Flowfield measurements of reverse flow on a high advance ratio rotor

The role of rotary motion on vortices in reverse flow

“…The experiments were intended to replicate the flow field of a rotor blade element at r/R = 0.40 passing through reverse flow over a range of advance ratios (0.60 < µ < 0.90). Kirk and Jones compared their results to the articulated rotor measurements of Lind et al (2018) and noted a significant difference in the stability of the reverse flow vortex between the surging and rotating cases, an observation originally attributed to the effects of rotary motion. It was theorized that the understanding of 3-D physics on a rotating wing in hover, where spanwise convection, vortex tilting and Coriolis effects combine to stabilize vortices formed at a sharp edge, may also be applicable to the reverse flow region of a rotary wing in forward flight.…”

“…The current work argues that such a discrepancy is unlikely to result from rotation. Instead, it is believed that the discrepancy is more likely due to the presence of dynamic pitching and flapping on the articulated rotor tested by Lind et al (2018). Figure 14 attempts to isolate the role of blade pitching and flapping, motions inherent to an articulated rotor in forward flight, on circulation production in the reverse flow region.…”

“…2017; Lind et al. 2018). This rotor is outfitted with four untwisted rotor blades, each with a radius of 0.85 m, an aspect ratio of 10.63 and a NACA0012 airfoil cross-section.…”

“…For sufficiently high pitch angles, an additional 'dynamic stall' vortex may form on regions of the blade that do not pass through reverse flow, but the current study has chosen to focus only on the sharp-edge vortex that persists in reverse flow. The simulations presented in this study are intended mirror a real rotor system, owned by the University of Maryland, that has been subject to considerable experimental investigation in past works (Berry & Chopra 2014;Trollinger et al 2017;Lind et al 2018). This rotor is outfitted with four untwisted rotor blades, each with a radius of 0.85 m, an aspect ratio of 10.63 and a NACA0012 airfoil cross-section.…”

Vortex formation on a pitching aerofoil at high surging amplitudes

“…This regime also represents a reasonable approximation of many real-world flow scenarios, including the inboard elements of a high-speed helicopter shown to exhibit flow separation during the deceleration (or 'retreating') portion of their oscillation cycle (Lind et al 2018), and the blades of vertical-axis wind turbines when encountering low tip speed ratios (Parker & Leftwich 2016). The current work seeks to experimentally investigate the onset of LEV formation in this high-amplitude surge regime.…”