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

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How do we measure mean part or time average of velocity field?

How do we measure mean part or time average of a (known) velocity field? In other words, if I know velocity field how can I measure its time average or mean part? What is time average in general?
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2k views

Why is pressure gradient assumed to be constant with respect to radius in the derivation of Poiseuille's Law?

Poiseuille's Law relies on the fact that velocity is not constant throughout a cross-section of the pipe (it is zero at the boundary due to the no-slip condition and maximum in the center). By ...
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2k views

What is the Coandă effect? How is it defined, and what causes it?

Can anybody explain to me the Coandă effect well? I am finding that many definitions and explanations conflict. I am particularly confused about the following points regarding the Coandă effect: ...
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1k views

What are the limitations of Smoothed-Particle Hydrodynamics?

I've been excited by some of the possibilities of Smoothed-Particle Hydrodynamics (SPH). I have seen some very exciting demonstrations of their use in 3D graphics, but I am wondering how well the ...
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255 views

Power in hydraulic analogy

In hydraulic analogy one compares electrical circuits with water circuits. For the electric case the formula $P = U \cdot I$ for the electric power holds. The analogous formula for water flow would ...
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3k views

How does a sponge “suck” up water against gravity?

If I take a sponge and place it in a shallow dish of water (i.e. water level is lower than height of sponge), it absorbs water until the sponge is wet, including a portion of the sponge above the ...
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202 views

Acceleration of a steady line vortex

In a question, I have to find the acceleration of a fluid parcel in a steady line vortex. I am given that $u_\theta=\frac{A_0}{r}$. So for a steady line vortex, the parcels are following circular ...
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342 views

Lorentz force for electrically conducting fluid flow in homogeneous magnetic field

I am mathematician and have paper which models situation when homogeneous magnetic field is applied to moving electrically conducting fluid. There is such Lorentz force formula on which all the work ...
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616 views

Addes mass forces: can a force depend on acceleration?

My friend and I had a little discussion about added mass forces. I always interpreted $F=ma$ as a cause-effect relationship, so I find rather uneasy to accept that the cause can instantaneously ...
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614 views

How to calculate flow rate from given stream function

I am stuck on a homework question, all because I dozed off in class. I understand what a stream function is but I don't know how to apply it to calculate the flow in pretty much anything. Here's the ...
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65 views

Modeling the formation of a stellar system and matter accretion

I am trying to figure out what do I need to know to properly simulate the creation of a solar system from a particle cloud with random distribution of hydrogen atoms. Being more of a programming ...
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99 views

Can convection cells evolve in stably stratified fluid?

Assume stably stratified fluid but not in equilibrium, e.g. with non-constant temperature gradient for example. Can convection cells be present? Typical example of convection cells is Rayleigh–Bénard ...
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5k views

What about negative Pressure?

Here is something I see : Let's say the ideal fluid(water here) of density $\rho$ is drawn from a source by a motor and thrown upwards with a velocity $v$. Now we take the power of motor be ...
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Is the meander ratio of a river $= \pi$?

To get from point $A$ to $B$, a river will take a path that is $\pi$ times longer than as the crow flies (I think this result is from Einstein). What is the proof of this, and how well does it hold ...
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201 views

How to find outflow pressure/force of air through a pipe

I have been trying to find the correct method for determining the outflow force resulting from a certain amount of air being forced into a pipe of a certain dimension. Essentially i'm trying to ...
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953 views

rate of flow out of a reservoir

A reservoir contains a fluid, and there is an aperture at the bottom through which the fluid flows out as shown in the illustration: I understand that the rate of flow is based on at least these ...
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183 views

Ducted or open fluid flow, which is best for aerodynamics and lift [closed]

I'm designing a copter and trying to decide if the propellers should be ducted or open axial flow. I've read some theory on ducted and open air flow but I can't find any where that compares the two. I ...
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2answers
477 views

How do we measure velocity field?

How do we measure velocity field u(r,t)? i know that how to measure ordinary velocity. $v=\frac{dr}{dt}$ but what about velocity field? what is difference between them?
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808 views

Why doesn't a bus blow due to internal pressure?

When one travels in a bus, if he's sitting at any window, he will feel that the air is coming inside. If someone is standing at the open door of the bus, he'll also feel that the air is coming ...
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163 views

Why is there no UV catastrophe (divergence) in turbulence?

I have just read that as the Reynolds number is increased, the separation of macroscopic and microscopic scales increases and that this also means that there is no UV catastrophy (or equivalently UV ...
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221 views

What is the minimum pressure difference for your ears to pop?

I'm assuming the answer to this largely varies from person to person. Assuming you could instantly change the pressure around your head by amount $\Delta p$, what is the minimum $\Delta p$ for your ...
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1answer
108 views

Distribution of pressure inside a capsule

How would pressure of an ideal gas be distributed over the inside of a capsule (a cylinder with semi-spheres on the ends)? What about the strain on the material? Is there a general formula for how ...
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2answers
367 views

Water draining from a height into the bottom of a reservoir

If I hang one 5 gal. Bucket directly above another, put a hole in the bottom of the upper bucket with a tube inserted just inside hanging all the way to the bottom of the lower bucket and fill the ...
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264 views

Absolute Viscosity of Water at certain temperatures

I just started my class on fluid mechanics. There's a problem that requires absolute viscosity $u$ of water at 25C. I looked up my table in the book and I only have it for water at 20C and 30C. I ...
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2k views

SHM of floating objects

If we consider an object undergoing who has an acceleration proportional to the displacement of the object, it is going simple harmonic motion. In terms of Newton's second law, this is $$ -\dfrac k ...
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5k views

What does the Reynolds Number of a flow represent physically?

What does the Reynolds Number of a flow represent physically? I am having trouble understanding the meaning and the utility of the Reynolds number for a certain flow, could someone please tell me how ...
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1answer
100 views

Water ripples and nonzero divergence

In 2-d, one ripple would mean the velocity of water particles move out radially forming a circular wavefront. The Navier Stokes equations say the divergence of velocity has to be zero, but this ...
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221 views

Pressure in fluids

Fluids exert hydrostatic pressure because their molecules hit each other or the immersed body, but why is that at a greater depth pressure is higher when molecules are the same ? Assume density of ...
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279 views

(How) Does A Vortex Speed Drainage?

When a sink drains it forms a vortex. Does that vortex speeds drainage. If yes, how does the vortex speed drainage? The only relevant thing I could find was an expensive article and a science fair ...
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59 views

What is the relationship between complex time singularities and UV fixed points?

In this paper it is described how the turbulent kinetic energy spectrum and the flatness (a measure for intermittency) are governed by the position of the (dominant) singularities of the solutions of ...
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3answers
2k views

How can I understand a Vortex Tube and its efficiency?

A Vortex Tube takes a pressurized input stream, most typically of a gas, and creates two output streams with a temperature differential. Apparently, it has been described as a Maxwell's Demon. Both ...
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522 views

Finding Surface Tension of water at certain Temperature and Pressure

The question is: Using the Young-Laplace Equation (if applicable), find the surface tension (dynes/cm) for water at 20 degrees Celsius with 2.5 psi. Round to the nearest tenth. ...
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3answers
2k views

Can Increasing the Turbulence Inside a Pipeline Economically Increase Flow?

"The velocity gradient in turbulent flows is steeper close to the wall and less steep in the center of the pipe than it is for laminar flows (Blatt p.97)." Does this mean that some degree of ...
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1answer
2k views

Stress energy tensor of a perfect fluid and four-velocity

In the following demonstration, there is an error, but I cannot find where. (I explicitely put the $c^2$ to keep track of units). We consider a metric $g_{\mu\nu}$ with a signature $(-, +, +, +)$ : ...
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7k views

What is Convective acceleration of flow velocity?

I know that $\frac {dv}{dt}=a$ is acceleration, but: what is convective acceleration of a flow velocity? what is difference between $(v\cdot \nabla) v$ and $v\cdot (\nabla v)$, ?
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210 views

Will a helicoper which is hovering inside a closed box move with the box when we move it? [duplicate]

I got this micro helicopter to play with, you know that cheap double bladed ones you can find for $25 or so. Anyway last night I was on bed trying to sleep and I got this idea, what if we put the ...
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1answer
134 views

Computational Fluid Dynamics methods

I have read some articles about the finite difference method on a cartesian orthogonal grid. I understand how it works when Dirichlet boundary conditions are used, or when Neumann boundary conditions ...
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125 views

Shock Rarefaction Interaction

I am interested to see/know if there exist analytical solutions for shock/rarefaction interaction. A rarefaction wave can catch up to a shock wave from behind. The shock will decay and the motion ...
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2answers
779 views

Are the hypotheses of the Bernoulli equation satisfied for a bird or airplane wing at low Mach number?

A previous question by David Zaslavsky was a request for a broad, "how things work" type of explanation of the lift of an airfoil. The answers given there are enlightening, but don't address a more ...
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1answer
193 views

I read a book saying bernoulli's flight equations didn't have as much impact on lift as most people think [duplicate]

I'm a computer scientist that likes to read about math and physics occasionally. A local author at a nearby aviation center brought bernoulli's flight equations into question. It was clear enough ...
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1answer
543 views

Enginering question about F1 car

In formula F1 car, air coming infront of the car goes up with very high velocity as a result pressure difference is created which is very high from Bernoulli theorem.. The F1 car is very light so ...
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55 views

Books describing motion of objects in fluid [closed]

I'm looking for any resources that would help me model the behavior of objects moving in fluid. My end goal is to be able to describe the motion of irregularly shaped objects in a river environment. ...
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1answer
815 views

What is Relativistic Navier-Stokes Equation Through Einstein Notation?

Navier-Stokes equation is non-relativistic, what is relativistic Navier-Stokes equation through Einstein notation?
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256 views

Additional boundary conditions for inclined flow?

I am solving an inclined flow problem, and am stuck. The problem is to find the volumetric flow rate of inclined flow in a square channel. Once I have the velocity profile, I can just integrate over ...
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1answer
181 views

What is a good reynolds number for this process?

I’m trying to convince my boss that the mixers we are using are too much. I’m trying to prove that we are over-mixing our product. Our product is ink…just your basic ink found in your printer at home. ...
2
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1answer
69 views

CMB anisotropies and tightly coupled limit

Sorry if this is a technical question. I am studying the origin of CMB anisotropies and the tightly coupled limit of the Boltzmann equations. We have a fluid composed of ionized electrons and photons. ...
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2answers
607 views

General procedure for solving fluid flow problems

Could someone help me devise a short series of steps for solving an arbitrary fluid flow problem? Often the most difficult part of these problems is just figuring out what path to take in solving ...
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1answer
195 views

Application of plasma actuators for flow control of moving objects

I've recently been to a converence on plasma physics were, to my surprise, a lot of presentations were concerned with plasma actuator. Could someone, preferably in the field, tell me how long people ...
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2answers
661 views

Hydrostatic pressure on a teapot spout

The phenomenon where water flows on the outside side of a teapot spout is named "The teapot effect", and occurs due to a difference in pressure between water and the atmosphere. Consider the image of ...
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146 views

Equilibrium of a sphere in a water tank

A rigid sphere of radius $R_S$ made from a material with specific gravity $SG_s$ is completely submerged in a tank of water with radius $R_t$ and initial depth $L$ as shown in the figure The ...