On viscoelastic cavitating flows: A numerical study

The effect of viscoelasticity on turbulent cavitating flow inside a nozzle is simulated for Phan-Thien-Tanner (PTT) fluids. Two different flow configurations are used to show the effect of viscoelasticity on different cavitation mechanisms, namely, cloud cavitation inside a step nozzle and string cavitation in an injector nozzle. In incipient cavitation condition in the step nozzle, small-scale flow features including cavitating microvortices in the shear layer are suppressed by viscoelasticity. Flow turbulence and mixing are weaker compared to the Newtonian fluid, resulting in suppression of microcavities shedding from the cavitation cloud. Moreover, mass flow rate fluctuations and cavity shedding frequency are reduced by the stabilizing effect of viscoelasticity. Time averaged values of the liquid volume fraction show that cavitation formation is strongly suppressed in the PTT viscoelastic fluid, and the cavity cloud is pushed away from the nozzle wall. In the injector nozzle, a developed cloud cavity covers the nozzle top surface, while a vortex-induced string cavity emerges from the turbulent flow inside the sac volume. Similar to the step nozzle case, viscoelasticity reduces the vapor volume fraction in the cloud region. However, formation of the streamwise string cavity is stimulated as turbulence is suppressed inside the sac volume and the nozzle orifice. Vortical perturbations in the vicinity of the vortex are damped, allowing more vapor to develop in the string cavity region. The results indicate that the effect of viscoelasticity on cavitation depends on the alignment of the cavitating vortices with respect to the main flow direction

A fast computational algorithm to evaluate large transmit-arrays

Transmit-arrays (TAs) have become popular solutions for point-to-point communications due to their lowprofile and high-efficiency. However, simulation of these large and detailed structures demands a great amount of memory and time. Here, we propose a fast algorithm to estimate the performance of a beam-steering TA to accelerate its design optimization process. The results and the required resources of the algorithm are compared with the ones of Ansys HFSS. It is shown that this algorithm can fairly evaluate the key factors of the performance of a TA within couple of minutes with memory resource of only 1 GB.info:eu-repo/semantics/acceptedVersio

3D CFD Simulation of a Bubble Column with Internals: Validation of Interfacial Forces and Internal Effects for Local Gas Holdup Predictions

CFD Models (Turbulent Models and Interfacial Forces) Incorporated with the Population Balance Model (PBM) Have Been Validated, Azimuthally, with the Gamma-Ray-Computed Tomography (CT) Results to Address the Effect of the Presence of Internals with Different Arrangements and Diameters. the Superficial Gas Velocity Applied Was Varied from 0.05 to 0.45 M/s. the Results Exhibit the Capability to Predict the Hydrodynamics of the Bubble Column, Further Incorporating the Population Balance Model and Promoting the Prediction of Simulation in High Superficial Gas Velocity. the Effect of Internals Revealed that the Gas Holdup Was Significantly Enhanced in the Bubble Column\u27s Wall Region, While the Gas Holdup Was Increased Remarkably in the Center and the Wall Regions of the Bubble Column Equipped by Internals of 1 In. Diameter More Than in Internals of 0.5 In. However, Internals with a Hexagonal Arrangement Increase the Gas Holdup in the Central Region and Less in the Wall Than in the Circular Arrangement

A Methodological Framework for the Resilience Analysis of Road Transport Networks Exposed to Freezing Rain

Road transport network is one of the major critical infrastructures to be maintained resilient, since its disruption would not only have direct economic consequences, but it could also lead to domino effects on other critical infrastructures. This study proposes a methodological framework for the analysis of the resilience of road transport networks exposed to natural hazards, in particular to freezing rain, that is a precipitation event wherein the supercooled droplets of rain freeze upon contact with any surface, creating a glaze of ice and contributing, in particular, to icy roads conditions. A numerical example is presented that considers a road transportation network exposed to freezing rain, whose probability of occurrence is estimated using historical meteorological conditions; the network disruption and recovery are evaluated for the resilience analysis of the selected road transport infrastructure

Antenna-filter-antenna-based cell for linear-to-circular polarizer transmit-array

In this paper, we propose a dual-band linear-to-circular polarization converter element based on the antenna-filter-antenna (AFA) structure. The traits of this cell include thin three-layer structure and ability to convert a linear incident wave to two orthogonal circular polarizations at two non-adjacent frequency bands. This combination of physical and electrical specifications makes this cell a novel solution. An example of this cell suitable for satellite communication is designed to operate at 20GHz and 30GHz (satellite Ka-band) with 4% and 8% bandwidth, respectively.info:eu-repo/semantics/acceptedVersio

Beam-steering ka-band phase rotation cells-based transmit-array for circular-polarization

Planar transmit-arrays (TAs) have been an attractive solution as gain-enhancers for various applications, e.g. satellite communications. For the first time, a TA composed of sequentially rotated cells is employed as beam-steering circular polarization Ka-band satellite ground terminal. Moreover, it is shown that the intrinsic filtering effect of phase rotation (PR) cells for this kind of TA enables improvement in the axial ratio bandwidth. The performance of the TA is evaluated through simulation. The TA presents only 2.7dB loss over zenith scan between 15° and 50° while its structure rotation provides 360° azimuth scan. The TA offers 2GHz (28.730.7GHz) combined 3dB axial ratio and 3dB gain bandwidth.info:eu-repo/semantics/acceptedVersio

Phase-delay versus phase-rotation cells for circular polarization transmit arrays - application to Satellite Ka-Band Beam steering

Planar transmit arrays (TAs) have been an attractive solution as gain enhancers for various applications, e.g., satellite communications. The TA performance directly depends on its composing unit-cell characteristics. Planar unit cells can be categorized into two main types: phase-rotation (PR) and phase-delay (PD) cells. There is no hint in the literature about the relative merits of these two types of cells for circular polarization when assessing the final TA performance. This paper offers a systematic comparison between the cells’ working principles and analyzes their impacts on TA performance. Examples of a PR-based TA and a PD-based TA are designed for single-band wide-angle beam steering operating at the satellite Ka-band. They are evaluated by simulation and measurement to quantify performance differences. No previous work employed a PR TA for wide-angle beam steering. This paper shows that PR TA offers a filtering effect toward the cross-polarization component of the source. This leads to better axial ratio and combined 3 dB axial ratio and 3 dB gain bandwidth. However, PD cells are easier to design and insensitive to feed polarization. The analysis in this paper allows a more informed decision when selecting the unit-cell category for any given TA application

Equivalent circuit modeling to design a dual-band dual linear-to-circular polarizer surface

The working principle of a thin dual-band dual-linear to circular polarizer is presented here. This polarizer not only converts incident linearly polarized (LP) waves to circularly polarized (CP) waves in two frequency bands, but it also reverses the handedness of each signal. The electromagnetic behavior of the cell is carefully analyzed and two equivalent circuit models (ECMs) are presented to model the responses of the cell to linearly polarized waves at normal incidence. The ECMs show how utilizing interlayer coupling can be leveraged to achieve reversed CP senses in two bands using a compact design. Analytical formulas are presented to provide initial values of the ECM components including the mutual coupling inductances. We present measurement results that agree well with the full-wave simulation and the ECM results, thus validating the proposed ECM model.info:eu-repo/semantics/acceptedVersio

New concept for multibeam antennas based on two cascaded Ka-band transmit-array

The paper presents a new two-beam per-feed approach for Ka-band multi spot-beam satellites. It is based on a dual-stacked transmit array (TA) arrangement, fed by a 3×3 standard waveguide array. The TAs are co-designed so that the first performs a beam collimation and tilting; whereas the second splits the incoming beam into two orthogonal circular polarizations, with symmetrical output angles relative to the incident angle. Therefore, the number of feeds is half the number of output beams, and the linearly polarized feed reduces significantly the antenna complexity. As a proof-of-concept, we design an antenna for the 30 GHz up-link band. The focal length of the first TA is 50 mm, and the aperture size of both TAs is 196×148 mm. A new type of phase correction, the multifocal approach, is used to minimize aberrations for all beams, which would be otherwise impossible with such low F/D. Using fullwave simulations, we confirm that the 18 beams present low SLL (~15 dB), pure circular polarization and good crossover (~3 dB) with gains between 21 and 22 dBi.info:eu-repo/semantics/publishedVersio

Dual-band dual linear to circular polarization converter in transmission mode-application to K/Ka-band satellite communications

Many wireless communication applications such as satellite communications use circularly polarized (CP) signals, with the requirement for easy switching of the polarization sense between uplink and downlink. Specifically, in satellite communications, the trend is also to move to higher frequencies and integrate the receiving and transmitting antennas in one dual-band terminal. However, these simultaneous demands make the design and fabrication of the composing parts very challenging. We propose, here, a dual-band dual-linear polarization (LP)-to-CP converter that works in the transmission mode. The working principle of this polarizer is explained through an example for Ka-band satellite communications at 19.7–20.2 and 29.5–30 GHz. The LP-to-CP converter is a single panel composed of identical unit cells with a thickness of only 1.05 mm and a size of 5.3 mm ×5.3 mm. Due to its operation in the transmission mode, the polarizer can be combined with a simple dual-band dual-LP antenna to obtain the desired dual-band dual-CP single antenna. However, the unique property of this polarizer is yet the fact that it converts a given LP wave, e.g., x-polarization, to orthogonal CP waves at the two nonadjacent frequency bands, e.g., left-handed CP at lower band and right-handed CP at higher band. The polarizer is tested both with 20 and 30 GHz LP rectangular horns to illuminate a dual-band transmit array (TA) to obtain wide-angle steering of CP beams. The performance of the polarizer and its association with the TA is evaluated through simulation and measurements. We also present design guidelines for this type of polarizer.info:eu-repo/semantics/acceptedVersio