Experimental characterization of wingtip vortices in the near field using smoke flow visualizations

In order to predict the axial development of the wingtip vortices strength, an accurate theoretical model is required. Several experimental techniques have been used to that end, e. g. PIV or hot-wire anemometry, but they imply a significant cost and effort. For this reason, we have performed experiments using the smoke-wire technique to visualize smoke streaks in six planes perpendicular to the main stream flow direction. Using this visualization technique, we obtained quantitative information regarding the vortex velocity field by means of Batchelor's model for two chord-based Reynolds numbers, Re-c = 3.33 x 10(4) and 10 5. Therefore, this theoretical vortex model has been introduced in the integration of ordinary differential equations which describe the temporal evolution of streak lines as function of two parameters: the swirl number, S, and the virtual axial origin, z(0). We have applied two different procedures to minimize the distance between experimental and theoretical flow patterns: individual curve fitting at six different control planes in the streamwise direction and the global curve fitting which corresponds to all the control planes simultaneously. Both sets of results have been compared with those provided by del Pino et al. (Phys Fluids 23(013): 602, 2011b. doi: 10.1063/ 1.3537791), finding good agreement. Finally, we have observed a weak influence of the Reynolds number on the values S and z(0) at low-to-moderate Re-c. This experimental technique is proposed as a low cost alternative to characterize wingtip vortices based on flow visualizations.