Dust Settling and Clumping in MRI-turbulent Outer Protoplanetary Disks

Planetesimal formation is a crucial yet poorly understood process in planet formation. It is widely believed that planetesimal formation is the outcome of dust clumping by the streaming instability (SI). However, recent analytical and numerical studies have shown that the SI can be damped or suppressed by external turbulence, and at least the outer regions of protoplanetary disks are likely weakly turbulent due to magneto-rotational instability (MRI). We conduct high-resolution local shearing-box simulations of hybrid particle-gas magnetohydrodynamics (MHD), incorporating ambipolar diffusion as the dominant nonideal MHD effect, applicable to outer disk regions. We first show that dust backreaction enhances dust settling toward the midplane by reducing turbulence correlation time. Under modest level of MRI turbulence, we find that dust clumping is in fact easier than the conventional SI case, in the sense that the threshold of solid abundance for clumping is lower. The key to dust clumping includes dust backreaction and the presence of local pressure maxima, which in our work is formed by the MRI zonal flows overcoming background pressure gradient. Overall, our results support planetesimal formation in the MRI-turbulent outer protoplanetary disks, especially in ring-like substructures.

Heat Transfer in Highly Turbulent Separated Flows: A Review

The study of flows with a high degree of turbulence in boundary layers, near-wall jets, gas curtains, separated flows behind various obstacles, as well as during combustion is of great importance for increasing energy efficiency of the flow around various elements in the ducts of gas-dynamic installations. This paper gives some general characteristics of experimental work on the study of friction and heat transfer on a smooth surface, in near-wall jets, and gas curtains under conditions of increased free-stream turbulence. Taking into account the significant effect of high external turbulence on dynamics and heat transfer of separated flows, a similar effect on the flow behind various obstacles is analyzed. First of all, the classical cases of flow separation behind a single backward-facing step and a rib are considered. Then, more complex cases of the flow around a rib oriented at different angles to the flow are analyzed, as well as a system of ribs and a transverse trench with straight and inclined walls in a turbulent flow around them. The features of separated flow in a turbulized stream around a cylinder, leading to an increase in the width of the vortex wake, frequency of vortex separation, and increase in the average heat transfer coefficient are analyzed. The experimental results of the author are compared with data of other researchers. The structure of separated flow at high turbulence—characteristic dimensions of the separation region, parameters of the mixing layer, and pressure distribution—are compared with the conditions of low-turbulent flow. Much attention is paid to thermal characteristics: temperature profiles across the shear layer, temperature distributions over the surface, and local and average heat transfer coefficients. It is shown that external turbulence has a much stronger effect on the separated flow than on the boundary layer on a flat surface. For separated flows, its intensifying effect on heat transfer is more pronounced behind a rib than behind a step. The factor of heat transfer intensification by external turbulence is most pronounced in the transverse cavity and in the system of ribs.

Near Wake Development Behind Marine Propeller Model in Presence of Freestream Turbulence

Three-dimensional particle image velocimetry (PIV) experiments were conducted in the immediate near wake and up to seven diameters downstream of a three-bladed marine propeller model operating in two different inflow conditions: one with imposed freestream turbulence with intensity of 7% and streamwise integral length scale comparable to propeller geometry, and the second experiment with a quiescent inflow conditions as a reference. The resulting Reynolds number based on propeller chord and relative velocity is Re0.7R = 4.7 × 105. All components of radial transport of mean flow kinetic energy are analyzed and the largest contributor to the fluxes is found to be correlated to Reynolds shear stresses, resulting in radially outward flux in the wake. Two regions of the near wake are distinguishable with downstream extent dependent on the level of external turbulence. In the first region, immediately behind the propeller, shed tip vortices are very coherent and undergo grouping and roll-up around each other and the second region where the vortex merger process is complete and characterized by breakdown of vortices into small-scale turbulence. The latter region was found to occur earlier in the experiment with external turbulence. Conditional statistics of velocity fluctuations were employed and they show that outward interactions and sweep events contribute the most to the transfer of mean flow kinetic energy from the inner wake to the freestream.

Ionic Wind Application to Vortex Ring Generator and its Transportation Efficiency

In this study, ionic wind generated in corona discharge is focused for producing an air flow without having mechanical actuators. First, the kinetic energy conversion efficiency to ionic wind from electric power is experimentally estimated to be 0.32%. Then, it is confirmed that intermittent blows of ionic wind enable to produce vortex rings without using mechanical system. We adopt novel sub-chamber structure to avoid the concentration of the substance in a vortex ring low, so that the substance concentration transported to the target distance of 200 mm increases by 9%. As an application, the efficiency for moisture transportation is evaluated through experimental measurements. As a result, it is shown that the substance (moisture) can be transported at an efficiency of about 85% to target distance of 200 mm under conditions where the influence of external turbulence is small.

VISUAL ART AS AN INSTRUMENT OF REORGANIZATION OF POST-CATASTROPHIC MEMORY

This article provides analysis of the Art approach towards transformation of longstanding traumatic experience in different countries, suggests possible consequences for the society in case if the catastrophic memory remains un-reorganized and grounds conclusion as for need to review approaches towards the later problem in Ukraine. During last three decades of statehood one can observe dynamic movement of Ukraine toward cultural assertion. One of its important elements is the comprehension of public traumatic experience. The latter bares immense importance for the success of Ukraine as a state due to the fact that for generations Ukrainians have been accumulating memory of mournful events that took place in the lives of their ancestors without the possibility to carry out the proper work of sorrow. Topicality of the mentioned problem has a special meaning nowadays considering that the state of Ukraine apt for new bouleversements. In the circumstances of internal and external turbulence conclusion of a new social contract is crucial, at the same time the negative experience of gen- erations which was not proper transformed, does not allow to address current social problems rationally and to consolidate society. Much is already said as for the role of memory in recreation of the past for the sake of the future, but the place of artistic practices in this process remains complex and uncomprehended. How do artistic practices objectivate social life and assist heeling? Visual art resonates with private or collec- tive memory allowing new means of acceptation as well as perceptional rectification and conceptualization of experience associated with sorrow. This article suggests the theoretical and methodological analysis of the “work of sorrow” through the prism of artistic expressions, character- izes their influence of re-organization of traumatic memory and demonstrates the role of visual art as an instrument to operate with reminiscence.