Unsteady flow phenomena in industrial centrifugal compressor stage

The results of an experimental investigation on a typical centrifugal compressor stage running on an atmospheric pressure test rig are shown. Unsteady flow was invariably observed at low flow well before surge. In order to determine the influence of the statoric components, the same impeller was repeatedly tested with the same vaneless diffuser, but varying return channel geometry. Experimental results show the strong effect exerted by the return channel, both on onset and on the behavior of unsteady flow. Observed phenomena have been found to confirm well the observed dynamic behavior of full load tested machines when gas density is high enough to cause appreciable mechanical vibrations. Therefore, testing of single stages at atmospheric pressure may provide a fairly accurate prediction of this kind of aerodynamic excitation

Resource Windfalls, Political Regimes, and Political Stability

We study theoretically and empirically whether natural resource windfalls affect political regimes. We document the following regularities. Natural resource windfalls have no effect on the political system when they occur in democracies. However, windfalls have significant political consequences in autocracies. In particular, when an autocratic country receives a positive shock to its flow of resource rents it responds by becoming even more autocratic. Furthermore, there is heterogeneity in the response of autocracies. In deeply entrenched autocracies the effect of windfalls on politics is virtually nil, while in moderately entrenched autocracies windfalls significantly exacerbate the autocratic nature of the political system. To frame the empirical work we present a simple model in which political incumbents choose the degree of political contestability by deciding how much to spend on vote-buying, bullying, or outright repression. Potential challengers decide whether or not to try to unseat the incumbent and replace him. The model uncovers a reason for the asymmetric impact of resource windfalls on democracies and autocracies, as well as the differential impact within autocratic regimes.

Adaptability Checking in Multi-Level Complex Systems

A hierarchical model for multi-level adaptive systems is built on two basic levels: a lower behavioural level B accounting for the actual behaviour of the system and an upper structural level S describing the adaptation dynamics of the system. The behavioural level is modelled as a state machine and the structural level as a higher-order system whose states have associated logical formulas (constraints) over observables of the behavioural level. S is used to capture the global and stable features of B, by a defining set of allowed behaviours. The adaptation semantics is such that the upper S level imposes constraints on the lower B level, which has to adapt whenever it no longer can satisfy them. In this context, we introduce weak and strong adaptabil- ity, i.e. the ability of a system to adapt for some evolution paths or for all possible evolutions, respectively. We provide a relational characterisation for these two notions and we show that adaptability checking, i.e. deciding if a system is weak or strong adaptable, can be reduced to a CTL model checking problem. We apply the model and the theoretical results to the case study of motion control of autonomous transport vehicles.Comment: 57 page, 10 figures, research papaer, submitte

Resource Windfalls, Political Regimes, and Political Stability

We study theoretically and empirically whether natural resource windfalls affect political regimes. We document the following regularities. Natural resource windfalls have no effect on the political system when they occur in democracies. However, windfalls have significant political consequences in autocracies. In particular, when an autocratic country receives a positive shock to its flow of resource rents it responds by becoming even more autocratic. Furthermore, there is heterogeneity in the response of autocracies. In deeply entrenched autocracies the effect of windfalls on politics is virtually nil, while in moderately entrenched autocracies windfalls significantly exacerbate the autocratic nature of the political system. To frame the empirical work we present a simple model in which political incumbents choose the degree of political contestability by deciding how much to spend on vote-buying, bullying, or outright repression. Potential challengers decide whether or not to try to unseat the incumbent and replace him. The model uncovers a reason for the asymmetric impact of resource windfalls on democracies and autocracies, as well as the differential impact within autocratic regimes.natural resources, elections, political accountability

A Graph Grammar for Modelling RNA Folding

We propose a new approach for modelling the process of RNA folding as a graph transformation guided by the global value of free energy. Since the folding process evolves towards a configuration in which the free energy is minimal, the global behaviour resembles the one of a self-adaptive system. Each RNA configuration is a graph and the evolution of configurations is constrained by precise rules that can be described by a graph grammar.Comment: In Proceedings GaM 2016, arXiv:1612.0105

Oil Price Shocks, Income, and Democracy

We examine the effect of oil price fluctuations on democratic institutions over the 1960-2007 period. We also exploit the very persistent response of income to oil price fluctuations to study the effect of persistent (oil price-driven) income shocks on democracy. Our results indicate that countries with greater net oil exports over GDP see improvements in democratic institutions following upturns in international oil prices. We estimate that a 1 percentage point increase in per capita GDP growth due to a positive oil price shock increases the Polity democracy score by around 0.2 percentage points on impact and by around 2 percentage points in the long run. The effect on the probability of a democratic transition is around 0.4 percentage points.democracy, oil price shocks, persistent income shocks

A uniqueness criterion for measure-valued solutions of scalar hyperbolic conservation laws

We prove existence and uniqueness of Radon measure-valued solutions of the Cauchy problem $$\begin{cases} u_t+[\varphi(u)]_x=0 & \text{in } \mathbb{R}\times (0,T) \\ u=u_0\ge 0 &\text{in } \mathbb{R}\times \{0\}, \end{cases}$$ where $u_0$ a positive Radon measure whose singular part is a finite superposition of Dirac masses, and $\varphi\in C^2([0,\infty))$ is bounded. The novelty of the paper is the introduction of a compatibility condition which, combined with standard entropy conditions, guarantees uniqueness

Trust and racial income inequality: evidence from the U.S.

Existing studies of trust formation in U.S. metropolitan areas have found that trust is lower when there is more income inequality and greater racial fragmentation. I add to this literature by examining the role of income inequality between racial groups (racial income inequality). I find that greater racial income inequality reduces trust. Also, racial fragmentation is no longer a significant determinant of trust once racial income inequality is accounted for. I also show that racial income inequality has a more detrimental effect in more racially fragmented communities and that trust falls more in minority groups when racial income inequality increases. The results hold under both least squares and instrumental variable estimation