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The water tunnel has long been recognized as one of the best tools for flow visualization. That is because, unlike in a wind tunnel, the dye does not dissipate rapidly and will persist downstream.

With the RHRC submersible water tunnel balance, flow visualization and force measurements can be conducted simultaneously, giving unprecedented insight into the causes of aerodynamic behaviors. An example of this insight is given below for the case of an F/A-18 at high angles-of-attack. The force data shows that there is very little yawing moment present at low AOA, but suddenly at angles above 45 degrees, there is a strong negative yawing moment. What has caused this behavior?

F/A-18 yawing moment coefficient vs. AOA

We now examine the flow visualization photos captured simultaneously to the force measurements. The photo at 45-degrees AOA shows the forebody vortex in red, which is following the general shape of the aircraft. As the aircraft increases AOA to 50 degrees, the forebody vortex on one side separates from the body aft of the canopy. The presence of a vortex close to an aerodynamic surface causes localized low pressure, and a force toward that low pressure. When the pressures are symmetric, such as over a wing, they can create strong vortex lift forces. However, when the vortices are asymmetric, they can create a large side force and yawing moment, exactly as we observed in the yawing moment plot above.

F/A-18 at 45 degrees AOA

F/A-18 at 50 degrees AOA


Comparison Between Wind Tunnel and Water Tunnel Flows

Water Tunnel Flow Visualization

Water Tunnel Vortex Patterns

Wind Tunnel Flow Visualization

Wind Tunnel Vortex Patterns





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