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This paper describes a combined experimental and computational effort to characterize the cycle-to-cycle variations previously observed in oscillating airfoil experiments. It is common in the dynamic stall community to assume that the variation in loads between pitch cycles is primarily due to turbulent fluctuations, which in turn justifies the assumption that the loads can be accurately represented using simple phase averages. However this work, which numerically and experimentally models an oscillating modified VR-12 airfoil at two different conditions, shows that various aspects ranging from model setup to inherent fluid dynamics may lead to furcation in the loads that can have first-order impacts on the flow. These clusters of loads are poorly represented by simple phase averaging techniques and require special consideration. This study successfully captures dynamic stall furcation using computational fluid dynamics (CFD) and shows good agreement for cluster-averaged quantities when compared with experimental data. Furthermore, this work also presents several lessons learned concerning best practices for the dynamic stall community toward improving future correlations between CFD and experimental data and the quality of future dynamic stall datasets in general.

Numerical and experimental analysis of a synthetic jet actuator in quiescent air is reported. The study focuses on the actuator itself and on the vorticity field as well as on structures that are generated by the actuator. Large eddies simulation was used for numerical analysis and phase-locked, two-dimensional microscopic-particle image velocimetry technique was used in the experiment. In the numerical simulation, the equations were integrated using a nondissipative scheme that enforces discrete conservation of kinetic energy. The internal cavity flow of the actuator was part of the simulation. The actuator under consideration is of a unique design, fabricated to accommodate practical issues associated with helicopter rotor application. However, it has the same operating principles as a conventional synthetic jet actuator. Harmonics of the forcing frequency were traced in the velocity field. Insight into the evolution of the vortical structures, jet flapping, and onset of turbulence in the jet was retrie...

Performance and flowfield measurements were conducted on a small-scale cyclorotor for application to a micro air vehicle. Detailed parametric studies were conducted to determine the effects of the number of blades, rotational speed, and blade pitching amplitude. The results showed that power loading and rotor efficiency increased when using more blades; this observation was found over a wide range of blade pitching amplitudes. The results also showed that operating the cyclorotor at higher pitching amplitudes resulted in improved performance, independently of the number of blades. A momentum balance performed using the flowfield measurements helped to quantify the vertical and sideward forces produced by the cyclorotor; these results correlated well with the force measurements made using load balance. Increasing the number of blades increased the inclination of the resultant thrust vector with respect to the vertical because of the increasing contribution of the sideward force. The profile drag coefficient of the blade sections computed using a momentum deficit approach correlated well with typical values at these low chord Reynolds numbers. Particle image velocimetry measurements made inside the cage of the cyclorotor showed that there are rotational flows that, when combined with the influence of the upper wake on the lower half of the rotor, explain the relatively low efficiency of the cyclorotor.

The aerodynamic interaction between an unsteady, inclined synthetic jet and a crossflow boundary layer was studied as a precursor toward applying active flow control concepts for rotor applications, such as dynamic stall control and fuselage drag reduction. Because the flowfield offered numerous challenges from a measurement perspective, several experiments were carried out using a phase-locked, two-dimensional microscopic particle image velocity technique in a building block approach, by adding one complexity after another. The procedure began with boundary layer measurements made on a simple flat plate using the microscopic particle image velocity technique. Velocity measurements were made deep in the viscous sublayer, as close as 20μm from the surface. Following this, the synthetic jet actuator was characterized while operating in quiescent air as well as in crossflow. The results showed that the evolution of the synthetic jet in crossflow was substantially different from its evolution in quiescent air, suggesting that any flow physics or performance prediction (for example, the depth of penetration of the jet into the boundary layer) made based on the quiescent flow conditions may not be applicable in crossflow. All the momentum added to the boundary layer had its source from the synthetic jet actuator, and the penetration of the jet was limited to the viscous sublayer and log layer; the outer layer was unaffected, despite using a jet to freestream velocity ratio of four. Significant effort was also made to validate the microscopic particle image velocity technique and evaluate its capability to accurately resolve such a complex flowfield. To this end, microscopic particle image velocity measurements were compared with hot-wire measurements made on a simple steady jet, as well as an unsteady, periodic synthetic jet. Excellent correlation was found between the two techniques, validating microscopic particle image velocity measurements.

The formation and evolutionary characteristics of the blade tip vortices generated by a hovering rotor were studied using dual-plane stereoscopic particle image velocimetry. The dual-plane stereoscopic particle image velocimetry technique permitted noninvasive measurement of the three components of the velocity field and the nine components of the velocity gradient tensor, a capability not possible with classical particle image velocimetry. The dual-plane stereoscopic particle image velocimetry method is based on coincident flow measurements made over two differentially spaced laser sheet planes. A polarization-based approach was used in which the two laser sheets were given orthogonal polarizations, with filters and beam-splitting optical cubes placed so that the cameras imaged Mie scattered light from only one or other of the laser sheets. The digital processing of the images used a deformation grid correlation algorithm that was optimized for the high velocity gradient and small-scale turbulent flows found inside the blade tip vortices. High-resolution flow imaging of the vortex sheet in the wake behind the rotor blade, combined with detailed turbulence measurements, revealed the presence of several turbulent flow features during the vortex roll-up process that appear to play an important role in its final evolution. The dual-plane stereoscopic particle image velocimetry measurements also included the fluctuating terms involved in the Reynolds-averaged stress transport equations. The results confirm that an isotropic assumption of turbulence is invalid inside blade tip vortices and that stress should not be represented as a linear function of strain. The dual-plane stereoscopic particle image velocimetry measurements were also compared with velocity and turbulence measurements made using a laser Doppler velocimeter system, with good correlation.

An experiment was conducted to identify and measure the sources of uncertainty that are associated with the application of particle image velocimetry to the measurement of the vortical wakes trailing from helicopter rotor blades. Phase-resolved, three-component particle image velocimetry measurements were performed in the wake of a subscale rotor operating in hover, and were compared with high-resolution three-component laser Doppler velocimetry measurements obtained with the same rotor under identical operating conditions. This helped formulate the essential experimental conditions that need to be satisfied for particle image velocimetry to accurately resolve the high-velocity gradient, high streamline curvature flows that are present inside the rotor wake and in the blade tip vortices. Uncertainties associated with the calibration, acquisition, and processing of the particle image velocimetry images were analyzed in detail. It was shown that the optimization of laser pulse separation time is fundamental to reduce the errors associated with acceleration and curvature effects. Similarly, the interrogation window size was shown to play a critical role in determining the velocity gradient bias errors. The correlation between the laser Doppler velocimetry and particle image velocimetry measurements of the tip vortex characteristics, such as core radius and peak swirl velocity, were found to be excellent.

The unsteady aerodynamics of a biomimetic inspired flapping-wing mechanism has been analyzed by performing detailed phase-locked diagnostics of its flow field. Flow visualization and particle image velocimetry results have shown the presence of a shed dynamic stall vortex that spans across most of the wing span. The shedding of this type of leading-edge vortex was accompanied by the formation of another leading-edge vortex before the first vortex reached the midchord, resulting in multiple shedding leading-edge vortices on the top surface of the wing during each wing stroke. A strong starting vortex was also formed at the trailing edge of the wing during the early part of its translational stroke. This vortex continuously gained strength from shed vorticity as the wing accelerated into its stroke. The starting vortex remained close to the trailing edge until the wing reached midstroke. A pair of vortices that continuously trailed from the root and tip of the wing were identified, both of which induced a significant downwash velocity over the wing surface. These trailed vortices were found to exhibit a contracting wake structure as they convected into the wake below the wing, consistent with an increase in slipstream velocity. The evolution of the tip and root vortex pair showed rapid diffusive characteristics with an increase in time (wake age).

Afl ow-visualization experiment was conducted on an insect-based flapping-wing mechanism. This enabled greater understanding and insight to be gained on the unsteady aerodynamic phenomena that are responsible for the enhanced lift of wings operating at low Reynolds numbers in hovering flapping flight. Flow-visualization images were acquired with a strobbed laser sheet to illuminate the flow, which was seeded with a mineral oil fog. The general flowfield structure was found to consist of a folded wake, with a relatively large starting vortex at the end of each half-stroke. A large flow recirculation region was generated in the plane of flapping, which was centered around the two extreme flapping displacements. These general flowfield features were enhanced by detailed observations of the local flowfield around the wing section. One observation was the presence of multiple vortices on the top surface of the wing as it underwent translation. Furthermore, the local flowfield images clearly showed the growth of the leading-edge vortex as a function of span and identified the presence of separated flow on the outboard regions of the wing. These experimental results were supported by a free vortex modeling of the wake developments. The model was found to predict similar wake flowfield dynamics to that found in the experiments. This research has contributed to a better understanding of the unsteady aerodynamic mechanisms that are responsible for the enhanced lift of insect-based flapping wings in hover.

An experiment was performed to help quantify the interdependence of viscous/turbulent diffusion and straining effects on the development of helicopter rotor tip vortices. The properties of the blade tip vortices were measured in the wake of a small-scale hovering rotor and compared to the results for the case when the wake approached a solid boundary. The presence of the boundary created velocity gradients that forced the tip vortex filaments to strain, allowing the effects of this process on the vortices to be measured relative to the baseline case without the boundary. It is shown that vortex stretching begins to decrease the viscous core size, and when the strain rates become large, this can balance the normal growth in the vortex core resulting from diffusion. The present results were used to help develop a more general tip vortex model suitable for use in a variety of helicopter rotor aeroacoustic applications. The proposed engineering model combines the effects of turbulent diffusion and strain on the vortex core growth. The empirical coefficients of this model have been derived based on the best available results from rotating-wing tip vortex measurements.