Non-radial instabilities and progenitor asphericities in core-collapse supernovae

Since core-collapse supernova simulations still struggle to produce robust neutrino-driven explosions in 3D, it has been proposed that asphericities caused by convection in the progenitor might facilitate shock revival by boosting the activity of non-radial hydrodynamic instabilities in the post-shock region. We investigate this scenario in depth using 42 relativistic 2D simulations with multigroup neutrino transport to examine the effects of velocity and density perturbations in the progenitor for different perturbation geometries that obey fundamental physical constraints (like the anelastic condition). As a framework for analysing our results, we introduce semi-empirical scaling laws relating neutrino heating, average turbulent velocities in the gain region, and the shock deformation in the saturation limit of non-radial instabilities. The squared turbulent Mach number, 〈Ma2〉, reflects the violence of aspherical motions in the gain layer, and explosive runaway occurs for 〈Ma2〉 ≳ 0.3, corresponding to a reduction of the critical neutrino luminosity by ∼25∼25 per cent compared to 1D. In the light of this theory, progenitor asphericities aid shock revival mainly by creating anisotropic mass flux on to the shock: differential infall efficiently converts velocity perturbations in the progenitor into density perturbations δρ/ρ at the shock of the order of the initial convective Mach number Maprog. The anisotropic mass flux and ram pressure deform the shock and thereby amplify post-shock turbulence. Large-scale (ℓ = 2, ℓ = 1) modes prove most conducive to shock revival, whereas small-scale perturbations require unrealistically high convective Mach numbers. Initial density perturbations in the progenitor are only of the order of Ma2progMaprog2 and therefore play a subdominant role.

From brown envelopes to community benefits: the co-option of planning gain agreements under deepening neoliberalism

The provision of physical and social infrastructure in the form of roads, green spaces and community facilities has traditionally been provided for by the state through the general taxation system. However, as the state has been transformed along more neoliberal lines, the private sector is increasingly relied upon to deliver public goods and services. Planning gain agreements have flourished within this context by offering another vehicle through which local facilities are privately funded. Whilst these agreements reflect the broader dynamics of neoliberalism, they are commonly viewed as a tool which can be employed to challenge these very dynamics by empowering local communities to secure more just planning outcomes. This paper counters such claims. Based on evidence gathered from 80 interviews with planners, councillors, developers and community groups in Ireland, the paper demonstrates how planning gain agreements have been strategically redeployed by the holders of political and economic power to serve their own ends. In seeking to understand why and how this has occurred, specific consideration is given to the changing power dynamics between the state and private capital under neoliberalism. The paper highlights how institutional arrangements have enabled developers to infiltrate the political sphere in more subtle and implicit ways than ever before. We conclude by arguing that planning gain must be understood as a mechanism which has been manipulated in ways which essentially work to preserve and enhance, rather than redress, existing power imbalances in the planning system by facilitating large scale transfers of wealth upwards in society.

Simulation of Distributed Contact in String Instruments: a Modal Expansion Approach

Impactive contact between a vibrating string and a barrier is a strongly nonlinear phenomenon that presents several challenges in the design of numerical models for simulation and sound synthesis of musical string instruments. These are addressed here by applying Hamiltonian methods to incorporate distributed contact forces into a modal framework for discrete-time simulation of the dynamics of a stiff, damped string. The resulting algorithms have spectral accuracy, are unconditionally stable, and require solving a multivariate nonlinear equation that is guaranteed to have a unique solution. Exemplifying results are presented and discussed in terms of accuracy, convergence, and spurious high-frequency oscillations.