Phase change and cooling characteristics of microjets measured using microcantilever heaters

This paper reports novel microcantilever metrology tools to investigate free microjets emanating from a micromachined nozzle of diameter 10 μm. Microcantilevers with an integrated resistive heater-thermometer were used to study microjet cooling and phase change characteristics. While the microcantilever heater was aligned to and impinged upon by hydrocarbon microjets, the heater temperature was modulated by a constant voltage source. Successive heating and cooling cycles captured the microjet phase change hysteresis properties. When the microcantilever heater temperature was controlled, it was possible to construct a full boiling curve. Measurements were made on microjets of butane, hexane, and octane. The convective heat fluxes accompanied by microjet impingement boiling were 2.9–7.6 kW/cm² for heater temperatures of 25–350 °C. Overall, the techniques reported herein show promise for characterizing microscale jet flows.     

Investigation and modeling of the structure of texting language

Language usage over computer mediated discourses, such as chats, emails and SMS texts, significantly differs from the standard form of the language and is referred to as texting language (TL). The presence of intentional misspellings significantly decrease the accuracy of existing spell checking techniques for TL words. In this work, we formally investigate the nature and type of compressions used in SMS texts, and develop a Hidden Markov Model based word-model for TL. The model parameters have been estimated through standard machine learning techniques from a word-aligned SMS and standard English parallel corpus. The accuracy of the model in correcting TL words is 57.7%, which is almost a threefold improvement over the performance of Aspell. The use of simple bigram language model results in a 35% reduction of the relative word level error rates.     

Investigation and application of Laplace spectra of orgraphs with the ring structure

The Laplace matrix is a square matrix L = (? _ij ) ∈ ? ^n×n in which all nondiagonal elements are nonpositive and all row sums are equal to zero. Each Laplace matrix corresponds to a weighted orgraph with positive arc weights. The problem of reality of Laplace matrix spectrum for orgraphs of a special type consisting of two “counter” Hamiltonian cycles in one of which one or two arcs are removed is studied. Characteristic polynomials of Laplace matrices of these orgraphs are expressed through polynomials Z _n (x) that can be obtained from Chebyshev second-kind polynomials P _2n (y) by the substitution of y ² = x. The obtained results relate to properties of the product of Chebyshev second-kind polynomials. A direct method for computing the spectrum of Laplace circuit matrix is given. The obtained results can be used for computing the number of spanning trees in orgraphs of the studied type. One of the possible practical applications of these results is the investigation of topology and development of new Internet protocols.     

Analysis of unsteady supersonic two-phase flows by the particle-in-cell method

A procedure for analyzing the unsteady flow of gas-particle suspensions, based on the Particle-In-Cell (PIC) method, is presented. Interactions between the phases are incorporated in the Lagrangian portion of the PIC calculation, while discrete ‘mass points’ are used to represent each of the phases of the suspension during the material transport. Test calculations presented to demonstrate the properties of the PIC method as applied to suspension flows include studies of an impulsively accelerated solid boundary moving with constant velocity into a suspension initially at rest, and of suspension flows in shock tubes. Shock tube calculations are also used to demonstrate the need for a fine mesh in order to resolve details of the relaxation zone behind the shock, and to investigate instabilities arising from too large a time increment.     

Design of supersonic airfoils by numerical optimization

A practical procedure for the design of low-drag supersonic airfoils is demonstrated, using an optimization program based on a gradient algorithm coupled with an aerodynamic analysis program which incorporates a unitary compression/ expansion formula for inviscid C_p distribution valid over a wide range of supersonic Mach numbers. Results are presented for low-drag nonlifting supersonic airfoils.     

Relationship of a Landsat cumulative disturbance index to canopy nitrogen and forest structure

Spatially-explicit knowledge of the timing, frequency, and intensity of forest disturbances is essential for forest management, yet little is known about how disturbances such as forest harvests and insect outbreaks might accumulate in their effects over time. Capturing the many forest harvest and insect defoliation events occurring over twenty-five years, we transformed a series of Landsat images into cumulative disturbance maps covering Green Ridge State Forest (GRSF) and Savage River State Forest (SRSF) in western Maryland. These maps summed yearly ΔDI images, which were defined as the change in a yearly tasseled cap disturbance index (DI), relative to a synthetic reference condition map created by finding the minimum DI value for all years. Intensive field-plot surveys and AVIRIS imagery collected during the summer of 2009 provided measurements of forest structure and canopy nitrogen. With these data, we found that while the most recent year's ΔDI had little relation, increases in the cumulative DI were related to decreased field-measured current canopy cover (R² = 0.66 at GRSF, 0.67 at SRSF and 0.34 combined) and watershed-averaged AVIRIS canopy N (R² = 0.40 at GRSF, 0.57 at SRSF and 0.54 combined). The latter relationship was obscured at the field-plot level of analysis, suggesting that fine scale studies will also need to account for other drivers (e.g. species composition) of variability in canopy N. Nevertheless, our study demonstrates that Landsat time series data can be synthesized into cumulative metrics incorporating multiple disturbance types, which help explain important cumulative disturbance-mediated changes in ecosystem functioning.     

多超音速目标被动声定位技术研究

首先提出了基于激波信号的超音速目标被动声定位模型,针对多超音速目标被动声定位研究存在的困难,提出了一种基于遗传算法的多目标定位技术,解决了多目标被动声定位时的虚假目标问题。通过采用多测量站信息融合的方法,解决了定位系统对某些方向的目标存在探测精度低的问题。系统仿真结果表明,该方法可以提高定位系统的精度和可靠性。     

Numerical investigation of supersonic flow for axisymmetric cones

An existing curvilinear finite-volume code with robust shock-capturing scheme was modified to allow for simulations of supersonic flow for axisymmetric cone geometries. It is shown how for an axisymmetric coordinate system the convective and viscous flux derivatives in the circumferential direction reduce to a y-momentum equation source term. The advantage of this approach over an axisymmetric code is that the governing equations and the discretization do not need to be changed. This paper provides a detailed derivation of the axisymmetric source terms from the full Navier-Stokes equations. Results are shown for a sharp and a blunt cone for approach flow Mach numbers of M = 3.5 and M = 7.99.     

利用声波进行流量测量的原理及应用

本文阐述了利用声波进行流量测量的原理及特点,并介绍了超声波流量计的应用及应用进展。     

Investigation of Ni-based thermal bimaterial structure for sensor and actuator application

Thermal bimaterial structures made of Ni and Ni-diamond nanocomposite for sensor and actuator application are proposed, fabricated, and tested. Two deflection types of thermal bimaterial structures, including upward and downward bending types, can be easily fabricated by controlling electroplating sequence of Ni and Ni-diamond nanocomposite. According to thermal performance measurement, the tip deflection of upward and downward types can reach about 82.5 μm and ?22.5 μm for a temperature change of 200 °C, respectively. In the condition, the thermomechanical sensitivity and output force are 412.5 nm/K and 97.0 μN for upward type thermal bimaterial structure; and ?112.5 nm/K and ?26.5 μN for downward type one. Due to the low electroplating process temperature (～50 °C) for both Ni-based layers, diminutive pre-deformation of as-fabricated structure and strong interlaminar bonding strength are verified by SEM and vibrational test. The resonant frequency of the structure remains unchanged after 10⁹ cycles.     

Investigation of novel tapered hybrid defected ground structure (DGS)

Novel defected ground structures (DGSs) have been investigated to achieve improved low‐pass filter (LPF) properties. A chronological development in designs ranging from conventional square‐patterned photonic‐bandgap structure (PBGS) and conventional dumbbell‐shaped DGS to proposed novel nonuniform DGS has been investigated theoretically and experimentally using computer‐aided design (CAD) tools. It can be seen that the proposed novel DGS having Chebyshev distribution provides excellent performances in terms of ripples in the passband, 10‐dB return‐loss bandwidth (RL‐BW), and 20‐dB rejection bandwidth, as compared to conventional DGSs and PBGSs. Thus, a perturbed surface below a standard 50Ω microstrip transmission line provides LPF characteristics. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2005.     

A contribution to the computation of transonic supersonic flows over blunt bodies

Stationary inviscid transonic supersonic flowfield around sphrese, ellipsoids and hemisphere-cylinders are calculated. The freestream Mach numbers considered are between $\text{M}{}_{\mathrm{\infty }}\text{= 1.02}$ and $\text{M}{}_{\mathrm{\infty }}\text{= 1.1}$. A special coordinate system is created which is adjusted to the subsonic stagnation point region for transonic freestream Mach numbers. The integration of the governing equations is carried out by means of a time-dependent finite-difference procedure. All the calculations discussed were stable and converged uniquely to the asymptotic stationary state. The influence of an artificial dissipation term added to the finite-difference scheme is studied. It is further shown, that in all cases the limiting characteristics come from the front part of the bodies. The results include bow shocks, sonic lines, characteristics, lines of constant pressure, density and Mach numbers. A comparison is made between such flow quantities which are calculable by analytical functions and the predicted ones. The quality of the results is checked by considering the conservation of the total enthalpy. For some examples the present solutions are compared with other theories and experimental data.     

利用声波进行流量测量的原理及应用

本文阐述了利用声波进行流量测量的原理及特点,并介绍了超声波流量计的应用及应用进展.     

Identification of large coherent structures in supersonic axisymmetric wakes

Direct numerical simulation data of supersonic axisymmetric wakes are analysed for the existence of large coherent structures. Wakes at Ma=2.46 are considered with results being presented for cases at Reynolds numbers Re_D=30,000 and 100,000. Criteria for identification of coherent structures in free-shear flows found in the literature are compiled and discussed, and the role of compressibility is addressed. In particular, the ability and reliability of visualisation techniques intended for incompressible shear-flows to educe meaningful structures in supersonic wakes is scrutinised. It is shown that some of these methods retain their usefulness for identification of vortical structures as long as the swirling rate is larger than the local compression and expansion rates in the flow field. As a measure for the validity of this condition in a given flow the ‘vortex compressibility parameter’ is proposed which is derived here. Best ‘visibility’ of coherent structures is achieved by employing visualisation techniques and proper orthogonal decomposition in combination with the introduction of artificial perturbations (forcing of the wake). The existence of both helical and longitudinal structures in the shear layer and of hairpin-like structures in the developing wake is demonstrated. In addition, elongated tubes of streamwise vorticity are observed to emanate from the region of recirculating flow.     

Multiresolution analysis of density fluctuation in supersonic mixing layer

Due to the difficulties in measuring supersonic density field, the multiresolution analysis of supersonic mixing layer based on experimental images is still a formidable challenge. By utilizing the recently developed nanoparticle based planar laser scattering method, the density field of a supersonic mixing layer was measured at high spatiotemporal resolution. According to the dynamic behavior of coherent structures, the multiresolution characteristics of density fluctuation signals and density field images...     

Modeling a supersonic flowfield in the core of the wingtip vortex at Mach 6

In this paper numerical calculations of a wingtip vortex for various wing angles of attack are presented, and the resulting flow modes are compared with each other and with the experimental data. Numerical computations were performed at the Keldysh Institute of Applied Mathematics, making use of the Spalart-Allmaras turbulence model. Experiments were carried out at the Institute of Theoretical and Applied Mechanics, Siberian Branch (SB), Russian Academy of Sciences (RAS), Khristianovich, Russia.     

Sliding mode robust control of supersonic three degrees-of-freedom airfoils

The robust aeroelastic control of a three-degrees-of-freedom (3DOF) linear and non-linear wing-flap system under sliding mode control (SMC) and operation in supersonic flight speeds is presented. Open- and closed-loop aeroelastic responses to blast and sonic-boom excitation in the wing-flap system with uncertainty, as well as flutter analysis, are investigated along with the implementation of a Sliding Mode Observer (SMO). The effectiveness of control in reducing the amplitude of oscillation and preventing flutter instability is demonstrated.     

超音速下热壁板的颤振分析

首先研究了热效应对壁板结构动力学特性的影响.将基于超音速活塞理论的非定常气动力模型与壁板的结构动力学方程相结合,得到了热壁板的颤振方程.利用p-k法进行了热壁板的颤振计算,讨论了气流偏角对颤振速度的影响.数值结果表明,热效应对壁板的固有特性有较大影响,进而影响壁板的颤振特性.