Transient Turbulent Flow Research Articles

Detached eddy simulation of pedestrian-level wind and gust around an elevated building

Wind flow turbulence is known to have a major influence on the pedestrian-level wind (PLW) environments, particularly around a building. The elevated design of a building, as a special feature, proved to improve pedestrian-level weak wind conditions in high-density cities. The present study aims to assess three turbulence models, the detached eddy simulation (DES), the steady-state RANS (SRANS), and the unsteady-state RANS (URANS), in their simulation of the PLW flow turbulence concerning wind gust. The simulated mean wind velocities around isolated buildings with and without an elevated design were compared with those obtained from a wind tunnel experiment. The effects of mesh resolution and inflow fluctuating algorithm on the performance of the DES model were thoroughly evaluated. The DES model can better reproduce the mean flow fields than the other two models. Finally, the unsteady fluctuations of wind flow around the buildings with and without the elevated design are analyzed in terms of instantaneous wind velocity, lift coefficient, energy spectral density, and turbulence intensity. The predicted lift coefficient and Strouhal number are approximately 0.01 and 0.09, respectively, which is consistent with what are reported in the literature. Modifications of the frequency of vortex shedding, periodical wind flow pattern, and the normalized wind gust flow fields around the two types of buildings are compared in detail. The work reveals that transient turbulent flow pattern can be reasonably obtained with the DES model, indicating the potential of using the DES for PLW gust assessments in urban planning.

Analysis of triboelectric charging of particles due to aerodynamic dispersion by a pulse of pressurised air jet

Triboelectric charging of powders causes nuisance and electrostatic discharge hazards. It is highly desirable to develop a simple method for assessing the triboelectric charging tendency of powders using a very small quantity. We explore the use of aerodynamic dispersion by a pulse of pressurised air using the disperser of Morphologi G3asa novel application. In this device particles are dispersed by injection of a pulse of pressurised air, the dispersed particles are then analysed for size and shape analysis. The high transient air velocity inside the disperser causes collisions of sample particles with the walls, resulting in dispersion, but at the same time it could cause triboelectric charging of the particles. In this study, we analyse this process by evaluating the influence of the transient turbulent pulsed-air flow on particle impact on the walls and the resulting charge transfer. Computational Fluid Dynamics is used to calculate particle trajectory and impact velocity asa function of the inlet air pressure and particle size. Particle tracking is done using the Lagrangian approach and transient conditions. The charge transfer to particles is predicted asa function of impact velocity and number of collisions based on a charge transfer model established previously for several model particle materials. Particles experience around ten collisions at different velocities as they are dispersed and thereby acquire charges, the value of which approaches the equilibrium charge level. The number of collisions is found to be rather insensitive to particle size and pressure pulse, except for fine particles, smaller than about 30µm. As the particle size is increased, the impact velocity decreases, but the average charge transfer per particle increases, both very rapidly. Aerodynamic dispersion by a gas pressure pulse provides an easy and quick assessment of triboelectric charging tendency of powders.

Transient behaviors of wall turbulence in temporally accelerating channel flows

The effects of mean flow acceleration on near-wall turbulent structures were investigated by performing direct numerical simulations of transient turbulent flows in a channel. The simulations were initiated with a fully developed turbulent channel flow and then temporal accelerations were applied. During the acceleration, almost linearly increasing excursions of the flow rate were imposed between the steady initial and final values. The initial Reynolds number (based on the friction velocity) was fixed to Reτ,i=180, and four different final Reynolds numbers (Reτ,f= 250, 300, 350, and 395) were selected to show the effects of the Reynolds number ratio (Reτ,f/Reτ,i) on the transient channel flows. To elucidate the effects of the flow acceleration rates on the near-wall turbulence, a wide range of acceleration durations has been examined. Various turbulent statistics and instantaneous flow fields revealed that the rapid increase in the flow rate invokes bypass-transition-like phenomena in the transient flow. In contrast, the flow evolves progressively and the transition does not occur clearly for the mild flow acceleration. When the increase in the Reynolds number is small during the acceleration, distinct bypass-like transition phenomena do not appear in the transient flows, regardless of the acceleration rate. The present study proposed new criteria based on the impulse of the acceleration in order to explain the transition to the new turbulence in the transient channel flow. The bypass-like transition is primarily due to the larger contribution of the impulse to the increase in the flow rate compared with that in viscous friction.

Effects of the impeller–volute tongue interaction on the internal flow in a low-specific-speed centrifugal pump with splitter blades

To investigate the effects of the main blades and splitter blades interacting with the volute tongue on the internal flow in a low-specific-speed centrifugal pump, the Reynolds-averaged Navier–Stokes equation, coupled with SST k-ω turbulence model, is employed to simulate the transient turbulent flow in the whole flow passage. The numerical simulation results have been verified with the experimental measurements by comparing the head and efficiency. The pressure fluctuation caused by impeller–volute tongue interaction, including time–history and frequency characteristics, is calculated and analyzed at five monitoring points adjoining the impeller outlet and tongue, as well as the torque of a single main blade and a single splitter blade. After that, both the energy loss and vorticity distributions on the middle section are discussed when the impeller rotates to four circumferential positions relative to the cutwater. The results show that the maximum pressures at the monitoring points occur before the blades reach the closest circumferential position with respect to the cutwater, and the peak pressure near the trailing edge of splitter blades is larger than main blades. There is only one torque peak of a single blade in one revolution when the angle between the monitoring blade and tongue is about 15°. Additionally, the torque peak arises before the torque valley, but the pressure valley at monitoring points in the impeller comes earlier than the pressure peak. Both the energy loss and vorticity are enlarged around the volute tongue evidently after the blades pass by the cutwater, and the splitter blades produce more unsteadiness and energy dissipation than main blades.

Mathematical Modeling of Liquid Slag Layer Fluctuation and Slag Droplets Entrainment in a Continuous Casting Mold Based on VOF-LES Method

Abstract The slag behaviors, directly relating with the qualities of the final cast products, are influenced by the transient surface flow of liquid steel in a continuous casting mold. A one-half scale model is used to investigate the slag behaviors and their droplets entrainment. The model based on Volume of Fluid (VOF) multiphase coupled with Large Eddy Simulation (LES) is established to further illuminate the phenomena of the liquid oil layer fluctuation, the slag “eye” regions, and the slag entrainment observed in water experiment. The effects of casting speeds on the slag behaviors and their entrained droplets are investigated. The results show that the fluctuation of the oil layer is influenced by the transient flow. The calculations for the oil layer profile, the accumulation, and protrusions of oil layer are consistent with the water experiment. The asymmetry of the slag “eye” regions is also influenced by the asymmetry of free surface and transient turbulent flow. The “eye” regions near the narrow wall show distinct asymmetric change at different casting speeds. At a lower casting speed, the slag “eye” regions change irregularly and display the alternate process of open and collapse at the two sides of the narrow walls of the model. While at a relative higher casting speed, the slag layer gathers toward the nozzle, and the slag “eye” regions gradually grow and always open. The simulation model can reveal that the mechanism of the slag entrainment includes two main modes: the cutting or dragging mode and shear layer instability. The average diameter and amount of the entrained droplets are calculated through the UDF codes of ANSYS FLUENT software, and the size distribution of the entrained droplets is also counted. When the casting speed is lower, the dominant diameters of the entrained droplets range between 2 and 3 mm. With casting speed increase, the distribution of the droplets becomes wider, and there is a gradually increase in the percentage of larger droplets with a size of 4–6 mm.

A fully coupled OpenFOAM® solver for transient incompressible turbulent flows in ALE formulation

ABSTRACTIn this article, the previously developed single block fully coupled algorithm [1,2] for solving three-dimensional incompressible turbulent flows is extended to resolve transient flows in multiple rotating reference frames using the arbitrary Lagrange–Euler (ALE) formulation. Details on the discretization of ALE terms along with a recently developed extension to the conservative and fully implicit treatment of multi-block interfaces into three-dimensional space are presented. To account for turbulence, the kω − SST turbulence model in ALE formulation is solved using Navier–Stokes equations. This multi-block transient coupled algorithm is embedded within the OpenFOAM® Computational Fluid Dynamics (CFD) library, and its performance evaluated in a real case involving a turbulent flow field in a swirl generator by comparing numerical predictions with experimental measurements.

Large-Eddy Simulation of Transient Horizontal Gas–Liquid Flow in Continuous Casting Using Dynamic Subgrid-Scale Model

Euler–Euler simulations of transient horizontal gas–liquid flow in a continuous-casting mold are presented. The predictions were compared with previous experimental measurements by two-channel laser Doppler velocimeter. Simulations were performed to understand the sensitivity to different turbulence closure models [k–ɛ, shear stress transport (SST), Reynolds stress model (RSM), and large-eddy simulation (LES)] and different interfacial forces (drag, lift, virtual mass, wall lubrication, and turbulent dispersion). It was found that the LES model showed better agreement than the other turbulence models in predicting the velocity components of the liquid phase. Furthermore, an appropriate drag force coefficient model, lift force coefficient model, and virtual mass force coefficient were chosen. Meanwhile, the wall lubrication force and turbulent dispersion force did not have much effect on the current gas–liquid two-phase system. This work highlights the importance of choosing an appropriate bubble size in accordance with experiment. Finally, coupled with the optimized interfacial force models and bubble size, LES with a dynamic subgrid model was used to calculate the transient two-phase turbulent flow inside the mold. More instantaneous details of the two-phase flow characteristics in the mold were captured by LES, including multiscale vortex structures, fluctuation characteristics, and the vorticity distribution. The LES model can also be used to describe the time-averaged gas–liquid flow field, giving reasonably good agreement with mean experimental data. Thus, LES can be used effectively to study transient two-phase flow inside molds.

Frequency characteristics of a vortex flowmeter in various inlet velocity profiles

Although vortex flowmeters are widely used in various industries, the accuracy of vortex flowmeter measurements depends on inlet flow conditions. In this study, flow fields of a vortex flowmeter with various inflow conditions are simulated numerically. In order to model transient turbulent flows, Large Eddy Simulation and vortex shedding frequencies are determined using fast Fourier transform techniques. To determine the frequency characteristics in the area of downstream of the vortex generator, both steady and unsteady velocity profiles are used as inlet conditions, including two different shear flows and various sinusoidal plug flows. For steady inlet conditions, the flow qualities along the pipe are quantified using a proposed definition to express a velocity profile which departs from a fully developed velocity profile. For unsteady inlet conditions, the predominant frequency pattern is successfully predicted and it is linked to the measurement accuracy of the vortex flowmeter.

Numerical simulation of fluid flow and heat transfer in an industrial continuous furnace

Abstract We present a three-dimensional computational study of the transient heat transfer and turbulent fluid flow inside an arc-welding electrode continuous furnace. The model is implemented in FLUENT, a finite volume commercial code. Large difference in the length scales of the electrodes and the furnace, movement of the electrodes, and existence of various modes of heat transfer are the major factors influencing the accuracy and efficiency of the simulation. Two modeling strategies are used to overcome these difficulties. First, the electrode geometry and material composition are simplified using a thermally equivalent model. Second, the electrode movement inside the furnace is avoided by implementing time-dependent boundary conditions applied to a fixed domain. A fairly close agreement is obtained in comparing the load’s temperature history against the experimental data with an absolute relative difference below 2.7%. The space between the electrode trays in the furnace is very limited. The analysis shows that the input air does not circulate effectively between the trays. This lowers the thermal efficiency of the furnace and leads to uneven treatment of the electrodes. The present study provides guidelines to improve future furnace designs.

Numerical simulation of wind-induced turbulence over precipitation gauges

To better quantify measurement uncertainty associated with solid precipitation gauges, time averaged and transient turbulent flow over gauges is studied using a Computational Fluid Dynamic (CFD) approach. By numerically solving the Reynolds-Averaged Navier-Stokes (RANS) equations, vortex formation in the bulk structure of the flow is examined. To study transient effects, Large-Eddy Simulation (LES) quantified the turbulence and eddy dynamics. The simulated gauge is a Geonor T200B gauge shielded with Alter slats for a wind velocity ranging from 1 to 10m/s. A multi-layer mesh generation technique allows the capture of a wide range of turbulent length scales. Gauge exposure effects on the wind characteristics are investigated for unshielded and shielded cases. Updrafting results in an effective collection area of only 30% of the orifice for capturing precipitation. The RANS results show wind momentum reduction above the gauge orifice from shielding with vertical slats assists catch efficiency of the gauge. However, tilting the slats in high winds degenerates shielding benefits. LES results show considerable production of turbulence kinetic energy near the inlet orifice area. Transient analysis with LES demonstrates that the shields intensify the turbulence near the gauge. With the addition of wind shields, the flow momentum above the gauge is reduced.

レイノルズ数を変えた過渡的チャネル乱流に対する直接数値計算

レイノルズ数を変えた過渡的チャネル乱流に対する直接数値計算

Large Eddy Simulations of the Effects of EMBr and SEN Submergence Depth on Turbulent Flow in the Mold Region of a Steel Caster

Transient turbulent flow in the mold region during continuous casting of steel is related to many quality problems, such as surface defects and slag entrainment. This work applies an efficient multi-GPU based code, CUFlow, to perform large eddy simulations (LES) of the turbulent flow in a domain that includes the slide gate, SEN, and mold region. The computations were first validated by comparing the predicted surface velocity with plant measurements. Then, seven LES simulations were conducted to study the effects of casting speed, electromagnetic braking (EMBr) field strength, and submerged entry nozzle (SEN) depth on the transient flow. The results show that EMBr has an important influence on flow inside the SEN, in addition to flow in the mold. With EMBr, an “M-shaped” flow profile is seen inside the SEN. The swirling flow behavior in the SEN and ports is more symmetrical at high casting speed and with higher EMBr strength. The position of the SEN ports relative to the peak magnetic field affects the EMBr performance. The results confirm and quantify how applying EMBr greatly lowers both the magnitude and turbulent variations of the surface velocity and level profile.

Numerical modelling of carbon dioxide leakage from an offshore pipeline

A three-dimensional transient computational model was developed to simulate the break up of carbon dioxide escaping from a pipeline leak to the seawater. Important physical phenomena such as turbulence and mass transfer during evaporation of the initially liquid CO2 stream are included in the model. Sensitivity of the model for mesh size, turbulence model and the order of the spatial discretisation scheme are shown for different scenarios. A three stage approach is used to achieve sufficient level of convergence for the transient multiphase turbulent flow simulation using the Span and Wagner equation of state and a high-order discretisation schemes. The scenarios considered in this study include a wide range of pipeline operating pressure, ambient seawater depth and leakage size.

Detached eddy simulation of turbulent flow in isolated street canyons of different aspect ratios

Air quality management in urban areas requires the use of advanced modeling tools, able to predict and evaluate the pollution level under different traffic and meteorological conditions. In the present paper, the Artificial Compressibility version of the Characteristic Based Split algorithm (AC–CBS) was used to assess the performance of the Spalart–Allmaras based Detached Eddy Simulation (SA–DES) model, for the calculation of incompressible turbulent flow in different urban street canyon configurations. To our knowledge, the DES version of the SA turbulence model was applied in this work for the first time for the simulation of turbulent flow in a street canyon. The proposed DES model was able to accurately reproduce the turbulent characteristics of the flow compared with results from real street canyon experiments, wind tunnel experiments, and also to that obtained with RANS simulations. These results are very similar to the ones obtained from Large Eddy Simulation (LES) of street canyons flow reported in some recent publications, but with the potential characteristic of reduced computational costs. The DES approach is very promising for the simulation of transient turbulent flows in urban areas when complex three–dimensional domains are considered. The performance of the DES model evaluated for the mean dimensionless streamwise velocity profiles was comparable to that of Reynolds–Averaged Navier–Stokes RANS approach if referred to Hit Rate (HR) validation metric, and even better if referred to Factor of two observation (FAC2) validation metric. An accurate reproduction of the turbulent flow is crucial for urban pollutant dispersion simulations, since the distribution of the pollutant concentrations could differ by order of magnitude in the different points of the street canyon. DES approach results were able to accurately predict the unsteadiness characteristic of the flow, and to reproduce some minor vortex structures, which were not observed in the RANS cases, that will lead to a more accurate reproduction of the pollutant concentrations.

Instability and Periodicity of Asymmetrical Flow in a Funnel Thin Slab Continuous Casting Mold

Characteristics of the transient turbulent flow in a funnel thin slab continuous casting mold are studied using a large eddy simulation (LES) computational approach. Validations were done through comparison with previous experimental data of the mean velocities and the instantaneous velocities, and good qualitative and quantitative agreements are obtained. The turbulence flow inside the mold consists of various scales vortices; many pronounced large scale vortex structures were clear found inside the mold, containing various small scale vortices between them. The boundary shear flow can separate from the narrow wall and transport vorticity into the interior flow, and then possibly develop into a vortex. The intermittent chaotic vortex formation on both sides of the SEN is found at the meniscus. Three types of vortex phenomena are identified: one big vortex, two vortices and three small vortices. The positions and sizes are different, and the vortexes are located at the low velocity sides adjacent to the SEN. Significant asymmetry is seen in the instantaneous flow in the two halves of the thin slab mold cavity, especially in the lower recirculation zone. The periodical behavior of asymmetric flow inside the liquid pool was identified and characterized. The spectrum of velocities shows that the mean time interval for periodical changeover is 14.28 seconds.

Analysis of Transient Transport and Entrapment of Particle in Continuous Casting Mold

A mathematical model has been developed to simulate the transient turbulent flow, solidification, transport and entrapment of particles in a continuous casting mold. The turbulence of molten steel inside the liquid pool is calculated using the large eddy simulation (LES). The enthalpy-porosity approach is used to simulate the solidification of steel. A Laplacian equation is used to approximate the pull velocity in the solid region. The transport of particle (bubble and non-metallic inclusion) inside the liquid pool is considered according to the Lagrangian approach. A criterion for particle entrapment in the solidified front is developed using the user-defined functions (UDF) of FLUENT software, considering force balance involving eight different forces acting on a particle at the mushy-zone. The predicted asymmetrical flow pattern is compared with the water model experiment results. The entrapped positions of particles in the solidified shell are predicted and compared with the ultrasonic flaw results of rolled steel plates and the previous study of inclusions distribution in slabs. The model has successfully reproduced many known phenomena and other new predictions, including the transient asymmetrical flow pattern, transient asymmetrical distribution of particles inside the liquid pool, transient entrapment ratios and asymmetrical positions of different particles in the solidified shell.

Transient Asymmetric Flow and Bubble Transport Inside a Slab Continuous-Casting Mold

A one third scale water model experiment was conducted to observe the asymmetric flow and vortexing flow inside a slab continuous-casting mold. Dye-injection experiment was used to show the evolution of the transient flow pattern in the liquid pool without and with gas injection. The spread of the dye was not symmetric about the central plane. The flow pattern inside the mold was not stationary. The black sesames were injected into water to visualize the vortexing flow pattern on the top surface. The changes of shape and location of single vortex and two vortices with time had been observed during experiments. Plant ultrasonic testing (UT) of slabs was used to analyze the slab defects distribution, which indicated that the defects are intermittent and asymmetric. A mathematical model has been developed to analyze the time-dependent flow using the realistic geometries, which includes the submerged entry nozzle (SEN), actual mold, and part of the secondary cooling zone. The transient turbulent flow of molten steel inside the mold has been simulated using the large eddy simulation computational approach. Simulation results agree acceptably well with the water model experimentally observed and plant UT results. The oscillating motions of jet and the turbulence naturally promote the asymmetric flow even without the effects of slide gate nozzle or the existence of clogs inside the SEN. The periodic behavior of transient fluid flow in the mold is identified and characterized. The vortexing flow is resulted from asymmetric flow in the liquid pool. The vortices are located at the low-velocity side adjacent to the SEN, and the positions and sizes are different. Finally, the model is applied to investigate the influence of bubble size and casting speed on the time-dependent bubble distribution and removal fraction from the top surface inside the mold.

Transient numerical analysis of rotor–stator interaction in a Francis turbine

Pressure fluctuation due to rotor–stator interaction and occurrence of vortex rope in draft tube at partial load operation are obvious phenomena in Francis type reaction hydro turbines. These hydrodynamic effects are important issues and should be addressed during the design of hydraulic machines. A 3-dimensional transient state turbulent flow simulation in the entire flow passage of a 70 kW-Francis turbine having specific speed of 203.1 is conducted to investigate the rotor–stator interaction by adopting kω based SST turbulence model. The commercial 3D Navier–Stokes CFD solver Ansys-CFX is utilized to study the flow through this vertical shaft Francis turbine in its stationary and transient passages, at 100% optimum load and 72% of part load. The investigated turbine consists of a spiral casing with 16 guide vanes, 8 stay vanes, a runner with 13 blades and a draft tube. With a time step of 2° of a rotational period of the runner for 10 full rotations, the time dependent pressure and torque variations are monitored at the selected locations during the unsteady state calculation. A periodical behavior is observed for the pressure distribution in guide vanes, runner blades and torque in the runner blades. The pressure distribution curve in runner blades reveals the two dominating frequencies – the lower peaks due to runner speed and the upper peaks corresponding to the number of guide vanes interacting with the flow. The flow acceleration toward inside of the runner is depicted by the expanding wakes behind the stay vanes. Vortex rope is observed in draft tube, downstream the runner, at part-load operation.

Numerical Simulation of the Turbulent Flow in Ultra-Precision Aerostatic Bearings

In this paper, the transient turbulent flow field in ultra-precision aerostatic bearings is numerically investigated, with the focus on vortex structures and pressure distribution in the bearing clearance. In order to capture details of transient turbulent flow, large eddy simulation (LES) is employed. It is found that with the increase of air supply pressure, the flow in the recess becomes turbulent and vortex shedding occurs near the orifice outlet. It is also demonstrated that this unsteady vortex shedding results in pressure fluctuation in the bearing clearance, which is related to the experimentally observed small vibration of the bearing on the order of nanometers.

Large Eddy Simulation of Turbulent Fluid Flow in Liquid Metal of Continuous Casting

The transient turbulent flow in continuous casting steel plays a key role in minimizing defects. Compared with the k-ɛ model, the large eddy simulation (LES) of turbulence provides much more accurate representation of turbulent flow by resolving large-scale dynamics. The turbulent flow in a liquid metal model of continuous casting has been simulated by LES and measured using ultrasonic Doppler velocimetry (UDV). The result of measurement and LES has been compared to validate the LES model and furthermore enhance the understanding of the transient turbulent feature in the flow field. The results show that the jet exiting from the nozzle port swings, which is not steady, and turbulent velocity variation frequencies decreased with distance from the nozzle port region and also the LES mode can capture the high frequency fluctuation, which the measurement cannot detect.