This image shows a plastic 1/48-scale model of an F-18 aircraft inside the "water tunnel" more formally known as the NASA Dryden Flow Visualization Facility. Water is pumped through the tunnel in the direction of normal airflow over the aircraft; then, colored dyes are pumped through tubes with needle valves. The dyes flow back along the airframe and over the airfoils highlighting their aerodynamic characteristics. Water tunnels such as the one at Dryden are used as a "first step" in providing important information about anomalies and phenomena revealed in wind tunnel tests or in actual flight tests.