Time-resolved Large-scale Volumetric Flow Measurement of a Helicopter Model Using Helium-filled Soap Bubbles

This study describes time-resolved large-scale volumetric flow measurement of a helicopter model (1.55 m rotor-span, cord length = 61 mm) in close-to-ground condition (height = 0.5 D) at Reynolds number 4.1 x 10^5 (rotational frequency = 20.83 Hz) to examine CFD predicted secondary vortex structures (originating from primary rotor-tip vortices), as well as ground effects. Five high-repetition-rate cameras are used to image tracer particles (Helium-filled soap bubbles HFSB) in a 400 x 600 x 400 mm³ measurement volume at 1.7 kHz recording rate. HFSB tracer particles are illuminated by an array of over-pulsed high-power LED's. 50 sequences, with 3800 images each, at different angles of attack and different particle densities are recorded. The images are analysed using Shake-the-Box (time resolved particle tracking) to gather dense Lagrangian particle tracks. Further, data assimilation techniques (Flow Fit, Vortex-In-Cell VIC#) are applied for high-resolution grid-reconstruction of velocity, vorticity, acceleration and pressure (pressure from PTV) from the track data. First analysis confirms the presence of secondary vortex structures and reveals spanwise oriented vortex tubes close to the ground. Low pressure and high acceleration (150 g) are measured in the primary tip vortices