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Questions tagged [fluid-mechanics]

Questions relating to fluids at rest (fluid statics) or the forces and effects of fluids in motion (fluid dynamics).

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34 votes
8 answers
9k views

Is it possible to "pump" a powder?

Is it possible to "pump" a powder the same way liquids can be pumped? If so, what are the challenges? If not, what are some alternatives?
peterh's user avatar
  • 2,359
29 votes
11 answers
9k views

Is it possible to build an air compressor with no moving parts?

Is it physically possible to build an air compressor with no moving parts? I'm envisioning a thermodynamic cycle capable of compressing air with no moving parts and operated in a stationary way. There ...
user3368561's user avatar
23 votes
12 answers
51k views

How to stop water flow in a siphon?

I have assembled a small DIY drip-irrigation system for my terrace garden. Please have a look at the attached image. I switch on a small pump to start the drip-irrigation system and then switch it off....
javed's user avatar
  • 333
22 votes
3 answers
2k views

How to know whether the flow is supersonic in a nozzle?

For a project I had built a convergent divergent nozzle designed for Mach number = 3. In that project, I could know the flow has gone supersonic by seeing the manometer fixed between the throat and ...
Subodh's user avatar
  • 1,413
20 votes
3 answers
5k views

Windmills in empty fields. Why no trees?

I have noticed that windmills are generally built in empty fields with no trees around, and I've been wondering why... A windmill is generally way taller than trees and I can imagine that trees don'...
Physther's user avatar
  • 614
18 votes
2 answers
816 views

What is the physical interpretation of the second term in the viscous stress tensor in the Navier-Stokes equations?

I have been searching for this answer for awhile. I've read numerous texts and even watched some lectures online, but often times this is never explained and just given. The viscous stress term in the ...
Adam O'Brien's user avatar
15 votes
3 answers
267k views

How to calculate flow rate of water through a pipe?

If a water pipe is 15 mm diameter and the water pressure is 3 bar, assuming the pipe is open ended, is it possible to calculate the flow rate or water velocity in the pipe? Most of the calculations I ...
Richard's user avatar
  • 341
15 votes
3 answers
58k views

How to determine the characteristic length in reynolds number calculations in general?

I understand that the reynolds number is given by the expression $Re=\frac{\rho v L}{\mu}$, where $\rho$ is the density, $v$ is the fluid velocity and $\mu$ is the dynamic viscosity. For any given ...
Paul's user avatar
  • 2,569
15 votes
5 answers
10k views

Purpose of spheres and fins on submarine propeller

On a recently launched Russian diesel-electric submarine, the rear propeller has two distinct features. You can see spheres at the base of every propeller blade: Also, the trailing edge of the shaft ...
Rupert Reckless's user avatar
14 votes
1 answer
15k views

Why is Mach 0.3 the threshold separating compressible and incompressible flow?

I've read that Mach 0.3 is pretty much the upper limit for treating air as an incompressible fluid. The sources I've read seem to treat this as a given, without proof or justification. Why is this ...
Paul's user avatar
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13 votes
3 answers
7k views

What makes the material travel through Archimedes' Screw?

...or in other words, why doesn't the material just stick to the screw, rotating in place, without progressing along its length? In the simplest case, the answer is obvious: gravity. If it's a ...
SF.'s user avatar
  • 6,105
13 votes
1 answer
436 views

Estimating whether the flow through a valve or nozzle cavitates

My understanding is that cavitation occurs in the flow of a liquid when the static pressure drops below the vapor pressure, even intermittently. So even if the time-averaged static pressure (what you ...
Ben Trettel's user avatar
13 votes
3 answers
6k views

How to prevent deposition of dust on ceiling fan blades

It is well known that dust gets deposited on a rotating ceiling fan because formation of a boundary layer over fan blades. The air layer next to blade surface is stagnant due to no-slip condition and ...
Debanshu Thakur's user avatar
13 votes
2 answers
270 views

How do the bubble oscillations in two-phase flows affect the dissipation rate of turbulent kinetic energy?

I am investigating the effects of bubble oscillations (calculated using the Rayleigh-Plesset equation for bubble dynamics) on the turbulence in two-phase flows. Since I am using cryogenic fluids, the ...
Karthik Venkatesh's user avatar
12 votes
3 answers
5k views

Which turbulence models are suitable for CFD analysis on a streamlined vehicle body?

Many commercial and open-source CFD codes implement several closure methods for the non-linear convective acceleration term of the Reynolds-averaged Navier-Stokes (RANS) equations. Common methods (...
Paul Gessler's user avatar
12 votes
1 answer
15k views

What is the meaning of vorticity and circulation?

In fluid kinematics I can't understand the meaning of these terms : vorticity and circulation. Can somebody give me a description of these terms so that a lay person can understand them easily?
M.Tarun's user avatar
  • 375
12 votes
3 answers
511 views

Exploiting fluid dynamics for the optimal design for gas mixing in a furnace

Background This is the standard design for a thermal furnace used in the Clauss process, which converts H2S to SO2. The main problem with the furnace is that gas mixing is rather poor and results in ...
22134484's user avatar
  • 331
12 votes
4 answers
6k views

Would an object at the bottom of the sea still experience buoyancy? [closed]

So I understand that buoyancy happens because the fluid exerts larger amounts of pressure from beneath an object compared to above, like in this picture So my question is, what if the object is ...
M. Wother's user avatar
  • 351
12 votes
2 answers
11k views

How long does it take for dust to settle out of the air?

In order to make this a manageable question, let's add a few simplifications. The dust particles can be well described as uniform spheres of radius $R$ and density $\rho$. The space is enclosed ...
Chris Mueller's user avatar
12 votes
5 answers
3k views

Low cost, moderately accurate water depth measurement

tl;dr: After an extended conversation with an old-timer, I realized a few things: The single most valuable measurement for the majority of people will be water-depth-in-well. The second most valuable ...
Peter Rowell's user avatar
11 votes
6 answers
5k views

What fundamentally distinguishes cavitation and boiling as different phenomena?

Cavitation and boiling are names for phenomenon that both involve sudden appearance of bubbles of vapor within a liquid, and in both cases they happen when the local hydrostatic pressure is lower than ...
uhoh's user avatar
  • 319
11 votes
5 answers
7k views

How is lift related to airspeed?

Lift produced by an airplane wing is related to airspeed - this much is clear; a plane moving too slowly will stall. But what is that relation? Linear? Quadratic? Exponential? I don't need the exact ...
SF.'s user avatar
  • 6,105
11 votes
2 answers
2k views

Can you use the Hagen-Poiseuille equation for a pipe which radius is in the sub-millimeter region?

As this will depend on Pressure drop $\Delta p$, assume that it does not leave the range from 0 to 100 bar. The Hagen-Poiseuille equation for an incompressible fluid is defined as: $$ \dot{V} = \frac{...
JHK's user avatar
  • 1,618
10 votes
2 answers
585 views

How is gas flow through (extremely long) pipelines monitored and controlled?

(This is closely linked with measuring the Mach number inside a nozzle but it is not regarding the supersonic flow) Friction and heat transfer have effects on the Mach number of the compressible flow ...
Subodh's user avatar
  • 1,413
10 votes
5 answers
1k views

Can a riverboat tack upriver?

Note: This question originally appeared on the Physics site (see here) and was closed for being off-topic. Whoops! If ships at sea can tack in a zigzaggy pattern by converting backwards force from the ...
Palbitt's user avatar
  • 239
10 votes
2 answers
925 views

How are wind tunnel models used when designing supersonic aircraft?

There are two things involved: wave drag and boundary layer separation. The wave drag depends on the Mach number while the latter depends on the Reynolds number of the flow. It's easy to maintain the ...
Subodh's user avatar
  • 1,413
10 votes
2 answers
661 views

Hydraulic siphon spillway entry and exit velocity magnitude and profiles

I am working on the hydraulic seawater flow though a siphon spillway. My task is to determine at a location in front of the spillway the flow velocity and profile when The flow enters the spillway ...
Ruudc's user avatar
  • 101
9 votes
2 answers
2k views

Why does filter efficiency decrease with thickness?

I understand that the most important parameters for a filter are basically pressure drop and thickness, both of which affect efficiency. However why does thickness cause a change in efficiency? I'm ...
User2341's user avatar
  • 313
9 votes
1 answer
781 views

Thrust distribution of engine

I am studying the thrust distribution of jet engines. But I'm now a bit confused. In Rolls-Royce's "The Jet Engine" book, http://aeromodelbasic.blogspot.com/2012/05/thrust-distribution-distribution-...
Jono's user avatar
  • 91
8 votes
2 answers
10k views

How Can I Determine Multiple Discharges from a Pipe Network? (Knowing the Inlet Conditions)

I have little/no experience with pipe networks and have been searching for a methodology to calculate the discharge from multiple outlets of an oil pipe network with a single inlet. As far as i can ...
Petrichor's user avatar
  • 508
8 votes
1 answer
187 views

Scale models of thermal plumes

I want to look at the thermal heat plume of a human in still air in a room. So I have a tank of water with a heating coil/cylinder in it. How do I calculate what the heat output of my heating coil ...
HCAI's user avatar
  • 205
8 votes
5 answers
5k views

Torque converter torque multiplication

I keep reading all over the internet that a torque converter multiplies torque at stall speed. Every explanation I have found sounded more like hand-waving than quantitative physical explanation. ...
gsimard's user avatar
  • 181
8 votes
2 answers
6k views

Why can we neglect the inertial (but not viscous) term in Navier-Stokes at low flow and high viscosity?

Why can we neglect the inertial (but not viscous) term in Navier-Stokes at low flow and high viscosity? Complete Navier-Stokes: $\rho \frac{D\vec{v}}{Dt}=\rho g - \nabla P+ \mu \nabla ^2 \vec{v}$ ...
Raoul's user avatar
  • 83
8 votes
1 answer
2k views

Contours of radial-flow impeller vane

I've been studying the design of turbopumps and their impellers, all the design equations and what-not; this question concerns the contours of the impeller vanes. I'm going to attempt to enter ...
ecfedele's user avatar
  • 519
8 votes
1 answer
375 views

How do I select an appropriate turbulence model?

Turbulent flow involves extremely small scale fluctuations which are usually too small to resolve by using the Navier-Stokes equations directly. To overcome this, we model each transient variable $\...
Paul's user avatar
  • 2,569
8 votes
1 answer
1k views

Estimating Darcy Friction Factors for Critical Flow

The Darcy-Weisbach equation is used to calculate the frictional pressure losses in pipes transporting incompressible fluids. This equation uses a dimensionless Darcy friction factor, also known as the ...
Jim Hargreaves's user avatar
8 votes
1 answer
277 views

How to design the most hydro/aerodynamic shape for a 'vehicle' that goes back and forth (using ANSYS)?

I am interested in making a very small scraper blade that moves back and forth along a submerged surface, however I need to make it as hydrodynamic as possible so I disturb the liquid as little as ...
user88720's user avatar
  • 265
8 votes
0 answers
672 views

Incompressible 2D pressure-driven flow: for Navier-Stokes equations in nondimensional form, how should I express pressure boundary conditions?

*I have updated this question. I'm using two codes for modeling incompressible, two-dimensional flow out of a tank. My fluid dynamics experience is pretty basic; I'm using the codes to understand a ...
Ant's user avatar
  • 245
8 votes
0 answers
7k views

Residuals of continuity not converging

I'm simulating a turbine. The convergence of solution is monitored by checking the residuals of the numerically solved governing equations. The "viscous-spalarat-almaras" model is used, and used air ...
user19061's user avatar
  • 899
7 votes
4 answers
4k views

Why is the friction factor in laminar flow independent of pipe roughness?

The Darcy friction factor is defined as: $$f_D =\frac{64}{Re}$$ with $Re$ I mean Reynolds number (global). I was wondering why Darcy in laminaire flow is independent of pipe roughness (wall roughness)?...
user avatar
7 votes
2 answers
131 views

What makes something like water so hard to change its density?

So I'm taking a compressible flow class soon, and I've been thinking beforehand about the topic. A substance like water can easily be pressurized, but its density doesn't change easily, meaning there ...
Jesse Batson's user avatar
7 votes
3 answers
295 views

What is "flow instability" in porous media?

Pressure drop across a porous medium can be described by the Forchheimer extension to Darcy’s law: $$-\frac{dp}{dx} = \frac{\mu}{K_1}\cdot v + \frac{\rho}{K_2}\cdot v^2$$ where $K_1$ and $K_2$ are ...
Algo's user avatar
  • 2,150
7 votes
4 answers
15k views

Ways to reduce turbulence in internal flows

Often it's desirable to reduce the turbulence level in an internal flow without changing the Reynolds number or fluid. This could be measured by the turbulence intensity, though other measures might ...
Ben Trettel's user avatar
7 votes
2 answers
8k views

Is inviscid flow necessarily turbulent?

The Reynolds Number is defined as $Re = \frac{\rho v L}{\mu}$, where $\rho$ is the density, $v$ is the fluid velocity, $L$ is the characteristic length, and $\mu$ is the dynamic viscosity. By ...
Paul's user avatar
  • 2,569
7 votes
3 answers
16k views

Force required to empty a syringe

How would you go about solving the following problem. I believe Bernoulli's equation needs to be employed, but I'm not sure how. Find the magnitude of the force that needs to be applied to a piston ...
thelastpanda's user avatar
7 votes
1 answer
421 views

Is it possible to replicate a Tesla valve in large scale?

Supposedly a Tesla valve will impede fluid or air flow in one direction whilst allowing flow to move freely through the valve entering from the opposite direction, without using any moving parts. Also ...
James's user avatar
  • 121

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