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Alex Poirot

@alex.beloug

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Fluid Mechanics Engineer | CTO @crestsurfclubs
2011-2022 Science @ Kelly Slater Wave Co & WSL
Surf Ranch wave design
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My full story is out. 12 questions and 37 pages later. I don’t surf. I spent years designing Kelly Slater's Surf Ranch wave. 15 years ago, this was just a dream. Today, @surfer_magazine told my story. Honestly, this feels unreal. Grateful beyond words. Link in bio for full article. I only posted here a few screenshots. Thank you @dashel3000 for the opportunity! And thank you Noah for the picture! by @alex.beloug
79
5 months ago
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If you wipeout, this is what you're falling into. And it feels like chaos for sure, but it's not! It's actually beautifully organized. Actual CFD simulation of velocity vectors in a 7ft barreling wave. Real speeds. Strong flows. Wave particles speeds hit 30m/s, i.e. 60mph. The most terrifying part? These massive arrows pointing down underwater are making sure you spend some extra time beneath the surface. The splash up rises above waveheight, which is rare and a tell of a very powerful wave. See the under waterline vortex in the back? It all clearly means trouble if you fall. CFD simulation RANS-VOF free surface Simcenter STAR-CCM+ (Siemens) by @alex.beloug
189
7 months ago
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That's me! Feels like yesterday. by @alex.beloug
80
6 months ago
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The Wave-Bending Equation. Snell's law, and still the simplest way to predict where a swell will bend and focus. Waves can travel thousands of kilometers across the ocean, only to change direction in the final stretch, once they start to feel the seafloor. As these waves enter shallower water, they slow down. One part of the crest slows before the rest, causing the entire wavefront to rotate. The equation works not only for waves, but for light as well. Imagine the seafloor as a lens. The swell brings the energy, the bottom decides where it goes. That's why one point is firing while the next bay is flat. But this equation is only the foundation. It is linear theory, a simplified version of reality, for a single period and a single direction. Real refraction depends on much more: directional spread, currents, diffraction, and the nonlinear speed variations of big waves in shallow water. I have learned a lot about this after running thousands of real prototype tests and full-scale 3D CFD simulations. And even though reality is a little bit more complex, that is the scientific process: we need to start somewhere, with the basics. And this is a great example of it. #surfing #surfscience #pointbreak #physics #oceanography by @alex.beloug
21
3 days ago
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From 269% drag to 25%! The first surfers had feathers. Long before humans rode waves, ducklings did. Fluid mechanics shows how: Every swimming duck makes waves. Making waves costs energy. That cost is wave drag. Here we line up 7 ducklings behind their mom. Wrong spacing: the ducklings average 269% of the wave drag they'd feel swimming alone. Right spacing: 25%. Same ducks. Same speed. 91% less wave drag. The only difference: 1.60 vs 1.15 body lengths between ducklings. -> Their waves either pile up or partly cancel. The second duckling goes further: negative wave drag, −27%. It sits on the front face of a wave made by mom and the other ducklings. The slope pushes it forward. Same trick as a surfer. Real ducklings do save energy in formation: up to 63% less than swimming alone, measured behind a decoy (Fish, 1994). Feel free to share this, these are my own simulations and I think it's a fun experiment. [SCIENCE]: The waves that keep pace with a duck repeat every λ = 2πU²/g. Here, 0.81 body lengths. 1.60 apart = 2.0 λ. Crests line up. Waves add. 1.15 apart = 1.4 λ. Crests miss. Waves partly cancel. Calm wake, small drag. Swim faster and the waves get longer, so the winning spacing moves. Linear potential-flow ship-wave model, deep water, Fr = 0.36 (a 12 cm duckling at about 0.4 m/s). Mom is 2.3× a duckling's length. Modeled wave drag only, not total swimming resistance. Percentages: each duckling vs the same duckling swimming alone. Inspired by the duckling research that won the 2022 Ig Nobel Prize in Physics: Fish (1994, 1995); Yuan, Chen, Jia, Ji & Incecik, J. Fluid Mech. 928, R2 (2021). #FluidMechanics #WavePhysics #CFD #SurfScience #Hydrodynamics by @alex.beloug
35
6 days ago
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The Iribarren number. Proposed by Iribarren and Nogales in 1949, turned into a breaker classification by Battjes in 1974, and still one of the simplest ways to predict how a wave will break. It works. But the thresholds are empirical and fuzzy. Real breaking depends on much more than slope and wave steepness: 3D bathymetry, wave groups, and the nonlinear evolution of the wave itself. My take: there will never be one simple formula that captures everything. Same story as surfboard fins. I've been running CFD tests at Crest that hint at what might be missing. Updated visual coming. Stay tuned. by @alex.beloug
99
14 days ago
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Grip isn't free! Surfboard fin setups, same speed, same turn. Only the fins change: Single: one fin, one clean vortex street. Nothing to interfere with. Smooth, drawn-out, predictable. Twin: two fins, two independent wakes, and a calm lane between them. Each fin works alone. Loose and fast. Thruster: the centre fin works in water the side fins have already bent. In steady runs, the three fins split the grip almost exactly in thirds*. Balanced. 2+1: the big centre fin carries the most load. The side fins steer. Stable down the line. Quad: the back two never touch clean water. They ride in the front fins' wake. Four fins of grip, four wakes of chaos. Grip is water pushed sideways. Every swirl you see is speed your board paid for. It was never about how many fins. It's about how they share the water. Send this to the friend who swears by their quad ✌️. [SCIENCE] 2D hydrodynamics CFD. Note that this is only a side quest, my passion is to make 3D barreling waves above 2m for surfing. And that's where I allocate all resources. *From a separate steady-state study: fixed 10° angle, same fin geometry. Drag and lateral force (grip) were computed for every case but aren't shown: at this Reynolds number, only relative comparisons are meaningful. One trend should carry over to real conditions: grip and drag rise together as fin area grows, but not in proportion. Each extra fin adds less, working in water other fins have already disturbed. This shows how fins interact, not how a board performs: a visualisation to start the discussion, not a scientific report. by @alex.beloug
566
24 days ago
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And this is what I do when I'm not on the computer running CFD simulations. by @alex.beloug
200
6 months ago
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Same wave. Different reef. Two completely different breaks. I almost never leave the colorbars in. This time I could. Can anyone make an educated guess on the depth difference over the reef between the 2 waves? Scientists, non-scientists, friends, everyone is welcome! I want to see how many different ways people approach this 👇 by @alex.beloug
166
5 months ago
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Barrel in real time on the Moon! Everyone pictures a giant, floaty barrel on the Moon. I did too. So I ran the simulation at lunar gravity, with extra headroom above the still water line for that big wave. But it just never came. Then I realized why, and added Jupiter for good measure, just to make sure! • Gravity sets the clock, not the shape: Moon, 1.62 m/s²: every motion 2.46× slower. Jupiter, 24.79 m/s²: every motion 1.59× faster. The lip launches slower on the Moon. It also falls slower. Throw distance x ≈ v·√(2h/g). With v ∝ √g, gravity cancels. Same arc. Same barrel. Barrel closes 2.36 s after entry on Jupiter. 3.75 s on Earth. 9.23 s on the Moon. So it hit me: this is Froude scaling, running the other way. The same law that makes the smaller scale prototypes I work on produce the same wave as full scale. • Conclusion? I started with the same wave for Earth, Moon, and Jupiter. That's not how I should have done it. I should have used the same wave-generating system, not the same wave. But here's the hard truth, error messages light up from everywhere and the simulation crashes right away. • What would really happen then if we bring a wave system to the moon? The machine now has far less weight to lift. A pneumatic caisson at the same pressure pushes about 6× harder relative to the water's weight, and a piston at the same stroke makes a wave about 2.5× taller. The wave really does get giant, but it's too big for that reef. It breaks early in deeper water and turns into a closeout or a bore, not a cleaner barrel. • Thought experiment only: same water, same air, only g changes. The real Moon has no atmosphere. Open water would boil. 2D VOF simulations in STAR-CCM+. by @alex.beloug
41
10 days ago
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Anatomy of a wave. Wait for the spit at 6s. Overhead view. A drone only sees the surface. CFD sees inside. Watch the spit being loaded and then released in one jet blast. Actual CFD simulation, 43 million cells, run in under 24h on 48 CPUs. by @alex.beloug
19
3 months ago
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Wave engineering after dark. Looks like Blade Runner. Runs on Navier-Stokes. RANS-VOF. 38 million cells. by @alex.beloug
13
17 days ago
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