Hydrodynamic role of longitudinal dorsal ridges in a leatherback turtle swimming

Leatherback sea turtles (Dermochelys coriacea) are known to have a superior diving ability and be highly adapted to pelagic swimming. They have five longitudinal ridges on their carapace. Although it was conjectured that these ridges might be an adaptation for flow control, no rigorous study has been performed to understand their hydrodynamic roles. Here we show that these ridges are slightly misaligned to the streamlines around the body to generate streamwise vortices, and suppress or delay flow separation on the carapace, resulting in enhanced hydrodynamic performances during different modes of swimming. Our results suggest that shapes of some morphological features of living creatures, like the longitudinal ridges of the leatherback turtles, need not be streamlined for excellent hydro- or aerodynamic performances, contrary to our common physical intuition.ope

Effects of rotor???rotor interaction on the wake characteristics of twin rotors in axial descent

In this study, the effects of rotor???rotor interaction on wake characteristics were investigated experimentally for a twin-rotor configuration in axial descent. The wake velocities were measured at descent rates (descent speed/induced velocity at the rotor disk during hover) from 0.87 to 1.52, and the rotor???rotor interaction strength was controlled by adjusting the distance between the rotor tips. As the descent rate increased, the wake of the isolated rotor gradually entered the vortex ring state (VRS), where the flow established an extensive recirculation zone. Correlation analysis was performed to distinguish the rotor wake between tubular and VRS topologies. The flow states for the isolated rotor were classified into pre-VRS, incipient VRS, and fully developed VRS, depending on the probability of vortex ring formation. The results reveal that the effects of rotor???rotor interaction on the wake characteristics of twin rotors differ depending on the descent rate, distance between rotor tips, and wake region. In the outer region, the flow state of the rotor wake remains consistent with that of the isolated rotor, irrespective of the distance between rotor tips. Conversely, the strong rotor???rotor interaction changes the flow state in the inner region by disrupting the vortex ring structure, intensifying the wake asymmetry about the rotational axis. The thrust measurements show that under the VRS, as the two rotors get closer, the thrust coefficient increases until vortex ring disruption occurs, and then decreases after the vortex ring is disrupted

Influence of the surface roughness on inner-outer interactions in a turbulent Couette-Poiseuille flow

When rod surface roughness is introduced in a turbulent Couette-Poiseuille flow (CP-flow), it is known that the Reynolds stresses near the centerline decrease due to weakened very-large-scale motions (VLSMs) and roll-cell motions [Lee, Y. M. et al., "Direct numerical simulation of a turbulent Couette-Poiseuille flow with a rod-roughened wall," Phys. Fluids 30, 105101 (2018)]. In the present study, we examine the origin of the weakened turbulent structures near the centerline in a CP-flow with roughness (CPR-flow) using a dataset from direct numerical simulation. The top-down and bottom-up interactions to organize a CP-flow are very similar to those found in earlier studies in turbulent channel/pipe and boundary layer flows. The circulation of roll-cells in the outer region induces the spanwise congregation of negative streamwise velocity fluctuating structures (u) near the wall, leading to a large-scale ejection into the outer region. This large-scale ejection contributes to the formation of a negative VLSM when two adjacent negative large-scale motions merge, and the VLSM induces the circulation of roll-cell motion due to the pure kinematics. A similar process for the inner-outer interactions is found for a CPR-flow. However, because the impact of the surface roughness suppresses the collective motion of negative u-structures near the surface roughness, strong congregation by roll-cells is observed to occur far from the wall, indicating that relatively few negative u-structures with low strength contribute to the formation of a large-scale ejection for the CPR-flow. The weakened large-scale ejection decreases the strength of the VLSM, resulting in weakened roll-cell motion

Effects of Rotor-Rotor Interaction on the Wake Structure and Thrust Generation of a Quadrotor Unmanned Aerial Vehicle

In this paper, the effects of rotor-rotor interaction on the wake structure and thrust generation of a quadrotor unmanned aerial vehicle (UAV) are experimentally investigated in the rotor tip Reynolds number range of 34000 ??? 54000. The interaction strength is manipulated by varying the number of rotating rotors and the normalized rotor separation distance. A stronger rotor-rotor interaction places the inner tip vortices between rotors closer to each other, forming an upflow region through vortex pairing and intensifying the turbulence intensity between rotors. To comprehensively evaluate the effect of interaction on the wake structure, we propose a modified Landgrebe???s model that accurately describes the wake boundary of UAV, given the number of rotating rotors and the normalized rotor separation distance. The wake analysis based on the model shows that the stronger the rotor-rotor interaction, the less the wake contracts and the closer the vena contracta moves to the rotor-tip path plane. The momentum theory combined with the modified Landgrebe???s model shows that the loss of axial momentum transfer due to the wake inclination is insufficient to account for the thrust loss caused by the rotor-rotor interaction. This paper shows that the shift of the inner tip vortex away from the rotational axis and the corresponding increase of induced axial velocity followed by a decrease in the local effective angle of attack is another important mechanism for the thrust loss

Distorted policy transfer? South Korea’s adaptation of UK social enterprise policy

This study draws upon communicative processes in policy transfer to consider the ways in which policy may be adapted to context or distorted. The theoretical framework is used to investigate exactly what the South Korean government borrowed from UK social enterprise policy. Despite claims that the UK was the source of both the general policy direction and the particular regulatory device, the Korean government did not learn about the specific contexts of the British policy, nor attempt two-way communication with domestic stakeholders. Rather, the UK policy was interpreted in accordance with the Korean government’s own ideas about how to utilize social enterprise. Historical legacies of top-down decision-making played an important role in this process, as did the state’s role as a regulator which mobilizes the private sector to achieve policy goals. The consequences have been negative for those organizations refused social enterprise status under the Ministry of Labor’s strict approval system, as well as for the original target population: the socially disadvantaged and vulnerable. It is suggested that the model advanced may help to illuminate the reasons why some borrowed policies differ considerably from the originals, and the use of policy transfer as a means of legitimizatio

Identification of gut dysbiosis in axial spondyloarthritis patients and improvement of experimental ankylosing spondyloarthritis by microbiome-derived butyrate with immune-modulating function

IntroductionDysbiosis is an environmental factor that affects the induction of axial spondyloarthritis (axSpA) pathogenesis. In the present study, we investigated differences in the gut microbiota of patients with axSpA and revealed an association between specific gut microbiota and their metabolites, and SpA pathogenesis.MethodUsing 16S rRNA sequencing data derived from feces samples of 33 axSpA patients and 20 healthy controls (HCs), we examined the compositions of their gut microbiomes.ResultsAs a result, axSpA patients were found to have decreased α-diversity compared to HCs, indicating that axSpA patients have less diverse microbiomes. In particular, at the species level, Bacteroides and Streptococcus were more abundant in axSpA patients than in HCs, whereas Faecalibacterium (F). prausnitzii, a butyrate-producing bacteria, was more abundant in HCs. Thus, we decided to investigate whether F. prausnitzii was associated with health conditions by inoculating F. prausnitzii (0.1, 1, and 10 μg/mL) or by administrating butyrate (0.5 mM) into CD4+ T cells derived from axSpA patients. The levels of IL-17A and IL-10 in the CD4+ T cell culture media were then measured. We also assessed osteoclast formation by administrating butyrate to the axSpA-derived peripheral blood mononuclear cells. The CD4+ IL-17A+ T cell differentiation, IL-17A levels were decreased, whereas IL-10 was increased by F. prausnitzii inoculation. Butyrate reduced CD4+ IL-17A+ T cell differentiation and osteoclastogenesis.DiscussionWe found that CD4+ IL-17A+ T cell polarization was reduced, when F. prausnitzii or butyrate were introduced into curdlan-induced SpA mice or CD4+ T cells of axSpA patient. Consistently, butyrate treatment was associated with the reduction of arthritis scores and inflammation levels in SpA mice. Taken together, we concluded that the reduced abundance of butyrate-producing microbes, particularly F. prausnitzii, may be associated with axSpA pathogenesis

Politics of social policy-making in South Korea and Japan

Until recently, welfare politics in Korea and Japan tended to be dominated by conservative forces "from above." This paper investigates the formation and domination of such forces from above, with a focus on the interaction between institutional arrangements and strategic maneuvering by political actors which, I argue, constitutes the politics of (social) policy-making. The notion of the politics of social policy-making aims to provide a more politically and institutionally sensitive framework than the pluralist analysis of policy-making. Korea and Japan share some crucial institutional legacies of the "developemental state" and "group-coordinated market economies." At the same time, attention should also be paid to different institutional configurations, such as a strong presidential system and first-past-the-post voting in Korea and a parliamentary system and single non-transferable vote in Japan. The main research question is, what are the similarities and differences in the politics of social policy-making between these two countries

Welfare Reform in Korea after the Economic Crisis

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