All Questions
7,191 questions
17
votes
10
answers
6k
views
Why is “water takes the steepest path downhill” a common approximation?
Googling around, I see people saying that water takes the steepest path downhill. Of course, this is just an approximation. Things like inertia, friction, etc. will cause deviations. I’m curious about ...
- 325
2
votes
0
answers
38
views
Relating Velocity to refractive index
First time posting so I'm sorry if I mess this up but, I am working on a project for work and I am wondering if you could relate the refractive index of a fluid to the velocity of the fluid as it ...
- 21
0
votes
1
answer
55
views
What metric to use for this dark matter simulation?
I am reading this paper https://arxiv.org/abs/1901.08064 which uses the GR version of euler equations in fluid dynamic to simulate the evolution of a perfect fluid system. (PDF)
and this is the paper ...
- 1
0
votes
1
answer
45
views
How does Newton's third law apply to forces between fluids and solid surface?
What is the direction of the force exerted by the fluid on the solid surface and what is the direction of the force exerted by the solid surface on the fluid?
If I am not wrong, I think the force ...
0
votes
1
answer
47
views
An intuitive molecular picture of Bernoulli’s principle?
I’m trying to grasp Bernoulli’s prediction that pressure decreases when
air passes from a wide pipe into a narrow pipe, as in a Venturi meter,
from the standpoint of molecules. I think in both pipes ...
- 159
2
votes
1
answer
74
views
Why do the forces on a Feynman Sprinkler balance out?
Apparently, in a Feynman sprinkler, the momentum imparted to the tube from sucking the fluid in is exactly balanced by the momentum imparted in the other direction when the fluid turns the bend. I get ...
- 23
1
vote
0
answers
39
views
Power Number of Impellers in non-Newtonian fluids
I have been working on a project involving mixing a non-Newtonian fluid.
To begin with, the fluid exhibits a shear-thinning behaviour that follows the power law:
$$\tau = K \dot{\gamma}^n$$
As for the ...
- 11
1
vote
0
answers
48
views
Non-linear Eigenvalue problem with an ODE to solve for the Linear Stability Theory of a Boundary Layer
I am working on the Linear Stability Theory (LST) for analysing the stability of a boundary layer in fluid mechanics. I have conducted CFD simulations and extracted the base flow data. To form the LST ...
- 11
0
votes
1
answer
45
views
How to calculate the surface tension limit theoretically?
When an object is submerging in a fluid with strong surface tension (e.g. a plastic cup sinking in a lake), it's very easy to see the surface tension in action. However, at some point, the surface ...
- 1
-2
votes
1
answer
94
views
Is a humans gravitational pull strong enough to attract germs and make us sick all the time?
Since humans are so much bigger than bacteria, and large objects attract small objects, are we attracting bacteria and virus?
- 37
1
vote
1
answer
84
views
Aerodynamic Center of Pressure in 3D not defined?
I have a background in aerospace engineering and, therefore, have dealt with the concept of a "center of pressure" for some time and have always found it quite intuitive and helpful. ...
- 31
3
votes
2
answers
79
views
Simulating nebulae
How can we simulate nebulae shapes computationally?
What are the governing equations or forces acting on nebulae? Is it suitable to just use Navier Stokes fluid dynamics and solar winds or is there ...
- 131
1
vote
0
answers
56
views
Why does the energy density of a conformal field theory scale as $T^4$ in $3+1$ dimensions?
I'm trying to understand hydrodynamics of relativistic CFTs. A paper I'm referring to is this article published in PRL by Itzhak Fouxon and Yaron Oz in 2008.
The paper states that hydrodynamics ...
1
vote
2
answers
55
views
Stretching vs bending of thin membrane
Stretching refers to the deformation of a material when a tensile (pulling) force is applied. The material elongates along the direction of the applied force.
Bending involves the deformation of a ...
- 241
0
votes
0
answers
31
views
Effect of condensation pressure drop in open flow
If saturated gas flows through a tube that is being cooled, the gas will start to condense, forming droplets at some point in the tube. Looking at the individual condensation "cells", I ...
- 3
1
vote
0
answers
28
views
What are some good resources to learn fluid mechanics? [duplicate]
I know that there are a lot of resources out there to be explored and I have gone through several of them. What I want is some resource where fluid mechanics is treated,
from a geometric viewpoint,
...
0
votes
0
answers
38
views
Is vorticity in ordinary liquids such as water quantized?
In superfluid helium, vortexes are quantized. A whirlpool of superfluid helium contains an integer number of these individual vortex quantas.
But all physics comes from quantum mechanics. So it would ...
- 6,665
0
votes
1
answer
70
views
Interesting Aerofoil Logical Fallacy
I am new to physics in general having just finished AP Mechanics and have limited knowledge of how fluid dynamics work. But just using forces and a simplified understanding of drag if have come to the ...
9
votes
1
answer
1k
views
Open or closed windows in a tornado? [closed]
Should one have the windows open or closed when a tornado is nearby? B.S. in Physics but very rusty in this day and age. I have always been taught that one should keep windows cracked open because of ...
- 101
0
votes
2
answers
164
views
Are there any substances that exist as supercritical fluids at STP?
If not, why
If so, please give some examples.
I know that some gases can be made into supercritical fluids by compression alone however I am not aware of any substance that is supercritical at stp
- 69
0
votes
2
answers
109
views
Why don't disturbances/wind move at the speed of sound in air?
Why don't sound waves move at the same speed as air that has a pressure differential? Imagine you have a box that is filled with air. One half high pressure air, the other half low pressure. They are ...
- 355
1
vote
1
answer
44
views
Can siphons improve the efficiency of hydroelectric dams?
Suppose we have built a hydroelectric dam where the water inlet is 100m above the outlet. We can now use the gravitational potential of the water to spin a turbine. However, suppose we install a ...
- 174
-1
votes
1
answer
75
views
Why will low pressure pulling higher pressure air towards it make "thrust"? [closed]
Low pressure air means a lack of air molecules. Normal pressure air means a normal amount of air molecules.
Say you have a stationary (can't move no matter what) cube of say steel, with low pressure ...
- 355
37
votes
5
answers
5k
views
Why does the pet's water bowl overflow?
So when i give the pet fresh water in a stainless steel bowl that i place on a mat according to the attached picture, from $t=0$ the bowl is at rest, the water normally oscillates in the bowl like a ...
- 483
0
votes
0
answers
57
views
Calculate number of air particles going through a hole in high vacuum at molecular flow
I have a vacuum chamber, connected to a vacuum pump with a pipe of diameter 50mm and need to create a high (0.0001mBar) vacuum.
I need to calculate the total suction time, and since I was not in the ...
- 1
1
vote
2
answers
55
views
Why there is a delay before canned fruits float to the top?
This question may appear silly compared to others asked on this site, yet, I can't get my head around it.
After pouring boiling water into a canning jar with berries, berries stay at the bottom of the ...
- 171
0
votes
0
answers
58
views
Is air resistance truly linear in surface area?
Air resistance is generally said to be linear in the surface area of the moving object. Out of pure curiosity, I am wondering to what degree this is an approximation, i.e., when this assumption begins ...
- 113
1
vote
4
answers
96
views
Incompressible flow condition intuition
The derivation of the continuity equation leads to
$$\frac{D\rho}{Dt}=-\rho(\vec\nabla\cdot\vec u)$$
as shown in this wiki article.
If we assume that the flow is incompressible, it implies that $\rho$ ...
0
votes
1
answer
24
views
Can one make a jet of air bounce without spreading?
Similar to this reaction on this image (left most) how it keeps its mass without spreading:
(Video Link)
Is there a way to make a jet of air bounce without spreading?
(source)
- 191
0
votes
1
answer
47
views
Is there a way to achieve a long jet of air?
As can be seen in the image bellow from a fog/smoke generator machine, the smoke spreads as soon as it leaves the nozzle.
(source)
Is there a way to achieve a longer jet of air with less dissipation ...
- 191
0
votes
1
answer
159
views
Is there any phenomenon where opposite reaction (Newton's 3rd Law) is not fulfilled?
I'm wondering if there is any case in nature/physics where it has been observed "where there is an action, there is not necessarily an exact equal and opposite reaction".
Or is there some ...
- 191
0
votes
0
answers
29
views
Diffusive momentum transport as overlap of acoustic peaks?
In the context of molecular dynamics simulations of soft or hard spheres in the fluid phase (e.g., with Lennard-Jones interactions), it is known that the velocity autocorrelation function (VACF) ...
- 905
1
vote
0
answers
50
views
Does the isotropy definition of a perfect fluid imply no heat conduction?
Weinberg defines a perfect fluid (Chapter 2, Section 10) as one where each fluid element appears isotropic in a reference frame moving with that element.
From the definition of the stress-energy ...
- 467
1
vote
0
answers
37
views
Wave interference and fluid dynamics question
I'm curious to find an answer to this question:
In the college canteen I would often slide my Styrofoam cup of milky coffee across the melamine table top. (unlike paper or plastic, Styrofoam is a ...
-1
votes
3
answers
62
views
Is the pressure at the top of the wings on Mars also lower than the local atmospheric pressure on Mars?
Generally speaking, on Earth, in order to fly, the pressure on the top of aircraft wings is lower than the local atmospheric pressure. So if we fly an airplane on Mars, will the pressure on the top of ...
- 2,165
1
vote
0
answers
53
views
Boat efficiency [closed]
Recently I was watching a boat race for "all electric boats" that use solar power to drive the engine (no sail only electric motor with solar system). in this race all boats have to be less ...
- 21
2
votes
0
answers
39
views
What happens with the water in a pipe when I quickly turn the tap
Assume I have a long pipe of diameter $D$ (e.g. 2cm) and length $L$ (e.g. 10m). The pipe is filled with water.
There is a tap at the start of the pipe, which regulates its flow rate.
I measure the ...
- 211
0
votes
2
answers
74
views
Is Steady Flow necessarily Streamlined Flow? or Vice Versa?
Is Steady Flow necessarily Streamlined Flow? What about vice versa? What is the difference between the two?
What conditions should be satisfied for a flow to be streamlined?
- 2,557
1
vote
0
answers
47
views
Add pressure gradient to Falkner-Skan (FS) boundary layer (BL) equations and solve numerically for arbitrary $P(x)$
I have read quite a few tutorials / watched several clips on the derivation of the Falkner-Skan boundary layer equations using similarity and then solution using RK solvers such as ...
- 21
1
vote
1
answer
55
views
Why the density have anything to do with initiation of subduction?
No matter wherever I read the process of subduction, I always find this one sentence which says "when the two tectonic plates collide the denser plate gets subducted under the lighter plate ...
0
votes
1
answer
43
views
Which is the cause and which one is the effect between the shear stress and velocity gradient in a fluid flow?
Take a Newtonian fluid flow between two parallel plates, one at rest and one at constant velocity, then there will be a linear velocity gradient between those two plates. Is it due to the no-slip ...
1
vote
0
answers
30
views
Bubble behaviour during water / vapour coexistence in the most idealised scenario without boundaries
Consider the following statement: during a 1st order phase transition, the temperature of the system stays constant and any extra heat goes into turning a larger portion of the system into the new ...
- 781
0
votes
2
answers
159
views
Energy of a flowing fluid
A pump transfers mechanical energy to a fluid by raising its pressure. The pressure of a flowing fluid is associated with its mechanical energy.
$\mathrm{Pa}=\mathrm{N}/\mathrm{m}^2=\mathrm{N}.\mathrm{...
- 105
-1
votes
2
answers
145
views
Why rain drops are spherical while water droplets on a glass surface are flat?
Why are raindrops spherical when falling through the air, but lose their spherical shape when they are on a flat surface?
- 15
9
votes
5
answers
2k
views
How exactly does a seashell make the humming sound?
My little brother asked me where the ocean-like sounds came from inside of a sea shell. I told him that the air trapped inside the shell vibrates, making a sound. But then he asked me why the air had ...
- 12.6k
0
votes
0
answers
15
views
Zero stress in $z$ components for thin surfaces
We can write the stress tensor as:
\begin{equation}
T= \left [
\begin{array}{ccc}
\sigma_r & \tau_{r\theta} & \tau_{rz} \\
\tau_{\theta r} & \sigma_\theta & \tau_{\...
- 241
0
votes
0
answers
28
views
Does the fluid need to pass through a converging nozzle with greater pushing pressure?
Under the condition that both the converging nozzle and the parallel nozzle maintain constant flow rate, does the fluid need to pass through the converging nozzle under greater pushing pressure than ...
- 2,165
0
votes
2
answers
103
views
Where does the pressure go when the fluid leaved the pipe?
I have an elementary question on Bernoulli's law. Consider the pipe as depicted below.
At point $1$, the pipe has the cross-section $A_1$ and the incompressible, non-viscous fluid is flowing with the ...
- 315
0
votes
0
answers
31
views
How to find out if Water Droplet will peneatrate Water Umbrella?
Imagine a scenario. we are pouring water from sink on the spoon, because of that some kind of water umbrella is created by water.
Like in this video: https://www.jsme-fed.org/experiment-e/2017_6/001....
- 131
-1
votes
1
answer
52
views
Will the divergent section of the tube generate pressure below atmospheric pressure?
I noticed the Venturi tube, which is said to generate pressure below atmospheric pressure. It has a convergence section and a divergence section, and I believe that the convergence section will ...
- 2,165