Fluid Dynamics Lab

A real 2D Lattice Boltzmann (D2Q9) wind tunnel. Air flows left to right. Drag across the flow to stir it, or paint your own obstacles.

Flow

Inflow speed U 0.070 Viscosity ν (lower = higher Re) 0.0126 Angle of attack 0° Obstacle size 18 cells Simulation speed 5 steps/frame

Obstacle

NACA profile

Mouse tool Stir Move Draw Erase

Brush radius 5 cells

Live measurements

Reynolds number: —
Mach / relaxation τ: —
Strouhal number: —
Mean drag C_d: —
Lift C_l: —
Time (D/U units): 0

Phenomena detected

Laminar flow
Boundary layer
Flow separation
Kármán vortex street
Stall

Drag and lift coefficients

Probe signal (transverse velocity behind obstacle)

Strouhal number St = f·D/U is measured from zero crossings of the probe signal. Theory for a cylinder is about 0.2 (0.165 at Re 100).

What am I seeing?

Reynolds number Re = U·D/ν compares inertia to viscosity. Below about 5 the flow hugs the body; near 20 to 47 a steady pair of vortices sits behind a cylinder; above about 47 the wake becomes unstable and sheds vortices.
Boundary layer is the thin film next to the surface where the fluid is slowed to zero by friction (no-slip). It gets thinner as Re rises.
Separation happens when the boundary layer, slowed down by rising pressure behind the widest part of the body, reverses and peels off the surface, leaving a recirculating wake.
Vortex shedding at Re above about 47 makes alternating vortices peel off the two sides, the Kármán vortex street. Its frequency gives St = fD/U, close to 0.2 over a wide range of Re. The body feels an oscillating side force at that frequency.
Drag and lift come from pressure and viscous forces integrated over the surface (here by momentum exchange of the lattice particles). C_d = F_x / (½ρU²L), and C_l likewise for the cross-flow force. Streamlined airfoils have small C_d; bluff bodies have large C_d.
Bernoulli: along a streamline, faster flow means lower pressure. Over a cambered or tilted airfoil the flow speeds up on the upper surface (low pressure) and slows beneath (high pressure), producing lift. Switch to the Pressure view to see it.
Stall: raise the angle of attack and lift grows until the boundary layer cannot follow the upper surface. It separates, lift collapses and drag climbs.
Numerics: D2Q9 BGK with a Smagorinsky eddy viscosity for stability at low viscosity, bounce-back obstacles, equilibrium inlet, zero-gradient outlet, mach kept below 0.15. Free-slip tunnel walls except in the step channel. At low resolution (obstacle only 10 to 60 cells wide) quantitative values like C_d are approximate and blockage by the walls raises St a bit.