Inertia controlled instability and small scale structures of sheet and cloud cavitation

The present investigation focuses on the numerical simulation of inertia driven dynamics of 3-D sheet and cloud cavitation on a 2-D NACA 0015 hydrofoil. Special emphasis is put on the numerical analysis of the re-entrant flow, the break-up of the sheet cavity and the formation of clouds. We demonstrate that our CFD-Tool CATUM (CAvitation Technische Universität Mu?nchen) is able to predict even delicate 3-D flow features such as irregular break-up patterns, cavitating hairpin and horseshoe vortices, 3-D instabilities in spanwise direction and the formation and propagation of shocks due to collapsing clouds close to the trailing edge of the hydrofoil. The numerically predicted flow features agree well with the experimental observations of Kawanami et al [1].http://deepblue.lib.umich.edu/bitstream/2027.42/84219/1/CAV2009-final17.pd

Numerical simulation of cavitating ship propeller flow and assessment of erosion aggressiveness

Our contribution focuses on the evaluation of cavitation aggressiveness on the cavitating model propeller VP1304 by numerical simulation. To this respect, we employ a density-based, finite volume method, based on a barotropic, homogeneous mixture model. Fully accounting for two-phase compressibility, collapse-induced instantaneous peak pres- sures and associated wave dynamics are captured by the chosen approach. The maximum instantaneous pressures registered on material surfaces allow for a quali- tative identification of erosion-sensitive areas. Furthermore, a collapse detection algorithm is applied for an automated recording of isolated cavity collapse events. Impact load spectra showing rate and intensity distributions of recorded collapses is utilized for a quantitative evaluation of cavitation aggressiveness. While the cavitating tip vortex is stable and does not lead to collapse events in the vicinity of the propeller, material erosion can be expected in the suction side root region due to a highly unsteady root cavitation

Unsteady bubbly cavitating nozzle flows

Unsteady quasi-one-dimensional and two-dimensional cavitating nozzle flows are considered using a homogeneous bubbly flow model. For quasi-one-dimensional nozzle flows, the system of model equations is reduced to two evolution equations for the flow speed and bubble radius and the initial and boundary value problems for the evolution equations are formulated. Results obtained for quasi-onedimensional nozzle flows capture the measured pressure losses due to cavitation, but they turn out to be insufficient in describing the twodimensional structures. For this reason, model equations for unsteady two-dimensional bubbly cavitating nozzle flows are considered and, by suitable decoupling, they are reduced to evolution equations for the bubble radius and for the velocity field, the latter being determined by an integro-partial differential system for the unsteady acceleration. This integropartial differential system constitutes the fundamental equations for the evolution of the dilation and vorticity in twodimensional cavitating nozzle flows. The initial and boundary value problem of the evolution equations are then discussed and a method to integrate the equations is introduced. Due to a lack of an algorithm to compute two-dimensional bubbly cavitating flows presently, the numerical simulation of 2D cavitating nozzle flows is obtained by the CFD-Tool CATUM, which is based on an equilibrium phase transition model. Results obtained for a typical cavitation cycle show instantaneous high pressure pulses at instances of cloud collapses.http://deepblue.lib.umich.edu/bitstream/2027.42/84228/1/CAV2009-final18.pd

The photospheric solar oxygen project: III. Investigation of the centre-to-limb variation of the 630nm [OI]-NiI blend

The solar photospheric abundance of oxygen is still a matter of debate. For about ten years some determinations have favoured a low oxygen abundance which is at variance with the value inferred by helioseismology. Among the oxygen abundance indicators, the forbidden line at 630nm has often been considered the most reliable even though it is blended with a NiI line. In Papers I and Paper II of this series we reported a discrepancy in the oxygen abundance derived from the 630nm and the subordinate [OI] line at 636nm in dwarf stars, including the Sun. Here we analyse several, in part new, solar observations of the the centre-to-limb variation of the spectral region including the blend at 630nm in order to separate the individual contributions of oxygen and nickel. We analyse intensity spectra observed at different limb angles in comparison with line formation computations performed on a CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. The oxygen abundances obtained from the forbidden line at different limb angles are inconsistent if the commonly adopted nickel abundance of 6.25 is assumed in our local thermodynamic equilibrium computations. With a slightly lower nickel abundance, A(Ni)~6.1, we obtain consistent fits indicating an oxygen abundance of A(O)=8.73+/-0.05. At this value the discrepancy with the subordinate oxygen line remains. The derived value of the oxygen abundance supports the notion of a rather low oxygen abundance in the solar hotosphere. However, it is disconcerting that the forbidden oxygen lines at 630 and 636nm give noticeably different results, and that the nickel abundance derived here from the 630nm blend is lower than expected from other nickel lines.Comment: to appear in A&

The Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest: The X-ray Catalog

We present the X-ray catalog and basic results from our Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest field. Our 9 ACIS-I fields cover a contiguous solid angle of ~0.4 sq. deg. and reach fluxes of 5E-16 cgs (0.4-2 keV) and 3E-15 cgs (2-8keV). Our survey bridges the gap between ultradeep pencil-beam surveys, such as the Chandra Deep Fields (CDFs), and shallower, large area surveys, allowing a better probe of the X-ray sources that contribute most of the 2-10 keV cosmic X-ray background (CXB). We find a total of 525 X-ray point sources and 4 extended sources. At ~10E-14 cgs 2-8 keV, our number counts are significantly higher than those of several non-contiguous, large area surveys. On the other hand, the integrated flux from the CLASXS field, combined with ASCA and Chandra ultradeep surveys, is consistent with results from other large area surveys, within the variance of the CXB. Spectral evolution is seen in the hardening of the sources at fluxes below 1E-14 cgs Above 4E1-14 cgs(0.4-8 keV), ~60 of the sources are variable. Four extended sources in CLASXS is consistent with the previously measured LogN-LogS of galaxy clusters. We report the discovery of a gravitational lensing arc associated with one of these sources. (Abridged)Comment: 67 pages, 26 figures, accepted for publication in the Astronomical Journa

The dynamical evolution of the circumstellar gas around low-and intermediate-mass stars I: the AGB

We have investigated the dynamical interaction of low- and-intermediate mass stars (from 1 to 5 Msun) with their interstellar medium (ISM). In this first paper, we examine the structures generated by the stellar winds during the Asymptotic Giant Branch (AGB) phase, using a numerical code and the wind history predicted by stellar evolution. The influence of the external ISM is also taken into account. We find that the wind variations associated with the thermal pulses lead to the formation of transient shells with an average lifetime of 20,000 yr, and consequently do not remain recorded in the density or velocity structure of the gas. The formation of shells that survive at the end of the AGB occurs via two main processes: shocks between the shells formed by two consecutive enhancements of the mass-loss or via continuous accumulation of the material ejected by the star in the interaction region with the ISM. Our models show that the mass of the circumstellar envelope increases appreciably due to the ISM material swept up by the wind (up to 70 % for the 1 Msun stellar model). We also point out the importance of the ISM on the deceleration and compression of the external shells. According to our simulations, large regions (up to 2.5 pc) of neutral gas surrounding the molecular envelopes of AGB stars are expected. These large regions of gas are formed from the mass-loss experienced by the star during the AGB evolution.Comment: 43 pages, 15 figures. Accepted for publication in the Astrophysical Journa

Electro-optic techniques for longitudinal electron bunch diagnostics

Electro-optic techniques are becoming increasingly important in ultrafast electron bunch longitudinal diagnostics and have been successfully implemented at various accelerator laboratories. The longitudinal bunch shape is directly obtained from a single-shot, non-intrusive measurement of the temporal electric field profile of the bunch. Further- more, the same electro-optic techniques can be used to measure the temporal profile of terahertz / far-infrared opti- cal pulses generated by a CTR screen, at a bending magnet (CSR), or by an FEL. This contribution summarizes the re- sults obtained at FELIX and FLASH

Near-surface dynamics of a gas bubble collapsing above a crevice

The impact of a collapsing gas bubble above rigid, notched walls is considered. Such surface crevices and imperfections often function as bubble nucleation sites, and thus have a direct relation to cavitation-induced erosion and damage structures. A generic configuration is investigated numerically using a second-order-accurate compressible multi-component flow solver in a two-dimensional axisymmetric coordinate system. Results show that the crevice geometry has a significant effect on the collapse dynamics, jet formation, subsequent wave dynamics, and interactions. The wall-pressure distribution associated with erosion potential is a direct consequence of development and intensity of these flow phenomena.Comment: 21 pages, 16 figures, submitted to the Journal of Fluid Mechanic

Near-surface dynamics of a gas bubble collapsing above a crevice

The impact of a collapsing gas bubble above rigid, notched walls is considered. Such surface crevices and imperfections often function as bubble nucleation sites, and thus have a direct relation to cavitation-induced erosion and damage structures. A generic configuration is investigated numerically using a second-order accurate compressible multi-component flow solver in a two-dimensional axisymmetric coordinate system. Results show that the crevice geometry has a significant effect on the collapse dynamics, jet formation, subsequent wave dynamics and interactions. The wall-pressure distribution associated with erosion potential is a direct consequence of development and intensity of these flow phenomena