/AI1h ago

Physics Foundation Model Walrus Transfers RTI Simulation To Lab Turbulence

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Miles Cranmer@MilesCranmer#1275inAI

There are multiple theories for this. The popular one is that we are simply not modelling initial conditions correctly. However, our experimentalist friends argue that the Schmidt number, which measures molecular diffusivity, might have significantly more importance than people assume. This would complicate things, because our best simulations are *orders of magnitude away* from modelling real Schmidt numbers accurately!

Miles Cranmer@MilesCranmer

The system I am referring to is the "Rayleigh-Taylor Instability" (RTI). Pour creamer into coffee, and watch the plumes descend - a heavy fluid falling into a lighter one - this is an RTI. Simulation predicts that this process should be much *slower* than it actually is.

Why??

8:48 AM · Jun 3, 2026 · 121 Views
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Miles Cranmer@MilesCranmer

Keep in mind, this is the fast one it had NEVER seen. So, handed nothing but realistic initial conditions, Walrus seemed to reproduce a decades-old discrepancy, in addition to several other statistics adding believability.

Miles Cranmer@MilesCranmer

Then, we took the initial conditions from the *real* experimental RTI (see video), and asked Walrus to simply roll the dynamics forward from there.

And then, Walrus simply... replicated the faster experimental growth. WTF?!?!

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Miles Cranmer@MilesCranmer

Now, where do foundation models come into the story? Well, as you may know, foundation models (like chatbots) are taught using vast amounts of diverse data - "pretraining."

This makes them particularly good at generalizing to new problems, since the old and new often have conceptual overlap. Within the @PolymathicAI collaboration, we have developed one such foundation model: "Walrus," which has digested all sorts of fluids-related simulations.

Miles Cranmer@MilesCranmer

There are multiple theories for this. The popular one is that we are simply not modelling initial conditions correctly. However, our experimentalist friends argue that the Schmidt number, which measures molecular diffusivity, might have significantly more importance than people assume. This would complicate things, because our best simulations are *orders of magnitude away* from modelling real Schmidt numbers accurately!

1hViews 69Likes 1Bookmarks 1
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Miles Cranmer@MilesCranmer

One way to frame this is as purely data-driven evidence supporting the hypothesis that initial conditions play the most significant role in this longstanding sim-experiment gap. And, it's... so incredibly cool to me that this seemed to work. Anyways there are a lot more details in the paper, please do check it out!

Miles Cranmer@MilesCranmer

Keep in mind, this is the fast one it had NEVER seen. So, handed nothing but realistic initial conditions, Walrus seemed to reproduce a decades-old discrepancy, in addition to several other statistics adding believability.

1hViews 110Likes 3Bookmarks 0