Questions tagged [flow]
The motion of fluids (gases, liquids and granular material).
515 questions
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1answer
33 views
Can Bernoulli Equation be applied across two fluids?
There is a beaker with two liquids placed above another. The beaker has a hole at bottom. So can I use Bernoulli equation across the interface of liquids.
2
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2answers
86 views
Complex Potential describing inviscid fluid flow
I'm a Mathematics student, working through a homework sheet for a Fluid Mechanics module. The question is given:
Consider the flow described by the complex potential $$w=4z+\frac{8}{z}.$$
...
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2answers
33 views
Which way does the fluid flow? [on hold]
In the given siphon arrangement if a hole is made at D, will water leak out or will air leak in?
I think air should leak in as fluids flow from high to low pressure but I'm not sure...
Any help ...
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1answer
30 views
Potential Flow integration (no-voice and very beginner!)
Let's say I managed to compute a velocity field that is divergence-free. How do I integrate it over time? meaning, I created a potential function, set v to be curl of potential function, so I have now ...
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0answers
41 views
+100
How should energy loss in a hydraulic jump be calculated?
The energy loss in a hydraulic jump is still calculated with the old equation of Bresse from year 1860;
(I.e., equation 7 in this paper from 2017)
$$
\frac{\Delta E}{E_1} = \frac{(\sqrt{1+8Fr^2}-3)^3}...
0
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1answer
51 views
Why does the flame of a gas stove not spread backwards?
On lighting a gas stove the flame is active only above the burner. However, the gas input from beneath the burner and all the way to the pipeline contains gas fuel. Why doesn't it all ignite up ? I am ...
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2answers
55 views
How pressure and velocity is related? And its effect in cavitation
As per my knowledge I think when velocity increases and discharge is constant,the fluid(liquid) molecules get away from each other in the region where velocity is high and that's why they get ...
0
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1answer
60 views
Bernoulli Experiment- why is the pressure on the left greater? [closed]
Is the Pressure $P_2$ caused by a reaction force to the fluid-fluid collision, or is it the result of the collisions of the water molecules with the walls at the neck of this experiment. The part ...
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1answer
38 views
Energy balance in a thermodynamic system
I think I've missed this point in class but I was wondering how to determine for example if I have a turbine, the Energy balance would look something like this:
$W$ as work $h_1$ as intake and $h_2$ ...
1
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1answer
26 views
Expression of mass flow rate for steady flow
According to Wikipedia the mass flow rate is given by:
$${\dot {m}}=\iint _{CS}\rho \mathbf {v} \cdot {\rm {d}}\mathbf {A}$$
And using the Reynolds Transport Theorem (if there is no sources):
$$\...
0
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1answer
31 views
Streamlines equation in a fixed frame
In this wikipedia article about potential flow around circular cylinder, the stream function is given by:
$$\psi =U\left(r-{\frac {R^{2}}{r}}\right)\sin \theta \tag{1}$$
But in another article we ...
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2answers
79 views
What causes my “kitchen witch” to move? [closed]
Not a physics person but curious about something going on and hoping someone can explain.
I have a kitchen witch hanging from my kitchen ceiling on some fishing line and it’s constantly swaying and ...
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1answer
28 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 ...
1
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2answers
98 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 ...
0
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2answers
85 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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0answers
37 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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0answers
15 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
46 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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0answers
18 views
With Bernoulli effect how to calculate pressure drop in adjacent tube to the air flow?
A possibility to draw crank case gases through an oil catch tank under lower pressure (rather than feed back into the carburettor) might be to take a tube to a hole made in the exhaust pipe.
Question.....
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1answer
29 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 ...
3
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3answers
76 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 ...
1
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1answer
34 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 ...
1
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0answers
48 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 ...
1
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1answer
41 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.
2
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2answers
94 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 ...
0
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2answers
96 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 ...
5
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1answer
72 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 ...
2
votes
1answer
36 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 ...
8
votes
2answers
110 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 ...
0
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1answer
58 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 ...
0
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1answer
40 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 ...
0
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1answer
62 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{...
0
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2answers
37 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
99 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 ...
4
votes
1answer
45 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 ...
1
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1answer
47 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 ...
0
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1answer
37 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 ...
0
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1answer
56 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 ...
0
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0answers
21 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
53 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 ...
0
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3answers
47 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 ...
0
votes
1answer
64 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 ...
3
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3answers
116 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 ...
1
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0answers
30 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
60 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 ...
2
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0answers
51 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 ...
21
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9answers
6k 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 ...
0
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1answer
44 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 ...
2
votes
3answers
48 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?
