Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session P25: Aerodynamics: Fixed, Flapping and Rotating Wing IV |
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Chair: Justin Jaworski, Lehigh Room: North 225 AB |
Monday, November 22, 2021 4:05PM - 4:18PM |
P25.00001: The effect of chordwise flexibility on the aerodynamic performance of microrobot-inspired flapping wings Emma K Singer, Geoffrey R Spedding Unsteady aerodynamic mechanisms involved in the lift production of flapping-and-pitching wings is a developing topic of contemporary research and are yet not fully understood. For biologically-inspired flying machines such as sub-gram flapping wing micro-air-vehicles (FW-MAVs) which operate in flow with Re < 1000, these mechanisms are critical for the achievement of net positive lift, control authority and payload capacity. The aerodynamic performance is sensitive to the wing stiffness, with complex three-dimensional and time-varying flow structures evolving along and behind the wing. Wing flexibility is defined by the dimensionless parameter effective stiffness, the bending rigidity normalized by the dynamic fluid pressure, which is used to design four wings with distinct stiffness. This work uses scaled wings undergoing prescribed sinusoidal flapping and pitching motion with zero mean flow to study the induced flow and forces during hovering. Tomographic PIV is used to visualize the 3D vortex formation and force-generating flow structures while a 6-axis force-torque sensor at the wing root captures instantaneous data. The coefficients of lift and drag are computed for several cycles of flapping for all wing types and compared to the results of the flow visualization. |
Monday, November 22, 2021 4:18PM - 4:31PM |
P25.00002: Axial Descent of Variable-Pitch Multirotor Configurations: An Experimental Investigation under Free-Flight Settings for Mars Deployment Applications Marcel Veismann, Skylar Wei, Sarah Conley, Larry Young, Jeff Delaune, Joel Burdick, Morteza Gharib, Jacob Izraelevitz For future helicopter-only Mars missions, NASA-JPL has proposed a novel entry, descent, and landing technique, in which the rotorcraft is deployed from the aeroshell in mid-air before landing. However, this approach is likely to subject the rotorcraft to unfavorable vortex ring state aerodynamics during deployment. To address this, the performance of a variable-pitch multirotor in axial descent was experimental investigated using untethered, free-flight wind tunnel tests. This approach allow to examine the rotor performance as a function of descent rate without restricting vehicle dynamics with a rigid mounting. Results indicated significant mean thrust losses of up to 20% compared to hover conditions, as well as heavily amplified rotor thrust fluctuations and vehicle attitude oscillations with increasing descent rate. Experimental findings were compared to analogous computational efforts utilizing the tool RotCFD, showing considerable discrepancies in the critical descent rates at which maximum thrust losses occur. |
Monday, November 22, 2021 4:31PM - 4:44PM Not Participating |
P25.00003: Vortex Structure Comparison Between a Novel Reduced Tip Vortex Rotary Blade and a Traditional Blade Mateo Diaz, Theresa Saxton-Fox The flow of a novel 18-inch rotor designed to reduce tip vortex strength was analyzed. The rotor was observed using stereo particle image velocimetry (PIV). During data acquisition, the rotor maintained a steady number of rotations per minute (RPM). Data were obtained as instantaneous snapshots and phase locked averages at several distinct phase angles. Non-local vortex identification methods were employed to characterize the vortices generated. Results were compared to those of a traditional power-optimized rotor to assess the effectiveness of the reduced vortex design. Significant differences in flow and vortex structure were observed between the two rotors. |
Monday, November 22, 2021 4:44PM - 4:57PM |
P25.00004: How water and solid boundaries affect the dynamics of a hovering rotor Darius J Carter, Qiang Zhong, Daniel Quinn Micro Aerial Vehicles (MAVs) are poised to revolutionize urban planning and diagnostics, rescue operations, transportation of goods, and home/business management. However, before MAVs become mainstream, they will need to fly much more dependably in complex environments. Our current understanding of how rotors interact with boundaries is based primarily on helicopter models designed for high Reynolds-number flows. We studied small (~20 cm diameter) isolated rotors to support better flow models of MAV-boundary interactions as they hovered above solid and water surfaces. We modeled the rotor by combining blade element theory with the potential flow-based "image method." We compared the model to experiments for multiple rotor diameters and blade pitches. Particle image velocimetry revealed deflected momentum jets between the rotor and the planar boundary. We see agreement between our experimental data and simple blade element modeling with inviscid ground effect. There are only minor differences in the effectiveness of the model between the solid and water cases. These findings suggest easy adaptation of ground effect models to differently sized rotors over either water or solid surfaces. |
Monday, November 22, 2021 4:57PM - 5:10PM |
P25.00005: Volumetric velocity field measurements in the wake of a co-axial, counter-rotating helicopter rotor hub Jeff Harris, Adam Nickels, Rommel Pabon, David Reich, Nicholas Jaffa, Sven Schmitz The unsteady loading and turbulent wake shed by co-axial, counter-rotating helicopter rotor hubs are examined using tomographic Particle Image Velocimetry. Experiments were conducted in the 12-inch water tunnel at the Applied Research Laboratory at Penn Statue using a circular test section 30 inches in length. Four high-speed cameras were used to record the motion of glass microspheres in addition to 3-component laser Doppler velocimetry (LDV) measurements of the rotor wake. These two measurements provide independent comparisons of the mean and fluctuating statistics of the velocity in the rotor wake. Good statistical agreement between the two velocity measurement techniques was achieved and large scale vortical regions in the wake are identified, relating to physical processes of the counter-rotating rotor flow. |
Monday, November 22, 2021 5:10PM - 5:23PM |
P25.00006: Numerical and Experimental Investigation of the Addition of Surface Roughness on Micro-Scale Propeller Performance Justin P Cooke, Matthew F Campbell, Edward B Steager, Igor Bargatin, Mark H Yim, George I Park Micro aerial vehicles (MAVs) require efficient propellers when operating at low Reynolds (Re) numbers due to increased viscous forces. Numerical and experimental methods are combined to show that propellers with surface roughness exhibit enhanced and reduced thrust and power coefficients, respectively, compared to their smooth counterparts. A numerical investigation using an unstructured mesh finite volume flow-solver showed that surface roughness reduces the pressure on the suction side of a propeller's blades, increasing pressure force contributions to its thrust. Further, the decreased pressure contribution to the drag lowers the required torque, yielding a net input power reduction despite an increase in viscous drag. These findings are corroborated by in-house thrust stand-based experiments, which revealed that surface roughness increased and decreased the thrust and power coefficients each, respectively, when compared against a baseline smooth propeller. |
Monday, November 22, 2021 5:23PM - 5:36PM |
P25.00007: PIV Measurements of the Aerodynamic Interactions of Closely Spaced Phase Locked Propellers with Synchronized Rotation Rates Jared Erickson, Daniel Maynes, Nathan Welker Emerging advances in electric-propulsion technology are enabling aircraft to use distributed electric propulsion (DEP) to increase efficiency and maneuverability. The implementation of DEP effectively decreases the spacing between propellers, introducing complex aerodynamic interactions that are not well understood. This study presents the use of stereoscopic particle image velocimetry to obtain 3D velocity measurements of the flow fields of phase locked and synchronized propellers at close proximity in a side-by-side configuration. The current study focuses on the hover configuration and flow field measurements were made over a plane that is located immediately downstream of the propellers. Single and dual propeller cases were explored at varying phase locked rotation angles. Variations of phase offset, separation distance, and rotation direction between the interacting propellers were explored. The total momentum thrust as a function of propeller phase angle was computed and used as a basis of comparison between the cases. It was found that the total instantaneous momentum flux of the dual propeller cases is a function of propeller phase angle, propeller spacing, and angular offset. |
Monday, November 22, 2021 5:36PM - 5:49PM |
P25.00008: The Effect of Casing Grooves on the Turbulence in an Axial Compressor Near its Best Efficiency Point Ayush Saraswat, Subhra Shankha Koley, Joseph Katz Stereo-PIV measurements performed in a refractive index matched facility examine the effect of axial casing grooves (ACGs) on the flow and turbulence within a one and a half stage axial compressor near its best efficiency point. These grooves are installed to delay the onset of stall but often cause efficiency penalty at design flow rates. Data are acquired at multiple axial planes, within and between the blade rows spanning the entire machine. The data are used for calculating the ensembled-averaged turbulent kinetic energy (TKE), Reynolds stresses, and associated production rate terms. Without ACGs, the turbulence in the tip region is affected by the tip leakage flow, blade wakes, as well global radial and circumferential contraction and extension throughout the machine. With the grooves installed, the flow jetting out of the ACGs upstream of the rotor leading edge rolls up into axially aligned vortices, which interact with the tip region flow. Driven by shear production, these vortices cause a significant increase in TKE along the periphery and downstream of the rotor. However, the differences between turbulence levels near the treated and untreated endwalls diminish as the flow passes through the stator downstream of the rotor. |
Monday, November 22, 2021 5:49PM - 6:02PM |
P25.00009: Evolution of turbulence across the stages of an axial compressor under pre-stall conditions. Subhra Shankha Koley, Ayush Saraswat, Joseph Katz Stereo-PIV measurements performed in a refractive index matched facility examine the evolution of turbulence in a series of axial planes within and between the blade rows of a multi-stage axial compressor, consisting of an inlet guide vane (IGV), followed by a rotor, and a stator. The data are recorded at below design flow conditions when the machine is about to stall. In the IGV-rotor gap, the turbulent kinetic energy (TKE), which is dominated by axial velocity fluctuations, is low in most of the passage except for the IGV wake and in the tip region, the latter owing to upstream-propagating instabilities. Within the rotor, very high turbulence is generated at the interface between the tip flow, which has high circumferential and low axial mean velocity, and the main passage flow, which has high axial velocity. This region is characterized by large-scale vortical structures aligned diagonally upstream. Both the shear and axial contraction terms are significant contributors to the turbulence production rate, and the axial velocity fluctuations can reach 80% of the mean values. The turbulence level decreases in the rotor-stator gap, and within the stator passage to a significant part by axial extension of the flow, which creates a broad region with a negative TKE production rate. |
Monday, November 22, 2021 6:02PM - 6:15PM |
P25.00010: Effect of flow confinement on the hydrodynamic performance of a fully-passive oscillating-foil hydrokinetic turbine Sierra S Mann, Guy Sumas, Peter Oshkai The influence of flow confinement on the hydrodynamic performance of a fully-passive oscillating-foil turbine prototype was investigated experimentally at a Reynolds number of 19,000. Quantifying confinement effects is important for comparison between model-scale experiments and full-scale turbine performance. The kinematic parameters and energy harvesting performance of the turbine were measured at eight confinement levels, ranging from 33% to 78%. Planar particle image velocimetry (PIV) was used to observe the timing of leading-edge vortex (LEV) formation and shedding, which was used to relate the flow physics to the turbine performance. The efficiency and the power coefficient increased with increasing confinement, as is expected, due to the increased effective velocity incident upon the turbine. At the highest level of confinement, the turbine performance dropped due to the shift in kinematic parameters associated with the close proximity to the confining walls. |
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