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

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Entropy is constant. How to express this equation in terms of pressure and density?

In hydrodynamics of an ideal, non-compressive flow we use 5 variables: pressure $p$, density $\rho$ and velocity field $\mathbf{v}$. So we need 5 equations. Landau's "Hydrodynamics" states that the ...
2
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1answer
70 views

Vorticity - Paradox

I am having trouble grasping the concept of vorticity. Above is a picture of fluid element under deformation (rotational). Say we ignore the vertical motion, which is $\frac {\partial {v}}{\partial ...
2
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1answer
115 views

Feynman's derivation of Bernoulli (part of it)

OK, so what's up with the $\Delta M$ here? I don't get it. In fact, my textbook doesn't even put it in. I don't think it is a type-o because he continues on with the explanation. If anything I got ...
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2answers
51 views

How does the viscosity (besides density) of a fluid affects sinking

If water, air of whatever fluid had a different viscosity, but the same density, would things fall/sink differently?
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97 views

Explicit form of the entropy production in hydrodynamics

I'm trying to understand how hydrodynamics arise from a precise, mathematical formulation of thermodynamics, learning mostly from Landau's "Hydrodynamics". So Landau starts from formulating the ...
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2answers
75 views

Can we 'see' the Kolmogorv microscale

My understanding of Kolmogorov Microscale is that in a turbulent fluid, smaller eddies tend to be similiar to larger eddies - until you arrive at the microscale. My understanding (that may be wrong) ...
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72 views

Are there any substances that allow sound to travel better then air? [closed]

The question is the same as the one I put in the title but here it is in more words. I was wondering if there were any substances that allowed sound to travel more efficiently. For example water is ...
3
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1answer
91 views

Torque on a rotational cylinder in viscous fluid

I've been stuck on what I'm pretty sure is a simple part of a larger question. It's a cylinder (radius a) spinning in a viscous fluid. It's rotating at rate $\Omega$ .During this question we get that ...
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73 views

What is “hydrostatic uplift pressure”?

I was reading Wikipedia about the St. Francis Dam and came across this sentence. Water that collected in the drainage pipes under the dam to relieve the hydrostatic uplift pressure was carried ...
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49 views

Does the lift created by a wing change when the slats are deployed?

This is a conceptual question as much as an empirical one, but the question is: Does the lift of a wing change when the slats (or any other leading edge device) are deployed? I am stipulating that the ...
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2answers
84 views

Pressure $p$ in the Bernoulli equation

I know the Bernoulli equation that states that the energy along a streamline is constant: $$ \frac{v^{2}}{2}+gh+\frac{p}{\rho}=C $$ However I can't seem to figure out what exactly $p$ is in this ...
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67 views

Movement of compressed air inside a rolling tire?

Does it stay still wrt the axis? Does it rotate along with the tire? Is it drawn away from the center of rotation?
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202 views

Why do turbine engines work?

I know roughly how a turbine engine (let's say a gas turbine producing no jet thrust) is supposed to work: The compressor forces fresh air into a combustion chamber, where it reacts with fuel to ...
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2answers
83 views

What makes laminar cascade break?

Near my house there is a mall that have a cascade, which has a pratically constant flow, and doesn't seem to have perturbations (at least near the edge where water falls), between its two levels. ...
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1answer
108 views

Bernoulli's principle

In steady flow, the air that flows faster causes a lower pressure, the lower pressure causes it to move faster, or exactly both at the same time? Also how is it that we're talking about the pressure ...
3
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2answers
67 views

What would cause a spinning fluid to stop spinning?

I once saw a demonstration where an electric current caused a drop of mercury to spin. The drop contained bits of iron, which could be seen flowing around in a circular pattern. As soon as the ...
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74 views

Deriving Bernoulli's equation via conservation of E

So I'm not OK with how some people derive this equation. These people consider a pipe whose endings have cross-sectional areas and heights which are different. They then use the conservation of ...
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1answer
62 views

Measuring flow rate of gases

Flow rates of liquids could be easily measured with a stopwatch and a known volume container. But gases, if entrapped in a container will create a back pressure, and may lead to decreased flow. I ...
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1answer
121 views

Why is the pressure on top of a slab of water greater than the pressure on the bottom?

In the image below, my book seems to be saying that if you have a cup of water in complete equilibrium, then if you have a small cube of water inside the cup given by $dm=ρAdy$ there is a downward ...
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39 views

Liquid pressure equalization (fluid-dynamics)

Will filling a gas tank fill quicker with a 2 inch inlet flowing straight down into it or with the inlet starting at the bottom filling it from bottom to top. With pressure being equal will the fill ...
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68 views

Wavy stream of liquid

When pouring a liquid into a glass some streams have a wavy shape, like the one in the following photo: (Couldn't find a better picture, sorry.) What causes the stream to be of such a shape?
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1answer
117 views

Determine the maximum capillary rise

The problem I have is this: A capillary tube is immersed vertically in water. Knowing that water starts to evaporate below 2kPa, determine the maximum capillary rise and tube diameter. Assume the ...
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0answers
19 views

Degrees of freedom of particles in Lattice Boltzmann method

Is it true that in Lattice Boltzmann method particles have only one degree of freedom even in 3D case? Can someone explain that fact or provide a link? Thanks!
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96 views

Taylor expansion in Lattice Boltzmann Method derivation

Currently I'm trying to understand Lattice Boltzmann Method for solving CFD problems. In its derivation BGK approximation is used to get rid of complicated collision integral. But when they come to ...
2
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1answer
35 views

Is the viscous sublayer size universal?

We know the non-dimensionalized distance of the viscous sub-layer is 5 wall units for a turbulent boundary layer. It is common to see this in Fluid Mechanics books but seems somewhat arbitrary. I ...
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42 views

Have I calculated the air requirement for a pneumatic mail system correctly?

I googled How to calculate the air requirements of pneumatic conveying and found a clue to what might be a correct equation (http://uk.ask.com/question/pneumatic-conveying-calculation). Here is my ...
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1answer
92 views

Fluid mechanics of aircraft at 10000 m

I have come up with the answers to parts a and b as shown in the image above. However in part c, I can't seem to find the answer, even though it only involves fitting numbers into Bernoulli's ...
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2answers
83 views

Why is a material such as plastic air-tight?

I'm not too familiar with fluid mechanics, but i always wondered how let's say plastic containers would be able to contain a fluid, such as gas or water. How does plastic prevent the molecules of the ...
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1answer
145 views

Why is there no upper-bound on power-efficiency of Hydro-turbines?

We have Betz's law, an upper bound on power efficiency, for wind-turbines. But there is no theoretical upper limit on hydro-turbines, Why this is so? Is it mainly due to the incompressibility of ...
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1answer
39 views

Are Normal Stress Differences in Newtonian Fluids Equal?

I was reading about normal stress differences in the context of Non-Newtonian fluids. There was a remark that the normal stress differences in Non-Newtonian fluids are not equal whereas the normal ...
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1answer
109 views

What is the exact cause of flow separation in a viscous fluid?

I recently got into a lengthy debate about the exact nature of boundary layer separation. In common parlance, we have a tendency to talk about certain geometries as being too "sharp" for a viscous ...
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3answers
93 views

Why does the tea in the middle of the cup spin faster when I stir it?

Try this: Make a cup of tea, add sugar, and stir it. Now add some milk, while it's still spinning. What I noticed is that the milk appears to move around a lot quicker in the middle of the cup of ...
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0answers
52 views

Similarities between laminar-turbulence transition and others like BCS-BEC crossover, quark-hadron transition etc

From my limited readings on fluid dynamics, my understanding is that as the system changes from near-laminar flows to full turbulence, the dimensionless Reynolds number changes from $ R << 1$ to ...
3
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2answers
190 views

Bernoulli Equation and Flow in a pipe - paradox

I am studying Bernoulli equation and am facing a problem. The Bernoulli equation is applicable along a streamline and in steady flow condition (I guess this condition is to make sure it can be applied ...
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2answers
197 views

Is there an equation to calculate the average speed of liquid molecules?

I seem to remember from first year physics that we can calculate the RMS speed of a stationary, ideal gas with $v=\sqrt{\frac{3RT}{M}}$. Does a similar equation exist for liquids?
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65 views

What causes the 'tears' in yoghurt drinks?

Fluid dynamical instabilities are present in many different everyday things. The famous tears in wine for example are a classical example of a Marangoni effect, where surface tension gradients due to ...
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45 views

When rotating a liquid-filled jar what determines how much force goes to spinning the specimen floating inside?

Another way of stating it: When you have leaves in a (cylindrical) tea cup, if you rotate the cup (eg clockwise), the leaves largely stay in place relative to the table, but spin clockwise a little. ...
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1answer
65 views

Could you blow up a barrel (closed container) using a straw and water?

Imagine you fill up a barrel with water (or any other liquid) dig a small hole at the top and place a sufficiently large straw in it, so that there is no pressure loss or anything. Then you start to ...
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3answers
56 views

Upward bouyant forces?

When an object is submerged in a fluid (e.g., water), there is a pressure on the object due to weight of water by p=mgh, but why is there an upward buoyant force?
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40 views

An equivalent for the Bernoulli equation for viscous liquids

I was wondering whether there is any equation for viscous liquids (probably derived from energy and mass conservation principles) relating the pressure, rate of volume flow, area of cross section and ...
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23 views

Snowmaking in the tropics - an estimate of water evaporation

If I set up a snowmaker in the tropics and sprayed water with it how much water would I evaporate? How would I calculate?
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45 views

Stop Motion 3D model of airflow over an airfoil

I am looking for a 3D stop motion image of air represented as molecules over a wing. Does anyone know where I can find an image that I can build a model from?
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2answers
203 views

Analogy between fluid dynamics and electromagnetism

I've been reading a set of notes (Chapter 13 of Caltech Ph 136 notes from 2004, by Blandford and Thorne) that draw an analogy between fluid dynamics and electromagnetism, identifying the magnetic ...
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1answer
85 views

How to calculate differential pressure

A ventilator fan mounted on the wall of an air tight room(except for an air intake with a given cross sectional area) is blowing air out of the room. If air displacement rate of the fan is given(CFM ...
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0answers
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Kolmogorov/Energy spectrum for turbulent boundary layer

Previously, I have calculated energy spectrum for 3D isotropic turbulent flow data which is equally spaced in all three directions and then to compute the energy spectrum, one performs Fourier ...
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8 views

Poseuille number for flat channel?

For a flat channel with fully developed flow, the Poseuille number when using the hydraulic diameter is $$Po=C_fRe_{D_{H}}=24$$ But when using 2b as the length scale, the Poseuille number is ...
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1answer
75 views

Pressure in Fluids,in particular horizontal pressure

I am not sure about the pressure existing in fluids.If the vertical pressure acts due to gravity and the atmospheric pressure since it also changes due to height also acts due to gravity then why is ...
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3answers
131 views

How come water in a cup won't shoot out of a straw placed inside it?

I've been learning about pressure and fluid dynamics, and I've stumbled onto a problem. Say you have a straw in a cup. The surface area of the water in the cup is much greater than that of the straw. ...
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3answers
86 views

Helium and its unique behaviour

It is said that helium is lighter than air. What does that exactly mean ? If it is lighter than air,then there should be absolutely no trace of helium in earth's atmosphere(it should have flown into ...
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467 views

Why can, or can not, a perfectly incompressible fluid exist?

Water is normally assumed to be an incompressible fluid - for example in the context of calculations involving water pressure. I wondered whether that is strictly true, or an approximation? Later I ...