Determination of principal characteristics of turbulent swirling flow along annuli: part 4: an asymptotic solution

An asymptotic solution of the momentum equation is given that describes the decay of swirling flow passing along the annulus formed between two concentric, straight, circular-section pipes having a common starting point. The flow is considered turbulent and approximations are made consistent with the notion of fully developed flow conditions. Applications of this approach are reviewed and shortcomings highlighted. A series of calculations are presented and compared with experimental and theoretical data previously obtained by the authors. It is shown that acceptable predictions of the overall flow behavior can be obtained over a wide range of initial conditions provided the calculations procedure is applied in regions of validity, which has not been the case in some published work. Substantial errors are found if, for example, the procedure is allowed to commence at inlet to the annulus owing to the inconsistency of the assumptions in the initially developing-flow region, which for this work extends at least five outer-pipe diameters downstream from inlet. The authors' previous numerical integration scheme may be used to predict flows satisfactorily in the developing-flow region and the present asymptotic solution used subsequently to reduce computation time and cost.     

磁阻型电磁发射的建模分析与仿真研究

本文分析了磁阻型电磁发射的机理,得出了发射过程的等效作用.利用磁矩模型建立了磁阻型电磁发射的理论模型,并结合发射电路的模型导出了弹体的动力学方程,分析了影响弹体发射的主要因素,给出了MATLAB仿真结果.     

准光模式变换器研究与设计

研究了将回旋管及其他高功率微波器件的振荡输出模式转换成准高斯波束的准光模式变换器,实现了准高斯模TEM00的横向输出。利用几何光学分析了模式在波导中的传播,进一步分析了伏拉索夫（Vlasov）辐射器的工作机理,运用矢量绕射理论计算出辐射场; 通过编程设计了将94 GHz、模式为TE62的毫米波转化为准高斯波束的准光模式变换器。结果表明：通过设计Vlasov辐射器和两级反射镜,可以在窗口处得到准高斯波束, 整个系统的功率效率达到87.5%。     

Experimental and Numerical Investigation of the Effect of Turbulator on Heat Transfer in a Concentric-type Heat Exchanger

This article experimentally and numerically analyzes the effect of turbulators with different geometries (Type I, Type II, Type III, and Type IV) located at the inlet of the inner pipe in a concentric-type heat exchanger. Experiments were performed at parallel-flow conditions in the same and opposite directions to investigate the impact of manufactured turbulators on heat transfer and pressure drop. In the numerical study, ANSYS 12.0 Fluent code program was used, and basic protection equations were solved in the steady-state, three-dimensional, and turbulence-flow conditions. Results were obtained from numerical analysis conducted at different flow values of air (7, 8, 9, 10, 11, and 12 m³/h). The distribution of temperature, velocity, and pressure was demonstrated as a result of numerical analyses. Experimental and numerical results were compared, and it was observed that they were in conformity with each other. When the data obtained from the analyses were examined, the highest heat transfer, pressure drop, and friction factor increase were detected to be in the Type IV turbulator.     

Concentric toroids as a wideband and efficient substrate for surface-enhanced Raman spectroscopy applications

Designing a structure having high local field enhancement, wideband resonance, and large hot spot area is the key element to obtain a large enhancement factor for surface-enhanced Raman spectroscopy applications. Here, the concentric toroid structures in dimer configuration is proposed, which shows a large local field intensity in a wide spectral range and the region that leads to a high-surface-enhanced Raman scattering signal intensity. Calculations show that the average surface-enhanced Raman scattering enhancement is up to 60 times more compared to the conventional dimer toroid structures with similar size.     

Measurement of properties of a smooth-walled spline-profile feed horn around 840 GHz

For astrophysical applications at submillimeter wavelengths, efficient coupling of the feed horn pattern in the focal plane to the radiation pattern of the telescope is paramount. To date, at 840 GHz with high band width, corrugated feed horns have been used with very good results. The goal of developing instruments for KOSMA above 1 THz where state-of-the-art heterodyne systems are still governed by receiver noise rather than by radiative background, requires fabrication of corrugated structures of the order of λ/3 or less. This is non-trivial, and here we present the performance of a smooth-walled spline-profile feed horn, which is considerably easier to manufacture compared to a corrugated feed horn. The performance is assessed via radiation patterns in H, E and 45° D-Plane in co- and cross-polarization.     

Design of a Quasi-Optical Mode Converter for a Frequency Step-Tunable Gyrotron

The quasi-optical mode converter for a frequency step-tunable gyrotron which consists of a dimpled-wall antenna (Denisov-type launcher) and a beam-forming mirror system has been optimized for 9 modes from TE_17,6 at 105 GHz to TE_23,8 at 143 GHz. The first mirror is a large quasi-elliptical focusing one; the second and third are phase-correcting mirrors with a non-quadratic shape of the surface. The results of calculations show that for these modes the Denisov-type launcher has a well-focused beam with low diffraction losses, and the radiation pattern presents an almost identical field shape for all modes considered. A multi-mode optimization of the phase-correcting mirrors with two different methods has been tested. The simulations show that the phase-correcting mirrors can be used for broadband operation in the frequency range from 105 GHz up to 143 GHz in the various design modes. This quasi-optical mode converter can achieve efficiencies of 94%-98% for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode.     

同心环形光栅环状莫尔条纹在光学系统焦距测量上的应用

本文提出一个确定任何光学系统焦距的普遍方法，并从理论上论述了同心环形光栅环状莫尔条纹的产生机理，推导出由光栅副所产生的环状莫尔条纹测量光学系统焦距的定量表达式，并在实验上给出了测量结果。实验表明，使用该方法非常简便，有很广的应用范围。     

A Variation of Schwarzschild Telescope: Golden Section Solution with Two Concentric Spheres and Its Extension to Finite Distance Solutions

Imaging by two concentric spheres is studied by geometrical method. As the system is symmetric with respect to the common center of curvature, peripheral aberrations, namely coma, astigmatism, and distortion, do not appear. The imaging surface is a sphere that shares the center of curvature with two mirrors. As we solve the zero spherical aberration condition for imaging of an object at infinite distance, the golden section number appears as the ratio of the distance between two mirrors and the radius of the primary mirror. For imaging of an object at finite distance, we have to increase the radius of the second mirror. This system also has no peripheral aberrations. Making the object surface spherical, we can obtain a flat image surface. As the spherical aberration is zero in the original system, spherical aberration, coma, and astigmatism remain zero when the object surface is curved.     

Unsteady Flow Due to Concentric Rotation of Eccentric Rotating Disks

While two parallel disks are initially rotating with the same angular velocity about non-coincident axes, the axes are suddenly made coincident. The development of the flow is examined until the fluid rotates as a rigid body in the steady-state. The velocity field and the shear stress components on the disks are found exactly by a Fourier series solution. Furthermore, a series solution that converges rapidly at small times is obtained with the aid of the Laplace transform technique.