hr-studioPhysics · 05
A computational design primer · Fluids & Flow

Motion that has no shape of its own.

A fluid is a field, not a thing — a velocity at every point in space that carries whatever floats in it and can never pile up on itself. Store that field on a grid, push it forward, and keep it from compressing, and you get smoke curling, water swirling, and air streaming past a wing. It ends in a wind tunnel you can reshape and read like a real CFD run. Stir it.

Scroll to begin
i.

The field carries things along

Fluid motion is stored as a velocity field — a little arrow in every grid cell saying which way the fluid there is heading. Anything riding in the fluid, like a puff of dye, is simply moved along by that field each step: look up where a cell's contents came from and pull them forward. That moving-with-the-flow is advection.

Drag across the fluid to push it, and watch the dye follow.

Drag to push · dye rides the flow
ii.

Fluid can't pile up

Real fluid is incompressible — the same amount flows out of every region as flows in, so it can't bunch up or leave gaps. Advection alone breaks that; the fix is a pressure projection that finds the pressure field whose push exactly cancels any squeezing or spreading. Switch it off and the flow clumps and blows out; switch it on and it moves like a fluid.

Stir it with projection off, then on, and feel the difference.

Projection removes compression
iii.

Flow around an obstacle

Put a solid body in a steady stream and the fluid must go around it — and the streamlines tell the whole story on their own. They part at the nose where the flow stagnates, crowd together over the shoulders where it accelerates — closer lines mean faster flow — and relax again downstream. Reading spacing as speed is how engineers read a tunnel at a glance.

Raise the wind and watch the lines crowd the shoulders.

Streamlines only · closer lines = faster flow
iv.

Reading a flow in colour

CFD tells its story through colour maps, each a different question asked of the same flow. Speed shows where the fluid races or stalls; pressure shows where it pushes and pulls; vorticity — the local spin — reveals the swirls peeling off the body, the two-tone Kármán street trailing downstream. Same simulation, three ways of seeing.

Switch between the three views.

One flow · three CFD fields
v.

The wind tunnel

Now run your own. Drop a shape into the stream — a cylinder, a wedge, a square, a flat plate, even a star — spin it with the Angle slider (the round cylinder won’t show it — the others will), and read the result in any CFD view. Bluff shapes shed a Kármán vortex street; streamlined ones cut clean. Push the wind faster and the wake goes wild; add viscosity and it calms. This is a coarse, two-dimensional tunnel, so it's qualitatively true, not a certified number — but the physics you see is real.

Pick a shape · drag to move · rotate to set the angle
A velocity in every cell, carried forward and squeezed free of compression — that's a fluid, and everything from a smoke wisp to a vortex street falls out of it. No shape of its own, and yet it draws every shape the world puts in its way.