Streamlines are lines that represent the flow of a fluid around an object or through another fluid. Streamlines are often made visible by putting thin films of smoke in the air. They are used in wind tunnel testing of airfoils (wings) and automobiles. A wind tunnel is a closed chamber with vents in the front and rear of the tunnel that allow wind and the streams of smoke that make streamlines visible to pass around an object. If the streamline smoke appears to be flowing in a gentle pattern without breaking up, then the object is considered aerodynamic. If the smoke breaks up upon encountering sections of an object, the flow has become turbulent. If the flow is turbulent the drag may be increased.