Vortex-enhanced mixing through active and passive flow control methods

This study aims to understand the underlying physics of vortex-enhanced mixing through active and passive flow control methods. To find a best flow control method that enhances turbulent mixing through the generation of streamwise vortices, an experimental investigation was carried out to compare active and passive flow control methods of an incompressible axisymmetric jet. For active flow control, the lip of the circular jet was equipped with a single small flap deflected away from the jet stream at an angle of 30° to the jet axis. The flap incorporated a flow control slot through which steady and oscillatory suction were implemented. The active flow control methods require power input to the suction devices. For passive flow control, the lip of the circular jet was equipped with a single small delta tab deflected into the jet stream at an angle of 30° to the jet axis. The chord lengths of the flap and delta tab were one-sixth of the jet diameter. The momentum of jet increased in the case of active flow control by entraining the ambient fluid, whereas momentum decreased in the case of passive flow control. The effect of steady suction saturated for volumetric suction coefficient values greater than 0.82 %. The strength of streamwise vortices generated by the flap were greater than those generated by the delta tab. Steady suction produced positive pressures just downstream of the flow control slot in the central portion of the flap and negative pressures at the flap edges. Oscillatory suction was highly dependent on dimensionless frequency (F ⁺) based on the distance from the flow control slot to the flap trailing edge; the pressures on the central portion of the flap increased for F ⁺ ≤ 0.11 and then decreased for greater F ⁺; finally attained negative pressures at F ⁺ = 0.44. The increase in jet momentum and turbulence intensity, combined with the induced streamwise vorticity, makes steady suction a potential concept for increasing propulsion efficiency through vortex-enhanced mixing. The flow control methods modify the jet flow, which in turn would alter the jet noise spectra.     

结构动力学方程的二次加速度积分法

本文提出了结构动力学方程求解的一类二次加速度逐步积分法，推导了计算公式，分析了积分稳定性和精度。通过理论分析和具体算例表明，这种方法具有相当高的积分精度，但积分是条件稳定的。     

结构主动控制的LQG/LTR改进方法

提出一种改进的LQG／LTR(Linear Quadratic Gaussian synthesis with a Loop Transfer Recovery)结构主动控制方法,一种新的补偿器结构被用于回路传输恢复(LTR)。这个补偿器有以下优点：(1)它是开环稳定的;(2)它能保证整个闭环系统的稳定性;(3)更重要的是．对于相同的回路传输恢复度,它所需要的增益要小于传统LQG／LTR方法的基于观测器的控制器增益。还有,就是这个新的补偿器比传统的基于观测器要有较好的恢复性能。最后,数值算例验证了本文方法的有效性。     

A dual family of dissipative structure-dependent integration methods for structural nonlinear dynamics

Nonlinear Dynamics - A dual family of dissipative structure-dependent integration methods is proposed for structural nonlinear dynamics. It not only can be a family of two-step integration methods...     

Assessment of shunted piezoelectric devices for simultaneous noise and vibration reduction: comparison of passive,active and hybrid networks

Structural vibration and noise control of a cavity-backed three-layered smart piezo-coupled rectangular panel system under harmonic or transient loads is achieved by using purely active, passive, and hybrid active/passive piezoelectric shunt networks. Problem formulation is based on the classical lamination plate theory, Maxwell’s equation for piezoelectric materials, linear circuit theory, and wave equation for the enclosed acoustic domain. The orthogonal mode expansions along with the modal coupling theory are employed to obtain the coupled differential equations of the electro-mechanical-acoustic system, which are then put into the convenient state-space form, and subsequently solved numerically in both frequency and time domains. A triple-mode hybrid RLC shunt circuit, in series with an external active voltage source and connected to a single electroded piezoelectric segment, is tuned to the dominant resonance frequencies of the composite structure. The linear quadratic optimal control (LQR) theory is adopted for obtaining the active control gains. The frequency and time domain performances of the passive, active and hybrid multi-modal piezoelectric systems are calculated and discussed in terms of sensor output voltage, local sound pressure, and control effort. It is found that the hybrid control methodology with properly tuned circuit parameters can be an excellent candidate for simultaneous vibration and structure-borne noise control of the cavity-coupled smart panel with decreased control effort. Also, the active control strategy integrated in the hybrid control system is demonstrated to enhance the overall system damping characteristics and improve the control authority at frequencies where the passive shunt network performs weakly. Limiting cases are considered and correctness of the mathematical model is verified by using a commercial finite element software as well as by comparisons with the literature.     

Design method of multiple time-delay controller for active structural vibration control

An optimal control method for seismic-excited building structures with multiple time delays is investigated in this paper. The system state equation with multiple time delays is discretized and transformed into a standard discrete form without any explicit time delay by a particular augmenting for state variables. A time-delay controller is then designed based on this standard equation using the discrete optimal control method. Effectiveness of the proposed controller is demonstrated by numerical simulations. Simulation results indicate that a very small time delay may result in the instability of the control system if it is not compensated in the control design. Time delay may be compensated effectively by the proposed controller, in the mean time, an effective control may be obtained. The proposed controller is valid for both small and large time delays.     

The control of ventilated supercavity pulsation and noise

A technique to control ventilated supercavity pulsation and noise is explored analytically and verified experimentally. The technique, which has its roots in parametric oscillators, changes the stiffness and, therefore, resonance frequency of the ventilated supercavity gas/water system by modulating or adding a sinusoidal component to the ventilation rate. This results in the ventilated supercavity effectively being driven off-resonance as the frequency of the interface waves which force the supercavity gas/water system remain largely unchanged. A wide range of ventilation rate modulation frequencies cause the pulsating supercavity to transition into twin vortex closure, typically within 0.25 sec of modulation initiation. Accompanying the transition from pulsation to twin vortex closure is a reduction in the radiated noise, to the continuum at the pulsation frequency, often by 35 dB or more. Other modulation frequencies do not suppress pulsation, but are effective at changing the supercavity pulsation frequency.     

Improving power-extraction efficiency of a flapping plate: From passive deformation to active control

A flapping plate flow energy harvester in a viscous uniform flow is simulated using a two-dimensional numerical approach. Our focus is to study the effects of flexibility and active control on the power-extraction capability of the flapping plate, and consequently to find a strategy to increase its power-extraction efficiency. Four typical cases with optimal kinematics predicted by previous studies are presented and discussed: a rigid plate, a flexible plate, a plate with a flexible leading segment and a rigid trailing segment, and a rigid plate with a simple active control on the leading segment. Our simulations show that with the kinematic parameters considered, the plate flexibility cannot significantly improve the power-extraction capability of the plate while the active control on the leading segment of the rigid plate increases the power coefficient by 11.3%. The analysis of flow fields shows that the changes in the power coefficient and drag force are related to the vortex structures and pressure distributions near the plate, as well as the projection area of the plate in the direction of the translational movement.     

Controlling Rucklidge chaotic system with a single controller using linear feedback and passive control methods

In this paper, the control of chaos with a single controller, which provides simplicity in implementation, is investigated in continuous time nonlinear Rucklidge system. For this purpose, a linear feedback controller and a passive controller are constructed and added to the Rucklidge chaotic system. Lyapunov function is used to realize that the controller ensures the global asymptotic stability of the system. Owing to the controller, Rucklidge chaotic system can be regulated to its equilibrium points. Numerical simulations of the proposed methods and local relay control method have been demonstrated, compared and discussed.     

The efficiency of direct integration methods in elastic contact-impact problems

A comparison of direct integration methods is made and their efficiency is investigated for impact problems. Newmark, Wilson–θ, Central Difference and Houbolt Methods are used as direct integration methods. Impact analysis includes that of elastic and large deformation based upon updated Lagrangian including buckling check. The results show that the direct integration methods give different results in different contact-impact cases.The English text was polished by Keren Wang     

A comparison of integration and interpolation Eulerian-Lagrangian methods

Selected finite element Eulerian-Lagrangian methods for the solution of the transport equation are compared systematically in the relatively simple context of 1D, constant coefficient, conservative problems. A combination of formal analysis and numerical experimentation is used to characterize the stability and accuracy that results from alternative treatments of the concentrations at the feet of the characteristic lines. Within the methods analyzed, those that approach such treatment with the perspective of ‘integration’ rather than ‘interpolation’ tend to have superior accuracy. Exact integration leads to unconditional stability and excellent accuracy. Quadrature integration leads only to conditional stability, but newly derived criteria show that stability restrictions are relatively mild and should not preclude the usefulness of quadrature integration methods in a range of practical applications. While conclusions cannot be extended directly to multiple dimensions and complex flows and geometries, results should provide useful insight to the development and behaviour of specific Eulerian-Lagrangian transport models.     

The integration methods of Vacco dynamics equation of nonlinear nonholonomic system

ＴＨＥＩＮＴＥＧＲＡＴＩＯＮＭＥＴＨＯＤＳＯＦＶＡＣＣＯＤＹＮＡＭＩＣＳＥＱＵＡＴＩＯＮＯＦＮＯＮＬＩＮＥＡＲＮＯＮＨＯＬＯＮＯＭＩＣＳＹＳＴＥＭＬｕｏＳｈａｏｋａｉ（罗绍凯）（ＲｃｅｅｉｖｅｄＡｕｇ．２．１９９３：ｃｏｍｍｕｎｉｃａｔｅｄｂｙＷａｎ...     

The development of advanced computational methods for turbomachinery blade design

The paper describes the basic components of a turbomachinery blade design system in use within Rolls-Royce. A number of modelling aspects of the advanced computational methods in use and under development are reviewed together with areas for future research and development. A quasi-3D blade design system which is used for both compressors and turbines is described covering through-flow and blade-to-blade analysis. Various features of blade-to-blade analysis are discussed including the use of compatible design and analysis modes and coupled boundary layer analysis capable of handling attached and separated flow; examples are included to show capabilities. Advances being made in the development and application of Reynolds-averaged Navier–Stokes models are covered showing capabilities with regard to loss and heat transfer prediction. A fully coupled quasi-3D through-flow and blade-to-blade analysis system is described and results presented to show basic capabilities. The need for 3D flow analysis is discussed and the elements of a 3D blade design system presented showing how this links to the traditional quasi-3D system. Examples are given showing basic capabilities of the methods available and under development. Finally areas for future development are presented indicating the mathematical and numerical modelling problems to be addressed.     

求解桁架结构极值响应的提升投影方法

求解结构的真实极值响应是结构鲁棒优化设计问题的难点,本文提出了求解桁架结构极值响应的新方法.求解结构极值响应的优化模型通常是非凸规划,只有全局最优值才能满足设计要求.然而对于这类非凸规划,求解其全局最优解和最优值是NP困难的问题.在不确定参数为桁架结构的外荷载向量的情况下,本文提出了提升投影方法将原始的非凸规划放松为非线性非光滑的凸规划问题.求解这个凸规划问题的全局最优值就可以得到结构精确极值响应的上界,从而满足了设计要求.数值算例验证了提升投影方法的可行性和高效性.     

Adaptive panel methods and their integration with CAD

The work outlined below presents simple but effective adaptive meshing algorithms for boundary integral methods modelling inviscid flows (panel method) using the IGES standard for describing geometry. By using certain IGES entities in describing the boundary, CAD-derived geometry may be used such that the geometric integrity of the boundary is maintained after an adaptive redistribution of the mesh. Three types of error estimators are tested and all are shown to produce a more accurate representation of the flow phenomena for the same number of panels as compared with a uniform mesh distribution.     

The use of plasma actuators for bluff body broadband noise control

Experiments were conducted using plasma actuators to control broadband noise generated by a bluff body flow. The motivation behind the study was to explore the potential of plasma actuators to reduce landing gear noise during approach phase of an aircraft. The control effectiveness of both dielectric barrier discharge and sliding discharge plasma actuators were tested in laboratory environment, using a representative bluff body consisting of a circular cylinder and an oblique strut. Noise measurements were taken in an anechoic chamber using a phased microphone array and far-field microphones. Results showed that the upstream directed plasma forcing, located at ±90 deg on the upstream cylinder with respect to the approaching flow, could effectively attenuate the broadband noise radiated from the wake flow interaction with the downstream strut. With the same AC electrical power consumption, the sliding discharge with additional DC voltage was found to be more effective due to its elongated plasma distribution and higher induced flow momentum. Measurements using particle image velocimetry suggested that the flow speed impinging on the downstream strut was reduced by the upstream plasma forcing, contributing to the reduced noise.     

Displacement of oil by a solution of an active and a passive additive

A study is made of the frontal displacement of oil from a homogeneous porous medium by a solution of an active additive in the presence in the flow a component (referred to as the passive additive) that does not directly influence the ratio of the mobilities of the water and the oil but does interact with the active additive, changing its adsorbability and (or) its distribution between the phases [1]. Such a situation arises when oil is displaced by solutions of surface-active substances and other reactants, whose effectiveness in application is significantly reduced by their adsorption by the porous medium (if the adsorbed matter itself does not reduce the mobility of the water, as in the case of the polymer in [2]). Under these conditions, it may be worth adding to the solution of the surface-active substance some cheap ballast substance that does not change the ratio of the mobilities of the phases but is capable of taking away from the surface-active substance some of the adsorption centers on the surface of the porous skeleton. Another example of a passive additive is provided by salts dissolved in water that influence the distribution of the active impurity between the water and the oil; this occurs in the case of displacement by carbonated water due to the dependence of the solubility of carbon dioxide gas in water on the mineralization of the water [3]. In the present paper, an investigation is made of the structure of the displacement front, and a technique is developed for constructing a self-similar solution in the case when the adsorbability of the passive additive and its distribution between the phases do not depend on the concentration of the active additive.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 74–83, November–December, 1982.