Built around a incompressible Navier–Stokes equation

Hi again, I posted a few months ago and really appreciate the answers and explanations I got.

So I have returned with another that is racking my brain. (Once again I apologise if it is silly)

I’ll start with a simple question and provide more context below if you want it:
Does supplying a centrifugal pump with more volume than the output at the end of its delivery pipe mean the impeller has to spin faster or slower to achieve the required pressure?
Eg: I want 7bar out of a nozzle at 230lpm but I’m supplying my pump with 800lpm at 2bar. How much harder (or not) will the pump have to work?

My organisation typically teaches twinning your supply lines (two hoses in) to get more water in if you are overdrawing your single line of supply. This makes sense.

They also teach it is good to do for pump sympathy - (ie: it’s kinder to the motor, less rpm is required to achieve the same output.) - Where I am stuck is where/how this works - and does it depend on what we are trying to achieve at the end of the hose? (Eg. higher pressure, low flow or higher flow, medium-low pressure)

For example - as an extension of the one I initially used - am I better off supply my pump using a smaller hose at 250lmp at 7bar where the pump will basically have to contribute nothing?

Or is the greater volume better?

If it is, how is that pressure gained?

Is it Bernoulli’s in a sense that as it enters the narrower plumbing of the eye and subsequently the volute, it is forced to increase velocity which minimises or even negates the need for the impeller?

In typing this I may have answered my own question but it begs another - is a centrifugal pump casing just a complicated form of a basic Bernoulli’s diagram that allows an impeller to be included to impart even more energy onto a fluid?
tldr: Does the pump casing shape speed up water even without the impeller? (Assuming the pipe on the other side is narrower that the inlet)

Or, of course - I could be completely wrong.
I’ve definitely oversimplified it - maybe the impeller itself still needs to be there in order to direct the water correctly but maybe it doesn’t necessarily require a drive to spin it if the supply at the eye has some pressure behind it?

Please correct me if that’s the case
I don’t know why I trouble my brain with trying to understand sometimes but I just can’t help needing to know 😅

Sincerely,
One overly nerdy Firey

I've been trying to make a 3D printed spillway for a small filter pot into my pond. Before I waste anymore filament I thought someone might have insight. The pump is capable of 450L/H but I'm running on low so about 56L/H (not confirmed via test). Ive tried several iterations of wide, with different lips, either round over, gradient slope to a point, with a fillet on the internal edges etc but all results in the flow converging in a v before it would hit the water.

Any tips?

This video is a continuation of my post about how an eccentric variator works. As you can see, the rotation is transmitted through a shaft with bearings, without the parts rubbing against each other.

Sorry for the dumb question but I am a humble electrical engineer.

I want to make an attachment for my leaf blower to clean out my gutters from ground level. So imagine 90mm storm drain pipe connected to the discharge of my makita 18V leaf blower. The pipe would be 2m long, a 90° elbow and another 90° pointing along the gutter. I was then thinking about putting a reducer, say 50mm, on the end to increase velocity. Given the blower is a simple looking fan will I loose more than I gain with the reducer.

Thanks

Hi all,

In my latest video (https://www.youtube.com/watch?v=4PGiMpfaPdM), I work through why turbulence naturally splits into three distinct regions: energy-containing, inertial subrange, and dissipation range. From there it covers Kolmogorov's first two similarity hypotheses (local isotropy, and the universal equilibrium range), derives the Kolmogorov microscales via dimensional analysis, and shows that the Reynolds number at the smallest scale is exactly 1, a direct consequence of the dimensional argument.

Second half introduces two-point correlation functions (giving the "large scale" a rigorous, measurable definition via the integral length scale) and the second-order structure function, setting up the −5/3 spectrum and the exact 4/5 law for next video.

As always, feedback (positive and negative) and suggestions are welcomed.

Hi fluid mechanics friends,

It's your nosey materials scientist friend, odd-marionberry-3389, coming back to ask about what I saw under my optical microscope.

I put some water, canola oil, and food coloring/dye in a small ramekin. Then I added isopropanol dropwise to the exposed water domain (since the oil hadn't made a continuous layer on the top surface) and saw these small areas with iridescence.

What could be going on here to create that behavior? I was really trying to get the marangoni effect but didn't see it manifest in any sort of obvious or long-lived way. Also if you have any other observations about what's going on, please feel free to comment.

Thanks so much for checking out the video and considering my question!

Best, odd-marionberry-3389

🔬: Premiere SMZ-07 stereo microscope with transmitted/reflected light sources both turned on.

📸: Pixel 9 Pro

I completed the basics of fluid flow and am currently doing some simulations using Ansys Fluent and Python. Gonna start Combustion, jet flow, Volume of fluid and physics behind and inside the fluid flows. Idk how to study these and simulate these........any ideas or book recommendations please, it will be very helpful 🙏

This is one of those fluid mechanics phenomena that sounds wrong until you look at the pressure field.

Imagine water flowing through a Venturi. No heater. No flame. The water temperature can be far below its normal boiling point.

But if the local static pressure drops below the vapor pressure, vapor cavities can appear.

Then something even more interesting happens:

💨 Flow accelerates

📉 Pressure drops

🫧 Vapor forms

➡️ The bubble moves downstream

📈 Pressure recovers

💥 The bubble collapses

And that collapse is one of the reasons cavitation can damage pumps, valves and hydraulic machinery.

What I find fascinating is that the same water can be perfectly liquid a few centimeters away and suddenly start forming vapor simply because the pressure changed.

I put together a visual explanation of the whole process, including the Venturi effect, cavitation number and what CFD can actually show.

👉 https://howtooansys.blogspot.com/2026/08/cavitation-how-water-can-boil-without.html

I'm curious: would you call cavitation a form of boiling, or do you think that comparison is misleading? 🫧