Fluid-induced Forces Research Articles

Decoupled Boundary Handling in SPH

Particle-based boundary representations are frequently used in smoothed particle hydrodynamics (SPH) due to their simple integration into fluid solvers. Commonly, incompressible fluid solvers estimate the current density and corresponding forces in case the current density exceeds the rest density to push fluid particles apart. Close to the boundary, the calculation of the fluid particles’ density involves both neighboring fluid and neighboring boundary particles, yielding an overestimation of density, and, subsequently, wrong pressure forces and wrong velocities leading to the disturbed fluid particles’ behavior in the vicinity of the boundary. In this paper, we present a detailed explanation of this disturbed fluid particle behavior, which is mainly due to the combined or coupled handling of the fluid–fluid particle and the fluid–boundary particle interaction. We propose the decoupled handling of both interaction types, leading to two densities for a given fluid particle, i.e., fluid-induced density and boundary-induced density. In our approach, we alternately apply the corresponding fluid-induced and boundary-induced forces during pressure estimation. This separation avoids force overestimation and reduces unintended fluid dynamics near the boundary, as well as a inconsistent fluid–boundary distance across different fluid amounts and different particle-based boundary handling methods. We compare our method with two regular state-of-the-art methods in different experiments and show how our method handles detailed boundary shapes.

RESEARCH ON THE FLUID-INDUCED EXCITATION CHARACTERISTICS OF THE CENTRIFUGAL PUMP CONSIDERING THE COMPOUND WHIRL EFFECT

In order to study the correlation mechanism between the flow characteristics and the fluid-induced force under the compound whirl motion in the centrifugal pump, the RNG k-ε model is selected in this paper to simulate a low specific speed centrifugal pump with impeller eccentricity based on the N-S equation. The changes of fluid-induced force with impeller eccentricity and the unsteady flow characteristics of the internal flow field of centrifugal pump under different flow conditions and rotation speeds are investigated, and the relationship between the fluid-induced force of the impeller and the internal flow field characteristics is discussed. The results show that the trend of fluid-induced force and the pressure coefficient is similar. When the rotation speed changes and when the flow is similar, the pressure coefficient under different rotation speeds almost coincides. With the increase of impeller speed and impeller eccentricity, the dynamic and static interferences between the impeller and the volute tongue are more significant, the uneven distribution of the pressure around the impeller makes the internal flow of centrifugal pump more disordered and increases the fluid-induced force near the volute tongue. The research results can provide important reference value for accurately grasping the internal flow excitation principle of the centrifugal pump.

Evaluation of the flow state and static performance of smooth annular liquid seals

The static fluid-induced force and stiffness coefficient of the smooth annular seal directly affect the rotor system stability. In this paper, a computational fluid dynamics method is applied to investigate the flow characteristics of a smooth annular seal for various eccentricities, discharge/supply pressures and rotational speeds under different flow conditions (laminar, transition, and turbulent flow). The influence factors and formation mechanism of the static instability in the smooth annular liquid seal are analyzed. Results show that laminar flow dominates the flow state at a rotational speed of ω= 2000 rpm. As the rotational speeds increase, the transition flow (2000-7000 rpm) gradually transits to the turbulent regime (ω> 7000 rpm). The direct static stiffness decreases first and then increases from laminar to transition flow state, and the viscosity effect is the dominant factor. For transition and turbulent flow with high eccentricities (ε= 80 %), the dominant viscous effect and inertial effect lead to the negative radial force and negative direct static stiffness coefficients. The smooth annular liquid seal shows best performance in the laminar flow and worst performance in the turbulent flow.

Characterization of the Propulsion System for Submersible Multimedium Robotic Vehicles

Multimedium robots are a new class of bioinspired vehicles with the ability of cross-domain transition and maneuverability across multiple domains. Such hybrid robots can transverse both in air and water, overcoming the stark differences in the motion dynamics within these mediums. Differences in the density, viscosity, and additional fluid-induced forces of the medium make the propulsion system design for multimedium robots a crucial challenge. Both unmanned aerial and remotely operated underwater vehicles rely on electrical propulsion coupled with propellers to navigate in their own respective medium of operation. However, due to the density differences in water and air, which is approximately 103 times, an aerial propeller demands higher RPM to generate significant thrust, unlike an underwater propeller. Higher water density and fluid inertia, on the other hand, demand a greater in-drive torque to spin the propeller in underwater conditions. Other critical parameters to be considered while choosing a suitable propulsion system are propeller diameter, propeller pitch, shaft torque requirements, operational RPM of motor–propeller pair, cavitation, and propeller tip deflection. This article presents an in-depth investigation and evaluation methodology to identify the feasible operating range for propellers and motors that meet the aerial and underwater thrust requirements while minimizing cavitation and propeller deflection. The thrusters (motor-propeller pair) are investigated individually, where propeller performance and physical constraints on the propeller blade are analyzed using blade elemental momentum theory (BEMT), while the motor is studied using the three-constant motor model. These subsystems are then mapped using torque equilibrium to determine their specific operational regime and overall efficiency. Numerical simulation in QPROP demonstrates that aerial thrusters can be utilized for both the mediums but with a sacrifice of overall efficiency. The optimal motor efficiency was 73.06% in aerial operation and 8.1% in underwater operation, but with increased thrust underwater due to higher fluid density, which is essentially a design tradeoff. The results of the analysis are experimentally validated and further presented as a framework for selecting an appropriate propulsion subsystem for multimedium vehicles based on the specific user-defined design requirements.

Influence of tooth geometrical shape on the leakage and rotordynamic characteristics of labyrinth seals in a cryogenic liquid turbine expander

Cryogenic liquid turbine expanders have been used in many refrigeration systems to save energy, and rotor stability is a top concern in their development. The present study focuses on the influence of tooth geometrical shape on the leakage and rotordynamic characteristics of labyrinth seals in a cryogenic liquid expander, whose rotordynamic characteristics are typically special because of the thermodynamic effect of cryogenic liquid and a large pressure drop. Computational fluid dynamics (CFD) simulations are conducted in thirteen-teeth labyrinth seals with isosceles trapezoid shaped teeth and two types of right-angle trapezoid shaped teeth for a given geometric parameter through the whirling rotor method and total energy model. The flow behavior in the three labyrinth seals with different tooth geometrical shapes is explored, and their influence on the leakage and rotordynamic characteristics of the labyrinth seal is obtained. The results show that the leakage flow rate of the labyrinth seal with front right angle trapezoid shaped teeth is 1.1% and 0.6% less than that with rear right angle trapezoid shaped teeth and isosceles trapezoid shaped teeth, respectively, because of a larger divergence angle and more leakage flow involved in dissipation, while the labyrinth seal with isosceles trapezoid shaped teeth produces a larger fluid-induced tangential force opposite to the direction of whirling velocity and is more favorable for rotor stability. In addition, a parametric study is conducted to analyze the influence of clearance, tooth height, tooth thickness and inclination angle on the leakage and rotordynamic characteristics of the three labyrinth seals.

Study on the characteristics of the nuclear reactor coolant pump during the process-divided start-up period

The nuclear Reactor Coolant Pump (RCP) is the main spinning equipment in the primary cooling system of the pressurized water reactor. The current start-up period of the RCP includes the variation of operating pressure, temperature, and rotating speed. A Computational Fluid Dynamics (CFD) model of a whole primary system is used to investigate the performance change and transient effects during this period. Results show that the start-up power and force could be limited by dividing the start-up period into processes. Meanwhile, the impeller blade sweeps over the guide vane, leading to pressure pulsation and impeller axial force fluctuation. This Blade Passing Frequency is dominant in the whole pulsation frequency band and is varying linearly with the rotating speed. The medium density and viscosity decline with temperature rise, further leading to a significant decrease in motor power and fluid-induced force, which is the main aim to apply process-divided start-up.

Research on Influence Factors of Fluid-induced Force in labyrinth liquid Seals for pump

A 3D model of straight-through labyrinth liquid seal for water pump was built to analyse the fluid-induced forces. We found that the eccentricity and pressure difference have great impact on both the tangential force and the radial force. The tangential force increases along with the rotating speed. When tooth hight is Th=2.4mm, the tangential force comes to the minimum. When tooth width Tw =2.0mm, the radial force comes to the maximum. The tooth numbers have a great influence on the fluid-induced force. The more number of sealing teeth will cause the instability the sea.

Energy Loss and Radial Force Variation Caused by Impeller Trimming in a Double-Suction Centrifugal Pump.

Impeller trimming is an economical method for broadening the range of application of a given pump, but it can destroy operational stability and efficiency. In this study, entropy production theory was utilized to analyze the variation of energy loss caused by impeller trimming based on computational fluid dynamics. Experiments and numerical simulations were conducted to investigate the energy loss and fluid-induced radial forces. The pump’s performance seriously deteriorated after impeller trimming, especially under overload conditions. Energy loss in the volute decreased after trimming under part-load conditions but increased under overload conditions, and this phenomenon made the pump head unable to be accurately predicted by empirical equations. With the help of entropy production theory, high-energy dissipation regions were mainly located in the volute discharge diffuser under overload conditions because of the flow separation and the mixing of the main flow and the stalled fluid. The increased incidence angle at the volute’s tongue after impeller trimming resulted in more serious flow separation and higher energy loss. Furthermore, the radial forces and their fluctuation amplitudes decreased under all the investigated conditions. The horizontal components of the radial forces in all cases were much higher than the vertical components.

Dynamic interaction between valve-closure water hammer wave and centrifugal pump

Water hammer is a principal cause of pipeline and equipment failure in pumping systems. The numerical method and experiments are used to investigate the dynamic interaction between the valve-closure water hammer wave and pump, to study the pressure variation and fluid-induced force on the pump components. The impeller-volute interaction and impeller position are taken into consideration. Results show that the valve-closure water hammer wave generates a substantial fluid-induced force on the pump and leads to a pressure surge in the pipeline. Meanwhile, the impeller-volute interaction also causes pressure and force variation in the pipeline and pump. The force amplitude caused by this factor in the axial direction is similar to that caused by the water hammer wave but is much smaller in the radial direction. The different interaction position of the water hammer wave on the blades can weaken the force, by 13.11% and 13.18% for the impeller and volute when the blades are under the most optimal position, respectively.

Orbit Decomposition Method for Rotordynamic Coefficients Identification of Annular Seals

The elliptical orbit whirl model is widely used to identify the frequency-dependent rotordynamic coefficients of annular seals. The existing solution technique of an elliptical orbit whirl model is the transient computational fluid dynamics (CFD) method. Its computational time is very long. For rapid computation, this paper proposes the orbit decomposition method. The elliptical whirl orbit is decomposed into the forward and backward circular whirl orbits. Under small perturbation circumstances, the fluid-induced forces of the elliptical orbit model can be obtained by the linear superposition of the fluid-induced forces arising from the two decomposed circular orbit models. Due to that the fluid-induced forces of circular orbit, the model can be calculated with the steady CFD method, and the transient computations can be replaced with steady ones when calculating the elliptical orbit whirl model. The computational time is significantly reduced. To validate the present method, its rotordynamic results are compared with those of the transient CFD method and experimental data. Comparisons show that the present method can accurately calculate the rotordynamic coefficients. Elliptical orbit parameter analysis reveals that the present method is valid when the whirl amplitude is less than 20% of seal clearance. The effect of ellipticity on rotordynamic coefficients can be ignored.

Influence of Fluid-Induced Forces on Cavitating Turbopumps Rotordynamics in Forced Whirl Experiment

As typically highlighted in the experimental results, in unshrouded axial inducers the fluid-induced rotordynamic forces are significantly dependent on the flow coefficient and do not present a quadratic behavior with respect to the whirl ratio, as commonly reported in radial pumps. Moreover these forces show destabilizing peaks that typically increase with decreasing cavitation number. The present paper illustrates the development of a theoretical model of the rotordynamic system capable of assessing the influence of fluid-induced rotordynamic forces on the dynamic response of turbopumps under cavitating/non-cavitating conditions. The implementation of the fluid-induced rotordynamic forces in the dynamic model has been proposed through the so-called F.I.R.F. model, which is applied to a test set-up for forced whirl experiments, in order to characterize the whirl natural frequencies and the relative mode shapes. The model has been successfully applied to a rotordynamic system including an unshrouded axial inducer and the corresponding computed critical speeds are summarized in the Campbell diagrams pointing out the difference between dry and wet analysis results.

Research on leakage performance and dynamic characteristics of a novel labyrinth seal with staggered helical teeth structure

A novel labyrinth seal with staggered helical teeth (SHT) structure is proposed in this paper. The influence of seal clearance, pressure drop, tooth number and eccentricity on the novel model is studied by numerical simulation. In addition, the differences of static characteristics between the labyrinth seal with SHT structure and labyrinth seal with ordinary helical teeth (OHT) structure are also compared and analyzed. The results show that the numerical model for the labyrinth teeth structure has better accuracy compared with the experimental results. The SHT structure has better leakage performance than OHT structure because the former can induce higher turbulent energy dissipation at the staggered position. Besides, the fluid-induced forces increase with the increase of eccentricity and pressure drop. The novel model and corresponding results can provide meaningful references for the research of ring seal structure.

Analysis of fluid-induced force of centrifugal pump impeller with compound whirl

In order to research the fluid-induced force of centrifugal pump impeller with compound whirling, based on the N-S equations, the RNG k-ε turbulence model is applied to simulate the low specific speed centrifugal pump under eccentric assembly condition. The effects of different flow rates, impeller eccentricities and whirl ratios on the fluid-induced force of impeller were investigated. Finally, the fitting models of fluid-induced force with compound whirl were obtained by least square method (LSM). The results imply that the calculated results considering the impeller eccentricity are closer to the experimental data than those without considering the eccentric whirl. In addition, the extremum value of fluid-induced force of impeller has a significant positive correlation with the impeller eccentricity. With the increase of flow rate, the fluid-induced force is non-monotonic. The whirl ratio can change the numbers of peak and valley of fluid-induced force, and the nonlinear fitting model of fluid-induced force using LSM presents high precision. This study can provide important references for the design and fluid-structure interaction (FSI) vibration of centrifugal pump.

Continuous ES/Feeder Cell-Sorting Device Using Dielectrophoresis and Controlled Fluid Flow

Pluripotent stem cells (PSCs) are considered as being an important cell source for regenerative medicine. The culture of PSCs usually requires a feeder cell layer or cell adhesive matrix coating such as Matrigel, laminin, and gelatin. Although a feeder-free culture using a matrix coating has been popular, the on-feeder culture is still an effective method for the fundamental study of regenerative medicine and stem cell biology. To culture PSCs on feeder cell layers, the elimination of feeder cells is required for biological or gene analysis and for cell passage. Therefore, a simple and cost-effective cell sorting technology is required. There are several commercialized cell-sorting methods, such as FACS or MACS. However, these methods require cell labeling by fluorescent dye or magnetic antibodies with complicated processes. To resolve these problems, we focused on dielectrophoresis (DEP) phenomena for cell separation because these do not require any fluorescent or magnetic dyes or antibodies. DEP imposes an electric force on living cells under a non-uniform AC electric field. The direction and magnitude of the DEP force depend on the electric property and size of the cell. Therefore, DEP is considered as a promising approach for sorting PSCs from feeder cells. In this study, we developed a simple continuous cell-sorting device using the DEP force and fluid-induced shear force. As a result, mouse embryonic stem cells (mESCs) were purified from a mixed-cell suspension containing mESCs and mouse embryonic fibroblasts (MEFs) using our DEP cell-sorting device.

Darrieus vertical-axis water turbines: deformation and force measurements on bioinspired highly flexible blade profiles

The characteristics of the fluid–structure interaction on the flexible blades of a horizontal-axis water turbine are studied; this bioinspired technology features mechanical simplicity, performance and lifetime improvement, and low fish impact risk, all characteristics of a truly sustainable renewable energy exploitation. A surface-tracking method synchronized with force measurements was applied on a surrogate model of single-bladed, vertical-axis water turbine in a water channel. This allows for the characterization of the structural deformations and their link to the hydrodynamic forces, over a large range of turbine designs and operating points. It is shown that the phase angles of the maxima in blade deformation coincide with those of the load maxima on a rigid blade in identical flow conditions. The influence of the turbine’s tip/speed ratio on the blade deformation and blade load is investigated. With the chosen blade design, hydrofoil deformation is found to be maximum in the operating points where the performance improvement is maximized (k_o=0.3). With lower values of reduced frequency (corresponding to lower rotation speed), fluid-induced forces dominate the fluid–structure interaction. Conversely, at higher frequencies, structural inertia dominates the interaction, and blade deformation is again reduced. Results suggest that the optimal blade rigidity may depend on the operating point; nevertheless the potential of the flexible-blade design is demonstrated through clear linking of fluid-induced forces and blade deformation in the complex flow conditions of the Darrieus water turbine.Graphic abstract

The Static Instability Characteristics of Labyrinth Seals: Experiments and Computational Fluid Dynamics Verification

Abstract The paper proposed an experimental measurement to identify the fluid-induced force and static stiffness coefficients. A three-dimensional fluid model of the labyrinth seals was also established. And the static characteristics under different eccentricities and inlet pressure were studied. Theoretical results show a good agreement with the experiment results. The labyrinth seal with different eccentricity ratios produces a de-centered fluid-induced force and a negative direct static stiffness, which cause the rotor deviating from the geometric center of the stator. Both the fluid-induced force and negative direct static stiffness coefficient with a value of −13.5 kN/m to −72 kN/m increase with increasing eccentricity and inlet pressure. Particularly, the direct static stiffness coefficient shows a relatively significant increase with a high eccentricity ratio (∼>40%). The static instability is mainly due to the fact that the fluid velocity in the small clearance increases more rapidly along the leakage path. The inertial force is increased significantly and the pressure decreases. This results in a greater pressure distribution in the larger clearance. The fluid force and direct static stiffness coefficient that tend to push the rotor away from the stator center are produced, and eventually lead to the static instability of the labyrinth seal.

Numerical Modeling of Static and Rotordynamic Characteristics for Three Types of Helically Grooved Liquid Annular Seals

Abstract This paper deals with numerical predictions of the leakage flowrates, drag power, and rotordynamic force coefficients for three types of helically grooved liquid annular seals, which include a liquid annular seal with helically grooved stator (GS/SR seal), one with helically grooved rotor (SS/GR seal), and one with helical grooves on stator and rotor (GS/GR seal). A novel transient computational fluid dynamics (CFD)-based perturbation method was proposed for the predictions of the leakage flowrates, drag power, and rotordynamic force coefficients of helically grooved liquid annular seals. This method is based on the unsteady Reynolds-averaged Navier–Stokes (RANS) solution with the mesh-deformation technique and the multiple reference frame theory. The time-varying fluid-induced forces acting on the rotor/stator surface were obtained as a response to the time-dependent perturbation of the seal stator surface with the periodic motion, based on the multiple-frequency elliptical-orbit stator whirling model. The frequency-independent rotordynamic force coefficients were determined using curve fit and fast Fourier transform (FFT) in the frequency domain. The CFD-based method was adequately validated by comparisons with the published experiment data of leakage flowrates and fluid response forces for three types of helically grooved liquid annular seals. Based on the transient CFD-based perturbation method, numerical results of the leakage flowrates, drag powers, and rotordynamic force coefficients were presented and compared for three types of helically grooved liquid annular seals at five rotational speeds (n = 0.5 krpm, 1.0 krpm, 2.0 krpm, 3.0 krpm, and 4.0 krpm), paying special attention to the effective stiffness coefficient and effective damping coefficient.

Effects of Four Types of Pre-swirls on the Leakage, Flow Field, and Fluid-Induced Force of the Rotary Straight-through Labyrinth Gas Seal

The labyrinth seal in turbomachinery is a key element that restricts leakage flow among rotor-stator clearances from high-pressure regions to low-pressure regions. The fluid-induced forces on the rotor from seals during machine operation must be accurately quantified to predict their dynamic behavior effectively. To understand the fluid-induced force characteristics of the labyrinth seal more fully, the effects of four types of pre-swirls on the leakage, flow field, and fluid-induced force of a rotary straight-through labyrinth gas seal (RSTLGS) were numerically investigated using the proposed steady computational fluid dynamics (CFD) method based on the three-dimensional models of the RSTLGS. The leakage, flow field, and fluid-induced force of the RSTLGS for six axial pre-swirl velocities, four radial pre-swirl angles, four circumferential positive pre-swirl angles, and four circumferential negative pre-swirl angles were computed under the same geometrical parameters and operational conditions. Mesh analysis ensures the accuracy of the present steady CFD method. The numerical results show that the four types of pre-swirls influence the leakage, flow field, and fluid-induced force of the RSTLGS. The axial pre-swirl velocity remarkably inhibits the fluid-induced force, and the circumferential positive pre-swirl angle and circumferential negative pre-swirl angle remarkably promote the fluid-induced force. The effects of the radial pre-swirl angle on the fluid-induced force are complicated, and the pressure forces and viscous forces show the maximum or minimum values at a specific radial pre-swirl angle. The pre-swirl has a negligible impact on the leakage. The four types of pre-swirls affect the leakage, flow field, and fluid-induced force of the RSTLGS to varying degrees. The pre-swirl is the influence factor affecting the leakage, flow field, and fluid-induced force of the RSTLGS. The conclusions will help to understand the fluid-induced force of labyrinth seals more fully, by providing helpful suggestions for engineering practices and a theoretical basis to analyze the fluid–structure interaction of the seal-rotor system in future research.

Numerical and Experimental Research on the Fluid-Induced Forces of Clearance Flow in Canned Motor Reactor Coolant Pump

Reactor coolant pump (RCP) is one of the most important equipment of the coolant loop in a pressurized water reactor system. Its safety relies on the characteristics of the rotordynamic system. For a canned motor RCP, the liquid coolant fills up the clearance between the metal shields of the rotor and stator inside the canned motor, forming a long clearance flow. The fluid-induced forces of the clearance flow in canned motor RCP and their effects on the rotordynamic characteristics of the pump are numerically and experimentally analyzed in this work. A transient computational fluid dynamics (CFD) method has been used to investigate the fluid-induced force of the clearance. A vertical experiment rig has also been established for the purpose of measuring the fluid-induced forces. Fluid-induced forces of clearance flow with various whirl frequencies and various boundary conditions are obtained through the CFD method and the experiment. Results show that clearance flow brings large mass coefficient into the rotordynamic system and the direct stiffness coefficient is negative under the normal operating condition. The rotordynamic stability of canned motor RCP does not deteriorate despite the existence of significant cross-coupled stiffness coefficient from the fluid-induced forces of the clearance flow.

Discrimination of normal skin fibroblast and malignant melanocytes using dielectrophoretic force and fluid-induced shear force

Cell analysis is an important technology that is widely used for medical diagnosis in hospitals and cell engineering research. Among cell analysis technology, dielectrophoresis (DEP) is one of the most promising approaches for discriminating biological particles because this phenomena requires no labeling procedure with a fluorescent dye or magnetic beads. In this study, we developed a precise cell analysis system by evaluating the DEP force with a liquid flow system. The DEP forces acting on a cell was characterized using a microfluidic chamber containing an electrode-array and fluid-induced shear forces. On the basis of this characterization, the discrimination between healthy skin cells and cancer cells was performed using our novel DEP cell sorting system. As a result, the healthy cells and cancer cells could be distinguished even if the dielectrophoretic properties of those cells were similar.