# Tagged Questions

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

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### Fluids in motion and pressure

We are taught in the lessons of fluid mechanics that for an ideal non compressible and non viscous fluid $at-rest$, the pressure at any point within the fluid is independent of the direction. But I ...
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### Unsmooth behavior of water waves

I happened to come across this rather interesting picture of a very deformed (or perhaps intricate) wave. The structure looks like it illustrates a superposition of multiple waveforms, as intuitively ...
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### Why is the solution to the Blasius boundary layer problem self-similar?

In every course or textbook that I encountered so far, the authors transform the Navier-Stokes equations of the Blasius boundary layer problem into the Blasius ODE. The problem with many of those ...
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What is the physical meaning of $v\times n$, where $v$ is a velocity vector and and $n$ is a unit normal vector of a interface? Why at the free surface between two fluids, $$v^{(1)}\times n=v^{(2)}\... 1answer 78 views ### Why does plugging a hole in an airplane's fuselage result in a huge (10x) atmospheric pressure spike? While reading about uncontrolled decompression, I came across one famous airline incident, Aloha Airlines Flight 243, where the article I was reading discussed a 1-square-foot hole created in the ... 1answer 331 views ### What are premultiplied energy spectra? When speaking about turbulence in fluid mechanics many times authors speak about "premultiplied energy spectra" but they never explain what really it is. I have searched for a definition but I did not ... 1answer 204 views ### How to make a structure of which water flows out at a constant rate? [closed] Is there any way to make a structure of which water flows out of at a constant rate? Any way could be used to build the structure, including 3D printing. 2answers 74 views ### Using fluid mechanics to show that force is directly proportional to velocity So I am writing a paper about viscous dampers in harmonic oscilators, however I was looking at some old fluid mechanic notes and I thought I had come across what I needed although I have gotten stuck. ... 3answers 2k views ### Distance traveled by a water jet I helped my kid in a science fair project, where we punctured holes in a water bottle at various heights and then measured the distance traveled by the water jets before they hit the ground. The ... 2answers 198 views ### Does the definition of compressibility depend on the frame of reference? According to many authors, a fluid is defined to be incompressible if the material derivative of the density \frac{D\rho}{Dt} is zero, that is to say, that in an frame of reference following the ... 3answers 695 views ### Boat Hull Drag in Shallow Water I paddle several different types of small craft in the ocean and bays near my home. One phenomenon I've observed is beyond my understanding of drag on a narrow displacement hull. When paddling in ... 1answer 51 views ### Monoatomic fluids and free space around atoms In monoatomic fluids the atoms can move quite freely around each other. Is there any thermodynamic/statistical mechanic equation how much free space there is between the atoms? This has to be ... 2answers 284 views ### Do rotating non-circular projectiles also experience Magnus lift? The Magnus effect has been studied on spherical projectiles such as golf balls, tennis balls, and soccer balls. The backspin of a golf ball leads to Magnus lift that opposes gravity, thus allowing the ... 1answer 224 views ### Is Archimedes' principle valid with moving objects? I was solving a classic application of Archimedes' principle: a body partially submerged that is made to oscillate vertically and perform a simple harmonic oscillations. The equations turn out to be ... 0answers 125 views ### Why is the water in a cup not inclined (opposite the cup) when the plane takes a turn? When a aircraft takes a turn: Why is the water in a cup not inclined (opposite the cup)? 2answers 429 views ### What is the pressure of falling water? Imagine a breaking wave 10m high. The wave breaks onto a beach. What is the maximum pressure created by the wave on the beach, ignoring effects such as might be cause by air? 1answer 39 views ### Reference for passive scalar I would like to learn about turbulence of a scalar advected by a random velocity field. I know that this problem can be solved analytically and that the statistics of the scalar field are intermittent.... 1answer 44 views ### the different settling speed of two solid particles in fluid In a container full of fluid A, which can be water or oil, I have two solid particles, both of which are of the same material. These two particles are of different size. One is bigger than the other ... 2answers 198 views ### Ice when melted at zero gravity [closed] I wish to know the behavior of ice at zero gravity, how ice will behave when melted at g=0? Thanks 2answers 642 views ### How to increase flow rate? [closed] I am coming up with ideas of how to increase grain (corn, soybeans etc,) flow in a tube. My first idea is to have spiral ribs inside of the tube to create a vortex effect that will allow the grain to ... 1answer 381 views ### Could Navier-Stokes equation be derived directly from Boltzmann equation? I know how to derive Navier-Stokes equations from Boltzmann equation in case where bulk and viscosity coefficients are set to zero. I need only multiply it on momentum and to integrate it over ... 3answers 3k views ### Why does the lake surface appear darker in some areas? What causes the surface of the lake to appear darker in some places? Firstly, I know that it's not fishes that cause it. Secondly, the dark surfaces move from time to time and thus are not ... 2answers 573 views ### Patterns in laminar flow of tap water This is a simple experiment that anyone can do at home. Open your tap so that the water maintains a laminar flow, and the cross section of flow is considerably thin. Place your finger 3-4 cm below the ... 2answers 430 views ### Fluid flow: Force acting on the fluid and the Navier-Stokes equation Consider a one dimensional fluid flow in a rectangular tube. Typical streams are the poiseuille streams. Consider the case in wich we apply a force on the fluid. The Navier-Stokes equation (for ... 2answers 483 views ### Deriving equation for skipping stones over water Why can rocks skip over water (stone skipping)? For instance, if you conduct an experiment where you drop both rocks from the same height, but give one considerable acceleration in the x-direction, ... 0answers 52 views ### Are there any in depth superfluid mechanic analyses of spacetime? Has there been much work done that treats particles as vortexes in a fluid, or dark matter as bubbles in this fluid (bending space in the same way massive particles (vortexes) are observed to do, but ... 2answers 116 views ### Does air density influence a football player's ability to “bend” the ball? Whilst reading an article on nasa.gov, there was a claim that I found interesting: At higher altitudes, the density r is lower producing a larger radius of curvature and a straighter path. The ... 0answers 35 views ### Fluid approximation for inflaton? In cosmology there is a widely used fluid approximation which applies when the mean free path is very small compared to the scale of observation and then all the properties of stress energy tensor can ... 0answers 59 views ### Why do clouds not fall? [duplicate] The density difference between the clouds and the atmosphere favours the clouds to fall. Why don't they? 1answer 141 views ### Calculate the pressure required to stop a pipette (or pipe) from dripping [closed] Suppose that I have a pipette with a length of 10 cm that is filled with water, with a diameter of 1 cm. If it is held vertically, how would I calculate the pressure required to stop any water from ... 1answer 136 views ### Change in radius of a charged bubble I was thinking about what'd happen to the radius of a bubble, if a charge is uniformly distributed over it. At the beginning, I thought that the bubble would expand, for sure, because of the like-... 2answers 386 views ### Why pouring milk from a height makes a chain shape? [duplicate] I have noticed a chain-like shape when milk is poured from a height into a cup. Actually the chain pattern repeats itself after some distance and again till it reaches the milk in the cup. Any ... 1answer 137 views ### Why Benard cells are usually hexagonal? Solution of equations of convection (hydrodynamics equations in Boussinesq approximation) involves existence of convective cells, so-called Benard cells. They are represented by superposition of ... 1answer 85 views ### Is, in terms of sound, pouring water in a bottle the analogous of blowing into it? When we pour some liquid in a closed container, for instance a jar or a bottle, we usually hear two (acoustical) noises: one is the classical turbulent (I guess white) noise, that in my view is ... 1answer 53 views ### Amount of unknown parameters in compressible Euler equations I'm looking at this page for the compressible Euler equations. To me it seems, in the 1-dimensional case, there should be 3 unknowns: density, velocity, and pressure. This is because the energy E ... 0answers 30 views ### The free surface of the wave is a material surface If we define the free surface by: F(x,y,t)=y-h(x,t)=0 Then for this to be a material surface \frac{DF}{Dt}=0 on y=h(x,t) However on y=h(x,t), F=0, so doesn't this just imply \frac{DF}{... 0answers 568 views ### Static pressure in relation to CFM I’ve scoured the internet in search of an answer to no avail, so it’s time to ask the experts! My background is more Chemistry and Biology, not Physics, or specifically fluid dynamics, so bear with ... 1answer 235 views ### Metric inside a sphere of uniform density? Is an exact solution to Einstein's Field Equations known for the interior of a sphere of uniform density (to approximate a star or planet, for example?) 1answer 265 views ### Rate of evaporation of water with mass loss at set temperature and pressure I have a cylindrical tank with an 'm' amount of water in it. The tank is sealed except through an exhaust with a fan that keeps a lower than atmospheric pressure (but not vacuum) inside the tank. The ... 2answers 764 views ### Given airplane mass, velocity of air under wing, and a wing area, find velocity of air over wing I attempted to solve this problem as a tutor for a student and struggled, but want to be convince the professor didn't provide enough information. The problem is essentially: We wish to maintain a ... 2answers 65 views ### Eulerian mass conservation on a stream line to Lagrangian mass conservation if the density of a fluid particle is conserved on a streamline,$$\frac{d\rho}{dt}=0.$$Why does this mean$$\frac{\partial \rho}{\partial t}+(\mathbf{v}\cdot\nabla)\rho=0$$is true everywhere? Why ... 0answers 30 views ### How can a rotating cylinder produce lift? [duplicate] In the air, the velocity and angular momentum that causes a force against gravity is lift. How can a rotating cylinder make lift with angular momentum? 1answer 77 views ### Can the fluid volume in a control volume change over time? For a control volume, am I right in thinking it's just the boundary of the volume that is fixed? The volume of fluid inside the control volume can change, over time? 0answers 35 views ### The velocity of air on an airfoil They say that velocity of the top layer on an airfoil is higher than the lower layer. But, according to the continuity principle of fluid, the velocity of air is inversely proportional to the surface ... 2answers 586 views ### Backwards airflow in a Venturi tube When air is flowing through a Venturi tube, it's speed increases at the narrow section. If the airflow is arranged by moving the Venturi tube through calm air, this means air will momentarily flow ... 1answer 94 views ### Does air speed vary in a jet engine axial flow compressor? Does it get slower and slower after each stage because of the pressure increase? Or does the shrinking of the space after each stage compensate for this and the air speed at the last stage of the ... 1answer 227 views ### Using the continuity equation against gravity Studying Fluid Mechanics right now and in my textbook there is an example of getting water up to a bathroom in a house. We're given the diameter of the inlet pipe and bathroom pipe, but only the ... 2answers 251 views ### Viscosity for steam in the Reynolds number In calculating the Reynolds number for a flow of steam in a pipe, this is the general formula I am trying to use:$$Re=\frac{\rho d v}{\mu} with density $\rho$, pipe diameter $d$, flow steam $v$ ...
The question is about sphere like surfaces, like the hemispherical base of a cylinder as shown. The cylinder is filled with fluid till the given height $h$, and the density of the fluid is $\rho$ ...