Plane Channel Research Articles

Direct Numerical Simulation of Turbulent Heat Transfer with Slip and Temperature Jump

The fully developed turbulent flow and heat transfer in a plane channel at a nominal shear Reynolds number of 180 is studied via the direct numerical simulation technique. A linear velocity-slip boundary condition is assumed on one of the channel walls, whereas the classical no-slip condition is imposed on the other wall. The heat transfer at the slip wall is assumed to occur with and without temperature jump, whereas a classical first-kind temperature boundary condition is assumed on the no-slip wall. Various statistical quantities are shown and discussed. It is observed that the velocity fluctuations are attenuated in the proximity of the slip wall but they are amplified in the vicinity of the no-slip wall. The behavior is somewhat different for the temperature fluctuations. For the case without temperature jump, the temperature fluctuations are amplified near the slip wall, whereas they are attenuated close to the no-slip wall. For the case with temperature jump, the modifications of turbulent heat fluxes are more dramatic and they decrease by increasing the amount of wall slip. In general, the velocity slip condition will result in an enhanced heat transfer if no temperature jump is taken into account. This behavior is reversed when the temperature jump at the slip wall is taken into account. The budget of temperature variance is shown and discussed for the case with temperature jump.

First observation of ion beam channeling in bent crystals at multi-TeV energies

Planar channeling in bent crystals has been observed in LHC with multi-TeV proton beam in 2015. Two crystals, mounted on novel high-accuracy goniometers (one in the horizontal and one in the vertical plane), are integrated in the LHC collimation system, for studying the feasibility of the crystal-based collimation scheme. Using this experimental setup, tests with fully-stripped lead ion beams at both 450 Z and 6500 Z GeV were carried during dedicated LHC beam time. Planar channeling was observed for the first time with lead ions at these unprecedented energies surpassing by more than 1 order of magnitude the previous state-of-the-art for lead heavy ions and providing an important experimental basis for future applications of bent crystals in beam manipulations. The set of measurements performed to confirm this observation, as the local loss reduction in presence of channeling and the evidence of a deflected beam downstream of the crystal, are presented.

Flows between orthogonally stretching parallel plates

Navier–Stokes equilibrium solutions of a viscous fluid confined between two infinite parallel plates that can independently stretch or shrink in orthogonal directions are studied. It is assumed that the admissible solutions satisfy spatial self-similarity in the stretching or shrinking perpendicular coordinates. The nonlinear steady boundary-value problem is discretized using a spectral Legendre method, and equilibrium solutions are found and tracked in the two-dimensional parameter space by means of pseudo-arclength continuation Newton–Krylov schemes. Different families of solutions have been identified, some of which are two-dimensional and correspond to the classical Wang and Wu self-similar flows arising in a plane channel with one stretching–shrinking wall [Wang, C.-A. and Wu, T.-C., “Similarity solutions of steady flows in a channel with accelerating walls,” Comput. Math. Appl. 30, 1–16 (1995)]. However, a large variety of three-dimensional solutions have also been found, even for low stretching or shrinking rates. When slightly increasing those rates, some of these solutions disappear at saddle-node bifurcations. By contrast, when both plates are simultaneously stretching or shrinking at higher rates, a wide variety of new families of equilibria are created and annihilated in the neighborhood of cuspidal codimension-2 bifurcation points. This behavior has similarities with the one observed in other planar and cylindrical self-similar flows.

Simulation of propagation and diffraction of a shock wave in a planar curvilinear channel

Subject of Research. Numerical simulation of a shock wave propagation in a plane curved channel is considered on the basis of numerical simulation data. Method. Calculations of an inviscid compressible gas were carried out on the basis of unsteady two-dimensional Euler equations. Discretization of the basic equations was carried out using the finite volume method. Calculations were carried out for different channels with different radius of curvature and Mach numbers of the initial wave. To find the angular position of the front at the current time, the absolute value of the derivative of the density with respect to the angular coordinate was used. The calculation results were compared with the data of a physical experiment. Main Results. The features of the emerging shock-wave flow pattern and its development in time are discussed. The shock-wave configuration observed in channels with different radii of curvature is compared. Some differences in the curvature change of the front of shock waves formed in channels with different radius of curvature are shown. The size of the Mach leg and its change with time depending on the intensity of the initial wave and the size of the annular gap is the angular coordinate function corresponding to the position of the shock wave at the current time. While the maximum Mach number on the outer wall is relatively weakly dependent on the initial wave velocity, the Mach number on the bottom wall decreases with increasing Mach number at the channel entrance. The performed numerical studies show that in all variants there are no non-physical oscillations of the solution. Practical Relevance. The study of shock-wave and detonation processes is of interest for using their potential in pulsed installations and power systems for aircraft and rockets. The calculation results are important for the search of the new flow patterns that guarantee the formation of self-sustained detonation combustion in the combustion chambers of promising propulsion systems. Adjusting the size of the annular gap gives the possibility to select a geometric configuration that will provide the formation of an optimal triple shock wave structure, as well as the required intensity and size of the Mach wave.

Direct numerical simulation of bidisperse inertial particles settling in turbulent channel flow

The behavior of settling velocity and clustering of bidisperse inertial particles in a turbulent channel flow is investigated through direct numerical simulation. The particle-laden planar channel flow has a friction Reynolds number at Reτ = 180. Eulerian–Lagrangian method is used to study the dynamic properties of bidisperse and monodisperse inertial particles with 16 different simulation sets, which are distinguished by Stokes numbers ranging from St+ = 1.31 to 52.58 and particle number ratio from 1:1 to 1:8. Momentum exchange between fluid and particle phases is considered in the simulation as the chosen initial volume fraction at 5 × 10−5 is in the two-way coupling regime. The gravity is set at the direction normal to both the wall normal direction and the streamwise direction. We observe that in the bidisperse cases the turbophoresis effect of inertial particles with the smaller diameter is significant even though it is very weak in the corresponding monodisperse cases. We use radial distribution function (RDF) to investigate the degree of clustering and turbophoresis. The results indicate that RDF is larger in the bidisperse cases for both large and small particles and it is greatly affected by the bulk particle number ratio and the Stokes number ratio. Unlike clustering, the terminal settling velocities of inertial particles in the bidisperse cases are affected by the final volume fraction at the dynamic equilibrium state. When their final volume fractions are lower than those in the corresponding monodisperse cases, the settling velocity of either particle becomes reduced from the monodisperse value. We also investigate the relationship between settling velocity and vortex strength. The results show that the preferential sweeping mechanism is strengthened with Stokes number decreasing and the mechanism can be quantified by the slope of the curve of settling velocity variation with vortex strength.

Flow visualization and skin friction determination in transitional channel flow

The present study experimentally determines the transitional Reynolds number range for plane channel flow and characterizes its transitional state. The pressure along the channel is measured to determine the skin friction coefficient as function of Reynolds number from the laminar state, through the transitional region into the fully turbulent state. The flow structure was studied through flow visualisation which shows that as the Reynolds number increases from the laminar state the transitional region starts showing randomly occurring turbulent spots. With increasing Reynolds number the spots shift into oblique patches and bands of small scale turbulence that form across the channel width, together with large-scale streaky structures found in areas between the turbulent regions. An image analysing technique was used to determine the intermittency factor, i.e. the turbulence fraction in the flow, as function of Reynolds number. It is found that the skin friction coefficient reaches its turbulent value before the flow is fully turbulent (the intermittency factor is still below one). This suggests that the observed streaky structures in non-turbulent regions contribute to the enhancement of the wall-normal transfer of momentum. Also above the Reynolds numbers where the turbulent skin friction coefficient has been established large-scale features consisting of irregular streaky structures are found. They have an oblique shape similar to the non-turbulent and turbulent patches in the transitional flow indicating that the transition process is not fully complete even above the Reynolds number where the skin friction reaches its turbulent level.Graphic abstractProcessing of flow visualization images to determine the turbulenceintermittency in the transitional region. Flow is from left to right, (a)unprocessed, (b) filtered (c) standard deviation of (b), (d) binary picture

Assessing Wall-Modeled Large-Eddy Simulation for Low-Speed Flows with Heat Transfer

This paper reports wall-modeled large-eddy simulation (WMLES) results of low-speed turbulent flows in a plane channel, in ribbed ducts, and around a film cooling jet. We compare our WMLESs to Pirozzoli et al.’s direct numerical simulations (DNSs) of low-speed plane channel flow (Pirozzoli, S., Bernardini, M., and Orlandi, P., “Passive Scalars in Turbulent Channel Flow at High Reynolds Number,” Journal of Fluid Mechanics, Vol. 788, Feb. 2016, pp. 614–639), our own DNSs of ribbed ducts with various pitch-to-height ratios, and Milani et al.’s WRLES and water-tunnel experiment of film cooling (Milani, P. M., Gunady, I. E., Ching, D. S., Banko, A. J., Elkins, C. J., and Eaton, J. K., “Enriching MRI Mean Flow Data of Inclined Jets in Crossflow with Large Eddy Simulations,” International Journal of Heat and Fluid Flow, Vol. 80, Dec. 2019, Paper 108472). We consider Mach number effects below the often-quoted low-Mach-number limit of . The results show that the Mach number has significant effects on the normalized mean temperature profile, even below the often-quoted low-Mach-number limit of , due to the associated viscous heating. In addition, we compare the first-grid point implementation (FGI) and the third-grid point implementation (TGI) of the equilibrium wall model. We show that, by placing the large-eddy-simulation/wall-model matching location away from the wall, TGI practically reduces the near-wall resolution seen by the wall model, which in turn leads to underperformance of the wall model. By considering three types of flows with increasing levels of complexities, the objective of this study is to systematically assess WMLES in terms of its ability to predict heat transfer for low-speed flows. For the flows considered here (that is, plane channel, ribbed duct, and film cooling), we show that WMLES with FGI is able to accurately model heat transfer at a much more reduced cost than WRLES and DNS.

Experimental study on the lock-in of an oscillating circular cylinder confined in a plane channel

The results of an experimental study concerning wall boundary effects on the lock-in behavior of the shedding vortex of a circular cylinder, which is forced to oscillate transversely in a uniform flow at moderate Reynolds number Re = 2000,are presented in this paper. A height-adjustable flow channel is designed to study the boundary effect. Two sets of experiments are organized to study the lock-in behavior by changing the blockage ratio and oscillation amplitude separately. When the blockage ratio exceeds a critical value, the boundary effects are observed and the Strouhal number of the stationary cylinder increases non-linearly. As the blockage ratio increases, both the lower and upper limits of the lock-in region move to the lower value of excitation frequency, and the extent of the lock-in region decreases first and then increases. The boundary changes the effects of oscillation amplitude on lock-in behavior. When free from the boundary effect, as the oscillation amplitude increases, a V-shaped boundary of the lock-in region is observed, and the extent of the lock-in region increases monotonically. In comparison, under the influence of the boundary, the extent of the lock-in region first increases with the increase of the oscillation amplitude, and then decreases with the increase of the oscillation amplitude. Hysteresis loops are first observed near the lower limits of the lock-in region for some ranges of blockage ratio and oscillation amplitude in a forced-vibration system, and the extent of the hysteresis loop is affected by the two parameters.

Моделювання в’язких течій методом граткових рівнянь Больцмана на графічних процесорах з використанням WebGL API

This work is dedicated to the modeling methodology of a viscous fluid flows with the lattice Boltzmann method on graphic processors based on the technology of images rendering in web browsers WebGL. A two-dimensional nine-velocity LBM model (D2Q9) with a collision integral in a Bhatnagar-Gross-Kruk approximation form is shown. The possibilities of calculation acceleration using WebGL technology is described, namely features of using textures to contain values of some physical quantities in numerical algorithms and using fremebuffers to storage the textures, influence of the texture parameters on the numerical algorithms, features of shaders programming. The questions of shader programs using for carrying out stages of physical modeling were considered. The proposed methodology was used to develop an original web program for modeling of classical test problems. Simulations of the Poiseuille flow in a plane channel and the flow around a circular cylinder in a plane channel were performed. The obtained results were compared with the results of calculations performed in the original verified modeling program based on the lattice Boltzmann method and in the Comsol Multiphysics package with the finite element method. Comparisons of the values of the velocity magnitude showed the consistency of the obtained results with the data of other numerical experiments. The analysis of computational speed in comparison with modeling using the optimized algorithm of a method with use of the technology of parallel calculations on CPU OpenMP in the original program is carried out. It is shown that the acceleration of calculations depends on the number of cells of the calculation grid. The results of the fluid flow modeling around a circular cylinder at Re = 1000 are demonstrated, which are obtained 30 times faster than with the calculations obtained with optimized lattice Boltzmann method and OpenMP technology.

Velocity of suspended fluid particles in a low Reynolds number converging flow

We studied a pressure-driven, low Reynolds number fluid flow through a planar channel whose spanwise width along the flow varied inversely as the streamwise coordinate such that the extensional rate on the centerline was near constant. The effect of the near constant extensional rate on an immiscible droplet of silicone oil was studied by tracking its deformation. The droplet rapidly deformed into an ellipsoid and displayed a consistent lag velocity compared to the single phase background flow at the same point. The observations were attributed to the flow induced deformation of the immiscible droplet, which was a function of the magnitude of the initial capillary number. The streamwise component of the single phase velocity along the centerline of the converging flow was also estimated as leading order using lubrication theory. The estimated velocity is compared favorably with numerical simulations; validation with experimental measurement of the flow of castor oil through the channel by tracking tracer particles is performed. The accuracy of the determination of the velocity field by the lubrication theory allowed for the careful measurement of the velocity difference between the drop and suspended fluid velocities. This research validated lubrication theory predictions of the flow velocity through a converging channel and provided an experimental insight into the behavior of a suspended phase.

Flow-induced orientation of an aqueous polymer solution in planar channels with an abrupt contraction.

Flow-induced orientation of an aqueous polymer solution in planar channels with an abrupt contraction.

TANDEM: biomicrofluidic systems with transverse and normal diffusional environments for multidirectional signaling.

Biomicrofluidic systems that can recapitulate complex biological processes with precisely controlled 3D geometries are a significant advancement from traditional 2D cultures. To this point, these systems have largely been limited to either laterally adjacent channels in a single plane or vertically stacked single-channel arrangements. As a result, lateral (or transverse) and vertical (or normal) diffusion have been isolated to their respective designs only, thus limiting potential access to nutrients and 3D communication that typifies in vivo microenvironments. Here we report a novel device architecture called "TANDEM", an acronym for "T̲ransverse A̲nd N̲ormal D̲iffusional E̲nvironments for M̲ultidirectional Signaling", which enables multiplanar arrangements of aligned channels where normal and transverse diffusion occur in tandem to facilitate multidirectional communication. We developed a computational transport model in COMSOL and tested diffusion and culture viability in one specific TANDEM configuration, and found that TANDEM systems demonstrated enhanced diffusion in comparison to single-plane counterparts. This resulted in improved viability of hydrogel-embedded cells, which typically suffer from a lack of sufficient nutrient access during long-term culture. Finally, we showed that TANDEM designs can be expanded to more complex alternative configurations depending on the needs of the end-user. Based on these findings, TANDEM designs can utilize multidirectional enhanced diffusion to improve long-term viability and ultimately facilitate more robust and more biomimetic microfluidic systems with increasingly more complex geometric layouts.

Features of the flow of concentrated suspensions of solid particles

Concentrated suspensions of solids, widely used in the pharmaceutical, cosmetic and food industries, exhibit complex rheology. In the rheometric one-dimensional flows of concentrated suspensions, a smooth or abrupt increase in stresses with a smooth increase in the strain rate intensity can be observed. This is related to the appearance of a first-order phase transition. In our previous work, a phenomenological rheological model of a concentrated suspension of solid particles in a Newtonian dispersion liquid has been developed. This model is characterized by an S-shaped flow curve and describes both continuous and abrupt increases in the stress intensity with a uniform increase in the strain rate intensity. In this work, exact analytical formulas are obtained for the flow velocity profiles of suspensions measured using rotary viscometers. The model proposed before is modified to take into account the non-Newtonian properties of the dispersion medium, which demonstrates pseudoplastic properties at low stress intensities and dilatant properties at high stress intensities. Based on the developed numerical model, the features of the flow of highly concentrated suspensions in plane and axisymmetric channels are analyzed. It is shown that in a flat diffuser, in contrast to Newtonian and pseudoplastic fluids, the longitudinal velocity of the suspension slows down near the walls, where the stresses are maximum, and accelerates in the central part of the channel. Calculations for the submerged jet in the area bounded by the walls show that, with an increase in the average velocity of the incoming pure liquid by more than 0.01 m/s, there occurs a local velocity vortex bounded by a layer with high particle concentration and more viscous medium; particle concentration inside the vortex is minimum.

Изучение критического угла каналирования ионов активных металлов через тонкие пленки алюминия

The spatial distributions of ions (K+, Na+) passed through thin polycrystalline and single-crystalline Al films with the thickness from 180 to 600 Å and critical channeling angles have been studied. The ion energies have been varied within the range E0 = 10-30 keV. It has been shown that an increase in the energy of the primary ion beam leads to a decrease in the width of the maxima of the angular distribution, which is associated with a decrease in the critical channeling angle ψcr. It has been found that the value ψcr does not exceed 4-50 for axial channeling and 9-100 for planar channeling.

Fabrication and Characterization of GaN-Based Fin-Channel Array Metal-Oxide-Semiconductor High-Electron Mobility Transistors With Recessed-Gate and Ga₂O₃ Gate Insulator Layer

In this work, the properties of gallium oxide (Ga2O3) and its excellent interface properties to GaN-based materials are explored as a gate insulator layer for GaN-based metal-oxide-semiconductor high-electron mobility transistors (MOSHEMTs). A novel vapor cooling condensation system was used to deposit the high quality Ga2O3 films with high insulation and low defect suitable for gate insulator layer. The characteristics of the Ga2O3 films were further explored by implementing GaN-based fin-channel array MOSHEMTs with recessed-gates and different channel widths. Compared to planar channel structure, the direct current, high frequency, and flicker noise performances were enhanced in the fin-channel MOSHEMTs with Ga2O3 gate insulator layer. For the GaN-based fin-channel array MOSHEMTs with 300-nm-wide channel, the devices exhibited superior performances of maximum extrinsic transconductance of 194.2 mS/mm, threshold voltage of −1.4 V, extrinsic unit gain cutoff frequency of 6.4 GHz, maximum oscillation frequency of 14.8 GHz, and normalized noise power of 8.45 $\times \,\, 10^{-15}$ Hz $^{-1}$ . It was also demonstrated that the associated performances were improved by reducing the width of fin-channel array.

Research on User Plane Redundant Transmission Technology for Highly Reliable 5G URLLC

Based on the research of the fifth generation of mobile communications, this paper focuses on supporting the reliable transmission technology of URLLC, and studies how to achieve transmission with higher reliability than the single-user plane channel and network function in the user plane path. Supports highly reliable transmission schemes through redundant transmission in the user plane. This transmission scheme focuses on the improvement of backhaul reliability, that is, there is no need to change the radio interface and related protocols; only a single UE is required, that is, no UE redundancy, the network nodes and related interfaces and the redundancy of concurrent PDU sessions are introduced into the network; network transmission The transport layer on the protocol is redundant, with a single network node, without UE influence.

Study of laminar convective heat transfer through a sinusoidal corrugated channel with a zero degree phase shift using different nanofluids

The study shows numerical analysis of convective heat transfer of a corrugated channel for laminar flow in the fully developed region. Aluminum-oxide (Al2O3) and Copper-oxide (CuO) water nanofluid is used to investigated the relation of heat transfer characteristics with corrugated geometry and nanofluid. Second-order single-phase mass, momentum, and energy equation is solved using Computational Fluid Dynamics software ANSYS 16. A 3 m long corrugated channel having a wavelength of 0.5 m is used as the physical model. The nanofluid flows at a uniform speed with a temperature of 300 K. 500 W/m2 constant heat flux is applied at the lower corrugated channel. Difference between the two channel walls is 0.5 m. The sinusoidal corrugated channel has a phase shift (φ) of 0° which ensures symmetrical corrugated area. The investigation is done for fully developed flow with an added 2.5 m of plane channel geometry at the entrance of the corrugated section which ensure fully developed flow. Different volume fractions (1%–5%) was considered here with base fluid water to increase thermal properties in a Reynolds number range of 50–100 to ensure laminar flow through the corrugated section. Pumping power, Nusselt Number and Friction factor is calculated and all the results are compared with pure water. The results reveal that CuO-water has a higher heat transfer rate and Nusselt Number which is 12.2208% more than pure water. The required pumping power is 71.78343% higher for Al2O3 water nanofluid. The study shows how the difference in thermo-fluid properties of the nanofluids affect the overall heat transfer performance when used in a sinusoidal corrugated geometry.

Calculation of Linear and Nonlinear Stability of Two-Layer Liquid Flow in a Horizontal Plane Channel

Calculation of Linear and Nonlinear Stability of Two-Layer Liquid Flow in a Horizontal Plane Channel

Experimental investigation on the propagation characteristics of detonations in a semi-confined straight channel

In a rotating detonation engine, detonation waves propagate in the combustor chamber with one side unconfined which will introduce lateral relief and, therefore, results in deficits of the propagation velocity and the peak pressure. To study the critical mixture height for detonation propagation, a simple combustor without considering the curvature has been used, which provides a planar semi-confined channel with different channel heights, e.g., 10 mm, 20 mm, and 30 mm. Effects of the nitrogen dilution ratio, the equivalence ratio, and the channel height on the propagation modes, the velocity deficit, and the cellular structure have been investigated when ethylene was utilized. The non-dimensional channel height (h/λa) was also adopted to evaluate the impacts of lateral relief. Experimental results indicated that three different propagation modes existed in the semi-confined channel. Velocity deficits resulted from the lateral relief mechanism were observed and the maximum velocity deficit is around 10–15%. Besides, the cell width increased and became more irregular when reducing the channel height. Successful detonation propagation can be obtained with a minimum normalized channel height of h/λa > 2, which will provide information for practical detonation operations.

Direct numerical simulation of turbulent flow separation induced by a forward-facing step

Direct numerical simulation is used to investigate turbulent flow separations generated by a forward-facing step exposed to an incoming fully-developed turbulent plane channel flow at Reτ=180. The step height is 25% of the inlet channel height. The results are analyzed in terms of the topology of the Reynolds stresses, transport of turbulence kinetic energy (TKE) and unsteadiness of separation bubbles using a broad range of post-processing techniques, including the Lumley triangle, quadrant analysis, auto-correlation, joint probability density function, proper orthogonal decomposition and linear stochastic estimation. The incoming turbulence intensity is elevated as the step is approached and peaks along the mean separating streamline over the step. The topology of the Reynolds stresses switches from two-component to axisymmetric along the mean separating streamline over the step. The production term of TKE peaks in the separated shear layer, while the turbulent diffusion term tends to transport TKE away from the shear layer. An expanded separation bubble upstream of the step is typically associated with a contracted separation bubble over the step. This pattern is captured by the first proper orthogonal decomposition (POD) mode. In the first POD mode, the streamwise and vertical fluctuating velocities switch signs abruptly near the mean separating streamline over the step. This is characterized by two pairs of counter-rotating vortices: one pair of counter-rotating vortices leaning over the step and another pair of opposite-signed counter-rotating vortices near the top surface of the step.