# Tagged Questions

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

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### How does the dissolution of salt affect the solution density?

Suppose you have a container of water as a solvent and you a certain amount of salt as a solute sitting at the bottom of the container that has yet to start dissolving. Supposing temperature and ...
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### A few questions related to bernoullis principle & fluid dynamics in a pipe

I've been struggling with a few things as i've been introduced to bernoulli's principle, hopefully someone can help me clear things up(in somewhat plain, easy to understand language) In the examples ...
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### Fluid speed and fluid density

How does fluid density affect fluid speed? Basically I am trying to figure out if, with all other quantities remaining constant, would an increase in fluid density cause the fluid speed to increase/...
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### Viscous Burgers equation physical meaning

The viscous Burgers' equation: $$q_{t}+q\:q_{x}~=~\nu\:q_{xx}, \mbox{ where } \:\:\nu >0,$$ combines the nonlinear propagation of $q(x,t)$ and the diffusion. What is this equation for? (in ...
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### The name of effect of liquid flow inducing flow to neighboring layers of liquid?

How do you call an effect, when liquid or gas stream is involving the neighboring layers of matter also move? Like on videos of rocket engines testing, when exhausting gas sucks an air and steam from ...
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### Force of water hitting a wall [closed]

If you had a 8" pipe with 500psi stream of water exiting it and hitting a wall at 90 degrees 8 feet away, what would the force of the water on the wall be? Thank you all. Non-mathematician.
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### How does an hydraulic ram pump work?

Hydraulic ram (wikipedia)(youtube) uses the water hammer (wikipedia) to pump water. Please help me understand how it works. I understand that the check valve catches the pressure from the water ...
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### '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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### 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 ...
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### 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, $3$ ...
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### 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 ...
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### 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, ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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.
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### 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 ...
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### 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 ...
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### 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) ...
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### 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. ...
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### 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.
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### 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 '...
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### 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 ...
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### 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 ...
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### 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.
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 $A$...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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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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### 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 ...
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### 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 ...
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### 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 $A_2$ ...
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### 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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### 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 ...
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, \$\...