# Questions tagged [fluid-dynamics]

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

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### Does air filling a vacuum produce a force?

Imagine you have a vacuum chamber set at a complete vacuum, and there is a valve to let air in. If you open this valve, the air molecules will start moving into this empty area, eventually filling it. ...
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### Bussman biplane wave drag

The Bussman Biplane consists of two diamond shapes that cancel each-others shockwaves. Which means that it generates no sonic boom or wave drag. It can be modified to generate lift with a slight sonic ...
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1 vote
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### Why is there no Betz type limit for fans?

Betz's law says that only 16/27 of the power in a wind stream can be converted into electrical power by a wind turbine. However, it appears that 100% of the power supplied to a fan motor can be ...
45 views

### Mathematical Proof for Atmospheric Pressure in a Non-Viscous, Incompressible Fluid Flowing Through a Tap

I am studying fluid dynamics and encountered a concept that I am struggling to understand fully. We are dealing with a non-viscous, incompressible fluid (like water) flowing steadily through a tap. My ...
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### Streamlines due to equal and opposite rectilinear vortices in infinite liquid

The question goes: When an infinite liquid contains two parallel, equal and opposite rectilinear vortices at a distance 2b, prove that the streamlines relative to the vertices are given by the ...
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### Jet of Water Entering a Reservoir Below Water Surface [closed]

Let's say that a 0.33-ft diameter pipe enters the wall of a reservoir, 20-ft below the surface of the water, with the incoming water temp being equal to the reservoir water temp (assume 60 degrees F). ...
1 vote
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### How to Measure Temperature Difference in Fluids? [closed]

I have an assignment, in which I have to measure the difference of temperature between the base and the surface of a fluid (e.g. oil) that is being gently heated from its bottom, in order to explain ...
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### Would it be possible to use Differential Pressure to create a fission reaction using a fluid enriched with fission fuel?

Would it be possible to use Differential Pressure to create a fission reaction using a fluid enriched with fission fuel? (To preface this, this is all theoretical. I’ve been playing with the concept ...
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### Ideal gas equation: Bernoulli's original derivation

I'm reading through some of Stephen Brush's books and i found Daniel Bernoulli's (1738, Hydrodynamica) original derivation of the Ideal gas law. In particular, this first part refers to the derivation ...
70 views

### Why are swimmers slower in shallow pools?

When I heard that swimming pools in the Olympics were slow due to shallowness ($\approx 2 \,\text{m}$ instead of $3 \,\text{m}$ deep), I thought it was due to the no-slipping condition, as for example ...
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### 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 ...
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### Comparison of the emptying time of 2 water columns [closed]

I want to compare the time in which 2 water columns would empty themselves under the same circumstances. We have one Waterhose with 25mm inner diameter and one with 15mm Both are aligned vertically ...
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### Optimization of heat and mass transfer in a gas turbine combustion chamber using CFD and ml [closed]

I am working on optimization of heat and mass transfer in a gas turbine combustion chamber using CFD and ml, and I need to develop a methodology, to be precise mathematical formulation for: maximize ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
1 vote
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### 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 ...
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1 vote
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### 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 ...
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### 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 ...
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### 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?
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### 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. ...
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### 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 ...
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### 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
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### 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 ...
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### 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 ...
1 vote
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### 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, ...
35 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 ...
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### 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 ...
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 ...
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### 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
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 vote
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### 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 ...
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### 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 ...
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### 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 $\rho$ ...
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### 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)
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### 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 ...
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### 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 ...
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### 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) ...
1 vote
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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?
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