Biomagnetic fluid flow in a channel with stenosis

In this study, the fundamental problem of the biomagnetic (blood) fluid flow in a channel with stenosis under the influence of a steady localized magnetic field is studied. The mathematical model used for the formulation of the problem is consistent with the principles of ferrohydrodynamics (FHD) and magnetohydrodynamics (MHD). Blood is considered as a homogeneous Newtonian fluid and is treated as an electrically conducting magnetic fluid which also exhibits magnetization. For the numerical solution of the problem, which is described by a coupled, non-linear system of PDEs, with appropriate boundary conditions, the stream function-vorticity formulation is adopted. The solution is obtained by the development of an efficient pseudotransient numerical methodology using finite differences. This methodology is based on the development of a semi-implicit numerical technique, transformations and stretching of the grid and proper construction of the boundary conditions for the vorticity. Results concerning the velocity and temperature field, skin friction and rate of heat transfer indicate that the presence of the magnetic field influences the flow field considerably.     

Temporal and spatial assessment of normal cerebrospinal fluid dynamics with MR imaging

The purpose of this study was to measure normal cerebrospinal fluid (CSF) pulsations within the intracranial and upper cervical subarachnoid spaces and the ventricular system. Phase contrast cine MR sequences were performed in sagittal and axial planes on 13 volunteers with flow encoding in the craniocaudal direction. CSF pulsations displayed considerable variations in healthy subjects, depending both on measurements localization and subjects, with CSF peak velocities ranging from 0 to 7 cm/s. In the subarachnoid spaces, the highest velocities occurred in the anterior location and increased from the cerebellar pontine angle cisterns towards the lower cervical spaces. In the ventricular system, the highest velocities occurred through the aqueduct of Sylvius. CSF flow within the third ventricle seemed to reflect a circular motion. There was a caudal net CSF flow in the aqueduct whereas in the upper cervical spaces net CSF flow was caudal anteriorly and cranial laterally. Velocity profiles of CSF pulsations demonstrated arterial morphology. After the R wave, caudal systolic motion was first observed in the posterior subarachnoid spaces, soon after in the anterior subarachnoid spaces and later in the ventricular system. Considering the morphology of CSF pathways, three successively initiated phenomena may explain the temporal course of CSF motion: the systolic expansion of the main arteries at the base of the brain, the systolic expansion of the cerebrospinal axis and, finally, the systolic expansion of the choroid plexuses.     

Experimental results on a hydrostatic bearing lubricated with a magnetorheological fluid

Magnetorheological fluids can be used as a smart lubricant as a result of the fact that its properties can be changed with the use of a magnetic field. Local flow resistance and local pressure can be generated by applying a local magnetic field. This work presents a hydrostatic bearing in which the pressure profile of a conventional hydrostatic bearing is recreated with solely the use of a magnetic field and a magnetorheological fluid. The magnetic field is applied only locally at the outer edges of the bearing with the use of an electromagnet. The principle is demonstrated with the use of an experimental setup and a model from literature.     

Vertical rotation effect on turbulence characteristics in an open channel flow

This paper solves the three-dimensional Navier-Stokes equation by a fractional-step method with the Reynolds number Reτ=194 and the rotation number Nτ=0-0.12. When Nτ is less than 0.06, the turbulence statistics relevant to the spanwise velocity fluctuation are enhanced, but other statistics are suppressed. When Nτ is larger than 0.06, all the turbulence statistics decrease significantly. Reynolds stress budgets elucidate that turbulence kinetic energy in the vertical direction is transferred into the streamwise and spanwise directions. The flow structures exhibit that the bursting processes near the bottom wall are ejected toward the free surface. Evident change of near-surface streak structures of the velocity fluctuations are revealed.     

Lattice Boltzmann simulation of fluid flows in two-dimensional channel with complex geometries

Boundary conditions (BCs) play an essential role in lattice Boltzmann (LB) simulations. This paper investigates several most commonly applied BCs by evaluating the relative L₂-norm errors of the LB simulations for two-dimensional (2-D) Poiseuille flow. It is found that the relative L₂-norm error resulting from FHML's BC is smaller than that from other BCs as a whole. Then, based on the FHML's BC, it formulates an LB model for simulating fluid flows in 2-D channel with complex geometries. Afterwards, the flows between two inclined plates, in a pulmonary blood vessel and in a blood vessel with local expansion region, are simulated. The numerical results are in good agreement with the analytical predictions and clearly show that the model is effective. It is expected that the model can be extended to simulate some real biologic flows, such as blood flows in arteries, vessels with stenosises, aneurysms and bifurcations, etc.     

Mechanism of controlling turbulent channel flow with the effect of spanwise Lorentz force distribution

A direct numerical simulation(DNS) is performed to investigate the control effect and mechanism of turbulent channel flow with the distribution of spanwise Lorentz force. A sinusoidal distribution of constant spanwise Lorentz force is selected, of which the control effects, such as flow characters, mean Reynolds stress, and drag reductions, at different parameters of amplitude A and wave number k_x are discussed. The results indicate that the control effects vary with the parameter A and k_x. With the increase of A, the drag reduction rate D_r first increases and then decreases rapidly at low k_x,and slowly at high k_x. The low drag reduction(or even drag increase) is due to a weak suppression or even the enhancements of the random velocity fluctuation and mean Reynolds stress. The efficient drag reduction is due to the quasi-streamwise vortex structure induced by Lorentz force, which contributes to suppressing the random velocity fluctuation and mean Reynolds stress, and the negative vorticity improves the distribution of streamwise velocity. Therefore, the optimal control effect with a drag reduction of up to 58% can be obtained.     

Effects of magnetic field and wall slip conditions on the peristaltic transport of a Newtonian fluid in an asymmetric channel

The effects of both magnetic field and wall slip conditions on the peristaltic transport of a Newtonian fluid in an asymmetric channel are studied analytically and numerically. The channel asymmetry is generated by propagation of waves on the channel walls travelling with different amplitudes, phases but with the same speed. The long wavelength and low Reynolds number assumptions are considered in obtaining solution for the flow. The flow is investigated in a wave frame of reference moving with velocity of the wave. Closed form expressions have been obtained for the stream function and the axial velocity component in fixed frame. The effects of phase difference, Knudsen number and magnetic field on the pumping characteristics and velocity field are discussed. Several known results of interest are found to follow as particular cases of the solution of the problem considered.     

带引流条导电矩形管磁流体湍流大涡数值模拟

为研究引流条对磁流体湍流的影响，采用自主开发的低磁雷诺数流固耦合磁流体相干结构模型大涡模拟求解器，对均匀磁场作用下平行层内带引流条导电矩形管和标准导电矩形管中液态金属湍流进行了数值模拟研究。结果表明，外加垂直流动方向的均匀磁场与流动的导电流体相互作用产生与流动方向相反的洛伦兹力，能够抑制磁流体的湍流脉动，这种抑制作用随着哈特曼数增大而增强。在弱导电率条件下，当Re=16350、Ha=212 时，两种管道中的流动均转换为层流流动状态。管道内壁面摩擦系数随着哈特曼数的增大而增大。引流条能在其近壁局部区域增强横向速度，有效激发湍流，但在弱壁面导电率条件下，带引流条导电矩形管壁面摩擦系数较标准矩形管大。     

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.     

Influence of flow on the structure and dynamics of clusters in complex plasma systems

The influence of flow with different strengths, positions, and widths on the structure and dynamics of clusters is studied by two-dimensional (2D) Langevin molecular dynamics simulations. The particles are confined by a quadratic confining potential. The horizontal position of the system centre, average inter-particle distance, and coupling parameter are calculated to characterize the effect of changing the strength, position, and width of the flow on the cluster structure. The trajectories, the velocity autocorrelation function, and the mean square displacement are obtained to further uncover the dynamic properties of the 2D dusty plasma system.     

Experimental study of heat transfer characteristics in oscillating fluid flow in tube

In the present work, an experimental investigation of heat transfer enhancement parameters of the oscillating flow heat exchanger under different frequencies, tidal displacement, and heat fluxes is carried out. The effect of different parameters on experimental effective thermal conductivity and convective heat transfer coefficient in cooling region is studied using correlations given by different researchers. A correlation for experimental effective thermal conductivity in terms of S²√ω is derived based on experimental data. This correlation is useful for predicting the optimum value of S²√ω before onset of turbulence.     

Effects of rotation and magnetic field on the nonlinear peristaltic flow of a second-order fluid in an asymmetric channel through a porous medium

In this paper, the effects of both rotation and magnetic field of the peristaltic transport of a second-order fluid through a porous medium in a channel are studied analytically and computed numerically. The material is represented by the constitutive equations for a second-order fluid. Closed-form solutions under the consideration of long wavelength and low Reynolds number is presented. The analytical expressions for the pressure gradient, pressure rise, friction force, stream function, shear stress, and velocity are obtained in the physical domain. The effects of the non-dimensional wave amplitude, porosity, magnetic field, rotation, and the dimensionless time-mean flow in the wave frame are analyzed theoretically and computed numerically. Numerical results are given and illustrated graphically in each case considered. Comparison was made with the results obtained in the presence and absence of rotation, magnetic field, and porosity. The results indicate that the effects of the non-dimensional wave amplitude, porosity, magnetic field, rotation, and the dimensionless time-mean flow are very pronounced in the phenomena.     

强梯度复杂流场中的粒子动力学响应试验研究

示踪粒子在(高)超声速流场中的动力学响应是粒子成像测速等粒子示踪测量技术的关键问题之一.现有文献对粒子动力学响应的试验测量往往是通过单个斜激波响应的测量方法.然而,当示踪粒子用于测量高速飞行器发动机内部复杂的激波串流场时,粒子将经历由多道激波导致的速度、压力、黏性等剧烈变化.本文结合目前(高)超声速飞行器的研究热潮,重点关注示踪粒子在应用于发动机内部具有连续激波的复杂流场测量中存在的跟随性评估方面,开展了一系列的相关试验研究.包括测量超声速风洞的喷管出口速度分布以验证测试系统的性能,在马赫4.2和3.0流场中测量了粒子对二维10°和15°单斜劈绕流中的斜激波动力响应,并测量了模拟发动机内部连续梯度的双斜劈粒子斜激波动力响应.结合粒子动力学的理论模型,得到了各状态的粒子弛豫时间、弛豫距离、Stokes数.基于图像方法、统计学规律分析了激波非定常抖动对测量结果的影响,并对测量结果进行了修正.结果显示,相同斜劈角度下,马赫数越高,粒子的弛豫时间、弛豫距离就越大.但是在相同的来流马赫数下,斜劈角度越大,粒子的弛豫时间、弛豫距离反而减小.在强梯度之后由于流场的雷诺数和黏性系数变化剧烈,粒子的跟随性降低了大约5.7%,stokes数增加了约1%.虽然在本文条件下Stokes数仍满足超声速流场对粒子跟随性的要求,但粒子响应的降低无疑是值得关注的,尤其是当其被应用于具有更多连续梯度的复杂流场测量中.     

Oblique stagnation slip flow of a micropolar fluid towards a stretching/shrinking surface: A stability analysis

A numerical solution is obtained for the steady oblique stagnation-point flow of a micropolar fluid over a stretching or shrinking surface with velocity slip condition. Results are obtained for representative values of slip parameter, micropolar parameter and stretching/shrinking parameter for strong particle interaction micropolar fluid. Dual solutions are found for the case of shrinking surface. An analysis of stability of these dual solutions shows that the solution branch that proceeds to large stretching case is stable. The streamlines are not symmetric for the oblique stagnation-point flow and reversed flow are observed near to the shrinking surface. The streamlines plots show that increase of slip parameter will reduce/eliminate the existing of rotating flow near the surface that caused by the shrinking effect.     

Flow separation behind a rib in a channel with laminar flow

Visualization data and results of combined measurements of flow quantities in flow with separation past a rib at nominally laminar regime of channel flow are reported. In the separation region, the flow is found to be essentially three-dimensional and unsteady, exhibiting a distinct cellular structure and flow zones with transverse motion. It is shown that the rib-induced flow separation gives rise to low-frequency fluctuations of flow velocity and initiates the turbulence transition in the channel flow. The critical Reynolds number at which flow instability starts developing in the channel is estimated. It is shown that at Reynolds numbers higher than the critical Reynolds number the linear integral scale of flow velocity fluctuations in the channel is defined by the duct size.     

槽道湍流展向振荡电磁力控制的实验研究

对槽道湍流的展向振荡电磁力控制进行了实验研究,讨论了展向振荡电磁力对宏观流场、近壁湍流结构以及壁面阻力的影响.采用谱方法进行了数值模拟的对比.数值模拟和实验结果均表明展向振荡电磁力能够使近壁区域的宏观流场产生周期性振荡,并影响壁湍流的条带结构,使其在展向上发生倾斜,从而使壁面阻力减小.     

On the effect of an anisotropy-resolving subgrid-scale model on large eddy simulation predictions of turbulent open channel flow with wall roughness

A large eddy simulation (LES) was conducted of turbulent flow in a channel with a rough wall on one side and a free surface on the other by adopting an anisotropy-resolving subgrid-scale (SGS) model. A shear Reynolds number of Re_τ = 395 was used based on the mean friction velocity and channel height. To investigate the grid dependency of the LES results caused by the SGS model, three grid resolutions were tested under the same definition of a roughness shape by using the immersed boundary method. The results obtained were compared with direct numerical simulation data with and without the wall roughness and those without the extra anisotropic term. The primary focus was on how the present anisotropic SGS model with coarser grid resolutions can properly provide the effects of roughness on the mean velocity and turbulent stresses, leading to a considerable reduction of the computational cost of LES.