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A strong wave manifestation may be a dominant constituent of the bar structure in barred spiral galaxies : large-scale gaseous density waves and shocks are identified as important phenomena in the gas flow. We generalize the steady-state gas-dynamical studies, previously limited to tightly wound normal spirals, to include barred and open-armed normal spirals. The steady-state response of the gas (non-self-gravitating) to a 5-107 o perturbing potential that is barlike in the inner parts and spiral-like in the outer parts is found to be strong and capable of inducing the formation of large-scale gaseous density waves and shocks in the bar and along the spiral arms. Highly oval streamlines characterize the gas circulation in the inner regions of the disk where large noncircular motions are of the order of 50 km s _1 to 150 km s-1. Strong velocity gradients in the gas flow are particularly pronounced across the bar near the shock. Two types of shocked gas flow are found to be possible in the inner regions of barred spirals. In one type, a shock-focusing phenomenon is discovered which focuses gas in the inner parts outward and gas in the outer parts inward, in the region of convergence where the spiral arm bends from the bar. This shock-focusing phenomenon is thought to account in part for the enhanced star formation activity observed at the ends of the bar structure in many barred spirals. Our analysis constitutes an essential complement to the recent time-evolutionary, numercial hydrodynamical calculations which lack the resolution necessary to compute the detailed structure of the shock and the gas streamlines in the bar region. The present study provides this resolution together with the critical forcing amplitude required to produce offset shocks along the bar. The dark, narrow dust lanes observed along the leading edges of the bar structure in many barred spirals are identified in this study as tracers of such shocks. The interstellar gas-itself a tracer and an important factor influencing the formation of stars and other tracers (e.g., offset dust lanes)-actually provides an outstanding interconnecting link between the overall appearance of a barred spiral and the underlying dynamics of its stellar component. Interpretation of the observed velocity field of one sample barred spiral, NGC 5383, through the application of the theoretical velocity field derived in this steady-state gas flow study indicates that a deeper understanding of barred spirals is obtained. Subject headings: galaxies : internal motions-hydrodynamics-shock waves

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In this paper we study the behavior of the gas in the gravitational field of a barred spiral. The gas-code used permits a much higher resolution and consequently also a lower numerical viscosity than in earlier published results obtained with other fully two-dimensional gas-codes. This capability is obtained through the use of much larger grid sizes (about 12,000 points as compared to 3200 in most earlier calculations) and of a windowing technique which allows us to continue a calculation with a finer mesh on a subregion of the original grid. Further improvement was obtained by using Godunov's method instead of the beam scheme. Our calculations confirm, albeit with a smaller amplitude, the postshock outflow predicted by Roberts, Huntley, and van Albada in a steady state study employing the same potential. The derived position of the shock is strongly dependent on the mesh size, probably through its influence on the viscosity. For our finest mesh we find a strong shock that is not offset from the potential minimum; for less fine grids the strength of the shock is less, but it is offset in the right sense. The very strong decay of the gas flow lines into the center apparent in earlier fully two-dimensional calculations has been much reduced here. Though the gas transport toward the center still is significant, it is no longer a dominant aspect of the derived gas flow.

We investigate the performance of the Time Warp kernel APSIS when running on various communication layers, in particular on a wide-area Grid. Several cancellation strategies are tried, among them the lazy cancellation and a little known bulk anti-messages optimisation. Our experiments with an Ising spin simulation indicate that the slowdown caused by high latency networks, while significant, is not catastrophic, and that it can be significantly reduced using the lazy cancellation. Experiments suggest that further improvements can be expected if a more elaborate communication infrastructure is put in place.

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An Interactive Simulation System (ISS) allows a user to interactively explore simulation results and modify the parameters of the simulation at run-time. An ISS is commonly implemented as a distributed system. Integrating distributed modules into one system requires certain control components to be added in each module. When interaction scenarios are complicated, these control components often become large and complex, and are often limited in their reusability. To make the integration more flexible and the solution more reusable, we isolated these control components out of the system's modules and implemented them as an agent framework. In this paper we will describe the architecture of this agent framework, and discuss how they flexibly integrate distributed modules and provide interaction support.