Embedded Files
한국항공대학교 항공 모빌리티 및 공력음향학 연구실
Summary
Background
Multirotor configurations have been developed for various applications such as drones and urban air mobility (UAM) systems. Because multirotor configurations are composed of multiple rotors, various aerodynamic effects occur among the rotors, in contrast to conventional rotorcrafts. In particular, complex aeroacoustic characteristics could be generated as a result of the wake interaction. As noise regulations for aviation systems have been strictly regulated, it is necessary to design noise-efficient multirotor configurations. However, there are not enough studies on noise analysis based on the direct aerodynamic prediction considering the wake interaction effect of multirotor.
Besides, to realize the low-noise design of multirotor configurations, the wake interaction should be quantified in the preliminary design stage. For the design and control of multirotor configurations, it is necessary to quantitatively analyze the performance of each rotor. However, only a few researchers have quantitatively analyzed the wake interaction and correlated this aspect with the performance of each rotor.
Objective
The multirotor aerodynamic and aeroacoustic characteristics have differences from those of a single rotor because of the wake interaction. In this study, a numerical analysis based on the free wake vortex lattice method is used for identifying the wake interaction effect.
Although the wake interaction effect could be quantitatively verified by modeling the wake, conventional wake analyses were consequential, owing to which, the wake interaction effect between each rotor in various flight conditions could not be compared. In this study, the induced circulation is examined to quantify the wake interaction effects in multirotor configurations.
Contribution
The aerodynamic and aeroacoustic characteristics of the multirotor configuration were analyzed.
The wake interaction effect was quantified in various flight conditions, and the characteristics of each rotor of a multirotor were compared with those of the single rotor.
The induced circulation was set as the wake interaction factor and quantitatively compared under various flight conditions.
[Wake structure of cross-type (advance ratio = 0.13)]
Method
Aerodynamic prediction
Free-wake vortex lattice method (VLM)
Incompressible, irrotational flow assumption
The total flow field is induced by the vortex lattice of the blade and the wake
Constant vorticity contour wake model
It is possible to express the distribution of vortex strength and strength change as a single wake element
Curved vortex elements
Wake modeling with curved vortex elements
2D CFD aerodynamic coefficient table look-up & viscous vortex model
Considering the viscous effect and airfoil camber effect
Aeroacoustic prediction
Ffowcs-Williams Hawkings (FW-H) Acoustic analogy
The source surface of the impermeable FW-H was set to the blade surface
The pressure of the blade surface which was given by free-wake VLM solver was used
Discrete frequency noise components: Thickness noise & Loading noise (Steady/Unsteady)
In multirotor configurations, unsteady loading noise occurs in hovering flight because of wake interaction
Wake interaction and induced circulation
Among the aerodynamic interactions of multi-rotor configurations, the rotor–rotor, rotor–wake, wake–rotor, and wake–wake interactions are collectively called wake interaction. The rotor–rotor/wake interactions are directly related to the aerodynamic loads on a rotor. Induced circulation (IC) is a quantification factor for analyzing complex wake interactions.
This factor is derived from the Kutta–Joukowski theory, according to which the aerodynamic loads on the rotor blades are related to bound circulation. Rotor–rotor/wake interactions are expressed as rotor-induced circulation and wake-induced circulation, respectively. Motion-induced circulation expresses the aerodynamic loads due to rotating and forward flight motions. The total circulation is the sum of each IC component. Based on the Biot–Savart law, each IC component is derived from the induced velocity of the rotor/wake panel in the free-wake VLM solver.
[Cross-type formation quadrotor]
Results
Loading noise directivity pattern of multirotor configurations in hovering flight
Three configurations with different spacing and the single rotor in the multirotor configurations were considered
The loading noise in the rotor in-plane direction and up to 30 degrees is almost the same in all configurations
In the axis direction, a large loading noise occurs in a configuration with a small spacing
The reason for the difference in (b) and (d) is the wake interaction effect is totally ignored in the single rotor
[Loading noise directivity pattern]
(a: d/D = 0.1; b: d/D = 0.36; c: d/D = 0.8; d: d/D = 0.36 with single rotor)
Distributions of induced circulation
In figures, parts (a) and (b) show the distributions in the advancing/retreating sides of the front/rear rotors, respectively, and (c) and (d) show the results of the blades located in the forward and opposite directions, respectively.
Because the difference in the wake-rotor interaction was not considerable, in accordance with the high interaction region, the overall distribution of the induced circulation of the front/rear rotor could be identified through these four distributions.
The absolute value and sign of the wake-induced circulation owing to the other wakes depended on the flight conditions.
In the rear rotor, the wake-rotor interaction occurred actively in each type of flight. The effect of the self-wake was less than that for a single rotor, indicating that the rotor/wake-wake interaction tended to increase the aerodynamic load.
[Circulation distribution of cross-type (advance ratio = 0.13, incidence angle = 0 deg.)]
Conclusion
Aerodynamic & Aeroacoustic analysis
Unsteady loading features were apparent in the multirotor due to the wake interaction.
In the hovering, as the spacing between the rotor was reduced, unsteady loading noise apparently occurs.
In the forward flight, because the rear rotor is operated under the influence of the wake of the front rotor, aerodynamic and aeroacoustic characteristics were much different from those of the single rotor.
Quantification of wake interaction effects in multirotor configurations
The wake dynamics of each rotor are affected by the various wake interaction effects. The upwash/downwash effects among multiple rotors lead to wake variations in the out-of-plane.
The wake interaction effect can be quantified using wake-induced circulation because rotor-induced circulation does not notably influence the performance.
In general, the wake of the front and self-rotors reduces the total circulation of the rear rotor, and although the wake of the rear and side rotors has a minor effect, the total circulation of the front rotor increases.
Future work will be aimed at defining the generalized wake interaction factor of multirotor configurations by using IC.
Page updated
Google Sites
Report abuse