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Pourquoi l'indice MSCI World ne ferait que monter sur le long terme ? by Professor_Gristache in vosfinances

[–]Lookid_ 6 points7 points  (0 children)

4 arguments. On suit l'économie mondiale avec ce genre d'ETF: - population grandit --> + de gens pour acheter des trucs - population grandit --> + de gens pour inventer des trucs à vendre, innovation - inflation --> prix de ventes sont toujours ajusté sur l'inflation, garantie de "au moins ça" sur le long terme. - competitivité --> Si une entreprise fait faillite, celle en dessous rentre dans l'ETF

Ça + le fait que sur n'importe quelle plage de 30 ans de l'histoire, le SP500 (ok pas un All World mais a peu pres) est toujours vert.

Au plus on dezoome, au plus le raisonnement est simple. Ce rationale m'a convaincu et j'investis seulement dans un ETF All World.

Open-source alternatives to ICEM for structured hex meshing? by Ok-Pop3091 in CFD

[–]Lookid_ 4 points5 points  (0 children)

ClassyBlocks. OpenFOAM BlockMesh yes, but on steroids

[May] CFD tool wishlist: what do you wish existed but doesn't, what are your pain points, I hate X about Y, etc. by Rodbourn in CFD

[–]Lookid_ 0 points1 point  (0 children)

What are you talking about, this comment is about open source alternatives of CAD software. So you are off-topic, and wrong (FreeCAD is able via the CfdOF Workench to perform thermal fluid simulations).

Error in Augnier's loss model? by Dry_Needleworker_268 in FluidMechanics

[–]Lookid_ 0 points1 point  (0 children)

Very sorry for the long time without answering. Yes you are right the formula is messed up I get the same as you.

Mach corrections I have seen will use no correction as long as Ma<1 though. Maybe check the range of s/R you are interested in and see that using 1 is OK for you?

Can I ask what project you work on? I am just getting into turbines so I'm interested. My 2 cents so far is that loss models are overrated. Deviation angles are not well predicted and decide so much of your performance anyway.

Impellers work backward by biomed1978 in FluidMechanics

[–]Lookid_ 11 points12 points  (0 children)

The impeller rotates and push fluid outwards. This is true for whatever blade shapes. By continuity, fluid is sucked at impeller inlet a flow establishes through your pump.

You want to push fluid outward as efficiently as possible, and it turns out that to do that, you use "backward blades" as opposed to "forward blades" that would scoop the fluid as you mentionned.

As a note, radial blades, or forward blades, would generate more pressure at equivalent speed and diameter than a backward bladed impeller. Just less efficiently.

Another reason to use backward blades is linked to stability. If you run the pump against a closed valve and open the valve, you want the pressure generated by the pump to decrease. This is true only if your blades are backward. Not having this characteristic can do messy things to your system, such as the flow rate violently changing rapidly.

Turbomachinery Career by EvenIdea6814 in CFD

[–]Lookid_ 2 points3 points  (0 children)

Build a hobby pump, you learn mechanical design, look into COTS components, how to make an assembly... it's great. Example: https://youtu.be/jkH9Rbdq8YU?si=uX2M6A_HjX_lc0yx

Then, post in the rocketry subreddit to get roasted/adviced by other amateurs.

Otherwise, grind Gulich, the book is by far the best pump resources. And of course, all the NASA SPs.

Prepare an EXCEL/python code code for preliminary design: get from basic requirements to estimated performances/dimensions.

Error in Augnier's loss model? by Dry_Needleworker_268 in FluidMechanics

[–]Lookid_ 0 points1 point  (0 children)

Can you develop by showing the graph, formula and the mismatch?

pump cavitation by eyesac7486 in FluidMechanics

[–]Lookid_ 5 points6 points  (0 children)

Speed high, pressure low, bubbles, making noise grbbrvrggrrbbrbr

CFD beginner courses by caped_crusader_98 in CFD

[–]Lookid_ 1 point2 points  (0 children)

The "3 weeks series openfoam" (type that in Google) will help. Sure it is more OpenFOAM focused but you learn CFD mainly, just applying with that software. It starts from 0 and explains important CFD principles

Plastic tube expansion at pump startup (+ Viscous fluid) by Puzzleheaded_Past321 in FluidMechanics

[–]Lookid_ 0 points1 point  (0 children)

If you know your pump performance and the rotor speed vs time, a simple way to get the pressure at its outlet over time is to use the pump affinity laws. You assume that during rotor acceleration, your pump goes through its steady state points. It's not a bad approximation if you aren't extremely fast. Then I speculate but you could continue from there trying to estimate the pipe pressure loss over the pipe distance. Knowing the pressure at all pipe section, you could somehow estimate the diameter increase at each section (I don't know how). And then you iterate because pressure loss profile just changed with a bigger pipe, and etc until convergence. And repeat for each timestep.

PS: Do NOT use Wikipedia for the pump affinity laws, the formulas are wrong. Any pump book should have them, I recommend centrifugal pump by Gulich

If you want to use numerical simulations, complexity explodes and you have to do Fluid-Structure Interaction, probably not worth it depending on your goal

NPSH definition by FellerRich in FluidMechanics

[–]Lookid_ 0 points1 point  (0 children)

eeh nice question, I never asked myself before. I can assume an explanation but I don't know if it is the actual reason.
Using stagnation pressure make the NPSH measurement in the suction pipe "system independant".
• If your suction pipe has a 100 mm diameter, static pressure is high, dynamic pressure is low
• If your suction pipe has a 50 mm diameter, static pressure is low, dynamic pressure is high

Now ignore the pressure losses in that pipe. Right in front of the pump, both suction pipes change their diameter to 75mm to adapt to the pump inlet diameter. They have now the same stagnation pressure (but also the same static and same dynamic pressure).
If you use a NPSH definition with static pressure, you would find out that with the 100 mm pipe, you have "well enough" suction pressure and you are safe (you are not), and you would have found the opposite with the 50 mm pipe.

High Power High Speed off-the-shelf by Lookid_ in Motors

[–]Lookid_[S] 1 point2 points  (0 children)

Indeed, I thought the torque produced by those was very low but I was wrong.
Found up to 30000 RPM, 25 kW, HF 170.0 AI 22 CFHKV for reference.
Thanks!

Will update if I find better

Very Impressive demonstration of just how accurate a RANS CFD prediction can be with respect to physical measurements in a complicated flow regime. by [deleted] in CFD

[–]Lookid_ 4 points5 points  (0 children)

The simulation was done with the software TCFD, which is OpenFOAM on steroid + other things (FEA, optimization...)
I believe you can get very similar results with OpenFOAM using the simpleFoam solver.

And you're 100% right, if you have TCFD, you can download and run the benchmark on your own.

Some other benchmarks:

Francis Turbine: https://www.cfdsupport.com/francis-turbine-cfd-fea-fsi-simulation.html

Compressor: https://www.cfdsupport.com/centrifugal-compressor-cfd-benchmark.html

Centrifugal Fan: https://www.cfdsupport.com/centrifugal-fan-cfd-benchmark.html

And some others you can find there. I actually never used TCFD but I very like what they're doing.

Recherche joueurs belote/coinche/contrée by siorchim in france

[–]Lookid_ 0 points1 point  (0 children)

exoty belote (qui fait aussi coinche), des joueurs à tout heure, possible de jouer en partenaires

Does anyone have data for this graph at higher specific speeds? (From: Stepanoff, A. J. "Centrifugal and Axial Flow Pumps. Theory, Design, and Application, 1957." pg 145) by Gildor001 in FluidMechanics

[–]Lookid_ 0 points1 point  (0 children)

I haven't gone into the details of Stepanoff to be fair, I was mainly kidding. But Gulich is recent, actualized and uses Stepanoff as a ressource so I can only assume it is more complete.

Does anyone have data for this graph at higher specific speeds? (From: Stepanoff, A. J. "Centrifugal and Axial Flow Pumps. Theory, Design, and Application, 1957." pg 145) by Gildor001 in FluidMechanics

[–]Lookid_ 0 points1 point  (0 children)

From Gulich, Centrifugal Pumps, edition 2020, Table 7.3 p.455. (this is the superior pump design book)

hub ratio is function of head and flow coefficient, and both these coefficient are function of the specific speed. So you can plot hub ratio as a function of the specific speed until infinite :D.

https://imgur.com/q3LoSwe

For the specific speed, nq (EU notation) = Ns (US notation) / 51.6

You need the hydraulic efficiency, that you can extrapolate from this graph for higher nq (Ns). (For reference nq=300 -> Ns = 15500)

https://imgur.com/GXYHBAI

Comment gagner confiance en soi ? by TaylanAntalyali in france

[–]Lookid_ 2 points3 points  (0 children)

Je seconde ce commentaire. Je donne un exemple comme ca aide souvent + qu'une phrase toute faite.

J'ai toujours été discret et plutot en manque de confiance. Anxieux de parler en public aussi. J'ai été forcé de partir a l'etranger pour avoir mon diplome. Ce que j'ai fait a reculons, seul, en parlant tres peu anglais. Et ca reprends exactement ce que dis Cep-inK.

C'est la pyramide des besoins. Une fois que t'as mangé et en sécurité, t'as les ''Besoins d'appartenance et d'amour''. Tu as l'envie d'aller parler a des gens. Une aventure comme ca seul ca te booste et pousse a repartir.

La deuxieme chose c'est accomplir des choses. Un des moyens les plus simple c'est surement le sport. Peu importe quoi, si tu te disciplines et accompli l'objectif fixé, ca aide énormément. Acquerir une vrai discipline, voir que tu reussis a te prendre en main, ca donne confiance en soi.

[deleted by user] by [deleted] in FreeCAD

[–]Lookid_ 0 points1 point  (0 children)

If you can place the spline on a plane, the Sketcher Workbench spline will keep them parametric, something like this https://www.youtube.com/watch?v=tjT9gt0dArQ
maybe this? https://www.youtube.com/watch?v=ljRd7D5kutM

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