Instantaneous and time-averaged flow structures around a blunt double-cone with or without supersonic film cooling visualized via nano-tracer planar laser scattering

In a Mach 3.8 wind tunnel, both instantaneous and time-averaged flow structures of different scales around a blunt double-cone with or without supersonic film cooling were visualized via nano-tracer planar laser scattering (NPLS), which has a high spatiotemporal resolution. Three experimental cases with different injection mass flux rates were carried out. Many typical flow structures were clearly shown, such as shock waves, expansion fans, shear layers, mixing layers, and turbulent boundary layers. The analysis of two NPLS images with an interval of 5 μs revealed the temporal evolution characteristics of flow structures. With matched pressures, the laminar length of the mixing layer was longer than that in the case with a larger mass flux rate, but the full covered region was shorter. Structures like K-H (Kelvin-Helmholtz) vortices were clearly seen in both flows. Without injection, the flow was similar to the supersonic flow over a backward-facing step, and the structures were relatively simpler, and there was a longer laminar region. Large scale structures such as hairpin vortices were visualized. In addition, the results were compared in part with the schlieren images captured by others under similar conditions.     

Edge Plasma Performance of Lower Hybrid Wave Injection on the HL-1M Tokamak

Recently, the L-mode to H-mode (L-H) transition in tokamak plasma confinement was found to be related to the presence of the poloidal flow shear near the plasma edge. An important mechanism is the ion orbit loss caused by interaction with the limiter. A complementary explanation is the generation of poloidal flows by plasma fluctuations via the Reynolds stress and the poloidal spin-up of plasmas from poloidal asymmetryof particle and momentum sources.     

Large Eddy Simulation of Turbulent Channel Flow with 3D Roughness Using a Roughness Element Model

Large eddy simulation of turbulent channel flow with dense and small 3D roughness elements is carried out using a roughness element model Profiles of mean Reynolds stress, mean velocity and rms velocity as well as turbulent structures near the wall are obtained. The shear stress in the rough wall is larger than that in the smooth wall side and the rough wall has a larger influence on the channel flow. Profiles of mean streamwise velocity near the wall have logarithmic velocity distributions for both smooth and roughness walls, while there is a velocity decrease for the rough wall due to larger fractional drag. All the three components of rms velocities in the rough wall region are larger than that in the smooth wall region, and the roughness elements on the wall increase turbulent intensity in all directions. The s~reak spacing and average diameter of near wall quasi-s~reamwise vortices increase with the presence of roughness elements on the wall and it is shown that the rough wall induces complex and strong streamwise vortices. Results of dense and small 3D roughness elements in both turbulent statistics and structure, obtained with a relatively simple method, are found to be comparable to related experiments.     

Schamel equation in an inhomogeneous magnetized sheared flow plasma with q-nonextensive trapped electrons

An investigation is carried out for understanding the properties of ion–acoustic(IA) solitary waves in an inhomogeneous magnetized electron-ion plasma with field-aligned sheared flow under the impact of q-nonextensive trapped electrons. The Schamel equation and its stationary solution in the form of solitary waves are obtained for this inhomogeneous plasma. It is shown that the amplitude of IA solitary waves increases with higher trapping efficiency(β), while the width remains almost the same. Further, it is found that the amplitude of the solitary waves decreases with enhanced normalized drift speed, shear flow parameter and the population of the energetic particles. The size of the nonlinear solitary structures is calculated to be a few hundred meters and it is pointed out that the present results are useful to understand the solar wind plasma.     

Statistical Characterization of Boundary Fluctuations in the HT—7 Tokamak

The statistical properties of plasma fluctuations are characterized in the boundary region of HT-7 tokamak. A non-Gaussian feature is observed in fluctuations of ion saturation current and floating potential in most of the scrape-off layer regions. The statistical properties of fluctuations have a clear radial dependence, showing a near-Gaussian character in the proximity of the velocity shear layer location and another region where the poloidal velocity has a trend to zero. Fluctuations show a bursty character with pulses asymmetric in time and the time asymmetry reaches the minimum around the shear layer. From the results, we can see an obvious coupling of the pulses and the poloidal flow.     

Modification of Boundary Fluctuations by LHCD in the HT—7 Tokamak

Measurements of boundary fluctuations and fluctuation driven electron fluxes have been performed in ohmic and lower hybrid current drive enhanced confinement plasma using a graphite Langmuir probe array on HT-7 tokamak. The fluctuations are significantly suppressed and the turbulent fluxes are remarkably depressed in the enhanced plasma. We characterized the statistical properties of fluctuations and the particle flux and found a non-Gaussian character in the whole scrape-off layer with minimum deviations from Gaussian in the proximity of the velocity shear layer in ohmic plasma. In the enhanced plasma the deviations in the boundary region are all reduces obviously. The fluctuations and induced electron fluxes show sporadic bursts asymmetric in time and the asymmetry is remarkably weakened in the lower hybrid current driving (LHCD) phase. The results suggest a coupling between the statistical behaviour of fluctuations and the turbulent flow.     

Configuration with Low Central Magnetic Shear Formed in Pellet Injection Discharge on HL-2A

Control and optimization of the plasma current profile is a key point in enhancing the plasma performance. Although several tools have been identified to modify transport directly, the effect of the current profile on transport is large and remains an important transport control feature. In the tokamak experiments it is demonstrated that the configuration with flat q-profile in the central plasma region is beneficial to improve plasma confinement. Discharges with internal transport barrier （ITB） have been established with low magnetic shear （so called optimized shear operation mode） in JET and ASDEX Upgrade.     

Effects of the injector geometry on a sonic jet into a supersonic crossflow

Large-eddy simulation of a sonic injection from circular and elliptic injectors into a supersonic crossflow has been performed.The effects of injector geometry on various fundamental mechanisms dictating the intricate flow phenomena including shock/jet interaction,jet shear layer vortices and their evolution,jet penetration properties and the relevant turbulence behaviors have been studied systematically.As a jet issuing transversely into a supersonic crossflow,salient three-dimensional shock and vortical structures,such as bow,separation and barrel shocks,Mach disk,horseshoe vortex,jet shear layer vortices and vortex pairs,are induced.The shock structures exhibit considerable deformations in the circular injection,while their fluctuation becomes smaller in the elliptic injection.The jet shear layer vortices are generated at the jet periphery and their evolution characteristics are analyzed through tracing the centroid of these coherent structures.It is found that the jet from the elliptic injector spreads rapidly in the spanwise direction but suffers a reduction in the transverse penetration compared to the circular injection case.The turbulent fluctuations are amplified because of the jet/crossflow interaction.The vertical Reynolds normal stress is enhanced in the downstream of the jet because of the upwash velocity induced by the counter-rotating vortex pair.     

Experimental Study on Transitional Flow in a Circular Microtube

We investigate the characteristics of the transition from laminar to turbulent flow in the microtube with a diameter of 310μm. The microscopic particle image velocimetry is used to measure the water flow at Re =1600-2500 in the microtube. It is found that the flow transition occurs at Re=1600-1900, and the streamwise streaks and vortices appear in the transitional flow fields. These experimental observations provide a validation for the theoretical prediction of unstable travelling waves in pipe flow.     

Measurements of Reynolds stress and turbulence in the boundary plasma of the HT-7 tokamak

Measurements of electric field fluctuations, Reynolds stress and poloidal flow have been performed in the boundary region of the HT-7 tokamak using a Langmuir probe array.Sheared radial electric field and poloidal flow have been found in the vicinity of the limiter and the turbulence has been clearly modified in this region. Furthermore,the electrostatic Reynolds stress component shows a radial gradient close to the velocity shear layer location.All results here indicate that the radial gradient of Reynolds stress may play an important role in the driving of poloidal flows in the plasma boundary region.     

Out-of-plane shear flow effects on fast magnetic reconnection in a two-dimensional hybrid simulation model

The effects of out-of-plane shear flows on fast magnetic reconnection are numerically investigated by a twodimensional(2D)hybrid model in an initial Harris sheet equilibrium with flows.The equilibrium and driven shear flows out of the 2D reconnection plane with symmetric and antisymmetric profiles respectively are used in the simulation.It is found that the out-of-plane flows with shears in-plane can change the quadrupolar structure of the out-of-plane magnetic field and,therefore,modify the growth rate of magnetic reconnection.Furthermore,the driven flow varying along the anti-parallel magnetic field can either enhance or reduce the reconnection rate as the direction of flow changes.Secondary islands are also generated in the process with converting the initial X-point into an O-point.     

Effects of a liquid lithium curtain as the first wall in a fusion reactor plasma

This paper explores the effect of a liquid lithium curtain on fusion reactor plasma, such curtain is utilized as the first wall for the engineering outline design of the Fusion Experimental Breeder （FEB-E）. The relationships between the surface temperature of a liquid lithium curtain and the effective plasma charge, fuel dilution and fusion power production have been derived. Results indicate that under normal operation, the evaporation of liquid lithium does not seriously affect the effective plasma charge, but effects on fuel dilution and fusion power are more sensitive. As an example, it has investigated the relationships between the liquid lithium curtain flow velocity and the rise of surface temperature based on operation scenario II of the FEB-E design with reversed shear configuration and high power density. Results show that even if the liquid lithium curtain flow velocity is as low as 0.5 m/s, the effects of evaporation from the liquid lithium curtain on plasma are negligible. In the present design, the sputtering of liquid lithium curtain and the particle removal effects of the divertor are not yet considered in detail. Further studies are in progress, and in this work implication of lithium erosion and divertor physics on fusion reactor operation are discussed.[第一段]     

Simulation of Dynamics in Two-Dimensional Vortex Systems in Random Media

Dynamics in two-dimensional vortex systems with random pinning centres is investigated using molecular dynamical simulations. The driving force and temperature dependences of vortex velocity are investigated. Below the critical depinning force Fc, a creep motion of vortex is found at low temperature. At forces slightly above Fc, a part of vortices flow in winding channels at zero temperature. In the vortex channel flow region, we observe the abnormal behaviour of vortex dynamics： the velocity is roughly independent of temperature or even decreases with temperature at low temperatures. A phase diagram that describes different dynamics of vortices is presented.     

Role of Advance Refuelling and Heating on Edge Reynolds Stress—Induces Poloidal Flow in HL—1M

The radial profile of electrostatic Reynolds stress,plasma poloidal rotations,radial and poloidsal electric fields have been measured in the plasma boundary region of the HL-1M tokamak using a multiarray of Mach/Langmuir probes.In the experiments of ohmic discharge,lower hybrid current drive,supersoniuc molecular beam injection (SMBI)and multi-shot pellet injection,the correlation between the Reynolds stress and poloidal flow in the edge plasma is presented.The adial profile changes of the Reynolds stress and poloidal flow velocity Vpol with lower hybrid wave injection power and SMBI injection are obtained.The results indicate that the sheared poloidal flow can be generated in tokamak plasma due to the radially varying Reynolds stress.     

Pion transverse-momentum spectrum,elliptic flow,and Hanbury-Brown-Twiss interferometry in a viscous granular source model

We examine the evolution of quark-gluon plasma(QGP) droplets with viscous hydrodynamics and analyze the pion transverse-momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss(HBT) interferometry in a granular source model consisting of viscous QGP droplets. The shear viscosity of the QGP droplet speeds up and slows down the droplet evolution in the central and peripheral regions of the droplet, respectively. The effect of the bulk viscosity on the evolution is negligible. Although there are viscous effects on the droplet evolution, the pion momentum spectrum and elliptic flow change little for granular sources with and without viscosity. On the other hand,the influence of viscosity on HBT radius Rout is considerable. It makes Rout decrease in the granular source model.We determine the model parameters of granular sources using the experimental data of pion transverse-momentum spectrum, elliptic flow, and HBT radii together, and investigate the effects of viscosity on the model parameters. The results indicate that the granular source model may reproduce the experimental data of pion transverse-momentum spectrum, elliptic flow, and HBT radii in heavy-ion collisions of Au-Au at√s_(NN)=200 GeV and Pb-Pb at√s_(NN) =2.76 Te V in different centrality intervals. The viscosity of the droplet leads to an increase in the initial droplet radius and a decrease of the source shell parameter in the granular source model.     

Measurements of Boundary Plasma in Synergy Discharges of IBW and LHCD on the HT-7 Tokamak

By applying ion Bernstein wave (IBW) heating into the lower hybrid current drive (LHCD) plasma, improved confinements have been obtained in the HT-7 tokamak. The central electron temperature was doubled and the storage energy was increased significantly. The core electron density and temperature were broadened and ther profiles near the edge were steepened. A transport barrier has been formed in the vicinity of the limlter radial location. An enhanced shear in poloidal phase velocity was found in the same region with reduction of the fluctuation levels and the coherences between fluctuations. The results suggest that the improved confinement in the IBW and LHCD plasma is at least partially due to the modification of shear in poloidal velocity and then the suppression of fluctuations and fluctuation induced fluxes via de-correlation effect.     

Transient Appearance of Double Spontaneous-Snakes in EAST H Mode Plasma

Transient lasting of double spontaneous snakes is observed in energy confinement improved plasma on the EAST tokamak. The destabilization of snakes with superb particle confinement can lower plasma energy confinement and slow plasma toroidal rotation. The non-monotonic q profile with double q=1 surface, slightly above unity in central, is constructed. The prevailing harmonic modes are susceptible to the nearly zero magnetic shear. Finally, the role of anomalous particle transport and the formation of off-axis peaking electron profile on the excitation of double snakes are discussed.     

The effect of two-dimensional shear flow on the stability of a crystal interface in a supercooled melt

A model is developed based on the time-related thermal diffusion equations to investigate the effects of twodimensional shear flow on the stability of a crystal interface in the supercooled melt of a pure substance.Similar to the three-dimensional shear flow as described in our previous paper,the two-dimensional shear flow can also be found to reduce the growth rate of perturbation amplitude.However,compared with the case of the Laplace equation for a steady-state thermal diffusion field,due to the existence of time partial derivatives of the temperature fields in the diffusion equation the absolute value of the gradients of the temperature fields increases,therefore destabilizing the interface.The circular interface is more unstable than in the case of Laplace equation without time partial derivatives.The critical stability radius of the crystal interface increases with shearing rate increasing.The stability effect of shear flow decreases remarkably with the increase of melt undercooling.     

The effect of two-dimensional of a crystal interface in shear flow on the stability a supercooled melt

A model is developed based on the time-related thermal diffusion equations to investigate the effects of twodimensional shear flow on the stability of a crystal interface in the supercooled melt of a pure substance. Similar to the three-dimensional shear flow as described in our previous paper, the two-dimensional shear flow can also be found to reduce the growth rate of perturbation amplitude. However, compared with the case of the Laplace equation for a steady-state thermal diffusion field, due to the existence of time partial derivatives of the temperature fields in the diffusion equation the absolute value of the gradients of the temperature fields increases, therefore destabilizing the interface. The circular interface is more unstable than in the case of Laplace equation without time partial derivatives. The critical stability radius of the crystal interface increases with shearing rate increasing. The stability effect of shear flow decreases remarkably with the increase of melt undercooling.     

Late-Stage Vortical Structures and Eddy Motions in a Transitional Boundary Layer

A high-order direct numerical simulation of flow transition over a flat-plate at a free stream Mach number 0.5 is carried out. Formation and development of three-dimensional vortical structures, typically shown as λ-vortices, hairpin vortices and ring-like vortices, are observed. Numerical results show that there is a strong downdraft motion of fluid excited by every ring-like vortex in the late-stage of the transition process. At two sides of the vortical structure centerline, the downdraft motions induced by the ring-like vortex and the rotating legs superimpose. This is responsible for the appearance of a high-speed streak associated with the positive spike observed in a previous investigation and the appearance of a high-shear layer in the near wall region.