Influence of air-jet vortex generator diameter on separation region

Control of shock wave and boundary layer interaction continues to attract a lot of attention. In recent decades several methods of interaction control have been investigated. The research has mostly concerned solid (vane type) vortex generators and transpiration methods of suction and blowing. This investigation concerns interaction control using air-jets to generate streamwise vortices. The effectiveness of air-jet vortex generators in controlling separation has been proved in a previous research. The present paper focuses on the influence of the vortex generator diameter on the separation region. It presents the results of experimental investigations and provides new guidelines for the design of air-jet vortex generators to obtain more effective separation control.     

Numerical Analysis of Laminar Free Convection in Square Cavity Containing Heat Generating Kr-85 Gas

A laminar free convection of heat generating fluid in a horizontal square container with 10 cm × 10 cm cross section and infinite length was analyzed through the Newton iterative numerical solution of the equations of change without the Boussinesq approximation. Computations were performed for Kr gas in the case of charging pressure of 0.216, 0.540 and 1.081 MPa at 295 K. As the Newton iterating procedure requires initial values not far away from the true solution, the solution of slightly lower percentage of ⁸⁵Kr was used for the initial values. The percentage of ⁸⁵Kr (and, hence heat generating rate) was thus increased gradually untill the concentration of ⁸⁵Kr was so high (–13% at 0.216 MPa) that convergent solution could not be obtained. With increasing the concentration of ⁸⁵Kr, flow pattern is changed from “ordinary” free convection like one generated when heated from below to a complexed pattern with several vortices in the upper part of the container. Similarity of flow fields was shown among various sets of charging pressure and concentration of ⁸⁵Kr. Reason of an onset of the vortical flows was also discussed based on the temperature distribution within the cavity.     

A MULTIGRID-ACCELERATED THREE-DIMENSIONAL TRANSIENT-FLOW CODE AND ITS APPLICATION TO A NEW TEST PROBLEM

A multigrid-assisted solver for the three-dimensional time-dependent incompressible Navier-Stokes equations on graded Cartesian meshes is developed. The spatial accuracy is third-order for the convective terms and fourth-order for the viscous terms, and a fractional-step strategy ensures second-order time accuracy. To achieve good time-wise efficiency a multigrid technique is used to solve the highly time-consuming pressure-Poisson equation that requires to be solved at every time step. The speed-up achieved by multigrid is shown in tabular form. The performance and accuracy of the code are first ascertained by computing the flow in a single-sided lid-driven cubic cavity with good grid-economy and comparing the results available in the literature. The code, thus validated, is then applied to a new test problem we propose and various transient and asymptotically obtained steady-state results are presented. Given the care taken to establish the credibility of the code and the good spatio-temporal accuracy of the discretization, these results are accurate and may be used for ascertaining the performance of any computational algorithm applied to this test problem.     

Image simulations of kinked vortices for transmission electron microscopy

We present an improved model of kinked vortices in high-T_c superconductors suitable for the interpretation of Fresnel or holographic observations carried out with a transmission electron microscope. A kinked vortex is composed of two displaced half-vortices, perpendicular to the film plane, connected by a horizontal flux-line in the plane, resembling a connecting Josephson vortex (JV) segment. Such structures may arise when a magnetic field is applied almost in the plane, and the line tension of the fluxon breaks down under its influence. The existence of kinked vortices was hinted in earlier observations of high-T_c superconducting films, where the Fresnel contrast associated with some vortices showed a dumbbell like appearance. Here, we show that under suitable conditions the JV segment may reveal itself in Fresnel imaging or holographic phase mapping in a transmission electron microscope.     

The onset of instability in the flow induced by an impulsively started rotating cylinder

The onset of initial instability in a developing Couette flow following the impulsive starting of an inner rotating cylinder is analyzed using linear theory. It is well known that there is a critical Taylor number Ta_c at which Taylor vortices first appear between two concentric cylinders. For Ta>Ta_c Taylor-like vortices occur at a certain elapsed time. In the present study, the critical time t_c to represent the onset of this initiating instability, which then grows as toroidal vortices, is analyzed using propagation theory. For this purpose a self-similar transformation is forced through scaling analysis. The resulting stability criteria compare well with the available experimental data for vortices in water. The new measures represent the onset of the fastest growing instability and also suggest the detection time for the manifestation of secondary flow in the primary time-dependent Couette flow.     

A numerical study on pressure drop and separation efficiency of a new swirl tube cleaner

In this paper, a new design of Swirl Tube Cleaner (STC) was introduced. The performance of an STC in terms of the separation efficiency and pressure drop was numerically investigated for different vane angles, vane lengths, and inlet velocities. The Reynolds stress turbulence model was used to detect the main flow structures of the highly swirling flow inside the STC. The discrete phase model that employs the Lagrangian frame of reference was utilized for particle tracking. The results are in good agreement with experimental data available in the literature. For all cases under investigation, the main flow characteristics of the STC consist of a hub vortex flow downstream of the back cone and tip vortices in the wake flow of the vanes. Around the initiation point of the vortex flow, the centrifugal accelerations are one order of magnitude higher than that of the upstream flow around vanes. The results show that the overall separation efficiency with the specified particle size distribution can reach 88% at a pressure drop lower than 250 [Pa] for the highest flow rate. Compared to traditional STCs, the new STC decreases the pressure drop by about 50% while enhancing overall separation efficiency by approximately 2%, considering the numerical accuracy.     

Photo-Thermal Transport Effects in a Thick Film YBCO Bolometer Under Magnetic Fields

Thick (~50 m) YBCO superconducting film bolometers have been fabricated to operate in the temperature range 89–92 K. Responsivities greater than 1 V/W have been achieved by biasing with a current slightly less than that of the critical current, and observing the photo-voltage in the resistive state. The time response of these films due to photo-conduction is slow (ms). The present work concentrates on the resistive transition and the width of the photo-signal peak under the effect of applied magnetic fields. The width of the photo-signal peak over the transition is found to be strongly magnetic field dependent at low field values while at higher field values it has been used to determine a value for the vortex tilt modulus. Further, it is found that a temperature difference in the position of the photo-signal peak relative to that of the dR/dT peak changes with applied field.     

On a modified Kim-Anderson model of the current-voltage characteristics of high-temperature superconducting ceramics

In comparison with the classical Kim-Anderson model for the current-voltage characteristics of type-II superconductors, another model with a less sharp pinning potential is presented. In addition to the approximate solution, for currents close to the critical one, a complete solution of this model is derived, which better fits the experimentally obtained current-voltage characteristics.     

Pancake Vortices

I describe the magnetic-field and current-density distributions generated by two-dimensional (2D) pancake vortices in infinite, semi-infinite, and finite-thickness stacks of Josephson-decoupled superconducting layers. Arrays of such vortices have been used to model the magnetic structure in highly anisotropic layered cuprate high-temperature superconductors. I show how the electromagnetic forces between pancake vortices can be calculatated, and I briefly discuss the effects of interlayer Josephson coupling.     

非定常不稳定气固两相流动的离散涡数值仿真Ⅲ.非定常不稳定气固两相流动中颗粒运动与旋涡的相关结构

基于气固两相流动的离散涡模型与数值仿真方法,分别模拟了与重力方向垂直和与重力方向平行的平面内气固两相圆柱绕流(其中颗粒的St数为1.0),得到了流场中颗粒分布与旋涡分布的时间序列,由此进一步证明了高雷诺数下的二维气固两相流动中固体颗粒运动与流体旋涡存在着明确的相关结构:气体流经圆柱体时产生不稳定的剧烈分离流动,在圆柱绕流的尾迹区内存在着复杂的旋涡发展演化过程,由于流体旋涡对其周围固体颗粒的诱导作用,使得在尾迹区内中等尺度(St～o(1.0))的固体颗粒从均匀气固混合物中分离出来而往旋涡区运动,并最终在旋涡结构的外沿聚集.