Showing the Airflow in a Wind Tunnel. A technique often used in wind tunnels is to introduce smoke in front of the airfoil that is being tested. The smoke comes from regularly-spaced point sources, and the wind flow in the tunnel spreads it out into parallel lines, called streamlines. The streamlines make it possible to visualize the airflow over the airfoil. When the lines continue smoothly over and past the airflow, they show that the flow remains laminar, and that the airfoil is creating very little drag. When the streamlines show more chaotic, turbulent flow, they indicate that the airfoil is creating more drag. You can do something similar with a wind tunnel by stretching thin strings across the flow path, above and below your airfoil test zone. Clip them in place so you can move them up and down to fit different airfoil shapes. Attach a ribbon (about 25 cm long) to each string. Use a stick attached to your airfoil to hold it while you place it in the flow path, between the ribbons. The ribbons will act like the smoke streamlines, so that you can visualize whether the flow over your airfoil is turbulent or laminar. Try different airfoil shapes and measure which have the most laminar and the most turbulent flow. (Parker, 2005, 18–19)