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

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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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1k views

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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2answers
146 views

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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1answer
306 views

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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273 views

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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77 views

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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47 views

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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38 views

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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65 views

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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196 views

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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136 views

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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60 views

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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450 views

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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209 views

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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1answer
81 views

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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37 views

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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2k views

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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2answers
728 views

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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150 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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52 views

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 ...
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2answers
318 views

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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181 views

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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196 views

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 ...
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1answer
27 views

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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82 views

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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78 views

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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43 views

Thermodynamics about turbines

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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176 views

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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110 views

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 ...
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1answer
604 views

Convective boundary condition

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 ...
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528 views

How does the dissolution of salt affect the solution density?

Suppose you have a container of water as a solvent and you a certain amount of salt as a solute sitting at the bottom of the container that has yet to start dissolving. Supposing temperature and ...
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Is a convergent-divergent nozzle a perpetual motion machine of the second kind?

As answered in this question, a convergent-divergent nozzle (de Laval nozzle) accelerates a gas flow by converting its heat energy into kinetic energy. My intuition says that this violates the second ...
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51 views

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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140 views

What is the meaning of the point of inflection in this?

Consider the following flow velocity profiles near a boundary, and the boundary profile point of inflection,PI Regarding the effect of pressure gradient on boundary-layer profiles does the point of ...
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2answers
51 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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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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2answers
155 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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45 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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3answers
895 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
50 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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1answer
137 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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2answers
304 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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1answer
88 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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47 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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35 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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59 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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76 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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1answer
132 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 ...