f (L/s) and maximal pressure p (bar) and attach its output to a small tube having on the other end a nozzle with an area A (m^2). What is the equation that ...
2
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2answers
126 views

Why does pressure decrease as velocity increases?

Bernoulli's principle states that as velocity increase pressure decreases. But higher the velocity, greater is the temperature and pressure must be high. Can you explain the situation in both the ...
0
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0answers
57 views

Inverted Bottle Physics - Bubble in, water out

With all due respect, I would like a practical answer, hopefully, to point my untrained brain in the right direction as to understand the variable elements of such system. I am not looking for the ...
3
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2answers
768 views

Force required to move a fluid out of piston at depth

I have a cylinder filled with water in it at the bottom of the ocean. Say 100 meters down. In this cylinder is a piston that moves up and down. Its job is to squish the water out the bottom of the ...
3
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0answers
46 views

Fluidics and surface tension

If a one cent coin is set carefully onto the surface of water horizontally (flat side parallel to the water surface). What are the relevant forces to determine if it would sink.
3
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2answers
287 views

Bernoulli's equation for flow between cylinders

Let's consider the situation bellow noting that the water cylinders aren't equal and there will be water flowing to the right (As indicated by the arrow) Taking Bernoulli's equation at the tip of ...
1
vote
1answer
254 views

Position based dynamics constraint scaling factor

Reading through Müller et al., Position Based Dynamics, 2007 I got lost when passing from equation (5) $$\Delta p = \frac{C(p)}{|\nabla_pC(p)|^2}\nabla_pC(p)$$ to equation (6) (and applying the ...
0
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1answer
39 views

Matching Bernoulli equation and Hagen Poiseuille law for viscous fluid motion

I thought that Bernoulli equation could be used only in the case of non viscous fluid. But doing exercises on 2500 Solved Problems In Fluid Mechanics and Hydraulics (Schaum's Solved Problems Series) ...
6
votes
2answers
279 views

Rayleigh-Taylor Instability dependence on acceleration direction

I'm trying to bolster my understanding of the Rayleigh-Taylor instability, and I've gotten stuck on the point of which fluid (more or less dense) is being accelerated into the other. Cases of uniform ...
8
votes
1answer
136 views

What is the physics behind “mocha diffusion”?

A friend shared a video on Facebook of a process for glazing pottery called "mocha diffusion." A light-coloured glaze is applied to a pot and then, while it's wet, a black liquid is dripped into it. ...
0
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0answers
14 views

Scalar form equations for Non-Newtonian fluid

I am having trouble to write down the scalar form equations for this special class of non-Newtonian fluid (Oldroyd-B fluid), I really appreciate your input.
3
votes
2answers
78 views

What is the velocity in the Navier-Stokes equation?

I have been looking at the Navier-Stokes equation, and can't seem to find anywhere a clear description of what velocity it represents. From what I have read it could be any of the following: The '...
0
votes
2answers
60 views

Pressure due to weight of the fluid in fluid dynamics

In Bernoulli equation, the only term which corresponds to what one usually calls "pressure" is $p$, the other are still pressures dimensionally but their meaning is linked to kinetic and potential ...
0
votes
2answers
247 views

Viscosity for steam in the Reynolds number

In calculating the Reynolds number for a flow of steam in a pipe, this is the general formula I am trying to use: $$Re=\frac{\rho d v}{\mu}$$ with density $\rho$, pipe diameter $d$, flow steam $v$ ...
1
vote
1answer
49 views

Why does viscosity effect the normal component of stress in a Newtonian fluid?

The constitutive law for a Newtonian fluid is $$ \boldsymbol{\tau} = 2\mu \mathbf{D} + \lambda\left(\nabla\cdot\mathbf{v}\right)\mathbf{I}$$ where $\mu$ is the dynamic viscosity. Assuming we have a ...
0
votes
1answer
49 views

Why does laminar flow turn turbulent(*) in external fluid flow and why doesn't a fully developed laminar flow turn turbulent in flow through pipes?

(*:with increase in distance from the leading edge.) I would appreciate if you give detailed explanation for the first question and then apply the same logic to answer the second.
2
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0answers
38 views

Water floating on water [duplicate]

In the video made by the slo-mo guys as shown here, at time 1:47 comes a moment when a water droplet(almost completely spherical), is seen to 'float' on the surface of the water for a while. So why ...
1
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0answers
14 views

Modelling a flow due to an oscillating hemisphere?

Say I have a long cylinder filled with air, closed at one end and open at the other. Now say I place a compressible hemisphere at the closed end and make it oscillate (compression and expansion of the ...
5
votes
4answers
139 views

Could you filter coffee back to being pure water?

Okay, so coffee filters remove solid matter from the beverage, whilst leaving the remaining coffee intact for caffeiney goodness. But it's got me thinking. Is there a way we could filter the coffee ...
0
votes
1answer
363 views

How does a ducted fan suck in air?

I am thinking of a simple ducted fan with a single set of rotating blades, no stator vanes behind. If the effect of the fan is to raise the pressure of the air, how is there flow into the fan against ...
0
votes
6answers
94 views

Can I recirculate water from an open reservoir to the bottom of a bigger, closed one, without a pump?

A fountain head pumps water out of the main tank into a 'pond' reservoir. Can the water recirculate back into the main tank without the help of another pump? I'm sorry if this a dumb question. I'm ...
3
votes
3answers
78 views

Fluid mechanics paradox: The force needed to 'balance' a fluid using a piston

We know that the pressure in a fluid (like water) is dependent on the depth. Consider this example: For the first setup, solving the pressure at the bottom yields $P_1 = \rho gh = 1000kg/m^3 \bullet ...
1
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2answers
36 views

Conditions for flow incompressibility

The condition for flow incompressibility is usually stated as: $$ \frac{D\rho}{Dt}=0 $$ This sort of vaguely makes sense intuitively. However it seems more natural to me to understand ...
0
votes
1answer
171 views

How does a water-steam mixture behave?

Namely, if I mix a flow of steam that has density $\rho_s$, velocity $v_s$, and pressure $P_s$ with a flow of water that has pressure $P_w$, velocity $v_w$, and density $\rho_w$, how can I find the ...
0
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3answers
29 views

Application of Bernoullis identity for problem concerning hole in a container

Consider the following problem: We have a container of water with a hole on the side, and we want to determine the velocity of the stream coming out of the hole (at $S_2$): If we suppose $S_1>&...
3
votes
2answers
46 views

Prevention of fluid vortex formation

I've been looking for a way to prevent Vortex (whirlpool) formation in a pressurized vessel as it drains, but I am not having any luck finding the parameters at which vortex formation occurs. Is there ...
2
votes
2answers
41 views

Different locations of a pump in a tube

I'm confused about pumps in fluid dynamics. As far as I understand, the basic effect of a pump which deliver a power $\mathcal{P}$ can be described with modified bernoulli equation between a point $A$...
-1
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1answer
55 views

Buoyancy and natural convection in a stratified water tank

I'm working on a simple calculation-model for a hot-water-storage. At the moment I'm stuck on implementing natural convection. To calculate the temperature I've implemented a simple 1D-model in ...
3
votes
1answer
38 views

Is the system derivative for mass, in the Reynolds transport theorem for mass, always zero?

The Reynolds transport theorem for mass is $$\frac{dm_{sys}}{dt} = \frac{dm_{cv}}{dt} - \dot{m_{in}} + \dot{m_{out}}$$ My question is the term $\frac{dm_{sys}}{dt}$ always zero. Since this is the ...
5
votes
1answer
126 views

How can we theoretically calculate the number of balloons needed to stop a bullet?

After watching this video, I am interested in proving it mathematically. The problem is that how can we apply the conservation of momentum here to find the velocity of the bullet while we don't know ...
0
votes
1answer
38 views

Attractive force between hose and floor when a pool is filled

I was filling a rubber pool for my baby using a hose. But I noticed that if I put the hose outlet directly with the floor there is an attractive force! This is strange, because the hose is throwing ...
0
votes
1answer
59 views

How cold air is less dense in one case and more dense in another case when compared with warm air?

There are two different and contradicting explanations given. Which one is correct? Explanation 1: Cold air is more denser than hot air because ,when cold air of some density, say 'd' is heated, the ...
0
votes
1answer
51 views

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 ...
0
votes
0answers
39 views

How to make an equal flow rate in 4 glass tubes with different inner diameters?

I would like to ask a question regarding a flow rate project I've been working on that required pneumatic software to solve. Unfortunately, I don't have the software, and I don't know how to control ...
1
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1answer
36 views

History of Turbulence suggestions

I am looking for extensive historical accounts of turbulence that ideally is strong in math and physics (at least giving references). Any suggestions? The only book I found is "A Voyage Through ...
0
votes
1answer
67 views

If the pressure between the cross-sections of a pipe is equal, will $P_1$ and $P_2$ be cancelled out of Bernoulli's Equation?

a). Water is flowing through a pipe of two circular cross- sections area $A_1$ and $A_2$ lying horizontally as shown in figure below. The pressure difference between the cross-section $A_1$ and $A_2$ ...
3
votes
1answer
57 views

What happens to the flow rate when a water pump is pumping water through a tube that changes size

I'm going to apologize in advance if this is a rudimentary question. It's been 20 years since I took a fluid mechanics class. We are building a waterfall in my backyard. The water pump has a head ...
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votes
1answer
43 views

why do microscopic cracks happen?

2 weeks ago i asked a question about " Gas permeation through solid metal? " i have got a very good answer here .but microscopic crack was mentioned in the answer. i searched in Internet but i didnt ...
1
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1answer
39 views

Viscosity and mean free path

I have troubles understanding how to derive the formula for viscosity in terms of the mean free path $$\eta\sim \rho \lambda \bar v$$ where $\bar v$ is the average molecular velocity of the gas, $\...
3
votes
2answers
48 views

Can localized fluid perturbations be accelerated by pressure gradients?

I would like to know if there are any examples in fluid dynamics (or continuum dynamics) of small perturbations (or waves, solitons, or other "localized" solutions of the fluid) being accelerated in ...
40
votes
7answers
45k views

How can two seas not mix?

How can two seas not mix? I think this is commonly known and the explanation everyone gives is "because they have different densities". What I get is that they eventually will mix, but this process ...
0
votes
2answers
39 views

Conditions of validity of basic fluid equations

I easily get confused about the conditions of validity of basic fluid equations. I list the conditions for these equations to hold true, as I could understand them. $\rho v S=\mathrm{constant}$: ...
81
votes
6answers
10k views

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 ...
1
vote
1answer
43 views

What's the minimum time scale for a candle to be lit in order for it to establish a steady state?

My Understanding When a candle is lit initially, it goes through a few stages (see, e.g. this explanation from the National Candle Association): Wick burns. Crusted wax on wick melts, evaporates ...