Questions tagged [fluid-dynamics]

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

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14
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1answer
704 views

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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183 views

Simulating the spatial distribution of water droplets from a dripping tap

I saw this pattern under a leaky tap. (Recreated images) The pattern was interesting because it looked like a probability distribution. Bigger droplets lie in the centre, and smaller ones scattered ...
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1answer
173 views

What would be the optimal diameter of gold grains if you want to fill a pool with them and swim in it?

I was wondering, if I were to fill a pool with gold/gold dust, what would be the optimal diameter of the grains to minimalize the friction to a point at which you would be able to swim in it ((and if ...
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181 views

Why there is no chemical potential on this thermodynamic relation?

I was following the book by Landau and Lifshitz, Fluid Mechanics (2nd edition) and got stuck trying to understand one of their arguments. On the chapter about Relativistic Fluid Dynamics it is stated ...
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1answer
171 views

Extremum principle for gradient wind balance

Dear Physics StackExchange, as is well known, analytic solutions of the Navier-Stokes equations are only available for very special flows. Therefore, in most fields concerned with fluid dynamics, a ...
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72 views

Is there a scale at which all solids can be treated as fluids?

I was answering an Earth Science SE question that involved the reasons why Earth has an equatorial bulge, and wanted to make an offhand comment such as "Real planetary constituents aren't this strong; ...
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An interesting observation: Ordered, up and down movement of vortex rings in water

I was watching a video on David Tong's research work when I stumbled upon a peculiar movement of vortex rings in water. Around the 1:20 time mark, Baths and Quarks: Solitons explained, David Tong uses ...
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1answer
167 views

Do the Shallow Water Equations produce 2d vorticity/eddies? Why/Why not?

My understanding so far: given a small flow moving forward within a larger stationary body of water, the water ahead would pile up, creating hydrostatic pressure in all 2d directions (and thus ...
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Why is the Fokker-Planck equation only valid for the forward and backward velocities but not for the flux velocity?

I noticed that the Fokker-Planck equation is often only written for the forward velocity $\vec b$ and the backward velocity $\vec b^*$: \begin{align} \partial_t \rho + \nabla (\vec b \rho) &= D \...
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376 views

Minimum velocity required to “skip” a spacecraft across the atmosphere?

When, hopefully, Virgin Galactic spacecraft start regular operations in a few years time, they will be simply travelling upwards, then achieving weightlessness for a short while, followed by a ...
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2answers
3k views

Inverted U manometer

I have some doubts on inverted U manometer. In the picture there is one U inverted manometer, the space above the liquid in the manometer is filled with air which can be admitted or expelled through ...
5
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0answers
179 views

When beer tapping, why does only one of the beers foam up?

In the paper Physics of beer tapping the author go through an explanation of the phenomenon of beer tapping, i.e. the foaming up of beer in glass bottle after it has been hit at the top of the bottle ...
5
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2answers
623 views

Speed of pressurized gas escaping into vacuum

Does speed of pressurized gas escaping through a narrow nozzle into vacuum depend on the pressure? I've asked a question on Space.SE regarding utilizing gas at extreme pressures for propulsion. My ...
5
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1answer
430 views

Does gravity affect water permeability?

Suppose I have an approximately rectangular prism composed entirely of folded paper. If I place 600lbs on top of these rectangular sheets of paper, the paper should compress. How does this affect ...
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134 views

Where else in physics does one encounter Reynolds averaging?

Reynolds-averaged Navier–Stokes equations (RANS) is one of the approaches to turbulence description. Physical quantities, like for example velocity $u_i$, are represented as a sum of a mean and a ...
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Does laminar flow of a mixture tend to result in mass separation?

In a laminar flow regime, a fluid separates into streamlines with a parabolic flow profile. Do particles within such a flow tend to become aligned by mass (e.g. lighter molecules travelling together ...
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100 views

Are sound waves in fluids also Goldstone modes?

One usually says that sound waves in a crystal, ie. phonons, are Goldstone modes of the broken translation symmetries. However, fluids have sound waves and also translation invariance. In fact, it's ...
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Is this wire-melting phenomenon a manifestation of the Plateau–Rayleigh instability; have I done my math right?

In LECTURE 5: Fluid jets from MIT's 1.63J/2.26J Advanced Fluid Dynamics equation 23 is $$\omega^2 =\frac{\sigma}{\rho R_0^3} k R_0 \frac{I_1(k R_0)}{I_0(k R_0)}\left(1 - k^2 R_0^2 \right)$$ and is ...
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2answers
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How do floating wind turbines stay upright?

Some designs for floating offshore wind turbines have asymmetric bases. Why, however, is the turbine sometimes based on the downwind side of the float as seen here and here? And, related, what ...
4
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1answer
112 views

Hydrodynamic interaction between two spheres in $Re\ll 1$ flow

I am studying the interaction between two spherical particles of radius $a$ in a low Reynolds number flow. Because of linearity, I know that their respective velocities will be linear in the forces ...
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87 views

Vortex ring solutions in General Relativity

Vortex rings are captivating toroidal self-reinforcing fluid systems that can happen in turbulent fluids. I wonder if something similar can exist as a solution to Einstein's gravitational equations
4
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1answer
151 views

Flow through a hole in a sphere

I have a sphere of diameter $D$ (radius $R$) with a small hole of diameter $d$ (radius $a$) in it, with air flowing through the axis of the hole: I am trying to estimate the drag force due to ...
4
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1answer
554 views

Negative Magnus effect?

Supposedly, the Magnus effect, which is responsible for the curve of a curveball, and is the reason that tennis players hit their ground strokes with topspin, only acts above a critical Reynolds ...
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2answers
1k views

Variables in calculation of drag coefficient

Okay, so I looked up drag force equation, and I found that the equation involved the drag coefficient. Then I looked up the drag coefficient, and the equation for it involved the drag force. ...
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236 views

relation of vortex street structure to Stroudhal & Reynolds Numbers

While on an expedition to a lake, it was noticed that the wind speeds gusted violently in seemingly periodic wakes and the proposed explanation was Von Karman vortex shedding off of nearby mountains ...
4
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1answer
47 views

How fast do the aerodynamic forces on a surface build up?

I`m currently working on my thesis in the field of active car aerodynamics. In order to design a controller to move parts of the cars body I need to know how fast the drag and downforce acting on this ...
4
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2answers
107 views

Over how large an area does an airplane in flight increase the pressure on the ground underneath it?

A plane exerts downward force on the air, so the ground must exert greater upward force on the air while a plane is in flight than when it is not (in a steady state where the air has no net ...
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1answer
532 views

Fluid dynamics tsunami

I have been given a energy head (pressure) in meters for a given tsunami on land. If the tsunami hits a wall 1 meter by 1 meter, is it possible to calculate the height it will reach. I am thinking it ...
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38 views

Vortex study using video analysis?

I would like to know if they can recommend books or works where vortices are studied through video analysis (for example with TRACKER). I am interested in this because in an undergraduate experimental ...
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45 views

Is there a connection between bathtub drains and tornados?

In the MIT fluid videos (link with timestamp), I heard that a tornado is essentially the same physics as a bathtub drain, but upside down. It doesn't look that way to me. It looks like both fluids ...
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3answers
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Why is Pascal's law true?

Pascal's principle state that if an external pressure is applied to a confined fluid, the pressure at every point within the fluid increases by that amount. Is there a mathematical derivation or ...
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Is there a height limit to the 'inflatable dancer'?

From the Bernoulli equation $$\frac{p}{\rho} + \frac{v^2}{2} + gh = \mathrm{const.}$$ along streamlines, it seems that there should be an ultimate limit to the height of an 'inflatable dancer' when ...
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Implications of thermodynamic inconsistency in CFD calculations

During my PhD work I had to use tabulated values of thermodynamic properties of gases in some Computational Fluid Dynamics (CFD in short) simulations. CFD simulations consist in the numerical solution ...
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Dominant component of drag in a streamlined body (sounding rocket) at 0.8 Mach

We are currently working on CFD solutions on ANSYS Fluent for the drag on our sounding rocket. We are modeling its drag profile through out its velocity range. We currently hit a top speed of approx ...
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58 views

In fluid mechanics, why are the forces on the fluid the same as the forces on the control volume?

From the momentum equation of fluid mechanics: $${\frac{\partial}{\partial t} \int_{V_f(t)} \rho \vec{v} \text{d}\text{V}} = \int_{\sum_f(t)} \vec{\bar{\bar{\tau}}} \cdot \vec{n} \text{d}\sigma + \...
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Why does a spinning disc turn before it fades?

I have recently gotten into disc golf, and an interesting phenomenon happens when you throw a disc hard. If you throw a disc right hand back hand, so that the disc spins clockwise, at high velocities ...
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Will the beaker be acted upon by the force in the direction of the green arrow?

The black circle is the top view of the beaker. In the gravity field, the beaker is vertical down along the beaker axis. The beaker is still. The water in the beaker rotates at a uniform angular speed ...
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If a hollow sphere connected to a rope and half-filled with water forms a physical pendulum, what does the water surface do?

Suppose I make a pendulum consisting of a long string connected to a hollow sphere, then fill the sphere half way with water and set the pendulum in motion by giving all the water and the sphere some ...
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69 views

Why is hydrodynamics a diffeomorphism invariant theory?

The fact that general relativity is a diffeomorphism invariant theory means that there is no preferred co-ordinate system in GR. How is it possible to understand this in the context of relativistic ...
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How do vortex filaments move?

In fluid dynamics (Navier-Stokes, incompressible case), a vortex filament, as I understand it, is a curve of points $x\in\mathbb R^3$ such that the velocity field $v\colon \mathbb R^3\times\mathbb R\...
3
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1answer
95 views

Is a hole better for ceating a toroidal vortex than a pipe?

Does a hole work better for creating a toroidal vortex, where the fluid starts to turn as it exits, than a pipe, where the flow inside is more laminar. Some high power vortex guns have a cone shaped ...
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109 views

Characteristics of the Navier-Stokes equations as a set of PDE's

I am not entirely sure if I should ask this question here or not, but here goes: can anyone suggest any reference (book, article, etc.) about the Navier-Stokes equations from a mathematical point view?...
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Dimension reducing of heat equation 2D problem

I have two metal objects of different thickness and conductivity: 1) thicker, but poorly conductive. It's properties: $k_1, H$ 2) thin and much better conductive. It's properties: $k_2, h$ ...
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Fluid mechanics, symplectic structure, the Hamiltonian, and vorticity

Consider an inviscid irrotational fluid in two dimensions. There are some explicit connections with symplectic geometry that I do not understand. I am not well versed in the later topic, so please ...
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164 views

Swimming pool skimmer floating weir forces

How would I calculate the forces required to open the weir gate depending on the depth of the water? The pump is an impeller type, I want full flow to the pump. I assume there are also forces from ...
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0answers
156 views

Is there any use of a quadratic equation of state in FLRW cosmology?

Consider standard FLRW cosmology. Usually, the relation between energy density $\rho$ and pressure $p$ of a cosmological fluid component is linear: \begin{equation}\tag{1} p = w \, \rho, \end{...
3
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1answer
517 views

What is the most efficient way to pour a liquid through a funnel?

Be it in cooking or an experiment, when transferring liquid from one container to another using a funnel should you: (a) pour it all straight into the funnel, filling it up and waiting for the liquid ...
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40 views

Minimal drop size

I am trying to do something that requires very high precision, so I got stuck in a question: What is the lower bound of a size of a water droplet (and what is the equation, I want to use different ...
3
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0answers
133 views

Derive mechanical pressure from strain energy density

Similarly to thermodynamic pressure, when we have: $$P_{therm} = (\frac{\partial U}{\partial V})_{S}$$ Can we define the mechanical pressure for a fluid as: $$P_{mech} = \frac{\partial \psi}{\...
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Good resource to “deduce” bathtub vorticies from navier stokes?

I am a mathematician that is interested in how physicists justify the creation and persistence of vortices (such as bathtub vortex or tornados) from Navier Stokes. The explicit solutions I have come ...

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