# Questions tagged [fluid-dynamics]

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

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729 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 ...
178 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 ...
188 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 ...
190 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 ...
185 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 ...
82 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; ...
218 views

### 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 ...
178 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 ...
137 views

### 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 \...
389 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 ...
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 ...
180 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 ...
668 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 ...
449 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 ...
139 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 ...
42 views

### How to obtain hydrodynamics from many-body quantum mechanics?

It is known that the Schrodinger equation is equivalent to the Euler equation (with a "quantum potential" term) and to the probability conservation equation (which is formally identical to ...
37 views

### 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 ...
115 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 ...
94 views

### 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 ...
91 views

### 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 ...
117 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 ...
92 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
167 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 ...
611 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 ...
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. ...
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 ...
2k views

### What's the meaning of a pressure coefficient in fluid mechanics?

I have a jet of air leaving a contraction from a pressurized tank at a velocity of 28.8 m/s and it's being directed towards a small object equipped with static pressure holes on its surface that give ...
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 ...
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 ...
544 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 ...
34 views

### Splashing of Falling Sphere does not depend on Sphere Size

I am reading through a Nature article where the authors study whether a sphere dropped from height into a body of water has enough velocity to splash and form the classic effect where air is pulled in ...
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### 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 ...
57 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 ...
45 views

### 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 ...
63 views

### 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 ...
446 views

### What are some good books on fluid mechanics for the olympiad?

I have already checked out Book Recommendations on the community wiki, but the recommendations there don't suit what I ask for. I need problems/problems+theory book(s) on high-school-physics-olympiad/...
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### 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 ...
19 views

### 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 ...
76 views

### 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 ...
72 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 ...