Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D14: Rotors and Rotating Wings |
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Chair: Vrishank Raghav, Auburn University Room: Georgia World Congress Center B301 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D14.00001: How quadcopters could retain high precision near obstacles Darius Carter, Justin Robinson, Bruce Zhang, Daniel Quinn The growth of the Micro Aerial Vehicle (MAV) industry is outpacing our understanding of how MAVs behave in cluttered environments. Search and rescue and product delivery – two key MAV applications occur in tight, confined spaces filled with complex obstacles. Our current understanding of how MAVs interact with boundaries is based primarily on helicopter models, which were designed for high Re flows. To create better flow models of MAV-boundary interactions, we measured the lift forces and wakes of small quadcopters using a load cell and Particle Image Velocimetry. We found that side-wall proximity has minimal effect on flight performance, whereas ceiling and ground proximity cause significant disruptions. To generalize our results to multi-MAV systems, we also considered two quadcopters with interfering wakes. The resulting flow models could be incorporated into MAV controllers to improve the precision of their trajectories. When MAVs operate near humans or cooperate with humans centimeter-scale precision is the only safe option. We, therefore, believe our models can help MAVs be safer and more reliable, two attributes critical to their societal acceptance. Better flow models could also obviate the need for heavy sensors and cameras, which would free up payload on MAVs. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D14.00002: A Stereo-PIV Investigation of a Quadrotor Flow Field Jacob Connors, John-Michael Velarde, Mark N Glauser According to the FAA as of January 2018, the number of small Unmanned Aircraft Systems (sUAS) in the skies above America has surpassed 1 million. These aircraft provide a platform that sensors can use to take data in ways never thought possible a decade ago. One such possibility is the development of a sUAS based turbulence measurement device. For a device like this to be accurate an understanding of the flow field above and below the sUAS is necessary. In this project stereo-PIV and hotwire data were taken surrounding a DJI Phantom III sUAS to quantify the effects of the propeller interaction on the flow. Experiments were conducted in Syracuse University’s Indoor Flow Lab (IFL) which is a 15’x10’x18’ quiescent room. Data were taken with the sUAS at a simulated hover with cross stream stereo-PIV planes taken both downstream and upstream, and hotwire planes were taken in a grid extending downstream. Utilizing both hotwire data and stereo-PIV results allow for an analysis of both time and space dependent turbulent quantities. These results will make it possible to make recommendations about sensor placement, filtering requirements, and assist in the validation of sUAS CFD codes. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D14.00003: Aerodynamic characteristics of a quadrotor UAV in hovering mode Seungcheol Lee, Jooha Kim In the present study, we investigate the aerodynamic characteristics of a quadrotor UAV in hovering mode by measuring the lift force and multiple two-dimensional velocity fields in the wake. The experiment is conducted at Re = 30,000 in a chamber large enough to neglect the ground effect, where Re is the Reynolds number based on the rotor chord length and the rotor-tip speed. The strength of the rotor-tip vortex shedding is asymmetric with respect to the rotor axis due to the rotor wake interaction, and the wake centerline of each rotor is inclined to the center of the UAV. The wake from each rotor moves closer to each other while traveling in the streamwise direction, and then is merged together inducing large fluctuations in the transverse velocity. In addition, the effect of the distance between rotors on the aerodynamic performance will be also discussed in the presentation. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D14.00004: Wake characterization of thrust scaled counter-rotating coaxial rotor Lokesh Silwal, Karlyle Munz, Benjamin Sexton, Nathaniel Reed, Vrishank Raghav The flowfield characterization of co-axial multi-rotor configuration in an out-of-ground (OGE) effect in hover is presented. A modular experimental setup with both axially and laterally adjustable rotor spacing is used for the study. Thrust scaled rotors each consisting of three blades actuated by DC brushless motors operating at torque balanced condition is being used. Collective pitch setting of the blades is changed to achieve the required trim condition. Flow visualization and time resolved particle image velocimetry (PIV) are used for the flow field characterization of the wake of the coaxial rotor system. The interaction between the wake and the tip vortices from the upper and lower rotor is studied in near and far wake by changing the axial separation distance between the rotors and keeping the tip Reynolds number constant. Non-dimensional axial separation distance is varied between 0.05-0.1 of the rotor diameter, which is the range in currently operational, full scale co-axial rotorcraft. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D14.00005: Large eddy simulation of flow past a multirotor propeller in forward flight condition Young-Jin Yoon, Keuntae Park, Sehyeong Oh, Haecheon Choi We investigate flow characteristics of an isolated multirotor UAV propeller under various advance ratios using large eddy simulation. An immersed boundary method in a non-inertial frame of reference with a dynamic global subgrid-scale model is used for the simulations. The flow field and aerodynamic coefficients from the present simulations are in good agreements with those from experiments. The simulation clearly shows the interaction of trailing vortex sheets with hub and tip vortices, which results in complicated non-axisymmetric vortical structures. With increasing advance ratio, blade sectional analysis reveals complete loss of lifting capability near the hub region in the retreating side. Large variation of the effective angle of attack during the blade rotation produces large oscillatory longitudinal moment which may be detrimental to vehicle stability. The detailed effect of advance ratio on the flow structure will be discussed at the presentation. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D14.00006: Multi-rotors in descent– Capturing the true dynamics of a descending drone Marcel Veismann, Christopher Dougherty, Morteza Gharib When descending, rotorcraft inherently enter their own downwash, leading to a highly unsteady flow field which can cause a loss of thrust and reduced stability. Our work aims to understand the interaction of multiple rotors in vertical descent by investigating a multi-rotor craft in a low speed, vertical wind tunnel. Previous experiments have shown that the instabilities are caused by rotor tip vortices convected through the rotor disk at specific descent velocities. Furthermore, these results showed considerable deviations between a fixed mounted and a rotationally free mounted multi-copter in a vertical flow, suggesting that a restricted system is not an accurate representative of the actual descent dynamics. The presented material focuses on the investigation of a fully unrestricted, free flying drone in a vertically rising flow and its response to the disturbances. Supported by thrust measurements and PIV analysis, the study provides further insight into the nature of the instabilities. The resulting data is compared to previous experiments in order to validate the experimental technique and help us understand the effects of restricting a multi-rotor. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D14.00007: Experimental investigation of rotor-wing interaction at low disk loading and low Reynolds number Mingtai Chen, James Paul Hubner, Darnisha Detraniece Crane Hover performance and prediction of tiltrotor-like vehicles lack experimental data and computational models for rotor-wing interactions at low disk loading and low Reynolds number. An experimental investigation of rotor-wing interaction in hover near the micro-air-vehicle (MAV) scale (Re < 100,000 and thrust < 5 N) is conducted to determine how changes in rotor-wing geometry influence rotor-wing interaction. The test setup consists of counter-rotating rotors and a flat plate representing a generic wing. The vertical distance between the rotors and wing, the spanwise distance between the rotors and the blade pitch angle are considered to assess the rotor-on-wing and rotor-on-rotor effects. Rotor thrust and torque, wing download force and wing surface pressures are collected. These results will be used to compare with computational models. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D14.00008: Time-resolved large-scale volumetric flow measurement of a helicopter model using Helium-filled soap bubbles Dirk Michaelis, Clemens Schwarz, Johannes Bosbach, Uwe Dierksheide, Christian Lemke, Christian Wolf, Daniel Schanz, Andreas Schroeder This study describes time-resolved large-scale volumetric flow measurement of a helicopter model (1.55 m rotor-span, cord length = 61 mm) in close-to-ground condition (height = 0.5 D) at Reynold number 4.1 x 10^5 (rotational frequency = 20.83 Hz) to examine CFD predicted secondary vortex structures (originating from primary rotor-tip vortices), as well as ground effects. Five high-repetition-rate cameras are used to image tracer particles (Helium-filled soap bubbles HFSB) in a 400 x 600 x 400 mm³ measurement volume at 1.7 kHz recording rate. HFSB tracer particles are illuminated by an array of over-pulsed high-power LED’s. 50 sequences, with 3800 images each, at different angles of attack and different particle densities are recorded. The images are analysed using Shake-the-Box (time resolved particle tracking) to gather dense Lagrangian particle tracks. Further, data assimilation techniques (Flow Fit, Vortex-In-Cell VIC#) are applied for high-resolution grid-reconstruction of velocity, vorticity, acceleration and pressure (pressure from PTV) from the track data. First analysis confirms the presence of secondary vortex structures and reveals spanwise oriented vortex tubes close to the ground. Low pressure and high acceleration (150 g) are measured in the primary tip vortices. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D14.00009: Dynamic Stall and Vortex Dynamics of a Single Blade in Cycloidal Rotation around an Advance Ratio of One John A. Farnsworth, Nishant Agarwal Cycloidal Rotors have been studied for over 100 years with a focus on applications in energy production (VAWT) and for VTOL vehicles. Numerous experimental and analytical studies have been carried out over the years to prove their potential competency over the conventional horizontal-axis rotors. In contrast, the present study focuses on extending the fundamental understanding of the unsteady aerodynamics associated with cycloidal rotor operation. In particular, particle image velocimetry (PIV) measurements and analytical tools were used to understand the change in flow dynamics around a single, fixed-pitch cycloidally rotating NACA 0012 blade as the system translates across an advance ratio (μ = U∞/ω R) of 1. Note that μ = 1 represents a critical transition point for the self-start of VAWTs and the progression of VTOL vehicles into forward flight. Preliminary analysis of PIV data shows that the flow is self-similar for cases at the same advance ratio, and that the wake structures do not depend upon the Reynolds number. The phase-history, velocity contour plots of the wake structure show a distinct cycloidal pattern for an advance ratio of 1.25, a more stationary wake pattern for an advance ratio of 1, and a retarding pattern for an advance ratio of 0.75. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D14.00010: Suppression of vortex shedding by spinning for low Reynolds number flow over a circular disk at angle of attack Marcus Lee, Timothy E Colonius, Beverley J McKeon The stability issues faced by micro air vehicles in gusty winds motivate the study of spinning disks as a potential design for robust flight due to spin stabilization. To this end, we use a three-dimensional immersed boundary method to study how the tip-speed ratio affects the wake instabilities for flow over a circular disk at angle of attack for Reynolds number of O(102). Spin has a stabilizing effect on the flow, decreasing the vortex shedding amplitude and, for sufficiently high tip-speed ratio, suppressing it completely to give a steady wake. The resulting flow structures are significantly changed, with complex flows in the recirculation bubble and strengthened tip vortices. |
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