Comparison of different nonlinear solvers for 2D time-implicit stellar hydrodynamics

Time-implicit schemes are attractive since they allow numerical time steps that are much larger than those permitted by the Courant-Friedrich-Lewy criterion characterizing time-explicit methods. This advantage comes, however, with a cost: the solution of a system of nonlinear equations is required at each time step. In this work, the nonlinear system results from the discretization of the hydrodynamical equations with the Crank-Nicholson scheme. We compare the cost of different methods, based on Newton-Raphson iterations, to solve this nonlinear system, and benchmark their performances against time-explicit schemes. Since our general scientific objective is to model stellar interiors, we use as test cases two realistic models for the convective envelope of a red giant and a young Sun. Focusing on 2D simulations, we show that the best performances are obtained with the quasi-Newton method proposed by Broyden. Another important concern is the accuracy of implicit calculations. Based on the study of an idealized problem, namely the advection of a single vortex by a uniform flow, we show that there are two aspects: i) the nonlinear solver has to be accurate enough to resolve the truncation error of the numerical discretization, and ii) the time step has be small enough to resolve the advection of eddies. We show that with these two conditions fulfilled, our implicit methods exhibit similar accuracy to time-explicit schemes, which have lower values for the time step and higher computational costs. Finally, we discuss in the conclusion the applicability of these methods to fully implicit 3D calculations.Comment: Accepted for publication in A&

Negative Party Identification and the Use of Party Cues in the Direct Democratic Context

The use of party cues is a fundamental process of how voters adopt policy preferences. While research has shown that party identification is an important driver of political attitudes in general and policy positions in particular, we know little about how negative party identification (identifying as an opponent to a party) impacts voters’ political preferences. This article aims to fill this gap in the literature by combining an experimental and observational empirical analysis of the effect of negative party identification on voters’ issue preferences in the context of direct democratic decision‐making. First, we analyze a survey experiment conducted during a real‐world campaign on affordable housing for a popular ballot in Switzerland. Using continuous measures of party identification, we show a causal relationship between negative party identification and voters’ policy preferences. Second, we use longitudinal observational data of vote choice on direct democratic policy proposals and show that voters adopt policy preferences that contrast with the policy positions of parties they oppose. In sum, the two complementary designs show that voters tend to position themselves not only in alignment with their preferred parties but also in opposition to parties with which they negatively identify. Furthermore, the results indicate that, when adopting policy preferences, negative cues may carry as much weight as positive party cues. Our analysis has important implications for understanding voters’ adoption of policy preferences in general and specifically in the direct democratic context

A Jacobian-free Newton-Krylov method for time-implicit multidimensional hydrodynamics

This work is a continuation of our efforts to develop an efficient implicit solver for multidimensional hydrodynamics for the purpose of studying important physical processes in stellar interiors, such as turbulent convection and overshooting. We present an implicit solver that results from the combination of a Jacobian-Free Newton-Krylov method and a preconditioning technique tailored to the inviscid, compressible equations of stellar hydrodynamics. We assess the accuracy and performance of the solver for both 2D and 3D problems for Mach numbers down to $10^{-6}$. Although our applications concern flows in stellar interiors, the method can be applied to general advection and/or diffusion-dominated flows. The method presented in this paper opens up new avenues in 3D modeling of realistic stellar interiors allowing the study of important problems in stellar structure and evolution.Comment: Accepted for publication in A&

Negative Party Identification and the Use of Party Cues in the Direct Democratic Context

The use of party cues is a fundamental process of how voters adopt policy preferences. While research has shown that party identification is an important driver of political attitudes in general and policy positions in particular, we know little about how negative party identification (identifying as an opponent to a party) impacts voters’ political preferences. This article aims to fill this gap in the literature by combining an experimental and observational empirical analysis of the effect of negative party identification on voters’ issue preferences in the context of direct democratic decision‐making. First, we analyze a survey experiment conducted during a real‐world campaign on affordable housing for a popular ballot in Switzerland. Using continuous measures of party identification, we show a causal relationship between negative party identification and voters’ policy preferences. Second, we use longitudinal observational data of vote choice on direct democratic policy proposals and show that voters adopt policy preferences that contrast with the policy positions of parties they oppose. In sum, the two complementary designs show that voters tend to position themselves not only in alignment with their preferred parties but also in opposition to parties with which they negatively identify. Furthermore, the results indicate that, when adopting policy preferences, negative cues may carry as much weight as positive party cues. Our analysis has important implications for understanding voters’ adoption of policy preferences in general and specifically in the direct democratic context.</p

The Use of Partisan Cues in Direct Democracy: Registration of the experimental design.

This document presents the use of partisan heuristics in vote choice for direct democratic ballots. We will present our research design which consists of a survey experiment that aims to evaluate the causal effect of partisan heuristic on vote choice. The experiment is conducted in Switzerland on a representative sample of 2000 respondents with the survey company Link. Additionally, we run the survey through a local newspaper website (Bielertagblatt) on Facebook and with a list of email we collected in previous surveys. The survey concerns a single ballot issue on which the Swiss people will have to vote on the 9th of February 2020. The ballot is an initiative that aims to increase the supply of affordable housing