LINEAR ACTIVE STRUCTURES AND MODES （ Ⅱ ） — DISCRETE SYSTEMS AND BEAMS

The basic concepts about the active structures and some attributes of the modes were presented in paper “Liner Active Structures and Modes ( Ⅰ ) “. The characteristics of the active discrete systems and active beams were discussed, especially, the stability of the active structures and the orthogonality of the eigenvectors. The notes about modes were portrayed by a model of a seven-storeyed building with sensors and actuators. The concept of the adjoint active structure was extended from the discrete systems to the beams that were the representations of the continuous structures. Two types of beams with different placements of the measuring and actuating systems were discussed in detail. One is the beam with the discrete sensors and actuators, and the other is the beam with distributed sensor and actuator function. The orthogonality conditions were derived with the modal shapes of the active beam and its adjoint active beam. An example shows that the variation of eigenvalues with feedback amplitude for the homo-configuration and non-homo-configuration active structures.     

Material Property Differentiation in Indentation Testing Using Secondary Sensors

Current in vivo and in situ testing procedures are dominated by indentation. The major challenge for this testing technique is in finding a unique solution to the “inverse problem” i.e., defining an appropriate constitutive framework and obtaining material properties consistent with the indentation force–displacement data. Much of the information related to the interplay between shear and bulk compliance in the deformation field beneath the indenter is lost when capturing this single output. We propose a material testing method that follows the well proven path of conventional indentation methods, but enriches the signal by acquiring displacement data not only for the actuated indenter, but also for a set of offset, passive secondary sensors. We use finite element (FE) simulations involving three cases of materials: (a) linear elastic, (b) hyperelastic and (c) time-dependent to demonstrate the benefit of these additional sensors. The results indicate that the addition of these secondary sensors can help to discern between materials with varying degrees of compressibility.     

On the accuracy of simultaneously measuring velocity component statistics in turbulent wall flows with arrays of three or four hot-wire sensors

A highly resolved turbulent channel flow direct numerical simulation (DNS) with Re _τ = 200 has been used to investigate the ability of probes made up of arrays of three or four hot-wire sensors to simultaneously and accurately measure statistics of all three velocity components in turbulent wall flows. Various virtual sensor arrangements have been tested in order to study the effects of position, number of sensors and spatial resolution on the measurements. First, the effective cooling velocity was determined for each sensor of an idealized probe, where the influence of the velocity component tangential to the sensors and flow blockage by the presence of the probe are neglected. Then, simulating the response of the virtual probes to obtain the effective velocities cooling the sensors, velocity component statistics have been calculated neglecting the velocity gradients over the probe sensing area. A strong influence of both mean and fluctuation velocity gradients on measurement accuracy was found. A new three-sensor array configuration designed to minimize the influence of the velocity gradients is proposed, and its accuracy is compared to two-sensor X- and V-array configurations.     

Silicon based flow sensors used for mean velocity and turbulence measurements

Small and directional sensitive silicon based sensors for velocity measurements have been designed and fabricated using microelectronic technology. Single-chip as well as double-chip sensors for the determination of mean velocity and turbulent stresses have been developed. To determine the performance of these silicon sensors, comparisons with conventional hot-wire sensors were done in a well-defined two-dimensional turbulent flat plate boundary layer at a constant Reynolds number of 4.2 · 10⁶. All the silicon sensors were found to have a spatial and frequency resolution that makes them suitable for turbulence measurements. In the studied flow field the measured profiles of mean velocities and Reynolds stresses of all silicon sensors show the same accuracy as corresponding hot-wire measurements. The silicon sensors are also shown to operate with good resolution even when the temperature of the heated part of the chip is reduced considerably.     

Polyvinylidene fluoride film sensors for measurement of unsteady pressures on aerodynamic surfaces Part II: Application to a small-scale marine propeller in spatially distorted mean flow

 Specially-developed Polyvinylidene Fluoride film sensors were used to measure the unsteady surface pressure on a small-scale marine propeller. Pressure response of the propeller blades to a spatially-varying-mean axial velocity flowfield was measured. Raw data were corrected for the thermally-induced sensor output in the nominally stratified flow with the aid of a blade-mounted constant-current anemometer. Corrected pressure measurements showed a phase lag in the blade pressure response relative to the mean velocity cycle. The necessity to consider thermally-induced sensor response was demonstrated.     

A carbon nanotube sensor for wall shear stress measurement

A novel carbon nanotube (CNT) sensor is being developed to measure the mean and fluctuating wall shear stress (WSS) in a turbulent boundary layer. The CNT WSS sensor is based on the thermal principle and featured by high spatial and temporal resolutions (in the order of nm and kHz, respectively), low power consumption (in the order of μW), and a compact fabrication process compared with traditional WSS measurement sensors. The CNT WSS-sensing element was characterized in detail before its calibration. The CNT sensor was operated under a constant temperature (CT) operation mode and an overheat ratio range of −0.15 to −0.19 and calibrated in a fully developed turbulent channel flow. It has been observed for the first time in a macroscopic flow that the sensor output power is approximately proportional to the 1/3 powered WSS, as expected for a thermal-principle-based WSS sensor, and the wall shear stress measurement is demonstrated for a low Reynolds number flow.     

Prediction of separation flows around a 6：1 prolate spheroid using RANS/LES hybrid approaches

This paper presents hybrid Reynolds-averaged Navier–Stokes (RANS) and large-eddy-simulation (LES) methods for the separated flows at high angles of attack around a 6:1 prolate spheroid. The RANS/LES hybrid methods studied in this work include the detached eddy simulation (DES) based on Spalart–Allmaras (S–A), Menter’s k–ω shear-stress-transport (SST) and k–ω with weakly nonlinear eddy viscosity formulation (Wilcox–Durbin+, WD+) models and the zonal-RANS/LES methods based on the SST and WD+ models. The switch from RANS near the wall to LES in the core flow region is smooth through the implementation of a flow-dependent blending function for the zonal hybrid method. All the hybrid methods are designed to have a RANS mode for the attached flows and have a LES behavior for the separated flows. The main objective of this paper is to apply the hybrid methods for the high Reynolds number separated flows around prolate spheroid at high-incidences. A fourth-order central scheme with fourth-order artificial viscosity is applied for spatial differencing. The fully implicit lower–upper symmetric-Gauss–Seidel with pseudo time sub-iteration is taken as the temporal differentiation. Comparisons with available measurements are carried out for pressure distribution, skin friction, and profiles of velocity, etc. Reasonable agreement with the experiments, accounting for the effect on grids and fundamental turbulence models, is obtained for the separation flows. The project supported by the National Natural Science Foundation of China (10502030 and 90505005).     

Attitude control for part actuator failure of agile small satellite

The stability and singularity problem of agile small satellite （ASS） with actuator failure is discussed in this paper. Firstly, the three-axis stabilized controller of an ASS is designed, where micro control moment gyros （MCMG＇s） in pyramid configuration （PC） is used as the actuator. By using the same controller and steering law, the control results before and after one gyro fails are compared by simulation. The variation of singular momentum envelope before and after one gyro fails is also compared. The simulation results show that the failure intensively decreases the capacity of output torque, which leads to the emergence of more singular points and the rapid saturation of MCMG＇s. Finally, the parameters of system controller are changed to compare the control effect.     

A, φ-Ω METHOD FOR 3-D EDDY CURRENT ANALYSIS

After the field equations and the snonumuoo conditions between the interfaces for 3D eddy current problems Under various gauges were discussed, it was pointed cut in this paper that using the magnetic vector potential A. the electric scalar potential φ and Coulomb gauge ∇·A = 0 in eddy current regions and using the magntetic scalar potential Ω in the non-conducting regions are more suitable. All field equations, the boundary conditions, the interface continuity conditions and the corresponding variational principle of this method are also given.     

Embedded-strain-sensor development for composite smart structures

The winding or layup procedure for fiber-reinforced composites lends itself to convenient installation of embedded sensors during fabrication. These permanently installed and protected sensors could be used during the service lifetime of the structure to monitor real-time conditions and determine when loading or vibration is excessive, and when damage has occurred. Such ‘smart or intelligent’ structures could be used to provide continuous ‘health monitoring’ of the structure as well as provide input for active vibration control. In the present study, two sizes of constantan wire (0.15-mm and 0.025-mm diameter) with a very thin but tough polyimide insulation were embedded in graphite-epoxy bars and tubes. The 25-mm by 2.5-mm by approximately 300-mm long bars were fabricated from hand-laid-up panels and subjected to static four-point bending and cantilever bending. The tubes (42-mm diameter by 1.25-m long) were subjected to static cantilever bending. Output from the constantan wire was monitored with conventional strain-gage indicators. Results indicate accurate tensile and compressive measurements of the integrated strain along the length of the constantan wire when compared with beam formulas and surface mounted strain gages. The constantan strain wire shows promise as an embedded sensor for ‘smart structures’.     

Optimal-contour design for a plane hypersonic engine output unit with account for the boundary layer, length, outer flow, and base end effects

The direct method of solving the variational problem for determining the optimal contour of a jet engine output unit with account for gas viscosity is developed. The optimal contours of two-dimensional output units with account for the boundary layer, length, outer flow, and base end effects are calculated numerically. In particular, the influence of turbulent boundary layers on the shape and thrust of the optimal output unit is studied. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 174–184, July–August, 2000.     

Centralised Versus Decentralised Active Control of Boundary Layer Instabilities

We use linear control theory to construct an output feedback controller for the attenuation of small-amplitude three-dimensional Tollmien-Schlichting (TS) wavepackets in a flat-plate boundary layer. A three-dimensional viscous, incompressible flow developing on a zero-pressure gradient boundary layer in a low Reynolds number environment is analyzed using direct numerical simulations. In this configuration, we distribute evenly in the spanwise direction up to 72 localised objects near the wall (18 disturbances sources, 18 actuators, 18 estimation sensors and 18 objective sensors). In a fully three-dimensional configuration, the interconnection between inputs and outputs becomes quickly unfeasible when the number of actuators and sensors increases in the spanwise direction. The objective of this work is to understand how an efficient controller may be designed by connecting only a subset of the actuators to sensors, thereby reducing the complexity of the controller, without comprising the efficiency. If n and m are the number of sensor-actuator pairs for the whole system and for a single control unit, respectively, then in a decentralised strategy, the number of interconnections deceases mn compared to a centralized strategy, which has n ² interconnections. We find that using a semi-decentralized approach – where small control units consisting of 3 estimation sensors connected to 3 actuators are replicated 6 times along the spanwise direction – results only in a 11 % reduction of control performance. We explain how “wide” in the spanwise direction a control unit should be for a satisfactory control performance. Moreover, the control unit should be designed to account for the perturbations that are coming from the lateral sides (crosstalk) of the estimation sensors. We have also found that the influence of crosstalk is not as essential as the spreading effect.     

MODAL SYNTHESIS METHOD FOR NORM COMPUTATION OF H∞ DECENTRALIZED CONTROL SYSTEMS （II）

When using H∞ techniques to design decentralized controllers for large systems, the whole system is divided into subsystems, which are analysed using H∞ control theorybefore being recombined. An analogy was established with substructural analysis instructural mechanics, in which H∞ decentralized control theory corresponds to substructuralmodal synthesis theory so that the optimal H∞ norm of the whole system corresponds to thefundamental vibration frequency of the whole structure. Hence, modal synthesismethodology and the extended Wittrick-Williams algorithm were transplanted from structuralmechanics to compute the optimal H∞ norm of the control system. The orthogonality and theexpansion theorem of eigenfunctions of the subsystems H~ control are presented in part (I)of the paper. The modal synthesis method for computation of the optimal H∞ norm ofdecentralized control systems and numerical examples are presented in part (Ⅱ).     

Generation and registration of disturbances in a gas flow. 1. Formation of arrays of tubular microheaters and microsensors

A new method is proposed for creating “smart” surfaces for suppressing turbulence and retaining a laminar supersonic flow. Methods of formation of super-fast-response sensors and actuators for such surfaces are developed. Such sensors and actuators are structurally designed as microtubes made of SiO ₂ /Si ₃ N ₄ /Au and InGaAs/GaAs/Au heterofilms and suspended above a substrate; the wall thickness of these tubes is in the nanometer range; the tubes are connected to electrical contacts. Models of distributed arrays of tubular microsensors and microactuators are fabricated in a single technological process, which involves the well-established planar technology and the technology of rolling of stressed heterofilms. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 2, pp. 145–151, March–April, 2009.     

The surface hot wire as a means of measuring mean and fluctuating wall shear stress

 The paper describes some applications of a wall shear stress sensor technique which is based on hot-wire anemometry. The “surface hot wire” is a flush-mounted thermal resistive wire with a tiny slot underneath. The arrangement of this sensor guarantees an improved signal-to-noise ratio compared to a common surface hot film. The setup and the application of single sensors and of surface hot-wire arrays are shown. Some results are presented that were acquired in several experiments in the field of laminar-turbulent transition. Received: 26 May 2000/Accepted: 7 February 2001     

On the spatial resolution of velocity and velocity gradient-based turbulence statistics measured with multi-sensor hot-wire probes

A highly resolved turbulent channel flow direct numerical simulation with Re _τ = 200 has been used to investigate the ability of 12-sensor hot-wire probes to accurately measure velocity and velocity gradient based turbulence statistics. Various virtual sensor separations have been tested in order to study the effects of spatial resolution on the measurements. First, the effective cooling velocity has been determined for each sensor for (1) an idealized probe where the influence of the velocity component tangential to the sensors and flow blockage by the presence of the prongs and the finite lengths of and thermal cross-talk between the sensors are neglected and, (2) for a real probe, the characteristics of which have been determined experimentally. Then, simulating the response of the virtual probes for these two cases to obtain the effective velocities cooling the sensors, velocity and vorticity component statistics have been calculated by assuming the velocity gradients to be constant over the probe sensing area.     

Temperature and strain measurement by combining ILFE and bragg grating optical fiber sensors

This paper presents two optical fiber sensor configurations that are capable of simultaneously measuring temperature and strain. These sensor configurations use serial and parallel combinations of in-line fiber etalon (ILFE) and in-fiber Bragg grating sensors, along with wavelength division multiplexing concepts. The results obtained while simultaneously varying the temperature over 130°C and strain field over 1500 με showed favorable agreement with thermistors and resistance strain gages.     

Development of an array of pressure sensors with PVDF film

 A new type of an array of pressure sensors, composed of PVDF (polyvinylidenefluoride), was devised and evaluated. In order to obtain the system transfer function of the PVDF system, a dynamic calibration was performed utilizing the signal from a 1/8 inch B&K microphone as input. The time history of the unsteady pressure was then reconstructed from the output of the sensor by using this transfer function. The reconstructed pressure signals showed good agreement with the reference signal in both temporal and spectral sense. This sensor was then used to measure the wall pressure fluctuations in a two-dimensional turbulent channel flow. Various statistical moments were obtained from the measurement data set. Comparison of these quantities with the existing studies demonstrated satisfactory agreement. These tests give credence to the relevance and reliability of this sensor for applications in more complicated turbulent flows. Received: 13 August 1996 / Accepted: 26 January 1998     

The forced electroelastic vibrations of a planar piezoelectric transformer of longitudinal-transverse type

The electroelastic vibrations of a classical longitudinal-transverese piezotransformer are considered. A refined formula is derived for the no-load transformation ratio with regard for the difference of the elastic compliances in the input and output sections. Satisfactory matching of calculated and experimental data is observed for the first four vibration modes. S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 36, No. 7, pp. 92–98, July, 2000.