Numerical investigation and optimization for interior duct shape of ducted tail rotor

https://doi.org/10.1016/j.ast.2021.106778Get rights and content

Abstract

In order to improve the aerodynamic performance of the ducted tail rotor, parametric investigations and optimization of the interior duct surface are performed based on the Computational Fluid Dynamics (CFD) method. Firstly, to predict the aerodynamic characteristics of the ducted tail rotor with high precision and efficiency, the Reynolds-Averaged Navier-Stokes (RANS) solver coupled with body-fitted mesh around rotor blade is established based on the rotating reference frame. Then, novel parametric researches on the aerodynamic characteristics of the ducted tail rotor are conducted with different lip radii, transition section lengths, and duct outlet areas respectively, and the flow separation mechanism on the different interior duct surfaces are obtained. Finally, a new curve lip shape parameterized by the Class Shape Transformation method is employed to replace the conventional circular lip, besides, the transition length and the outlet area which have great influences on the flow separation phenomenon are combined to conduct the optimization of the interior duct surface. Attribute to the suppression of the flow separation, the required power and the FM of the optimized ducted tail rotor show a maximum decrement of 2.49% and a maximum increment of 2.44% with respect to the baseline SA365N1 ducted tail rotor respectively.

Introduction

The ducted tail rotor of the helicopter is a kind of application of the ducted fan, which has more advantages than the conventional tail rotor [1], [2]. The ducted tail rotor is safer and has lower noise level than the conventional tail rotor due to the blocking effect of the duct. The duct can produce additional thrust, which is caused by the negative pressure distribution on the duct lip and the larger static pressure around the duct outlet, making the ducted tail rotor more efficient than the conventional tail rotor. Besides, the flow inside the duct and the thrust produced by the duct are significantly influenced by the shape of the interior duct surface [3], [4], [5], [6]. Therefore, it is necessary to perform the parametric researches and optimization design of the interior duct surface for the ducted tail rotor.

In order to investigate the effects of the duct shape on the aerodynamic performance of ducted tail rotor and ducted fan, a series of experiments were conducted. Vuillet and Morelli [3] measured the aerodynamic characteristics of the Dauphin fenestron, and the aerodynamic performance of the ducted tail rotor was obtained, but the influences of the duct shape parameters on the aerodynamic characteristics were not involved. Keys [4] tested the effects of several lip radius variations on the performance of the 3/4 scale FANTAIL™ system for the Boeing Sikorsky Light Helicopter, and the range of lip radius with maximum hovering performance was obtained, but the transition length and outlet area variations were not taken into account. Black and Rohrbach [5] tested the effects of the inlet and outlet area on the aerodynamic performance of the ducted fan, but the effects on the flowfiled were not introduced in detail. Yilmaz [6] conducted a test on the aerodynamic characteristics of the ducted fan with different duct geometries obtained by combining the NACA airfoils, and the efficiency of the ducted fan were improved by using the front part of NACA7312 airfoil instead of NACA4312 airfoil as the duct lip, which indicated that the aerodynamic characteristics of the ducted fan were influenced by the duct lip and the aerodynamic performance of the ducted tail rotor could be improved by using a curve lip shape instead of the conventional circular lip.

In order to overcome the shortcomings of the experimental method, such as high cost, long time consumption, and limitations of the experiment environment, the CFD methods were gradually used to investigate the aerodynamic characteristics of the ducted tail rotor and the ducted fan. Marino [7] investigated the aerodynamic performance of a full-scale Dauphin fenestron by employing a steady-state RANS approach. Akturk and Camci [8], [9] simulated the flowfield of the ducted fan with various tip clearances and analyzed the effects of the tip leakage flow on the aerodynamic characteristics of ducted fans. Moghadam [10] simulated and analyzed the tip vortex system of the ducted fan, including the tip leakage vortex and induced vortex. However, the influences of the transition section on the tip vortex and induced vortex were not taken into account. Sheng [11] numerically investigated the effects of the ducted fan geometric parameters on the hover efficiency and stall margin of the ducted fan, but the duct geometric parameter only involved the duct inlet lip radius. Although many investigations of the aerodynamic characteristics for ducted tail rotor and ducted fan have been conducted, there are few detailed parametric researches on the interior duct surface of the ducted tail rotor based on CFD method, and the investigated shape parameters only involve the lip radius and the diffuser angle. However, the flowfield between the blade tip and the transition section is complicated due to the interaction of the blade tip vortex and transition section wall. Therefore, it is important to conduct more detailed parametric researches on the interior duct surface of the ducted tail rotor based on the CFD method.

In order to further improve the aerodynamic performance of the ducted fan, Biava and Barakos [12] employed a gradient based optimization coupled with the Unsteady Reynolds-Averaged Navier-Stokes (URANS) method to optimize the duct shape of ducted fan, and the efficiency of the optimized ducted fan had been increased by 2%. The duct outlet area and duct length were controlled by two design variables and the diffuser shape was described by CST parametrization, but the duct lip was not involved. Qing [13] employed a surrogate-based optimization method coupled with momentum source method to optimize the ducted fan with different rotor rotation speed, but the detailed flowfield of the rotor was not taken into account. Zhang and Barakos [14] carried out a optimization of the ducted fan including the duct shape and blade twist distribution, but the transition section was not allowed to deform. Therefore, it is necessary to establish a parametrization method to describe the interior duct surface of the ducted tail rotor in detail, and carry out the optimization design of the ducted tail rotor with high precision CFD method, including the shape of lip, transition section, and duct outlet.

In this paper, the effects of different interior duct surface shapes on the aerodynamic characteristics of the ducted tail rotor in hover are investigated, and the optimization design of the interior duct surface is conducted. In order to predict the performance of the ducted tail rotor with high efficiency and accuracy, the CFD methods coupled with rotating reference frame and periodic boundary conditions are developed to investigate the aerodynamic characteristics of the SA365N1 Dauphin fenestron, and the parametric analyses are conducted, such as different lip radii, transition section lengths, and duct outlet areas. Then, the parametrization method of the interior duct surface is developed and the optimization design of the interior duct surface is conducted to improve the hovering performance of the ducted tail rotor.

Section snippets

Grid generation

In order to predict the aerodynamic characteristics of the ducted tail rotor, multi-block structured body-fitted grids around the ducted tail rotor are generated in this paper. Fig. 1 shows the multi-block structured body-fitted grids of the ducted tail rotor. For the hovering state, the duct is simplified as an axisymmetric model, and the flowfield is also axisymmetric. In order to improve the optimization efficiency of the duct shape, the periodic boundary conditions are adopted to reduce the

Numerical analyses of shape parameter influences on the aerodynamic characteristics of ducted tail rotor

The influences of the shape parameters on the aerodynamic characteristics of ducted tail rotor are investigated based on the SA365N1 model. The duct is an important component of the ducted tail rotor, which plays an important role in improving the aerodynamic performance of the ducted tail rotor. In the hovering state, the additional thrust produced by the duct is almost equal to the thrust produced by the tail rotor. The shape parameters of the ducted tail rotor mainly include the lip radius,

Duct shape parameterization

Duct shape parameterization is an important basis for the optimization design of the ducted tail rotor. The thrust produced by the duct is mainly concentrated on the lip, thus, the lip shape needs to be well designed. At present, the lip shape of the ducted tail rotor is usually described by a simple circular radius. In order to obtain a more complicated curve lip shape, the mature class shape transformation (CST) method [19], [20], [21] is employed to parameterize the lip shape of the ducted

Optimized results and analyses

The SA365N1 ducted tail rotor is selected as the baseline. There are six design variables for the duct, four of which are defined by the CST fitting of the lip, and two are the Y coordinate of the control point for the transition section and the X coordinate of the control point for the diffuser section respectively. In order to obtain an optimized duct to improve the efficiency of the ducted tail rotor, the power coefficient of the ducted tail rotor is minimized and the thrust coefficient of

Conclusions

The influences of the shape parameters and the optimization of the interior duct surface for the ducted tail rotor are investigated by CFD method in this work, and some valuable conclusions can be summarized as follows.

(1) The CFD methods with rotating reference frame and periodic boundary conditions are reliable to simulate the aerodynamic characteristics of the ducted tail rotor and perform the optimization of the duct shape.

(2) The shape of the duct lip affects the pressure and velocity

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research is supported by the National Natural Science Foundation of China (12072156, 12032012) and the Foundation of Rotor Aerodynamic Key Laboratory (RAL20190102). Besides, this research is A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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