https://reddit.com/link/1idia58/video/whyj9cox93ge1/player

A professor explained using Bernoulli's principle that the gap between the disk and the nozzle in the circumferential direction is very small and the velocity is high, resulting in a pressure lower than the ambient pressure.

Diagram of nozzle usage status

I think it's because the fluid has viscosity, so the stagnant water in the cylindrical space of the nozzle will be drawn out of the nozzle space, resulting in the pressure of the fluid in the nozzle space being lower than the ambient pressure.

I am using this flow meter from McMaster. And I don't trust the reading. I am flowing shop air into it with these conditions:

• inlet pressure: 140 PSI
• ambient pressure: atmosphere
• ambient temp: 72 F

It is reading 13 SCFH (0.22 SCFM).

I have a digital gauge in series with the McMaster gauge and it reads 0.68 SCFM. I am trying to figure out which one to believe.

Thank you

I've recently been learning about air ejectors and how they operate. They accelerate steam up to the speed of sound by using a convergent nozzle, and then the steam goes through a divergent nozzle which increases the speed and lowers the pressure even more. What happens at Mach 1 that causes the steam flow properties to reverse like that?

I came across this NASA GRC page which mentions about the limitations of the Venturi theory which I am not able to understand.

This theory deals with only the pressure and velocity along the upper surface of the airfoil. It neglects the shape of the lower surface. If this theory were correct, we could have any shape we want for the lower surface, and the lift would be the same. This obviously is not the way it works – the lower surface does contribute to the lift generated by an airfoil. (In fact, one of the other incorrect theories proposed that only the lower surface produces lift!)

Why can't we simply extend the theory for the lower surface of the airfoil too?

https://preview.redd.it/j28kjf3v7cfe1.jpg?width=1600&format=pjpg&auto=webp&s=c79a14c8537882699f8ee6ca45e2050a63dd1436

The area of cross section through which the fluid flows decreases more in the upper region (for this positive cambered airfoil) which means the flow velocity will be more there (using continuity principle) which means less pressure in that region comparatively to the lower region. The difference in pressure in the upper and lower surface causes a net force for lift?

So, yes the shape of lower surface should matter? If the lower surface is more curved then it will make the area of cross section through which the fluid flows more smaller and thus more pressure decreasing net pressure difference and lift.

Even for a flat plate, we can do similar analysis (from this simulator)?

Sorry if all of this sounds dumb or if I missed something. Please correct me where I went wrong.

I have no experience with CFD but am familiar with navier stokes due to having a meteorology degree and mathematics master's. I know some python but wouldnt consider myself good.

Here is what I want to model: we know that 2-dimensional flows dont exhibit the turbulence cascade (lack of vortex stretching means vorticity is conserved) and therefore energy is brought away from small scales to larger scales. I can see this in the real atmosphere when small vorticity centers merge with large waves. Ive seen it on some youtube simulations of 2D flow as well. Yet at the same time, chaotic behavior is still evident. In fact, Ed Lorenz(not to be confused with Lorentz) showed that even in a simple 2D barotropic model of the atmosphere, this chaos creates a hard limit of numerical forcasting of around 15 days(and much less for smaller scales and features). I want to create a model of 2 dimensional flow starting with lots of vorticity at small scales and run a simulation of how the system evolves with different energy distributions and starting states. The setting would be in a 2-dimensional pipe in an inertial frame of reference(no coriolis like effect). I feel like this project may be well beyond me, but if I want to try. How? This is just for fun as Ive always wanted to do a CFD simulation but dont know where to even start.

Hi everyone,

I recently hosted an open session introducing a structured course on Computational Fluid Dynamics (CFD) and Computational Heat Transfer (CHT). The session covered approaches to solving problems in fluid mechanics, an overview of computational techniques, and details about the curriculum.

If you’re interested in learning CFD and heat transfer from the basics, focusing on writing your own codes in Python/MATLAB, the recording of the session is now available on YouTube: https://youtu.be/ym4KHgdaNaU

For more details, check out the recording or feel free to message me directly. I’ll be happy to share the curriculum and more details!

question

This is a device with an inlet pipe and cylindrical space. There is a gap on the lower wall of the cylindrical space. When compressed air is input into the cylindrical space through the inlet pipe, the air will move horizontally along the bottom of the cylindrical space and be discharged into the atmosphere through the gap (the path of compressed air is shown in the red curved surface). During this process, due to the viscosity of the air, the air inside the cylindrical space will rotate (as shown by the blue ring).

My question is, will this device also generate cold air like a vortex tube.

Vortex_tube

https://preview.redd.it/k6lcfjanoqee1.png?width=947&format=png&auto=webp&s=b0988edbdb33f69cf3ddf0dc9159ca75f5a9690f

https://reddit.com/link/1i82d7p/video/mn3r93p5nqee1/player

Unfortunately, nor can I recall exactly what the 'physical paper book' was.

The principle of the machine is quite simple: there's a cylinder with a piston in it, & the process begins with the piston stationary @ one end of the cylinder. Also, there is a closeable slit in the side of the cylinder; & water is introduced @ an 'ordinary' high speed into the cylinder tangentially, such that it itself constitutes a cylinder of water of some thickness (or depth, if we prefer) whirling around the inside of the mechanical cylinder, & kept against the interior surface of it by its own centrifugal force.

Lastly, the piston is rammed hard along the cylinder; & the consequent flow of the water is that it spirals inward across the face of the piston, & where it's within a very small radius of the absolute centre of the face it 'leaps of' into a very fine & very high speed jet.

I'm not altogether surprised that I haven't been able to find anything about such a machine: I suspect the jet thus produced is no faster than what can be achieved by the usual method, which is simply to use an extremely stout reciprocating pump to compress the water to extremely high pressure & to let it exit through a narrow orifice … there is also the obvious advantage with that method that we can have a continuous flow … or @least a flow that's almost continuous but with some pulsation to it.

But it would be nice to know in some detail about any such machine that's actually existed @ any time, if only as a 'proof of concept' … or maybe even, occasionally, such a machine has actually been used : maybe in circumstances in which it was easier to produce the jet that way rather than by handling the extremely high pressure required to be handled in the usual method of producing a high-speed jet … & also in which the obviously necessary intermittancy of the jet was not a huge problem, or no problem.

And also the theory of such a machine would be of interest: it doesn't seem to be necessarily the case that a jet would form: it seems plausible that the water could just remain, in the form of a vortex (perhaps as one with a hollow core), on the face of the piston, increasing in depth as the piston proceeds along the cylinder.

Hi, can anyone recommend a Computational fluid dynamics program for someone who has never used one before? I have a project i need to design for distilling dirty water into drinkable and want to test the most efficient design. I have a diploma in renewable energy so i know what to design and not just a tube over a fire. I have used sketchup for designs but cant test theoretical reality with that. I have a Threadripper, 3090 and ECC ram so quite powerful computing for this task. Any help appreciated, thanks.

Hello, can someone explain how to estimate the wave drag using the energy balance? For subsonic incompressible flow, the kinetic energy in the body’s wake is equal to the drag on It and if the flow is inviscous the drag is zero as expexted because of no wake. In the supersonic compressible case even with no viscous effects drag is present because of a lisa of energy via mach waves, but how can I estimate this energy? Is there an equation or something else?

If I have two pipes with water pressure of 50psi each, and they meet up into one pipe, is the resulting PSI 50 or 100?

Circulation_(physics)

And also when you put your hand into the air bubble, places where there is no air bubble actually begin to get air bubbles, the air almost ripples down and spreads

For an animatronic project, I have gotten parts for relatively small pneumatic cylinder, 7 in and 5 in stroke, and a valve to control it with arduino, but I need a portable way to supply air to it, I was stuck between using a air tank or cartridges/tank, if co2, how would a regulator connect to it? I have never work with fluid power so I don’t really know.

In this video it is claimed that high speed wind over a roof causes a low pressure zone due to Bernoulli's principle, which causes the roof to lift off. Is this an accurate explanation? Intuitively the deflection of the wind would instead cause a downward force.

https://preview.redd.it/rcvzdglw8yde1.png?width=2013&format=png&auto=webp&s=457aa57d906588cd5aa94fcab184a437574ac33c

I am almost graduating my mechanical engineering degree and I´m now faced with the difficult decision of chosing a Master´s. I have great interst in Fluid Dynamics/CFD/FEA but i don´t know what Master´s to choose. My main 2 options are Mechanical Engineering with a specialty in Fluids or Computer Mechanics. I worry about future job opportunities and also the fact that although I´m really intrigued by Computer Mechanics I have very low coding capabilities (I have only written "Hello world" in Java I think). I´d be glad to have the testimony of anyone with similar experiences or real world job knowledge about this theme.

Tried posting this in r/askengineers but it got removed cause my karma is too low.

So this is probably a pretty dumb question, as I'm not an engineer or scientist - but it popped into my head and now I must ask.

It is this: why do we use oils in a liquid state to lubricate engines internal components? Wouldn't it be better to use a gas like argon, nitrogen, or helium?

From my (extremely limited) understanding, gasses like this are inert, and are thermally stable across a wide range of temperates. Wouldn't they make for very good lubricants on moving components? I would think they could be pretty beneficial from an efficiency standpoint, could pretty much axe traditional cooling systems, get rid of oil pumps all together, and run at much higher rpms? Also wouldn't have to worry about contamination. Could make them sealed units from the assembly line

It certainly would be a different type of engine than we currently know. I'm not sure what type of considerations would go into manufacturing something like this - although it might require an ungodly amount of pressure to properly lubricate everything. Wouldn't the smaller particles size allow it to reach every crevice completely uniformily? Would the machining tolerances need to be impossibly tight that we couldn't manufacture one?

What am I missing here? Someone much smarter than I has certainly considered this and either clearly seen why this is a bad idea - or already done it. Maybe there are particular applications this would actually work in. Id love to know.

What exactly is the characteristic length which is present in many dimensionless numbers in Fluid Mechanics? For example, say Reynolds number or the Knudsen number.

For an airfoil, it is the chord length. For a sphere, it is the diameter. For a thin sheet, it is the length. All of these don't point me to some proper definition for characteristic length but rather some conventions used. Or, is there a proper definition?

Now, if I had a very complicated shape, how will I find the characteristic length of it?

Are the characteristic length present in various other dimensionless constants and equations same or do they differ?

To understand this characteristic length, I tried to derive Reynold's number if at all it was possible. Various sources pointed out a derivation whose general approach looks something like this,

Re = inertial forces/viscous forces = m * a/mu * A * (dv/dy)

So, I attempted to derive it in a similar way on my own,

Re = m * (dv/dt) / mu * A * (dv/dy) = m * (dy/dt) / m * A

Considering a fluid element of m = rho * A * L, we simplify the above equation to,

Re = rho * L * (dy/dt) / mu

Here, flow velocity u = dx/dt and we know Re = rho * L * u / mu, so by this u = dx/dt = dy/dt? Did I miss something here?

There is this YT video by Prof. Van Buren where he does some dx -> L, dy -> L which I don't understand? Does Reynolds number actually have any derivation or it was empirically observed which later people attempted to derive it mathematically?

Also, the length L I have used is for a fluid element, how is it the characteristic length?

If there are any errors, please correct me.

Hello there!

For those of you who know the Navier-Stokes equation, you might recall that in its cylindrical coordinate form, extra terms appear on the equations for the radial and angular components, which are said to be due to the effect of the geometry of the cylindrical coordinate system itself. Do any of you know of a source that shows how these extra terms are derived? Or, instead, would you be able to show me how they are derived? Sources I find would just usually explain what these extra terms mean and not exactly show how they were derived from scratch.

I have no problem with the rest of the terms, though, including those that naturally result from the divergence and Laplacian operators.

Edit: Extra terms are highlighted in blue below.

https://preview.redd.it/ibrt889dozce1.jpg?width=1364&format=pjpg&auto=webp&s=0acd4549abf22494359c6bfa8576c29659a8b177

The general definition is that Static Pressure is due to fluid being at rest while Dynamic Pressure is due to movement of fluid.

But then we define Pressure at a point in a fluid as Static Pressure? Like, even in a flowing fluid, the pressure at a point would be Static Pressure not Static Pressure + Dynamic Pressure?

So, is Dynamic Pressure not exerted on fluid element itself unlike Static Pressure? Is it like some imaginary term which just had units of Pressure?

Some mentioned that Static Pressure is due to Potential energy of the fluid while the Dynamic Pressure is due to Kinetic energy of the fluid. Is this correct or there are any exceptions?

Also, P + rhogh together in Bernoulli equation represent Static Pressure right?

If there are any errors, please correct me.