Questions tagged [fluid-dynamics]

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

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80 views

Ceiling fans with just one large winglet?

I am from India and in India ceiling fans have generally three wings. Today while laying on my bed a question came in my mind. First of all, I know that if we use just a single wing at a time of same ...
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1answer
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Do airplanes circulate the air sideways in flight?

When airliners fly through clouds it looks like they are circulating the air sideways. See Image. The wings are pushing the air flown through downwards, which circulates the air each side of the ...
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Pressure dependence of viscosity

Does viscosity depend on pressure? If depends, is there any equation by which I can calculate viscosity at a specific pressure?
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Total Energy of a Flowing Fluid

Does image I and image II have a different equation? or is it just PE difference?
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Why can we assume that Velocity at the top of large tank is zero?

When using Bernoulli's Equation we sometimes assume that the velocity is zero if the tank is very large. Why can we do this? Is it because the pressure is assumed to be zero as well?
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What happens when you stir a superfluid?

If I stir up some liquid helium in a container with an irregular shape, I'd expect to see a vortex break up into smaller and smaller vortices. Since liquid helium has no viscosity, I would also expect ...
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Total Pressure and Total Momentum

Bernoulli's equation indicates that : $P_o = \frac{1}{2}\rho_1 u_1^2 + P_1 = \frac{1}{2}\rho_2 u_2^2 + P_2$ However, when you use conservation of momentum integral equation, you get $\rho_1 u_1^2 ...
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1answer
28 views

Columns of liquid on top of gas, when and why does gas bubble up through the liquid?

I have a long straw ("small" diameter tube of constant cross-section) hanging vertically above me. The straw is filled with liquid (water). I blow a finite volume of air (gas) into the lower ...
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1answer
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Is the equation given here an approximation of the actual venturimeter equation?

The manometer contains a liquid of density $ \rho_m$. The speed $ v_1 $ of the liquid flowing through tube at a broad neck area to be measured from the equation of continuity (10.10). the speed of ...
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How to prove symmetry of stress tensor using torque equation? [closed]

** You have to write the angular momentum equation i.e. Torque = (Moment of Inertia)x(Angular Acceleration) for an infinitesimally small 3D element (about any axis of your choice). Finally, you have ...
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Calculate high pressure airflow from low pressure readings

How does one go about calculating the high-pressure airflow from an oxygen (assume 100%) bottle/supply. I only have low-pressure flow sensors available to me. I have high-pressure air coming from a ...
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Question on Reynolds number using Newton's definition of inertial force and how to obtain the common expression for Reynolds number?

The concept of inertial force is traced back to Newton’s Philosophiae naturalis principia mathematica, The Principia, Newton. At the beginning of his book, Newton presents eight definitions forming ...
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Do helicopters and planes fly by different reasons?

In the case of the helicopter, it is pretty clear (well, for me) that it gets lift by pushing air downwards. And as a consequence, there is more pressure below its blades than above. But on the other ...
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2answers
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Turn over a full bottle, why does it pour?

Why does a full bottle leak when you turn it over? Idea: Let's assume there is a full bottle which is not open, yet. You turn it over such that the opening points towards center of the earth. Now you ...
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1answer
18 views

How to divide the water flow into outs with equal flow each? [closed]

I'm designing a machine that uses water to boost fish in one phase of the process. My main concern is how to split the water that I get from a water pump in to $n$ outputs with same flow rate. The ...
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Can I transfer 20,000gals of water 3/4mile into another tank of equal size that is at 30' lower elevation? [closed]

original tank of water is 20,000gals Destination tank of water is 20,000gals The bottom of the original tank is 30' above the top of the destination tank. the Pipe must go down into a ravine and back ...
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The Integral Equation for Stokes flow

I am trying to understand and rederive why "The Integral Equations for Stokes Flow" is convoluted over just the surfaces of the rigid boundaries, and not the entire fluid volume. $$u_i(\...
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1answer
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Tensile strain produced on two thin rods of different lengths

Suppose there are two thin rods $Y$ and $Z$ with length $L_1$ and $L_2$ respectively. $L_2$ has larger magnitude than $L_1$. Both rods have same density $p$, cross sectional area $A$, Young's Modulus $...
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Sign convension in shell momentum balance

I was trying to understanding the shell momentum balance from https://www.youtube.com/watch?v=sMsw8BPT578. At 26:25 if you see the teacher is saying momentum in - out. Although I have seen the videos ...
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Relationship Between Specific Impulse and Expansion Ratio Equation

I am trying to find an equation with both Specific Impulse and Expansion Ratio (throat area is constant with exit area changing) of a rocket. This assumes the following constants (all of which I know):...
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1answer
34 views

Thermodynamic equation of a fluid [duplicate]

I have a system whose entropy is specified by three variables: Salinity $S$, pressure$p$, and density$\rho$. I have the following partial derivative: \begin{equation} (\frac{\partial\eta}{\partial p}...
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Lattice Boltzmann method: properties of D2Q37 stencil and beyond

I am new in the world of Lattice Boltzmann method (LBM). I have just found in this paper [1] the following statement: A D2Q37 Lattice Boltzmann scheme correctly reproduces the equation of state of ...
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Initial Conditions for no-slip velocity (Walls)

For CFD initial condition, I want to know if i have to apply a very high velocity gradient on the wall of a pipe (internal flow) as it's the case when you study the freestream flow which surrounds an ...
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1answer
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When pressing coffee, why does boiling water and coffee mixture appear to decrease after grounds are pressed to bottom?

When pressing coffee, why does total volume of boiling water and coffee appear to decrease after grounds are pressed to bottom?
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1answer
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Is only fluid velocity or both fluid velocity and sound speed discontinuous at a shock?

Formation of shock waves are common phenomena in Physics. Defining the Mach number as $M=v/c_s$ , where $v$ is the fluid velocity and $c_s$ is the sound speed, it is known that the flow jumps from ...
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Helmholtz Resonator: What are some factors that might affect the resonance frequency in a glass bottle?

I'm trying to understand what are some factors that may positively/negatively influence the resonance frequency in a glass bottle, if we wish to compute this frequency with Helmholtz's resonator ...
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How to pump liquid aerosol?

I have a use case where I want to produce fog (an aerosol) using an industrial grade high-volume fog generator and then I would like to direct the resulting fog to different locations i.e. small boxes ...
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Air bubble of any shape in a viscid liquid

I am wondering about the existence of a function\action that would move the box so that the air bubble would take a given shape. Consider a subset of spaces $\Omega\subset \mathbb{R}^3$ maybe for a ...
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1answer
35 views

How do I calculate the diameter of a liquid poured onto a flat surface?

I'm looking to calculate the diameter of a liquid after being poured on a flat surface, in terms of: Time - t Viscosity - η Density - ρ Volume poured - V Basically, assume you were to carefully pour a ...
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Strange shape of flowing Streamlined Water. ( I think it is somewhat related to Fluid Dynamics and Force )

Before i proceed to my question, I want the reader of this question to try out this little experiment at home. Take a cup or mug filled with water ( does not depend how much you fill the cup or the ...
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1answer
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How does air flow within a melodica when two key are pressed?

I'm trying to understand the air flow within a melodica. A melodica is a wind instrument that has a piano-like keyboard. Pressing on a key opens an airway so that air entering the melodica's air ...
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What causes the Coandă effect? [duplicate]

What causes the Coandă effect? Here's my understanding of it: When a fluid flows around a curved surface it has high velocity and so low pressure its pressure will be lower than the atmospheric ...
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Is this application of product rule for the definition of pressure correct?

$\vec F$ = force vector, $\vec A$ = area vector, $P$ = pressure Mathematically $\vec F = P \vec A$. By product rule we get, $$ {\rm d}\vec F = P {\rm d}\vec A + \vec A {\rm d}P $$ Why do we often ...
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How much can water compress air?

So I was thinking of using a barometric sensor to measure the compression of air in a tube in order to measure water level in a tank (Basic diagram below). So based on the equation $ \frac{P_1 V_1}{...
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If I blow into a tube, how long does it take for air to exit the other end?

I have a tube open on both ends. I blow into one end. How long does it take before I can detect air coming out the other end and why?
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How deep does the pool at the end of a waterslide need to be?

I am looking at a problem where a woman has travelled down a simple waterslide and into a pool, then hurt her knee by striking the floor of the pool. I need to figure out how deep the pool needed to ...
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2answers
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Stress and Strain lag for viscous materials

As stated here, In purely viscous materials, strain lags stress by a 90 degree phase. How can we derive this statement? Is it experimental? If not, which is its proof and what is the physical cause ...
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1answer
55 views

Treating expressions of the form $\vec x \cdot \nabla \vec y$: what is the order of operations?

This comes up in the context of a homework assignment. We're given the Euler equations for invisicid fluid flow. The variables at play: $p=p(x,y,z,t)$ is pressure $\rho = \rho(x,y,z,t)$ is mass ...
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Fanno flows and Energy generation

Fanno flows are a very interesting phenomenon in fluid dynamics: the flow (steady, 1D, adiabatic) is moving in a pipe where friction forces are not negligible. Due to the second law of thermodynamics, ...
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Literature references for why and how flow expands when it leaves a hole

So I’m in high school and I’m doing research on fluid dynamics. However I can’t find any references for how a fluid flow expands when it leaves a nozzle/hole. For exemple, when I blow, I can feel the ...
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What are some examples of moving a fluid changing only its kinetic energy?

Bernoulli's principle states that, in a steady, incompressible, inviscid flow, the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline. ...
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Why do fluids not accelerate?

A fluid flowing in a horizontal pipe must be flowing at a constant velocity because of the conservation of mass. However, considering how there would be a pressure and hence force acting behind the ...
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1answer
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Ellis's viscosity law: Demostration

Show that the velocity profile of a fluid that obeys Ellis's viscosity law and that flows through a tube of radius R and length L is given by expression: $$ v(i)-v(i-1)+\Delta r\left(\phi_{0}+\phi_{1}\...
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1answer
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FLRW Coupling to Perfect Fluid

In order to obtain the Friedmann equations from the Lagrangian formalism, as far as I understand, one way is to minimally couple a scalar field $\phi(t)$ to the FLRW metric, i.e. $S=\int d^4 x \sqrt{-...
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Why does poured honey move periodically when reaching the surface it's poured upon?

Here's the picture, known to most of us: When honey is poured out of a container, as in the picture (look here for the flow in motion), we see a kind of periodic flow of the honey when it comes to a ...
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Why is the atmospheric pressure under a roof and on open land the same?

If someone stands on the ground exposed to the open sky the atmospheric pressure experianced by him will be the same as someone under a roof (which is not part of a sealed box), this is very intuitive ...
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MHD instability: $\boldsymbol{k}\cdot\boldsymbol{B}=0$?

In this paper, on page 3, it states that to trigger an instability, a disturbance needs to have a wave vector, $\boldsymbol{k}$, which satisfies, $$\boldsymbol{k}\cdot\boldsymbol{B}=0,$$ where $\...
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Smaller droplets == smaller surface tension?

When applying soap liquid on the inner surface of swim goggles, the surface tension of the water decreases and small droplets of water on the surface won't form, therefore the fog won't form. In this ...
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How does gravity, viscosity, IMF help a liquid spread on a surface?

If I have a bottle full of water and I turn it upside down at a small height above a flat horizontal surface while it’s cap is open, the water spreads on the surface. If I used a metal solid in place ...

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