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

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Will a gas rotate as fast as the spherical container it is contained within?

Let's say I have a sealed spherical glass container 30 cm in diameter which contains plain air. The glass container is rotated about its axis at 1 revolution per minute. My question is, would the gas ...
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2answers
86 views

Why is the Reynolds number “the way it is?” Why is its order the way it is?

Why is the Reynolds number “the way it is?” Why is its order the way it is? I'm not sure if this is an appropriate question for this context, but I would like more intuition on this matter and so ...
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79 views

Can a nozzle increase the thrust of a water rocket?

First of all, what is the correct equation to determine the thrust produced by a water rocket? $$ F_T = \dot{m} V_e \tag{1} $$ $$ F_T = \dot{m} V_e + (p_e - p_0)A_e \tag{2} $$ Using the first ...
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204 views

Fluid dynamics tsunami

I have been given a energy head (pressure) in meters for a given tsunami on land. If the tsunami hits a wall 1 meter by 1 meter, is it possible to calculate the height it will reach. I am thinking it ...
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43 views

Calculate Potential [closed]

$\vec{w} = \begin{pmatrix}w_r \\ w_{phi} \end{pmatrix} = \begin{pmatrix}\frac{Q_0}{2 \pi r} \\ 0 \end{pmatrix} $ 1) Show that the flow satisfys the continuity equation 2) Show that $\vec{w}$ ...
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376 views

Is a venturi pump feasible at very low pressure?

I need assistance (or experience feedback) in estimating if a Venturi pump would work at low pressure. A boiler under vacuum (10% atmosphere) will produce steam (at a given speed depending on the ...
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1answer
26 views

How does a body accelerate that is put into a flow field with constant speed?

Imagine an infinitely long flow channel filled with water flowing at a constant speed everywhere parallel to the infinitely long dimension of this channel. Furthermore, imagine we place a body (say a ...
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182 views

Sound Propagation using Finite-Difference Time-Domain (FDTD) considering wind effects

I am trying to plot the propagation of sound from a fixed source in a 2D environment using Finite-Difference Time-Domain (FDTD) Method taking into account the effects of the wind velocity. I came ...
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26 views

Constant RPM of Gas Compressor

I would like to improve my understanding of gas compressors. If the compressor is moving at constant RPM, does it mean that it can only allow the same volumetric flow rate at all times? So the power ...
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176 views

Flow rate through hole at bottom of tank, estimate time for depth to reach 1 foot

I am a structural engineering student and have limited experience in fluid dynamics. I'm not quite sure if I'm going about the problem in the correct way. The question is: An open cylinder of ...
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36 views

Has there been any research done in fluid dynamics within a mobius strip?

I was wondering if there has been any research done with the flow of fluid within a mobius strip? The reason I am interested is, assuming the fluid would flow around a center axis point, at certain ...
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63 views

Laminar vs Turbulent flow separataion

(Left is a laminar separation, Right is a turbulent separation) I have been having a bit of trouble understanding this concept. How come the laminar separation involves a much larger wake than the ...
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2answers
856 views

Difference resultant aerodynamics force on an airfoil and a flat plate

From basic airfoil theory the following free body diagram can be determined for a two dimensional asymmetric airfoil: Here the direction of the resultant force is governed by the geometry of the ...
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1answer
3k views

Why, when one opens 1 car window, does that noise occur?

When you're driving and you open 1 car window, say the front one, there comes a horrible noise, but when you open another window just the slightest bit, this noise goes away (I'm sure most people know ...
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40 views

Car window vibrating noise

I was going to ask a question similar to: Why, when one opens 1 car window, does that noise occur? but saw that it was already asked and answered very helpfully. However, there are a couple other ...
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413 views

Pipe in a flowing river problem

I'm working on a certain problem in fluid mechanics, which isn't really my strongest area. The problem is as follows: Curved pipe is partially submerged in a flowing river so that one end is pointing ...
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45 views

Any applications of kinetic physics on cosmological scales?

On galaxy cluster scales, one is usually forced to use approximate theories such as hydrodynamics or magnetohydrodynamics for modelling cosmological phenomena. I wanted to ask if kinetic theory is ...
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1answer
51 views

What is this form of mass conservation equation?

I found the following equation of conservation of mass (continuity) in "Computational Fluid Dynamics Vol.III" by Hoffmann: $$ \frac{\partial \rho}{\partial t} + \frac{\partial}{\partial x}(\rho u)+ ...
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1answer
42 views

Drag force acting on a disk in a 2D system

I have a 2-dimensional system with behavior governed by Langevin dynamics in which disks (circles) move through a fluid. In the Langevin equation, there is a velocity-dependent term that accounts for ...
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186 views

Liquid column “recoils” in a sealed cylinder when hit by a piston — is it possible?

Consider a cylinder filled partially with a liquid (e.g. water). The cylinder is sealed, and is at held at room temperature (e.g 298K). At equilibrium (or when no external disturbance is imparted to ...
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1answer
28 views

Reynolds Average Navier Stokes equations and turbulence scale

To obtain the time average of an unsteady term like $\frac{\partial u_{i}}{\partial t}$ by definition we perform the following: \begin{align} \overline{\frac{\partial u_{i}}{\partial t}} &= ...
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40 views

Origin of pressure gradient in Navier-Stokes integral

I am not understanding the origin of the second term on RHS of momentum conservation equation (cf. the Wiki page), $$ \frac{\partial}{\partial t}\int_V\rho\mathbf u\,dV=-\oint_S\left(\rho\mathbf ...
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23 views

Difference between a tap and a hosepipe

When the flow of water through a tap is blocked by the fingers, the water does not increase in velocity whereas the velocity of water increases when the same is done tp a hosepipe. Both involve the ...
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143 views

How does a converging-diverging nozzle not violate conservation of energy?

Disclaimer: I know absolutely nothing about fluid dynamics, and very little about physics in general. THis may be a really dumb question. Now, at subsonic speeds, converging and diverging nozzles ...
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143 views

Understanding equation for force acting on stones skipping 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 this equation, $$ F = C_L\rho U^2S\sin({\alpha + \beta}) $$ which ...
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Magnus effect, Coanda effect, and the Bernoulli principle: What is balancing gravity on a spinning sphere vs. a stationary screwdriver?

I've seen videos of golf balls and screwdrivers levitated by a focused air stream. Reading the comments of these videos (and others), I see explanations of the machanics at work and their cause and ...
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How can rising bubbles shrink and disappear?

I was recently looking at a Wurlitzer juke box, and noticed something strange. It's decorated with liquid-filled tubes. Gas bubbles are injected at the bottoms of the tubes, and the bubbles naturally ...
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77 views

Can we reduce a hurricane's power using wind turbines?

Below is the abstract of an article in Nature: Taming hurricanes with arrays of offshore wind turbines, describing computer simulations that indicate that wind turbines could disrupt a hurricane ...
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320 views

Fluid Mechanics with calculus on manifolds

Fluid Mechanics is a branch of physics that uses a lot of vector calculus in $\mathbb{R}^3$ to describe phenomena mathematically. Calculus on manifolds, however, is the straightforward generalization ...
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25 views

Velocity change of air down a cone?

Does the velocity of fluids(specifically gasses) change when traveling down a cone from the wide opening to the narrow opening? If so, is there an equation used to calculate the acceleration or ...
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1answer
68 views

Flow in diverging channel is unstable according to Bernoulli's equation

So let's say we have a circular pipe with a variable cross section with an inviscid incompressible fluid flowing through it. The areas of the two ends are $A_1$ and and $A_2$. Let's say we also ...
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100 views

Strong and weak turbulence

As far as I understand, in general, turbulence describes the process of transfer of energy from a large scale, at which a system is perturbed, to a small scale, where dissipation happens. At ...
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60 views

Why does the vortex form in the von Karman Vortex Street?

Why does the vortex form in the von Karman Vortex Street? So far I have this for the reasoning: 'It occurs when a fluid flows around a cylindrical object. The pressure on a fluid particle rises to ...
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35 views

Calculating the drag force provided by a laminar flow [duplicate]

If I drop a ball into viscous fluid (e.g. oil), by calculation I know that the fluid flow is laminar. So which equation do I need to calculate the drag force provided by fluid? The drag equation, ...
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5answers
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Have we figured out how to analyze turbulent fluids?

I was surprised to read that we don't know how to analyze turbulent fluids. On page 3-9 of The Feynman Lectures on Physics (Volume One), Feynman writes: Finally, there is a physical problem that ...
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What is the mystery of turbulence?

One of the great unsolved problems in physics is turbulence but I'm not too clear what the mystery is. Does it mean that the Navier-Stokes equations don't have any turbulent phenomena even if we solve ...
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489 views

About turbulence modeling

I have some questions about this paper: Lagrangian/Hamiltonian formalism for description of Navier-Stokes fluids. R. J. Becker. Phys. Rev. Lett. 58 no. 14 (1987), pp. 1419-1422. After reading ...
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16 views

What is the critical diameter ratio of a cone shaped reducer before air overflows

Let's assume we have a cone shaped cylindrical reducer with $D_1$ inlet and $D_2$ outlet diameter (funnel). Air flows through it with velocity $v_1$ at inlet and $v_2$ at outlet. With the velocity ...
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746 views

What stops giant cruise ships toppling over in rough seas?

A week ago, 2 of the most gigantic cruise ships in the world docked near my city. If you have seen one, or been on one, you will know how large they are. They look extremely unsafe to me, although ...
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228 views

How does vasoconstriction/vasodilation of arterioles change blood pressure?

Background: I am coming at this question from an electrical engineering background, and I feel like I am missing certain assumptions that are going into the statement found in my physiology textbook, ...
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492 views

Breaking the sound barrier underwater

Theoretically, if I were to launch something faster than the speed of sound in water (around 5 times that of air), what would happen?
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3k views

Mechanics around a rail tank wagon

Some time ago I came across a problem which might be of interest to the physics.se, I think. The problem sounds like a homework problem, but I think it is not trivial (i am still thinking about it): ...
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1answer
147 views

Why do I sink then float in a swimming pool?

Was swimming this morning and observed something strange: if I stop treading water, hold my legs together, arms pressed against my sides, then I sink to about 3m below the surface, and then rise again ...
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76 views

Could a Jumbo Jet aircraft fly on paper wings?

My dad was an airline pilot and, during his training, his instructor told him that, in absolutely ideal conditions, (in practice obviously impossible) that the actual material composing the wing ...
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165 views

The optics of vortex (in water): why there is a bright ring?

This picture is from this YouTube video from "Physics Girl", which shows how you can generate vortices in a swimming pool by using a plate: And there she explains the creation of the shaded circle ...
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64 views

9-point stencil “equivalent” for advection equation

So I inherited from some people a code that solves the advection-diffusion-reaction equation for a particular system. The original code was first implemented in 1D which worked fine in cartesian ...
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53 views

Landau's Ideal fluild one dimentional flow equation

There is a similar Phys.SE question here, but I still didn't get the idea. The problem is: Write down the equations for one-dimensional motion of an ideal fluid in terms of the variables $a$ and ...
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42 views

What is the cause for turbulence?

The transition from laminar flow to turbulent flow is usually told interms of Reynolds numbers. The Reynolds number is $Re=UL/\nu$ where $U$ is the speed, $L$ is dimensional length and $\nu$ is the ...
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No diffusion term in conservation of mass in Navier-Stokes equations?

I have followed derivations of the Navier-Stokes equations and I can see how the various terms arise in the "main equation", the momentum conservation equation. However I don't understand why the ...
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Why and how strong are currents near ship side?

I read reports of people drowned swimming in rivers near medium-sized boats or flat-bottom barges. The idea is that an anchored ship stands still, and the surface current (which goes exactly along it ...