Measurements on a yawed rotor blade pitching in reverse flow

In forward flight, high advance ratio rotors encounter a large region of reverse flow on their retreating side. The reverse flow region is the cause of significant separation in sharp-trailing edge rotor blades and is believed to be dominated by a flow structure called the reverse flow dynamic stall vortex (RFDSV). This vortex incurs large, unsteady torsional loads that are not well-predicted by modern comprehensive rotorcraft codes. To gain a physical understanding of the impact of yaw on the RFDSV, a subscale, NACA0012 model rotor blade has been experimentally tested at two reverse flow yaw angles, ${\mathrm{\Lambda }}_{\mathrm{rev}}=0^{\circ }$ (unyawed) and ${\mathrm{\Lambda }}_{\mathrm{rev}}={30}^{\circ }$ (yawed), over a range of reduced frequencies, $0.160<k<0.450$. Pressure time histories were obtained from surface-mounted unsteady pressure transducers, and three-component velocity fields were obtained using stereoscopic particle image velocimetry at the midspan. The present work focuses on the impact of yaw on the strength, number, and behavior of vortices shed for a given set of pitch kinematics. In cases where the unyawed blade shed multiple vortices, the presence of yaw was found to suppress secondary flow structures, including the trailing-edge vortex, and thus delay the breakup of the RFDSV to later times in the pitch cycle. The suppression of secondary flow structures was consistent across multiple kinematic scalings of the flow field, suggesting that the flow at ${\mathrm{\Lambda }}_{\mathrm{rev}}={30}^{\circ }$ is subject to physical mechanisms not present at ${\mathrm{\Lambda }}_{\mathrm{rev}}=0^{\circ }$. This work proposes that spanwise flow, which was found to substantially increase at ${\mathrm{\Lambda }}_{\mathrm{rev}}={30}^{\circ }$, amplifies small spanwise vorticity gradients in the flow field and results in the net transport of vorticity along the blade span. The transport of vorticity, particularly in three-dimensional flows, is the subject of ongoing experimental efforts, and the current work represents a first step in understanding its role in reverse flow aerodynamics.

Figure 1

Top-down view of the aerodynamic environment encountered by a rotor blade element in forward flight at $\mu =1.0$. (a) The reverse flow region is highlighted on the retreating side of the rotor, and (b) isocontours of the yaw angle, adapted from [5], are sketched atop the rotor disk.

Figure 2

Experimental setup in the $20\times 28$ in. low speed wind tunnel at the University of Maryland. The field of view (FOV) is shown at the midspan of the tunnel. Unsteady pressure transducers, represented by black dots in the right-hand subfigure, are installed along the surface of the blade in the chordwise direction.

Figure 3

Photograph of the tunnel mounting apparatus for the blade at ${\mathrm{\Lambda }}_{\mathrm{rev}}={30}^{\circ }$. The blade was passed through the tunnel floor and ceiling by way of a circular bearing and was held in place by a spar housing structure.

Figure 4

Pitch input (combined collective and cyclic) for a Mach-scale rotor operating at $\mu =0.6$, from [17].

Figure 5

Pitching kinematics for the baseline reduced frequency in the current work overlaid with an exact sinusoidal fit.

Figure 6

(a) Sample reduced time and (b) effective reduced frequency distributions for a blade element passing through reverse flow. This figure corresponds to a rotor with $R=40$ in., $c=3.15$ in., and $\mathrm{\Omega }=900$ RPM, consistent with the experiments in [7, 17, 18].

Figure 7

Identification of relevant flow structures in the unyawed (top) and yawed (bottom) configurations at the baseline reduced frequency, $k=0.160$. A strong reverse flow dynamic stall vortex (RFDSV) is present at both yaw angles, but a trailing edge vortex (TEV) appears much more coherent in the unyawed case.

Figure 8

Evolution of nondimensional circulation generated by flow structures in the unyawed and yawed configurations. As evidenced by the PIV, the presence of spanwise flow appears to drain vorticity from secondary flow structures and in turn delays the breakup of the RFDSV.

Figure 9

Appearance of low pressure impulse in the unyawed configuration (left) and the yawed configuration (right) at the baseline reduced frequency, $k=0.160$.

Figure 10

Comparison of pressure impulse shape in the unyawed and yawed configurations at $k=0.160$. The difference in pressure impulse shape highlights the importance of the trailing edge vortex (TEV) in breaking down the reverse flow dynamic stall vortex (RFDSV).

Figure 11

Identification of relevant flow structures in the unyawed (top) and yawed (bottom) configurations at the “high” reduced frequency, $k=0.450$. Here, the behavior of the reverse flow dynamic stall vortex (RFDSV) is remarkably similar when the blade is either yawed or unyawed, as the trailing edge vortex is absent from both cases.

Figure 12

Comparison of surface pressure (top) and circulation (bottom) evolution for the unyawed and yawed cases at $k=0.450$. Since secondary flow structures are absent from both configurations, the time histories of pressure and circulation are very similar between ${\mathrm{\Lambda }}_{\mathrm{rev}}=0^{\circ }$ and ${30}^{\circ }.$

Figure 13

Vortex center as calculated from the ${\mathrm{\Gamma }}_1$ function. The results are nondimensionalized by the geometric chord of 5 in. for both cases. Note the similarity between the yawed and unyawed cases.

Figure 14

Comparison of pressure impulse shape in the unyawed and yawed configurations when the adjusted reduced frequency is matched ($k^{*}=0.160$). Even when the RFDSV is expected to reach the trailing edge at the same time, the difference in pressure impulse shape persists.

Figure 15

The “effective,” or streamwise, pitch angle for the unyawed and yawed configurations at the baseline kinematics (left, ${\alpha }_{\mathrm{mean}}=10.{75}^{\circ }$, ${\alpha }_{\mathrm{amp}}=10.{75}^{\circ }$) and the corrected kinematics (right, ${\alpha }_{\mathrm{mean}}={13}^{\circ }$, ${\alpha }_{\mathrm{amp}}={13}^{\circ }$). Note that the effective pitch angle nearly matches throughout the cycle for the corrected case.

Figure 16

Comparison of pressure impulse shape in the unyawed and yawed configurations when the effective pitch angle is matched at constant reduced frequency, $k=0.160$.

Figure 17

Comparison of pressure impulse shape in the unyawed and yawed configurations when the effective pitch angle is matched at constant adjusted reduced frequency, $k^{*}=0.160$.

Figure 18

Definition of expected chordwise and spanwise flow directions, with an illustration of the impact of spanwise variation on surface pressure.

Figure 19

Phase-averaged vector fields showing contours of spanwise flow for the unyawed (top) and yawed (bottom) conditions at $k=0.217$. Red regions correspond to flow toward the tunnel floor (the expected direction), while blue represents flow toward the tunnel ceiling.