Non-diffuseness of vibration fields in ribbed plates

This paper presents numerical simulations of structural intensity in a rib-reinforced plate, investigating the diffuseness. Many prediction models of building and structural acoustics, such as statistical energy analysis or energy flow methods, assume the vibrational wave fields to be diffuse. However, the diffuseness assumption is not always valid. One such example is a rib-reinforced plate typically found in a lightweight floor with wooden joists. Other examples can be found in aircraft and ship structures. The structural intensity of a ribbed plate is computed at low to mid frequencies using the Fourier sine expansion of the transverse displacement of the plate. Hamilton's principle is used in combination with thin plate theory and Euler beam theory. The model takes into account interactions between components. The Fourier sine modes are re-formulated as plane waves in a radial coordinate system, which can express the structural intensity in terms of the angular component of the modes. In the simulations, ensemble averages and rain-on-the-roof excitations are used. The numerical results show that the structural intensity varies significantly as the angle of propagation changes and cannot be assumed to form a diffuse field.     

CPU散热器数值模拟分析

运用CFD软件FLUENT对等截面直肋散热器侧送风强迫对流换热方式下,不同肋厚、肋高、肋间距的温度场进行模拟,获得了三种参数对散热器散热能力的影响规律和最佳参数;最终得到与计算结果吻合较好的模拟结果,验证了理论分析和数值方法的可靠性。     

Point defect mobility in irradiated GaAs

For the first time the process of primary radiation defect migration in GaAs at T = 300 K was detected using a direct experimental technique. The maximum value of the migration activation energy E_m ≈ 0.34 eV was determined.     

A study of modal characteristics and the control mechanism of finite periodic and irregular ribbed plates

An analytical solution is presented in this paper to investigate the control mechanism and modal characteristics of finite periodic and irregular ribbed plates. Peak responses of a finite periodic ribbed plate were examined where they were grouped into two sets of propagation zones according to the coupling mechanism at beam/plate interfaces. Details of modal characteristics in pass bands of the periodic ribbed plate were elucidated and the control mechanism was discussed. Modes in each pass band that are governed by shear force couplings were characterized by one of the beam flexural modes whose modal responses could be represented approximately by those of the corresponding orthotropic plate modes. Modes in the second set of pass bands were found to retain the resonance frequencies of the corresponding modes of the unribbed base plate. Higher order orthotropic plate modes were also identified, which could not be grouped into any pass bands defined by the classical periodic theory. The control mechanism leading to vibration confinement in disordered and irregular ribbed plates was also discussed. It was found that beam spacing irregularity attributes to localization of the group of modes associated with flexural wave couplings but not the group of modes associated with moment couplings.     

The admittance feature representation and impact sound feature extraction in the material identification of ribbed plates

The admittance features representing the physical attributes are used as the intermediates to extract the material-attributes-related impact sound features of ribbed plates.Firstly, the admittance feature representations of metal ribbed plates attributes are obtained and the relationship between the admittance features and the impact sound features are established via correlation analysis method. Then, material-attributes-related impact sound features are obtained indirectly. Finally, the performances of different sound features for the material recognition of ribbed-metal plates are verified through the Support Vector Machine classifier.The results indicate that the obtained four sets of features can effectively identify the materials of the metal ribbed plates, while the accuracy of a single feature depends on the separable degree of the corresponding material attribute. And the features extracted based on admittance functions have higher average accuracy than that of timbre features. Therefore, the proposed sound feature extraction method based on admittance features is valid, and the extracted sound features can effectively reflect the physical attributes.     

Estimation of broadband power levels radiated from turbulent boundary layer-driven ribbed plates having dissimilar sections

Previous papers developed an approach for estimating the vibration-related broadband acoustic power radiated by a ribbed steel plate in water driven by turbulent boundary layer pressures. This was extended to apply to plates composed of identical ribbed sections joined at infinite impedance supports. In this paper the basic model is modified so that it can be applied to plates composed of dissimilar sections joined at clamped supports. The response is formulated directly in terms of power and the effects of the supports are accounted for by power scattering coefficients, which are calculated through a Wiener-Hopf analysis. Sample calculations are shown to agree with results of other formulations. Only frequencies below bending coincidence are considered.     

Heat transfer and flow characteristics of turbulent slot jet impingement on plane and ribbed surfaces

A computational study is carried out to assess the suitability of various RANS based turbulence models for slot jet impingement on flat and ribbed surfaces with various values of Reynolds number and jet to plate spacing. The com-puted results are compared with the reported experimental data. It was observed that none of the turbulence models considered predicted the heat transfer data accurately. However, some models predicted the experimental data with good trends, e.g., secondary peak and several spikes in Nusselt number for ribbed surface, with a precise computation of the stagnation point Nusselt number. Further, the effects of slot width, rib pitch and jet to ribbed surface spacing were investigated for jet impingement on a ribbed surface. It was observed that the local Nusselt number increased with slot width and rib to plate spacing. It was also observed that increasing Reynolds number had a positive effect on the local heat transfer. With increasing rib pitch the local Nusselt number increased near the stagnation zone but de-creased downstream. The observed flow pattern was different for jet impingement on a ribbed surface than that on a flat surface.     

An approximation for the point mobility at the intersection of two perpendicular plates

The problem of point excitation at a T-intersection of two perpendicular plates is studied in order to establish expressions for the point mobility. It is found that the theory for point excitation of the free surface of a semi-infinite elastic solid is applicable in the frequency range associated with structure-borne sound transmission. From this theory the mobility for an infinite system is derived. Based on this model and on an experimental investigation an estimation procedure for the point mobility in the finite dimension case is developed. The agreement with measurements performed in situ is quite acceptable. Both the theoretical and the experimental investigations reveal that the real part of this mobility is small, although it is larger in the experimental results. This indicates that other components of excitation are difficult to eliminate and may contribute to the power input in practice. Because of the small real part of the mobility it is advantageous with respect to the reduction of structure-borne sound power transmission to locate the contact points between a source and the receiver at such intersections. Corrections are deduced for the measured magnitude of the mobility for the case when separate force and motion transducers are used.     

R410A和R22在水平内螺纹管内冷凝性能的实验研究

实验研究了环保替代制冷工质R410A和R22在冷凝温度40C时在内螺纹强化管(外径为9.52mm)内的冷凝换热特性,对二者的冷凝换热性能进行了对比,并研究了测试管外冷却水流量对换热系数的影响.结果表明:在管外冷却水流量相同时,R22的总换热系数K普遍比R410a小,而管内传热系数h,比R410A大.R22与R410A的总传热系数K均随管外冷却水流量的增加而增加,当制冷剂流量Gm大于300kg.s-1.m-2时,管外冷却水流量对总传热系数K的影响变小.     

Point of impact prediction in isotropic and anisotropic plates from the acoustic emission data

It is shown in this paper that the conventional triangulation technique is not very reliable for locating the impact point even in isotropic plates when the sensors are placed close to the point of strike for two reasons: First, it is difficult to pinpoint the exact time of arrival of the signal and, second, the Lamb modes in a plate are dispersive. Dispersive signals attenuate differently at various frequencies and propagate with different speeds causing distortions in the received signals, and thus introduce error in the time of flight measurement. The triangulation technique assumes that wave speeds in all directions are the same, which is not true for anisotropic plates. Here an alternative approach based on an optimization scheme is proposed to locate the point of impact in isotropic and anisotropic plates. A formulation is presented for the general anisotropic case. Experiments are carried out with an aluminum plate by dropping balls on the plate and picking up acoustic signals at different locations. The impact points predicted by the conventional triangulation technique and the proposed modified method are compared for this isotropic plate. Then it is investigated how the prediction would change if the plate is assumed to have some anisotropy.     

Laminar free convection heat transfer between vertical isothermal plates

The paper represents results on numerical investigation of flow and heat transfer between two isothermal vertical plates under laminar natural convection. A system of complete Navier–Stokes equations is solved for a two-dimensional gas flow between the plates along with additional rectangular regions (connected to inlet and outlet sections), whose characteristic sizes are much greater than the spacing between the plates. The calculations were performed over very wide ranges of Rayleigh number Ra = 10 ÷ 10⁵ and a relative channel length AR = L/w = 1 ÷ 500. The influence of the input parameters on the gas-dynamic and thermal structure of thermogravitational convection, the local and mean heat transfer, and also the gas flow rate between the plates (convective draft. We determined sizes of the regions and regime parameters when the local heat flux on the walls tends to zero due to the gas temperature approach to the surface temperature. It is shown that the mean heat transfer decreases as the relative channel length AR grows, whereas the integral gas flow rate (convective draft) and Reynolds number in the channel Re = 2wUm/ν increase. The use of a modified Rayleigh number Ra* = Ra · (w/L) (Elenbaas number) leads to generalization of calculation data on mean heat transfer. These data are in good agreement with the correlations for heat transfer [1, 2] and gas flow rate [3]. The reasons of variation of the data in the range of low Rayleigh numbers are discussed in detail.     

Point mobility of a cylindrical plate incorporating a tapered hole of power-law profile

The paper describes the results of experimental measurements of point mobility carried out on circular plates containing tapered holes of quadratic power-law profile with attached damping layers. The obtained results are compared to the developed numerical model, as a means of validation. The profiles of the tapered hole in the plates are designed to replicate near zero reflection of quasi-plane waves from a tapered hole in geometrical acoustics approximation, also known as acoustic black hole effect. The driving point mobility measurements are provided, showing a comparison of the results for a constant thickness circular plate, a constant thickness plate with a layer of damping film applied and a plate with a quadratic power-law profile machined into the center, which is tested with a thin layer of elastic damping material attached. The results indicate a substantial suppression of resonant peaks, agreeing with a numerical model, which is based on the analytical solution available for the vibration of a plate with a central quadratic power-law profile. The paper contains results for the case of free boundary conditions on all edges of the plates, with emphasis placed on the predictions of resonant frequencies and the amplitudes of vibration and loss factor.     

An extended transfer matrix-finite element method for free vibration of plates

A combination of extended transfer matrix and finite element methods is proposed for obtaining vibration frequencies of structures. This method yields the value of the frequency once a trial value is assumed. By using this technique, the number of nodes required in the regular finite element method is reduced and therefore a smaller computer can be used. Besides, no plotting of the values of the determinants corresponding to each assumed frequency is necessary. A worked example is given for the case of vibration of a cantilever plate. The results show fast convergence from the assumed value to the true natural frequency.     

Dynamics of mass transfer caused by the photoinduced spatially inhomogeneous modulation of mobility in a multicomponent medium

The reasons behind the three-dimensional mass transfer of the inert components of a mixture in which photoinduced spatially inhomogeneous polymerization takes place are discussed. Such a mass transfer is responsible for the laser-induced formation of controllable diffraction gratings in polymer composites containing liquid crystals [R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, and T. J. Bunning, Chem. Mater. 5, 1533 (1993); R. L. Sutherland, V. P. Tondiglia, and L. V. Natarajan, Appl. Phys. Lett. 64, 1074 (1994); R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, et al., J. Nonlinear Opt. Phys. Mater. 5, 89 (1996); R. Caputo, A. V. Sukhov, and C. Umeton, Mol. Mater. 12, 192 (1999)]. The semiphenomenological model proposed in this work is based on the assumption that mass transfer results from conventional Fickian diffusion which, however, does not occur over the entire volume of the sample, but only in regions free from clusters of long polymer chains. It is shown that such a “restriction of the active volume” is responsible for the conventional diffusion of inert components of liquid crystals as well as reagents and low-molecular products (short chains), in spite of the initial spatial homogeneity of concentrations in the sample. The qualitative predictions of the model coincide with the experimental results [R. Caputo, A. V. Sukhov, and C. Umeton, Mol. Mater. 12, 192 (1999)].     

Combined radiant and convective heat transfer to laminar steam flow between gray parallel plates with uniform heat flux

The non-linear integro-differential equations descrining combined radiation and convection heat transfer to non-gray non-isothermal steam in laminar flow between gray parallel plates are developed and solved numerically. The results are discussed and compared with a gray, linear, analytical solution and with available experimental data.     

Field-effect transistors with high mobility and small hysteresis of transfer characteristics based on CH3NH3PbBr3 films

Physics of the Solid State - Field-effect transistor (FET) structures based on soluble organometallic perovskites, CH3NH3PbBr3, were obtained and their electrical properties were studied. FETs made...     

An analytical and experimental study of the vibration response of a clamped ribbed plate

An analytical solution is presented in this paper for the vibration response of a ribbed plate clamped on all its boundary edges by employing a traveling wave solution. A clamped ribbed plate test rig is also assembled in this study for the experimental investigation of the ribbed plate response and to provide verification results to the analytical solution. The dynamic characteristics and mode shapes of the ribbed plate are measured and compared to those obtained from the analytical solution and from finite element analysis (FEA). General good agreements are found between the results. Discrepancies between the computational and experimental results at low and high frequencies are also discussed. Explanations are offered in the study to disclose the mechanism causing the discrepancies. The dependency of the dynamic response of the ribbed plate on the distance between the excitation force and the rib is also investigated experimentally. It confirms the findings disclosed in a previous analytical study [T.R. Lin, J. Pan, A closed form solution for the dynamic response of finite ribbed plates, Journal of the Acoustical Society of America 119 (2006) 917–925] that the vibration response of a clamped ribbed plate due to a point force excitation is controlled by the plate stiffness when the source is more than a quarter plate bending wavelength away from the rib and from the plate boundary. The response is largely affected by the rib stiffness when the source location is less than a quarter bending wavelength away from the rib.