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

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3answers
973 views

Effect of water pressure on sinking objects

As I understand, water pressure increases as we go towards bottom of the ocean. So if an object* is thrown into water and it starts sinking with some speed, does the sinking object's acceleration ...
-1
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1answer
148 views

free falling object with air resistance?

we already know that if we plot speed vs time of free falling object it will be y=gx graph because we know the acceleration is gravity. If it have air-resistance acceleration will be change right?. ...
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0answers
36 views

How to get discretization coefficients of matrix $A$ in Finite Volume Method (FVM)? [closed]

First we have Discretization of the Transport Equation $$ \frac{\partial \rho \phi}{\partial t} + \nabla(\rho U \phi) - \nabla (\rho \Gamma_\phi \nabla \phi) = S_\phi (\phi) $$ In Finite Volume ...
1
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2answers
117 views

Can an extremely small force lift a mountain in a hydraulic system? If not, then why not?

As you see in the picture, a car can be lifted with a small force F1. Here it is shown that 'F1 . A1 = F2 . A2'. What will happen if A1 is of the size of a pin's cross-sectional area. Can a child ...
0
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0answers
23 views

Explicit solutions to simple one-dimensional fluid flow problems? [duplicate]

In order to test and verify numerical simulation results, I'm looking for explicit (or approximate) solutions to simple fluid flow problems to recalculate (like e.g. the pressure development during ...
6
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4answers
207 views

If I bury a cylinder (top end sealed) in sand, how much force do I need to pull it out?

Let's say the cylinder has a diameter equal to its height of $25\;\mathrm{cm}$ the cylinder is sealed at the top and filled with sand the cylinder is also buried in the sand at sea at a depth of ...
1
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3answers
171 views

How do birds generate thrust?

I have been watching this video carefully and I want to know how the wings of birds generate thrust. This is because the wings are more or less flapping up and down --- generating the lift. But I do ...
2
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1answer
466 views

What is the difference between dispersion and diffusion?

What is the difference between dispersion and diffusion? Currently I believe, that diffusion is the mixture of molecules due to Brownian motion. So I read everywhere, that it happens with magnitude of ...
3
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2answers
105 views

Speed of Sound/Push in Supersonic Flight

I've learned from things like the Veritasium Slink Drop and the old "break the speed of light by pushing on the end of a very long pole" thought experiment that objects have a "speed of push" which is ...
2
votes
1answer
142 views

Can a human body supercavitate to survive water impact?

Inspired by this analysis of a human (OK, Captain America) hitting water feet first at terminal velocity, I'm wondering if supercavitation would be possible and whether it would improve your chances ...
3
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1answer
68 views

Mathematically impossible for a vortex line to have loose ends?

Could someone show the math behind it? Source : "A vortex is a bunch of air circulating around itself. The axis around which the air is rotating is called a vortex line. It is mathematically ...
0
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0answers
68 views

Why does air circulate on an airfoil — The Kutta Condition

Why does the air circulate on a flowing airfoil, thus giving rise to increased velocity (circulation + relative airspeed) above the wing and hence decreased pressure.
4
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0answers
58 views

Why do floating objects attract? [duplicate]

Objects floating on the surface of calm water seem to be attracted to one another and tend to come together in a drift. Is this attraction like gravity?
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2answers
92 views

Turbofan: Efficiency wrt. the bypass ratio: “a lot of slow air > a little faster air”?

Reading / viewing up on how jet engines work, this video explains at the 9:02 mark that, for turbofan engines, ".. it is more aerodynamically efficient to have a lot of air moving relatively slowly ...
0
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0answers
57 views

How do you calculate the air and water pressure inside the bell siphon during and after the vacuum is formed?

I am interested in the math/physics behind the simple bell siphon. In this YouTube video, the presenter says that the space in the top of the bell becomes a vacuum when the water flow eventually fills ...
3
votes
1answer
168 views

Stresslet coefficient in fluid dynamics

I encountered a parameter called stresslet coefficient in several literatures, which seems related with viscosity and stream function (also influencing the far field displacement). But there's no ...
1
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0answers
32 views

velocity function in slip effect

$$\frac{dp_l}{dx}-\mu_l\frac{\partial^2 u}{\partial y^2}=0$$ where $\mu_l$ and $p_l$ is the liquid phase viscosity and pressure, respectively; and $u$ is the flow velocity. The boundary ...
6
votes
1answer
140 views

Why will two bubbles floating on water surface attract each other?

Two identical bubbles floating on water surface will form clumps, according to the "cheerio effect". But what's the detail about the force? It's necessary to calculate the shape of water surface, in ...
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0answers
22 views

Efficiency increase via ducting a fan - why is this more in viscous media?

I am looking at mixers for liquids like water or slurries. This brochure claims the following: With the optional jet ring, it is possible to increase efficiency by up 15% in water - even higher in ...
2
votes
1answer
82 views

Can a long pipe stop the flow of a liquid from a pump?

If a pump is discharging a fluid into a pipe, is it possible to add a long enough pipe such that the flow of the liquid will be stopped and none will come out of the open end of the pipe?
3
votes
2answers
56 views

Buoyancy of a positively charged object inside positively charged fluid

An object with a density of $x$ inside a liquid with a density greater than $x$ would float. If we assume that both of these are positively charged and the object is in the middle of the liquid: ...
1
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1answer
129 views

What makes an abstract physical system describable by a “fluid” equations of motion?

We can describe (some of) the dynamics of many systems using fluid mechanics. Of course these include classical fluids like water, more exotic fluids like photon gases and the universe as a whole and ...
6
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0answers
79 views

Time inversion for Euler equation

Consider Euler equation for continuum body: $$\frac{\partial u^i}{\partial t}+\mathbf u\cdot \nabla u^i=- \frac{1}{\rho} \frac{\partial p}{\partial x^i} $$ where $\rho$ is the mass density, $p$ is ...
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0answers
14 views

Applications non-Newtonian fluid [duplicate]

Are non-Newtonian fluid difused in the "real world"? Which are most common or fantastic applications?
2
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2answers
131 views

Bernoulli's principle on a curve ball

I've seen a few excellent answers here on the Magnus force, which explains why balls with a spin will curve. However, my intuition is still telling me that the Bernoulli's principle would push it the ...
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3answers
123 views

Does the equation of continuity hold for turbulent flows?

My textbook mainly deals with laminar flows. The book derives the equation of continuity, which states that the cross-sectional area times the velocity of a flow is always constant. But nowhere in the ...
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0answers
116 views

Conformal maps and flow around corners

The complex potential $\Psi = \psi + i \phi$ ($\psi$ is the velocity potential, $\phi$ the stream function) is an analytical function for an irrotational inviscid flow, since its constituents in this ...
3
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1answer
72 views

Coanda effect and Teapot effect

According to many sources, the teapot effect and the Coanda effect are the same phenomenon. But some sources claim these are two different effects. Does anyone know a reference where this issue is ...
0
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1answer
92 views

How to model a rising helium balloon?

I'm trying to model the ascent of a helium filled weather balloon from 0km to 25km altitude. The plan is to eventually use a python script to calculate the time taken to reach 25km. However, I don't ...
5
votes
2answers
586 views

Do objects sink in ice?

The title explains it all. Do objects suspended in ice sink over time? You may remember the story of World War 2 planes that were found 260 ft under the ice which would be anachronistic with seasonal ...
3
votes
1answer
73 views

Viscous fluid flowing around obstacle: would it deflect earlier?

Consider a viscous fluid, flowing linearly (say, with velocity $\vec u = [1,0]$ everywhere). Then an obstacle is put in the flow. Would a highly viscous fluid start deflecting around the obstacle ...
1
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1answer
28 views

No-slip condition when one fluid flows through another

What sort of no-slip zone is in place when liquid water flows from a faucet through air at room temperature? Does the water drag a layer of air molecules along with it?
2
votes
1answer
164 views

Viscosity coefficients

I'm using the 2nd edition of "Transport Phenomena" by Bird and Stewart. I am having trouble with one of the equations: $$\tau_{ij} = \sum_k \sum_l \mu_{ijkl} \frac{\partial v_k}{\partial x_l} $$ ...
3
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1answer
91 views

Why viscosity is diffusive?

I'm studying fluid mechanics in more depth during my Ph. D. and there is something related with the diffusive term that has been bothering me for a long time. Looking at the convection diffusion ...
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1answer
846 views

How do I calculate pressure drop across a reduction in pipe diameter? [duplicate]

If reducing from one pipe size to another through a reducing fitting, say 6.25cm to 5cm, how do I calculate the pressure drop? Also, how does this impact a change in volumetric flow rate (or does it ...
0
votes
1answer
32 views

Equation of conservation of mass for isotropic fluid considering external forces [closed]

I have trouble doing the following problem: Consider a fluid isotropic in three dimensions such that one can ignore all dissipative forces effect, as would be the viscosity. By analyzing the flow ...
3
votes
1answer
256 views

Pascal's Principle and hydraulic lift

I am introducing myself to fluids through some physics textbooks. Most of them start with Pascal's principle.: If an external pressure is applied to a confined fluid, the pressure at every point ...
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0answers
46 views

How the pressure of a system changes in terms of density?

I am modeling a closed natural circulation loop, filled with water. Some parts of the loop are heated, some are cooled and other are assumed adiabatic. As an effect of heating and cooling the density ...
3
votes
2answers
124 views

Pressure versus Volume system problem

A rigid air tight container of volume 1.5 m$^3$ is filled with a gas of density 8 kg/m$^3$. The container has a density meter that allows us to watch the changes in density in the gas in the ...
3
votes
1answer
125 views

How much weight can a person carry to avoid being blown over by wind?

I wonder if there is a certain amount of weight that I could carry in a rolling bag to help prevent me from being knocked over by the wind. I have balance problems and some lack of muscle strength in ...
2
votes
2answers
98 views

What does k denote in this equation?

Recently, I was attempting to compare the rate of flow of different liquids using the formulas given below: $$P = \rho g h$$ $$ \frac{dh}{dt}\varpropto h$$ $$ \frac{dh}{dt} = -k h $$ $$ ...
4
votes
3answers
171 views

Physical interpretation of the change of diffusion term in navier stokes equations

In the Navier-Stokes Equations, there is one term accounting for convective flow and one term for diffusive flow. At high flow rates, the diffusive term becomes much smaller compared to convective ...
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0answers
71 views

Determining The Coefficient of Viscosity and Diffusivity in an Estuary?

I am curious and eager to discuss about the experience of determining viscosity and diffusivity coefficient for our coastal or estuary model. I myself, just start to use the range of: ...
1
vote
1answer
74 views

How to caculate the pressure of a fluid when there is no gravity

Well for example we put a ball filled with water, with a density of $1\cdot 10^3 kg/m^3$, in space. How to calculate the pressure the water is exposed to?
0
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0answers
124 views

Shape of the “flow head” in Laminar flow in pipe

Laminar flow is a streamlined steady flow with a uniform gradient of velocity across the diameter of the pipe. I am familiar with the elementary treatment of laminar flow, like basic velocity ...
3
votes
2answers
58 views

Estimating parameters to Falkenhagen equation for viscosity of solutes

In this answer to a question about viscosity of water in the presence of solutes, the Falkenhagen relation is given: $$\frac{\eta_s}{\eta_0}=1+A\sqrt{c}$$ where $\eta_s$ is the solution ...
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vote
2answers
115 views

What is a good model for computing water dropping on a surface?

Before introducing my question I would like to outline the fact that I'm a coder, so I can be wrong when using some kind of terminology. What is a good model for computing the flow, the shape and all ...
4
votes
2answers
107 views

Dynamics of Honey in hot water: Honey accelerates like it is falling through air?

Here is a Video showing the honey accelerating in the hot water. As you can see, there are also dynamics. Since the water started stationary, I guess the dynamics arise because of convection flow. ...
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0answers
64 views

Which body position causes the least amount of negative acceleration? [closed]

Let's say you are in the air 500 meters above flat land and you have no parachute. Which pose - forming a sphere or spreading all extremities to make yourself as wide as possible or other - is the ...
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1answer
74 views

How can a fly be in the space of a roofless car, get out of the car space and get back in while the car is driving at the speed of 80mph? [closed]

I was driving in a cabriolet car at the speed of 80mph, while a fly flew in and out of the frame of the car. My question is, How can the fly leave the frame and get back in? Is it possible that it ...