Exact solutions to one-dimensional acoustic fields with temperature gradient and mean flow

An exact solution for one-dimensional acoustic fields in ducts in the presence of an axial mean temperature gradient and mean flow is presented in this paper. The analysis is valid for mean Mach numbers such that the square of the mean Mach number is much less than one. The one-dimensional wave equation for ducts with axial mean temperature gradient and mean flow is derived. By appropriate transformations, the wave equation is reduced to an analytically solvable hypergeometric differential equation for the case of a linear mean temperature profile. The developed solution is applied to investigate the dependence of sound propagation in a duct on factors such as temperature gradient and mean flow. The results obtained using the analytical solution compare very well with the numerical results. The developed solution is also compared with an existing analytical solution.     

Current-driven electron drift solitons

The soliton formation by the current-driven drift-like wave is investigated for heavier ion (such as barium) plasma experiments planned to be performed in future. It is pointed out that the sheared flow of electrons can give rise to short scale solitary structures in the presence of stationary heavier ions. The nonlinearity appears due to convective term in the parallel equation of motion and not because of temperature gradient unlike the case of low frequency usual drift wave soliton. This higher frequency drift-like wave requires sheared flow of electrons and not the density gradient to exist.     

变延迟进动定制激发(DANTE)序列在工程中的优化设计

变延迟进动定制激发（Delays Alternating with Nutation for Tailored Excitation,DANTE）序列作为一种黑血预脉冲序列,通过连续施加小角度激发脉冲,以及结合散相梯度,使得流动物质和静态物质达到不同的稳态,从而抑制流动的血液.对于静态物质而言,施加DANTE序列后在图像等间隔的位置会出现暗条纹,暗条纹的宽度与梯度幅值和小单元持续时间乘积相关：乘积越大,暗纹宽度越小.对于动态物质而言,为达到较好的抑制效果,需要增加整个DANTE序列模块的准备时间,并且增大梯度幅值和小单元持续时间的乘积.因此,该方法对于梯度系统的要求较高,而实际梯度放大器（Gradient Amplifier,GPA）有一定的限额.在有限的GPA条件下,为使得DANTE序列具有更好抑制流动信号效果,本文在读出方向以及片层旋转两个方面进行了梯度优化,实现了更好的黑血效果.     

Analytic Hessian derivation for the quasi-one-dimensional Euler equations

The Hessian for the quasi-one-dimensional Euler equations is derived. A pressure minimization problem and a pressure matching inverse problem are considered. The flow sensitivity, adjoint sensitivity, gradient and Hessian are calculated analytically using a direct approach that is specific to the model problems. For the pressure minimization problem we find that the Hessian exists and it contains elements with significantly larger values around the shock location. For the pressure matching inverse problem we find at least one case for which the gradient as well as the Hessian do not exist. In addition, two formulations for calculating the Hessian are proposed and implemented for the given problems. Both methods can be implemented in industrial applications such as large scale aerodynamic optimization.     

Intermittent transport associated with the geodesic acoustic mode near the critical gradient regime

Turbulent transport near the critical gradient in toroidal plasmas is studied based on global Landau-fluid simulations and an extended predator-prey theoretical model of ion temperature gradient turbulence. A new type of intermittent transport associated with the emission and propagation of a geodesic acoustic mode (GAM) is found near the critical gradient regime, which is referred to as GAM intermittency. The intermittency is characterized by new time scales of trigger, damping, and recursion due to GAM damping. During the recursion of intermittent bursts, stationary zonal flow increases with a slow time scale due to the accumulation of undamped residues and eventually quenches the turbulence, suggesting that a nonlinear upshift of the critical gradient, i.e., Dimits shift, is established through such a dynamical process.     

Computational study of the laminar to turbulent transition over the SD7003 airfoil in ground effect

In this work, we present a numerical study of the laminar-turbulence transition flow around a symmetrical air-foil at a low Reynolds number in free flow and near the ground surface at different angles of attack. Finite volume method is employed to solve the unsteady Reynolds-averaged Navier–Stokes (RANS) equation. In this way, the Transition SST turbulence model is used for modeling the flow turbulence. Flow around the symmetrical airfoil SD7003 is numerically simulated in free stream and near the ground surface. Our numerical method can detect different aspects of flow such as adverse pressure gradient, laminar separation bubble and laminar to turbulent transition onset and the numerical results are in good agreement with the experimental data.     

Effect of shape factor on the electrokinetic response of slot-like capillaries

The effect of the shape factor on the electrokinetic response of pressure-driven liquid flow through slot-like capillaries is analyzed in this work. The electrokinetic response is found by first solving for the electrical potential using the Poisson–Boltzmann equation, and then using it as an input to construct an external force term in the Navier–Stokes equation. It is found that flow properties are significantly modified for small pore sizes with hydraulic radius in the range of microns and submicrons, according to the properties of the electrolyte solution. The modified flow rate in the presence of such electrokinetic effects can be less than 50% of the predictions expected with conventional flow models such as Darcy's equation. Apparent viscosities larger than expected are calculated using the reduced flow. Actual values are dependent on the capillary dimensions, pressure gradient, solid-surface conductivity, and properties of the electrolyte fluid. For a rectangular capillary cross-sectional geometry the apparent viscosity and friction coefficients values decrease with the shape factor in a nontrivial way. These results are relevant for the modelling of certain porous formations of interest to the oil industry.     

Phase diagram of speed gradient model with an on-ramp

In this paper, phase transitions are investigated in speed gradient model with an on-ramp. Phase diagrams of traffic flow composed of manually driven vehicles and adaptive cruise control (ACC) vehicles are studied, respectively. The traffic flow composed of ACC vehicles is modeled by enhancing propagation speed of small disturbance. The phase diagram of traffic flow composed of manually driven vehicles is similar to that in previous works, in which such states as pinned localized cluster (PLC), moving localized cluster (MLC), triggered stop-and-go traffic (TSG), oscillatory congested traffic (OCT), and homogeneous congested traffic (HCT) are reproduced. In the phase diagram of traffic flow composed of ACC vehicles, traffic stability is enhanced and such states as PLC, MLC, and TSG disappear. Furthermore, some interesting phenomena, such as stationary OCT upstream of on-ramp and appearance of second OCT in HCT, are identified.     

Investigation of Turbulent Transition in Plane Couette Flows Using Energy Gradient Method

The energy gradient method has been proposed with the aim of better understanding the mechanism of flow transition from laminar flow to turbulent flow. In this method, it is demonstrated that the transition to turbulence depends on the relative magnitudes of the transverse gradient of the total mechanical energy which amplifies the disturbance and the energy loss from viscous friction which damps the disturbance, for given imposed disturbance. For a given flow geometry and fluid properties, when the maximum of the function $K$ (a function standing for the ratio of the gradient of total mechanical energy in the transverse direction to the rate of energy loss due to viscous friction in the streamwise direction) in the flow field is larger than a certain critical value, it is expected that instability would occur for some initial disturbances. In this paper, using the energy gradient analysis, the equation for calculating the energy gradient function $K$ for plane Couette flow is derived. The result indicates that $K$ reaches the maximum at the moving walls. Thus, the fluid layer near the moving wall is the most dangerous position to generate initial oscillation at sufficient high $\operatorname{Re}$ for given same level of normalized perturbation in the domain. The critical value of $K$ at turbulent transition, which is observed from experiments, is about 370 for plane Couette flow when two walls move in opposite directions (anti-symmetry). This value is about the same as that for plane Poiseuille flow and pipe Poiseuille flow (385-389). Therefore, it is concluded that the critical value of $K$ at turbulent transition is about 370-389 for wall-bounded parallel shear flows which include both pressure (symmetrical case) and shear driven flows (anti-symmetrical case).     

Thermomechanical coupling in a cylindrical hybrid-aligned nematic liquid crystal

A solution to the system of equations describing a cylindrical hybrid-aligned nematic liquid crystal is obtained. The rotational flow driven by vertical temperature gradient in such a cell is investigated theoretically. The cell is suggested as a new experimental setup for determining an additional relation required to measure the twelve thermomechanical coefficients. It is shown that the terms in the expressions for thermomechanical stress and heat flux obtained in [8] are equivalent to those originally proposed in [7].     

Evaluating the modulated gradient model in large eddy simulation of channel flow with OpenFOAM

Recently, a new family of subgrid-scale (SGS) models, termed as gradient-based models, has been introduced to calculate the SGS stresses in large eddy simulation (LES). In the present work, the modulated gradient model (MGM) was implemented in the OpenFOAM package, and the pimpleFoam solver was improved to be adopted with non-eddy viscosity models. The MGM is a new, nonlinear model that uses the local equilibrium hypothesis to assess the SGS kinetic energy and the velocity gradient tensor to calculate the relative weight of the different components of the SGS stress tensor. To evaluate the accuracy of the MGM along with the modified pimpleFoam solver, a turbulent channel flow was simulated at the three different frictional Reynolds numbers of 180, 395 and 590. Furthermore, the results were compared with direct numerical simulation data, as well as the numerical results obtained by the established SGS models such as the dynamic Smagorinsky model (DSM). A suitable accuracy for the first- and second-order turbulence parameters was reported. Moreover, it was demonstrated that MGM is computationally efficient compared to the DSM in treating channel flow.     

Separation of velocity distribution and diffusion using PFG NMR

Pulsed field gradient (PFG) NMR is applied to investigate flow processes. In this case the NMR signal experiences phase modulation due to flow and signal attenuation due to the distribution of velocities. The velocity distribution consists of one part originating from diffusion and of a second part, the distribution of the directed motion. The usual PFG-experiment in which the gradient strength is incremented cannot distinguish between both. Incrementing velocity at constant gradient strength keeps the contribution from diffusion constant but changes the absolute width of the velocity distribution. So the signal is attenuated again, but only due to the distribution of the directed motion. The phase modulation as a signature of flow is not affected by this strategy, because velocity and gradient strength are Fourier conjugated. The key advantage of this approach is the possibility of measuring very low velocities, which only cause a very slight phase modulation that is easily covered by diffusion. The method is discussed here for very slow flow in a rheometer cell.     

A steady state, supersonic flow solution of the Boltzmann equation

Nonclassical effects which appear in the supersonic nonequilibrium gas flow of a nonuniform relaxation described by the Boltzmann equation are studied. For such a flow, in particular, the heat flux and the temperature gradient have the same signs. Analytical and numerical results are presented. Possible experimental verification is discussed.     

直流纯氩层流等离子体射流的长度变化

采用主要由阴极、阳极以及介于阴极和阳极之间的中间段组成的直流非转移式电弧等离子体发生器,在大气压条件下,比较系统地研究了纯氩层流等离子体射流的长度随着弧电流、气体流量以及发生器结构而变化的规律。结果表明:层流射流的长度随弧电流和工作气流量的增加而增长;层流向湍流流动转变的临界气流量值随弧电流增大而提高;在发生器的伏安特性呈大梯度变化的情况下,射流长度随弧电流的变化幅度增大。     

Spiral demystified

Spiral acquisition schemes offer unique advantages such as flow compensation, efficient k-space sampling and robustness against motion that make this option a viable choice among other non-Cartesian sampling schemes. For this reason, the main applications of spiral imaging lie in dynamic magnetic resonance imaging such as cardiac imaging and functional brain imaging. However, these advantages are counterbalanced by practical difficulties that render spiral imaging quite challenging. Firstly, the design of gradient waveforms and its hardware requires specific attention. Secondly, the reconstruction of such data is no longer straightforward because k-space samples are no longer aligned on a Cartesian grid. Thirdly, to take advantage of parallel imaging techniques, the common generalized autocalibrating partially parallel acquisitions (GRAPPA) or sensitivity encoding (SENSE) algorithms need to be extended. Finally, and most notably, spiral images are prone to particular artifacts such as blurring due to gradient deviations and off-resonance effects caused by B₀ inhomogeneity and concomitant gradient fields. In this article, various difficulties that spiral imaging brings along, and the solutions, which have been developed and proposed in literature, will be reviewed in detail.     

Dynamic evolution of transverse energy flow in focused asymmetric optical vector-vortex beams

We present here controlled generation of asymmetric optical vector-vortex beams using a two-mode optical fiber and study the dynamic evolution of the transverse energy flow (TEF) when focused through a spherical lens. The dependence of the TEF on various factors such as the vortex charge, vortex anisotropy and polarization structure around the vortex core is explored. It is found that the TEF is directly proportional to the phase gradient and its direction is governed by the vortex charge. The presence of C-point polarization singularity in the beam and the polarization structure around it results in vibrational phase gradient which is the major factor deciding the TEF in vector-vortex beams.     

A study of the effect of self-generated pressure gradient sources on the resistive filamentation instability

In the field of fast ignition scheme, self-generated magnetic fields via beam resistive filamentation have a significant role in the angular divergence of the relativistic electron beam, which can be affected by the intensity of other self-generated magnetic fields. In this context, the effects of pressure gradient sources arising from temperature and density gradient of the pellet along the beam flow direction are investigated. The results showed that the resistive filamentation instability can be strongly amplified compared to the fully homogeneous plasma. In this respect, for the distance away from the critical surface, the instability is protected for a longer wave number. Also, the beam and plasma properties such as the beam relativistic factor, the beam number density, and the degree of the plasma temperature anisotropy might be effective.     

Pulse sequence design for MR velocity mapping of complex flow: Notes on the necessity of low echo times

Lowering of the echo time (TE) has been proposed as a way to reduce effects of phase dispersion in MR velocity mapping, because a low TE reduces sensitivity to higher-order motion terms while first-order velocity sensitivity is maintained. Methods of lowering TE involves the use of extreme gradient ramp times and gradient strengths as well as reduction of the duration of transmit/receive windows, the latter method causing decrements in image resolution. When reducing higher-order sensitivity, however, it is not the overall TE that is the critical parameter, but rather the time pattern of the gradients used in the experiment. Hence, changes in TE without subsequent variations in gradient pattern would, according to theory, not affect quantitative measurements of complex flow and vice versa. In this study, we experimentally demonstrate this relation and utilize the experience to create a sequence robust towards complex flow without sacrifices in image resolution. Our experimental observations show that variations in TE alone while maintaining the time course of the velocity-encoding gradient does not significantly affect measurements of through-plane average complex flow in the studied velocity range. A parameter that cannot be measured as accurately if TE is increased is the peak flow. A phase mapping sequence with prolonged TE from 3 ms to 5 ms but with short duration of the velocity-encoding (section-selective) gradient and improved in-plane resolution was demonstrated in vivo.     

Nonlinear instability research of longitudinal structure generated by roughness in unswept wing boundary layer

The results of experimental study of a nonlinear varicose instability of the streaky structure generated by roughness element in unswept-wing boundary layer are presented. Features of the varicose breakdown of longitudinal steady streaky structure such as modulation of structure in transverse and streamwise directions by secondary disturbance, occurrence of the new streaky structures and A-structures downstream are shown. Spatio-temporal pictures of the hot-wire visualization of flow during spatial evolution of the streaky structures under influence of secondary high-frequency disturbance are discussed. Features of the adverse pressure gradient influence upon processes of the nonlinear varicose instability evolution and flow structure are revealed. Essential influence of the adverse pressure gradient on evolution of disturbances in shown. Comparison of varicose instability of the streaky structures generated in two different ways (the roughness element as in the given work, and continuous air blowing as in the earlier published work) is the carried out. The work supported by the Ministry of Education and Science of the Russian Federation (Grant No. RNP. 2.1.2.3370) and by the Russian Foundation for Basic Research (Grant No. 05-01-00034).