A three dimensional approach to blade packet vibrations

A finite element analysis for the free vibration of blade packets is discussed in this paper. A three dimensional isoparametric quadratic element with three degrees of freedom (u, v and w) per node is used for the analysis. The analysis indicates the effects of the presence of the shroud on the natural vibrations of independent blades and the blade packet. Since the blade packets are analyzed as three dimensional structures, all the natural modes can be determined. The natural modes thus determined include the batch modes arising due to the presence of the shroud. Batch modes occur both in bending and torsion. Two blade packets are illustrated here to outline the utility of the method.     

Effect of flow circulation on combustion dynamics of fire whirl

In this paper, the effect of flow circulation on the combustion dynamics of fire whirl is systematically investigated by experiments. New correlations for the burning rate, flame height, radial temperature and mass flow rate are established for fire whirl. It is clarified that flow circulation helps increase both the fuel-flame contact area and the actual fuel surface area, which in turn increases both the heat feedback to the fuel surface and the radial velocity in the ground boundary layer, leading to increase of burning rate. A novel idea for correlation of fire whirl flame height is proposed by assuming that the ratio of the fire whirl flame height to the flame height without circulation solely characterizes the effect of circulation. This idea is fully verified, thereby a new formulation for flame height is established, which successfully decouples the burning rate and the circulation. It is indicated that the fuel-rich core in the flame body of fire whirl significantly affects the radial temperature distribution in the continuous flame region, and the flame body can be described by the combination of a cylinder and a cone. The flow circulation significantly suppresses fire plume radius and thus decreases its increasing rate with vertical distance. It is also demonstrated that the fire whirl flame involves laminarized regions in its lower section, coexisting with turbulent regions in the upper portion. The flow circulation enhances the air entrainment in the ground layer by altering the radial velocity profile and increasing the radial velocity. In the low section of flaming region, the significant decrease of mixture between the combustion products and surrounding air dominates the pure aerodynamic effect of flow circulation on the flame height. Finally, it is clarified that fire whirls maintain higher centerline excess temperature than general pool fires due to the effect of less air entrainment.     

Stability analysis of a nonlinear rotating blade with torsional vibrations

This paper discusses the stability of a spinning blade having periodically time varying coefficients for both linear model and geometric nonlinear model. To obtain a reduced nonlinear model from nodal space, a standard modal reduction procedure based on matrix operation is developed with essential geometric stiffening nonlinearities retained in the equation of motion. For the linear model, the stability chart with various spinning parameters of the blade is studied via the Bolotin method, and an efficient boundary tracing algorithm is developed to trace the stability boundary of the linear model. For the geometric nonlinear model, the method of multiple time scale is employed to study the steady state solutions, and their stability and bifurcations for the periodically time-varying rotating blade. The backbone curves of steady-state motions are achieved, and the parameter map for stability and bifurcation is developed.     

非均匀桨直升机旋翼厚度噪声分析

分析了桨叶间距非均匀调制对直升机旋翼厚度噪声的影响。桨叶间距按照正弦调制和余弦调制变化时,根据FW-H方程的Formulation 1A公式,计算了直升机悬停状态下的旋翼厚度噪声。桨尖速度为亚音速时的计算结果显示,与桨叶间距均匀的直升机旋翼厚度噪声相比,非均匀桨可以改变厚度噪声的频谱特征,而且在基本保持厚度噪声总声压级不变情况下,降低厚度噪声桨叶通过频率及谐频的线谱幅值。本文的分析和结果有助于认识非均匀旋翼厚度噪声的频谱变化规律。     

Scale effects in the bending vibrations of helicopter rotor blades

Scale effects and dynamic similarity in the bending vibrations of helicopter rotor blades are examined by expressing the first three modes of bending vibration of a uniform, conventional rotor blade by a series of Legendre polynomials as suggested by Wilde and others. The natural frequency ratios for these three modes are then determined as functions of a dynamic similarity parameter over the entire range of designs and operating conditions from very flexible, rapidly rotating blades to highly rigid, slowly turning conditions.     

直升机旋翼挥舞、摆振的激光动态测试系统

提出一种直升机旋翼挥舞、摆振角的动态测试系统。在直升机桨毂安装与旋翼同步旋转的三叉件 ,每个三叉件悬臂上装有光学三角位移传感器和线阵 CCD位移传感器 ,实现了动态测量直升机旋翼桨叶的挥舞和摆振角。为保证系统在恶劣条件下正常工作 ,系统采用了加装干涉滤光片、L D调制技术和自动增益控制电路等一系列技术措施。最后给出了实验结果。由位移 -角度转换和传感器自身的非线性引起的误差用若干静态标定点为节点插值的方法修正。得到较高精度的测试曲线     

Effect of phase angle on multibladed rotor flutter

A theoretical technique for predicting the flutter characteristics of a helicopter rotor is presented. The effect of phase angle on flutter speed of a two-bladed rotor in hovering and axial flight is determined. For this purpose, a uniform and untwisted rotor blade with coupled flapwise bending and torsional degrees of freedom is considered. The transmission matrix method is used to obtain the natural vibration characteristics of the system. An unsteady aerodynamic theory is used to obtain the aerodynamic loading in compressible flow.     

Experimental research on combustion dynamics of medium-scale fire whirl

The medium-scale fire whirl was extensively investigated by experimental means, in order to establish correlations of the burning rate, flame height and flame temperature of fire whirl, and to clarify the difference between fire whirls and general pool fires. Experimental observations and data confirmed that a free burning fire whirl is a highly stable burning phenomenon with large quasi-steady periods. Burning rates of fire whirls depend on pool diameter similarly to those of general pool fires; however the transition turbulent burning occurs sooner as the pool diameter increases. The lip height seems to have little effect on the burning rate of fire whirls. The correlation was proposed to couple the height of fire whirl to the fire release rate and ambient circulation. It correlates the data from both this work and the literature. Radial temperature profiles in the continuous region of the fire whirl were confirmed to be hump-type, implying the existence of fuel-rich inner core. The pool diameter and heat release rate do not significantly affect the radial temperature profiles in non-dimensional radial coordinates. It was found that the fire plume of fire whirl involves three distinct zones just like that of pool fire, but with different normalized ranges. Fire whirls maintain a higher ratio of continuous flame height to the overall flame height, and also higher maximum centerline excess temperature in continuous flame region, as compared to general pool fires. It was further demonstrated that the fire whirl plume at its origin behaves like a turbulent jet with moderate swirling, and then tends to become buoyancy dominated downstream, with slight swirling. With an increase in dimensionless height adjusted by the plume origin, the plume centerline excess temperature decays rapidly and approaches the theoretical value of −5/3 for free buoyancy plume.     

Effect of lattice vibrations on scanning tunnelling microscope images

Tersoff and Hamann's theory of the scanning tunnelling microscope is used in a preliminary investigation of the effect of lattice vibrations on the atomic scale contour maps generated with this instrument. It is found that appreciable errors in inferred charge density corrugation amplitudes can arise if the mean-square atomic displacements vary significantly across the sample surface.     

Effect of high-frequency vibrations on oriented crystallization of binary alloys

The effect of high-frequency rotational vibrations with small amplitude on the oriented crystallization of binary alloys has been numerically studied. The calculations have been made in the framework of the quasi-static approach for succinonitrile-acetone and succinonitrile-salol binary systems. It has been shown that at a large solidification rate and/or high impurity concentration both for a lighter impurity (acetone) and for a heavier one (salol) a pit is formed on the axis of symmetry during growth which accelerates the development of morphological instability. Rotational vibrations make a strong stabilizing effect. The threshold for morphological instability increases with an increase in the vibration intensity, the radial segregation of an impurity decreases, and the pit is not formed.     

Effect of shroud eccentricity on suppression of flow induced vibrations

The object of the research described was to examine the vibrational characteristics (amplitude and frequency) and the resulting drag for an eccentrically shrouded cylinder. The results revealed that an eccentricity in any direction and of any magnitude always reduces the amplitude of vibration in the subcritical flow region in comparison with that of a concentrically shrouded cylinder. This trend was terminated within the transitional region where there was a converging of the concentric and eccentric vibrational responses. The drag of the cylinder enclosed in the axial-rod shroud was found to be less than the drag of the plain cylinder in the subcritical region but the opposite effect was obtained in the transitional region. The drag was measured on the eccentrically shrouded cylinder for various upstream, lateral and downstream eccentricities and the results were compared with those for the concentrically shrouded cylinder.     

Effect of heterogeneity on the axisymmetric vibrations of cylindrical shells

The influence of large heterogeneity on the axisymmetric vibration characteristics of thin, composite cylindrical shells is studied, both analytically and numerically. In the neighborhood of the axisymmetric breathing mode, frequency spectra for shells of infinite and finite length are shown to be influenced qualitatively as well as quantitatively by large deviations from material and geometric symmetry in layer arrangement. A study of mode coupling in a semi-infinite shell is made for both end modes and modes with stationary frequency with real finite wave number, the latter being uniquely generated by a special class of heterogeneity. In each case, analytical estimates are given for frequencies, wave numbers, and modal amplitudes as functions of material and geometric properties of the shell.     

An experimental study of the blue whirl onset

This paper focuses on the blue whirl's formation conditions and onset mechanism. The blue whirl is experimentally generated using a forced-ventilation fire whirl apparatus, where the swirl intensity can be actively adjusted. It also allows the blue whirl to form in the laminar regime, without requiring a turbulent fire whirl as the inducer. Transverse particle image velocimetry (PIV) measurement is performed to understand the swirling flow near the vortex core and to quantify the swirl intensity in terms of circulation. The result demonstrates that a stable blue whirl can be formed only when the circulation is above a threshold value of about 0.07 m²/s, for the current experimental setup. Then, by examining the formation process during the blue whirl onset, a strong correlation can be confirmed between the circulation jumping and the flame regime transformation from a weak fire whirl to a blue whirl. To understand the critical condition for vortex breakdown, we further propose a new swirl number, $S_P$, based on balancing the pressure differences associated with the centrifugal and the radial-converging effects. As a result, the turning point in the $S_P$ curve agrees well with the emergence of the recirculation zone, verifying the relevance of the proposed vortex breakdown mechanism in determining the blue whirl onset.