Speed |u| / U
Pick a preset or draw a shape. Air flows left to right.
What am I looking at?
- The simulation
- The tunnel is a grid of small cells. Instead of solving the equations of fluid motion directly, the lattice Boltzmann method (LBM) tracks little packets of fluid hopping between neighbouring cells in nine directions. Each step the packets move, then collide and relax toward equilibrium. Averaged over many packets, that produces the same behaviour as a real, slightly compressible fluid. Cells you draw are solid walls the packets bounce off.
- Reynolds number (Re)
- Re = speed x size / viscosity. It says how "syrupy" or "swirly" the flow is. Low Re (below about 40 for a cylinder) gives smooth, steady flow. Around 50 to 200 the wake starts to wobble and sheds alternating vortices, the von Karman vortex street. Here the size is the height of your obstacle in cells.
- Drag coefficient (Cd) and lift coefficient (Cl)
- Drag is the push along the flow, lift is the push across it (positive is up). They are scaled by the dynamic pressure of the oncoming air and the obstacle's frontal height so shapes can be compared: Cd = Fx / (0.5 U squared x height). The numbers are measured by adding up how much momentum the fluid hands to every solid cell. When vortices shed, Cl swings up and down; its average is near zero for a symmetric shape.
- The views
- Speed is how fast the air moves relative to the inflow. Vorticity is local spin (red anticlockwise, blue clockwise), great for seeing vortices. Pressure is the pressure relative to the inflow, in units of dynamic pressure: high in front of the body, low where the air accelerates. Tracers are bits of smoke carried by the flow.
- Caveats
- Tunnel walls at top and bottom are frictionless, so a big obstacle in a short tunnel squeezes the flow (blockage) and reads a higher Cd than in open air. It is a 2D slice, so real 3D effects like turbulence are missing.