Surface Vortex Formation Research Articles

Hydraulic performance of bottom intake velocity caps using PIV and OpenFOAM methods

The objective of this investigation is to obtain a more profound understanding of the effective parameters of the velocity caps for bottom intake systems, utilizing particle image velocimetry (PIV) and OpenFOAM. Observations indicate a higher probability of surface vortex formation in square types compared to circular ones, with the vortex being formed downstream of the caps. Additionally, the flow pattern reveals that the flow whirls in a more favorable path into the circular caps as opposed to the square ones. Through both experimental and numerical comparisons of three shapes (rhombus, square, and circle), it becomes evident that the circular type outperforms the other types in terms of discharges through the intake, showing an improvement of about 8%. The results indicate that flow depth and height of the velocity caps are positively effective parameters for the flow rate, with respective influences of 90% and 30%. In contrast, the interaction between the flow and caps intensifies with an increase in the distance of the intake opening from the bed, which plays a negative influence on the flow rate. Enhancing the number of blades in caps proves to be the optimal approach for generating a smoother flow with minimal impact on the flow rate. Numerical simulations show a 50% reduction in cap height leads to a significant 33% decrease in flow rate. Additionally, rotating the square cap by 45° into a rhombus aligned with the flow direction results in a 7% discharge flow rate increase.

Integrating mechanical mixing, headspace, and rheology in a computational model for a fermentation process

AbstractAlthough computational fluid dynamics (CFD) modelling has been applied to study bioprocesses where mechanical mixing and aerations are key, the effect of having a free surface has usually been ignored. This work studies a mixing tank for bioprocess applications to analyze the impact that the assumption of a flat liquid level has upon the modelling results, with and without aeration. The methodology takes an experimental and modelling approach. A dual impeller mixer with a speed of 400 rpm is used for batch systems with both non‐Newtonian and Newtonian fluids to characterize the axis torque and surface vortex formation. Several multi‐phase CFD models are applied to study the modelling and numerical effects of the headspace on the results. Regarding modelling accuracy, the CFD models are shown to be able to capture the effect of the free surface on the fluid dynamics of the stirred tank for different fluid rheologies. Therefore, the simplification of the liquid level as a flat and fixed surface should not be applied, especially for a process with aeration or when unaerated conditions can lead to a surface vortex. Regarding the numerical accuracy, it is concluded that the mixture model does not predict the interface shape as well as the Eulerian model. However, only the mixture model shows to be numerically stable. Overall, this work provides validated CFD configurations able to predict the effect of a free surface on the mixing mechanisms in stirred reactors.

EFFECT OF SUMP GEOMETRY AND SUBMERGENCE DEPTH ON FREE SURFACE VORTEX FORMATION OF PUMP INFLOW

This paper examines and discusses pump sump design as it affects unsteady pump inflow. It is shown that an improvement of the unsteady inflow characteristics can be accomplished by introducing some modifications to the pump sump design. The paper introduces a 3D numerical computation of unsteady flow through a water sump into the intake of a large vertical pump. The numerical simulation is performed using ANSYS CFX. Numerical simulations performed use the shear stress transport model to account for the effect of turbulence. The results obtained were verified by comparison with published experimental data. The effect of the sump geometry and water level on the internal flow in pump sump is presented. Four sump geometries were studied: a rectangular sump, a sump with inclined edge at 30°, a sump with inclined edge at 45°, and a half-circle-ended sump. Also, three submergence levels were studied: S/D = 1.015, 0.771, and 0.54. It is concluded that sumps of rectangular geometry are the best to avoid surface and floor vortex formation.

Gravitational Surface Vortex Formation and Suppression Control: A Review from Hydrodynamic Characteristics

The energy-conversion stability of hydropower is critical to satisfy the growing demand for electricity. In low-head hydropower plants, a gravitational surface vortex is easily generated, which causes irregular shock vibrations that damage turbine performance and input-flow stability. The gravitational surface vortex is a complex fluid dynamic problem with high nonlinear features. Here, we thoroughly investigate its essential hydrodynamic properties, such as Ekman layer transport, heat/mass transfer, pressure pulsation, and vortex-induced vibration, and we note some significant scientific issues as well as future research directions and opportunities. Our findings show that the turbulent Ekman layer analytical solution and vortex multi-scale modeling technology, the working condition of the vortex across the scale heat/mass transfer mechanism, the high-precision measurement technology for high-speed turbulent vortexes, and the gas–liquid–solid three-phase vortex dynamics model are the main research directions. The vortex-induced vibration transition mechanism of particle flow in complex restricted pipelines, as well as the improvement of signal processing algorithms and a better design of anti-spin/vortex elimination devices, continue to draw attention. The relevant result can offer a helpful reference for fluid-induced vibration detection and provide a technical solution for hydropower energy conversion.

CFD-Based and Experimental Hydrodynamic Characterization of the Single-Use Bioreactor XcellerexTM XDR-10.

Understanding the hydrodynamic conditions in bioreactors is of utmost importance for the selection of operating conditions during cell culture process development. In the present study, the two-phase flow in the lab-scale single-use bioreactor Xcellerex XDR-10 is characterized for working volumes from 4.5 L to 10 L, impeller speeds from 40 rpm to 360 rpm, and sparging with two different microporous spargers at rates from 0.02 L min to 0.5 L min. The numerical simulations are performed with the one-way coupled Euler–Lagrange and the Euler–Euler models. The results of the agitated liquid height, the mixing time, and the volumetric oxygen mass transfer coefficient are compared to experiments. For the unbaffled XDR-10, strong surface vortex formation is found for the maximum impeller speed. To support the selection of suitable impeller speeds for cell cultivation, the surface vortex formation, the average turbulence energy dissipation rate, the hydrodynamic stress, and the mixing time are analyzed and discussed. Surface vortex formation is observed for the maximum impeller speed. Mixing times are below 30 s across all conditions, and volumetric oxygen mass transfer coefficients of up to 22.1 h are found. The XDR-10 provides hydrodynamic conditions which are well suited for the cultivation of animal cells, despite the unusual design of a single bottom-mounted impeller and an unbaffled cultivation bioreactor.

An analytical model for vortex at vertical intakes

Abstract In the present paper, free surface vortex formation at intakes is investigated analytically. By assuming a spiral form for vortex streamlines, continuity and momentum equations were integrated and solved in a vortex flow domain. From this solution, velocity and pressure distributions were found above the intake under vortex action. An equation for the water surface profile was also found and compared with another research. By considering that in an air core vortex, pressure at the intake entrance drops to zero, a relationship was found for critical submerged depth and verified by experimental data and another analytical equation. It was concluded that the results of the proposed spiral analytical model had good agreement with the experimental data.

Physical and Mathematical Simulation of Surface‐Free Vortex Formation and Vortex Prevention Design during the End of Casting in Tundish

Herein, the formation of the vortex and slag entrainment in tundish is studied by using mathematical and physical simulation. This article aims at contributing to the understanding of the free surface vortex formation in continuous casting tundish at the end of casting. A 3D multiphase mathematical model using the renormalization group (RNG) k–ε turbulence model is developed to predict the vortex formation and slag layer behavior on the scale of the 1:4 water model of a two‐strand slab tundish. The Eulerian volume of fluid model is adopted to track the steel–slag–air free surface and vortex, and the effect of Coriolis force is considered in this simulation. An improved vortex prevention design is also proposed by changing the position of the diversion hole on the dam of the tundish, which is compared with the original scheme using numerical and physical simulation. The predicted results of the vortex structure and the critical height of the vortex formation agree well with the water modeling results. It is demonstrated that the critical height of the vortex formation is reduced significantly by regulating the position of the diversion hole on the dam near the outlet of the tundish.

Transient analysis of upstream wake inside turbine blade passage with purge flow

Secondary air bled from the compressor which bypasses the combustion chamber is used to seal the turbine components from incoming hot gas. Interaction of this secondary air (also known as purge flow) with the mainstream flow can alter the flow characteristics of turbine blade passage. This paper presents numerical investigation of interaction between ejected purge flow and mainstream flow in the presence of upstream disturbances/wakes. Steady as well as unsteady simulations are carried out using Reynolds Averaged Navier Stokes equations and SST turbulence model. The numerical results are validated with experimental measurements obtained at the blade exit region using an L shaped 5 hole probe and Scanivalve. Upstream wakes are generated by a circular cylinder, kept upstream of blade leading edge at different pitch-wise positions. For transient analysis cylinders are kept at stagnation line (STW) and middle of the blade passage (MW). The analysis reveals the interaction effects of two more additional vortices, viz. the cylinder vortex (Vc) and the purge vortex (Vp). Steady state analysis shows an increase in the underturning at blade exit due to the squeezing of the pressure side leg (PSL) of horse shoe vortex towards the pressure surface by the cylinder vortices (Vp). The unsteady analysis reveals the formation of filament shaped wake structures which breaks into smaller vortical structures at the blade leading edge for STW configuration. These filaments lead to the formation of additional pressure surface vortices. On the contrary, in MW configuration, the obstruction created by the purge flow causes the upper portion of cylinder vortices bend forward, creating a shearing action along the spanwise direction. In MW configuration, the horse shoe vortices generated from the upstream cylinder are broken by the purge vortex whereas in the STW configuration it slides over the purge vortex and move towards the suction surface under the influence of the pitchwise pressure gradient.

Large‐Scale Experiments on the Formation of Surface Vortices with and without Vortex Suppression

AbstractExperiments were carried out in a 50‐m3 cylindrical tank to determine the influence of strong momentum on the formation of large‐scale gas‐core vortices. Gas‐core lengths were measured for varying volume flow rates and submergence depths. The critical Froude numbers were also determined and the efficiency of different vortex suppressors on the gas‐core formation was investigated. The horizontal velocity field inside the vortex core region was additionally recorded using particle image velocimetry. The experimental results were used to verify numerical simulations and compared to vortex models and correlations from literature.

Investigation on the Surface Vortex Formation During Mechanical Stirring with an Axial-Flow Impeller Used in an Aluminum Process

The present study investigated the mechanism of surface vortex formation in an aluminum melt vessel stirred by an axial-flow impeller mechanically. The oxide film is formed at the aluminum melt/air interface, and the movement of the interface entrains the oxide film and inclusions. Hence, the transient movement of melt–air interface is significant. The present study conducted a water model experiment and numerical simulation focusing on the movement of gas–liquid interface. The present study found that the oxide film can be entrained by two phenomena: (1) local surface vortex and (2) sloshing near the vessel wall. The local surface vortex is formed due to the pressure distribution around the impeller, and the sloshing is caused by macroinstabilities, which is generated by the discharged flow of axial-flow impeller. Besides, the shape of gas–liquid interface is dependent on the impeller shape. The axial-flow impeller gives rise to steeply curved shape of gas–liquid interface near the impeller shaft.

Surface vortex formation and free surface deformation in an unbaffled vessel stirred by on-axis and eccentric impellers

The present study investigated the mechanism of surface vortex formation and free surface deformation in an unbaffled stirred vessel with on-axis and eccentric impellers having four paddle blades. To shed light on the mechanism of surface vortex formation and free surface deformation, we conducted experiments and numerical simulation in an unbaffled cylindrical stirred vessel. The surface vortex was found to be small in the eccentrically stirred vessel due to a vertical flow caused by the interaction between the discharged flow and sidewall. In the case of on-axis impeller, low-pressure zones at the rear faces of blades pulled the free surface, that causes the air entrainment. These phenomena could be seen in both the experimental observation and numerical simulation. The three types of transient free surface deformation were observed: local vortex formation due to the low-pressure zone mentioned above, entire rotation of free-surface elevation due to the macro-instabilities, and local elevation and deflection due to trailing vortices.

Numerical analysis of free surface vortices behavior in a pump sump

Recently, designers and engineers of pump stations realized that the efficiency and performance of a pumping station does not depend only on the performance of selected pumps but also on proper design of intake sumps. Most recurring problems faced in a pumping station are related to the sump design or intake design rather than pump design. Swirl angle parameter and free surface vortex formation are important parameters that determine the quality of flow ingested by sump. For swirl angle parameter, the hydraulic institute prescribes the method that needs to be employed for estimating this parameter. On the other hand, the free surface vortex formation also leads to a decrease in the efficiency of pump intake and pump especially when the vortex rope rotates opposite to impeller rotation. In this study, both numerical and experimental analysis of a rectangular pump sump was carried out to predict swirl angle and free surface vortices formation. The swirl angle experimentally was obtained using a traditional swirl meter installed at the suction pipe in rectangular single pump intake. The objective was to obtain the average tangential velocity at different suction pipe diameter. For free surface vortices the objective was to observe the different vortex formation, air entrance and location. The swirl angle and average tangential velocity estimated by CFD simulation was in agreement with experimental results. The four types of free surface vortices (Surface swirl, Surface dimple, air bubbles and full air core to intake) was also observed clearly in ANSYS CFX.

Numerical and experimental investigation of surface vortex formation in coolant reservoirs of reactor safety systems

Abstract The reliable operation of the emergency coolant pumps and passive gravitational injection systems are an important safety issue during accident scenarios with coolant loss in pressurized water reactors. Because of the pressure drop and flow disturbances surface vortices develops at the pump intakes if the water level decreasing below a critical value. The induced swirling flow and gas entrainment lead to flow limitation and to pump failures and damages. The prediction of the critical submergence to avoid surface vortex building is difficult because it depends on many geometrical and fluid dynamical parameters. An alternative and new method has been developed for the investigation of surface vortices. The method based on the combination of CFD results with the analytical vortex model of Burgers and Rott. For further investigation the small scale experiments from the Institute of Nuclear Techniques of the Budapest University of Technology and Economics are used which were inspired from flow limitation problems during the draining of the bubble condenser trays at a VVER type nuclear power plants.

New idea of suppressing free surface vortex during tapping liquid steel from converter

During tapping liquid steel from a converter to a ladle, slag may carry over due to a formation of free surface vortex. Such vortex should be suppressed in order to minimize contamination of the steel by slag. In the present study, a new idea is proposed to minimize the vortex formation by applying dual tapping hole system. The idea was tested using a rectangular shape water vessel with dual tapping holes, supplemented by a numerical simulations, in order to evaluate feasibility of the dual tapping hole system. Both the water model experiment and the numerical simulation showed similar tendencies that the dual tapping hole system is promising to prevent the vortex formation. Experimental results showed that the free surface vortex formation is highly sensitive to shape and configuration of the nozzle. From the numerical simulation, flow characteristics were obtained, and vortex canceling between two nozzles is confirmed. Effect of dual tapping hole increased with increasing distance between two holes in the present experimental condition. Finally, a 1/7.8 scale-down water vessel simulating a real converter was employed in order to confirm the validity of the above findings. The result showed almost the same tendencies with the previous findings.

Analysis of Factors Affecting Free Surface Vortex Formation during Steel Teeming

Analysis of Factors Affecting Free Surface Vortex Formation during Steel Teeming

PIV Measurements to Study the Effect of the Reynolds Number on the Hydrodynamic Structure in a Baffled Vessel Stirred by a Rushton Turbine

Turbulent flow inside a cylindrical baffled stirred vessel is studied experimentally for different Reynolds numbers. A set of speed was selected ranging from 100 rpm to 350 rpm. These speeds gave high turbulence but without significant surface vortex formation. Vector field’s maps and contours of time averaged velocities, for both radial and axial components in the impeller stream of a vessel stirred by a Rushton turbine, were determined by means of 2D PIV technique. This study reveals the importance of choosing the whole flow field of the entire vessel in order to provide comprehensive understanding of the flow pattern and mixing conditions which is essential for reliable design.

Large Eddy Simulations of Double-Ruler Electromagnetic Field Effect on Transient Flow During Continuous Casting

Transient flow during nominally steady conditions is responsible for many intermittent defects during the continuous casting of steel. The double-ruler electromagnetic field configuration, or “FC-Mold EMBr,” is popular in commercial slab casting as it provides independent control of the applied static field near the jet and free surface regions of the mold. In the current study, transient flow in a typical commercial caster is simulated in the absence and in the presence of a double-ruler magnetic field, with rulers of equal strengths. Large eddy simulations with the in-house code CU-FLOW resolve the important transient behavior, using grids of over five million cells with a fast parallel solver. In the absence of a magnetic field, a double-roll pattern is observed, with transient unbalanced behavior, high surface velocities (~0.5 m/s), surface vortex formation, and very large surface-level fluctuations (~±12 mm). Applying the magnetic field suppresses the unbalanced behavior, producing a more complex mold flow pattern, but with much lower surface velocities (~0.1 m/s), and a flat surface level with small level fluctuations (<±1 mm). Nail board measurements taken at this commercial caster, in the absence of the field, matched reasonably well with the calculated results, both quantitatively and qualitatively.

Surface vortex prevention at power intakes by horizontal plates

The efficiency of horizontal perforated and solid plates, installed on top of an intake for prevention of surface vortex formation, was experimentally investigated. The experiments were conducted on a horizontal intake of a reservoir. By changing the angle of the approach flow, submergence of the intake and the discharge, 36 air-core surface vortices with different strengths were generated. The strengths of these vortices were evaluated by measuring tangential velocities using an acoustic Doppler velocimeter. A relationship between the strength of the surface vortex and the necessary percentage of uniform opening of the perforated plate is presented. It was found that a perforated plate of width 1D and 1·5D length normal to the intake (D is the tunnel diameter) with 50% uniform opening and a solid plate of 1·5D width and 2D length eliminated all surface vortices at the intake. Pressure was also measured in a section immediately downstream of the intake by means of four transducers. By using the measured pressure values, the effect of anti-vortex plates on local loss at the intake was determined.

Intake shape and boundary-related considerations in the operation and design of storm-sewer drop structures

This paper presents the results of an experimental investigation into the hydraulic efficiency of a vertically oriented water intake. Under low submergence conditions, the operation of such an intake is affected significantly by the formation of surface vortices, which result in significant flow reduction and the downstream entrainment of air and swirl. The experimental work considered the fact that vortex formation is governed by the geometrical characteristics of the intake, including the diameter, local shape, submergence, surrounding boundary, etc, and the velocity distribution in the approaching flow. Therefore, an experimental study was conducted to investigate the influence that the local shape and the proximity to nearby boundaries have on the hydraulic performance of a vertical water intake. The results of this experimental investigation will hopefully lead to a better understanding of the phenomena governing the flow in such structures and to the elaboration of improved engineering design guidelines.

Minimum pump submergence to prevent surface vortex formation

Recent changes in the Hydraulic Institute pump station intake design standards prohibit the formation of dye-core vortex activity in vertical pump intakes. Existing equations used to predict submergence vortex activity are intended for stronger air entraining vortices. In this study, several physical hydraulic models of vertical turbine pump sumps were used to evaluate/modify existing empirical equations to assist engineers and pump manufactures in determining the minimum submergence required to prevent various degrees of surface vortex formation.