# Tagged Questions

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

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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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### 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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### 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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### Several suction pumps connected to same air flow source in parallel, is this set up unequal?

I have some sensors with built in vacuum pumps that are open to the room exhaust. I have a single sample source (a compressed air tank, connected to a T fitting and I side stream sample from one of ...
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### Calculating mass flow rate, force of thrust produced compressed air discharge

This one's a bit of a doozy, so please feel free to comment on any one single part or parts to this physics problem. The setup is simple--I have a container (an empty bottle) which I fill with a ...
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### Venturi “Turn Down” [closed]

I'm sorry if this isn't the right forum for this question, but physics is the only stackexchange which had many results for Venturi. I'm trying to measure the flow of air through a 16mm hose/pipe ...
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### Is there reduced water supply due to reduction in pipeline diameter?

I have a 3" water pipeline length 80'coming in from municipal supply, say pipe A, to my water meter. After the water meter, a 2.5" pipeline goes 160' to my underground water tank, say pipe B. Water ...
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### What is actually the characteristic length in fluid dynamics description?

In many fluid-dynamics models and quantities (Reynolds number, Strouhal number... generally various turbulent flow descriptions) there is a parameter (usually $L$ or $\ell$) descripted as ...
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### Extension of the advection diffusion equation with diffusion perpendicular to the flow

Assume you model particle flow in fluid in a cylinder by an advection-diffusion equation, thus taking into account both diffusion and bulk flow. What would happen to the model if you would, at some ...
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### Does groundwater flow depend on the geometry of the aquifer?

I consider two cases of a confined aquifer and want to calculate the pressure / hydraulic head in a quasi 1d case. These two cases are shown in my drawing. I assume homogenious, isotropic darcy flow, ...
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### Why does liquid flow along the mug [duplicate]

I have been wondering why situation described in the picture below happens Why does the water/milk or any liquid poured from the mug/bottle flow along the surface of the vessel instead of leaving it ...
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### Karhunen-Loeve transform of a repeating process

This question is inspired by measurements of an unsteady flow. I have some doubts about interpretation of principal component transform using Karhunen-Loeve theorem. I have (centered $\equiv$ zero ...
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### What shape is a vortex?

When I see a whirlpool form around the drain in my tub, or see a hurricane on the radar, to my naked eye it looks pretty much circular from above. Is it a perfect circle? An ellipse? If not, ...
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### What is the property which flows as described by the stress energy tensor in GR?

I found the following definitions: The stress–energy tensor (sometimes stress–energy–momentum tensor or energy–momentum tensor) is a tensor quantity in physics that describes the density and flux of ...
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### How to determine which one is dominant shear rate vs. flow rate

I have a question for the image below. Now we have a pump connecting to a votator for transferring some slurry. The pump yields a flow rate Q, and the votator yields a shear rate 𝛾. I wanna ...
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### Intuitive explanation for subsonic Fanno flow

In most situations in physics, the effect of kinetic friction is to reduce the macroscopic kinetic energy of a system and convert it into heat, thereby increasing its temperature. but in the case of ...
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### Does steady flow imply laminar?

Most secondary school textbooks, in their chapter about fluid dynamics, seem to suggest that "steady flow" and "laminar flow" are synonyms. Though I never received any fluid mechanics course when I ...
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### Fluid flow in porous medium with moving boundary

Everyone, I would like to numerically model axisymmetric fluid flow in a porous medium with a moving boundary, where the fluid pressure is specified on the boundary, whose radius increases with time. ...
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### Force of Leaky Bucket over time

Indie Lab Procedure: I attached a cylindrical can to a force sensor. On the bottom of the can I drilled holes of various diameters. I then added a volume of water while collecting data from the force ...
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### Reactor opening and total time to reach a specific temperature

Let's say I have a reactor (we assume that its shape can be approximated by a horizontal cylinder) in which there is air at the temperature of 1000K and at pressure of 1.5 bar. Now, I want to open it ...
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### Fluid Dynamics - How does air behave out a window?

I often smoke out a window, and I know that if you're driving a car, the passing air will affect the pressure inside and "suck" out the smoke for instance. But what if you're in a window pane? I have ...
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### Self similarity for slowpokes

I am struggling with the concept of self similarity. If I google search, I either find examples that don't explain all too much or detailed explanations that go waaaaay over my head. I've found a ...
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### Friction in a fluid

when an object is moving in a fluid(air for example), the air will resist the object's movement: molecules of the air will collide with the surface of the object (no slip condition) and then we will ...
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### Calculating flow rate of a syringe

I wanted to know how would I go about calculating the flow rate of a syringe with a metal tip that is dispensing water using a pressurized air. I have had a look on internet about this and on this ...
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### Finding diffeomorphism given vector fields [closed]

Given a vector field how do you find the associated diffeomorphisms? Say I am given a vector field in Minkowski space $$\xi = x \frac{\partial}{\partial t} + t \frac{\partial}{\partial x}.$$ How do ...
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### flow rate in split bottleneck

water flows from top to bottom. first there are two pipes, one with 10 LPM max flow rate and another with 20 LPM max flow rate. they both go to another pipe, which is 20 LPM max flow rate. the total ...
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### How viscosity and velocity relate in a pipe?

Assume we have a pipe that there is a fluid stream in it. By increasing the velocity of the fluid the resistance will increase either (because of the viscosity I think). My question is how are these ...
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### Can a compressible fluid flowing , change its speed in the absence of a pressure gradient?

I've heard that in a high velocity nozzle, any random 'fluid particle' experiences an increase in kinetic energy at the cost of internal energy and pressure energy.And I do understand that for any ...
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### If I was to poke two holes in a water filled balloon would it lose water twice as fast a a balloon with one hole?

if I was to assume that the only force behind water coming out of the holes is the pressure i.e., the height of the water remaining which is constantly changing (declining) as water leaves the balloon,...
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### Streamlines and ideal / real fluids

I was going through some basic description of streamlines in which I noticed that streamlines in an Ideal fluid where shown "equidistant" from each other at a place . whereas in the real fluid ...
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### Streamlines and line of flow of fluid particles

I am very confused with this thing can someone clarify these problems given below-- 1 //Do or can the streamlines of a fluid particle show the position of the particle at a time( using the streamlines)...
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### Drag Force on a cylinder with its long axis aligned with the flow

yesterday I started looking into equations that link a cylinders diameter $r$ and length $L$ to the force experienced in a highly viscous medium ($Re<<1$). I found the onseen-approximation but ...
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### Does water follow the path of least resistance

I am having hard time proving this either right or wrong. Let's take for example a river, does the water flowing down follow the least resistant path?
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### Angular momentum transfer in a bottle vortex

A colleague and I were discussing the following fluid mechanics experiment, which most people have probably tried in their kitchen: take a bottle full of water (a 2-litre transparent soda pop bottle ...
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### Piezoelectric water flow detection

Can a piezoelectric sensor detect constant water flow, or just the moment when the water is turned on and off?
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### What is the difference between hydraulic radius and geometrical radius in a cylindrical pore?

What is the difference between hydraulic radius and geometrical radius in a cylindrical pore? I was going through the 'stacks' present in an thermoacoustic refrigerators there I came across the ...
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### Is there a theoretical limit to the maximum flow through a pipe?

Assuming laminar flow, the Hagen–Poiseuille equation defines the flow rate as a function of the head pressure. However as the flow rate increases so does the head loss due to friction within a pipe, ...
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### What laws of physics apply when a flow of air is divided?

The question is rather simple, but I'm not sure if there is a simple answer to it. Let`s say I've got a flow of air (5 bar, 25 l/min) running through a 6mm hose. Then this flow hits a branch dividing ...
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### Turbulent and smooth flow of water from a tap

When water flows normally from a tap, we can say it is turbulent ( Pic 1 ). But when we attach a piece of cloth to the opening of tap, water flow becomes smooth ( Pic 2 ). We can say that by touching ...
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### How do cyclonic dust separators work? [closed]

I have a background in engineering so I am somewhat embarrassed to say that I don't understand how a seemingly simple cyclonic dust separator works. Here is a link to video of such a device in ...
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### Perturbations on a fluid thread and hydrodynamic instability

I know that due to fluid instability there are some perturbations ( picture below ) after all of these, my question is the reason of existence of these perturbations. Why exactly they appear? Shear ...
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### growth rate of the instability of a laminar flow?

I was just studying something about The Plateau-Rayleigh Instability that I saw something called Growth rate of the instability. My question is the exact physical definition of growth rate of ...
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### Is there an equivalent of probability current for the Wigner distribution?

I know that for a wavefunction, I can derive a probability current $\mathbf{J}$ that satisfies the continuity equation: $$\nabla \cdot \mathbf{J}=-\frac{\partial}{\partial t} \big|\psi\big|^2$$ Can a ...
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### Why in fluid interface, liquids tend to reduce their surface?

I've read something about fluid interface and know that liquids tend to reduce their surface but my question is that why do they tend to reduce their surface?
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### Stationary waves on a laminar liquid flow near collision with a surface [duplicate]

Look at the picture below I know about plateau rayleigh instability. And my opinion is that the reason of this phenomena is plateau rayleigh instability. But there's a question, in the picture ...
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### How to calculate the depth of a flow meter so that there are no air bubbles in the stream

A mix of air and water (foam) is flowing into a tank from the left and going out to the right. To the right there is a suction pump. The tank is divided by a middle vertical wall that stretches from ...
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### What is the difference between flow and expansion?

Fluids (both liquids and gases) will move from one point in space to another due to a potential gradient. Some examples may be: 1) horizontal pipe flow, a fluid will move from a region of high ...
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### How do I convert pressure and flow from Imperial to metric units?

I'm not a physicist or have much experience in engineering. However, for a programming task, I need to change code that calculates "pressure, head, feet of fluid, gauge" into "meters of head, absolute"...
### Why is temperature a function of $y$ and $t$ only?
Say you have an incompressible thermal conducting fluid contained between two infinite horizontal plates separated by a distance $H$. Initially both the plates and the fluid are at rest at ...