展弦比对薄片翻滚下落运动的影响

发布时间：2018-01-03 05:04

本文关键词：展弦比对薄片翻滚下落运动的影响　出处：《清华大学》2016年博士论文　论文类型：学位论文

【摘要】：本文通过实验测量与理论分析两种方法,研究了展弦比从2到10的自由下落薄片的翻滚运动,得到了展弦比对翻滚运动规律的影响。在实验研究中,我们设计并构造了薄片下落发射装置,可以以固定角度发射薄片。发射位置足够高,以便薄片下落能飘移足够距离,方便实验记录稳定翻滚下落状态。采用高速摄像机记录薄片翻滚下落的运动轨迹,并构造了实验结果处理方法,以此得到瞬时以及时间平均后的运动学和气动参数,包括下飘角、转速、下落速度、升力和阻力系数以及他们的波动谱。作为实验研究重要结果之一,我们发现,在某些范围的展弦比下,存在一个伴随着周期倍增的双周期旋转现象,对此现象,我们利用傅里叶分析明确了其频率的倍增。三维薄片即使在固定大攻角状态,流场也异常复杂。对于翻滚下落,除了旋转导致的复杂流场,瞬时大攻角状态下,也存在旋涡脱落以及非定常流动。因此,通过理论分析研究此类问题以获得展弦比对翻滚运动规律的影响,是具有较高复杂度的问题。在本文中,我们通过将考虑了翻滚的二维情况下的气动力与力矩估算公式,加入流向涡与转动环量的影响带来的修正,得到了三维情况下的升阻力以及力矩估算公式。其中涉及到的一些比例系数与展弦比无关,因此由展弦比足够大的实验结果标定。利用这些气动参数估算,我们构造了薄片下落的运动学模型。在翻滚下落处于平衡状态时,这样得到的运动学模型就可以给出下飘角、下落速度和旋转频率。通过与实验结果进行比较,我们验证了理论模型的有效性。以此理论模型,我们研究了各种参数尤其展弦比对下落规律的影响。实验和理论模型分析均表明:下飘角和下落速度垂直分量是展弦比的递减函数,转速和下落速度水平分量是展弦比的递增函数。原因是随着展弦比的减小,由翼尖涡和转动环量导致的诱导阻力加大,造成升阻比的减小,因此下飘角增大,而平动诱导阻力导致的耗散力矩,会使转速减小。
[Abstract]:By means of experimental measurement and theoretical analysis, the rolling motion of a freely falling sheet with a aspect ratio of 2 to 10 is studied, and the effect of the aspect ratio on the rolling motion is obtained. We have designed and constructed a wafer falling launcher that can fire the wafer at a fixed angle. The emission position is high enough so that the falling flake can drift far enough. It is convenient for the experiment to record the stable tumbling state. The high-speed video camera is used to record the motion track of the thin slice rolling and falling, and the processing method of the experimental results is constructed. The kinematics and aerodynamic parameters, including the angle of falling, rotation speed, falling velocity, lift and drag coefficient and their wave spectrum, are obtained from the instantaneous and time-averaged kinematics and aerodynamic parameters, which are one of the important results of the experimental study. We find that under some range of aspect ratio, there exists a double period rotation phenomenon with double periods. We use Fourier analysis to determine the multiplication of its frequency. The flow field is extremely complex even in the fixed large angle of attack state. For tumbling and falling, except for the complex flow field caused by rotation, the instantaneous large angle of attack state. There are also vortex shedding and unsteady flow. Therefore, it is a high complexity problem to study this kind of problems through theoretical analysis to obtain the effect of aspect ratio on the rolling motion. By taking into account the two dimensional aerodynamic force and torque estimation formula of rolling, we add the effect of flow vortex and rotation ring to the correction. The formulas for estimating the lift resistance and torque in three dimensional cases are obtained. Some of the proportional coefficients involved are independent of the aspect ratio, so the experimental results with a large aspect ratio are used to estimate the aerodynamic parameters. The kinematics model of thin sheet falling is constructed. When the roll is in equilibrium, the kinematics model can give the angle of falling, the velocity of falling and the frequency of rotation. The results are compared with the experimental results. We verify the validity of the theoretical model. The effects of various parameters, especially the aspect ratio, on the falling law are studied. The experimental and theoretical model analysis show that the vertical component of the falling velocity and the falling angle are the decreasing functions of the aspect ratio. The horizontal component of rotational speed and falling velocity is an increasing function of the aspect ratio, because with the decrease of the aspect ratio, the induced resistance caused by the vorticity of the wingtip and the amount of the rotation ring increases, resulting in the decrease of the ratio of lift to drag, so the falling angle increases. And the dissipative torque caused by the translational induced resistance will decrease the rotational speed.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O353.4

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