Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session A30: Aerodynamics: Rotating Wings and UAVs |
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Chair: Hui Hu, Iowa State University Room: 255 B |
Sunday, November 24, 2024 8:00AM - 8:13AM |
A30.00001: Understanding the influence of the Rossby number on the transient dynamics over a rotating wing Abbishek Gururaj, Mahyar Moaven, Brian S Thurow, Vrishank Raghav Studies have demonstrated that the Rossby number greatly affects LEV stability during the steady-state phase of rotating wing motion, but its influence on transient dynamics is less clear. This research explores the effects of the Rossby number on the transient dynamics over a rotating wing, using load measurements and rotating three-dimensional velocimetry. Previous studies have indicated that an increase in the Rossby number results in a reduction in the steady-state lift. In contrast, our findings show that the maximum lift during the transient phase increases as the Rossby number increases, with the difference between the maximum and steady-state lift increasing with an increase in the Rossby number. Preliminary flow field measurements suggest that in the transient phase, the LEV grows faster with increasing Rossby number, leading to higher circulatory and, thus, a higher maximum lift. With an increase in the Rossby number, the reduced effects of rotational accelerations result in a reduced out-of-plane transport of vorticity from the LEV and, hence, a faster growth of the LEV. Eventually, as the steady state is achieved, the reduced effects of rotational accelerations result in the LEV moving farther away from the wing, resulting in a significant drop in the steady-state lift at higher Rossby numbers. This study will further quantify the vorticity fluxes that mediate LEV growth and correlate them with the lift evolution in the different phases under different Rossby numbers. |
Sunday, November 24, 2024 8:13AM - 8:26AM |
A30.00002: Characteristics of tip vortex of a low Reynolds number rotor using large eddy simulation Young-Jin Yoon, Haecheon Choi We perform large eddy simulation to investigate the characteristics of tip vortices trailing from a low Reynolds number rotor at different pitch angles. The rotor consists of two blades featuring a circular-arc cambered profile with no twist along the span. The Reynolds number based on the tip speed and chord length is 34,200. The characteristics of the tip vortex is analyzed by aligning instantaneous flow fields with respect to the identified vortex center to account for the wandering motion of the vortex. During the formation of the tip vortex, separation of the crossing flow from the pressure side to the suction side at the blade tip creates complex flow containing multiple eddies. At a low angle of attack, the interaction of these small-scale vortices is observed inside the tip vortex even farther downstream, resulting in a wide high turbulence area inside the tip vortex. However, when the pitch angle is increased, small-scale vortices in the blade shear layer and at the tip merge into the primary vortex. In this case, the region with high turbulence intensity is confined only inside the core. The result from the snapshot POD method indicates that the dominant mode for the velocity fluctuations has two pairs of counter-rotating vortices. This mode rotates in the same direction as that of the vortex and modifies the core structure in a way that induces asymmetry inside the core. |
Sunday, November 24, 2024 8:26AM - 8:39AM |
A30.00003: Helicopter rotor simulation via actuator line method coupled with DYMORE Taeyeon Kwon, Junbeom Park, Yedam Lee, Kunhyuk Kong, Byeonguk Im, SangJoon Shin, Sang Lee The unsteady analysis of articulated helicopter rotor operation will require massive computational resources with traditional methods. To reduce the cost, Actuator Line Method (ALM) represents each rotor blade with a line and projects the aerodynamic force onto the computational mesh in isotropic Gaussian distribution. The Immersed Boundary Method (IBM) is implemented on the fuselage for Fluid-Structure Interaction (FSI) analysis. Spalart – Allmaras (SA) model is applied to capture the turbulence in rotor wake. Advection Upstream Splitting Method (AUSM), along with Monotonic Upstream-centered Scheme for Conservation Laws (MUSCL) scheme take part in simulating the advancing blade compressibility. ALM has a well-known problem of showing unphysical sectional thrust distribution at the blade tip. The tip loss correction model presented by previous works lacked performance in diverse collective pitch angles. To simulate the feathering motion of the blade, present study implements a newer tip loss correction function with improved performance in varying collective pitch angle. In house code was developed using the above schemes and coupled with DYMORE, a finite-element based analysis of nonlinear flexible multibody systems, to compute trim condition and aeroelasticity. |
Sunday, November 24, 2024 8:39AM - 8:52AM |
A30.00004: Role of three-dimensionality in navigation behind bluff-body wakes Vedasri Godavarthi, Kartik Krishna, Steven L Brunton, Kunihiko Taira Small-scale autonomous vehicles encounter unsteady flow conditions in rescue, surveillance, and sensing operations. Since they are equipped with finite actuation and sensory capabilities, trajectory planning in unsteady wakes is crucial. In this work, we study the role of three-dimensionality in navigation behind bluff body wakes. We use finite-horizon model-predictive control for trajectory planning in a three-dimensional cylinder wake at a Reynolds number of 300. We compare the 3D wake navigation performance with the 2D wake for several wake-crossing scenarios. Successful navigation is possible with a time horizon as low as one-tenth of the wake-shedding period. Further, the 3D wake navigation can be faster than the 2D wake navigation by taking advantage of the secondary vortices and lower spanwise coherence in 3D wakes. We find that the trajectory planners can leverage the secondary transverse vortices to effectively redirect toward the target. This study has the potential to develop sensor-friendly navigation strategies for autonomous vehicles in unsteady flows. |
Sunday, November 24, 2024 8:52AM - 9:05AM |
A30.00005: Steady and Unsteady Downwash Effects in Multirotor Interactions Anoop Kiran, Daniel Marella, Alex Wang, Nora Ayanian, Kenneth S Breuer Despite their versatility for search-and-rescue, precision agriculture, and infrastructure inspection, quadrotors operating in close proximity can suffer from catastrophic flight instabilities due to downwash wake interference. Understanding the characteristics of flow interactions and their effect on vehicle dynamics can inform drone stability and control. We have developed a dynamic testing platform that can rapidly accelerate a small quadrotor (Crazyflie) towards a nearby vehicle or a stationary boundary. By varying the amplitude and frequency of the motion as well as the thrust levels, we quantitatively map out the zones of downwash interaction by measuring the average and fluctuating forces and moments acting on each quadrotor - the one experiencing the downwash and the one generating the disturbance - as functions of vehicle separation and velocity. We extend these techniques to characterize dynamic ground effects. PIV measurements of the flow field around the vehicle provide the spatial structure of the downwash wakes, giving us insight into the flow features contributing to the destabilizing forces. |
Sunday, November 24, 2024 9:05AM - 9:18AM |
A30.00006: Experimental investigation of flow past arrays of propellers using multi-plane, time-resolved, 2D-2C PIV Prasoon Suchandra, Shabnam Raayai The expansions in the unmanned aerial vehicles (UAVs) technology have given rise to implementations of drone swarm strategies for a variety of applications, such as environmental monitoring, security, and surveillance to name a few. With fewer safety guidelines for drone swarms compared to formation flight with crewed aircraft, the opportunity has expanded in exploring tighter formation strategies for these vehicles. Thus, understanding the mechanics of flow around multi-propeller and vertical take-off and landing systems is an essential optimization tool to achieve certain outputs, like minimizing energy expenditure for a long-distance mission of multiple drones. In this talk, we focus on the dynamics of flow past multi-propeller systems, containing 5 propellers packed closely in a V-formation. We employ propellers comprised of two-blades with cross-sectional shapes following NACA 6430, with a maximum blade twist of 45° at the root. The performance of a single propeller is first characterized in a water tunnel with the axis of the propeller oriented vertically and the freestream in horizontal direction. Arrays of these propellers are then suspended in the water tunnel where we perform 2D-2C high-resolution, high-speed PIV at three horizontal planes: one below, one above and one at the height of the propellers. We use the PIV data to explore the interactions between the propellers, complex vortex dynamics, and the associated turbulence statistics. PIV data is used in conjunction with the Navier-Stokes equations to extract some flow field information in the third (vertical) direction. We also obtain the pressure fields and estimates of drag forces experienced by propellers. Lastly, we use the time-resolved PIV to characterize vortex shedding. |
Sunday, November 24, 2024 9:18AM - 9:31AM |
A30.00007: Abstract Withdrawn
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Sunday, November 24, 2024 9:31AM - 9:44AM |
A30.00008: Investigating the bubble bridge formed during an amphibious UAV’s water-to-air transition Logan Patrick Honts, Yuanhang Zhu, Daniel B Quinn Amphibious unmanned aerial vehicles (UAVs) provide great versatility in surveying, exploration, and even next generation delivery missions by eliminating the need for multiple vehicles to traverse the two media separately. Previous studies have shown that quadrotors can navigate in both air and water and cross the air-water boundary, but produce unsteady, turbulent waves at the water surface during the transition. Specifically, while exiting the water, a "bubble bridge" forms due to air entrainment and corresponds to a sharp change in the rotor's rotational frequency, thrust, drag, and power consumption. We present here a system that simulates one UAV rotor dynamically traversing the air-water interface in a highly repeatable manner. We explored how the bubble bridge's timing and depth are affected by exit velocity, rotor diameter, number of rotor blades, and input throttle. The rotor's re-entry to the water differs significantly from its exit, requiring two alternate approaches to the transition. Better understanding the dynamics of air-water rotor transitions will help to 1) analyze the amphibious capability of commercial UAV rotors, 2) shed light on the best exit and re-entry strategies, and 3) offer design ideas for rotors designed specifically for amphibious operation. |
Sunday, November 24, 2024 9:44AM - 9:57AM |
A30.00009: On the Performance Degradation of UAV Propellers Under Rainfall and Icing Weather Conditions Hui Hu, Anvesh Dhulipalla, Abdallah Samad, Nianhong Han Unmanned-Aerial-Vehicles (UAVs) pose a high risk of accidents due to rainfalls and inflight icing in cold weather. In this study, a series of fly test campaigns are conducted to fly an instrumented UAV under thunderstorms with heavy rainfalls and icing weather conditions to characterize the aerodynamic performance degradation of UAV propellers induced by rainfall and inflight icing, in comparison to those under normal weather conditions. By leveraging the unique Raining/Icing Research Tunnel available at Iowa State University, a comprehensive lab experiment campaign was also conducted to elucidate the underlying physics pertinent to the rain-induced and icing-induced performance degradation of the UAV propellers. The research findings are very helpful in bridging the knowledge gap between the idealized operation conditions commonly used in UAV propeller design and the realistic weather conditions experienced by UAVs to ensure more reliable and predictable UAV operations under all weather conditions. |
Sunday, November 24, 2024 9:57AM - 10:10AM |
A30.00010: Leading Edge Convection Heat Transfer for small UAS Alyssa S Avery, Zach Wattenbarger, James Masoner Convection heat transfer is one of the primary phenomena for determining aircraft ice classification and is needed to determine the weight and volume of ice accretion. The flight regime of unmanned aircraft, however, has substantial differences from manned aircraft. Therefore benefit little from their studies on convective heat transfer. A robust study on convective heat transfer for un- manned aerial systems(UAS) would provide a necessary foothold in addressing the icing problem in UAS. The work includes the design and development of a leading-edge convection heat transfer instrument. The instrument is a system of heated wires embedded in a custom built composite wing with a Clark Y airfoil. Initial experiments were conducted to verify sensor functionality and validate the process. The sensor is an assembly of 1/8 inch copper plate, a foil element heater, and thermocouple. Copper was selected as the heat transfer surface due to its high thermal conductivity. It was cut just large enough to accommodate the foil element heater and thermocouple. The heater is 0.3in by 3.5in, and an extra 0.5in was added to allow room for the thermocouple. A narrow sensor is preferred because it requires less modification to match the airfoil profile. For these initial tests, the copper strip is left with square edges, but it will be rounded off in future tests to match airfoil curvature. The results correlate well with previous cylinder experiments and prove a significant deviation from heat transfer functions used in manned aircraft icing models. |
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