Questions tagged [fluid-dynamics]

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

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Coupling Navier-Stokes and stochastic models for particle tracking in micro-scale free convection?

I have been using a commercially available software to simulate laminar free convection in a specific small domain (let's use channel w/ heated lower wall as an example). The scale is approx 50-100 ...
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Surface fluctuations in the combustion of ethanol in a woosh bottle

A whoosh bottle demonstration typically involves igniting ethanol vapor in a large bottle with a narrow opening. The rapidly expanding gas creates the whooshing noise as it vacates the container. This ...
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Indicator of the acoustic level in near field

Usually in acoustics, it is assumed that what we hear is the acoustic pressure. In far field - when the distance to the source is larger than the size of the source and than the wavelength - one has $...
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1answer
381 views

How can I derive the stress tensor for a Newtonian fluid in more physical terms?

The question is quite fundamental and more on a beginner's level (not sure if good in this high-level-forum, but I try): I have big problems in understanding the stress tensor for Newtonian fluids in ...
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535 views

Number of collisions per second a water molecule is experiencing in the fluid phase at ambient conditions

I am teaching a DNA course for lay people and I want to give them an indication of the number of collision per second a water molecule experiences in pure water. Similarily, I would like to know the ...
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Water vapour behaviour during hypersonic expansion

I have a question about the behaviour of air with a small mass fraction of water vapour at extremely low pressures and temperatures. I am currently running experiments in a large vacuum facility that ...
3
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1answer
76 views

Weyssenhoff fluid and Frenkel condition

A Weyssenhoff fluid is a continuos fluid with spin. The spin is described by an antisimmetric tensor $s{_{ab}}=s{_{[ab]}}$ satisfying the Frenkel condition \begin{equation} s{_{ab}}u{^b}=0 \end{...
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Two phase mixing by virtue of high Reynolds number alone

Consider a long horizontal tube into which I inject two immiscible fluids. Say, water and benzene. If one increases the pumping velocity i.e. flowrate / Reynolds number would one get intimate mixing ...
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2answers
287 views

How do we find force to derive $pV=Nmv^2$?

Since momentum is not changing in time t , but instantaneously in very small time at wall B. Why do we write $F=\frac{2mv}{t}$? Where $t$ is (twice the distance AB)/ velocity. Why don't we take the ...
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137 views

Time scale turbulence flow -distance between two particles

I want to estimate the time interval, $\tau$, during which the original distance $L$ between two fluid particles become much larger. I assume that the motion takes place in the inertial interval. I ...
3
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1answer
356 views

Why is the total force at a free surface zero?

I am looking into waves on a free surface for which their are two main conditions: Kinematic condition: Particles on the surface remain on the surface. Dynamic condition: Forces acting on the surface ...
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86 views

How are boundary consitions implemented correctly in time dependent hydrodynamics?

I posted this question more than one year ago and got an answer recently. This answer looks good to me, but indicates that something is wrong in my original approach to the problem. Can someone tell ...
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Why would oscillation of a gas column inhibit rather than promote mixing?

Many years ago I helped to support an experiment conducted in Japan which investigated the effects of high frequency oscillation ventilation (HFOV) on the mixing and distribution of gas into the lungs....
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A question about droplets formation

This picture shows a fuel ejector of a diesel engine that propels ships. When the fuel line pressure exceeds 380 bars, the tension of the spring is overcomed and fuel is ejected into the combustion ...
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475 views

Backing pressure for a turbo-molecular pump

It is known that turbo-molecular pumps are to be operated at already low levels of pressure. So the turbo is always backed by another pump (usually a scroll, rotary pump) that ensures this. One should ...
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130 views

How does air going through silica gel change its humidity

Assume that the properties(temperature,humidity,velocity and so on) of the incoming air flow stay constant. The air flow goes through a silica gel plate. Humidity of the air will decrease, but how to ...
3
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1answer
84 views

Could a hydrophobic surface increase a liquid's resistance to compression/displacement?

Imagine a quantity of an aqueous (yet slightly viscous) solution is resting on a hydrophobic surface with a contact angle around 100°. A downward force is then applied as a (repellant) surface is ...
3
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1answer
75 views

Comparing two versions of the same hydrodynamic code and their error

I have two versions of a hydrodynamic code that has the same underlying physics; let's call them code A and B. However, code B is more optimized and more object oriented than A. I was trying to ...
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2answers
960 views

What is the total velocity/force on a particle in fluid?

I am now facing a fluid-particle interaction problem. I would like to simulate particle motion in a fluid. I do know the external force acting on a particle (dielectrophoretic force in this case) ...
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551 views

Confused on how to apply the volume integral form of the Biot-Savart law

In Viscous evolution of point vortex equilibria (Jing, 2011) the author explains that the velocity equation for the Lamb-Oseen vortex is found by applying the 2D Biot-Savart law to the vorticity ...
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2answers
1k views

Why is the solution to the Blasius boundary layer problem self-similar?

In every course or textbook that I encountered so far, the authors transform the Navier-Stokes equations of the Blasius boundary layer problem into the Blasius ODE. The problem with many of those ...
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92 views

Bulk and dynamic viscosity in the atmosphere

I'm studying the physics of the atmosphere but I'm struggling with the matter of viscosity (Navier-Stokes equation) for gravito-acoustic waves. From Landau-Lifschitz : $$ (T)_{ij} = -p\delta_{ij} + \...
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224 views

What is the relationship between the angular speed and the diameter of the eye of a water vortex?

In a water vortex formed in a plastic bottle, what is the relationship between the angular speed of the water and the diameter of the hole in the cap? I would have expected that according to the ...
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2answers
9k views

How to calculate the velocity of fluid at the outlet when density and the pressure drop are known?

I have a U- like pipe. Its inlet has atmospheric pressure $p_o=10^{5} \, Pa$. Vacuum is applied to the other end with a pressure gradient $\nabla p_v=-30 \cdot 10^{3} \, kPa/s$. The total time of the ...
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Modeling the formation of a stellar system and matter accretion

I am trying to figure out what do I need to know to properly simulate the creation of a solar system from a particle cloud with random distribution of hydrogen atoms. Being more of a programming ...
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349 views

Conformal symmetry of Navier-Stokes?

This question is in reference to the paper arXiv:0810.1545 Can someone help understand this scaling argument and the proof(?) that there is a conformal symmetry in Navier-Stoke's equation? (..am I ...
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168 views

How do I simulate a constant velocity flow in porous media

I am modelling gas combustion in porous media. Most contemporary models assume that the pressure drop from the porous media is small enough to disregard, but I want to include that in my investigation....
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2k views

What's the meaning of a pressure coefficient in fluid mechanics?

I have a jet of air leaving a contraction from a pressurized tank at a velocity of 28.8 m/s and it's being directed towards a small object equipped with static pressure holes on its surface that give ...
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304 views

Flow behind a plate causes pressure drop?

I've done an experiment where we direct an air jet from a fan onto a set of electronic scales. By measuring the velocity of the air jet at the exit of the fan nozzle, and applying the conservation of ...
3
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2answers
118 views

Concept regarding Ventury Tube-Bernoulli application

I was recently studying applications of Bernoulli Equation and came across the Ventury tube. This is diagram I have used to analyse the venturimeter. I understand how we obtain the first equation ...
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1answer
642 views

Worthington jets explanation: fluid phenomenon

I don't understand the reason behind the formation of Worthington jets I've been reading a bit about Worthington jets Video 1, this phenomenon is caused when something is thrown to the water as we ...
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1answer
122 views

Can convection cells evolve in stably stratified fluid?

Assume stably stratified fluid but not in equilibrium, e.g. with non-constant temperature gradient for example. Can convection cells be present? Typical example of convection cells is Rayleigh–Bénard ...
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1answer
638 views

Explaining the shape of a raindrop and a drop of mercury

I saw on wikipedia that the shape of a raindrop is explained by using Laplace pressure. But why? Since the drop is in motion, we shouldn't be able to use an hydrostatic law, am I wrong? The ...
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1answer
41 views

How does air flow within a melodica when two key are pressed?

I'm trying to understand the air flow within a melodica. A melodica is a wind instrument that has a piano-like keyboard. Pressing on a key opens an airway so that air entering the melodica's air ...
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2answers
96 views

Why do fluids not accelerate?

A fluid flowing in a horizontal pipe must be flowing at a constant velocity because of the conservation of mass. However, considering how there would be a pressure and hence force acting behind the ...
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70 views

Why does poured honey move periodically when reaching the surface it's poured upon?

Here's the picture, known to most of us: When honey is poured out of a container, as in the picture (look here for the flow in motion), we see a kind of periodic flow of the honey when it comes to a ...
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34 views

Simple analytical model for fluid flow in “Mushroom cloud”

In potential flow theory there are simple analytical models (formulas) for velocity-field of elementary features (like source, sink, dipole, vortex etc.) Is it possible to write simple analytical ...
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4answers
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How to quantify evaporative cooling?

Water has a latent heat of vaporization equal to $2,260\ \mathrm{kJ/kg}$. While ethyl alcohol has a latent heat of vaporization equal to $846\ \mathrm{kJ/kg}$. Even though ethanol's latent heat is ...
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1answer
68 views

Is it possible to derive the equations of motion from the energy of a system alone, without knowing canonical coordinates or the Lagrangian?

Is it possible to derive the equations of motion from the energy of a system alone, without knowing canonical coordinates or the Lagrangian? I'm confused about which parts of the fundamental ...
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3answers
45 views

Is there an equation depicting a compressible fluid flowing into a cone, increasing in pressure and velocity as the cone narrows?

I'm trying to write an equation which shows a compressible fluid entering a cone with radius R and length L, where the pressure (density) and velocity of the compressible fluid increases as the exit ...
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1answer
63 views

Can I use Pascal's principle to destroy any airtight container?

I am looking at the following textbook problem (please note that I am not asking for a solution to the problem, only using it as a motivating example): A host pours the remnants of several bottles of ...
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Lubrication Theory Question

I am consideringflow between a rigid cylinder parallel to a rigid horizontal stationary surface. Assuming that we can apply the lubrication limit (so that the cylinder is very close to the surface) I ...
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63 views

Normal Viscous Stress (Newtonian Incompressible Flow)

I have been struggling to understand why normal viscous stresses are often neglected in elementary fluid mechanics analysis (see examples below). Usually, the justification is something in the lines ...
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3answers
68 views

how does bouyance work on a molecular scale (gas or liquid phases), or doesn't it?

a balloon filled with hottor water submerged in colder water (more dense) will rise due to bouyancy. But when we remove the balloon's wall why is there still bouyancy. I would think that the molecules ...
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Analytical Solution of the Sod Problem with Different Gammas?

Is there a known analytical solution to the Sod Shock Tube problem where the two gases have different specific heat ratios $\gamma$? I have only been able to find solutions where the $\gamma$ values ...
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Physical Interpretation of the Equation of Motion

Ocean dynamics can often be described by the equation of motion $$\frac{D\vec{u}}{Dt}=-\nabla\Phi-\frac{1}{\rho}\nabla p-\vec{f}\times\vec{u}.$$ I am searching for a physical meaning of this equation, ...
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Are dipole interactions considered short range in 2d

Every paper that I read about dipole-dipole interactions always call them long-range interactions. Is dipole-dipole interactions can be considered short-range in 2D? As we know it behaves as $\...
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Effective viscosity in bubbly liquid

Motivation: I have screwed some crepes big time this morning and it made me think... Consider a bubbly liquid (such as a crepes batter if you whisk it too much). Is there an approximate measure of ...
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1answer
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Adjoint operators and the method of variation for the Orr-Sommerfield problem

My question relates to the transient Orr-Sommerfield Squire problem, described on page 20 of the thesis by Eaves. Here I briefly describe the context of my question. We consider an infinitesimal ...
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Vertical spike like water drop

Slow motion guys created this Energy Spike ( at 1 min 53 sec ) that looks like a massive water drop or like a negative vortex in the Edinburgh Hydraulics laboratory. Using momentum of radial inward ...

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