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Questions tagged [fluid-dynamics]

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

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Variable mass pendulum (water filled in Bob)?

There is pendulum having Bob made of hollow sphere, having water filled in it. Now at t=0 , a small hole is made on bottom of sphere ,at the same time pendulum is given oscillation. (Assume no damping)...
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Physical interpretation of a singular pde [on hold]

Can you kindly have a look at the following question attached in the link below: https://math.stackexchange.com/questions/3058490/physical-interpretation-of-a-singular-pde?noredirect=1#...
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Flow generated by vortex near a rigid cylinder

I have a problem with complex potential and can't figure out how to approach it properly. The problem: Find flow around cylinder of radius $a$ generated by vortex of cicrculation $\Gamma$ at point $b$...
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How can mass flux be defined for a one-dimensional flow?

I am reading a textbook where an example of a one-dimensional binary diffusion is given. For a substance A we write: $\dot{m}_A'' = Y_A(\dot{m}_A'' + \dot{m}_B'') - \rho \mathcal{D}_{AB} \frac{dY_A}{...
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Problem about fluid flow in a pipe [on hold]

Water at 15°C flows by gravity from one deposit to another, which has its water level 20 meters lower than the first one, on a smooth pipe with a 50 mm diameter and a 100 meters length. Calculate the ...
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4answers
59 views

Calculating value of $g$ from Torricelli's law

For my Physics project, I decided to do an experiment with fluids. I'm using the Torricelli's law to verify that the value of acceleration due to gravity on earth is $9.81m/s^2$. I am using the ...
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Mean squared velocity in Poiseuille flow

For fully developed laminar Poiseuille flow in a tube, I came across this relation: $$ \bar{u^2} = 2 u_b^2 $$ where $u_b$ is the bulk or mean velocity, defined by: $$ u_b = \frac{1}{\rho A} \int_A \...
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What Is The Lowest Temperature At Which A Fluid Will Flow?

I’m studying Hydraulics, and this week we’re talking about viscosity and fluid properties and I came across this question but I can’t find the answer anywhere in my books. Please help.
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How long for released dye to uniformly saturate the oceans?

Suppose a supertanker filled with purple dye breaks open at a particular point in the middle of the Pacific Ocean and spills an enormous volume of purple dye into the ocean all at once. Imagine that ...
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How to generate force with air pressure [on hold]

I am trying to figure out how to generate a substantial force for a short period of time by releasing pressure in a small high-pressure air tank. A rough estimate of the force required is 40 ...
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How do you reckon my mattress works? [on hold]

So I have one of those "sleep number" mattresses, made by Select Comfort. Good mattress. It contains two air chambers, and has a pump which can inflate or deflate these air chambers to the desired ...
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Surviving only on another's exhales? [on hold]

While reading this - Surviving under water in air bubble - this question came to me: How long can an adult person survive on breathing in only exhales of another adult person? Can this be even ...
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1answer
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Energy recovery from refinery cooling tower

Is there any possibility of recovering exhaust energy from a cooling tower by putting a wind turbine above it? Would this affect the performance of the cooling tower?
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1answer
39 views

Laminar Flow From Outdoor Faucet

I ran across a video recently that really caught my attention. It's been circling the internet over the past couple weeks, but I'm trying to figure out what's going on. Basically, it's a video of an ...
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Pressure on a Heat Exchanger in a piping system

I have a piping system that is supplied by a centrifugal pump. The system consists of pipe, 2 heat exchangers, some elevation gain and loss and ends up in an atmospheric tank. If I wanted to know ...
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Swimming pool skimmer floating weir forces

How would I calculate the forces required to open the weir gate depending on the depth of the water? The pump is an impeller type, I want full flow to the pump. I assume there are also forces from ...
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1answer
32 views

Viscoelastic Constitutive Relation

In the Mori-Zwanzig formalism, the following identification for the generalised shear viscosity $\eta(t)$ is given: $$ \eta(t) = \frac{V}{k_B T} \langle \sigma(t) \sigma(0) \rangle, $$ identified as ...
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+100

Is there any use of a quadratic equation of state in FLRW cosmology?

Consider standard FLRW cosmology. Usually, the relation between energy density $\rho$ and pressure $p$ of a cosmological fluid component is linear: \begin{equation}\tag{1} p = w \, \rho, \end{...
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High school level experiments regarding fuild dynamics/mechanics? [closed]

For our physics Internal Assessment, we must perform an experiement and write a paper about it. We are free to choose any topic as long as it is within our syllabus. I am interested in fluids and ...
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3answers
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why do fluids become less dense when heated?

I have been told that fluids expand and become less dense when heated, but i never been told why. So my question is why do fluids become less dense when heated? What is going on in a microscopic level ...
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Does surface tension increase for a flowing fluid?

When a fluid is moving with velocity V, what becomes of its surface tension, does it increase or decrease in value
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Converting this navier stokes solution into a incompressible solution?

I have an equation for a non-viscous compressible fluid with density, pressure and velocity given by: $$ \begin{align} \rho(x, y, z) &= \frac{3B}{a^2 + x^2 + y^2 + z^2} \\ p(x, y, z) &= \frac{...
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1answer
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Hole affect to pressure between 2 rooms [closed]

I have 2 rooms and pump the pressure to 20mmHg and 2mmHg and have the plate in the middle. After that I change the plate to a plate with hole 6mm diameter can I calculate the pressure after I change ...
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1answer
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Is it possible to MAKE a Jet Pump with the Specs.? [closed]

(A) Must be able to pump high density and Low viscous fluid say for ex. Mercury (B) Jet pump must be as low in weight as possible. (C) An efficiency of >80%. i.e. the ratio of Kinetic energy ...
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Risk of dying (fluid dynamics)

This is both a theoretical problem (whose resolution I am asking here) and a practical one just to underline a risk for people. Let us assume that on a bank in a railway station a person of 100 Kg ...
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3answers
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Why tubes with lower cross sectional area has lower pressure than the tubes with high cross sectional area?

The tube with lower cross sectional area has a lower pressure than the tube with high cross sectional area, right? Why is that, how can I understand this intuitively and using bernoulli principle (or ...
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2answers
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Torricelli's law doesn't seem to apply in an experiment. Why could this be? What is going on?

For physics class we've done this experiment where we would measure the speed of the water escaping from a water bottle through a hole in a side of it. We would calculate the speed through kinematics ...
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1answer
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Non-conservative forces in Lagrangian mechanics

In the Lagrangian formalism with a dissipative frictional force $F$, we can write $$\frac{d}{dt}\frac{\partial\mathcal{L}}{\partial\dot{q}_{k}}-\frac{\partial\mathcal{L}}{\partial q_{k}}=Q^{(nc)}_{k}...
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Feasibility of tunneling vehicles as seen in cartoons [closed]

In cartoons, we sometimes see drilling vehicles that tunnel their way through the earth at top speed. Like this guy. Such vehicles are clearly preposterous. Real-life drilling machines progress no ...
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Adding a swirl to poiseuille flow

I came acroos this question and I have no clue how to answer it: If we have poiselle flow in a circular pipe of Length $L$ and pressure gradient $\Delta P$ and introduce a swirl $u_\theta=\omega r$, ...
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2answers
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Could the cosmic fluid be better described as a kind of van der Waals gas, or a kind of foam, instead of a dust gas?

In cosmology, it is usually assumed that the cosmological fluid made of galaxies could be described as a gas of "particles" without any pressure (the dust gas), of density $\rho_{\text{matter}} \...
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1answer
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How fast does liquid rise in a capillary tube?

Take a small diameter tube, stick it in water, and surface tension will drive the fluid up through the tube. How fast is the fluid moving through the tube during this process? Here's my attempt: ...
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Onsager reciprocal relations and Darcy's permeability tensor for anisotropic porous media

The wiki entry on soil tensor permeability states that The tensor is symmetric by the Onsager reciprocal relations Bear & Cheng (2010, p122)* state that: The hydraulic conductivity tensor, $...
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2answers
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What are the similarities/differences between the behaviors of Quantum particles and bouncing droplets? [duplicate]

Bouncing droplets on a fluid surface show many weird behaviors of the quantum world. Look at this for example: https://arxiv.org/abs/1307.6920 They can show tunneling, double-slit interference ...
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1answer
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On the evaporation of fluids

Q. In which portion (1,2,3) there will be maximum concentration of evaporated water droplets? Or there will be uniform distribution of droplets? Background: Today, I noticed something really strange ...
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1answer
310 views

Siphon water from washing machine into sink next to it [closed]

I have a top-loading washing machine that has broke down and is full of water. Instead of getting a bucket to manually collect the water, I was thinking I could use a tube to siphon the water out of ...
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Hydrostatic pressure vs static pressure in a fluid

hydrostatic pressure is the scalar and isotropic pressure exerted by a fluid at rest at a certain point inside the fluid itself on the neighboring fluid parcels or on on an elementary differential ...
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1answer
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Is the velocity gradient of a homogeneous viscous fluid constant?

The formula for the viscosity of a fluid (or even a gas) is equal to \begin{align*} \frac{F}{A} =\mu\frac{du}{dz} \end{align*} where $F$ is the force applied to the top layer of the fluid, $A$ is the ...
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Why do large eddies in turbulence break-up

This is probably a silly question. But, Why do large turbulent eddies break up into smaller ones? A possible argument Why can't they (large eddies) retain their size and gradually diffuse instead of ...
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1answer
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Hydrostatic pressure: clarifications

Hydrostatic pressure is due to the weight of liquid. Do liquid molecules exert pressure on their neighbours molecules or only in objects that are in contact with them?
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1answer
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Curvature of fluid surface next to a floating sphere

I want to estimate the curvature $\kappa$ of the fluid surface next to a floating sphere. The situation is static and shown here: The fluid density is $\rho$, downward gravity is $g$, sphere radius ...
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1answer
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Capillary length and Bond number confusion

The Bond number represents the ratio of gravity forces to surface tension forces, and is defined as $$ Bo = \frac{\rho g L^2}{\sigma}$$ where $\rho$ is the fluid density, $g$ is gravitational ...
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2answers
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Why does an ice-cube turn faster and faster while melting in water?

Whenever I put an ice-cube into a glass of hot water, so that it melts quickly, and it is initially rotating slowly, I noticed that its rotational speed increases as it melts and 'shrinks'. Why? I ...
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1answer
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When a carbonated drink is opened, why does the liquid tend to flow out?

In other words, why does the coke flow out of it's bottle/can. Isnt the air trapped inside the container, so the air must flow out? What pushes the liquid to go outside the container? For example, we ...
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Why there is a difference between same components of 2 parallel planes?

In this context the author is deriving the net outflow through 2 surfaces.to do so he is calculating the difference between x components ofthe vector V. so now my question is here the 2 surface are ...
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1answer
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Magnus effect without windspeed

Canagnus effect take place if windspeed is 0 ? Lets assume that our frame of reference a place an earth (e.g. soccer pitch) .We measure the windspeed and we find it zero.But if we make a knuckleshoot ...
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2answers
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Viscous fluid equivalent of hyperelasticity

A hyperelastic solid is defined as one for which the stress tensor $\sigma$ can be written as the derivative of some stored energy function $W$ w.r.t. the strain $\varepsilon$: $\sigma = \frac{\...
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Why do the colloidial particles in a viscuous fluid stop (stroboscopically) moving below a treshold as described in the experiment below?

In a nice experiment (inspired by the renowned physicist G.I. Taylor), which can be seen in this video (around 12 minutes), Professor David Pine and Professor Jerry Gollub filled a Couette cell with a ...
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Where does this equation come from?

I am investigating about viscosity of fluids and I found the following equation to calculate the viscosity: $$\nu = \frac{2(\rho_s-\rho_l)\cdot g \cdot r^{2}}{9\cdot\Delta v} $$ The experiment ...
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If a salt solution heated under a pressure slightly lower than critical pressure will it boil?

8.8% NaCl solution for example have a critical point of 450°C under 423 bar pressure. If we heat such solution to same temperature under slightly lower pressure i.e. 420 bar should it will boil or it ...