Hot-Film Measurements and Data Evaluation on Rotor Blade Tip Vortices in a Wind Tunnel

Abstract

Helicopter rotors produce helical vortex systems originating from the blade tips, whose structure and evolution affect the aerodynamic performance, vibrational loads and acoustic emissions of the rotorcraft. This thesis investigates vortex properties in controlled wind-tunnel conditions and establishes a bridge to hover-like conditions.

Constant-temperature hot-film anemometry was applied to a commercial two-bladed Aeronaut CAMcarbon 12×4.5" rotor operated in the 1-Meter Wind Tunnel Göttingen (1MG) at the German Aerospace Center (DLR). A dedicated, statistics-driven workflow overcomes the limitations of the pointwise technique by modeling the distribution of vortex detections across rotor radial locations at fixed axial stations with a Gaussian envelope, while probability ellipses quantify local dispersion and confidence. Core radii are estimated from near-diametral crossings using an upper-quantile selector, and centerline kinematics yield the convective speed required to extract swirl and circulation. Complementary thrust and velocity measurements with a downstream flow restrictor quantify facility-induced inflow and enable normalization for comparisons with hover datasets.

The measured trajectories exhibit blade-bound separation, pairing and leap-frogging at high vortex ages. Vortex stretching is observed, with the core radius of the inner filament growing faster than that of the outer filament, and the outer filament transferring circulation to the inner while total circulation remains approximately conserved. Measurements on an in-house single-bladed rotor showed noisy hot-film signatures and strong meandering; PIV snapshots indicate delayed, inboard roll-up at low tip Reynolds number, explaining the weaker near-tip signal.

The proposed methodology provides reliable vortex metrics from pointwise data and a practical path to reconcile standard facility measurements with hover-like free-flow conditions.