As unmanned aircraft or drones are becoming more commonplace in our ever-developing environment where a number of drones can be flying in the same given airspace, a drone is likely to experience wake vortex turbulence from another drone nearby. Therefore, there is a need to understand how the airflow is generated from one vehicle and to what extent this airflow might affect another vehicle. Results of such studies will be useful in establishment of safe separation of drone operations. In the present work, flow simulations are carried out using overset mesh on ANSYS Fluent 19.2 for both single and multiple propellers, and the flights in hovering and forwarding motion are considered. The simulation results obtained are in general agreement to the experimental results that are available from other studies. This provides support to the idea that the method of flow simulation can be acceptably reliable in predicting the resulting airflow from a flying drone and the proposed method can be further applied to various cases of drone flights.
With the increase interest in utilizing multirotor UAS in very low altitude airspace, the safety aspect associated with higher UAS density gains importance. One of the safety consideration is the separation requirements for UAS sharing the airspace, which consists of two major components: wake-based and CNS performance-based. To evaluate the wake-based separation requirements, the wake-field produced by the leading UAS and the resulting encounter response by the follower UAS must be obtained. While previous studies on aircraft wake vortex evolution showed that the fuselage wake have small impact on the overall wake vortex strength and structures, the more distributed lift generation with less distance between the initial formation point of wake vortex to the fuselage could lead to a larger impact. This paper investigates the impact of including multirotor fuselage on simulating the strength and structure of the resulting wake vortex.
As the use of multi-rotors is becoming common, understanding how the wake vortices develop and decay from a multi-rotor is important to reduce the effect of wake vortices on other multi-rotors and ensure their safety during operation by applying a wake separation minima. This paper proposes a method of flow simulation in ANSYS Fluent to study the decay of wake vortices from quadrotor propellers as the vortices evolve over distance and time. First, the near-field wake flow from the quadrotor propellers was simulated using Reynolds-Averaged Navier-Stokes (RANS) turbulence model. The velocity profile in the quadrotor frame of reference was then extracted to be applied for a flow simulation using Large Eddy Simulation (LES). The circulation strength as a function of distance could then be calculated, but it was limited by the length of the computational grid due to the wake flow being seen from the quadrotor frame of reference. To change into the stationary frame of reference, the interpolation data from the first stage LES was exported and modified by subtracting the freestream velocity from the velocity component in the freestream direction. The modified interpolation data was then used for the second stage LES, and the circulation strength in terms of time could be calculated from the resulting wake flow. Three cases of flow simulation are presented in this paper: first using 380-mm-diameter propellers based on the SUI Endurance quadrotor propellers with forward flight speed of 6 m/s, second using the 380-mm-diameter propellers with 12 m/s speed, and third using 240-mm-diameter propellers (similar to the propellers of DJI Phantom quadrotor) with 12 m/s speed. Findings regarding the decay of wake vortex from multi-rotor vehicles will help in determining the safe separation distance required to mitigate the hazards due to wake vortex encounters and ensure the safety of multi-rotor vehicles.
The operation of Unmanned Aerial Systems (UAS) is expected to keep increasing in the future with its rapid advancement and ever-expanding applications. One of the safety measures that need to be put into place for safe UAS operations is separation requirements. This article presents the results from flow simulations of quadrotor propellers using the Overset method in ANSYS Fluent 19.2 and the Virtual Blade Model (VBM) method in OpenFOAM as part of ongoing work to establish a wake vortex–based safe separation distance for UAS operations. The near-field flow simulation using the Reynolds-averaged Navier–Stokes turbulence model was validated with mesh convergence and turbulence model studies and verified with simulation and experimental data in the literature to ensure that the propeller simulation methods could generate a realistic near-field flow of multi-rotor UAS in forward flight. The far-field flow simulation using the Large Eddy Simulation turbulence model was validated with a mesh convergence study by calculating and comparing the circulation strength of the vortices in the far-field flow of each mesh setup but not verified due to a lack of existing experimental data.
Wake hazard is one of the factors affecting the separation requirement for same-track fixedwing aircraft in civil aviation. It could also be a contributing factor for UAS traffic management (UTM) with multirotors, especially with the widespread use of ADS-B like remoteID for UAS that greatly reduces the separation requirements due to communication, navigation, and surveillance (CNS) performance of the aircraft and the airspace. While the wake vortex structure and decay mechanism for fixed-wing aircraft has been well studied over the years, the same dynamics might not be observed in multirotor aircraft due to difference in lift generation mechanism. Additionally, the difference in wake vortex generation from different multirotor configurations were not as well studied. This paper presents the simulation results from two quadrotors (DJI Phantom 3 size, ∼ 1.2 kg, and Inspire 1 size, ∼ 3.5 kg) and one hexarotors (DJI M600 sized, ∼ 15 kg) for a fixed follow distance behind those aircraft.
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