# Tagged Questions

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

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### Flow rate calculation of small nozzles in a linear system

I am currently working on a system for feeding multiple very small reactors, of which a prototype is described in the figure below (for scale, the distance between each nozzle is 9mm). The liquid ...
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### What happens if a cylinder filled with propane gas is connected to an empty one of the same size

Lets say I have 2 100lb propane cylinders. One of the cylinders is filled to its maximum capacity while the other is empty. If I connect them to each other via a high pressure hose in an upright ...
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### Why is turbulence caused?

In high Reynolds numbers we have turbulent flow. This is because the inertial forces are much greater than the viscous forces. I understand inertial forces to be actually the fictional forces due to ...
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### Difference between inviscid and viscous flow

In my lecture notes, I have a load of examples and I want to sort out which egs are viscous flow and which are inviscid flow. It is not always said if the flow is viscous or inviscid. Please can ...
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### Newton and Stokes regime in settling velocity

Can you tell me please the differences between Newtonian and Stokes regime in settling velocities of particles in moving fluids? I know that Newtonian applies to higher Reynolds numbers. Does that ...
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### Viscous fluid boundary condition

Consider an incompressible viscous ﬂuid of kinematic viscosity $ν$ , dynamic viscosity $µ$ and density $ρ$ . A viscous boundary layer is located over a solid surface at $y = 0$ and $x > 0$. The ﬂow ...
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### Bernoullis in Parallel pipes

Im having trouble with a scenario where a flow 𝑄𝑆𝑡𝑎𝑟𝑡 splits into two parallel pipes 𝑄𝐴 and 𝑄𝐵 then rejoins 𝑄𝐸𝑛𝑑 before exiting the control volume what makes this scenario difficult is ...
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### Time Scales Of Processes In Molecular Dynamics

Suppose I run a molecular dynamics simulation of a fluid sandwiched between solid walls which are periodic in the lateral directions and finite in the direction of the fluid film thickness. Now, I ...
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### Finding boundary condition of stationary solid body

A ﬂuid ﬂows past a stationary solid body of arbitrary shape. Write down the boundary condition on the ﬂuid velocity $\textbf u$ for an inviscid ﬂuid and for a viscous ﬂuid, at the solid surface. We ...
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### Why is it easier to remove stains with water?

And I don't mean stain removers, but literally tap water. See here's the thing, you can't remove a stain without some sort of friction, and water reduces friction. Yet if I say, spill some coffee and ...
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### Water Jet scenario [duplicate]

So I have a question I came across that goes along the lines of Water is discharged from a 20 mm diameter nozzle to the atmosphere. The water jet leaves the nozzle with a velocity of 12 m/s and is ...
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### Pressure drop through a flat vs. pleated filter

Consider two identical pipes where two identical air flows take place. The pipes are "obstructed" by two filters made of the same fabric, one flat, the other pleated as shown in the following picture. ...
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### Physical Meaning of Over- / Underdetermined Acoustic Eigenvalue Problem

I am performing an eigenmode study on a system of acoustic ducts. The system consists of two large cylindrical volumes connected by several smaller cylindrical volumes (modeling a combustion chamber ...
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### How do ceiling fans cool or heat?

Why does a ceiling fan blowing air downward cool in the summertime: whereas a ceiling fan sucking air upwards heats in the wintertime: ? Why isn't it the other way around?
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### Three-dimensional centrifugal instability and the Rayleigh critertion

Lord Rayliegh postulated that for a two-dimensional symmetric swirling flow, a three-dimensional instability may occur if the angular momentum magnitude decreases in the radial direction with respect ...
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### Rotational and irrotational flow

Can a rotational flow take place if there is no viscosity? If yes then will it always be rotational or will it become irrotational after some time?
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### How relevant are Reynolds number and flow regime in very small tubing at very low flow rates?

I know that in bigger pipes, Reynolds number and the dominant flow regime can be important in further decisions. Now, I have an application with tubing of about 0.5 mm in diameter and a flow rate of ...
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### Contact Angle In Capillary

One Professor recently told me that in an insufficient length capillary, the contact angle doesn't change, its an inherent property which does not change. If the edges of capillary are extremely sharp ...
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### Rotating droplet of liquid

I'm interested in the fluid dynamics of a spherical droplet of liquid whose surface is forced to rotate at a constant velocity. I'm interested in either steady-state solutions (the internal flow ...
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### How does gas friction emerge?

The fluid parcel model states that a gas can be modelled as the sum of infinitesimal small parcels. According to this model, the friction of a gas with a solid surface (e.g. the wall of a pipe) ...
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### Do gases have phonons?

A phonon is a quantized unit of sound; they are encountered when quantizing lattice vibrations in solids. Now, even an ideal gas supports sound waves, but in such a case, interactions between atoms ...
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### acceleration of water when it travels through garden hose

How to derive average acceleration of water as it travels through nozzle of garden hose. Suggest some assumptions for better understanding
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### Why would oscillation of a gas column inhibit rather than promote mixing?

Many years ago I helped to support an experiment conducted in Japan which investigated the effects of high frequency oscillation ventilation (HFOV) on the mixing and distribution of gas into the lungs....
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### Is there any electric current in a continuous medium of moving charges?

Consider a continuous charged fluid (unlike in real life where charge is carried in discrete chunks like electrons) in a bottle. Suppose that the fluid is stirred in a circular manner and then left to ...
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### Water Moving Through Coffee Grinds

Like many of my peers, I rely on coffee as if it were prescribed to me. It might even be classified as an addiction if it weren't for the cultural norms surrounding a cup of coffee or tea in the ...
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### Pressure required for gas to flow through an orifice

Imagine I have an upside-down cone, which is filled with a gas - let's say it is air. At the bottom of this cone is an extremely small hole (a few microns in diameter). How do I go about calculating ...
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### Effect of wall friction on stagnation pressure and temperature

We have incompressible air flowing through a cylindrical pipe, from section A to section B. The process is considered to be stationary. My theory book says that, because of the frictional losses on ...
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### Intuitive explanation for lower pressure above airfoil

From Bernoulli's principle we know for an incompressible fluid (constant density $\rho$) in a gravitational potential $\psi=gz$, that we can state the equation along a streamline from point 1 to point ...
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### Can drag force and lift force be in the same direction in the following case?

In the attached picture, can I say that there is a lift force in the Y-direction, and a drag force too in the same Y-direction? $F_L$ proportional to $V_fx^2$ ? $F_D$ proportional to $V_py^2$ ? Is ...
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### Is drag force in the direction of particle motion or opposite to motion?

Suppose water is flowing in horizontal direction (positive $x$-direction) and a particle immersed in that water is also moving in the same direction. In this case, is the drag force $F_D$ in the ...
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### Trouble understanding a formula in Turbulence

I'm very new to turbulence.I'm reading this book on "Turbulence: An introduction for scientist and Engineers" by Davidson. I came across this formula for energy dissipation. This is in chapter one. I ...
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### Bent capillary tube

What would happen if I bend a capillary tube and use it as shown in figure? 1.Will the water(or any other adhesive liquid) level remain same as before bending? 2.or it will come up till end? 3.Or ...
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### Physical explanation of Pascal’s Law

I have a problem with physical explanation of Pascal’s Law. For example, when I was teaching my sister (a high school student) about force transmitting by a rope, I said her: “In a very simplified ...
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### A question about droplets formation

This picture shows a fuel ejector of a diesel engine that propels ships. When the fuel line pressure exceeds 380 bars, the tension of the spring is overcomed and fuel is ejected into the combustion ...
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### Approximating the pressure drop for a centrifugal fan/blower

Is it possible to approximate the pressure drop for a centrifugal fan by using the fan's characteristics, the airflow, and the speed at which the fluid is being ejected? Would it be possible to adapt ...
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### Convective heat transfer coefficients - how do you calculate them?

How do you calculate (or otherwise determine) h_i and h_o (convective heat transfer coefficients) for a heat exchanger? Standard thermodynamics equation below for reference. My specific situation ...
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### Calculating Pressure in Lattice Boltzman Equation

I'm new to the LBE, but have been finding them very helpful for doing some fluid dynamics simulations. I was wondering how pressure would be calculated. I'm guessing this is quite easy, but it's ...
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### Do you need to increase pressure to pump a fluid from a large pipe through a small pipe?

The application of this question is towards clogged arteries and blood flow. I am wondering why people with clogged arteries have high blood pressure if a smaller artery (assuming it is smooth and ...
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### Navier-Stokes Energy Equation

I've been assigned (for homework in a mathematical modelling course) the task of deriving the Navier-Stokes energy equation in one space dimension: Consider a fluid flowing through a cylindrical ...
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### Inertial force of fluids [duplicate]

Reynold's number is the ratio of inertial force to viscous force.I also know the definition of inertial force.but where does a fluid get this force from?what are the factors responsible for this force?...