Downwash airflow distribution pattern of hexa-copter unmanned aerial vehicles

In recent times,the use of vertical take-off and landing(VTOL)multi-rotor Unmanned Aerial Vehicle(UAVs)for spraying chemical pesticides against weeds and pests has recently become popular.The current aerial spray application research is primarily focused on examining the influence of UAV spraying parameters such as flight height,travel speed,rotor configuration,droplet size,payload and wind velocity.The downwash airflow velocity generated by the UAV rotor propeller has a significant impact on the droplet deposition process.A test rig was developed to measure the downwash airflow pattern generated by the rotor propeller of a UAV.In this investigation,a six-rotor electric autonomous UAV sprayer was used to investigate the parameters and distribution laws of downwash airflow velocity.The downwash airflow velocity was measured using portable anemometers mounted on the test rig at radial positions viz.,0 m,0.5 m,1 m,1.5 m and 2 m,perpendicular to(X)and parallel to the UAV’s flight direction(Y).The experiment was conducted at three levels of hover height,viz.,1 m,2 m and 3 m(Z)and three levels of payload,viz.,0 kg,5 kg,and 10 kg.The special downwash airflow distribution pattern was analysed using the Python programming language(Version 3.7).Results show that the downwash airflow velocity generated by the radial position of the UAV rotor is evenly distributed on the rotating loop and the standard deviation of the downwash airflow velocity is less than 0.5 m/s.The maximum downwash airflow velocity of 13.8 m/s was observed below the rotor at 10 kg payload capacity,1 m hover height(Z),and 0.5 m in the X-direction.The minimum downwash wind field of 0.3 m/s was observed at 0 kg pay load capacity,1 m height,and 2 m in the X-direction.The downwash airflow velocity along each position in the radial direction of the rotor increases initially and then decreases.This downwash airflow distribution results helps in mounting of spray nozzle configuration to drone sprayer which helps to understanding spray liquid distribution and other spray operational parameters.The influence of downwash airflow distribution combined with the spray operational parameters of the UAV sprayer viz.,flight height,travel speed,rotor configuration,payload and wind velocity on spray volume distribution was studied.A field experiment was conducted to study the effect of UAV sprayer downwash airflow distribution on spray droplet deposition characteristics in a rice crop compared with manual knapsack sprayer.

Downwash airflow field distribution characteristics and their effect on the spray field distribution of the DJI T30 six-rotor plant protection UAV

Spray characteristics are the fundamental factors that affect droplet transportation downward,deposition,and drift.The downwash airflow field of the Unmanned Aviation Vehicle(UAV)primarily influences droplet deposition and drift by changing the spray characteristics.This study focused mainly on the effect of the downwash airflow field of the UAV and nozzle position on the droplet spatial distribution and velocity distribution,which are two factors of spray characteristics.To study the abovementioned characteristics,computational fluid dynamics based on the lattice Boltzmann method(LBM)was used to simulate the downwash airflow field of the DJI T30 six-rotor plant protection UAV at different rotor rotational speeds(1000-1800 r/min).A particle image velocimetry system(PIV)was utilized to record the spray field with the downwash airflow field at different rotational speeds of rotors(0-1800 r/min)or different nozzle positions(0,0.20 m,0.35 m,and 0.50 m from the motor).The simulation and experimental results showed that the rotor downwash airflow field exhibited the‘dispersion-shrinkage-redispersion’development rule.In the initial dispersion stage of rotor airflow,there were obvious high-vorticity and low-vorticity regions in the rotor downwash airflow field.Moreover,the low-vorticity region was primarily concentrated below the motor,and the high-vorticity region was mainly focused in the middle area of the rotors.Additionally,the Y-direction airflow velocity fluctuated at 0.4-1.2 m under the rotor.When the rotor airflow developed to 3.2 m below the rotor,the Y-direction airflow velocity showed a slight decrease.Above 3.2 m from the rotor,the Y-direction airflow velocity started to drastically decrease.Therefore,it is recommended that the DJI T30 plant protection UAV should not exceed 3.2 m in flight height during field spraying operations.The rotor downwash airflow field caused the nozzle atomization angle,droplet concentration,and spray field width to decrease while increasing the vortex scale in the spray field when the rotor system was activated.Moreover,the increase in rotor rotational speed promoted the abovementioned trend.When the nozzle was installed in various radial locations below the rotor,the droplet spatial distribution and velocity distribution were completely different.When the nozzle was installed directly below the motor,the droplet spatial distribution and velocity distribution were relatively symmetrical.When the nozzle was installed at 0.20 m and 0.35 m from the motor,the droplets clearly moved toward the right under the induction of stronger rotor vortices.This resulted in a higher droplet concentration in the right-half spray field.However,the droplet moved toward the left when the nozzle was installed in the rotor tip.For four nozzle positions,when the nozzle was installed at 0 or 0.20 m from the motor,the droplet average velocity was much higher.However,the droplet average velocity was slower when the nozzle was installed in the other two positions.Therefore,it is recommended that the nozzle is installed at 0 or 0.20 m from the motor.The research results could increase the understanding of the downwash airflow field distribution characteristics of the UAV and its influence on the droplet spatial distribution and velocity distribution characteristics.Meanwhile,the research results could provide some theoretical guidance for the choice of nozzle position below the rotor.

Multi-UAV Cooperative Hunting in Cluttered Environments Considering Downwash E®ects

This paper presents a novel solution to the three-dimensional (3D) cooperative hunting ofmultiple drones that deals with surrounding a target simultaneously while navigating aroundobstacles in the cluttered dynamic 3D environment. Meanwhile, drones avoid the air°owdownwash force created by the spinning propellers on unmanned aerial vehicles (UAVs) andtheir e®ect on the other UAVs. This solution consists of a 3D Simultaneous Encirclementstrategy, the cooperative hunting objective with a novel revised particle swarm optimization(PSO*) path planning algorithm, a °ocking theory-inspired obstacle avoidance algorithm, and acascade PI controller. Simulation results with varying conditions were carried out to validatethe e®ectiveness of the proposed solution by successfully taking care of the downwash e®ects,and having multiple hunter UAVs hunt and encircle a moving or stationary target in a dynamicor static obstacle-rich cluttered environment.

Downwash modelling for three-lifting-surface aircraft configuration design

This paper introduces a semi-empirical model to predict the downwash gradient at the horizontal tail of a three-lifting-surface aircraft.The superposition principle applied to well established formulations valid for two lifting surfaces is not a reasonable approach to calculate the downwash of a canard-wing-tail layout,and this paper demonstrates that such a basic technique leads to incorrect results.Therefore,an ad hoc prediction model is proposed that considers the combined nonlinear effects of canard and main wing inductions on tail downwash,being based on a full factorial design sweep of CFD simulations obtained by varying the main geometrical parameters of the three lifting surfaces.A suitable analytical formula for the downwash gradient is established through a process of data analysis and factor extraction.The presented model extends the validity of the available models for traditional two-lifting-surface designs by means of a correction factor.The engineering estimation method introduced here exhibits an acceptable accuracy,as well as relatively small prediction errors,and it is suitable for conceptual and preliminary studies of threesurface layouts.The value of this methodology is confirmed by the validation with the results of numerical and experimental investigations on a case study aircraft.

一体化红外抑制器排气出流对其气动和红外辐射特性的影响

经地面模型实验验证,数值仿真展示了一体化红外抑制器排气出流、模拟旋翼下洗气流和模拟前飞来流相互作用下的流动和换热特征,通过改变排气出流角度和模拟前飞来流速度,计算分析一体化红外抑制器引射、混合特性以及红外辐射强度空间分布的变化规律。仿真结果表明:模拟的前飞来流与排气出流相互作用后会造成局部排气不畅,前飞来流甚至倒灌入红外抑制器内,从而引起抑制器壁面局部高温,增加了抑制器整体红外辐射强度;改变排气角度可将之改善,且存在气动和红外辐射特性兼顾的最佳角度。前飞来流速度从15 m/s提高到55 m/s,抑制器内部高温壁面温度降低,抑制器红外辐射强度峰值在3~5μm波段降低约50%,在8~14μm波段降低约20%。红外抑制器的下洗气流进口加装弯曲导流片可以对内部混合管高温壁面进行遮挡,有效降低顶部视角的红外辐射强度。最佳排气角度下,利用排气下游隔板的遮挡,正对抑制器排气出口方向(底部视角)上的3~5μm和8~14μm波段红外辐射峰值分别降低约50%和33%。

Numerical simulation and experimental verification on downwash air flow of six-rotor agricultural unmanned aerial vehicle in hover

Recently,multi-rotor unmanned aerial vehicle(UAV)becomes more and more significantly irreplaceable in the field of plant protection against diseases,pests and weeds of crops.The easy takeoff and landing performance,hover function and high spraying efficiency of UAV are urgently required to spray pesticide for crop timely and effectively,especially in dispersed plots and hilly mountains.In such situations,the current researches about UAV spray application mainly focus on studying the influence of the UAV spraying parameters on the droplet deposition,such as operation height,operation velocity and wind velocity.The deposition and distribution of pesticide droplets on crops which depends on installation position of nozzle and airflow distribution characteristics of UAV are directly related to the control effect of pesticide and crop growth in different growth periods.As a preliminary step,this study focuses on the dynamic development law and distribution characteristics of the downwash air flow for the SLK-5 six-rotor agricultural UAV.Based on compressible Reynolds-averaged Navier-Stokes(RANS)equations with an RNG k-εturbulence model and dynamic mesh technology,the efficient three-dimensional computational fluid dynamics(CFD)method was established to analyze the flow field distribution characteristics of UAV in hover.Then the unsteady interaction flow field of the wing was investigated in detail.The downwash wind speed of the marked points for the SLK-5 UAV in hover was also tested by weather tracker.It was found that the maximum velocity value of the downwash flow was close to 10 m/s;the z-direction velocity was the main body of the wind velocity in the downwash airflow,and the comparison of the wind velocity experiment test and simulation showed that the relative error was less than 12%between the experimental and simulated values of the z-direction velocity at the marked points.Then the flow characteristics of the longitudinal and cross section were analyzed in detail,the results obtained can be used as a reference for drift and sedimentation studies for multi-rotor unmanned aerial vehicle.

Downwash distribution of single-rotor unmanned agricultural helicopter on hovering state

The effective coverage and velocity of downwash are directly related to the assemblage of spraying system and spraying effect.The downwash of the unmanned agricultural helicopter(UAH)N-3 was discussed in the paper.The computational fluid dynamics(CFD)methods were used to simulate and analyze the distribution of the downwash,and a wind field measurement device had been designed to test the downwash of UAH N-3.In the tests,the UAH N-3 was raised up to 5.0 m,6.0 m and 7.0 m from the ground,“annular-radial-distribution-point”method was introduced,8 directions separated by an angle of 45°(the radial direction)with the intersection point of the main rotor shaft and the ground plane as the center,0.5 m as the step length for the longitudinal(to 2.5 m)and radial(to 4.0 m)direction to set the sample points,considering the range of the rotor rotating circular area mainly.The 5 m height results of N-3 were fully discussed to describe the downwash distribution with the longitudinal altitude increased and the radial distance increased.The standard deviations of five test altitudes for eight directions were comparatively analyzed,the results showed that the total standard deviation was not greater than 0.6 m/s.The overall relative maximum margin of error calculated from the simulation and measurement data was between 0.6 and 0.7,which verified the credibility of the simulation data.High-order polynomials were used to fitting the simulation and measurement data,the fitting results showed that the polynomial coefficient of determination R^(2) met or exceeded 0.75 when the altitudes were more than 1 m,indicating the fit equation having the reference values.When the altitudes equal or less than 0.5 m,the polynomial coefficient of determination R^(2) was smaller,ranging during 0.3 to 0.7.The study would provide some foundations for the optimization of the assemblage of spraying system on the single-rotor UAH,which would promote China aviation plant protection.

耦合风场四旋翼植保无人机下洗流与雾滴分布研究

旋翼植保无人机喷药过程中雾滴与旋翼旋转产生的下洗流场和外界风场高度耦合作用,直接影响施药品质。因此基于拉格朗日多相流模型对耦合风场的四旋翼植保无人机下洗流、有无下洗雾滴分布和含风场雾滴分布规律进行数值分析计算,通过旋翼风速测量试验验证数值计算方法的可靠性。结果表明,旋翼下洗气流在轴间中垂线上互相吸引,会发生气流聚合现象,最大气流速度为14 m/s;随着风场作用,迎风侧气流受到挤压逐渐向背风侧偏转,风速过大超过3 m/s下洗流已逐渐失去对风抗干扰能力;有下洗气流作用时沉积时间相比无下洗流作用时加快10倍,提高施药效率;风速的增加导致雾滴的沉积范围变大,当风速达到2~3 m/s时漂移增加,出现复喷现象更加严重。

The computational fluid dynamic modeling of downwash flow field for a six-rotor UAV

The downwash flow field of the multi-rotor unmanned aerial vehicle(UAV), formed by propellers during operation, has a significant influence on the deposition, drift and distribution of droplets as well as the spray width of the UAV for plant protection. To study the general characteristics of the distribution of the downwash airflow and simulate the static wind field of multi-rotor UAVs in hovering state, a 3 D full-size physical model of JF01-10 six-rotor plant protection UAV was constructed using Solid Works. The entire flow field surrounding the UAV and the rotation flow fields around the six rotors were established in UG software. The physical model and flow fields were meshed using unstructured tetrahedral elements in ANSYS software.Finally, the downwash flow field of UAV was simulated.With an increased hovering height, the ground effect was reduced and the minimum current velocity increased initially and then decreased. In addition, the spatial proportion of the turbulence occupied decreased. Furthermore, the appropriate operational hovering height for the JF01-10 is considered to be 3 m. These results can be applied to six-rotor plant protection UAVs employed in pesticide spraying and spray width detection.

Distribution regularity of downwash airflow under rotors of agricultural UAV for plant protection

In the plant protection spray operation of UAVs,the process of droplet from formation to sedimentation target is affected by airflow,easy to form uneven deposition.Accurately description of rotor downwash flow field,clarification of velocity vector distribution at different heights of the UAV rotor flow field,simulation of the flow field with high precision,which are the prerequisites for accurately analyzing the droplet deposition distribution in rotor downwash flow field.Based on CFD method,the detail of rotor flow field was numerically calculated.Taking LTH-100 single-rotor agricultural UAV as the research object,the three-dimensional solid model of UAV was established,the Reynolds average N-S equation was used as the control equation and the RNGκ-εas the turbulence model to simulate the flow field of UAV in hover and lateral wind conditions,the wind velocity distribution at different altitudes of rotor downwash flow field was studied.The simulation results of the hover state showed that:In the flow field,the peak velocity appears in a circular distribution below the distal axis of the rotor.With the decrease of height,the peak velocity distribution area showed a tendency to expand gradually after small shrinkage;When the distance from the rotor was not more than 1.5 m,the downwash flow field presented an axisymmetric distribution based on the rotor axis,and the variation rate of velocity in the peak velocity was basically the same,turbulence in downwash flow field made the flow field more complex when the distance from rotor was larger than 2.0 m.On this basis,the optimal flight altitude of UAV is 1.5 m.Wind velocity test of the flow field was carried out on a rotor test bench,wind velocities at four altitudes of 0.5 m,1.0 m,1.5 m and 2.0 m were measured to verify the coincidence between the simulated and measured values.The test results showed that:the relative error between the measured and simulated values at four measurement heights were between 0.382-0.524,and the overall average relative errors was 0.430,which verified the confidence level of simulated values for measured values.When the lateral wind velocity was 3 m/s,4 m/s and 5 m/s,the simulation results showed that:The distribution trend of airflow velocity at the same altitude in lateral-wind flow field with different wind speeds was similar;When the lateral wind speed was 5 m/s,the coupling field formed by the lateral wind and rotor airflow cannot reach the height of 2 m below the rotor.The results of this study can provide more accurate environmental conditions for theoretical analysis of droplet deposition regularity in the flow field,and also provide methodological guidance for the related research on rotor flow field of multi-rotor UAV.

Model migration for CFD and verification of a six-rotor UAV downwash

Currently,Computational Fluid Dynamics(CFD)has been used to investigate agricultural UAV downwash.However,the validations of CFD models are difficult to deal with.Current verification methods are to use either water-sensitive papers or wind-speed arrays,which could get wind distribution or speed only.In this study,model migration was used to develop and verify downwash CFD models.The basic idea is to try to use the results of a scaled-down drone to represent that of a real-used UAV.The CFD models of both a real-used six-rotor UAV,JF01-10,and a 1:10 scaled-down small drone were developed by ANSYS.Then,the scaled-down drone was utilized to conduct trials by particle image velocimetry(PIV),so that not only distribution and speed but also flowing direction of downwash could be obtained.Results indicated the relative error between the PIV tests and the CFD models of the small UAV was less than 12%,while that between the tests and the CFD models of JF01-10 was less than 34%.It could be indicated that model migration could reflect multiple downwash characteristics but should be optimized in some complex details.This study was a preliminary but fundamental attempt to investigate CFD modelling and validation of agricultural UAVs and provided a novel thinking of downwash verification.

Influence of the inner tilt angle on downwash airflow field of multi-rotor UAV based on wireless wind speed acquisition system

The downwash airflow field is an important factor influencing the spraying performance of plant protection UAV,and the structural design of rotors directly affects the characteristics of the downwash airflow field.Therefore,in this study,three-dimensional models of a six-rotor UAV with various inner tilt angles were established to simulate and analyze the influence of the inner tilt angle on the downwash airflow field based on the Reynolds average NS equation,RNG k-εturbulence model,etc..On this basis,a wireless wind speed acquisition system using the TCP server was developed to carry out the test through the marked points with real-time detection.The simulation results show that,the variation of inner tilt angles of the six-rotor UAV did not cause significant difference in the time dimension of the downwash airflow field,and with the change of the inner tilt angle from 0°to 8°,the distribution of downwash airflow field tended to obliquely shrink towards the central axis direction,and the amplitude of linear attenuation of airflow speed was also increased,which the difference of attenuation amplitude was 1 m/s.Besides,under the different inner tilt angle,the airflow velocity in“lead in area”was significantly greater than that in the“lead out area”,and the difference of air velocity distribution in space would affect the uniformity of droplet deposition.Through the calibration test,the measurement accuracy error of the developed system was lower than 0.3 m/s,and the adjusted R2 of the calibration fitting equation was higher than 0.99.The test and simulation values at test points from 0.2-2.3 m below the rotors exhibit the same variation trend,and the average relative error at the height of 1.1-2.3 m below the rotors and 0.2-0.8 m near the ground was within 10%and 20%,respectively.The simulation and test results were highly reliable,which could provide basis and reference for the design and optimization of plant protection drones.

高空菱形翼布局无人机气动特性

为了获取无人机执行情报、监视和侦察等特种任务时的长航时优势,菱形翼布局无人机受到了大家的极大关注。高空菱形翼布局无人机通过前后翼搭接,能够有效增加整机展弦比,同时由于飞行高度高,飞行速度低,流动存在层流-分离-转捩等特殊流动现象,流动具有复杂性。本文采用数值模拟方法,对菱形翼布局无人机在低雷诺数下的气动特性进行了研究。结果表明:菱形翼气动布局具有较好的气动特性,存在着前后翼相互干扰的问题。随着前后翼距离靠近,前翼受阻滞、后翼受下洗效应越严重;同时,在大迎角下,后翼受前翼下洗影响流动分离延迟,使整机气动焦点后移,纵向静稳定裕度增大。

基于牛顿运动定律的低速翼型升力分析及教学建议

基于牛顿运动定律定性分析了低速翼型的升力来源,从“整体”和“局部”两个角度分别阐释了升力的来源.从整体分析,机翼将空气向下加速而获得向上的升力;从局部分析,机翼改变其周围气压,使机翼受到向上的压力差.结合各阶段学生背景知识,给出了对应的教学建议.

Performance characterization on downwash flow and spray drift of multirotor unmanned agricultural aircraft system based on CFD

In recent years,multi-rotor Unmanned Aerial Vehicles(UAVs)have been employed in the field of plant protection in China.Spray drift has been considered a major impact in agriculture aerial spraying,and spray quality in the application of plant protection products.The downwash including wake vortices and downward wind field plays a major role in the dispersal and deposition of pesticide spray released by nozzle(s)equipped in aircraft.Differ from the fixed-wing UAV,the downwash flow of multi-rotor UAV was result from the rotation of rotor.Therefore,a study on off-target drift and ground deposit concerning the rotor rotation was simulated through a series of Computational Fluid Dynamics(CFD)simulations to obtain the influence of downwash.The discrete Phase Model(DPM)was taken to simulate the motion of droplet particles since it is an appropriate way to simulate discrete phases in flow field and can track particle trajectory.In this study,the parameters of CFD simulations were acquired by three kinds of actual replicated experiment.The simulation analysis mainly obtains the droplet drift and deposition rule,the influence of eddy current,and downwash flow caused by the rotor rotation.The results showed that the downwash distribution below different rotors was different owing to the flight angle of inclination,“behind”is the greatest,“middle”is secondly,and“forward”is smallest in value(behind,middle,and forward represent three regions below rotors along flight direction).According to the simulation results,two methods of reducing droplet drift were put forward and specific simulations were carried out to prove their feasibility.The results of this study can provide theoretical support for improving the spray quality of UAV and reducing the drift of droplets.

基于PINNs的单旋翼植保无人机下洗流场预测模型

植保无人机(unmanned aerial vehicle,UAV)进行喷施作业时,旋翼高速旋转所产生的下洗流场是影响雾滴飘移的重要因素。为了快速准确地预测单旋翼植保无人机下洗流场的速度等流场参数,提升无人机精准施药效果,该研究基于物理信息神经网络(physics-informed neural networks,PINNs)构建了单旋翼植保无人机下洗流场的预测模型。在全连接神经网络结构的基础上,嵌入纳维-斯托克斯(Navier-Stokes,N-S)方程作为物理学损失项来参与训练,减轻网络模型对数据依赖性的同时增强了模型的可解释性。通过最小化损失函数,使得该模型学习到流场中流体的运动规律,得到时空坐标与速度信息等物理量之间的映射关系,从而实现对单旋翼无人机下洗流场的速度等参数的快速预测。最后通过风洞试验验证该预测模型的可行性和准确性。结果表明:没有侧风的情况下,预测模型在旋翼下方0.3、0.7、1.1以及1.5 m共4个不同高度处各向速度的预测值和试验值的误差均小于0.6 m/s,具有较小的差异性;不同侧风风速情况下,水平和竖直方向速度的预测值与试验值的总体拟合优度R2分别为0.941和0.936,表明所提出的模型在单旋翼植保无人机下洗流场预测方面具有良好的应用效果,能够快速准确地预测下洗流场的速度信息。研究结果可为进一步研究旋翼风场对雾滴沉积分布特性的影响机理提供数据支撑。

小型植保无人机下洗气流场影响雾滴运动特性规律研究

为了研究小型无人机下洗气流场对雾滴运动特性的影响规律,以小型无人机为基础,采用Navier-Stokes(N-S)方程、realizable k-ε模型和Semi-Implicit Method for Pressure-Linked Equations算法,对施药过程中的下洗气流场和雾滴离散运动进行了详细的数值模拟分析。研究分析无人机下洗气流的速度特性、雾滴沉积特性以及作业高度对农药沉积效果的影响。通过试验结果可知,模拟值与试验值的相对误差在20%以内,验证了下洗气流场数值模型的可行性。进一步模拟分析结果表明,在无人机喷药平台的影响下,下洗气流场在距离旋翼1 m处达到速度峰值。随着作业高度的增加,雾滴逐渐分散并扩散。雾滴主要分布在两个“气流引入区”和两个“气流导出区”,该分布特征有助于优化施药效果和提高农药的使用效率。根据分析结果,当无人机飞行作业姿态与地表保持平行,且将作业高度调整至0.8~1.0 m之间时,可显著提高农药沉积量,从而提高植保无人机的施药效果。本研究验证了下洗气流场数值模型的可行性,为小型植保无人机的对靶雾滴漂移及沉积研究提供了参考依据。

八旋翼植保无人机下洗气流对雾滴运动与分布特性的影响

多旋翼无人机由于其质量轻、操作灵活、飞行稳定等优势,已成为中国植保无人机市场上的主流机型。为探究多旋翼无人机旋翼气流分布特性及其对雾滴运动和沉积行为的影响,本文基于大疆MG-1P型八旋翼植保无人机的下洗气流场进行测定,并对比分析了旋翼气流对喷雾雾滴的速度、粒径和沉积分布均匀性的影响。结果表明,无人机旋翼下洗气流的强度伴随测试层高度的下降而降低,机体正下方的气流速度方向近似于竖直向下且呈现先增加后减小的趋势,位于机体两侧区域的气流呈现“先收缩、后扩张”的喇叭状,近地位置测试点L和M的气流方向均指向测试区域外侧斜下方,其与竖直方向的夹角分别为71.3°和81.5°。整体而言,无人机机身两侧旋翼下洗气流的速度和方向呈对称分布。在旋翼静止时,喷雾雾滴的沉降速度较慢,各测试层及测试点的雾滴速度均低于1 m/s。无人机悬停时,其旋翼风场极大地提高了雾滴速度,且雾滴速度分布特性与旋翼风场强度高度吻合。与旋翼静止相比,无人机悬停时产生的高速下洗气流可致使雾滴粒径增大。雾滴的沉积分布效果在距离地面20 cm处最好,其在旋翼静止和悬停条件下的平均沉积量分别为4.69μL/cm^(2)和5.85μL/cm^(2),沉积量的变异系数分别为51.81%和36.87%。无人机旋翼下洗气流可有效提高喷雾雾滴的沉积量和分布均匀性。本研究可对多旋翼植保无人机下洗气流优化、雾流场与气流场的耦合以及提高喷雾液的利用率和分布均匀性提供参考。

基于欧拉方程解摄动的时差导数快速算法

使用基于分区对接结构网格获得的Euler方程基本流场解,通过远场边界处理方法(物面法线Riemann不变量方法)摄动,对时差导数与自由来流合成攻角、舵偏角和气流滚转角变化率构建摄动关系,提出了一种时差导数快速估算方法。本文利用国际动导数标准模型(SDM)进行算例验证,计算结果与参考文献结果具有很好的一致性,表明本文方法耗时少、精度高、工程实用,适用于复杂气动外形飞行器的时差导数快速计算。利用本文方法在亚/跨/超声速范围内获取了5°攻角时SDM俯仰旋转导数和洗流时差导数。结果表明:在亚/跨声速范围内,是否考虑时差导数结果之间存在一定差别,随着来流马赫数增大,时差的影响逐渐减小,在超声速条件下时差影响基本可以忽略。

多旋翼植保无人机悬停下洗气流对雾滴运动规律的影响

多旋翼植保无人飞机在农药喷洒和授粉作业等相关领域已开展广泛应用,但存在风场分布不明晰导致的分布不均问题。针对六旋翼植保无人飞机,结合雷诺平均N-S方程及Realizable k-ε湍流模型,建立了下洗气流三维数学模型,风场测试及非定常计算表明特征点z向速度的测量和模拟值相对误差在9%以内,验证了风场数值计算的可靠性;在机翼旋转诱导及外界气压的挤压下,下洗气流纵向主截面呈现出"收缩-扩张-再收缩"现象;"旋翼间干扰"使得下洗风场湍流效应明显,横截面的旋翼间区域出现了气流"引入"及"导出"区。引入雾滴离散相,并对连续相进行动量、能量方程修正,建立喷头含雾滴离散相的两相流模型,结合喷头喷幅试验来验证了该两相流模型计算雾滴运动轨迹的有效性;结果表明,粒径越小,雾滴水平方向分速度衰减越快,喷幅越小;雾滴粒径越大,竖直方向的最终分速度越大。建立了六旋翼植保无人飞机悬停条件下含雾滴离散相的三维两相流模型,计算分析表明,雾滴主要分布在"旋翼间干扰"明显的3个"引入区"、3个"导出区",下洗区内侧雾滴群交织,外侧大雾滴周向水平行程更大进而分布在外围;当雾滴粒径小于200?m时,雾滴运动范围无法覆盖全部的"引入区",雾滴多分布在下洗区中心;当雾滴粒径大于250?m时,雾滴运动区域逐渐覆盖所有"引入区"、"导出区"。该研究可为飞行施药过程中迎风气流、下洗气流、瞬时横风耦合风场扰动下雾滴的漂移、沉积研究提供参考。