The effect of the Coriolis force on Kelvin-Helmholtz-driven mixing in protoplanetary disks

We study the stability of proto-planetary disks with vertical velocity gradients in their equilibrium rotation rates; such gradients are expected to develop when dust settles into the midplane. Using a linear stability analysis of a simple three-layer model, we show that the onset of instability occurs at a larger value of the Richardson number, and therefore for a thicker layer, when the effects of Coriolis forces are included. This analysis also shows that even-symmetry (midplane-crossing) modes develop faster than odd-symmetry ones. These conclusions are corroborated by a large number of nonlinear numerical simulations with two different parameterized prescriptions for the initial (continuous) dust distributions. Based on these numerical experiments, the Richardson number required for marginal stability is more than an order of magnitude larger than the traditional 1/4 value. The dominant modes that grow have horizontal wavelengths of several initial dust scale heights, and in nonlinear stages mix solids fairly homogeneously over a comparable vertical range. We conclude that gravitational instability may be more difficult to achieve than previously thought, and that the vertical distribution of matter within the dust layer is likely globally, rather than locally, determined.Comment: Accepted for publication in Ap

Dust sedimentation and self-sustained Kelvin-Helmholtz turbulence in protoplanetary disk mid-planes. I. Radially symmetric simulations

We perform numerical simulations of the Kelvin-Helmholtz instability in the mid-plane of a protoplanetary disk. A two-dimensional corotating slice in the azimuthal--vertical plane of the disk is considered where we include the Coriolis force and the radial advection of the Keplerian rotation flow. Dust grains, treated as individual particles, move under the influence of friction with the gas, while the gas is treated as a compressible fluid. The friction force from the dust grains on the gas leads to a vertical shear in the gas rotation velocity. As the particles settle around the mid-plane due to gravity, the shear increases, and eventually the flow becomes unstable to the Kelvin-Helmholtz instability. The Kelvin-Helmholtz turbulence saturates when the vertical settling of the dust is balanced by the turbulent diffusion away from the mid-plane. The azimuthally averaged state of the self-sustained Kelvin-Helmholtz turbulence is found to have a constant Richardson number in the region around the mid-plane where the dust-to-gas ratio is significant. Nevertheless the dust density has a strong non-axisymmetric component. We identify a powerful clumping mechanism, caused by the dependence of the rotation velocity of the dust grains on the dust-to-gas ratio, as the source of the non-axisymmetry. Our simulations confirm recent findings that the critical Richardson number for Kelvin-Helmholtz instability is around unity or larger, rather than the classical value of 1/4Comment: Accepted for publication in ApJ. Some minor changes due to referee report, most notably that the clumping mechanism has been identified as the streaming instability of Youdin & Goodman (2005). Movies of the simulations are still available at http://www.mpia.de/homes/johansen/research_en.ph

Effect of a span wise flow on the laminar-turbulent transition

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.The boundary-layer transition to turbulence has been the subject of research for a long time. The transition process, however, has not yet been fully explained though the final stage of the boundary-layer transition has been explained that some small turbulent sources (turbulent spots) occur suddenly in the boundary layer. As these turbulent spots fill up the boundary layer, they induce a transition to turbulence of a laminar boundary layer. The most important process is the turbulentspot appearance in the transitional boundary layer because a progression to turbulence that is not present in the laminar state is promoted. The process of the transition from the prior state to turbulent spots, however, has not been ascertained. Thus, the mechanism of turbulent-spot appearance has only been explained as the word "breakdown". The computational simulation by Brandt [1] demonstrates that the breakdown is induced by the interaction of streaks which move laterally and slowly. Meanwhile, we investigate a downstream development of a single hair-pin-type vortex generated by an artificial small jet. From the velocity field measured in detail, this vortex grows and increases in number downstream, and finally the developed vortices constitute a spot. In the initial stage of downstream development where the vortices propagate in the streamwise direction, the velocity perturbations in a spot reiterate the in-phase wave form. In addition, the low- and the high-speed streaks in the spot are elongated straight in streamwise direction. In the transition stage, it is shown that the amplitude in the instantaneous velocity signals in the spot become irregular locally, where the low- and the high-speed streaks distort laterally. Further downstream, it is clarified that the occurrence of the momentum-transfer accompanied with local and temporary ejection movements and sweep movements become irregular in the spot, where the low-speed and the highspeed streaks cross one another and switch their positions with each other in spanwise direction. The appearance of the crossover of the streaks shows the break-up of the spot structure, i.e., the beginning of its breakdown. A crossover of the streaks produces a new crossover in a chain reaction, so that the transition to turbulence (breakdown) progress rapidly. And finally the spot enter into a turbulent region. The irregularity in the velocity field, showing the other distinct feature of the spot, is occurred owing to the distortion of streaks in spanwise direction and, therefore, the streaks cross one another. Thus, we considered that these characteristics of velocity field are induced by a spanwise flow. In this study, we pay much attention to a spanwise flow and investigate its effect on the boundary-layer transition. From the measurement of the streamwise and spanwise component of velocity using a small X-type hot-wire probe, we found that the spanwise distortion of the velocity field and the irregularity of the velocity perturbation are caused by the spanwise flow in the spot. These results show that the spanwise flow have a critical role in laminar-turbulent transition.dc201

Laboratory study on heterogeneous decomposition of methyl chloroform on various standard aluminosilica clay minerals as a potential tropospheric sink

International audienceMethyl chloroform (1,1,1-trichloroethane, CH3CCl3) was found to decompose heterogeneously on seven types of standard clay minerals (23 materials) in dry air at 313 K in the laboratory. All reactions proceeded through the elimination of HCl; CH3CCl3 was converted quantitatively to CH2=CCl2. The activities of the clay minerals were compared via their pseudo-first-order reaction rate constants (k1). A positive correlation was observed between the k1 value and the specific surface area (S) of clay minerals, where the S value was determined by means of the general Brunauer-Emmett-Teller (BET) equation. The k1 value was anti-correlated with the value of n, which was a parameter of the general BET equation and related to the average pore size of the clay minerals, and correlated with the water content that can be removed easily from the clay minerals. The reaction required no special pretreatment of clay minerals, such as heating at high temperatures; hence, the reaction can be expected to occur in the environment. Photoillumination by wavelengths present in the troposphere did not accelerate the decomposition of CH3CCl3, but it induced heterogeneous photodecomposition of CH2=CCl2. The temperature dependence of k1, the adsorption equilibrium coefficient of CH3CCl3 and CH2=CCl2, and the surface reaction rate constant of CH3CCl3 were determined for an illite sample. The k1 value increased with increasing temperature. The amount of CH3CCl3 adsorbed on the illite during the reaction was proportional to the partial pressure of CH3CCl3. The reaction was sensitive to relative humidity and the k1 value decreased with increasing relative humidity. However, the reaction was found to proceed at a relative humidity of 22% at 313 K, although the k1 value was about one-twentieth of the value in non-humidified air. The conditions required for the reaction may be present in major desert regions of the world. A simple estimation indicates that the possible heterogeneous decomposition of CH3CCl3 on the ground surface in arid regions is worth taking into consideration when inferring the tropospheric lifetime of CH3CCl3 and global OH concentration from the global budget concentration of CH3CCl3

NN-body Simulation of Planetesimal Formation Through Gravitational Instability of a Dust Layer

We performed N-body simulations of a dust layer without a gas component and examined the formation process of planetesimals. We found that the formation process of planetesimals can be divided into three stages: the formation of non-axisymmetric wake-like structures, the creation of aggregates, and the collisional growth of the aggregates. Finally, a few large aggregates and many small aggregates are formed. The mass of the largest aggregate is larger than the mass predicted by the linear perturbation theory. We examined the dependence of system parameters on the planetesimal formation. We found that the mass of the largest aggregates increase as the size of the computational domain increases. However the ratio of the aggregate mass to the total mass $M_\mathrm{aggr}/M_\mathrm{total}$ is almost constant $0.8-0.9$. The mass of the largest aggregate increases with the optical depth and the Hill radius of particles.Comment: 34 pages, 11 figures. Accepted for publication in Ap

Monoiodoacetic acid induces arthritis and synovitis in rats in a dose- and time-dependent manner: proposed model-specific scoring systems

SummaryObjectiveIn a rat monoiodoacetic acid (MIA)-induced arthritis model, the amount of MIA commonly used was too high, resulting in rapid bone destruction. We examined the effect of MIA concentrations on articular cartilage and infrapatellar fat pad (IFP). We also established an original system for “macroscopic cartilage and bone score” and “IFP inflammation score” specific to the rat MIA-induced arthritis model.DesignMale Wistar rats received a single intra-articular injection of MIA in the knee. The amount of MIA was 0.1, 0.2, 0.5, and 1 mg respectively. Articular cartilage was evaluated at 2–12 weeks. IFP was also observed at 3–14 days.ResultsMacroscopically, low MIA doses induced punctate depressions on the cartilage surface, and cartilage erosion proceeded slowly over 12 weeks, while higher MIA doses already induced cartilage erosion at 2 weeks, followed by bone destruction. MIA macroscopic cartilage and bone score, OARSI histological score, and Mankin score increased in a dose- and time-dependent manner. The IFP inflammation score peaked at 5 days in low dose groups, then decreased, while in high dose groups, the IFP score continued to increase over 14 days due to IFP fibrosis.ConclusionsPunctate depressions, cartilage erosion, and bone destruction were observed in the MIA-induced arthritis model. The macroscopic cartilage and bone scoring enabled the quantification of cartilage degeneration and demonstrated that MIA-induced arthritis progressed in a dose- and time-dependent manner. IFP inflammation scores revealed that 0.2 mg MIA induced reversible synovitis, while 1 mg MIA induced fibrosis of the IFP body

On the Formation of Planetesimals via Secular Gravitational Instabilities with Turbulent Stirring

We study the gravitational instability (GI) of small solids in a gas disk as a mechanism to form planetesimals. Dissipation from gas drag introduces secular GI, which proceeds even when standard GI criteria for a critical density or Toomre's $Q$ predict stability. We include the stabilizing effects of turbulent diffusion, which suppresses small scale GI. The radially wide rings that do collapse contain up to $\sim 0.1$ Earth masses of solids. Subsequent fragmentation of the ring (not modeled here) would produce a clan of chemically homogenous planetesimals. Particle radial drift time scales (and, to a lesser extent, disk lifetimes and sizes) restrict the viability of secular GI to disks with weak turbulent diffusion, characterized by $\alpha \lesssim 10^{-4}$. Thus midplane dead zones are a preferred environment. Large solids with radii $\gtrsim 10$ cm collapse most rapidly because they partially decouple from the gas disk. Smaller solids, even below $\sim$ mm-sizes could collapse if particle-driven turbulence is weakened by either localized pressure maxima or super-Solar metallicity. Comparison with simulations that include particle clumping by the streaming instability shows that our linear model underpredicts rapid, small scale gravitational collapse. Thus the inclusion of more detailed gas dynamics promotes the formation of planetesimals. We discuss relevant constraints from Solar System and accretion disk observations.Comment: Accepted for publication in the Astrophysical Journal; 20 pages, 10 figure

Solar neutrino-electron scattering as background limitation for double beta decay

The background on double beta decay searches due to elastic electron scattering of solar neutrinos of all double beta emitters with Q-value larger than 2 MeV is calculated, taking into account survival probability and flux uncertainties of solar neutrinos. This work determines the background level to be [1-2]E-7 counts /keV/kg/yr, depending on the precise Q-value of the double beta emitter. It is also shown that the background level increases dramatically if going to lower Q-values. Furthermore, studies are done for various detector systems under consideration for next generation experiments. It was found that experiments based on loaded liquid scintillator have to expect a higher background. Within the given nuclear matrix element uncertainties any approach exploring the normal hierarchy has to face this irreducible background, which is a limitation on the minimal achievable background for purely calorimetric approaches. Large scale liquid scintillator experiments might encounter this problem already while exploring the inverted hierarchy. Potential caveats by using more sophisticated experimental setups are also discussed