Effects of Wake Separation on Aerodynamic Interference Between Rotors in Urban Low-Altitude UAV Formation Flight
Abstract
:1. Introduction
2. Experimental Approach
2.1. Wind Tunnel and Test Bench
2.2. Propeller Geometry
2.3. Test Conditions and Parameter Definition
3. Numerical Approach
3.1. CFD Methodology
3.2. Meshing
3.3. CFD Validation
4. Result and Analysis
4.1. The Effect of Aerodynamic Interference During Hover Flight
4.2. The Effect of Aerodynamic Interference During Vertical Flight
4.3. The Effect of Aerodynamic Interference During Horizontal Flight
5. Conclusions
- (1)
- Hover flight: When the lateral separation is X = 0D, and the axial separation is between 1.0D and 3.0D, the aerodynamic performance of the bottom rotor is significantly reduced, with the thrust coefficient decreasing by nearly 45%. As the lateral and axial separations increase, the bottom rotor gradually moves out of the severe interference region, leading to a gradual rise in its thrust coefficient. While the axial separation remains constant, the thrust coefficient continues to increase with lateral separation, although the rate of increase diminishes as the axial separation grows. The simulation results indicate that when the lateral separation between the two rotors is X = 0D, the velocity field of the rotors exhibits good symmetry. However, when a lateral separation exists, the flow field around the bottom rotor becomes distinctly asymmetric. The inner wake of the rotor features higher velocity but a shorter development distance, while the outer wake has a lower velocity but a longer development distance. The wake flow field of the bottom rotor is highly sensitive to changes in the inflow over the upper surface of the rotor disk. In hovering conditions, it is recommended to maintain a lateral separation of X ≥ 1D or an axial separation of 11D.
- (2)
- Vertical flight: When the rotor separation is small, the thrust coefficient decreases the most at the lateral separation X = 0D. For example, the thrust coefficient at the axial separation Z = 1D is reduced by nearly 50%. When the bottom rotor is affected by the wake, the decrease in the thrust coefficient becomes significant as the flight speed increases. As the axial separation increases, the rotor’s thrust coefficient at X = 1D is the largest. When the rotor is rotating at 5000 RPM, and the flight speed is 2 m/s, the rotor’s thrust coefficient can reach 87% of the isolated rotor’s thrust coefficient, and at 5 m/s, it can remain above 83%. In addition, the flow field pattern of the rotor’s cross-section speed is affected by the flight speed and rotor rotational speed. For a rotor with a speed of 5000 RPM and a flight speed of 2 m/s, only the low-speed airflow on the outer periphery of the wake is affected and gradually disappears. When the flight speed increases to 5 m/s, the rotor wake shows a tendency to shrink. It is recommended that when the axial separation is small, the lateral separation should be greater than or equal to 1D; when the lateral separation is X = 0D, the axial separation should be greater than or equal to 11D. In order to avoid a significant decrease in the thrust coefficient due to high-speed flight, the flight speed should be controlled appropriately.
- (3)
- Horizontal flight: The wake from the top rotor significantly affects the thrust coefficient of the bottom rotor, even at larger lateral separations. The extent of this influence is closely related to the rotor’s RPM and horizontal flight speed. The experimental results indicate that under wake interference, the thrust coefficient can decrease by up to 40%. For horizontal flight at a speed of 2 m/s, it is recommended that the lateral separation between the rotors be X ≥ 4.0D. At flight speeds of 5 m/s or higher and axial separation of Z = 1.5D, the rotor thrust coefficient remains above 90% of the isolated rotor’s thrust coefficient. When Z = 2.5D, the thrust coefficient stays above 95% of the isolated rotor’s thrust coefficient across the entire lateral separation. The velocity field analysis shows that during horizontal flight, the rotor wake inclines toward the side and rear. As the flight speed increases, the angle of inclination of the wake also increases, concentrating the flow region and expanding the range of lateral interference. Additionally, at higher flight speeds with lower rotor RPM, the wake intensity decreases, and the wake development distance shortens. Based on both experimental and simulation results, it is recommended to increase the lateral separation to reduce wake interference during high-speed horizontal flight while the axial separation can be appropriately reduced.
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
References
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Flight State | Variable | Values |
---|---|---|
Hover | RPM | 2000–8000 |
X/D | 0, 0.25, 0.5, 0.75, 1 | |
Z/D | 1, 3, 5, 7.5, 11 | |
Vertical | RPM | 2000–8000 |
X/D | 0, 0.25, 0.5, 0.75, 1 | |
Z/D | 1, 3, 5, 7.5, 11 | |
Velocity | 2 m/s, 5 m/s | |
Horizontal | RPM | 2000–8000 |
X/D | 1, 2, 3, 4, 6, 8, 10 | |
Z/D | 0.5, 1.5, 2.5 | |
Velocity | 2 m/s, 5 m/s, 10 m/s, 15 m/s |
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Liu, C.; Li, B.; Wei, Z.; Zhang, Z.; Shan, Z.; Wang, Y. Effects of Wake Separation on Aerodynamic Interference Between Rotors in Urban Low-Altitude UAV Formation Flight. Aerospace 2024, 11, 865. https://doi.org/10.3390/aerospace11110865
Liu C, Li B, Wei Z, Zhang Z, Shan Z, Wang Y. Effects of Wake Separation on Aerodynamic Interference Between Rotors in Urban Low-Altitude UAV Formation Flight. Aerospace. 2024; 11(11):865. https://doi.org/10.3390/aerospace11110865
Chicago/Turabian StyleLiu, Cong, Baiqing Li, Zhiqiang Wei, Zongwei Zhang, Zezhong Shan, and Yu Wang. 2024. "Effects of Wake Separation on Aerodynamic Interference Between Rotors in Urban Low-Altitude UAV Formation Flight" Aerospace 11, no. 11: 865. https://doi.org/10.3390/aerospace11110865
APA StyleLiu, C., Li, B., Wei, Z., Zhang, Z., Shan, Z., & Wang, Y. (2024). Effects of Wake Separation on Aerodynamic Interference Between Rotors in Urban Low-Altitude UAV Formation Flight. Aerospace, 11(11), 865. https://doi.org/10.3390/aerospace11110865