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

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Normal Modes for Standing Waves in 1-D Acoustic Ducts with Arbitrary (but real) Impedance Jumps

Let's say we have a 1-D duct, such as this: Where $Z_i \equiv \frac{P}{US}$ is the acoustic impedance, L is the length of the duct in question, and S is the area of the cross-section. In general, ...
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33 views

Identity in continuum mechanics [closed]

For a problem in the textbook I am reading, I need to prove that $$\int_Vw_{i,j}v_jdV = \int_Sw_iv_jn_jdS,$$ where $S$ is the boundary of the volume $V$, $v_i$ is the velocity vector field of a ...
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3k views

How much lift does an airplane get from its wings, vs the rest of the airframe?

Consider a big commercial airliner, like a 727, 747, or a 787. At cruising altitude, under standard conditions, how much of the lift of the aircraft comes from the wings, and how much from the rest ...
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838 views

Integrating pressure over a surface

Consider the 2D airfoil below. In engineering (and maybe physics) you will often see something like the following as an expression for the pressure force acting on a surface (in this case a curve ...
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1answer
42 views

vorticity not zero - why

I am quite new to fluid dynamics and I cannot seem to understand the concept of vorticity. It is defined as $\nabla\times\vec{v}$ where $\vec{v}$ is the velocity vector. Now (working in cartesian ...
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19 views

Standing waves and acoustic levitation

i am doing a new personnal project about acoustic levitation with standing waves into ambiant air using a piezo ultrasonic tranceiver and a reflective pannel. I modelised both of the emission and the ...
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3answers
56 views

Calculate acceleration based on differential pressures

In my example, water is in a tube and is being accelerated by a compressed air. I already know that $P = \frac{F}{A}$ so if you know the pressure and you know the surface area its acting on, you can ...
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1answer
36 views

Water coming out of a vertical tube lifts a weight to a certain height [closed]

Before I even start the question I would like to apologize in advance for some grammar mistakes since I translated this question from another language. Water is coming out of a vertical tube at a ...
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268 views

Why are lubicants like grease or oil slippery?

I know that grease or oil are slippery in nature but never thought of why? I want to know what causes this tendency.
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23 views

Why the meandering ratio of river different from different study?

Referring to this question Is the meander ratio of a river $=π$? According to the accepted answer, based on a paper by Leopold and Langbein The resulting curve is not quite a semi-circular ...
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2answers
21 views

Would a hybrid mixed medium balloon allow you to use less helium to lift equivalent weight?

Suppose you had two balloons, one inside the other (a inside b). Would filling balloon "a" with the 80% nitrogen/20% oxygen mix you find in the surrounding air at ground level, and then filling ...
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40 views

Example of basic(?) fluid dynamics [closed]

I am completely new to the world of fluid dynamics, I have tried to come up with what I hope will be a simple case. How does one go about calculating the following? I am far more interested in the ...
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1answer
34 views

What causes fizz?

When you go to a soda fountain and pour some soda into your cup, frothy fizz is generated at the top of the cup. What causes this? Obviously has something to do with carbonation, but when my cup is ...
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24 views

Resonant frequency(s) of system of resonant objects?

I am trying to gain a better understanding of resonance when we are dealing with a coupled system of resonating objects. For example, say we have a single gas bubble in liquid and suppose it ...
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0answers
10 views

Given these equations how would I go about finding the velocity potential? [closed]

An organ pipe of length $L$ is closed at $x=L$. the pressure at $x=0$ is made to vary according to the law $\rho_1 = \rho_0 \sin(nt)$. I just wanted to know what law $\rho_1 = \rho_0 \sin(nt)$ means, ...
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1answer
28 views

External v.s. internal Reynolds number for cylindrical pipe

When calculating Reynolds number for cylindrical pipe, we were taught: $Re=vd/\nu$, where $v$ is the fluid velocity, $d$ is diameter, and $\nu$ is kinematic velocity. If you reorient the cylinder so ...
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1answer
34 views

How do I find a stream function given a volumetric flow rate?

How do I find a stream function given a volumetric flow rate? The flow only occurs in one direction, between 2 plates, and I have no knowledge of velocity. I know that volumetric flow rate = ...
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1answer
551 views

Why do bowlers rub the ball before bowling in a cricket match?

Why do bowlers rub the ball before bowling in a cricket match? What is the science behind it? Is it because of friction?
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1answer
222 views

Rate of evaporation of water with mass loss at set temperature and pressure

I have a cylindrical tank with an 'm' amount of water in it. The tank is sealed except through an exhaust with a fan that keeps a lower than atmospheric pressure (but not vacuum) inside the tank. The ...
4
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1answer
53 views

Why does wind sound the way it does?

You can obviously hear wind. From everyday experience, the stronger the wind, the louder its sound. But this sound is always similar, and quite distinctive, noise-like. Can one compute the spectrum ...
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2answers
86 views

How is temperature and pressure related to when a tire rolls on the ground?

Actual observation: Here in Canada it is known that even when 'dry' snow is on the ground, and when a car drives over it that small amount of dry snow instantly becomes ice (NOT GOOD). Anyways as an ...
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21 views

Pulse wave velocity

I heard that the pulse wave velocity is higher than the speed of blood flow through the arteries. This is obviously due to the ellastic properties of vessel walls. I would like to have the most ...
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1answer
46 views

Derivation of Kelvin's circulation theorem

In the derivation of Kelvin's circulation theorem, I take the material derivative of circulation, or \begin{align} \dfrac{D\Gamma}{Dt} = \dfrac{D}{Dt}\oint_C \vec{u} \cdot d\vec{\ell}. \end{align} ...
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1answer
30 views

What is the force exerted by a fluid on the corner of a right-angled tube? [closed]

It is to be considered that the water is entering and leaving at the same velocity v. I have found a solution but cannot understand it as it is using a term dM/dt which I cannot understand in this ...
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2answers
94 views

Is surface tension characteristic of a fluid?

Does surface tension depend only upon the nature of the fluid or on other factors too like the liquid-solid or liquid-gas interface? I saw in my book definition that $$S = U/A,$$ where $U$ is the ...
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1answer
42 views

Reconciling the Temperature and Pressure Relationship in Gasses vs Fluids

I've been trying to wrap my mind around this for the last couple of days to no avail. I suspect that much of my confusion comes from the fact that I have not yet taken a class specifically focused on ...
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55 views

Flow in a cylindrical pipe

A cylindrical pipe is, at the left of section A, divided by a plane passing through its axis. The upper half is supplied with water at a velocity $V_1$, the lower half is supplied with water at a ...
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1answer
38 views

Entropy in ideal fluids [closed]

Can you derive a conservation law for entropy per unit mass? I know that there is a proof for this conservation. My assumptions are: $(\nabla p)/n = D \vec{v} + \vec{v} \cdot \nabla\vec{v}$ (Euler ...
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1answer
2k views

Paper plane between two fans - is this possible?

The setup: two fans facing each other, distance around 1m. Both are turned on. In between them, place a simple paper plane and according to this video, it will fly. ...
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2answers
36 views

How many water wheel can fit on a river until another one produces zero energy?

Mostly what I'm after are the factors I need to know to do the problem. Imagine we're in the days of Leonardo Da Vinci. And pizza is everyone's favorite food. And there's a river that goes on ...
0
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1answer
41 views

Would a Tesla Valve create a pressure differential?

Start with two sealed boxes that are the same in every way. Then connect them by a tesla valve so that air could flow between them. Over time through entropy would the one-way preferential flow of ...
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1answer
30 views

reynold's transport theorom contradicting bernoulli equation

I am considering a funnel with two areas $A_1$ and $A_2$ with velocity at the interface as $V_1$ and $V_2$ respectively. let pressures at those areas be $P_1$ and $P_2$ . For simplification we assume ...
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36 views

Total energy of the fluid

Consider a one dimensional system with fluid in it. Mass and momentum balance equation of the system are (in the absence of external forces and assuming Newtonian behaviour valid for viscocity), ...
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101 views

If I Blow smoke out my window, will any come inside my room?

My girlfriend and I have got into a lively debate about whether or not my flatmates can smell the smoke I blow out my window. Can you help us? Here is the scenario. The smoker, places himself ...
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29 views

What is the definition of governing equation? [closed]

What is the exact definition of the governing equation? What is its purpose? What makes it different than other equations of physics? Why they are necessary?
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24 views

physical constraints on fluid models based on shannon entropy/information

Is it possible to use the idea of entropy to constrain mathematical objects meant to describe physical fluid motion which is virtually incompressible and virtually without friction. More precisely ...
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20 views

Drag in Free Fall [duplicate]

I was recently thinking about how I might go about describing projectile motion with air reistance. The problem I encountered was that air resistance might cause some acceleration for a short period ...
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52 views

Mathematical understanding of vortex solitons

I am wondering if anyone has ever come up with a mathematical description of something that (to me, and I am no expert) look like soliton vortexes. The example I can think of is if you create two ...
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1answer
24 views

Relativistic blast waves in medium of non uniform density

The relativistic outflow of a gamma ray burst drives a shock into the circumburst medium. This medium could have a uniform density $n_0$ or a density profile of the form, $n(r) \propto r^{-2}$. For ...
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90 views

What is meant by “surge” in fluid mechanics?

When reading about the action of opening a volume at vacuum submerged in a fluid at pressure, the fluid is said to "surge", i.e. to fill the void space (the vacuum). Per Google, surge is defined as a ...
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1answer
29 views

Diffusion (2 species) versus advection (1 species)

Why do we see diffusion with 2 species but advection with a single species if both cases involve molecules spreading to increase entropy? To take a physical example: Let's say I have a box with two ...
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34 views

What is the use of thermal boundary layer and velocity boundary layer?

In heat transfer by convection, why must we show the thermal boundary layer and the velocity boundary layer. What do they signify? Also how are these terms related to the Prandtl number? And what is ...
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2answers
1k views

Why bodies lose heat faster in water than in air?

A simple question. In several places, including this very recent and already famous paper http://www.sciencemag.org/content/348/6236/786.short it is stated that bodies lose heat in water very fast ...
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12 views

How does submerging tubing affect 'airflow' when the tube is filled with water also?

The question probably isn't asked well and I feel sort of stupid asking it but... So I have a small piece of tubing connecting two chambers (where both the chambers are partly filled with water and ...
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0answers
23 views

What is the “flow area” for a screen/mesh? [closed]

I'm trying to find out, for a given duct area, how large an area of woven mesh (like bug screen) do I need for minimal pressure drop? Is there a rule of thumb or approximation for "effective flow ...
6
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2answers
155 views

Why do stellar boundaries exist?

What theorems or insights are available to decide wether a star with some given interdependence between its density, pressure and temperature distributions ought to have a boundary at a finite ...
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3answers
75 views

Can a submarine be powered by a wind turbine?

This is not intended as a realistic way for propulsion of submarines, it is just an interesting thought experiment. Say I want to travel directly towards the wind, only powered by the wind. This is ...
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31 views

Is this fluid-dynamics problem complete?

A wind tunnel test must be conducted to study an aircraft prototype with the following specs: $$ V_{prototype}=300\frac{Km}{h} $$ $$ \rho_{prototype}=1\frac{Kg}{m^3} $$ $$ MaxScale=1:5 $$ $$ ...
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1answer
21 views

Does moving air suck in air from left and right of its path? [closed]

When air is made to move (e.g. by a fan) it has a lower pressure than when it is still. Essentially, this moving air should become a low pressure zone relative to the static air around it and the ...
12
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1answer
325 views

Could Navier-Stokes equation be derived directly from Boltzmann equation?

I know how to derive Navier-Stokes equations from Boltzmann equation in case where bulk and viscosity coefficients are set to zero. I need only multiply it on momentum and to integrate it over ...