Flow Visualization Techniques

While the wake of a helicopter rotor is sometimes rendered visible through natural condensation effects, flow visualization using subscale rotor models has been the primary method used to study the wake physics. Some of the earliest studies were by Taylor (1950) and Dingeldein (1954), who used balsa dust to visualize the rotor wake. A more popular method is to use a dense white smoke, which is entrained into the wake, thereby rendering it visible when illuminated by a suitable strobed lighting source. Density gradient flow visualization methods, such as strobed (phase-resolved) shadowgraphy and schlieren (and to a lesser extent, interferometry), have also been used to study helicopter rotor wakes The phenomena that have been studied include tip vortex formation, blade-vortex interac­tions, vortex-airframe surface interactions, main rotor wake-tail rotor interactions, ground interference, multirotor flows, and the wake roll-up in forward flight.