# Tagged Questions

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

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### Vortex in a Pipe / Solutions or approximations?

I want to study a vortex flow in a pipe; in other words helical flow. A couple introductory texts on fluid mechanics (e.g. Shames, Mechanics of Fluids) describe the solution to vortex flow in 2D: ...
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### Why do stellar boundaries exist?

What theorems or insights are available to decide wether a star with some given interdependence between its density, pressure and temperature distributions ought to have a boundary at a finite ...
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### Origin of spin and direction in the magnus effect

If you solve the Bernoulli equation: $$p=p_0-\rho_0{v^2 \over 2}$$ using a complex flow potential for a flow around a cylinder: $$W(z)=v_0 z + {v_0 R^2 \over z} - {\Gamma \over 2 \pi } \ln(z)$$ you ...
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### Is there a difference between solid-to-fluid convection and fluid-to-fluid convection?

I found an online calculator relating to convective heat transfer, but the calculator's description says it's for heat transfer between a solid surface and a moving fluid. However, the definitions ...
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### Physical intuition for Lamb vector

In many aerodynamical descriptions involving vortex structures (e.g. the Crocco's theorem) there is a useful physical quantity called Lamb vector: $$\vec{\omega} \times \vec{v}$$ where ...
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### Prove thrust is double the dynamic pressure across area A [on hold]

Hey, I would be really thankful if someone could answer this for me. Prove that the value of the reaction thrust F equals twice the total dynamic pressure across the area A for an incompressible ...
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### Force required to move a fluid out of piston at depth

I have a cylinder filled with water in it at the bottom of the ocean. Say 100 meters down. In this cylinder is a piston that moves up and down. Its job is to squish the water out the bottom of the ...
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### Water bombing - risks and possibilities

I just saw a video on youtube, where an excavator drops a load of water on a car, destroying it completely. I also saw a video with a fire-fighting plane dropping even more water on a burning truck. I ...
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### Why does a propeller suck in air from the front?

A rotating but fixed propeller sucks in air from the front. What is the cause for this acceleration of air particles? Since propeller blades act like rotating wings, my best guess is that the blades ...
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### Liquid jet in the absence of gravity [on hold]

On earth, we have liquid jets which are falling and accelerating under influence of gravity ( this cause their radius to decrease ) My question is about liquids in the absence of gravity. Can we make ...
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### Changing the Density of a liquid while its surface tension is constant

I'm working on Plateau-Rayleigh waves and I want to investigate the effect of density of the liquid on the wavelength of these waves How can I change the density of the liquid while its surface ...
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If a tornado forms not contacting the ground and then moves over a large flat roof and contacts that surface, producing low pressure above that surface and having normal, higher atmospheric pressure ...
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### Water volume vs. pressure [on hold]

I have a pump that can deliver 14000 liter per hour. It is rated for a lift of 6m and a push of 36m. It has a 4m long 25mm hose as inlet(fed from a 30,0000l tank) and a 25mm outlet connected to 2m ...
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### Vortex filaments in Stokes flow

Suppose that due to some sort of active process (like a moving boundary or an array of pumping source-sink dipoles) it was possible to create lasting vortex filament at low Reynolds number. While ...
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### Nomenclature clarification concerning solitons

My experience with solitons is restricted to the classical setting, namely solutions to the quartic interaction $\phi^4$, the Sine-Gordon equation, and Korteweg–de Vries equations. I was explicit to ...
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### finding work done by the buoyant force on a submerged object [on hold]

A flotation device is in the shape of a right cylinder, with a height of 0.500 m and a face area of 4.00 m2 on top and bottom, and its density is 0.320 times that of fresh water. It is initially held ...
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### How to restict air flow without reducing pressure

I have a large volume of lightly pressurized air say 10000 cubic feet at 1 psi above atmospheric pressure. I want to move it to another area through a duct which is 20 in by 20 in. Airflow will be ...
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### inlet velocity calculation from mass flow rate [on hold]

I am struggling for following calculation, Could somebody please help me with this Mass flow rate and inlet velocity at 300 K and 40 mbar are 4.246 gm/minute and 32.8 cm/sec respectively, if the ...
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### Interpretation of Eigenvalues and Eigenvectors of an hyperbolic conservation law $\partial_t W + A \partial_x W = 0$

I read in a article dealing with a hyperbolic partial differential equations this statement : For any system of hyperbolic partial differential equations (pde), expressed as (1) ...
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I am confused about Reynolds number. I am trying to use the formula to see if NASCAR cars have laminar or turbulent flow. But I am not sure how exactly to use the equation in order to do such ...
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### Angle of rotation of an ellipsoid in a linear shear flow field

I am modeling the motion of an ellipsoid in a linear shear flow field. The ellipsoid is rotating about its shortest semi-principal axis which I have designated the $z$-axis in the body-fixed frame, ...
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### Adding cinnamon to my coffee makes it stop whirling much faster: Why is thats?

When I stir my coffee in a cup, the fluid spins a long time, but when I pour cinnamon into the cup, the spinning fluid slows and stops very rapidly - why?
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### Dyson air blade principle as a propulsion system for an autonomous underwater vehicle?

Any thoughts on applying this principle to an AUV (autonomous underwater vehicle) ? if the power source is a battery, would its efficiency be beneficial to that of a propulsion system?
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### What is the difference between vortexing and centrifuging?

I understand that vortexing will result in mixing / re-suspension of particles, and that centrifugation will result in the separation of particles. However, what is the difference in the physics that ...
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### Fluid diodes for use with cryogens

My aim is to measure the pressure difference across a fluid diode (a tube which allows a fluid to flow easily in one direction and with difficulty in the other direction). Is there an equation which ...
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### Why is an airplane propeller so different from a boat propeller in shape? [closed]

Examples: Airplane propeller Source:http://www.sensenich.com Boat propeller Source:http://www.nauticexpo.com It is obvious that the airplane propeller is drastically different from the boat ...
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### Explicit form of the entropy production in hydrodynamics

I'm trying to understand how hydrodynamics arise from a precise, mathematical formulation of thermodynamics, learning mostly from Landau's "Hydrodynamics". So Landau starts from formulating the ...
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### Is there a relationship between maximum flow rate of a fluid through perforations in a cavity and the flow rate of the fluid in the cavity itself?

I have a scenario where water is flowing horizontally through a (rectangular) cavity. The underside of the cavity has a set of perforations that look something like: and I need to determine the ...
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### Water Waves in the Wake of a Boat

As a boy I noticed that the waves from the wake of my model boat would fan out. If I looked at the end furthest from the boat the front had turned so that it was almost running in the same direction ...
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### Explanation of the waves on the water planet in the movie Interstellar?

We will ignore some of the more obvious issues with the movie and assume all other things are consistent to have fun with some of these questions. Simple [hopefully] Pre-questions: 1) If the water ...
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### Why are mass flow rates found using Venturi meter and pitot tube different?

Regarding an experiment to find the mass flow rate of a fluid (air) using a venturi meter and a pitot tube: What could be the cause of a variance in results obtained using the 2 instruments mentioned? ...
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### Physical meaning of multipole moment

Is there a physical interpretation for multipole moments? For a quantity governed by the Laplace equation ($\nabla^2 \omega = 0$), I understand that the general solution is given by the multipole ...
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### Does the definition of compressibility depend on the frame of reference?

According to many authors, a fluid is defined to be incompressible if the material derivative of the density $\frac{D\rho}{Dt}$ is zero, that is to say, that in an frame of reference following the ...
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### Why does water flowing from a tap become thinner as it flows down

When I opened the tap, water flowing out from the openening became thinner as it flowed down. The same thing also happened when weather came out of a bottle. What is the reason for this? Is it ...
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### Bernoulli Equation for isentropic ideal gas flow

If one derives the Bernoulli Equation for the isentropic ideal gas flow you get: $$\frac{1}{2}v^2 + g z + \left(\frac{\kappa}{\kappa -1} \right) \frac{p}{\rho} = const.$$ Two questions: Is ...
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### Effect of initial suction power on siphon performance

Background I have a flooded crawl space and I'm using a long hose to siphon the water out. I do this as follows: One side of the hose is in the crawl space, and the other side down a hill in the ...
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### How to model/simulate pressures and flows in a network of pipes

I'm having a hard time finding information on how to model/simulate this. I attached a couple files, both of which show an example tank & pump network. It's just nonsense that I made up for this ...
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### Helmholtz decomposition allows incompressible flow with an irrotational component?

A vector field can be written in terms of irrotational and a divergence-free components. Using a 2D velocity field as an example, $\vec v = -\nabla \phi + \nabla \times \vec \Psi$ Where $\vec \Psi$ ...
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### Rotating fluid boundary layer [closed]

This question has probably already been answered but nevertheless: Water is brought to rotate in a glass placed on a rotating disk. In the steady state the water surface is parabolic. What is the ...
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### Is flow speed between two pressures dependent on absolute or relative pressure difference?

Imagine you have a pressure chamber which is divided by a removable wall in two equal parts. Scenario A) Air pressure on the left side is 1 bar, on the right side it is 2 bar. Scenario B) Air ...
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### Does irrotational imply inviscid?

Let us consider a 2D irrotational flow, such that $\nabla\times\boldsymbol u =\boldsymbol 0$. Defining the stream function such that $\boldsymbol u =\nabla\times\psi \boldsymbol n$ where \$\boldsymbol ...
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### Silicone tube with three holes, flow rate, pressure

I have a silicone tube -- a saline solution flows in from one end, and then flows out of three holes of equal diameter and equal distance from each other that are along the side of the tube. What can ...
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### Some hints/introductions/textbooks for LBM (Lattice Boltzmann methods) fluid simulation?

I come from a computer science background and I want to learn details about LBM in fluid simulation. I have been searching around for a concise tutorial that tells me how LBM is used to solve a ...
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### Simulating supersonic flight in game

So this question about how to implement a good looking simulation of super-hypersonic flight of a rocket from launch point to orbit. I have an idea how to implement a good looking aerodynamic on ...
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### Boundary layer theory in fluids learning resources

I'm trying to understand boundary layer theory in fluids. All I've found are dimensional arguments, order of magnitude arguments, etc... What I'm looking for is more mathematically sound arguments. ...
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### How to estimate the Kolmogorov length scale

My understanding of Kolmogorov scales doesn't really go beyond this poem: Big whirls have little whirls that feed on their velocity, and little whirls have lesser whirls and so on to viscosity. ...
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### Kolmogorov/Energy spectrum for turbulent boundary layer

Previously, I have calculated energy spectrum for 3D isotropic turbulent flow data which is equally spaced in all three directions and then to compute the energy spectrum, one performs Fourier ...
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### Rayleigh-Taylor Instability dependence on acceleration direction

I'm trying to bolster my understanding of the Rayleigh-Taylor instability, and I've gotten stuck on the point of which fluid (more or less dense) is being accelerated into the other. Cases of uniform ...