Added Mass Effect Research Articles

Modal characteristics analysis of a variable-speed pump-turbine rotor system: a perspective of resonance

Abstract The safety and stability of the rotor system is crucial to the pump-turbine unit. The present paper analyzed the modal characteristics of a variable-speed pump-turbine rotor system based on the rotor dynamics theory and FEM method. The results show that the bending mode of the rotor system exhibits the forms of forward whirling (FW) and backward whirling (BW). The added mass effect will reduce the natural frequency of the rotor system, and it is more obvious for the torsional mode Torsional 1. In addition, the critical speed analysis shows that both the critical speed of the dry mode and wet mode of the rotor system is greater than 700rpm, which proves the safety of the unit. The results of the analysis can be used as a reference for the optimized design of similar variable-speed pump-turbine rotor system, which is conducive to the improvement of the reliability and life of the rotor system.

3D dynamics and control of a snake robot in uncertain underwater environment

Abstract The snake robot can be used to monitor and maintain underwater structures and environments. The motion of a snake robot is achieved by lateral undulation which is called the gait pattern of the snake robot. The parameters of a gait pattern need to be adjusted for compensating environmental uncertainties. In this work, 3D motion dynamics of a snake robot for the underwater environment is proposed with vertical motion using the buoyancy variation technique and horizontal motion using lateral undulation. “The neutral buoyant snake robot motion in hypothetical plane and added mass effect is negligible”, these previous assumptions are removed in this work. Two different control algorithms are designed for horizontal and vertical motions. The existing super twisting sliding mode control (STSMC) is used for the horizontal serpentine motion of the snake robot. The control law is designed on a reduced-ordered dynamic system based on virtual holonomic constraints. The vertical motion is achieved by controlling the mass variation using a pump. The water pumps are controlled using the event-based controller or Proportional Derivative (PD) controller. The results of the proposed control technique are verified with various external environmental disturbances and uncertainties to check the robustness of the control approach for various path following cases. Moreover, the results of STSMC scheme are compared with SMC scheme to check the effectiveness of STSMC. The practical implementation of the work is also performed using Simscape Multibody environment where the designed control algorithm is deployed on the virtual snake robot.

Dynamics, stability and bifurcations of a planar three-link swimmer with passive visco-elastic joint using “ideal fluid” model

Articulated swimming robots have a promising potential for various marine applications. A common theoretical model assumes ideal fluid, where the viscosity is negligible and the swimmer–fluid interaction is induced by reactive forces originating from added mass effect. Some previous works used this model to study planar multi-link swimmers under kinematic input prescribing all joint angles. Inspired by biological swimmers in nature that utilize body flexibility, in this work we consider an underactuated three-link swimmer where one joint is periodically actuated while the other joint is passive and viscoelastic. Analysis of the swimmer’s nonlinear dynamics reveals that its motion depends significantly on the amplitude and frequency of the actuated joint angle. Optimal frequency is found where the swimmer’s net displacement per cycle is maximized, under symmetric periodic oscillations of the passive joint. In addition, upon crossing critical values of amplitude or frequency, the system undergoes a bifurcation where the symmetric periodic solution loses stability and asymmetric solutions evolve, for which the swimmer moves along an arc. We analyze these phenomena using numerical simulations and analytical methods of perturbation expansion, harmonic balance, Floquet theory, and Hill’s determinant. The results demonstrate the important role of parametric excitation in stability and bifurcations of motion for flexible underactuated locomotion.

On the unsteady aerodynamics of flapping wings under dynamic hovering kinematics

Hummingbirds and insects achieve outstanding flight performance by adapting their flapping motion to the flight requirements. Their wing kinematics can change from smooth flapping to highly dynamic waveforms, generating unsteady aerodynamic phenomena such as leading-edge vortices (LEV), rotational circulation, wing wake capture, and added mass. This article uncovers the interactions of these mechanisms in the case of a rigid semi-elliptical wing undergoing aggressive kinematics in the hovering regime at Re∼O(103). The flapping kinematics were parametrized using smoothed steps and triangular functions and the flow dynamics were simulated by combining the overset method with large eddy simulations. The analysis of the results identifies an initial acceleration phase and a cruising phase. During the former, the flow is mostly irrotational and governed by the added mass effect. The added mass was shown to be responsible for a lift first peak due to the strong flapping acceleration. The dynamic pitching and the wing wake interaction generate a second lift peak due to a downwash flow and a vortex system on the proximal and distal parts of the wing's pressure side. Conversely, aerodynamic forces in the cruising phase are mainly governed by the growth and the establishment of the LEV. Finally, the leading flow structures in each phase and their impact on the aerodynamic forces were isolated using the extended proper orthogonal decomposition.

Effects of cross flow on the launching process of submarine-launched vehicle considering the 6-DOF motion at barrel-exit stage

Submarine-launched vehicle (SLV) is a research hotspot due to its strong strike capability, among which underwater launching technology is a key issue hindering its development. In this paper, a numerical model is proposed to simulate the launching problem of SLV. The high nonlinearity induced by the coupling of multi-degree-of-freedom motions and the interaction between fluid and structure is solved. Specifically, the 6-DOF motion model of structure considering its strong added-mass effect, cavitation model and collision model are adopted. This numerical model is firstly validated by simulating the launching of SLV in still water. And then, the applicability of this model is discussed, as well as the effect of cross flow and initial launching velocity on the launching process. The results indicate that proposed numerical model could be used under both low-speed and high-speed cross flow situations. Cross flow changes the attitude and locomotion state of SLV. When flow velocity exceeds 1.25 m/s, the collision occurs between SLV (with the initial launching velocity of 40 m/s) and its barrel. This obtained locomotion of SLV after leaving its barrel causes the launching failure. Besides, a method of increasing launching velocity is introduced to promote the success rate of launching in high-speed cross flow situation.

Experimental and numerical investigation on the hydroelastic response of barge and KVLCC2 ship

In this paper, the hydroelasticity of two segmented ship models (Barge and KVLCC2) is investigated by experimental and numerical methods. A two-way Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) coupled method is adopted and further validated its accuracy against experimental results. The current approach emphasizes the overall convergence between two solvers, maintaining a strongly coupled manner to comprehensively address the fluid-structure interaction phenomenon, including the added mass effect. A series of experiments of a segmented Barge and KVLCC2 under various wave conditions were firstly conducted. The motions of the models are measured and the displacement Response Amplitude Operators (RAOs) are calculated. The CFD-FEA coupled method is demonstrated in agreement with experimental results by comparing the numerical prediction with the tank test results in terms of motion, affirming the validity and robustness. Finally, the hydroelastic response is discussed and investigated.

Unraveling the nature of sensing in electrostatic MEMS gas sensors

This paper investigates the fundamental sensing mechanism of electrostatic MEMS gas sensors. It compares among the responsivities of a set of MEMS isopropanol sensors before and after functionalization, and in the presence and absence of electrostatic fields when operated in static and dynamic detection modes. In the static mode, we found that the sensors do not exhibit a measurable change in displacement due to added mass. On the other hand, bare sensors showed a clear change in displacement in response to isopropanol vapor. In the dynamic mode, functionalized sensors showed a measurable frequency shift due to the added mass of isopropanol vapor. In the presence of strong electrostatic fields, the measured frequency shift was found to be threefold larger than that in their absence in response to the same concentration of isopropanol vapor. The enhanced responsivity of dynamic detection allows the sensors to measure the vapor mass captured by the functional material, which is not the case for static detection. The detection of isopropanol by bare sensors in static mode shows that change in the medium permittivity is the primary sensing mechanism. The enhanced responsivity of dynamic mode sensors when operated in strong electrostatic fields shows that their sensing mechanism is a combination of a weaker added mass effect and a stronger permittivity effect. These findings show that electrostatic MEMS gas sensors are independent of the direction of the gravitational field and are, thus, robust to changes in alignment. It is erroneous to refer to them as ‘gravimetric’ sensors.

Validation of the added mass effect in the narrow gaps of nuclear reactor pressure vessels

Nuclear reactor pressure vessels (RPVs) must be robust to external environmental excitations such as seismic loads. Precise numerical models that predict the dynamic responses to external loads are crucial for evaluating the safety of RPVs. Notably, in RPVs submerged in reactor coolant, it is necessary to examine the added mass effect of the fluid via fluid-structure interaction (FSI) analysis, employing either an analytical approach or a numerical analysis. The present study focuses on modeling and analyzing the added mass effect in the narrow gaps associated with the dense structural layout in the reactor. In this context, this paper validates analytical and numerical models that accurately describe the added mass effect for two cases: (i) simple concentric cylinders with a narrow gap filled with fluid and (ii) a complex small-scale RPV with narrow gaps also filled with fluid. Initially, the analytical model, employing Lagrange's equation of motion and velocity potential, and the numerical model, utilizing FE modeling in acoustic FSI through ANSYS, were validated through experimental modal testing for the added mass effect in the narrow gap between simple concentric cylinders. The analytical model for the concentric cylinders indicates that the added mass diverges to infinity as the gap approaches zero, which should be examined carefully. Through experiments for the concentric cylinders, the analytical and FEM-based added mass effects were validated with a natural frequency error of 3 %. Subsequently, based on the results for the concentric cylinders, a complex numerical model of the small-scale reactor with complex boundary conditions was developed and cross-validated through corresponding experimental modal testing of the small-scale RPV. The added mass model for the narrow gaps in the RPV was validated by substantial agreement of eight representative modes, as demonstrated by a 5 % natural frequency error.

Effect of skull morphology on fox snow diving

Certain fox species plunge-dive into snow to catch prey (e.g., rodents), a hunting mechanism called mousing. Red and arctic foxes can dive into snow at speeds ranging between 2 and 4 m/s. Such mousing behavior is facilitated by a slim, narrow facial structure. Here, we investigate how foxes dive into snow efficiently by studying the role of skull morphology on impact forces it experiences. In this study, we reproduce the mousing behavior in the lab using three-dimensional (3D) printed fox skulls dropped into fresh snow to quantify the dynamic force of impact. Impact force into snow is modeled using hydrodynamic added mass during the initial impact phase. This approach is based on two key facts: the added mass effect in granular media at high Reynolds numbers and the characteristics of snow as a granular medium. Our results show that the curvature of the snout plays a critical role in determining the impact force, with an inverse relationship. A sharper skull leads to a lower average impact force, which allows foxes to dive head-first into the snow with minimal tissue damage.

Undulatory Propulsion at Milliscale on Water Surface.

The oscillatory pitch motion at the leading edge of a millimeter-scale flexible sheet on the water surface can generate undulatory locomotion for swimming, similar to a honeybee vibrating its wings for propulsion. The influence of various parameters on such swimming strategy remains unexplored. This study uses magnetic milliswimmers to probe the propulsion mechanics and impact of different parameters. It is found that this undulatory propulsion is driven by capillary forces and added mass effects related to undulatory waves of the milliswimmers, along with radiation stress stemming from capillary waves at the interface. Modifying the parameters such as actuation frequency, pitch amplitude, bending stiffness, and hydrofoil length alters the body waveform, thus, affecting the propulsion speed and energy efficiency. Although undulatory motion is not a prerequisite for water surface propulsion, optimizing body stiffness to achieve a proper undulatory waveform is crucial for efficient swimming, balancing energy consumption, and speed. The study also reveals that the induced water flow is confined near the water surface, and the flow structures evolve with varying factors. These discoveries advance the understanding of undulatory water surface propulsion and have implications for the optimal design of small-scale swimming soft robots in the future.

Numerical theory and method on the modal behavior of a pump-turbine rotor system considering gyro-effect and added mass effect

The pump-turbine unit is the core equipment of pumped storage power station, and the rotor system is the key component of the pump-turbine unit. In this paper, the numerical method considering the gyro-effect and added mass effect is proposed, the effect of the added mass, gyro-effect and bearing stiffness are discussed, respectively, and a “transfer” phenomenon of the mode shape is found. A coupling influence coefficient is proposed to analyze the coupling influence of gyro-effect and added mass effect on the rotor system mode, and the nodal diameter (ND) mode of runner is discussed. For the forward whirling ND mode, the coupling influence coefficient increases with the increase of the rotational speed, while for the reverse whirling ND mode, the coupling influence coefficient decreases with the increase of the rotational speed. The results in this paper can provide a theoretical basis for revealing the complex dynamic behaviors of the rotor system, and provide scientific guidance for the research and development, design, and safe and stable operation of pump-turbine units.

Hydrodynamic benefits of passive flexibility in a self-propelled oscillatory ray

In the present study, numerical simulations are conducted to investigate the hydrodynamic benefits of a self-propelled oscillatory ray with passive flexibility compared to those with active flexibility. For the active flexibility case, the prescribed motion is applied to the entire surface of the ray. On the other hand, for the passive flexibility case, the motion of the leading edges is only prescribed, whereas that of the rest parts is determined by the interaction with the surrounding fluid. The cruising speed and input power of the ray with passive flexibility increase as the horizontal bending rigidity decreases, and its propulsive efficiency is maximized at a specific horizontal bending rigidity. Compared to the active flexibility case, the propulsive performance of the oscillatory ray with passive flexibility is improved by not only enhanced circulation and added-mass effects but also by the favorable repartition of the resultant force caused by a large deflection angle. Strong vortical structures induced by a large deformation over the entire region of the fin generate strong negative pressure on the forward side of the overall surface, even near the central body (i.e., increased circulation effect). Furthermore, the positive pressure on the backward side increases in the passive flexibility case due to high fin acceleration caused by more intense oscillating motions (i.e., increased added-mass effect). When the oscillating frequency and the Reynolds number vary, the performance of the ray with passive flexibility is confirmed to be higher than that with active flexibility.

A fully coupled regularized mortar‐type finite element approach for embedding one‐dimensional fibers into three‐dimensional fluid flow

SummaryThe present article proposes a partitioned Dirichlet‐Neumann algorithm, that allows to address unique challenges arising from a novel mixed‐dimensional coupling of very slender fibers embedded in fluid flow using a regularized mortar‐type finite element discretization. The fibers are modeled via one‐dimensional (1D) partial differential equations based on geometrically exact nonlinear beam theory, while the flow is described by the three‐dimensional (3D) incompressible Navier‐Stokes equations. The arising truly mixed‐dimensional 1D‐3D coupling scheme constitutes a novel approximate model and numerical strategy, that naturally necessitates specifically tailored solution schemes to ensure an accurate and efficient computational treatment. In particular, we present a strongly coupled partitioned solution algorithm based on a Quasi‐Newton method for applications involving fibers with high slenderness ratios that usually present a challenge with regard to the well‐known added mass effect. The influence of all employed algorithmic and numerical parameters, namely the applied acceleration technique, the employed constraint regularization parameter as well as shape functions, on efficiency and results of the solution procedure is studied through appropriate examples. Finally, the convergence of the two‐way coupled mixed‐dimensional problem solution under uniform mesh refinement is demonstrated, a comparison to a 3D reference solution is performed, and the method's capabilities in capturing flow phenomena at large geometric scale separation is illustrated by the example of a submersed vegetation canopy.

Experimental analysis of liquid vertical slosh damping at vacuum and atmospheric pressures

Experimental data are presented for a cantilevered vertically vibrating beam supporting a tank partially filled with liquid, inside a vacuum chamber where the air pressure can be reduced. Results are presented with and without the tank and contained liquid, as well as under two different gas pressures (atmospheric and vacuum). When the liquid is absent from the tank, aerodynamic damping and added mass effects are quantified. When the tank is partially filled with liquid, the damping versus tank amplitude curves indicate differences that are mainly due to aerodynamic effects, with more noticeable effects in the 50% fill case. The results support the observation that at the density ratios presented here, two-phase liquid/ gas modelling may not be needed for the evaluation of net damping due to violent sloshing flows.

Development of a Simulation Model for Digital Twin of an Oscillating Water Column Wave Power Generator Structure with Ocean Environmental Effect

This research article focuses on developing a baseline digital twin model for a wave power generator structure located in Yongsu-ri, Jeju-do, South Korea. First, this study performs a cause analysis on the discrepancy of the dynamic properties from the real structure and an existing simulation model and finds the necessity of modeling the non-structural masses and the environmental factors. The large amounts of the ballast are modeled in the finite element model to enhance the accuracy of the digital twin. Considering the influence of environmental factors such as tide level and wave direction, the added mass effect of structural members, one of the hydrodynamic effects, depending on the change of the ocean environments is calculated based on the rule of Det Norske Veritas and applied. The results indicate that non-structural mass components significantly impact the dynamic characteristics of the structure. Additionally, environmental factors have a greater effect on the dynamic behavior of the box-type structure compared to lightweight offshore structures.

Dynamic response of composite materials subjected to low velocity impacts in water: A numerical study

The damage tolerant design of marine composite structures for impacts is dependent on empirical data to ensure continuing seaworthiness. Impact studies on composite laminates are typically performed according to ASTM D7136 which was originally developed by the aerospace industry. The marine operating environment, however, is vastly different. Impact characterisation and damage prediction of underwater composite structures is hindered by the limited understanding on the effects of fluid-structure interaction of a surrounding water medium with the composite structure and impactor. A coupled Eulerian-Lagrangian (CEL) numerical model is presented in this work with the aim of advancing fundamental understanding of the effects of a fluid on the transient dynamic response of composite plates subjected to foreign body impact. The added mass effect of the water increased the peak impact force while reducing the free vibration frequency and peak displacement. The virtual added mass factor also reduced significantly with increasing laminate thickness. This work will pave the way in improving the damage tolerant design of underwater composite structures as will computationally efficient methods of simulating such events.

Experimental study on the influence of vibration amplitude on the fluid damping of a submerged disk

Accurate determination of modal parameters for disk-like structures submerged in heavy fluids is a topic of interest in many fields. For instance, some type of hydraulic turbomachinery can be modeled as disk structures due to the similarity in their mode shapes. Although in the recent years many studies have analyzed and correctly predicted the fluid added mass effect on the natural frequencies and mode shapes of these types of structures, it is particularly challenging to determine fluid damping and characterize its most influential effects. In this article, the influence of vibration amplitude on the fluid damping ratio of a submerged disk, when the disk gets close to a rigid wall, is analytically and experimentally studied. Particularly, the first diametrical modes of the disk have been analyzed at different vibration amplitudes and at very small distances away from a rigid surface. The disk is put in a resonant state by means of a piezoelectric patch generating a harmonic excitation and the response is measured by means of a Laser Doppler Vibrometer (LDV). The damping ratio is obtained directly from the vibrating free decay following a sudden stop of the excitation. It is shown that the fluid damping ratio values are related to the vibration amplitude: there is a trend of increasing damping ratio as the amplitude of the vibration increases. This effect is found to be much larger for small distances between the disk and the rigid surface.

Experimental and numerical studies on hydrodynamic effects of ice floe drift and impact process in uniform current

The safety of offshore structures is often threatened by the impact load of isolated ice floes. However, the accurate estimation of the impact load can be challenging due to the unquantified near-field hydrodynamic interaction. In this paper, the hydrodynamic interactions between ice floes and offshore structures are investigated by a model experiment and the numerical calculation. The model experiment is conducted to study the whole process of a model ice drifting and impacting a cylinder under the influence of uniform flow. Meanwhile, the interaction process is also numerically simulated by using the Computational Fluid Dynamics (CFD)-Finite Element Method (FEM) coupling method. The experimental and numerical results show good agreement in terms of velocity time histories, peak impact forces and peak accelerations. The influences of current velocity, ice floe size, and ice floe shape on various interactions are discussed based on numerical results, including the velocity attenuation effect before impact, the added mass effect during collision, the kinetic energy and motion attitude of the ice floe. The main findings of present research suggest that the viscous effect in near-field and the bow wave are noteworthy factors causing the velocity attenuation of the ice floe and ignoring these influences can lead to underestimation of the velocity attenuation effect to some extent. Additionally, the (effective) added mass coefficients calculated based on the ice floe motion equation and the momentum balance equation are found to be close to each other. However, with the decreasing of the ice floe size, the difference between the added mass coefficients calculated by the two methods gradually increases.

Analysis of the immersed boundary method for turbulent fluid-structure interaction with Lattice Boltzmann method

An efficient and new methodology to deal with fluid structure interaction at high Reynolds number flows is presented in this article. It relies on the coupling of the lattice Boltzmann method and the immersed boundary method with a second order predictor corrector model for the structure. The effect of the lagrangian weight of the immersed boundary method is also analyzed in the context of fluid structure interaction. Both curved and moving boundaries are considered, and several methods to calculate the lagrangian weight are compared on relevant test-cases: the laminar flow around a cylinder at Reynolds number Re=100, the Poiseuille flow in a 2D channel and a 3D fluid-structure interaction test case: a deformable flapping flag immersed in a laminar flow. A convergence study in space and time is performed and the three following parameters are investigated: the number of lagrangian markers along the boundary, the value of the lagrangian weight and the shape of the discrete delta function used in the immersed boundary. Finally, the novelty of the paper is two-fold: a new expression of the lagrangian weight which is found to reduce the error by 20% in the case of fluid-structure interaction, and the coupling of a turbulence model with a second order predictor corrector model for improved stability and accuracy. This numerical methodology is found here to be accurate for challenging cases with high added mass effect and high Reynolds number flows.

Effect of wing planform on plunging wing aerodynamics

The unsteady aerodynamics of three plunging wings with different planform shapes was investigated in water tunnel experiments by means of force/moment and three-dimensional volumetric velocity measurements at a post-stall angle of attack. In contrast with a rectangular wing, a tapered wing does not exhibit a local minimum of the time-averaged lift coefficient at a reduced frequency of k≈ 1.7. This is due to the oblique shedding of trailing-edge vortices, which prevents the coupling with leading-edge vortices. The wing with a round-tip exhibits the same double maxima that correspond to the subharmonic and the fundamental frequency of vortex shedding in steady freestream. However, combined leading-edge and tip vortex produce larger time-averaged lift force with increasing reduced frequency. The variation of the bending moment is similar to that of the lift force in general. However, the mean bending moment arm shifts outboard with increasing reduced frequency due to the increased three-dimensionality of the vortex filaments and the tip-vortex effect. Maximum lift and bending moment become more similar for all three wings at higher reduced frequencies as the added mass effect becomes more significant.