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Questions tagged [flow]

The motion of fluids (gases, liquids and granular material).

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1answer
43 views

Two containers that have the same volume (let say 500 liter) but are different in shape

I remember from school, one time a teacher explain us the following: two containers of the same volume, but one is thin and tall, and the second is short and wide. then if we fill them with water from ...
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2answers
327 views

Continuity equation, differential form vs integral form in 1D flows

The continuity equation for fluid flow assuming steady-state and constant fluid density, that is: $\rho = \text{constant}$ and $\partial_t=0$ can be written in the two following forms (differential ...
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2answers
96 views

Why does the flow of charge even create electricity? [closed]

Okay this is a question I’ve asked a lot of places but I always get its the flow of charges and it’s like a property. What I don’t really understand is how is this flow of charges creating electric ...
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39 views

Air pressure drop off rate

I’ve been thinking about this for a while, and at first, I assumed that the inverse square law applied, but seeing as how air isn’t a form of radiation, I’m not sure that it still applies. So if we ...
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23 views

Heat transfer in Steady vs Oscillating flow through a Matrix material (Conceptual Input needed)

I have been trying to model heat transfer and pressure drop for oscillatory flows inside a closed channel flowing past a matrix material. My intuition and experimental results (in journal papers) tend ...
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2answers
118 views

Pressure in a moving fluid

I’m having some quite large conceptual difficulties which I hope can be resolved by considering this example. When considering the pressure in the sheet very far away from the impact zone the ...
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1answer
109 views

Heat flow for an ideal gas in Isothermal Expansion

I'm having a bit of difficulty grasping the concept of heat flow in isothermal expansion -- most likely because I don't fully grasp the concepts yet. We were given a problem that the professor ...
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3answers
212 views

Why is blowing so different than sucking? [duplicate]

Why is it so easy to blow out a candle from a significant distance, but nearly impossible to suck enough air to do the same? Even without focusing the airflow through a nozzle or something, this ...
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1answer
45 views

Unclear assumption in deriving fluid energy conservation laws

I am currently working through Alexandre Chorin's Mathematical Introduction to Fluid Mechanics. In the first chapter, he treats the change in Kinetic energy of a fluid region $W\subset D$ subject to ...
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0answers
105 views

Pressure buildup in a pipe

How to model the fluid's pressure evolution over time and distance in a pipe, from inlet to outlet, assuming the fluid is water. Say that I have a pipe filled with a fluid with pressure P1 and ...
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1answer
122 views

What is a precise definition for laminar flow that includes “laminar vortex-shedding”?

I can't find a definition for laminar flow that includes vortex structures. See paper: Numerical calculation of laminar vortex-shedding flow past cylinders and countless others when googled.
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2answers
140 views

Bernoulli Experiment, unanswered questions

In the bernoulli experiment, the pressure on one side is greater than the other, thus the fluid flows faster through the thinner nozzle.In the image, is the pressure created shown correctly? Since ...
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2answers
135 views

Potential flow pressure on a smooth surface

For an incompressible potential flow around a smooth rigid body, is it true that the pressure on the surface of the body is proportional to $a\cos^2\theta+b$ where $\theta$ is the angle the inward ...
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1answer
100 views

Hydrodynamic interaction between two spheres in $Re\ll 1$ flow

I am studying the interaction between two spherical particles of radius $a$ in a low Reynolds number flow. Because of linearity, I know that their respective velocities will be linear in the forces ...
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1answer
167 views

What is the relation between incompressible flow and laminar flow?

I have a small question about dynamics. My textbook shows me the velocity profile for an incompressible flow, which has a parabolic profile. Does this automatically mean it's laminar flow? I don't ...
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2answers
166 views

Are there any known models with limit cycles in their RG flow?

The text-book presentation of the renormalization group (RG) leaves one with the impression that all systems will eventually flow to a fixed point. This is somewhat enforced by the phenomenological ...
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1answer
62 views

By which physical mechanism does the continuity equation in fluid mechanics work?

The Navier-Stokes equations consist of the momentum equation and the continuity equation. Consider the incompressible versions for the purpose of this question. Continuity is always talked about as a ...
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1answer
41 views

Bring energy into mixing hot and cold taps

Are you able to bring energy into your equation? I like to know if there is a way to calculate the required energy necessary to obtain a given temperature and flow at the outlet of the mixer when hot ...
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1answer
138 views

Confused about Navier-Stokes equation

Just look at the L.H.S of the compressible navier-stokes equation from wiki $$\rho(\partial_t \vec{u}+\vec{u}\cdot\nabla\vec{u})=...$$ How can I add a vector $\partial_t \vec{u}$ and a scalar $\vec{...
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2answers
39 views

How do I derive a formula for the water level inside a container when it is filled up at a constant flow rate? [closed]

When a container (e.g. Vase, Water Glass etc.) is filled up at a constant flow rate the water height changes differently over time depending on the shape of the container. This video shows some ...
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4answers
196 views

Calculating value of $g$ from Torricelli's law

For my Physics project, I decided to do an experiment with fluids. I'm using the Torricelli's law to verify that the value of acceleration due to gravity on earth is $9.81m/s^2$. I am using the ...
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1answer
54 views

How long for released dye to uniformly saturate the oceans?

Suppose a supertanker filled with purple dye breaks open at a particular point in the middle of the Pacific Ocean and spills an enormous volume of purple dye into the ocean all at once. Imagine that ...
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1answer
62 views

Laminar Flow From Outdoor Faucet

I ran across a video recently that really caught my attention. It's been circling the internet over the past couple weeks, but I'm trying to figure out what's going on. Basically, it's a video of an ...
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1answer
45 views

Water / Air flow via thermal difference

This is a simple water/air flow thought experiment I had that I'd like to understand better... The basic idea is that you have a closed loop of water or air standing vertically. One side is heated to ...
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1answer
103 views

Is the velocity gradient of a homogeneous viscous fluid constant?

The formula for the viscosity of a fluid (or even a gas) is equal to \begin{align*} \frac{F}{A} =\mu\frac{du}{dz} \end{align*} where $F$ is the force applied to the top layer of the fluid, $A$ is the ...
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22 views

Why there is a difference between same components of 2 parallel planes?

In this context the author is deriving the net outflow through 2 surfaces.to do so he is calculating the difference between x components ofthe vector V. so now my question is here the 2 surface are ...
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0answers
187 views

Shear stress and surface heat transfer rate with various Pr values

I'm revising for my Fluid Mechanics exam, and came across a question that ask us to explain the behaviour of the shear stress and surface heat transfer rate with various Pr values. The question is ...
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3answers
56 views

Question on mass flow and Betz's law

On Wikipedia I was reading an article on Betz's law, where has a statement that lost me: So the density times the area and speed should be equal in each of the three regions, before, while going ...
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1answer
113 views

How can one experimentally measure the velocity of water flowing through a pipe?

I am planning to investigate the effect of bend curvature of a pipe on velocity of water. For this, I will need to find the velocity of water through various pipe bends. Would placing an object in the ...
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3answers
128 views

Strange behavior of water flow

So recently when I was in a toilet, I noticed a strange behavior of the water that flowed down a nearly vertical pipe of my friend house. Instead of flowing straight down on the nearly vertical pipe, ...
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0answers
20 views

How would you model a single point outputting fluid in all directions whilst enclosed in a sphere?

The idea is this: There is a point at the center of a sphere. This point is releasing a fluid (say water) in all directions towards the edges of the sphere. The fluid can collide with the sphere's ...
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0answers
60 views

Difference between von Karman vortex street & shedding

I want to know if there are any differences between von Kármán vortex street and vortex shedding ? If so, what are they?
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0answers
96 views

Levitating water wheel/ball

Recently I have come across this interesting YouTube video regarding this phenomenon. A huge ball is able to be levitated when a part of it is in contact with a water jet shooting upwards. After ...
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0answers
55 views

Proof that pressure rises in a fixed control volume

I am looking for a proof using Bernoulli's equation that that for $Q=(Area)(Velocity)$, $Q_{in}>Q_{out} \Rightarrow P_{inside}>P_{outside}$. I read that for a computer, if the case fans blow a ...
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9answers
7k views

The water analogy seems to imply that power = current. Why is this incorrect?

Many people think of the water analogy to try to explain how electromagnetic energy is delivered to a device in a circuit. Using that analogy, in a DC circuit, one could imagine the power-consuming ...
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1answer
80 views

How to use General Energy Equation in parallel pipes?

My question comes from exercises with parallel pipes such as this. I know that to analyse internal flow, we use general energy equation which origins from the 1st law so each time we apply it we are ...
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3answers
50 views

Would it generate a lift if wind is circulating inside a nearly closed tube?

For a setup like the below image, would there be a lift generated to the tube, due to bernoulli effect?
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1answer
69 views

Angle of Attack of a propeller blade

What will be the best angle of Attack for a propeller blade that is in the airfoil shape?
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1answer
25 views

Question about Divergence of Disturbance Flow

If we break down a flow past a particle into both the incident and disturbance flow like:$$\mathbf{u}^\infty+\mathbf{u}^D=\mathbf{u}^{total} $$ Can we show that: $$ \nabla\cdot\mathbf{u}=0 \...
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1answer
61 views

Why are the diagonals of the pressure tensor non-negative?

I understand that the pressure tensor is simply the momentum flux which makes sense to me (pressure is force per unit area which is momentum change per unit time per unit area). From this, a simple ...
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2answers
254 views

Combined Poiseuille-Couette flow

I stumbled upon this exercise in James Fay "Fluid Mechanics" book, which I'm using to learn fluid dynamics by my own, and I am struggling a bit with it, any help will be appreciated: The figure ...
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2answers
201 views

How do “divergence” and “curl” relate to the three states of matter?

A fluid is said to have divergence (the ability to flow) and curl {the ability to rotate). Do these two characteristics fully define a fluid, or are there other important properties that I am missing? ...
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0answers
27 views

At what concentration do dispersed particles begin behaving like a fluid?

Suppose that I have a nearly insoluble metallic compound in a solution, and that what little of this compound does dissolve aggregates to form nanoparticles. The number of nanoparticles is ...
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2answers
287 views

Relationship between flow rate and pressure in fire service pumps

I'm trying to get an explanation (at a suitable level please!) for something we have recently come accross. In general in the Fire Service we are taught that if you increase the presure of water ...
4
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3answers
205 views

What is the shape of a stream of water coming out of pipe?

As shown in the photograph below, when water comes out of a pipe (located on the surface of the earth) it travels downwards in an arc. What is the shape and size of this arc as a function of D, the ...
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0answers
65 views

Doubts in work done in a control volume [closed]

For a control volume, in a general steady state steady flow process, we can write the generalized equation as- \begin{align} \dot{Q}_\text{CV}+\sum\dot{m}_\text{in}\times\theta_\text{in}&=\sum\dot{...
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3answers
72 views

Why does the increase in velocity result in decrease in pressure in a flow? Are there any explanations behind?

So I have been searching for this answer that why would increase in velocity result in pressure drop and found no explanation, please help, thanks!
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1answer
43 views

How fast do the aerodynamic forces on a surface build up?

I`m currently working on my thesis in the field of active car aerodynamics. In order to design a controller to move parts of the cars body I need to know how fast the drag and downforce acting on this ...
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1answer
24 views

Reconciling various expressions of flux and flow in kinetic theory

There are a few expressions I have been shown to describe the flux $\vec{\Phi}_A$ of some quantity $A$ described in kinetic theory. I am having trouble understanding how they are related or if they ...
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2answers
258 views

Pressure distribution around an aerofoil

Some notes that I'm reading right now show a picture of the distribution of pressure around an aerofoil with zero angle of attack. The notes say that the pressure is identical at the top/bottom ...