Questions tagged [fluid-dynamics]

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

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What are some good books on fluid mechanics for the olympiad?

I have already checked out Book Recommendations on the community wiki, but the recommendations there don't suit what I ask for. I need problems/problems+theory book(s) on high-school-physics-olympiad/...
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1answer
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Relationship between bridge pylon and water flow

Recently, I was met with the following question: When observing water flow around a bridge pylon, of the following statements, which one(s) are true: a) The bridge pylon applies force to ...
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1answer
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Diver's air torus rising from 10m underwater (edited)

[Edit: earlier version was even more messy] I just watched a video of a jellyfish caught in a diver's 'air ring' - a torus blown for the sake of watching it rise. The jellyfish gets drawn into the ...
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Is the force due to mass flow proportional to the square of the mass velocity?

I'm trying to calculate the force on a surface due to some flow of water against that surface- for simplicity, assume that the mass flow vector is orthogonal to the surface. Using dimensional ...
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1answer
64 views

Uranus: The Spin

In his prog. , on the outer planets, Brian Cox stated that Uranus spins on its side because it once endured an interplanetary collision. Such a cataclysm would normally be devastating for both ...
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Entropy in non-homogeneous equilibrium states and local thermodynamic equilibrium

My question concerns the possibility of using the hypothesis of local thermodynamic equilibrium for calculate the entropy of a non-homogeneous equilibrium states. For example, let's imagine the ...
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48 views

Are conservation of mass and conservation of energy *independent* constraints for physical systems?

I'm modeling fluid systems and want to use conservation of mass (aka 'continuity') and conservation of energy as constraints to help solve for certain system parameters. But it occurred to me that the ...
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The physics of the startup of a lava lamp

I noticed a strange phenomenon observing a lava lamp. We all know the complex flow of bubbles when the lamp is "hot" and running, but I would like to have some explanation about the startup behavior ...
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245 views

Viscous Force On A Rotating Cylinder

In this question asked in Irodov, it is taken that the friction force acting on a unit area of a cylindrical surface of radius $r$ is given by $σ = ηr(∂ω/∂r)$. A fluid with viscosity $η$ fills the ...
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Does a thickness of a bubble become constant over time?

Assuming no gravity, imagine you have a bubble floating in the air, does its thickness have a tendency to become constant? State a bit differently, if we start with a bubble with variable thickness, ...
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417 views

Can you explain the perfume bottle with the Bernoulli effect?

Someone explained the perfume bottle with the Bernoulli effect: squeezing the bulb over the fluid creates a low pressure area due to the higher speed of the air, which subsequently draws the fluid up. ...
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Is there a physical circumstance where the flow of a system changes from a nodal source/sink to a saddle?

Phase portraits for sinks can often like eddies in water, so I would assume there is some fluid or aerodynamic system where these curves appear. But, the only difference between a saddle and a nodal ...
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Does tilting a liquid develop shear stress on its surfaces?

Please help me these confusions I have. If I tilt a cuboidal container containing an ideal fluid (non viscous liquid), does a shear stress develop on the liquid due to some shearing force acting ...
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Boltzmann transport equation for granular gases

I am researching about granular gases and their collisions, and have come across this Boltzmann transport equation: $$\frac { \partial f ( v ) } { \partial t } = \iint d u _ { 1 } d u _ { 2 } f \left( ...
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2answers
192 views

Why does air get trapped below water pouring into a glass?

The drinking-water-tap at my workplace has a low pressure continuous stream of water. When I pour water into a glass which has at least about (depth) 5cm of water in it, surprisingly large air bubbles ...
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125 views

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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Proof that pressure rises in a fixed control volume

I am looking for a proof using Bernoulli's equation that that for $Q=(Area)(Velocity)$, $Q_{in}>Q_{out} \Rightarrow P_{inside}>P_{outside}$. I read that for a computer, if the case fans blow a ...
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126 views

How does the velocity of the fluid layers gradually increase towards the axis of a tube?

For a fluid moving through a cylindrical tube in a streamlined motion, a fluid layer in immediate contact with the wall of the tube remains effectively at rest. The layers away from the wall has ...
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4answers
377 views

Can someone explain why there is water in this vacuum chamber?

I work at a water treatment plant that uses a vacuum system to pull the water into the filters and backwash the filters. In the middle of the vacuum system there is the vacuum chamber. What I don't ...
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1answer
986 views

Continuity equation in the Lagrangian flow picture approach

In deriving continuity equations using Lagrangian. We consider the element of fluid which occupied a rectangular parallelopiped having its centre at the point $(a,b,c)$ and its edges $\delta a$ , $\...
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143 views

At what critical Reynolds number does vortex shedding begin?

In: "Fluid Dynamics", Chapter 3 (Turbulence), Section 26, Landau and Lifchitz analyze the problem of the stability of a steady flow past a body of finite size. The fluid is assumed to be ...
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1answer
82 views

How can a vortex be created?

i have seen this video. https://www.youtube.com/watch?v=pnbJEg9r1o8&t=96s It is not completly clear how a difference of velocity can cause a vortex, can you explain me?
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Computing the velocity of fluid along the length of a cylinder

I am modelling the flow of fluid through a cylinder of length L.The cylinder is divided into three segments(each segment of length L/3), there are four nodes(i=0,1,2,3). I'm trying to compute the ...
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Forces acting upon body on impact with water

I am investigating the water-entry of a steel ball-bearing at different speeds. Ideally, I would like to model (to an extent) the forces experienced by the ball bearing as it transitions from air to ...
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When can we say $x$ and $p$ are “independent variable”, in order to find the Vlasov equation?

I have a question about "independent variable" in physics, and especially variable in Lagrangian or Density Function. I read several questions about it in this forum and although I have the feeling I ...
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Why do bubble rings stop rising near the surface, and then spontaneously dissipate?

When I've blown bubble rings in my pool I've noticed that every single time a ring rises to the surface it stops rising about 1-2 centimeters from the surface, stopping in place. Then, it ...
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How to model pressure loss over time for air in a cylinder with a hole

Related to an engineering project I will be working on. The situation is that essentially I will have a closed cylinder with a small hole on the side, which allows the air inside to be at equilibrium ...
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2answers
335 views

In what cases should plasmas be considered compressible?

I was listening to a lecture where it was mentioned incompressibility was assumed during derivation of an electrostatic instability, but incompressibility was not assumed for an electromagnetic ...
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Is it possible to have a vortex with more than one attachment point?

In my study of two vortexes, fire whirls and tornadoes, I have come to hear a bit about vortex attachment. As far as I can understand, a vortex is attached to its "power source" for lack of a better ...
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59 views

Thermodynamic formulation of pefect fluid with spin

The spin phenomena in an asymmetric spin-related Einstein-Cartan candidate solution is due to microscopic spin of small volumes of a globally "ordinary" perfect fluid. The only description I have ...
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How to determine the maximum size of a hole that can keep ink/water stay in an ink-cartridge for fountain pens?

What can be the maximum size of the hole? The physical forces to be taken into consideration are: external pressure, internal pressure, surface tension of the fluid (ink=water). What is the formula?
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1answer
731 views

How to calculate Shear Rate magnitude for Compressible Flow?

I'd like to calculate the shear rate formula for CFD (Non-newtonian Fluid) and want to know if the following formula is the good one: Viscious Stress General Equation (Tensor): So the magnitude of ...
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How to prove that Navier-Stokes equations are Lorentz invariant?

I've been hearing recently that the Navier-Stokes (NS) equations are invariant under a Lorentz transformation, so I tried to prove it just changing terms of transformed velocity instead of the ...
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Can Normal Shock Waves be stationary in a flow?

When solving problems of converging diverging nozzles, a shock wave is formed in the flow. For the purpose of problem solving we consider it to be stationary in one point of the flow. Is this ...
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5answers
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If I replace water molecules with microscopic solid spheres, will they still reach the same height in communicating vessels?

I'm having an hard time understanding why liquids take the shape of the container or why they reach the same height in communicating vessels. I'm wondering if I try to simulate their behaviour in ...
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What is the “sweeping velocity” of a turbulent flow?

Background: While reading Rubenstein and Zhou 1997 I encountered an unfamiliar term, "sweeping velocity". To quote the authors (pg. 3): ...the sweeping hypothesis makes the decorrelation depend ...
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Why does water flow up the sides?

Ive noticed this on my faucet and wonder why water flows up the sides? Is it dark magic?
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Confused about applications vs misconceptions about Bernoulli principle

I've been reading on wikipedia that the Bernoulli principle is often incorrectly applied to the example of blowing on pieces of paper because A third problem is that it is false to make a ...
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Total force generated in a pneumatic cylinder-piston with known gap size

Assuming that we have a pneumatic cylinder-piston with arbitrary but known surface area A, a known clarence c, and a known length L. What is the best model to describe the total steady-state force ...
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Why is it said that vorticity gets produced at a wall?

I am currently taking a fluids mechanics class and during the lecture the Prof. explained the following: when dealing with a 2D incompressible newtonian flow without external forces (gravity), ...
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Derivation of the energy equation in the differential form

I'm trying to understand the derivation of the energy equation from fluid mechanics, that is presented in the book "Fluid Mechanics" (4th ed.) by Frank M. White (as you can see here, page 231). I can ...
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Settling Velocity of Particles in a Flowing Fluid

I'm trying to calculate the settling velocity of particles traveling through a horizontal section of pipe. I've not been able to find information about settling velocities in moving fluids, only still ...
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In what sense can a “manned model” boat handle identically to a much larger boat?

A YouTube video(relevant part is 1:24 - 1:46) suggests that if you take an oil tanker, scale it down 25 times, and give it an engine with 0.4 horsepower, "they behave exactly like a [larger] ship". ...
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Power in a wind turbine

An wind turbine starts turning by a wind speed of $v_b$ and it stops turning (rated wind speed, to prevent overloading) by a wind speed of $v_g$. By wind speeds between $v_b<v<v_g$, the power ...
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Pressure and velocity change at window

When the pressure and velocity of atmosphere(out side of room) are Pa and Va, how do pressure and velocity of wind change at window? Set the window size 'A'. Please explain this. Thank you.
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Frequencies of stellar g-mode oscillations in astroseismology

It's a simple derivation that in a gravitating fluid, if the density decreases with height then a perturbed fluid element should oscillate at the Brunt-Vaisala Frequency, $$N^2 = - \frac{g}{\rho_0} \...
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1answer
203 views

Bernoulli equation

Can we apply Bernoulli equation to a closed system the same way we apply it to an open system? In an open system increasing the speed of the fluid implies that either the pressure or GPE should ...
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What is the maximum gap that water won't pass through for hydrophobic surfaces?

If I had 2 hydrophobic surfaces (in this particular case, 2 sheets of coated glass), placed edge-to-edge, what is the maximum gap between the sheets where water would not drip between the sheets? Is ...
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How to scale up a boat design?

I've been trying to research how to scale up a boat design. Specifically I am trying to find out how much power will be needed to "drive" the boat? Suppose I know that a 250 mm model boat, with 100 W ...
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Simplest model of simplest organism swimming in fluid

(Hopefully this question isn't too elementary for this StackExchange.) Given a micro-organism we'll define as two "cells" (which are the atomic things in this model): A "main" larger cell: ...

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