Airflow & vortex wakes
Before you start
Air is a fluid too. A little tracer smoke reveals how invisible air bends around an obstacle, transfers momentum and leaves a wake.
The setupIn a two-dimensional, low-speed wind tunnel, air enters from the left. A cylinder or streamlined ellipse obstructs the flow. Smoke follows the computed velocity field; drag the body, change its angle or inject smoke with a click.
What to do and noticeObserve a low-Re wake first, then increase Re and wait for alternating vortex shedding. · Replace the cylinder with a streamlined ellipse and compare drag, then rotate it. · Smoke represents tracers carried by the flow, not visible air molecules.
Predict firstCan a stationary obstacle experience a force?
Build an intuition for velocity fields, drag and wake vortices using smoke.
Prerequisites: Fluids flow and include both liquids and gases. Viscosity describes frictional effects between neighbouring layers.Make airflow visible
Predict firstCan a stationary obstacle experience a force?
Yes. Air changes momentum as it meets and passes the body. Pressure and viscous stresses exert a force on the body; the component along the incoming flow is drag.
How to observe
- Observe a low-Re wake first, then increase Re and wait for alternating vortex shedding.
- Replace the cylinder with a streamlined ellipse and compare drag, then rotate it.
- Smoke represents tracers carried by the flow, not visible air molecules.
Model notes
D2Q9 lattice Boltzmann flow includes viscosity and a no-slip solid boundary. This is a two-dimensional, nearly incompressible, low-Mach model; finite resolution and tunnel boundaries affect drag estimates, so it is not an aircraft-design tool. Re uses the original cylinder diameter as a common reference across shapes. Alternating wake vortices are not full three-dimensional turbulence. Gases are compressible; shocks and high-speed flight require a different model.