Complex Vortex Flow Research Articles

Analysis of the Flow Behavior and Pressure Fluctuation of a Pump Turbine with Splitter Blades in Part-Load Pump Mode

The internal flow of a pump turbine is unstable in part-load pump mode for small guide-vane openings, and the strong vibration caused by pressure pulsation is related to the safe and stable operation of the unit. A pump turbine with a six-splitter-blade runner was chosen for unsteady simulation analyses. A standard k-epsilon turbulence model was adopted to study the unsteady flow and pressure pulsation in part-load pump mode. The predicted results show that the flow in the draft tube and the runner with splitter blades was relatively stable and the flow of the blade-to-blade channel was symmetrical. When the inlet and outlet velocity distribution of the vanes was not uniform, a vortex began to form in the stay-vane domain. The reason for this vortex formation is explained, and it is pointed out that the existence of the vortex and backflow leads to uneven velocity distribution. The unsteady calculation results showed that the pressure-pulsation peak-to-peak amplitudes in the vaneless area and guide vanes were much higher than those of other monitor points because of rotor–stator interference between the rotating runner and the vanes. In addition, the pulsation characteristics of the monitor points at different circumferential positions in the vaneless region were quite different. In the vaneless area, the velocity gradient along the circumferential direction was very large, and there was a phenomenon of backflow. Also, the pressure pulsation was 0.2 times that of the runner rotational frequency, and the blade-passing frequency was a third-order frequency. At the outlet of the guide vane, the pressure pulsation was mainly of a low frequency with a complex vortex flow. Finally, the pressure pulsation began to decrease rapidly in the stay-vane region.

MODELLING THE EVOLUTION OF CASTING DEFECT CLOSURE BY RADIAL SHEAR ROLLING

This paper studies the behavior of a transverse through defect in an ingot during complex vortex flow of metal deformed by the radial shift rolling (RSR) method. The aim of the work was to investigate the applicability of the RSR method for the deformation treatment of oxide-dispersed strengthened steel (ODS-steel) ingots into a final semi-finished product. Vortex flow of metal promotes dispersion of alloying solid oxides and refinement of steel structure. However, the question of development or closure of possible casting defects of ODS-steel ingot remains open. For this purpose, an ODS-steel ingot with a through-hole modelling the defect was rolled according to the assumed process scheme. Also, to visualize the evolution of the defect in more detail during the rolling process, a simulation of thesame defect development in an aluminum bar has been performed in 2 mm steps. The results of both rolls showed good correlation. Based on many cross-sections of the experimental bars, detailed 3D models of the defect evolution were built. An average reduction of 50 % in defect volume was found. Welding of the outer areas occurs immediately, but the central areas remain unclosed, stretching proportionally with the drawing of the rod during rolling. For this reason, RSR cannot be used with large cavity defects in the axial zone, but this method is guaranteed to weld surface defects even at low reductions.

Cyclic Analysis of In-Cylinder Vortex Interactions Based on Data-Driven Detection and Characterization Framework

Abstract The complex vortex flow interactions are critical to affect the fuel–air mixing and combustion stability in direct-injection engine. However, due to the strong cyclic variations inside engine, the multiscale swirl flow characteristics with cyclic details are difficult to be sufficiently revealed. Therefore, a vortex detection and characterization framework, including physical and data-driven methods, is implemented to elucidate the cyclic vortex interaction process. In this study, a high-speed time-resolved particle image velocimetry is applied to record the spatiotemporal flow behavior under three different swirl ratio conditions. First, the presence of vortex motion is detected at each crank angle for each engine cycle. Results show that the vortex interaction processes under different swirl ratio conditions exhibit distinctive characteristics. The presence of multiple vortices and their interactions are found to trigger dramatic changes and variations in swirl flow behavior. Then, the individual-cycle analysis of the vortex interaction effects on flow characteristics is conducted. The vortex characteristics including vortex location, strength, and size are examined with cyclic detail using data-driven unsupervised clustering. Results indicate that the vortex merging is the main source inducing the vortex characteristics variations. Furthermore, the occurrence and duration of the vortex merging process are found to be closely related to the intake swirl ratio and valve lift profile. Increased swirl ratio and valve lift cause vortex to merge earlier and reduce the merging duration. This finding provides a potential idea to alleviate the cyclic variation issue by controlling the vortex merging process.

Study of changes in the structure of zirconium alloy E125 after deformation by radial shear rolling

The rheological properties of the alloy E125 (Zr‑2.5 %Nb) were studied in the range of deformation rates 0.5–15 s‑1 and the temperature range 20–770 °C. A database for computer FEM modeling was created in the specified temperature and speed ranges. In the DEFORM‑3D software package, numerical simulation of a complex radial shear rolling process was carried out using the obtained database. The conditions conducive to the grinding of the alloy structure into an ultrafine‑grained state were determined. Based on the simulation results, a full‑scale experiment of rolling a bar made of alloy E125 on a radial shear rolling mill RSP‑14/40 in 7 passes from a diameter of 37 mm to 20 mm with a total compression in diameter of ε = 85 % was carried out. At the same time, according to the modeling, the total accumulated deformation in the most developed peripheral zone was 27,5 mm/mm. Due to the complex vortex flow of the metal, the distribution of accumulated deformation over the cross section was uneven with a gradient to the axial zone. This should also affect the change in the structure. The changes and the gradient of the structure were studied by the method of EBSD mapping of the sample section with a resolution of 2 mm. The microhardness gradient of the cross section was also investigated by the HV method. The axial and central zones of the sample were studied on the. The structure has a pronounced gradient from the formed equiaxed ultrafine‑grained (UMZ) structure on several outer millimeters of the peripheral section to an elongated rolling texture in the center of the rod. The work shows the possibility of processing with the formation of a gradient structure with enhanced properties for the E125 alloy, and also presents a database for FEM calculations.

Heat-driven vortical flow in microsized liquid crystal film

ABSTRACTThe theoretical description of vortex dynamics in a hybrid aligned nematic (HAN) microvolume with an orientational defect excited by a temperature gradient, ∇T, is given based on the appropriate nonlinear extension of the classical Ericksen–Leslie theory. We have carried out a numerical study of the system of hydrodynamic equations including director reorientation, fluid flow v and the temperature redistribution across the HAN microvolume under the influence of ∇T, when the HAN microvolume is heated from above. Calculations show that, due to the interaction between the gradient of the director field ∇nˆ and ∇T, the HAN microvolume settles down to a stationary complex-vortex flow regime.PACS numbers: 61.30.Cz, 65.40.De

Vortex Dynamics in a Hybrid Aligned Nematic Microvolume with an Orientational Defect

The aim of this theoretical paper is to investigate the physical mechanism responsible for the appearance of vortex flow in a hybrid aligned nematic (HAN) microvolume with an orientational defect, excited by a temperature gradient ∇T. This was done in the framework of the classical Ericksen-Leslie theory, supplemented by thermomechanical correction of the shear stress and Rayleigh dissipation function, as well as taking into account the entropy balance equation. We have carried out a numerical study of the system of hydrodynamic equations including director reorientation, fluid flow v, and the temperature redistribution across the HAN microvolume under the influence of ∇T, when the HAN microvolume is heated from above. Calculations show that, due to the interaction between the gradient of the director field ∇n^ and ∇T, the HAN microvolume settles down to a stationary complex vortex flow regime.

Vortex Structurization of Swirl Flows in Energy Systems

The physical factors and conditions leading to the formation of stable vortex structures in internal turbulent swirling currents occurring in complex channels of various energy and heat exchangers are considered. The possibility of applying the theory of screw flows and the equation for the rate of helicity change to determine the critical conditions for the occurrence of a deterministic vortex structure of turbulent flows and to predict the effects of the occurrence of large-scale vortex formations in various elements of energy systems is substantiated. The phenomenon of the swirl flow crisis is described. It is shown that the use of thermodynamic stability analysis to describe complex vortex flows makes it possible to expand the applicability of mathematical modeling methods to describe the processes of formation of vortex structures. The results of computational and theoretical modeling of vortex formation processes during the flow of a swirl flow in pipelines of variable cross-section, which are elements of pipeline systems of a nuclear power installation, are presented.

Modeling of Two-Phase Flow Parameters of a Multi-Channel Cylindrical Cyclone

The variation in the two-phase flow parameters in a cylindrical body of new geometry and principle of operation are considered for a device for separating solids from air flow, solving the problem of numerical flow modeling. The aim of this research was to analyze the changes in the parameters of a multi-channel cylindrical cyclone in a mathematical model and to compare it with the results of the examined physical model. Studies on the numerical modeling of cyclones are reviewed, and models and equations for complex vortex flow description are applied. Differential equations were numerically solved by the finite volume method using the standard turbulence models of k–ε and RNG k–ε. Numerical modeling of the velocities, pressures, and volumes of both phases of the two-phase flow was performed. The simulation of the volume distribution of the second phase (glass particles) in the cyclone structure at flow rates of 10.9 m/s, 13.9 m/s, and 21.9 m/s was performed. The values obtained were compared with the physical model of the cyclone in question. The mean relative error was ±6.9%.

Influence of pipeline geometry on hydrodynamics and heat transfer processes by an example of a ship steam generator

The theoretical and computational analysis proposed in this work is aimed at identifying the features of thermal and hydrodynamic processes carried out in the steam-generating channels of the ship type water-moderated nuclear power installations. It is shown that the complex geometry of the thermohydraulic tract curvilinear channels of the steam generating system has a significant effect on the efficiency of the transport nuclear power installation. In addition to the formation of large-scale vortex structures and swirling flow in the pipeline, the phenomenon of the swirling flow crisis is revealed, under which the low-frequency component of the acoustic spectrum is enhanced. The scientific and applied significance of the proposed research is associated with the need to ensure a wide range of operational changes in efficient and safe operation power modes of icebreaker nuclear power installations. The research, aimed at developing the principles of physical and mathematical modeling of complex vortex flows, is necessary to optimize the design parameters of the thermal power equipment elements of new generation ship nuclear power installations in order to ensure increased safety and reliability of their operation.

Helicity dynamics in reconnection events of topologically complex vortex flows

Abstract

A comparison between advanced hybrid machine learning algorithms and empirical equations applied to abutment scour depth prediction

Complex vortex flow patterns around bridge piers, especially during floods, cause scour process that can result in the failure of foundations. Abutment scour is a complex three-dimensional phenomenon that is difficult to predict especially with traditional formulas obtained using empirical approaches such as regressions. This paper presents a test of a standalone Kstar model with five novel hybrid algorithm of bagging (BA-Kstar), dagging (DA-Kstar), random committee (RC-Kstar), random subspace (RS-Kstar), and weighted instance handler wrapper (WIHW-Kstar) to predict scour depth (ds) for clear water condition. The dataset consists of 99 scour depth data from flume experiments (Dey and Barbhuiya, 2005) using abutment shapes such as vertical, semicircular and 45° wing. Four dimensionless parameter of relative flow depth (h/l), excess abutment Froude number (Fe), relative sediment size (d50/l) and relative submergence (d50/h) were considered for the prediction of relative scour depth (ds/l). A portion of the dataset was used for the calibration (70%), and the remaining used for model validation. Pearson correlation coefficients helped deciding relevance of the input parameters combination and finally four different combinations of input parameters were used. The performance of the models was assessed visually and with quantitative metrics. Overall, the best input combination for vertical abutment shape is the combination of Fe, d50/l and h/l, while for semicircular and 45° wing the combination of the Fe and d50/l is the most effective input parameter combination. Our results show that incorporating Fe, d50/l and h/l lead to higher performance while involving d50/h reduced the models prediction power for vertical abutment shape and for semicircular and 45° wing involving h/l and d50/h lead to more error. The WIHW-Kstar provided the highest performance in scour depth prediction around vertical abutment shape while RC-Kstar model outperform of other models for scour depth prediction around semicircular and 45° wing.

Investigation of vortex flow patterns at the meniscus in a water caster mould

ABSTRACTVortices at the meniscus of the continuous casting mould can entrain mould powder into the liquid pool. The entrained globules will form slivers in the subsequent downstream processing of the slabs. This article presents a three-dimensional computational study of the vortexing at the meniscus with an emphasis on capturing flow behaviours of vortices in continuous casting mould. We have used ANSYS FLUENT 19.2 a commercial CFD package. Two turbulence models, namely standard k- model and shear stress transport (SST) k- model, are employed in this work. The performance of the two turbulence models has been evaluated by comparing the flow measurements from 0.4-scale water model experiments. Both the turbulence models could capture vortices at the top surface. The transient vortex flow characteristics such as formation, expansion (and sustenance), thinning, merging and dissipation of the rotational flow are also predicted by the numerical simulations. In general, the standard k- model and the SST k- model exhibited good capability in predicting the complex, unsteady and turbulent vortex flow phenomena at the meniscus in continuous casting mould.

Expanding the concept of end effect in vortex chambers

Background. Mechanisms of vortex structures generation in geometric inhomogeneities of streamlined surfaces as the basis for the development of low-cost control methods for hydromechanical and thermophysical characteristics of technical devices, aircraft and water vehicles.Objective. Studying the features of the formation and interaction of vortex formations inside and near a cross streamlined semi-cylindrical groove on a flat surface, as well as the structure, kinematic and dynamic characteristics of the boundary layer using statistical analysis.Methods. Experimental investiganions in a wind tunnel of the fields of averaged and fluctuation velocity components in semi-cylindrical groove and its near wake on a hydraulically smooth flat surface by hot-wire measurements in the range of Reynolds numbers calculated along the plate length to the front edge of the recess Rex=U0x/n from 3,8×104 up to 6,9×105.Results. According to measurements in the mid-section of the groove of the fields of actual velocities, the probability density functions of the longitudinal velocity fluctuations, their coefficients of asymmetry and excess are obtained. The profiles of the averaged longitudinal velocity and the isotaches of the averaged and fluctuation velocity components inside the groove are constructed. The regions of formation of the reverse flow in the groove, the velocity field in the shear layer, and the features of its interaction with the stern wall of the groove were found. In the transition from the laminar flow regime of a groove with a low vortex formation intensity to a turbulent one, at first an intense flow is formed in the near-bottom region of the cavity, directed towards the direction of the main flow. That is the reason of generation of the large-scale vortex structures that are periodically ejected out from the groove. As a result of the transition, the intensities of the vortex motion and the circulation flow in the groove volume are significantly increased, and in the near wake there is a separation region with intense velocity fluctuations. It has been established that in the boundary layer above the groove, the law of the probability density distribution of the longitudinal velocity fluctuations is close to normal one, but in the recess to the Maxwellian one. With the deep of semi-cylindrical groove the asymmetry and excess coefficients increase.Conclusions. A complex vortex flow that forms inside a half-cylindrical cavity leads to a change in the structure of the boundary layer above the plate. In the transition and turbulent flow regimes in the boundary layer on the plate, the features and regularities of both the generation of large-scale vortex structures inside the groove and their ejection out from the groove into the near wake are revealed. Thus, the conditions are established under which the transversely streamlined semi-cylindrical groove can serve as an effective generator of vortices, which can be used to control hydroaerodynamic drag and heat and mass transfer processes in structural elements of vehicles, power machines and apparatuses.

Velocity fields of vortex flow inside cross streamlined semi-cylindrical groove

Background. Mechanisms of vortex structures generation in geometric inhomogeneities of streamlined surfaces as the basis for the development of low-cost control methods for hydromechanical and thermophysical characteristics of technical devices, aircraft and water vehicles.Objective. Studying the features of the formation and interaction of vortex formations inside and near a cross streamlined semi-cylindrical groove on a flat surface, as well as the structure, kinematic and dynamic characteristics of the boundary layer using statistical analysis.Methods. Experimental investiganions in a wind tunnel of the fields of averaged and fluctuation velocity components in semi-cylindrical groove and its near wake on a hydraulically smooth flat surface by hot-wire measurements in the range of Reynolds numbers calculated along the plate length to the front edge of the recess Rex=U0x/n from 3,8×104 up to 6,9×105.Results. According to measurements in the mid-section of the groove of the fields of actual velocities, the probability density functions of the longitudinal velocity fluctuations, their coefficients of asymmetry and excess are obtained. The profiles of the averaged longitudinal velocity and the isotaches of the averaged and fluctuation velocity components inside the groove are constructed. The regions of formation of the reverse flow in the groove, the velocity field in the shear layer, and the features of its interaction with the stern wall of the groove were found. In the transition from the laminar flow regime of a groove with a low vortex formation intensity to a turbulent one, at first an intense flow is formed in the near-bottom region of the cavity, directed towards the direction of the main flow. That is the reason of generation of the large-scale vortex structures that are periodically ejected out from the groove. As a result of the transition, the intensities of the vortex motion and the circulation flow in the groove volume are significantly increased, and in the near wake there is a separation region with intense velocity fluctuations. It has been established that in the boundary layer above the groove, the law of the probability density distribution of the longitudinal velocity fluctuations is close to normal one, but in the recess to the Maxwellian one. With the deep of semi-cylindrical groove the asymmetry and excess coefficients increase.Conclusions. A complex vortex flow that forms inside a half-cylindrical cavity leads to a change in the structure of the boundary layer above the plate. In the transition and turbulent flow regimes in the boundary layer on the plate, the features and regularities of both the generation of large-scale vortex structures inside the groove and their ejection out from the groove into the near wake are revealed. Thus, the conditions are established under which the transversely streamlined semi-cylindrical groove can serve as an effective generator of vortices, which can be used to control hydroaerodynamic drag and heat and mass transfer processes in structural elements of vehicles, power machines and apparatuses.

Analytical and experimental study of the substance transport in the vortex flow

The characteristics of the vortex flow with a free surface formed in a vertical cylindrical container filled with water, where the source of motion is the disk at the bottom endwall, are studied experimentally and analytically. Different types of oils and petroleum are used in experiments as admixtures. The problem of an “oil body” form in a complex vortex flow is considered on the basis of the analysis of equations for the mechanics of immiscible liquids of different densities with physically based boundary conditions. The calculations of the free surface forms are carried out in cases of one- and two-component fluid. The relations obtained are in satisfactory agreement with the experimentally registered forms of the interfaces. The flow structure near a rotating disk is studied, the shapes of the trajectories of moving liquid particles in the water column near the disk are determined. The coincidence of the types of spiral movement of liquid particles on the surface and near the disk indicates that the characteristic features of the vortex flow are determined in the boundary layer on the rotating disk and then transferred to the entire spatial region occupied by the liquid. Experimental and theoretical study of the vortex flow showed that the trajectories of moving liquid particles near the water surface are spatial spirals along which these particles move from the periphery to the center. The expressions obtained for the form of trajectories of liquid particles accelerated by the rotating disk coincide with the type of registered tracers’ trajectories in the depth of liquid and on the free surface of the flow.

Experimental and numerical investigations of unsteady aerodynamic derivatives for a generic lambda wing UCAV configuration

Experimental and numerical investigations have been conducted to analyze the unsteady aerodynamic and flight mechanical behavior of a 53° flying wing configuration representing a generic unmanned combat air vehicle (UCAV). The considered vehicle is named SACCON/DLR-F17 and the here presented results have been collected within an internal project of the German Aerospace Center in collaboration with the NATO task group AVT-201 on “Extended Assessment of Reliable Stability & Control Prediction Methods for NATO Air Vehicles”. Both projects have the aim to conduct studies of an integrated approach to predict the stability and control characteristics for a generic UCAV configuration based on both experimental and numerical investigations. Systematic investigations have been performed to determine the dynamic derivatives for the longitudinal and lateral motion. A comparison between data of experimental wind tunnel investigations and numerical simulations under real flight conditions is given. In wind tunnel tests, performed in the DNW-NWB wind tunnel, the SACCON configuration was studied at a reduced scale of 1:8 and a Mach number of M∞ = 0.15. Forced oscillation motions were performed during these tests to calculate the dynamic derivatives from force and moment data. For the numerical calculations, the same reduced frequencies were applied with the full-scale configuration under real flight conditions using the RANS method DLR-TAU. The planform aerodynamics is governed by complex non-linear flow phenomena due to leading edge vortex flow separation at moderate to high angles of attack. Based on the numerical results, an interpretation of the complex vortex flow is given to understand the unsteady flow phenomena for a variety of roll, pitch, and yawing motions.

Numerical study of the material transport in the viscous vortex dipole flow

This paper presents a numerical study of the material transport of Lamb dipole(s) in the two-dimensional viscous flow. We focus on the properties of the rate of strain tensor, which has received less attention in the literature. It is noted that the eigenpairs of the tensor explicitly indicate the strength and direction of material stretching and compressing. The tensor provides a clear map of the material motion regardless of the complexity of the vortical flow. The strain rate field displays a rich structure as it contains five elliptic points and six hyperbolic points. It is interesting to observe that the left elliptic point of the strain rate field bifurcates into two at t > 0. Two kinds of material curves, circular and vertical, are used to illustrate the flow transport. The transport mechanism discussed here can be employed to explore the transport in more complex vortex flows.

Abutment scour depth modeling using neuro-fuzzy-embedded techniques

ABSTRACTBridge pier scour hole is formed during major flood events due to a complex vortex flow around the abutments and causes the failure of foundation. Therefore, accurate prediction of the scour hole depth is a major factor in the hydraulic structure design. This paper offers an adaptive neuro-fuzzy-embedded subtractive clustering (ANFIS–SC) method for estimating abutment scour hole depth under clear water condition with uniform bed sediments. The accuracy of ANFIS–SC method is compared with two other ANFIS methods embedded with fuzzy C-mean clustering and grid partitioning (ANFIS–FCM and ANFIS–GP). The decisive factors on the abutment scour hole depth include the ratio of the average diameter of particle size to abutment transverse length (d50/l), excess Froude number of the abutment (Fe), shape factor (Ks), and the ratio of approach stream depth to abutment transverse length (h/l). Eleven different input combinations were evaluated with four, three, and two input variables to survey the effect of each dimensionless parameter on the abutment scour depth. The results indicated that the use of all effective input variables (Fe, d50/l, h/l, Ks) results in the best prediction of abutment scour depth. The comparison of ANFIS models showed the superiority of ANFIS–SC over the other methods.

Application of scheme with customizable dissipative properties for gas flow calculation with interface instability evolution

We have performed testing of the scheme with customizable dissipative properties as applied to problems for complicated gas flow structure with the evolution of interface instability.The scheme is implemented by the splitting into two phases. The first one uses the central differences of both deformation and gradient terms of the Euler equations with artificial viscosity TVD limiters. TVD type reconstructions for convective terms with flux limiters are used in the second phase. A two-order TVD Runge-Kutta algorithm is applied to march the solution in time. We address the issue of numerical resolution and efficiency of the proposed scheme in comparison with some high resolution schemes through numerical examples: the implosion problem, the explosion problem and the double Mach reflection problem. The scheme with customizable dissipative properties in terms of cost (cost of machine time) – quality (resolution) is at the level of modern high order schemes. For some numerical examples, the proposed scheme is superior. The scheme can be recommended for numerical studies of both complex shock wave and vortex flows with the evolution of interface instability.

Dynamic Simulation in Guide Vane Opening Process of a Pump-Turbine in Pump Mode

During the process of switching conditions in pump-turbine, unstable flow would take place and seriously impact on the stability and safety. This paper deals with the guide vanes’ moving process in pump mode through unsteady numerical simulations. Dynamic mesh methodology is used for simulations in which GVO (Guide Vane Opening) from 9mm to 26mm. Simulation results approve that, there are many complex vortex structures and flow blockages phenomena under small GVO condition. Through the mathematical method of Fast Fourier Transform (FFT) and Short-Time Fourier Transform (STFT), the dominant pressure fluctuation frequencies are from runner blade passing frequency (BPF) and its harmonic frequency, as well as some other low frequencies are from unsteady flow states. Furthermore, the flow states are more unstable and complex under small guide vanes moving process. Compared with fixed GVO position, the flow states are more unstable and complex under guide vanes moving process, while the amplitude of main frequencies becomes higher.