Tagged Questions

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

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Approximation for rotational drag on a box?

The drag equation gives us an approximation for the drag on an object caused by its velocity in a fluid: $F = \frac{1}{2} \rho A v^2 C_d$ Is there any similar approximation for the rotational drag ...
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How do shock waves form in space?

Today my aerodynamics professor mentioned that the equations we were learning for atmospheric shock waves can also be applied by astrophysicists to study black hole-related shock waves in space. How ...
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Funnel theory exists?

I have a funnel and I notice when I try to pour powdered sugar through it, it blocks up unless I tap it. If I put sand in the funnel, it goes through. Is there a theory of funnels that can predict ...
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Is there a dynamic prelude to the state equation?

If I'm not mistaken, the state equation $$PV=nRT$$ also known as the 'ideal gas law' serves as a mathematical model relating pressure-volume-mass-temperature states of an assumed ideal gas. In other ...
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How is viscosity related to the forces in fluid?

I think it was that the forces are only perpendicular but do not know how this is related and from where it comes from. EDIT: I am asking for the difference between viscous and non-viscous fluid in ...
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Can we regard dark matter as a special fluid rather than individual particles?

If we assume that dark matter is an $N$-body system of sizeless and collisionless particles which only interact via newtonian gravity, do these huge quantity of particles manifest some collective ...
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while travelling in a bus or train, air comes inside the vehicle but it seems it goes nowhere

I have been amazed observing that while travelling in a train or bus , we feel air comming into the vehicle. I know this is due to difference in pressure but my question is that we dont feel air going ...
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Turbulent and smooth flow of water from a tap

When water flows normally from a tap, we can say it is turbulent ( Pic 1 ). But when we attach a piece of cloth to the opening of tap, water flow becomes smooth ( Pic 2 ). We can say that by touching ...
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movement of water in plants

Books say that water moves from places with a high water potential to places with a low water potential. The movement of water in plants are explained because water potential is lowest in leaves and ...
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Can water be made to flow through a pipe using only gravity and resulting in a higher elevation at the end of the pipe from its starting point?

I am not a physics guy, so be nice to the layman, guys! Please note: not a homework type problem. This is a scenario being considered for real-world application. I have not yet considered details ...
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How do cyclonic dust separators work? [closed]

I have a background in engineering so I am somewhat embarrassed to say that I don't understand how a seemingly simple cyclonic dust separator works. Here is a link to video of such a device in ...
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Sound in stirred hot fluids

If a hot beverage in a cup gets stirred, the sound of the spoon changes. You can easily hear this if you repeatedly cling the spoon to the cup ground after stirring. The cling sound will raise in tune ...
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Stokes law in 2-dimensions

Stokes' law states that force on slow moving sphere (i.e. $Re\ll1$) in liquid is $$F_d = 6 \pi \mu R V$$ In two dimensions we are in trouble (flow around disk in 2d or around cylinder in 3d), ...
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Pressure as a scalar? [duplicate]

Why do treat pressure as a scalar? We know that $P=F/A$ (Pressure = Force/Area) and force is vector quantity, so then why should pressure not be a vector?
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Problems implementing Lattice Boltzmann method

I want to make a basic 2D implementation of the Lattice Boltzmann method (in javascript) to simulate gas/fluid flow, but I have run into some problems and I can't seem to find the cause of it. ...
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Balancing a bowl of water

Let's say you have a mildly elliptical bowl balanced on top of a stick. This bowl is filled with water, not necessarily to the top but fairly full (I'm not 100% sure this makes a difference). You then ...
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Velocity of water from tap to fill a cracked bucket to brim

Let us consider a real life situation. The bucket in figure below (drawn using MS PAINT) has an irregular crack of essentially small area. And I decide to open a tap above it and start filling it ...
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Bernoulli's Equation for flow of gas and changing area

My task is to determine the size of piping needed in a system. In this system, hydrogen gas flows horizontally in a $\frac{3}{4}\,\mathrm{inch}$ pipe and into a vertical pipe of a larger diameter. My ...
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Is it possible that any spatial variation in press disturbance without gravity?

When gravity (and buoyancy), acting as the only body force of a liquid, is ignored. Is it possible that any spatial variations in pressure perturbation are the result of fluid motion or some other ...
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Why velocity potential?

(Aero-)Acoustics (among other parts of fluid-dynamics) loves the velocity potential $\varphi$ defined as $\vec{v} = \nabla \ \varphi$ with the condition of $\nabla \times \ \vec{v} = 0$. I am ...
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Centripetal forces acting on fluid flow in a bent pipe?

Yesterday I answered this question, using analysis of forces from this web page. But today I started having doubts regarding the completeness of this analysis. It occurred to me this approach does ...
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How to visualize that the critical flow conditions in a heat added flow remains constant?

In a variable area nozzle, where compressibility is observable, if the flow goes from point A to point B and heat addition is considered. How do we see that the critical speed of sound (imaginary ...
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Can we tune the exhaust system of a gasoline car to be as quiet as an electric car? [closed]

Electric cars are quiet, so quiet that legislation is proposed in many countries, to make them audible to pedestrians, for obvious safety reasons. My question: is it possible, without getting into ...
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Force applied to a 90 degree elbow by hydraulic flow

I have a question about forces applied to a 90 degree elbow. Essentially we have a 90 degree pipe bend used to redirect discharge from a water system, which can discharge high volumes and pressures ...
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Help me in the measurement of small deformation of a gas-liquid interface

I am new to optical measurements and sorry for any stupid question? I am also not sure whether this is a right place to discuss the experimental issues... Recently, I am trying to measure the small ...
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Equating $\frac{dp}{dt}$ and $F$ at a hydraulic jump?

A journal I am reading about circular hydraulic jumps provides the equation and I quote: $H$= Depth after hydraulic jump, $h$= depth before hydraulic jump, $V$= velocity after hydraulic jump, $v$= ...
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Is the reason why water in a cup rises if a solid is placed within it the ideal gas law?

Let's say you have a cylinder with water that takes up volume V and has temperature T. When a solid mass is placed within the cylinder, the water momentarily takes up a smaller volume within the ...
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fluid dynamics of two streams in single pipe

Background: The fetus needs to deliver both high and low oxygen tension blood within the same pipe (inferior vena cava) back to the right atrium. Blood entering the right atrium is able to cross an ...
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How does a water-steam mixture behave?

Namely, if I mix a flow of steam that has density $\rho_s$, velocity $v_s$, and pressure $P_s$ with a flow of water that has pressure $P_w$, velocity $v_w$, and density $\rho_w$, how can I find the ...
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Lift and drag coefficients on other planets

The question I'm trying to answer seemed simple: how hard would it be to fly on a planet with lower gravity but also thinner atmosphere compared to Earth. If the answer could hint me at how much ...
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can gasoline be compressed

Simple question, can gasoline be compressed? Is it compressible as in how water is incompressible. Would one be able to store x gallons of gasoline in a pressurized container that is x-y size. For ...
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Does charged saltwater repel itself?

if you take a volume of saltwater, and charge it to a high voltage, will the water molecules repel each other and climb the walls of the container slightly, or does the fact that water is polar ...
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How does a 1,000 mph car maintain vertical stability with tiny front wings?

Formula 1 cars have wings to help keep them on the road during cornering, they don't need them whilst moving in a straight line. And that's ok , they "only" travel at 220 mph. In a different speed ...
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How does a ball cause a splash? (With the relevant math)

Problem Statement: Imagine a spherical ball is dropped from a height $h$, into a liquid. What is the maximum average height of the displaced water? For instance, although one particular drop of water ...
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Crown shape in water waves?

Watching this slow motion video of waves caused by a droplet falling on water, I am puzzled by the first wave shape. The first wave rises with circular symmetry as I expected, but is not even in ...
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Forces of reaction and torque of water flowing out of a vessel

Consider the problem wherein we are told to find out the force of reaction of water flowing out of a vessel through an orifice. I attempt to compare it with a shotgun and a bullet as a conservation ...
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Calculating the 'splash zone' of a water droplet?

How would you calculate the probabilistic area for a water droplet splash zone? So far my guess would be to find out how much the wind is acting upon it horizontally, find out how fast its falling, ...
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Coffee Straw Physics

When I put my little, cylindrical coffee straw into my coffee, the liquid immediately rises about half a centimeter up the straw without provocation. This is also the amount of coffee that the surface ...
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Perturbations on a fluid thread and hydrodynamic instability

I know that due to fluid instability there are some perturbations ( picture below ) after all of these, my question is the reason of existence of these perturbations. Why exactly they appear? Shear ...
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Fluid speed and fluid density

How does fluid density affect fluid speed? Basically I am trying to figure out if, with all other quantities remaining constant, would an increase in fluid density cause the fluid speed to increase/...
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How can fast moving particles gain energy from slow moving ones?

Imagine a large diameter piston filled with water connected to a small funnel. When you press on the piston slowly but with considerable force the water will move very quickly from the funnel in form ...
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growth rate of the instability of a laminar flow?

I was just studying something about The Plateau-Rayleigh Instability that I saw something called Growth rate of the instability. My question is the exact physical definition of growth rate of ...
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What happened here in this Taylor-Couette Flow experiment?

I came across this video of Taylor-Couette Flow on YouTube. Originally I was looking for a visualization of the wavy Taylor vortices induced by the angular motion of the inner cylinder. However, I ...
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Maximum pressure in a die-less wire drawing apparatus

I ran across this patent: http://www.google.com/patents/US4549421 and was interested by the idea of reducing the cross sectional area of a wire with only shear stress caused by the wire being pulled ...
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Will there be a drag for a solid body already dragged due to reaction?

By learning Fluid Structure Interaction tutorials, I managged to simulate a 2d transient fsi simulation using Comsol fsi fully coupled module. A solid body due to motion in vertical direction ...
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Why in fluid interface, liquids tend to reduce their surface?

I've read something about fluid interface and know that liquids tend to reduce their surface but my question is that why do they tend to reduce their surface?
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How to calculate maximum wind speed in a funnel (Venturi Effect)?

It must be in an open area, so nothing forces the wind, just himself. Lets assume that the wind is blowing only one direction. I assume that at some point the speed will stop increasing and it will ...
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What law of physics did these people think they disproved?

I was trying to understand the Physics of how water gets to the leaves of a tree and found this article which claims some experiment disproved a 300 year old law of Physics. Sounds pretty unlikely... ...