Numerical investigation on unsteady characteristics of ducted fans in ground effect

Abstract Ducted fans are widely used in various applications of Unmanned Aerial Vehicles (UAVs) due to the high efficiency, low noise and high safety. The unsteady characteristics of ducted fans flying near the ground are significant, which may bring stability problems. In this paper, the sliding mesh technology is applied and the Unsteady Reynolds Averaged Navier-Stokes (URANS) method is adopted to evaluate the influence of ground on the aerodynamic performance of ducted fans. The time-averaged results show that the ground leads to the decrease of duct thrust, the increase of rotor thrust and the decrease of total thrust. The transient results show that there exist small-scale stall cells with circumferential movements in ground effect. The stall cells start to appear at the blade root when the height is 0.8 rotor radius distance, and arise at both the blade root and tip when the height drops to 0.2. It is found that the unsteady cells rotate between blade passages with an approximate relative speed of 30%-80% of the fan speed, and lead to thrust fluctuations up to 37% of the total thrust. The results are essential to the flight control design of the ducted fan f lying vehicle, to ensure its stability in ground effect.

摘要管道风机因其高效、低噪音和高安全性而广泛应用于无人机的各种应用中。管道风机在近地飞行时的非定常特性显著，这可能会带来稳定性问题。本文应用滑动网格技术，采用非定常雷诺平均Navier-Stokes（URANS）方法来评估地面对涵道风扇气动性能的影响。时间平均结果表明，地面导致导管推力减小，转子推力增大，总推力减小。瞬态结果表明，地面效应中存在具有周向运动的小尺度失速单元。当高度为0.8转子半径距离时，失速单元开始出现在叶片根部，当高度降至0.2时，失速细胞在叶片根部和叶尖都出现。研究发现，非定常单元在叶片通道之间以风扇速度的30%-80%的相对速度旋转，并导致推力波动高达总推力的37%。这些结果对飞行器涵道风扇的飞行控制设计至关重要，以确保其在地面效应下的稳定性。

5.Conclusions and futurework

In this paper,the sliding mesh technique is used to simulate the rotation of the propeller, and SST model is used for numerical calculation.The time-averaged and transient performances of the ducted fan in ground effect at different heights are compared and analyzed indetails. The conclusions are drawn as follows:

(1)Time-averaged characteristics

When the ducted fan hovers in ground effect, the ground causes the flow in the duct diffuser section to shift to the inner wall of the duct.The down wash wake flow is then turned and stretched outward. A high-speedwall jet along the ground plane and a dead water zone at the center of the outlet are thereby formed.As the height above the ground decreases, on the one hand, the intake distortion induced by the rolled up ground vortex increases the angle of attack of the blade, and pushes the low-pressure zone on the suction side and the high-pressure zone on the pressure side to extend along the span wise direction from the blade tip to the blade root,resulting in an increase in the rotor thrust.On the other hand, the narrowing of the low-pressure zone at the duct lip section and the deviation of the minimum pressure point cause a drop in the duct thrust.The total thrust shows a down ward trend

(2)Transient characteristics

There are significant thrust fluctuations of the ducted fans in ground effect,and the amplitude of the fluctuation increases with the decrease of the ground height.When the height drops below h/R=0.3, the duct thrust may even be negative.The results of FFT of the static pressure and thrust in time series at the monitoring points show that the thrust fluctuation is directly related to the stall cells travelling between bladepas sages in the flow field.The stall cells initially appear at the blade root when h/R drops to 0.8,and the irrelative rotating speed decreases with the decrease of the height.When the height above the ground h/R decreases to 0.2, the stall cells appearat the blade tip as well.The spatial FFT results show that the stall cell of the ducted fan in ground effect is a small-scale disturbance in the circumferential direction,which occurs near the leading edge of the blade.

The present study comprehensively analyzes the aerodynamic characteristics of the ducted fan in ground effect, and clarifies its flow mechanisms based on CFD methods.However, the unsteady effect of the ducted fan under other conditions such as forward flight in ground effect and ceiling effect and hovering in crosswinds have not been explored yet, and the flow mechanisms under dynamic conditions may be much more complicated, which require further investigations. Additional efforts should be made to develop the corresponding control methods in order to reduce the thrust fluctuation.

5.结论和未来工作

本文采用滑动网格技术模拟螺旋桨的旋转，并采用SST模型进行数值计算。对不同高度下管道风机在地面效应下的时间平均和瞬态性能进行了详细比较和分析。结论如下：

（1） 时间平均特征

当管道风扇在地面效果中悬停时，地面会导致管道扩散器部分的气流转移到管道内壁。然后，下洗尾流被转向并向外拉伸。由此形成沿地面的高速壁射流和出口中心的死水区。随着离地高度的降低，一方面，卷起的地面涡流引起的进气畸变增加了叶片的攻角，并推动吸入侧的低压区和压力侧的高压区沿翼展方向从叶片尖端延伸到叶片根部，导致转子推力增加。另一方面，管道唇缘部分低压区的变窄和最小压力点的偏差导致管道推力下降。总推力呈下降趋势

（2） 瞬态特性

导管风扇在地面效应下存在显著的推力波动，波动幅度随着地面高度的降低而增加。当高度降至h/R=0.3以下时，管道推力甚至可能为负。监测点静压和推力时间序列的FFT结果表明，推力波动与流场中叶片之间的失速单元直接相关。当h/R降至0.8时，失速单元最初出现在叶片根部，无关转速随着高度的降低而降低。当离地高度h/R降至0.2时，叶片尖端也会出现失速细胞。空间FFT结果表明，地面效应下涵道风扇的失速单元是周向的小尺度扰动，发生在叶片前缘附近。

本研究全面分析了涵道风扇在地面效应下的气动特性，并基于CFD方法阐明了其流动机理。然而，导管风扇在其他条件下的非定常效应，如地面效应中的前飞、天花板效应和侧风中的悬停，尚未得到探索，动态条件下的流动机制可能要复杂得多，需要进一步研究。应进一步努力开发相应的控制方法，以减少推力波动。