Smoke Flow Visualization

Smoke flow visualization studies of subscale helicopter rotor wakes has been performed by Gray (1956), Pizialli & Trenka (1970), Landgrebe (1971,1972), Brand et al. (1990), Mercker & Pengel (1992), Ghee & Elliott (1995), Martin et al. (1999), and many other investigators. A dense white smoke is entrained into the rotor wake and blade tip vortices. When stroboscopically illuminated, the smoke particles reflect light, allowing a photograph of the flow structure to be recorded.[39] The tip vortices appear as circular regions devoid of smoke, which is a result of centrifugal forces produced on the smoke particles near the vortex cores (see later). Ideally, to give accurate spatial information, the wake must be illuminated using a thin light sheet, preferably using a laser. In another form of smoke flow visualization, smoke is ejected from the blade tip directly into the tip vortices. This renders the vortices visible as 3-D tubular trails with central voids. Such methods have been used to visualize the tip vortices in hover [see Gray (1956)] and in forward flight [see Muller (1990a)]. In a water tunnel, dye ejected from the blade tips gives similar results – see Werle & Armand (1969). Projected smoke filament techniques, such as those developed by Steinhoff (1985) and Muller (1990b, 1994), have also received some attention.

While the application of smoke visualization techniques has had good success for sub­scale rotor models, experiments on full-scale helicopter rotors are rare. A general limitation

of the smoke flow technique, however, is that smoke particles are quickly dispersed, and lower particle concentrations make the tip vortices harder to visualize. However, by using smoke “bombs” attached to the blade tips, the method has been used to visualize part of the wake structure of a CH-46 tandem rotor helicopter – see Spencer (1969) and Stemfeld & Schairer (1969). Using the same technique, the wake generated by a coaxial Ka-34 helicopter has been documented by Akimov et al. (1994).