Session T03: Aerodynamics: Rotating Wings

Analysis of the two-way interactions between an actuating line and a body-meshed end-plate

Actuator Line Methods (ALM) have been successfully used in the wind energy sector and rotorcrafts for performance prediction of open rotor geometries. However, their applicability to ducted fan geometries, in which the end points of the actuating line are in close proximity to the solid duct, is still to be explored. The potential (inviscid) and viscous interactions of the duct on the line, and of the line on the duct are of interest. To explore this problem, we consider in this work a three-dimensional finite-span wing equipped with attached end-plates and detached end-plates at different spacings. The lifting wing is modelled with an actuator line at different discretizations and different non-isotropic Gaussian kernel parameters. The results are compared to the wall-resolved simulations of Villeneuve, Boudreau & Dumas (2019) for the same geometries. The physical analysis is carried out on the basis of the vorticity structures and dynamics, and on the spanwise loading of the wing.   

Wake characteristics of a twin rotor in axial descent

In the present study, we investigate how the wake characteristics of a twin rotor change as the descending speed and the spacing between the rotors change. For this purpose, the velocity fields measurements are performed with varying the normalized rotor separation distance (0.25 ≤ s/R ≤ 2.5, where s is the distance between the adjacent rotor tips and R is the radius of the rotor) and the descent rate (0.87 ≤ V_D/ V_h ≤ 1.52, where V_D is the descent speed and V_h is the induced velocity at the rotor disk in hover). For the isolated rotor, the wake generates extensive recirculation zones near each rotor tip at a high descent rate of more than 1.08, entering the vortex ring state (VRS). For the twin rotor, the wake has different flow characteristics depending on the descent speed and the rotor separation distance. In the region between rotors (inner region), the rotor-rotor interaction collapses the vortex ring structure at the critical s/R, which has been lowered as the descent rate increases. On the other hand, the flow state in the outer region does not change regardless of the rotor separation distance. These results reveal that the strong rotor-rotor interaction causes the flow states in the inner and outer regions to differ from each other, leading to an asymmetrical wake structure about the rotational axis. Some more details will be discussed in the presentation.

    

Investigation of dynamic stall on a cross-flow turbine blade using modal decomposition

A cross-flow turbine blade, that rotates on an axis perpendicular to the flow, undergoes a complex dynamic stall cycle due to the non-sinusoidal variation of relative flow speed and angle of attack. Furthermore the blade passes through flow speeds much lower than the freestream on the downstream end of the turbine, hence significantly altering the stall cycle. Modal decomposition is performed on high resolution flow fields from large-eddy simulation to explain the stall process and to estimate the relative flow as a step towards reduced-order modeling of the turbine. Distinct flow regimes are explored such as a high angle of attack deep stall regime, a moderate angle of attack regime with optimal power generation, and an intracycle variation of angular velocity that drastically alters the relative flow. Common modes based on velocity and vorticity fields are extracted across these regimes to compare the corresponding temporal coefficients and hence explain the stall dynamics. Additionally, the modes can isolate the relative flow variation from influences such as flow induction and vortices created or encountered by the blade. Relative flow estimation methods using mode temporal coefficients, or probing the reconstructed low-order flow field, are compared for accuracy.   

Aerodynamic Scaling of Rotorcraft Blades in Hyperbaric Conditions

Novel multirotor aircraft have been developed for applications ranging from urban transportation to military use. Techniques for modelling the performance of aircraft rotors continue to improve, but they still fall short of perfectly replicating the full highly complex flow physics, including unsteady aerodynamics, scale effects, and wake dynamics. Blade element momentum and vortex wake methods are commonly used to analyze rotorcraft performance but resort to simplified models based on assumptions about the flow physics, such as prescribed inflow or wake geometries, or quasi-steady conditions. Experimental testing provides higher fidelity data, but the cost of fabricating and testing models in a suitably large wind tunnel can be prohibitively expensive due to the sheer scale of rotor blades. Testing with scale models has been done, but small models fail to replicate full-scale performance. The compressible nature of the flow means Reynolds number (Re) and Mach number (Ma) cannot both be matched. Conducting experiments on scaled rotors in compressed air allows density to be tuned to match Re and Ma across a range of scales. Data from single rotors in hover will be presented to examine the dependence of performance, such as thrust and power coefficients on Re, as density varies.   

Aerodynamic effects of rotor-rotor interaction on a tandem rotor in ground effect

In this study, we investigate the effect of rotor-rotor interaction on the thrust generation and the wake structure of a tandem rotor operating near the ground. For this purpose, the force and velocity field measurements are performed with varying the normalized rotor separation distance (0.25 ≤ s/R ≤ 2.45, where s is the distance between the adjacent rotor tips and R is the radius of the rotor) and the normalized ground height (0.50 ≤ h/R ≤ 2.00, where h is the distance between the rotor tip-path plane and the ground). The interaction between the rotors causes the thrust generated by a tandem rotor to be less than that generated by a single rotor at the same h/R. The thrust loss is more pronounced at lower s/R as the tandem rotor approaches the ground. The measured velocity field shows that the thrust loss is due to the reinjestion of tip vortices rising above the rotor tip-path plane by the fountain flow formed between the rotors. As the tip vortices are reinjested into the rotor disk, the turbulence intensity of the rotor inflow is increased compared to a single rotor, which in turn reduces the thrust generated. The flow characteristics described above appear in the form of a vortex ring between the two rotors, which will be discussed in detail in the presentation.   

Flow visualizations on a propeller with co-rotating shroud for MAV applications

Stationary shrouds are placed around propellers in many aeronautics applications to improve thrust efficiency, reduce noise, and provide protection from debris. However, their installation requires additional support structures and tight tolerances for blade-tip clearance. For small-scale aerial vehicles, it may be feasible to eliminate these drawbacks by attaching the shroud directly to the blade tips such that they rotate together. We experimentally investigate the potential benefits of such a design, dubbed "prop-shroud", under hover conditions. Various prop-shroud prototypes based on standard NACA airfoil cross-sections are 3D printed to explore the effect of blade and shroud geometry on performance. Particle image velocimetry on select configurations provides insight into the interesting vortex dynamics and changes in the flow field that arise from the addition of a rotating shroud. The observed flow features are interpreted with measurements of thrust, torque, and noise level.   

The Role of Vortices in Aeroacoustic Noise from Rotating Wings: Insights from Force and Acoustic Partitioning

Drones have several important applications such as wildlife monitoring, product delivery, security etc. but the noise they generate can be a major factor limiting their operation. The two main categories of drone aeroacoustic noise are tonal noise and broadband noise. This noise is generated due to periodic variation in aerodynamic force, blade vortex interaction, blade gust interaction, boundary layer-trailing edge interaction, stall and flow separation noise. Determining the relative importance of these various mechanisms is difficult since the surface pressure variations that are the source for this noise, are induced simultaneously by all these mechanisms.  Vortex-induced pressure fluctuations are particularly important, but the vorticity field in these flows is highly complex, making it difficult to pinpoint the flow structures that are important for noise generation. We have used direct numerical simulations to simulate the flow associated with a canonical drone rotor and further used POD to decompose flow field into dominant modes. We then apply the recently developed force and FW-H based acoustic partitioning to examine the role of different dominant flow structures on the blade loading noise.   

Turbulence and Associated Production Mechanisms in an Axial Compressor Rotor Near its Stall Point

The complex structure of turbulence in an axial compressor is studied based on Stereo PIV measurement performed in a refractive index-matched facility. The analysis focuses on pre-stall conditions, when the mean flow is still stable, but the instantaneous flow is already affected by large scale intermittent fluctuations. The TKE is particularly high in regions surrounding the tip leakage vortex (TLV), and at the interface between the main passage flow and the region with low axial momentum, including region with backflow, along the tip. The high TKE around the blade tip is a statistical manifestation of spillover of large intermittent (backflow) vortices from one passage to the next in the vicinity of the blade leading-edge. The production rate terms help in identifying dominant mechanisms affecting the inhomogeneous and anisotropic distribution of Reynolds stresses. For example, near the casing, axial contraction, and shear production terms (-2〈u_θ^'u_z^'〉∂U_θ/∂z) are major contributors, whereas, near the TLV, circumferential contraction and (-2〈u_z^'u_r^'〉∂U_z/∂r) are important, while the radial contraction term changes form negative to positive values. Similar features appearing in other turbomachines highlight the prevalence of underlying flow phenomenon near the stability limit.

    

The Influence of Cycle-to-Cycle Variation on Cross-Flow Turbine Performance

Cross-flow turbines are subject cycle-to-cycle variability due to inflow fluctuations, potential hysteresis from previous cycles, and dynamic stall's sensitive and stochastic nature. We seek to understand the extent of cycle-to-cycle variability in turbine performance and flow-field hydrodynamics, as well as the links between the two.

Two-component, planar particle image velocimetry data, obtained inside the turbine swept area, is examined in concert with simultaneously captured turbine performance data. Flow fields are investigated via a hierarchical clustering pipeline featuring a principal component analysis preprocessor. This enables clustering based on all of the dynamics present, weighted by their importance, in an interpretable, low dimensional subspace. The results of the flow field clustering are compared to those from hierarchical clustering of the simultaneous turbine performance measurements. The data set utilized here has relatively small performance variations overall (standard deviations on the order of 1.5% of the mean), yet our results show a clear performance and hydrodynamic dependence on assigned cluster. Correlations between assigned cluster and inflow velocity are likely explanatory.

    

Efficiency of an intermittent, flexible propeller

In this talk, I will present a study addressing the problem of propulsion using intermittent dynamics.  The system consists of a body moving forward using a propeller that has the possibility of changing shape depending on whether the propeller is rotating or not. The nature of the intermittent dynamics consists in alternating active phases (when the propeller in rotating) and passive phases where the body uses inertia to sustain the motion. The control parameters of the intermittency are the duty cycle (DC) the burst time and the coast time. We show that, for optimal sets of those parameters, the efficiency of the intermittent propeller is improved compared to classical models.  

    

Aerodynamic Effects of Phase Offset Between Synchronized Propellers in Hover

Developments in aviation technology are creating new opportunities to use distributed electric propulsion (DEP) on novel aircraft concepts in support of urban air mobility (UAM). The propulsion system for DEP vehicles spread many small electric motors and propellers around the aircraft. The inevitable decrease of propeller spacings on DEP vehicles causes complex interactions that can be detrimental, with some researchers reporting thrust fluctuations, decreased performance, and large noise signatures. This study aims to explore the propeller-propeller aerodynamic interactions between two counter-rotating, synchronized propellers in close proximity. This study uniquely phase locks the propellers and quantifies the impact of phase control on overall performance under hover conditions. 2-D and 3-D PIV experiments were performed to characterize the flow generated by the interacting propellers at different relative phases. Variations in tip vortices, momentum fluxes, and streamwise velocities were explored between the phase offset cases. The interactions between the propellers causes significant alteration to the tip vortex trajectories and rate of tip vortex decay when compared to a single propeller control test. Larger momentum fluxes through the propellers and induced streamwise velocities were also observed for the dual propeller cases. However, our current PIV results show very minor aerodynamic differences between dual propeller cases at varying phase offsets. Because other researchers have reported potential noise benefits realized by controlling phase offset, the results of this study suggest that the benefits of phase control on the directivity patterns and overall sound pressure levels (OASPLs) can be realized with negligible sacrifice in aerodynamic performance.   

Aerodynamic Loading of High Advance Ratio Rotors with Blunt Trailing-Edged Blades

The reverse flow region developing on high-speed edgewise rotors is an inherent aerodynamic limitation, confining the flight speed and the efficiency of modern rotorcraft. To enable high advance ratio flight, the impact of the rotor blade geometry on the aerodynamic blade loads in the reverse flow region was investigated in the present study. A conventional, sharp trailing-edged NACA0012 was compared to a novel, blunt edged elliptical airfoil. A single-bladed rotor rig equipped with a six-axis force-torque sensor was used to perform water tow tank experiments. Advance ratios between 0.40 and 1.00 at collective pitch angles from 13° to 25° with 0° cyclic pitch were studied. Highly unsteady blade forces were measured over the rotor azimuth due to the superposition of the rotational and translational blade motions in the three-dimensional rotor environment. At an advance ratio of 1.00, negative lift and drag forces were detected on more than 75% of the azimuth angle range on the retreating side of the rotor disk due to flow reversal and separation effects. The lift decreased to approximately zero on the retreating blade at an azimuth angle slightly above 270° at an advance ratio of 0.40. The elliptical airfoil reduced the lift magnitude, lowering the blade performance on the advancing side of the rotor disk. However, the elliptical airfoil decreased the magnitude of the drag, the rotor torque, and the pitching moment in the reverse flow region which increases the rotor performance. The current results highlight the significance of the blade cross-sectional geometry, specifically the geometric trailing edge curvature, and its impact on the rotor performance.