Transient Flow Patterns Research Articles

Experimental study of full field riser hydrodynamics by PIV/DIA coupling

A full-field hydrodynamic study under riser flow conditions is performed, by using a combined Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA) technique. The employment of a temporal histogram-based method (THM) enables an accurate measurement of the solids volume fraction over the full field of a pseudo-2D riser unit. The full visual access to the riser section enables a complete characterization of the complex transient particulate phase flow patterns. Full-field riser hydrodynamics are quantified at different operating conditions defined by the superficial gas velocity. Under these conditions, a cluster detection method is utilized to characterize the heterogeneity of the riser flow.

Transient inhomogeneous flow patterns in supercooled liquids under shear.

Supercooled liquids and other soft glassy systems show characteristic spatial inhomogeneities in their local dynamical properties. Using detailed molecular dynamics simulations, we find that for sufficiently low temperatures and sufficiently high shear rates supercooled liquids also show transient inhomogeneous flow patterns (shear banding) in the start-up of steady shear flow, similar to what has already been observed for many other soft glassy systems. We verify that the onset of transient shear banding coincides quite well with the appearance of a stress overshoot for temperatures in the supercooled regime. We find that the slower bands adapt less well to the imposed deformation and therefore accumulate higher shear stresses compared to the fast bands at comparable local shear rates. Our results also indicate that the shear rates of the fast and slow bands are adjusted such that the local dissipation rate is approximately the same in both bands.

Численное решение задачи обтекания клина потоком стратифицированной жидкости с использованием OpenFOAM

Based on the open source software 2D numerical simulations of incompressible stratified fluids flows have been performed. They are characterized by a wide range of values of internal scales that are not in a homogeneous liquid. Mathematical model is based on the fundamental set of differential equations of inhomogeneous multicomponent fluid mechanics. The problem is solved using the finite volume method in an open source package OpenFOAM. The method allows analyzing in a single formulation the dynamics and fine structure of flow patterns past obstacles in a wide range of flow parameters. A particular attention is focused at construction of a high quality computational grid which satisfies basic requirements for resolution of all the microscales of the problem in high-gradient regions of the flow. The calculations were performed in parallel regime on computational facilities of the web-laboratory UniHUB (www.unihub.ru). The same system of equations and a general numerical algorithm were used for the whole range of the parameters under consideration. The calculation results are in a qualitative agreement with the data from laboratory experiments. Transient flow patterns past obstacles are analyzed, and physical mechanisms are determined, which are responsible for formation of vortices in regions with high density gradients near the edges of an obstacle. For all the velocities of the body motion, the flow field is characterized by a complicated internal structure. In the flow pattern around motionless body dissipative gravity waves are manifested at the edges of the strip. Around the slowly moving body a group of attached waves, are formed in opposite phases at the edges of the wedge. Then, the main flow components become vortices, which are formed around the edge of the wedge and manifested downstream in the wake. With further increase in velocity of the body motion, the flow pattern becomes more non-stationary.

Unsteady CFD simulation for bucket design optimization of Pelton turbine runner

To investigate flow patterns on the bucket of Pelton turbine runners is one of the important issues to improve the turbine performance. By studying the mechanism of loss generation on the flow around the bucket, it becomes possible to optimize the design of inner and outer bucket shape. For making it into study, computational fluid dynamics (CFD) is quite an effective method. It is normally used to simulate the flow in turbines and to expect the turbine performances in the development for many kind of water turbine including Pelton type. Especially in the bucket development, the numerical investigations are more useful than observations and measurements obtained in the model test to understand the transient flow patterns. In this paper, a numerical study on two different design buckets is introduced. The simplified analysis domain with consideration for reduction of computational load is also introduced. Furthermore the model tests of two buckets are also performed by using the same test equipment. As the results of the model test, a difference of turbine efficiency is clearly confirmed. The trend of calculated efficiencies on both buckets agrees with the experiment. To investigate the causes of that, the difference of unsteady flow patterns between two buckets is discussed based on the results of numerical analysis.

LBM-LES Simulation of the Transient Asymmetric Flow and Free Surface Fluctuations under Steady Operating Conditions of Slab Continuous Casting Process

Transient flow structures in a continuous casting mold can strongly influence the slag entrainment in liquid steel and the bubbles capture in the initial solidified shell, both of which are associated with the quality of the final product. This paper presents a numerical study of the turbulent flow with a top free surface in the continuous casting mold at a meso-scale level by a three-dimensional combined approach of Free Surface Lattice Boltzmann Method and Large Eddy Simulation (FSLBM-LES). The validity of the model is verified by the good agreement between the calculated results and the measurements from various water experiments in terms of the flow velocity and free surface profile. The mathematical model is then used to reveal the transient and spatiotemporal asymmetric characteristics associated with the transient flow field and the free surface fluctuation, although the steady state operation is considered during the continuous casting process. The results show that the locations of the jets of liquid steel from the two out ports of the Submerged Entry Nozzle (SEN) always fluctuate alternatively within a certain range, and periodically deviate from the design angle of the SEN within the same time period. The oscillating behavior of the jets promotes the asymmetric flow patterns and multi-scale vortices at both sides of the SEN. By introducing the Q-criterion in the results analysis, the formation, development, and shedding of the coherent structure (CS) of the turbulent flow are quantitatively characterized. The interaction between the transient flow patterns and the fluctuations of the top free surface as well as the evolution of the transient profile and velocities of the free surface are also demonstrated. The results obtained from the current study suggest that the FSLBM-LES model offers a promising way to study the complex flows and related transfer phenomena in the continuous casting process.

Natural convection subject to sinusoidal thermal forcing on inclined walls and heat source located on bottom wall of an attic-shaped space

Natural convection within an attic space under diurnal temperature condition on the sloping wall and a heat source located on the insulated bottom wall have been studied to show the basic flow features in the attic space over diurnal cycles. The finite volume numerical method has been employed to solve the governing equations. Effect of Rayleigh number (Ra), attic aspect ratio, heater location and its size are discussed on the transient flow pattern and heat transfer phenomenon after grid independency and time interval selection tests with a fixed prandtl number of 0.72 (air). Results are presented as a form of isotherms and streamlines. Also, heat transfer is presented as a form of Nusselt number. The numerical experiments show that, the flow in the attic space is stratified during the daytime heating stage; whereas the flow becomes unstable at the night-time cooling stage.

Assessment of Different Turbulence Models for the Motion of Non-metallic Inclusion in Induction Crucible Furnace

Turbulent fluid flow due to the electromagnetic forces in induction crucible furnace (ICF) is modeled using k-ϵ, k-ω SST and Large Eddy Simulation (LES) turbulence models. Fluid flow patterns calculated by different turbulence models and their effects on the motion of non-metallic inclusions (NMI) in the bulk melt have been investigated. Results show that the conventional k-ϵ model cannot solve the transient flow in ICF properly. With k-ω model transient flow and oscillation behavior of the flow pattern can be solved, and the motion of NMI can be tracked fairly well. LES model delivers the best modeling result on both details of the transient flow pattern and motion trajectories of NMI without the limitation of NMI size. The drawback of LES model is the long calculation time. Therefore, for general purpose to estimate the dynamic behavior of NMI in ICF both k-ω SST and LES are recommended. For the precise calculation of the motion of NMI smaller than 10 μm only LES model is appropriate.

Large Eddy Simulation of Transient Flow and Inclusions Transport in Continuous Casting Mold under Different Electromagnetic Brakes

A mathematical model has been developed to analyze transient fluid flow and inclusions transport in a slab continuous casting mold, considering the effects of electromagnetic brake (EMBr) arrangement and magnetic field strength. Transient flow of molten steel in the mold is calculated by using the large eddy simulation. The electromagnetic force is incorporated into the Navier–Stokes equation. The transport of inclusion inside the mold is calculated using the Lagrangian approach based on the transient flow field. The predicted results of this model are compared with the measurements of the ultrasonic testing of the rolled steel plates and the water model experiments. The transient asymmetrical flow pattern and inclusion transport inside the mold exhibits satisfactory agreement with the corresponding measurements. With electromagnetic brake effect, the velocities around the braking region are significantly suppressed, and the recirculating flow in the lower part drops and tends to develop a plug-like flow. The EMBr arrangement has an insignificant effect on the overall removal fraction of inclusions, especially for larger inclusions. The inclusion removal rate for the flow-control mold (FCM arrangement) reduces instead compared with no EMBr, especially for smaller inclusions.

Looping Dynamic Characteristics of a Pump-Turbine in the S-shaped Region During Runaway

During transients, hydroturbines may demonstrate some dynamic characteristics that differ from the corresponding static characteristics in steady operating conditions. To study the dynamic characteristics of a pump-turbine, we simulated the runaway transients in a model pumped-storage plant by coupling one-dimensional (1D) water conveyance system and three-dimensional (3D) pump-turbine. The results show that the runaway dynamic trajectories form loops in the S-shaped region in the unit discharge and unit torque charts of the pump-turbine, not following the corresponding static characteristics. Theoretical analysis and flow patterns comparisons illustrate that the looping trajectories are mainly caused by the successive features of transient flow patterns, namely, the transient flows in the pump-turbine are influenced by their previous status. These features induce different performances between similar dynamic operating points in different moving directions. Furthermore, through comparing the transient parameters calculated by the dynamic and static characteristics separately, we found that both methods are available to capture the unstable behaviors of the pump-turbine, but the dynamic method displays more accurate results when simulating the critical transient parameters. Therefore, in practical engineering applications, we suggest to use the static characteristics method for stability analysis while dynamic characteristics method for transient parameters, which is important for optimizing the layout of water conveyance systems.

Influence of impeller-tongue interaction on the unsteady cavitation behavior in a centrifugal pump

Purpose – The purpose of this paper is to analyze the cavitation dynamics in the blade channel of a centrifugal pump with a particular focus on the direct influence of the pump’s volute. Design/methodology/approach – A homogeneous multiphase model, namely the Zwart-Gerber-Belamri cavitation model, is employed to numerically describe the evolution of the process of cavitation within the pump. The RNG k-e turbulence model is applied to analyze the unsteady turbulent flow. A second order implicit formulation is used for the time discretization for the unsteady flow calculation and a finite volume algorithm is used for the space discretization. Findings – The cavities in the passage exhibit an obvious life cycle which includes initiation, growth, contraction, and separation, and collapse with a frequency corresponding to the impeller rotation frequency under off-design conditions. This phenomenon arises through an alternating interaction between reverse flow with the cavity interface and is associated with the response of the vortex region to the effect of uneven pressure distribution on volute and impeller-tongue interaction. Originality/value – This study simulated and analyzed the complex transient cavitation flow patterns inside a centrifugal pump and explains the reason for the unsteadiness. This knowledge is instructive in achieving the stable operation of pumps and in trouble shooting rough or cavitating operation.

0522 高度化手法による下掛け式クロスフロー水車の自由表面流れの解明

We have developed water turbines appropriately for low head open channels to effectively utilize the hydro power energy. To improve the performance of the turbine, it is necessary to determine the free surface flow patterns around the turbine. We introduced the new equipment system and the more precise computational grid of numerical analysis, it was possible to specify more precise flow patterns than old ones. Time-averaged velocity distribution at inner or outer circumference of runner was almost consistent. The flow direction at the second stage outlet had rapidly changed against the mainstream. At the inner circumference of runner at the first stage outlet, the small scale vertical flow toward the same rotational direction of runner occurred. In addition, we investigated the transient flow patterns at outer circumference of the runner.

Hillslope Hydrology and Stability

Ning Lu, Jonathan W. Godt Hillslope hydrology and stability forms an important addition to the literature on hydromechanical processes in hillslopes. The book is divided into five parts and ten constituent chapters. Part one provides an introduction and state-of-the-art. Important background on landslide terminology, classification, occurrence and socio-economic impacts are introduced with a summary of rainfall induced landslides. The authors then perhaps somewhat ambitiously cover hillslope geomorphology in one chapter predominantly describing the hillslope hydrologic cycle, topography, soil classification, hillslope hydrology and streamflow generation and mechanical processes in hillslopes. The second part of the book concerns hillslope hydrology with chapters on steady and transient infiltration. Water movement mechanisms, Darcy's law and vertical flow in one layer and two layer soil systems are all discussed in detail. Hydrologic barriers including flat and dipping capillary barriers are introduced as well as hydrologic barriers due to heterogeneity. The governing equations for transient water flow are presented for saturated and unsaturated flow. One-dimensional transient flow is discussed with reference to Richards equation and the Green-Ampt and Srivastava and Yeh infiltration models. Numerical solutions for multi-dimensional problems are briefly outlined. A detailed account of transient flow patterns includes the factors controlling flow direction, a conceptual model for wetting and drying states and flow patterns under constant rainfall intensity. Discussion of transient hillslope flow patterns concludes with consideration of patterns after cessation of rainfall and resulting from transient rainfall. The third part of the book moves from hydrology to a state-of-the-art discussion of total and effective stress in hillslopes. Total stresses are first defined …

Computational model of heat exchange between dairy cow and bedding

The article presents computational model for transient heat exchange between the body of a dairy cow and bedding thermally coupled with floor and the ground beneath. The finite volume technique is used to determine heat exchange and solve for 3-dimensional, transient heat flow and temperature pattern in bedding, floor and ground. A comparative analysis of calculation results has revealed good agreement with own temperature measurements (correlation factor above 0.8) and observations of thermal performance of bedding.The proposed mathematical model has been applied exemplary to calculate the role of bedding in heat exchange between a lying cow and two alternative bedding throughout the year and to evaluate thermal comfort of the lying area.The model can be used to analyze the development of thermal conditions in lying areas, which is regarded by many researchers as one of the core factors influencing lying behavior and welfare of cattle.

A full transient three-dimensional study on the effect of pulsating exhaust flow under real running condition on the thermal and chemical behavior of closed-coupled catalyst

The engine-out flow is highly transient and hot, and may place tremendous thermal and inertial loads on a closed-coupled catalyst. Therefore, time-dependent and detailed flow and thermal field simulation may be crucial. The aim of this study is to develop combined chemical reaction and multi-dimensional fluid dynamic mathematical model and to study the effect of unsteady pulsating thermal and flow characteristics on thermal reliability of closed-coupled catalyst. The effect of cell density on the conversion performance under real running condition is also investigated. Unlike previous studies, the present study focuses on coupling between the problems of pulsating flow pattern and catalyst thermal response and conversion efficiency. The results are expressed in terms of temporal evolution of flow, pollutant and temperature distribution as well as transient characteristics of conversion efficiency. Fundamental understanding of the flow and thermal phenomena of closed-coupled catalyst under real running condition is presented. It is shown that instants of significantly low values of flow uniformity and conversion efficiency exist during exhaust blow-down and the temporal variation of flow uniformity is very similar in pattern to one of conversion efficiency. It is also found that the location of hot spot in monolith is directly affected by transient flow pattern in closed-coupled catalyst.

Large Eddy Simulation of Transient Flow, Solidification, and Particle Transport Processes in Continuous-Casting Mold

The current study developed a coupled computational model to simulate the transient fluid flow, solidification, and particle transport processes in a slab continuous-casting mold. Transient flow of molten steel in the mold is calculated using the large eddy simulation. An enthalpy-porosity approach is used for the analysis of solidification processes. The transport of bubble and non-metallic inclusion inside the liquid pool is calculated using the Lagrangian approach based on the transient flow field. A criterion of particle entrapment in the solidified shell is developed using the user-defined functions of FLUENT software (ANSYS, Inc., Canonsburg, PA). The predicted results of this model are compared with the measurements of the ultrasonic testing of the rolled steel plates and the water model experiments. The transient asymmetrical flow pattern inside the liquid pool exhibits quite satisfactory agreement with the corresponding measurements. The predicted complex instantaneous velocity field is composed of various small recirculation zones and multiple vortices. The transport of particles inside the liquid pool and the entrapment of particles in the solidified shell are not symmetric. The Magnus force can reduce the entrapment ratio of particles in the solidified shell, especially for smaller particles, but the effect is not obvious. The Marangoni force can play an important role in controlling the motion of particles, which increases the entrapment ratio of particles in the solidified shell obviously.

Dynamic characteristics of a pump-turbine during hydraulic transients of a model pumped-storage system: 3D CFD simulation

The runaway process in a model pumped-storage system was simulated for analyzing the dynamic characteristics of a pump-turbine. The simulation was adopted by coupling 1D (One Dimensional) pipeline MOC (Method of Characteristics) equations with a 3D (Three Dimensional) pump-turbine CFD (Computational Fluid Dynamics) model, in which the water hammer wave in the 3D zone was defined by giving a pressure dependent density. We found from the results that the dynamic performances of the pump-turbine do not coincide with the static operating points, especially in the S-shaped characteristics region, where the dynamic trajectories follow ring-shaped curves. Specifically, the transient operating points with the same Q11 and M11 in different moving directions of the dynamic trajectories give different n11. The main reason of this phenomenon is that the transient flow patterns inside the pump-turbine are influenced by the ones in the previous time step, which leads to different flow patterns between the points with the same Q11 and M11 in different moving directions of the dynamic trajectories.

Tidal stream energy impacts on estuarine circulation

Among the impacts on the marine environment associated with the operation of a tidal farm, the alteration of the transient and residual flow velocities must be assessed in detail, for they constitute the driving force of important environmental processes such as sediment and pollutant transport, and nutrient dispersion. The objective of this study is to assess the impacts caused by the operation of a tidal farm on the transient and residual flow by means of a case study: a tidal stream farm in Ria de Ribadeo, an estuary in NW Spain. For this purpose a 3D numerical model of the estuary is implemented and successfully validated based on field data of tidal levels and flow velocities. The energy extracted by the tidal stream farm from the flow is accounted for by adding a momentum sink term in the equations of the model. Two scenarios representative of typical winter and summer conditions are considered. The results show that the disturbances to the transient flow patterns are concentrated in the proximity of the farm, with a weakening of the flow upstream and, especially, downstream of the farm (up to 0.25ms−1) and an intensification on both sides (up to 0.10ms−1). As for the residual flow, we find that the operation of the tidal farm does not disrupt the complex 3D residual circulation of the ria, but it does lead to modifications of the residual flow of up to 0.025ms−1, or approximately 10% of the baseline residual flow, which affect a much larger area than in the case of the transient flow (up to approximately 2km from the farm). The repercussions of these alterations of the transient and residual flow patterns of the estuary for its sedimentary and biological processes warrant further research. Overall, these results confirm the importance of assessing the impacts of a tidal stream farm on the flow.

Visualization and measurement of periodic reverse flow and boiling fluctuations in a microchannel evaporator of an air-conditioning system

This paper presents a simultaneous visualization and measurement on periodic reverse flow and boiling fluctuations of R-134a in a parallel flow microchannel evaporator used for air-conditioning systems. The visualization study shows that the microchannel repeats the transient flow patterns with two stages: liquid rewetting stage, and transient annular evaporating/dewetting stage. Correspondingly, five transient flow patterns are identified along the microchannel: bubbly flow, elongated bubble, liquid lump flow, annular flow, and mist/dryout flow. The reverse flow is observed to begin with the elongated bubble flow at the upstream channel and continue until the transient annular film evaporating/dewetting stage, indicating that the existing immeasurable high pressure within the elongated bubble or at a certain location in the annular flow exceeds the channel inlet and exit pressures. The period of the pressure drop fluctuation and flow pattern alternation are almost identical with that of the reverse flow, and it decreases from 1.49s to 1.13s when the average heat flux increases from 14.1kW/m2 to 18.3kW/m2. The decrease in oscillation period is mainly due to the shortened time interval for the annular film evaporating/dewetting stage.

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.

Long-term transient thermal analysis using compact models for data center applications

Reduced-order thermal models are necessary to enable real-time assessment of the optimum operating and control conditions to improve data center energy efficiency. A 3D computational fluid dynamics (CFD) and heat transfer model of a basic raised floor, air cooled, hot aisle/cold aisle data center configuration was developed and simulation results were used to generate snapshots for initializing compact models based on the Proper Orthogonal Decomposition (POD) and the Nonlinear Principle Component Analysis (NLPCA) methods. The specific focus of the study was to numerically investigate how the thermal trends can be affected by the long-term transient flow patterns associated with leakage and how the compact models (e.g. POD and NLPCA) can be utilized for much faster implementations and characterizations of the transient flows. Using both the POD and the NLPCA method, good agreement was achieved between the full simulation results and the compact models for predicting the dynamically developed local flow structure over a range of transient cooling air supply operating conditions. In addition, the NLPCA method was implemented to better characterize the nonlinear aspects of the CFD results. The benefits of using both the POD and NLPCA methods are discussed in relation to constructing compact models as real-time predictive tools. A systematic use of the compact models has also been proposed, to enable more robust thermal management and control of data centers.