Three-dimensional self-adaptive grid method for complex flows

A self-adaptive grid procedure for efficient computation of three-dimensional complex flow fields is described. The method is based on variational principles to minimize the energy of a spring system analogy which redistributes the grid points. Grid control parameters are determined by specifying maximum and minimum grid spacing. Multidirectional adaptation is achieved by splitting the procedure into a sequence of successive applications of a unidirectional adaptation. One-sided, two-directional constraints for orthogonality and smoothness are used to enhance the efficiency of the method. Feasibility of the scheme is demonstrated by application to a multinozzle, afterbody, plume flow field. Application of the algorithm for initial grid generation is illustrated by constructing a three-dimensional grid about a bump-like geometry

The role of Parliamentary Budget Offices in participatory budgeting considerations for the South African Parliamentary Budget Office

Abstract: The role of Parliamentary Budget Offices (PBOs) in supporting parliaments by enhancing participatory budgeting, is the key focus of this article. Participatory budgeting is recognised as a tool for empowering the public to actively participate in the fiscal and budgetary process. Parliaments are increasingly strengthening their capacities to exercise oversight over the public finance management process. PBOs are being established to empower parliaments to strengthen the budgetary oversight process. The South African Parliamentary Budget Office (SAPBO) provides the necessary technical support to Parliament regarding the budget process. A critical challenge is to obtain the involvement of the broader public in the budgetary process. While space for public participation has been established by the legislature, broader public involvement in budgeting should be enhanced. The article found that the SAPBO is well placed to ensure meaningful public engagement and build public capacity in support of participatory budgeting. In addressing the SAPBO mandate, access to information and engaging in budgetary capacity building should be a core function. Through communication with the public and soliciting their inputs, the SAPBO can play a key role in enhancing participatory budgeting

Firewall black holes and echoes from an action principle

It is often said that there is no gravity theory based on local action principles giving rise to firewall black hole solutions. Additionally, Guo and Mathur [Int. J. Mod. Phys. D 31, 2242009 (2022).] have cast doubt on the observability of firewall echoes due to the closed trapped surface produced by a backreaction of macroscopic in-falling wave packets. In this paper, we bring Einstein-Maxwell-Dilaton action as a toy model that serves as counterexample to these assertions. Actions with Maxwell and dilaton fields emerge from several fundamental theories, such as the low energy limit of (super) string theory or Kaluza-Klein compactifications. In these systems, the black hole solution has two curvature singularities. We will show that the outer singularity inside the event horizon can cause significant change to the outside, close to the extremal limit, making a macroscopic reflective barrier near the event horizon that would lead to “observable” gravitational wave echoes in this toy model. Additionally, we also call into question the argument by Guo et al. [J. High Energy Phys. 07 (2018) 162.] claiming that a very small fraction of the backscattered photons will be able to escape back to infinity from the firewall, using these black holes as a counterexample.publishedVersio

VCSEL and Integration Techniques for Wavelength-Multiplexed Optical Interconnects

GaAs-based vertical-cavity surface-emitting lasers (VCSELs) are dominating short-reach optical interconnects (OIs) due to their high modulation speed, low power consumption, circular output beam and low fabrication cost. Such OIs provide the high bandwidth connectivity needed for interconnecting servers and switches in data centers. With the rapidly increasing use of Internet-based applications and services, higher bandwidth connectivity and higher aggregate capacity VCSEL-based OIs are needed. Until now, this has been achieved mostly through an increase of the lane rate by higher speed VCSELs and higher order modulation formats. Furthermore, spatial-division-multiplexing has proven effective for increasing the aggregate capacity. Much higher capacity can be achieved by multiple wavelengths per fiber, known as wavelength-divisionmultiplexing (WDM). Moreover, smaller footprint and higher bandwidth density WDM transceivers can be built using monolithic multi-wavelength VCSEL arrays with densely spaced VCSELs. This requires a VCSEL technology where the wavelength of individual VCSELs can be precisely set in a post-epitaxial growth fabrication process and a photonic integrated circuit (PIC) for multiplexing and fiber coupling. Flip-chip integration over grating couplers (GCs) is considered for interfacing VCSELs with waveguides on the PIC. In this thesis, an intra-cavity phase tuning technique is demonstrated for setting the resonance wavelength of VCSELs in a monolithic array with an accuracy in spacing of <1 nm. Uniform performance over the array is achieved by spectral matching and balancing of mirror reflectances, optical confinement factor and optical gain. Single transverse and polarization mode VCSELs, as required for flip-chip integration over GCs, with a record output power of 6 mW are also demonstrated.Finally, an investigation of angled flip-chip integration of a VCSEL over a GC on a silicon photonic integrated circuit (Si-PIC) is presented. Dependencies of coupling efficiency and optical feedback on flip-chip angle and size of the VCSEL die are studied using numerical FDTD simulations. Moreover, flip-chip integration of a VCSEL over a GC on a Si-PIC is experimentally demonstrated. The insertion loss from the VCSEL at the input GC to a singlemode fiber, multimode fiber or flip-chip integrated photodetector over the output GC was measured and quantified. The latter forms an on-PIC optical link