# Tagged Questions

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

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### Stationary waves on a laminar liquid flow near collision with a surface [duplicate]

Look at the picture below I know about plateau rayleigh instability. And my opinion is that the reason of this phenomena is plateau rayleigh instability. But there's a question, in the picture ...
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### How do hydrofoils tack without a keel

I asked a while back about how sails produce a forward force on a boat and got some good answers and (I thought) a good understanding: essentially the sail produces a force that has some non-zero ...
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### What is the difference between flow and expansion?

Fluids (both liquids and gases) will move from one point in space to another due to a potential gradient. Some examples may be: 1) horizontal pipe flow, a fluid will move from a region of high ...
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### Question Regarding torricelli's theorem/Law

I recently studied about bernoulli's equation/principle. After the derivation of the said equation , my book gave some applications of the principle, which include torricelli's theorem/law. In ...
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### Can solids behave like liquids?

Basically, the more I look at things the more it seems like there's entirely no difference between a liquid and gas, it all just depends on the relative density of what's around it. This being said, ...
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### conservation of energy for a viscous fluid

I have a pair of questions about conservation of energy and momentum in a viscous fluid. First, energy. Symon's Mechanics states an equation for conservation of energy in a droplet of a non-viscous ...
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### volumetric flow rate due to Bernoulli's principle

for calculating the volumetric flow rate of a liquid coming out of a tube, I know that If the pipe is too short, the Hagen–Poiseuille equation may result in unphysically high flow rates; the flow is ...
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### Which travels further, a football or a soccer ball, when thrown?

I recently answered this question: Laminar Versus Turbulent Flow and it caused me to think of the following situation. Given that an (American) football weighs between Football 400 to 430 grams, and ...
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In scientific literature, we know that the condition for incompressible flow is that the particular derivative of the fluid density $\rho$ is zero: $$\frac{D\rho}{Dt}=0$$ and for steady flows the ...
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### Why is temperature a function of $y$ and $t$ only?

Say you have an incompressible thermal conducting fluid contained between two infinite horizontal plates separated by a distance $H$. Initially both the plates and the fluid are at rest at ...
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### Why small droplet goes upward after pinch-off?

This small droplet moves against gravity. How to calculate its initial upward velocity exactly after pinch-off?
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### total vorticity generation in 3D

In 2D flows, the total vorticity generation for an impulsively started plate can be found by measuring the circulation of the flow. This gives the vorticity generated per unit length. I would like ...
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### Why are racing catamarans' hulls not teardrop shaped? [closed]

Looking at the America's Cup 72 class catamarans, I was wondering about their hull shape, which has a sharp wave piercing bow. My question is: since an airfoil shape experiences less drag than a sharp ...
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### Shaken vs stirred, is there a difference? [closed]

I was looking up tequila Drink recipes for the up coming holiday, and I kept seeing ones calling to be shook and other to be stirred. So I wondered: is there really a difference? does stirring provide ...
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### Height of Water 'Splashing'

Suppose from a height $H$, I throw a ball of mass $M$ and radius $R$ with initial velocity $u$ into a pool of depth $x$ having a liquid with density $\rho$ and coefficient of viscosity $\eta$. ...
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### If the surface tension is reduced to half(say by using a surfactant), what would be the effect on the size of an air bubble in the liquid

If I reduce the surface tension to half of the original value, what would be the effect on the size of an air bubble in the fluid? What would be the effect on buoyancy and drag forces? Also, if I ...
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### Find the separation distance for a line of oil being squashed between two flat plates [closed]

I was wondering if someone could give me some help on how to start this problem, I'm really struggling to get my head around it. A long line of oil is being squashed between two flat plates of length ...
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### Efficiency of a gravity feed hose

If I am trying to fill a large cylindrical container with a gravity feed hose, would it be more efficient, time-wise, to drill a hole in the bottom of the container for a greater initial pressure ...
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### Why does a sheet of paper fall slower than the same when rolled into a ball?

It depends on volume but both the sheet and the sphere have the same volume. Does surface area play any role here?
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### Show that the boundary layers diffuse out from the plate with speed $\sqrt{\frac{\nu}{t}}$ [closed]

I was wondering if somebody would be able to help me with this problem. I know how to solve it using dimension arguments but I'm unsure what is meant by transformation techniques. Any help would be ...
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### Why can't it work? Perpetual motion machine [duplicate]

Lets assume that the red ball is more dense than the white fluid and less dense than the blue fluid. The fluids are of very low viscosity and the machine can be of any height. The flaps open when ...
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### How deep can a whirlpool descend?

My question is based around comparing the physics of a tornado against, what I imagine/assume to be the marine equivalent of a tornado, that is a whirlpool located either at sea, in a tidal region or ...
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### How to make two equations dimensionless? [closed]

I recently got set this problem and am having trouble scaling the resulting equations. Any help would be appreciated. An incompressible thermal conducting fluid is contained between two infinite ...
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### Measuring Steam Flow with Venturi Effect?

I understand steam is a compressible fluid and that the Bernoulli effect has been generalised for compressible fluids but I'm struggling understand how the Venturi effect can be generalised to allow ...
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### How do i calculate the amount of fluid flowing through a tube?

If I have a tube with an internal diameter of 3.5mm in inside diameter and 30cm in length with a viscosity of 1.77cp, and a pressure of 200kpa, how fast will the liquid flow through? It the tube is in ...
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### Flow between two infinite plates [closed]

I recently got set this problem and I was wondering if anyone would be able to give me some hints/intuition on how to solve it. Thanks. An incompressible thermal conducting fluid is contained between ...
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### Work done by flow of a fluid

I'm looking at the power a flow of steam in a turbine. I originally assumed that the kinetic energy of a moving section of steam was equal to (let $V$ be volume, $\rho$ be density, and $v$ be velocity ...
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### Why do not the ripples on a pond form a 3 dimensional wave?

I can see that that the expansion of the the circles happens in a two dimensional plane. Therefore, we say that a two dimensional wave is formed. But, why cannot we count the vertical disturbance so ...
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### Pouring oil on choppy water to calm it , does it work and if so how?

Near where I live, local fishermen often bring cans of castor oil with them, to calm the water around their boats, if they feel bad weather is due. They claim this method of sea calming works, (...
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### Aerodynamic noise created by cooling fan

What is the reason behind the noise created by fan blades while moving? Some paper says that it is because of the Kármán vortex street but it can be formed in static bodies only. Most of the ...
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### The pressure in a container of water is based on depth. So what happens if I remove the bottom of the container?

So I understand that if we have a system that involves a container of water the pressure will equal atmospheric pressure at the top and as we go further down the container the pressure will increase ...
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### Would opening a car door at high speed change the path of the car? [closed]

If I were to open a standard, four door, four wheel, car on the highway, would the door swing open and possibly tear off? Would the drag that it creates 'reroute' the car (turn it to the direction of ...
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### Under what conditions is air is in chemical equilibrium?

I am considering the flow field around a hypersonic glide vehicle at $M=7$. As I understand it, air is in chemical equilibrium if the fluid time scale $t_f \ll t_c$, where $t_c$ is the chemical time ...
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### Anti-centrifugal force? [duplicate]

I put some balls (denser than water) in a bucket. I took a stirring rod, and swirled the water in one direction. The water became low in the middle, and high along the edges (centrifugal force). But ...
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### Can I move an oil tanker?

Let's say that I'm standing at the edge of a basin made of concrete. This basin is filled with salt water and a ship floats on the water close to the edge of the basin. The ship is neither anchored ...
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### Why is raindrop spherical in shape? [duplicate]

As the topic suggests why the raindrop is spherical in shape? Why it is not triangular or bipyramidal or tetrahedral? Is centre of mass or density of water related to it?
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### 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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### Conservation of Mometum in integral form

Water is pumped from one reservoir into a another. The pipe expands after a lenght of $L/2$ to a diameter of $d$. Given: $\dot{V}_p, \rho_w, d, \eta, L, p_p$ The previos tasks allowed to solve ...
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### Why does a continuous water stream form ripples when colliding with a surface? [duplicate]

I was in the shower one day washing my hair, and noticed that the water falling off my hair formed perfect streams that ceased to break into droplets for quiet some distance (probably about 5-6 cm). ...
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### What is the physical fundamentals of Pascal's law

Pascal's law or the principle of transmission of fluid-pressure (also Pascal's Principle) is a principle in fluid mechanics that states that pressure exerted anywhere in a confined incompressible ...
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### Vacuum pressure equivalent in a siphon hose filled with water?

So, I was thinking about how siphons worked today, and no matter how high or low the tube goes, as long as the end is lower than the start, water will flow. So, if you took a hose, ran it up 50 feet,...
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### Adding fluctuations to a hydrodynamic simulation to trigger instabilities

I'm doing a 2D combustion hydrodynamic simulation and there's a hydrodynamic instability that should be triggered because of the particular physical properties of my system. The key to the instability ...
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### Good book for learning fluid statistics

I'm currently using Gray and Gubbins Theory of Molecular Fluids to learn about the statistical physics of fluids. It may be a fine reference text, but I'm not impressed with it as an introduction to ...
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### On deriving a fluid flow pressure drop model using either physical theory or empirical methods

I'm trying to derive a model from physical principles of fluid flow, a simple quadratic model that tends to well fit actual pressure-flow data. For (real) subsonic air flow (ideally adiabatic) ...
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1.A turbine is rated at 650 hp when the flow of water through it is 0.85 m3/sec. Assuming an efficiency of 84%, what is the head acting on the turbine.
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### Euler equation: momentum conservation in terms of enthalpy?

So I stumbled upon this equation: $\frac{\partial h v}{\partial t}= - \nabla(hv \cdot v)- \nabla P$ where $h$ is enthalpy, $v$ is fluid velocity, $t$ time, and $P$ pressure. It seems to have the ...
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### What does it mean for a physical quantity if its mixed second partial derivatives are not equal?

This goes for every problem (either in electromagnetism or fluid dynamics) that has to do with vector fields. Say we have a fluid flowing in a closed circular pipe (or an electromagnetic field, the ...
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### Shape of water jet

Suppose you are shooting a water jet in a vacuum vertically onto a flat plate in a gravity-less room. The jet makes a right angle to the plate and water is assumed to be incompressible. The mass ...
Consider a fluid over a sheet which is placed horizontally along the x-axis. The lower face of the sheet is in contact with another fluid at temperature $T_f$ ( it is heating the sheet). The sheet is ...