Measurements of the dynamic lift force acting on a circular cylinder in cross-flow and exposed to acoustic resonance

A piezoelectric transducer is developed to perform direct measurements of the dynamic lift force acting on a circular cylinder in cross-flow, in the presence and absence of acoustic resonance. Details of the force transducer design are presented in the paper. The dynamic lift force is measured for a single cylinder with two different diameters, D=12.7 and 15.8 mm. During the tests, the first transverse acoustic mode of the duct housing the cylinder is self-excited. The fluctuating pressure on the top wall of the duct is measured simultaneously with the dynamic lift force. In the absence of acoustic resonance, the measured dynamic lift coefficients agree favorably with those reported in the literature. However, when the acoustic resonance is initiated, the dynamic lift experiences a drastic increase in amplitude associated with abrupt changes in the phase between the lift force and the acoustic pressure. A methodology to extract the hydrodynamic lift component from the total lift measured during acoustic resonance is also proposed. The hydrodynamic lift force is then decomposed into in-phase and out-of-phase components, with respect to the resonant sound pressure. This decomposition procedure provides new insights into the nature of the aeroacoustic sources in the cylinder wake. The proposed methodology, together with the test results provide a general design approach to assess the increase in the dynamic fluid loading on bluff bodies in cross-flow due to the excitation of acoustic resonance.     

Active feedback control of stall in an axial flow fan under dynamic inflow distortion

This work proposes an active feedback control strategy using cross-correlation technique in a single stage axial flow fan operating under dynamic inflow distortion. Experiments were carried out under dynamic inflow distortion at the design speed with only three sensors and actuators, each. The stall inception mechanism studies under dynamic inflow distortion were carried out using 1-D continuous Morlet wavelet transform. It was observed that stall inception under co- (in the same direction of rotor rotation) and counter-rotating (in the opposite direction of rotor rotation) inflow distortion occurred through long and short length-scale disturbances, respectively. The knowledge of the nature of instabilities under dynamic inflow distortion was used to set the threshold of the correlation coefficient. It was observed that the active feedback control strategy resulted in a stall onset delay of 125 (?3.125 s) and 65 (?1.625 s) rotor revolutions under co- and counter-rotating inflow distortions, respectively. The highest delay under co-rotating inflow distortion was attributed to the substantially higher stall warning time as compared to counter-rotating inflow distortion.     

Effect of acoustic resonance on the dynamic lift of tube arrays

The effect of acoustic resonance on the dynamic lift force acting on the central tube in square and normal triangle tube arrays is investigated experimentally. For each array pattern three different tube spacing ratios, corresponding to small, intermediate and large spacing ratios, are tested. The resonant sound field in the tube array is found to cause two main effects. First, it generates a “sound-induced” dynamic lift due to the resonant acoustic pressure distribution on the surface of the tube, and secondly, it synchronizes vorticity shedding from the tubes and thereby enhances the hydrodynamic lift force due to vortex shedding. The combined effect of these two unsteady lift forces depends on the phase shift between them, which is dictated by the frequency ratio of the acoustic mode to the natural vortex shedding frequencies. When the flow velocity is increased during the coincidence resonance range, the phase shift increases rapidly and therefore the effects of the two lift components change from reinforcing to counteracting each other. For the pre-coincidence lock-on range, the frequency ratio remains larger than unity and the two lift components always reinforce each other. Numerical simulations are also performed to compute the sound-induced lift force, and sound-enhancement coefficients are developed to estimate the effect of sound on the vortex shedding forces. The simulation and experimental results are implemented in a simplified design guide, which can be used to evaluate the dynamic lift forces acting on the tubes during acoustic resonances.     

Design of active flutter suppression and wind-tunnel tests of a wing model involving a control delay

In this study, a delayed controller was designed for active flutter suppression of a three-dimensional wing model. The design of controller can be divided into two steps. At the first step, a short time delay was artificially introduced into the control loop and the dynamic equations of the aeroelastic system with delayed control were converted into a set of delay-free state-space equations by using a state transformation. At the second step, the control law was synthesized by using the theory of optimal control for the delay-free state-space equations. To demonstrate the performance of the delayed controller, the margin of time delay was studied. The numerical results showed that the delayed controller had good robustness with respect to the time delay. Moreover, the delayed controller was digitally implemented and tested for the three-dimensional wing model in NH-2 subsonic wind-tunnel. The experimental results illustrated that the critical flow speed of flutter instability of the wing model could be effectively increased from 36.5 m/s to 39 m/s.     

A new reduction-based LQ control for dynamic systems with a slowly time-varying delay

Time delays in the feedback control often dete- riorate the control performance or even cause the instability of a dynamic system. This paper presents a control strategy for the dynamic system with a constant or a slowly time-varying input delay based on a transformation, which sire-plifies the time-delay system the relation is discussed for into a delay-free one. Firstly, two existing reduction-based linear quadratic controls. One is continuous and the other is discrete. By extending the relation, a new reduction-based control is then developed with a numerical algorithm presented for practical control implementation. The controller suggested by the proposed method has such a promising property that it can be used for the cases of different values of an input time delay without redesign of controller. This property provides the potential for stabilizing the dynamic system with a time-varying input delay. Consequently, the application of the proposed method to the dynamic system with a slowly time-varying delay is discussed. Finally, numerical simulations are given to show the efficacy and the applicability of the method.     

Analysis of effects of differential gain on dynamic stability of digital force control

The paper presents an analytical investigation of the dynamics of digital force control. A one degree-of-freedom (DoF) mechanical system with low viscous damping is subjected to proportional-derivative (PD) force control. Analytical results are presented in the form of stability charts in the parameter space of sampling time, control gains and mechanical parameters. Simple closed form results include the largest stable proportional gain and the least steady state force error that provide synthesis of mechanical and control system parameter influences for the design of digital force control. Also, a novel analytical explanation is given why even the properly filtered force derivative signal is rarely used in practice, and why the occurring vibrations have frequencies one range smaller than that of the sampling frequency of the digital control.     

Effect of compressibility on plasma-based transition control for a wing with leading-edge excrescence

Compressibility effects were numerically investigated for use of plasma-based flow control, which was applied to delay transition generated by excrescence on the leading edge of a wing. The wing airfoil section incorporates a geometry that is representative of modern reconnaissance air vehicles, and has an appreciable region of laminar flow at design conditions. Modification of the leading edge can be caused by the accumulation of debris, insect impacts, microscopic ice crystal formation, damage, or structural fatigue, resulting in premature transition and an increase in drag. A dielectric barrier discharge (DBD) plasma actuator, located downstream of the excrescence, was employed to delay transition, mitigate the effects of turbulence, decrease drag, and increase energy efficiency. Solutions were obtained for several Mach numbers, up to the transonic range. The effect of compressibility on transitional behaviour was explored, and the effectiveness of plasma-based control to delay transition with increasing Mach number was determined.     

An analytical study of time-delayed control of friction-induced vibrations in a system with a dynamic friction model

We investigate the control of friction-induced vibrations in a system with a dynamic friction model which accounts for hysteresis in the friction characteristics. Linear time-delayed position feedback applied in a direction normal to the contacting surfaces has been employed for the purpose. Analysis shows that the uncontrolled system loses stability via. a subcritical Hopf bifurcation making it prone to large amplitude vibrations near the stability boundary. Our results show that the controller achieves the dual objective of quenching the vibrations as well as changing the nature of the bifurcation from subcritical to supercritical. Consequently, the controlled system is globally stable in the linearly stable region and yields small amplitude vibrations if the stability boundary is crossed due to changes in operating conditions or system parameters. Criticality curve separating regions on the stability surface corresponding to subcritical and supercritical bifurcations is obtained analytically using the method of multiple scales (MMS). We have also identified a set of control parameters for which the system is stable for lower and higher relative velocities but vibrates for the intermediate ones. However, the bifurcation is always supercritical for these parameters resulting in low amplitude vibrations only.     

Numerical investigations of the vortex interactions for a flow over a pitching foil at different stages

The fluid–structure interaction is investigated numerically for a two-dimensional flow (Re=2.5·10⁶) over a sinusoid-pitching foil by the SST (Shear Stress Transport) k–ω model. Although discrepancies in the downstroke phase, which are also documented in other numerical studies, are observed by comparing with experimental results, our current numerical results are sufficient to predict the mean features and qualitative tendencies of the dynamic stall phenomenon. These discrepancies are evaluated carefully from the numerical and experimental viewpoints.In this study, we have utilized Λ, which is the normalized second invariant of the velocity gradient tensor, to present the evolution of the Leading Edge Vortex (LEV) and Trailing Edge Vortex (TEV). The convective, pressure, and diffusion terms during the dynamic stall process are discussed based on the transport equation of Λ. It is found that the pressure term dominates the rate of the change of the rotation strength inside the LEV. This trend can hardly be observed directly by using the vorticity transport equation due to the zero baroclinic term for the incompressible flow.The mechanisms to delay the stall are categorized based on the formation of the LEV. At the first stage before the formation of the LEV in the upper surface, the pitching foil provides extra momentum into the fluid flows to resist the flow separation, and hence the stall is delayed. At the second stage, a low-pressure area travels with the evolution of the LEV such that the lift still can be maintained. Three short periods at the second stage corresponds to different flow patterns during the dynamic stall, and these short periods can be distinguished according to the trend of the pressure variation inside the LEV. The lift stall occurs when a reverse flow from the lower surface is triggered during the shedding of the LEV. For a reduced frequency k_f=0.15, the formation of the TEV happens right after the lift stall, and the lift can drop dramatically. With a faster reduced frequency k_f=0.25, the shedding of the LEV is postponed into the downstroke, and the interaction between the LEV and TEV becomes weaker correspondingly. Thus, the lift drops more gently after the stall. In order to acquire more reliable numerical results within the downstroke phase, the Large Eddy Simulation (LES), which is capable of better predictions for the laminar-to-turbulent transition and flow reattachment process, will be considered as the future work.     

Vibration control for the primary resonance of the van der Pol oscillator by a time delay state feedback

We investigate the primary resonance of an externally excited van der Pol oscillator under state feedback control with a time delay. By means of the asymptotic perturbation method, two slow-flow equations on the amplitude and phase of the oscillator are obtained and external excitation-response and frequency-response curves are shown. We discuss how vibration control and high amplitude response suppression can be performed with appropriate time delay and feedback gains. Moreover, energy considerations are used in order to investigate existence and characteristics of limit cycles of the slow-flow equations. A limit cycle corresponds to a two-period modulated motion for the van der Pol oscillator. We demonstrate that appropriate choices for the feedback gains and the time delay can exclude the possibility of modulated motion and reduce the amplitude peak of the primary resonance. Analytical results are verified with numerical simulations.     

On the convergence and causality of a frequency domain method for dynamic structural analysis

Venanico-Filho et al. developed an elegant matrix formulation for dynamic analysis by frequency domain (FD), but the convergence, causality and extended period need further refining. In the present paper, it was argued that: (1) under reasonable assumptions (approximating the frequency response function by the discrete Fourier transform of the discretized unitary impulse response function), the matrix formulation by FD is equivalent to a circular convolution; (2) to avoid the wraparound interference, the excitation vector and impulse response must be padded with enough zeros; (3) provided that the zero padding requirement satisfied, the convergence and accuracy of direct time domain analysis, which is equivalent to that by FD, are guaranteed by the numerical integration scheme; (4) the imaginary part of the computational response approaching zero is due to the continuity of the impulse response functions. The English text was polished by Yunming Chen     

Effects of vertical exciting force of a tracked vehicle on the dynamic compaction of a high lifted decomposed granite

The main purpose of this paper is to evaluate the effects of a jumping action of a tracked vehicle mounted with a vertical oscillator on vibro-compaction of a high lifted decomposed granite. A vibro-compaction test was executed using a model tracked vehicle of 4.9 kN weight under a condition of frequency of 16 Hz and load ratio of maximum vertical exciting force to vehicle weight of 0.2–2.0. As a result, it was observed that both the amount of sinkage of terrain surface and the dry density of soil increased hyperbolically with increment of the load ratio and the dry density distribution with depth became uniform for the whole depth of the soil stratum. It was confirmed that the volume shrinkage of soil was succeeded by the propagation of acceleration to deep stratum due to the jumping action and the dilatancy phenomenon due to an alternative shear stress. The optimum load ratio obtaining a maximum dry density at the frequency of 16 Hz was judged to be 2.0 within this experiment. In the application of these test results to an actual prototype tracked vehicle of 39.2 kN weight, it was estimated that the degree of compaction of a high lifted soil stratum of 90 cm became over 90% at the load ratio of 2.0.     

动力响应数值分析中的hourglass现象

结构动力响应数值分析中，通常采用高斯积分一类的数值积分。单点高斯积分的采用可能引起hourglass模态。控制hourglass模态是结构动力响应数值分析中必须解决的问题之一。本文详细分析了hourglass模态产生的机理，并在此基础上讨论了hourglass模态的控制措施、具体应用及对正常应模态的影响。     

An experimental investigation on the dynamic axial buckling of cylindrical shells using a Kolsky Bar

Several experiments were performed with a Kolsky Bar (Split Hopkinson Pressure Bar) device to investigate the dynamic axial buckling of cylindrical shells. The Kolsky Bar is a loading as well as a measuring device which can subject the shells to a fairly good square pulse. An attempt is made to understand the interaction between the stress wave and the dynamic buckling of cylindrical shells. It is suggested that the dynamic axial buckling of the shells, elastic or elasto-plastic, is mainly due to the compressive wave rather than the flexural or bending wave. The experimental results seem to support the two critical velocity theory for plastic buckling, withV _c1 corresponding to an axisymmetric buckling mode andV _c2 corresponding to a non-symmetric buckling mode. The project supported by National Natural Science Foundation of China     

Energy consumed by a bearing supported spindle in the presence of a dynamic cutting force and of defects

A machine tool spindle system model is proposed in this paper to investigate the non-linear face-milling cutting forces behavior, which are neglected in the literature, in order to predict the total mechanical power of a spindle. A simulation of the structure of the spindle based on the finite-element method is elaborated to estimate the variable cutting forces and then the variable power loss generated by bearings, considering the angular position and contact angles of the variable balls. Experiments are elaborated to compare the experimental power values with the predicted results. Particular attention is paid to different types of defects (inner ring spalling, outer ring spalling, eccentricity, and unbalance) in order to study their impact on the power consumed by the spindle during the approach and cutting phases under different rotating conditions.     

A theoretical investigation of the development of stationary crossflow vortices in the boundary layer on a swept wing

Crossflow instability plays very important role in the transition of the boundary layer on a swept wing, typical in the engineering applications. Experiments revealed that the linear stability theory well predicted the form of the crossflow vortices, but usually much overpredicted their growth rate. Using nonlinear theory of hydrodynamic stability, combined with some other considerations, we were able to obtain the growth rate in good agreement with experimental observations. The project supported by the National Natural Science Foundation of China, Grant No. 19572048     

车辆悬架控制臂液压衬套区间动刚度模型研究

已有文献的车辆悬架控制臂液压衬套动刚度模型仅能给出动刚度变化理想曲线,不能给出动刚度的上下变化曲线。为了解决这个问题,将流体惯性系数和流量阻尼系数定义为区间变量,运用区间不确定性理论建立了液压衬套的动刚度模型。将初始区间动刚度模型获得的仿真结果与已有的实验结果对比,吻合较好,验证了该模型的正确性。采用子区间组合法,在一定的区间精度下获得优化的区间动刚度模型,进而获得优化的动刚度上下限变化曲线。区间动刚度模型为动刚度的全面描述提供了一种方法,对文献的动刚度模型进行了改进。