Vortex motion around a circular cylinder above a plane

The study of vortex flows around solid obstacles is of considerable interest from both a theoretical and practical perspective. One geometry that has attracted renewed attention recently is that of vortex flows past a circular cylinder placed above a plane wall, where a stationary recirculating eddy can form in front of the cylinder, in contradistinction to the usual case (without the plane boundary) for which a vortex pair appears behind the cylinder. Here we analyze the problem of vortex flows past a cylinder near a wall through the lenses of the point-vortex model. By conformally mapping the fluid domain onto an annular region in an auxiliary complex plane, we compute the vortex Hamiltonian analytically in terms of certain special functions related to elliptic theta functions. A detailed analysis of the equilibria of the model is then presented. The location of the equilibrium in front of the cylinder is shown to be in qualitative agreement with the experimental findings. We also show that a topological transition occurs in phase space as the parameters of the systems are variedComment: 17 pages, 8 figure

Multi-strange particle production in relativistic heavy ion collisions at sNN=62.4\sqrt{s_{NN}}=62.4 GeV

We present preliminary STAR results on measurements of multi-strange particles $\Xi$, $\Omega$ and their anti-particles from Au+Au and Cu+Cu at $\sqrt{s_{NN}}=62.4$ GeV collisions. In order to better understand the role of strangeness enhancement in nucleus-nucleus collisions and its scaling properties with system size, we compare the results from Au+Au and Cu+Cu reactions for different event centrality classes. Strangeness enhancement is discussed in the context of multi-strange to pion ratios. Finally, $\Omega/\phi$ ratio is shown for different systems and energies for a systematic study

Platform Pricing Structure and Moral Hazard

We study pricing by a two-sided platform when it faces moral hazard on the sellers? side. In doing so, we introduce an equilibrium notion of platform reputation in an infinite horizon model. We find that with transaction fees only, the platform cannot eliminate the loss of reputation induced by moral hazard. If registration fees can be levied, moral hazard can be overcome. The registration fee determines the participation threshold of sellers and extracts them, while (lower) transaction fees provide incentives for good behavior. This provides a motivation for platforms to use registration fees in addition to transaction fees