Hydraulic Intakes Research Articles

Numerical Simulation of Air-Core Surface Vortex at Critical Submergence for Dual Horizontal Intakes

Abstract Air-entrainment from surface vortex at critical submergence for hydraulic intakes adversely affects the withdrawal efficiency and intake performance. In this study, flow at the lateral dual square bottom intake placed in a row operating under uniform approach flow is numerically simulated. The dual intakes are subjected to perpendicular approach flow making the withdrawal condition more complex. The interface of the air–water phase is tracked using the volume of fluid (VOF) model, both coupled and decoupled with the level-set method with different spatial discretization methods for identifying the best interface interpolation. This study is focused on the development of a methodology for the determination of critical submergence for dual intakes numerically. Surface vortex causing air-entrainment at lateral dual intake was identified using three distinct approaches and a novel methodology involving the use of volume fraction study combined with a swirling-strength based vortex detection mechanism is proposed to compute the critical submergence for the safe operation of the intakes. Further, the effect of intake pipe blockage on the critical submergence is studied using differential intake protrusions and identified that a differential protrusion of the downstream intake can reduce the critical submergence with an enhanced withdrawal capacity of the upstream intake. The computed critical submergence is validated using experimental results on lateral dual intakes and found to be within ±10% error. The results of this study will be helpful to practicing engineers in the rational design of hydraulic intakes for various diversion projects.

Characteristics and mechanisms of air-core vortex meandering in a free-surface intake flow

The air-core vortex generated at hydraulic intakes is usually associated with efficiency reduction and performance deterioration of the hydraulic machinery. Numerical simulations of the air-core vortex in a horizontal intake system are performed for various intake submergence depths and flow rates. Our study focus on the characteristics and mechanisms of the air-core vortex meandering phenomenon. Statistical analyses of the meandering coordinates of the vortex core are carried out, which characterize the meandering scope, dispersion radii and frequencies of the meandering coordinates of the air-core vortex. Parametric studies of the effects of the flow parameters on the air-core vortex meandering indicate that the dispersion radii increase as the intake flow rate increases or the intake submergence depth decreases, and the meandering frequencies decrease with increasing flow rate. Snapshot proper orthogonal decomposition (POD) analyses of the velocity and vorticity fields reveal that the large-scale energetic coherent structures in the first two modes cause streamwise and spanwise displacement of the air-core vortex, and the small-scale coherent structures with low energy fractions in the third to sixth modes contribute to elliptical deformation of the vortex core. Multiple relationships among the temporal frequencies of the decomposed POD modes and the meandering coordinates are discovered, which further proves that the large-scale energetic coherent structure in the turbulent flow field is the intrinsic reason for the meandering phenomenon of the air-core vortex.

Air-entrainment in hydraulic intakes with a vertical pipe: The mechanism and influence of pipe offset

Air-entrainment is a negative phenomenon in hydraulic intakes with a vertical pipe. Researchers sometimes chose to offset the pipe from the intake center to trigger the air-entrainment, while the underlying mechanism on the formation and propagation is not fully understood. In this paper, the mechanism in both pipe-offset and pipe-centered cases is studied in depth. In addition, the asymmetric flow generated in the symmetric pipe-centered cases is analyzed. It is found that the air-entrained vortex formation procedures are consistent despite of the different pipe positions. The formation and enhancement of the air-entrainment is decided by the stretching term in the enstrophy transport equation. The movement of the vortex is closely related to the tilting term. Asymmetric flow is classical characteristics no matter whether the pipe located on the centerline of the intake or not. For the pipe-offset cases, the natural asymmetry makes the air-entrainment formation more easily and quickly. The relative air-entrainment rate is found to be larger in pipe-offset cases than in pipe-centered cases. For the pipe-centered cases, the asymmetric flow arises due to the asymmetric flow separation when the flow past the pipe surface. The study extends the generality of the formation procedure proposed by Guo et al. (2020). In addition, the flow separation effect on the air-entrained vortex is revealed. If one can suppress the separation, the air-entrained vortex is expected to be suppressed simultaneously.

Investigation on the air-core vortex in a vertical hydraulic intake system

Air-core vortex occurred at the hydraulic intakes deteriorates the performance and operation stability of hydraulic machineries. To investigate the evolution process and generation mechanism of the air-core vortex formed at the vertical intake, large-eddy simulation with the coupled level-set and volume-of-fluid method is performed in a pump sump. This paper elucidates the evolution of velocity distribution caused by pump suction, and associations between vortices motions and velocity evolution and the accompanying free-surface deformations are clarified, which reveals the generation mechanism and evolution characteristics of air-core vortex. Vortices are created by the disturbances of small recirculation zone around the intake pipe. The recirculation is caused by the interaction of the wall of vertical pipe and converging flows with opposite directions driven by the pump suction. The enhancement of vortices due to vortex distortion dominates the air-core vortex formation, and the vertical stretching effect accounts for about 70%. The meandering characteristics of air-core vortex are characterized through analyses of meandering scope and velocity distribution, and the convection of mean flow and sump wall take the leading role in causing its meandering.

Numerical simulation of an air-core vortex at a hydraulic intake using OpenFOAM

A vortex is a ubiquitous everyday phenomenon that is observed in nature and it is formed due to the rotational motion of fluid around an axis perpendicular to the free surface. Free surface vortices are a common unwanted occurrence at hydraulic intakes which can cause serious detrimental impacts on mechanical devices such as turbines and pumps. In this paper, an experimentally observed air-core vortex is numerically simulated using the OpenFOAM LTSInterFoam solver. The LTSInterFoam solver has hitherto been mainly used for hydrodynamic studies relating to ship manoeuvrability by researchers. This solver uses a local time stepping approach to speed up convergence towards steady state conditions thus overcoming some of the challenges associated with the use of the conventional interFoam solver for the simulation of free surface vortices. The Shear Stress Transport (SST) k−ω Model was used for the simulation. There was generally good agreement when results from the study were compared with other vortex-related analytical models and experimental data. Overall, the study concludes that the OpenFOAM LTSInterFoam solver is capable of simulating free surface vortices at hydraulic intakes. However, being a steady state solver, the solver cannot account for the transient process involved in the evolution of free surface vortices.

Modeling of the Free-Surface Vortex-Driven Bubble Entrainment into Water

The recently developed GENTOP (Generalized Two Phase Flow) concept, which is based on the multifield Euler‒Euler approach, was applied to model a free-surface vortex—a flow situation that is relevant for hydraulic intake. A new bubble entrainment model has been developed and implemented in the concept. In general, satisfactory agreement with the experimental data can be achieved. However, the gas entrainment can be significantly affected by several parameters or models used in the CFD (Computational Fluid Dynamics) simulation. The scale of curvature correction C s c a l e in the turbulence model, the coefficient in the entrainment model C e n t , and the assigned bubble size to be entrained have a significant influence on the gas entrainment rate. The gas entrainment increases with higher C s c a l e values, which can be attributed to the stronger rotation captured by the simulation. A smaller bubble size gives higher gas entrainment, while a larger bubble size leads to a smaller entrainment. The results also show that the gas entrainment can be controlled by adjusting the entrainment coefficient C e n t . Based on the modeling framework presented in this paper, further improvement of the physical modeling of the entrainment process should be done.

유압부하와 흡기스로틀을 이용한 트랙터용 기계식 디젤엔진의 DPF 강제재생에 대한 연구

유압부하와 흡기스로틀을 이용한 트랙터용 기계식 디젤엔진의 DPF 강제재생에 대한 연구

Theoretical and experimental study on the vortex at hydraulic intakes

ABSTRACTTo study the characteristics of the vortex at a hydraulic intake, an experiment with a free surface vortex was conducted in a cylindrical tank. The vortex flow field was measured using particle image velocimetry (PIV) and extensive experimental data describing the vortex characteristics (radial and tangential velocity distributions, vortex core radius variation, water surface profile and circulation distribution) have been obtained and analysed. Based on the detailed experimental data, an empirical model describing the key vortex characteristics is developed. In addition, using the pressure distributions inside and outside the vortex core, a dimensionless equation for the critical submergence requiring only the Froude number and the circulation number is derived. Results using this equation are in agreement with the experimental data.

Numerical evaluation of gas core length in free surface vortices

The formation and evolution of free surface vortices represent an important topic in many hydraulic intakes, since strong whirlpools introduce swirl flow at the intake, and could cause entrainment of floating matters and gas. In particular, gas entrainment phenomena are an important safety issue for Sodium cooled Fast Reactors, because the introduction of gas bubbles within the core causes dangerous reactivity fluctuation. In this paper, a numerical evaluation of the gas core length in free surface vortices is presented, according to two different approaches. In the first one, a prediction method, developed by the Japanese researcher Sakai and his team, has been applied. This method is based on the Burgers vortex model, and it is able to estimate the gas core length of a free surface vortex starting from two parameters calculated with single-phase CFD simulations. The two parameters are the circulation and the downward velocity gradient. The other approach consists in performing a two-phase CFD simulation of a free surface vortex, in order to numerically reproduce the gas- liquid interface deformation. Mapped convergent mesh is used to reduce numerical error and a VOF (Volume Of Fluid) method was selected to track the gas-liquid interface. Two different turbulence models have been tested and analyzed. Experimental measurements of free surface vortices gas core length have been executed, using optical methods, and numerical results have been compared with experimental measurements. The computational domain and the boundary conditions of the CFD simulations were set consistently with the experimental test conditions.

Experimental study on unstable free surface vortices and gas entrainment onset conditions

Free surface vortices formation has been, for many years, a relevant issue in many engineering applications with hydraulic intakes, since strong whirlpools introduce swirl flow at the intake causing entrainment of floating matters and gases (air or other gases). The Gas Entrainment (GE) phenomenon has also been an important topic in nuclear industry in the last years, due to the possibility of free vortex formation at the surface, especially in sodium-cooled fast reactors (SFRs). This phenomenon may result in unlikely positive reactivity insertion accident, affecting the safety performances of the reactor itself. The GETS experimental facility (Gas Entrainment Test Section) has been built in the thermal–hydraulics laboratory of the DIAEE in order to study free surface vortices occurrence. The main purpose of this facility is to understand the influence of different parameters on free surface vortices formation and evolution. Experimental tests and preliminary observations, carried out, with cold water as working fluid, different water level in the tank, circulating flow rate and outlet tube diameter, are presented as occurrence maps in this paper. The observed vortices have been qualitatively classified in different “formation stages” and the most important parameters that influence the physical phenomenon have been identified through a dimensionless analysis. Empirical correlations based on dimensionless parameters of the transition boundaries between stages, including the onset of gas entrainment, have been developed and discussed, and very preliminary considerations on the sodium–water similitude, based on the Froude and Weber numbers, are introduced.

Three-dimensional Numerical Simulation of Vertical Vortex at Hydraulic Intake

To investigate an effective numerical model for simulating vertical vortex, turbulent models with VOF method are compared. The effects of the turbulent kinetic energy, the turbulent dissipation rate and the turbulent viscosity on formation and development of vertical vortex are analyzed. It shows that RNG k-ɛ model is more suitable than standard k-ɛ model to the rapidly strained and great curving streamline flows. The calculated spiral flow agrees with the laboratory tests. It may be helpful to the study on generating mechanism of vertical vertex.

On the hydraulic intake shape factor for a circular opening located at an impervious boundary: Influence of inclined stratification

AbstractIn this paper, we develop an exact closed form solution for a circular entry point located at the interface between an impermeable material and a stratified porous medium, where the principal plane of hydraulic isotropy is inclined to the interface. Copyright © 2010 John Wiley & Sons, Ltd.

Comparison between empirical formulae of intake vortices

The hydrodynamic properties of free surface vortices at hydraulic intakes were investigated. Based on the axisymmetric Navier–Stokes equations and empirical assumptions, two sets of formulations for the velocity distributions and the free surface profiles are proposed and validated against measurements available in the literature. Compared with previous formulae, the modifications based on Mih's formula are found to greatly improve the agreement with the experimental data. Physical model tests were also conducted to study the intake vortex of the Xiluodu hydroelectric project in China. The proposed velocity distribution formula was applied to the solid boundary as considered by the method of images. A good agreement was again observed between the prediction and the measurements.

Investigation of air-core vortex at hydraulic intakes

Hydrodynamic properties of the surface vortex have been investigated. Based on the Navier-Stokes equations, three sets of the new formulations for the tangential velocity distributions are derived, and verified against the experimental measurements in the literature. It is shown that one modification greatly improves the agreement with the experimental data. Physical model experiments were carried out to study the intake vortex related to the Xiluodu hydropower project. The velocity fields were measured using the Particle Tracking Velocimetry (PTV) technique. The proposed equation for tangential velocity distribution is applied to the Xiluodu project with the solid boundary being considered by the method of images. Good agreement has been observed between the formula prediction and the experimental observation.

Hydraulic characteristics of vertical vortex at hydraulic intakes

The trace of vertical vortex flow at hydraulic intakes is of the shape of spiral lines, which was observed in the presented experiments with the tracer technique. It represents the fluid particles flow spirally from the water surface to the underwater and rotate around the vortex-axis multi-cycle. This process is similar to the movement of screw. To describe the multi-circle spiral characteristics under the axisymmetric condition, the vertical vortex would change not only in the radial direction but also in the axial direction. The improved formulae for three velocity components for the vertical vortex flow were deduced by using the method of separation of variables in this article. In the improved formulae, the velocity components are the functions of the radial and axial coordinates, so the multi-circle spiral flow of vertical vortex could be simulated. The calculated and measured results for the vertical vortex flow were compared and the causes of errors were analyzed.

The entrainment envelope of dye-core vortices at submerged hydraulic intakes

Each winter in Canada, operational difficulties are encountered at various water works resulting from intake blockages caused by frazil ice entrainment. In a lake setting, frazil is a surface phenomenon, the strong downward current produced by a swirling flow, with an intake vortex present, provides a mechanism by which frazil is transported from the water surface to the submerged intake below. Laboratory experiments were conducted to study the entrainment envelope associated with swirling and non-swirling flows into submerged water intakes. Three-dimensional velocity measurements were made with an acoustic Doppler velocimeter. The results clearly show that the entrainment envelope for swirling flow is several times larger than that for non-swirling flow. This paper details, for a given set of conditions, the differences in the non-swirling and swirling flow entrainment envelopes and emphasizes the potential difficulties with frazil ice that vortices can cause at intakes.Key words: vortex, dye-core vortex, submerged hydraulic intake, entrainment envelope, three-dimensional velocity measurements, acoustic Doppler velocimeter.

Velocity of Air-Core Vortices at Hydraulic Intakes

Velocity of Air-Core Vortices at Hydraulic Intakes

Velocity of Air-Core Vortices at Hydraulic Intakes

Velocity of Air-Core Vortices at Hydraulic Intakes

Velocity of Air‐Core Vortices at Hydraulic Intakes

Concise equations for the tangential, radial, and axial velocities as well as the water surface profile of air-core vortices at hydraulic intakes have been derived. The equations were developed by modifying the equation for tangential velocity originally proposed by Rosenhead in 1930. Laboratory experiments were conducted on strong air-core vortices near a water intake. The results indicate the equations agree with experimental measurements and are applicable to vortex motion in general. The equations for radial and axial vortex velocities contain an eddy-viscosity term, which should be proportional to vortex circulation. Dimensionless parameters describing the circulation and intake Froude numbers and the intake submergence are also used in the analysis. Using these equations, the depth and three velocity components of an air-core vortex at certain hydraulic intakes can be predicted. This information may be helpful to the practicing engineer in determining the submergence required to avoid air-entraining vortices at intakes.

Field and laboratory cavitation studies of the sluice at the Bukhtarma hydroelectric power plant

1. The results of field and model spillway cavitation studies at the Bukhtarma hydroelectric power plant indicate that a reliable prediction of spillway cavitation phenomena is possible only with the use of vacuum-cavitation tunnels. The prediction of cavitation phenomena by testing models in an open hydraulic arrangement, i.e., without modelling the pressure on the free surface of the stream, may lead to a significant reduction in the actual vacuum, and in the long run, to underestimation of cavitation risks. 2. Analysis of the cavitation and hydrodynamic flow characteristics in the field and on the model provides the basis for us to assume that with Reynolds numbers of the order of 105 and above, a condition of Reynolds self-similarity is observed in modeling cavitation at hydraulic intake structures. The effect of factors that cannot be modeled (air saturation, the chemical composition of the water, the presence of impurities insoluble in water, etc.) on the prediction of cavitation phenomena with the use of vacuum stands is negligible. 3. The cavitation process both in the field and on the model is accompanied by highfrequency pressure pulsations. In testing models of structures in a vacuum stand, the amplitude characteristics of these pulsations are modelled according to Froude numbers under conditions where the external pressure above the free surface of the stream is modelled in a stand. The frequency spectra of the high-frequency component of pressure pulsations with cavitation are virtually coincident in the field and on the model. 4. The studies indicated that the Bukhtarma hydroelectric plant spillway has an insatsifactory profile, which significantly deforms the stream and produces cavitation. It is possible to prevent this by varying the profile of the springboard surface and, primarily, the lateral walls.