909 research outputs found
Investigations of Protostellar Outflow Launching and Gas Entrainment: Hydrodynamic Simulations and Molecular Emission
We investigate protostellar outflow evolution, gas entrainment, and star
formation efficiency using radiation-hydrodynamic simulations of isolated,
turbulent low-mass cores. We adopt an X-wind launching model, in which the
outflow rate is coupled to the instantaneous protostellar accretion rate and
evolution. We vary the outflow collimation angle from =0.01-0.1 and
find that even well collimated outflows effectively sweep up and entrain
significant core mass. The Stage 0 lifetime ranges from 0.14-0.19 Myr, which is
similar to the observed Class 0 lifetime. The star formation efficiency of the
cores spans 0.41-0.51. In all cases, the outflows drive strong turbulence in
the surrounding material. Although the initial core turbulence is purely
solenoidal by construction, the simulations converge to approximate
equipartition between solenoidal and compressive motions due to a combination
of outflow driving and collapse. When compared to a simulation of a cluster of
protostars, which is not gravitationally centrally condensed, we find that the
outflows drive motions that are mainly solenoidal. The final turbulent velocity
dispersion is about twice the initial value of the cores, indicating that an
individual outflow is easily able to replenish turbulent motions on sub-parsec
scales. We post-process the simulations to produce synthetic molecular line
emission maps of CO, CO, and CO and evaluate how well
these tracers reproduce the underlying mass and velocity structure.Comment: Accepted to ApJ, 17 pages, 15 figure
An Alternative Accurate Tracer of Molecular Clouds: The "-Factor"
We explore the utility of CI as an alternative high-fidelity gas mass tracer
for Galactic molecular clouds. We evaluate the X-factor for the 609
m carbon line, the analog of the CO X-factor, which is the ratio of the
H column density to the integrated CO(1-0) line intensity. We use
3D-PDR to post-process hydrodynamic simulations of turbulent, star-forming
clouds. We compare the emission of CI and CO for model clouds irradiated by 1
and 10 times the average background and demonstrate that CI is a comparable or
superior tracer of the molecular gas distribution for column densities up to cm. Our results hold for both reduced and full chemical
networks. For our fiducial Galactic cloud we derive an average of
cmKkms and of cmKkms.Comment: 5 pages, 4 figures, 1 table, accepted to MNRAS Letter
Turbocharger Dynamic Analysis: Advanced Design Simulation in Time Domain Using CFD Predicted Thermal Boundary Conditions
Small changes of surface temperature, clearance and bearing profile can significantly change stiffness and damping characteristics of slider bearings. This may influence dynamics and in case of turbochargers the rotor radial deflection. Noise, Vibration, Harshness (NVH) or durability issues like rotor colliding with housing may be implied as a consequence.
This paper presents a new methodology for dynamic turbocharger investigation. It considers multi-body dynamics (MBD) of flexible rotor and housing structures coupled with elasto-hydrodynamics (EHD) of the inner and outer oil film. The energy equation for calculation of oil film temperature is considered in EHD using hermal boundary condition obtained from 3D computational fluid dynamics (CFD) simulation. Typical targets for CFD simulation within the turbocharger development process are flow and thermal investigation as well as specifically providing accurate thermal boundary condition for thermo-mechanical fatigue analysis. For this purpose CFD analysis considers a fully coupled fluid-structure interaction. However, the same CFD model can be used to provide the required boundary conditions for dynamic analysis as well. The bearing profiles under thermal load are derived from Finite Element (FEM) analysis based on same thermal boundary conditions.
The authors demonstrate the application of the methodology for a typical turbocharger design study applied for heavy-duty engines with full floating bushings that have radial bore connections between inner and outer oil films. The rotor operating speed reaches up to 110 krpm. Dynamic simulation results with nominal clearance and temperature are compared with the results obtained when CFD and FEM predicted boundary conditions are used. Based on results for the oil film pressure and flow through each oil film as well as flow between inner and outer oil film a valid conclusion about the rotor dynamic behaviour, bearing mechanical and thermal loads can be made. The presented methodology proves to be a next level approach in prediction of turbocharger simulation in the development process
Coupled Elastodynamics of Piston Compression Ring Subject to Sweep Excitation
The piston compression ring's primary function is to seal the combustion chamber, thus mitigating gas leakage to the crankcase and avoiding loss of pressure loading. As a result, the ring is meant to conform closely to the cylinder surface which promotes increased friction. The compression ring is subjected to combustion pressure loading, ring tension, varying inertial force and friction. It is a slender ring of low mass, thus undergoes complex elastodynamic behaviour, when subjected to a multitude of forces. These motions occur in the ring's radial in-plane and axial out-of-plane dynamics, which comprise flutter, ring axial jump, compression-extension, ring twist and rotational drag. An implication of these motions can be loss of sealing, gas blow-by, loss of power and lubricant degradation/oil loss, to name but a few. Consequently, understanding and accurately predicting ring dynamic behaviour under transient conditions is an important step in any subsequent modelling for evaluation of cylinder system efficiency. There have been a plethora of investigations for ring dynamics, often decoupling the ring behaviour in its in-plane and out-of-plane motions. This approach disregards any transfer of dynamic energy from one degree of freedom to another which is only applicable to rectangular ring cross-sections. Alternatively, there are computationally intensive approaches such as finite element analysis which are not conducive for inclusion in any subsequent system level engine modelling where ring response alters in an instantaneous manner. This would require embedded finite element analysis within a transient analysis. This paper presents a finite difference numerical analysis for coupled in-plane and out-of-plane motions of compression rings with practical cross-sectional geometries, which are mostly not rectangular. The formulated method can be integrated into a system level transient cyclic analysis of ring-bore contact. The presented approach takes into account the energy transfer between different degrees of freedom. The predictions are validated against precise non-contact measurements of ring elastodynamic behaviour under amplitude-frequency sweeps. This approach has not hitherto been reported in literature and constitutes the main contribution of the paper
ALMA Cycle 1 Observations of the HH46/47 Molecular Outflow: Structure, Entrainment and Core Impact
We present ALMA Cycle 1 observations of the HH46/47 molecular outflow using
combined 12m array and ACA observations. The improved angular resolution and
sensitivity of our multi-line maps reveal structures that help us study the
entrainment process in much more detail and allow us to obtain more precise
estimates of outflow properties than previous observations. We use 13CO(1-0)
and C18O(1-0) emission to correct for the 12CO(1-0) optical depth to accurately
estimate the outflow mass, momentum and kinetic energy. This correction
increases the estimates of the mass, momentum and kinetic energy by factors of
about 9, 5 and 2, respectively, with respect to estimates assuming optically
thin emission. The new 13CO and C18O data also allow us to trace denser and
slower outflow material than that traced by the 12CO maps, and they reveal an
outflow cavity wall at very low velocities (as low as 0.2km/s with respect to
the cores central velocity). Adding with the slower material traced only by
13CO and C18O, there is another factor of 3 increase in the mass estimate and
50% increase in the momentum estimate. The estimated outflow properties
indicate that the outflow is capable of dispersing the parent core within the
typical lifetime of the embedded phase of a low-mass protostar, and that it is
responsible for a core-to-star efficiency of 1/4 to 1/3. We find that the
outflow cavity wall is composed of multiple shells associated with a series of
jet bow-shock events. Within about 3000AU of the protostar the 13CO and C18O
emission trace a circumstellar envelope with both rotation and infall motions,
which we compare with a simple analytic model. The CS(2-1) emission reveals
tentative evidence of a slowly-moving rotating outflow, which we suggest is
entrained not only poloidally but also toroidally by a disk wind that is
launched from relatively large radii from the source.Comment: Accepted for publication in ApJ. 26 pages, 20 figure
Etudes glaciaires, géographiques et botaniques dans le massif des Grandes Rousses - Alpes françaises
Article publié dans Etudes Glaciologiques : Tirol autrichien Massif des grandes Rousses. Ministère de l'Agriculture-Direction de l'Hydraulique- Services d'Etudes des grandes forces hydrauliques ( Région des Alpes) - Grenoble 1909 - pp 33-112Ce travail de 1905 repose sur un aperçu géomorphologique, phytogéographique et des mesures glaciologiques du massif des Grandes Rousses dans les Alpes françaises
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