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This paper presents a numerical simulation utilizing the Smoothed Particle Hydrodynamics (SPH) methodology to analyze the impact of water flow on bridge superstructures. The focus of the study is the Canh Nang bridge, which experienced significant damage during a severe flood in Vietnam. The SPH model accounts for flow morphology, velocity fields, and flow pressure around the submerged superstructure, providing insights into areas of high flow pressure and the water resistance coefficient (Cd). By employing modified dynamic boundaries for solid surfaces and the inflow-outflow conditions, the model effectively addresses fluid-bed and fluid-structure interactions. The results highlight elevated flow pressure on specific surface locations of the superstructure, while lower pressures are observed on the bottom surfaces and between adjacent girders. The calculated Cd values are evaluated against those from various bridge design standards, including the Indian code, Eurocode, AS5100, and TCVN 11823. This comparison reveals discrepancies and suggests the potential for refining current design practices. Future research directions include the experimental validation of SPH model results and the exploration of how structural parameters influence superstructure response during flood events. [ABSTRACT FROM AUTHOR]

The subject of this work is the development of a nonlinear model of the interaction of an unsteady fluid flow with a structure and finding analytical solutions for the system of equations that correspond to the specified model. The convection effect of the fluid velocity field was already considered in the previous works of the authors of this paper. These studies are devoted to the water hammer without considering the "flow-structure" interaction. This work expands the possibilities of modeling and considers four equations instead of two equations of the theory of the water hammer (equations of conservation of mass and momentum), two of which relate to the motion of particles of a solid body (pipes or structures). The novelty of this work lies in the consideration of the model that describes the interaction of the flow with the structure, the convection in the velocity field, and the effect, together with the friction of the fluid against the solid wall, on the dynamics of the shock pulse propagation process both in the fluid and in the solid body. It should be noted that the solution of the nonlinear system of differential equations as a whole is carried out by an analytical method, which makes it possible to obtain an exact (rather than numerical) solution of the problem. Since the effects of various factors should be evaluated by comparison with the main components, dimensionless equations containing six parameters (dimensionless combinations) were obtained in this study. Two of these parameters were named after scientists – Darcy and Weisbach (steady friction) and Bruno (unsteady friction). Particular cases of the general (full) model were considered, and the effects of various factors on the dynamics of the interaction of the flow with the structure during the propagation of the shock pulse were determined. Research methods are purely theoretical. The concepts of a self-similar equation and a system of equations, balances of forces acting on particles of a fluid and a solid body, and a standard method of reducing a system of equations to a single equivalent equation are used. Conclusions. An extended model of the interaction between the unsteady fluid flow and the structure is proposed. The transition to a self-similar variable makes it possible to solve a nonlinear system of differential equations and obtain an analytical (exact) solution. The functions of longitudinal stress in a solid body, pressure disturbance, and velocity of motion of particles in a solid body (pipe) are linearly expressed by the velocity of shock pulse propagation in the fluid. It should also be noted that the results for the particular case of the linear model completely agree with the already known ones. The advantage of using a self-similar solution is that it is easy to obtain. The results of previous studies on the water hammer problem were qualitatively consistent. As the fluid viscosity increases, the shock pulse domain becomes more concentrative.

The recognizable shapes of landforms arise from processes such as erosion by wind or water currents. However, explaining the physical origin of natural structures is challenging due to the coupled evolution of complex flow fields and three-dimensional (3D) topographies. We investigate these issues in a laboratory setting inspired by yardangs, which are raised, elongate formations whose characteristic shape suggests erosion of heterogeneous material by directional flows. We combine experiments and simulations to test an origin hypothesis involving a harder or less erodible inclusion embedded in an outcropping of softer material. Optical scans of clay objects fixed within flowing water reveal a transformation from a featureless mound to a yardang-like form resembling a lion in repose. Phase-field simulations reproduce similar shape dynamics and show their dependence on the erodibility contrast and flow strength. Through visualizations of the flow fields and analysis of the local erosion rate, we identify effects associated with flow funneling and the turbulent wake that are responsible for carving the unique geometrical features. This highly 3D scouring process produces complex shapes from simple and commonplace starting conditions and is thus a candidate explanation for natural yardangs. The methods introduced here should be generally useful for geomorphological problems and especially those for which material heterogeneity is a primary factor. [ABSTRACT FROM AUTHOR]

In many catchments prone to debris flows, prevention structures such as check dams and retention basins have been installed to prevent debris flows from impacting the nearby infrastructures. The SPH model adopting the Herschel–Bulkley–Papanastasiou (HBP) constitutive law has shown good potential in modeling the interaction between debris flow and prevention structures. However, the accuracy of this model is not fully satisfactory when modeling the deposition process of debris flow, because the original HBP law is a viscoplastic model which does not consider frictional dissipation. Therefore, in this paper, we proposed a novel SPH model for analyzing the interaction between debris flow and prevention structures, by incorporating a modified HBP law with frictional dissipation into the original SPH model. The proposed model is validated by column collapse and flume benchmark experiments first and then utilized to analyze a real debris flow and its interaction with the prevention structures in the Cancia catchment in northern Italian Alps. The results of the column collapse experiment show that our model exhibits a better performance in simulating the collapse process compared with the original SPH model, and the simulation results of the sand flume test illustrate that the proposed model can accurately predict the impact force of debris flow on the prevention structure. The simulation results of the Cancia debris flow demonstrate that the check dams can dramatically diminish the discharge and the frontal flow velocity of the debris flow, and the peak impact force of debris flow generally decreases with gentler channel slope. Furthermore, various prevention structures show different interaction mechanisms with debris flows: the flat deposition platform mainly dissipates the kinetic energy of the flow, the check dam mainly reduces the peak discharge of the debris flow and intercepts the debris mass, and the retention basin at the outlet contributes to the deposition of debris flow. The proposed novel SPH model is helpful for guiding the optimization design of multiple prevention structures in debris flow gullies. [ABSTRACT FROM AUTHOR]

While the conus elasticus is generally considered a part of continuation of the vocal ligament, histological studies have revealed different fiber orientations that fibers are primarily aligned in the superior-inferior direction in the conus elasticus and in the anterior-posterior direction in the vocal ligament. In this work, two continuum vocal fold models are constructed with two different fiber orientations in the conus elasticus: the superior-inferior direction and the anterior-posterior direction. Flow-structure interaction simulations are conducted at different subglottal pressures to investigate the effects of fiber orientation in the conus elasticus on vocal fold vibrations, aerodynamic and acoustic measures of voice production. The results reveal that including the realistic fiber orientation (superior-inferior) in the conus elasticus yields smaller stiffness and larger deflection in the coronal plane at the junction of the conus elasticus and ligament and subsequently leads to a greater vibration amplitude and larger mucosal wave amplitude of the vocal fold. The smaller coronal-plane stiffness also causes a larger peak flow rate and higher skewing quotient. Furthermore, the voice generated by the vocal fold model with a realistic conus elasticus has a lower fundamental frequency, smaller first harmonic amplitude, and smaller spectral slope. [ABSTRACT FROM AUTHOR]

In this study, we consider the Oseen structure of the linearization of a compressible fluid–structure interaction (FSI) system for which the interaction interface is under the effect of material derivative term. The flow linearization is taken with respect to an arbitrary, variable ambient vector field. This process produces extra "convective derivative" and "material derivative" terms, which render the coupled system highly nondissipative. We show first a new well‐posedness result for the full incorporation of both Oseen terms, which provides a uniformly bounded semigroup via dissipativity and perturbation arguments. In addition, we analyze the long time dynamics in the sense of asymptotic (strong) stability in an invariant subspace (one‐dimensional less) of the entire state space, where the continuous semigroup is uniformly bounded. For this, we appeal to the pointwise resolvent condition introduced in Chill and Tomilov [Stability of operator semigroups: ideas and results, perspectives in operator theory Banach center publications, 75 (2007), Institute of Mathematics Polish Academy of Sciences, Warszawa, 71–109], which avoids an immensely technical and challenging spectral analysis and provides a short and relatively easy‐to‐follow proof. [ABSTRACT FROM AUTHOR]

The Minbaklu Bridge, an essential segment of the southern cross-island expressway in Taiwan, was destroyed owing to debris flow in the Yusui Stream. To elucidate the development of debris flow in the Yusui Stream and the bridge collapse event, this study proposed a hybrid model of the discrete element method and depth-averaged model to simulate the event through three stages: formation of the natural dam, debris flow triggered by dam braking, and bridge collapse owing to the debris flow. The simulation results obtained indicate that the proposed model can provide reasonable simulations of the formation of natural dam in both time line and spatial distribution. Further, it indicated that the erosion developed at the river bed of the flow region, and illustrated the failure process of the Minbaklu Bridge, which revealed that it was uplifted before being pushed away. Furthermore, the final deposition of the alluvial fan deposition was demonstrated as well. Overall, the proposed hybrid model provides a useful tool for investigating debris flow and interaction with engineering structures. [ABSTRACT FROM AUTHOR]

Seals can detect prey up to 180 m away using only their flow‐sensing whiskers. The unique undulating morphology of Phocid seal whiskers reduces vortex‐induced vibrations (VIVs), rendering seals highly sensitive to biologically relevant flow stimuli. In this work, digital models of harbor and grey seal whiskers are extracted using 3D scanning and a mathematical framework that accurately recreates their undulating geometry is proposed. Through fluid–structure interaction studies and experimental investigations involving a whisker array mounted on 3D‐printed microelectromechanical systems sensors, the vibration characteristics of the whisker array and the interaction between neighboring whiskers in steady flows and fish‐wake‐like vortices are explained for the first time. Results reveal that the downstream vortices intensity and resulting VIVs are consistently lower for grey than harbor seal whiskers and a smooth cylinder, suggesting that the grey seal whisker geometry can be an ideal template for the biomimetic design of VIV‐resistant underwater structures. In addition, neighboring whiskers in an array influence one another by resulting in greater flow vorticity fluctuation and distribution area, thus causing increased vibrations than an isolated whisker, which indicates the possibility of a signal‐strengthening effect in whisker arrays. [ABSTRACT FROM AUTHOR]

A 3D numerical model is developed to investigate the flow motion and sediment transport around a spur dike. In this model, fluid motion is described by the Navier–Stokes equations, adopting large eddy simulation to capture turbulent transport and dissipation. The spur dike and sand bed are treated by the porous media method. The suspended load concentration and the bed load transport rate is calculated separately, and then the bed variation is updated using the mass-balance equation. A series of flume experiments are employed to validate the model's performance before being applied for the case of partially emergent spur dikes and submerged spur dikes, respectively. Detailed analyses on the spatial-temporal variation of flow intensity, sediment concentration and shapes of scour holes are made, based on which some innovative findings are discussed such as the scouring process patterns, as well as the influence of flow conditions on the maximum scour depth and location, and then useful engineering suggestions are provided to improve structural safety. [ABSTRACT FROM AUTHOR]

Arrays of baffles are widely used to prevent and mitigate granular flows (e.g., debris flows and landslides) in mountainous areas. A thorough understanding of the decelerating effect and the impact force of the baffle arrays is essential for engineering design and hazard mitigation. However, the interaction mechanism of granular flows and baffles is still not fully understood. In this work, numerical simulations based on the smoothed particle hydrodynamics (SPH) method are performed to investigate the influence of baffle height and inclination on the interaction between granular flows and baffles. It is found that the SPH model can well capture the flow kinematics of granular materials through the baffles and can obtain the impact force acting on the baffle structures. The results indicate that the performance of baffles is affected by the overflow of granular flows and increasing baffle height can effectively improve the deceleration effect on granular flows. However, the impact force analysis shows that the strength of higher baffle structures also needs to be increased in engineering design. In addition, the peak impact force is found to be closely related to the Froude number Fr. [ABSTRACT FROM AUTHOR]

We address the capability of large eddy simulation (LES) to predict the physics of density currents interacting with bluff obstacles. Most density currents of interest in engineering and geophysical applications interact with obstacles or topographic features. Validating LES solutions in these contexts is crucial to establish it as a trusted tool. We thus propose a validation effort based on simple geometries that nonetheless pose challenges common to more complex systems, including boundary layer separation and convective instabilities. We focus on lock-exchange gravity currents in the slumping phase interacting with an emergent vertical circular cylinder. Our main investment was in ensuring that the comparison of experimental data and numerical results include, at least, the velocity and the density fields , and derived quantities (e.g., second order moments). Measurements of both density and velocity fields were performed in the side and plan views for cylinder Reynolds numbers, R e d , in the range 1300 to 3475. It was found that the LES accurately predicts the temporal evolution of the current front position. The computed front velocity exhibits a maximum relative error less than 8%. A good agreement between the LES and the experimental size and shape of the current head, and billows was found. The overall features upstream the cylinder, including a reflected wave, adverse pressure gradient and backflow, and downstream the cylinder, including the backflow, wake and the formation of a new head are well reproduced by LES. The agreement between the LES and the experimental time-space evolution of current spanwise- and depth-averaged density contours and the instantaneous velocity fields are not affected by R e d . [ABSTRACT FROM AUTHOR]

Debris-flow disaster has caused large casualties and tremendous economic loss. Check dams, flexible barriers, silt dams and baffle arrays are most used disaster prevention countermeasures. For a better design strategy, we made a thorough review and discussion about the achievements and challenges in four important aspects, including impact force estimation, run-up height prediction, failure analysis and plain configuration planning. The impact force exerted by debris flow on structures is the most crucial design parameter, while most widely used models are based on hydraulic theory and lack physical mechanisms, especially in accounting for the effect of nonstationary flow regimes, impact patterns and barrier characteristics. Current methods of designing protection structures mainly depend on static and deterministic theory to address dynamic problems that are highly stochastic, which reveals a great research gap in understanding the response and failure under impact of structures. In future, physically based design strategy should be highlighted, for which robust physical modelling methods and numerical simulation tools are needed for the better understanding of flow–structure interaction mechanism and the verification of structure design strategy. Furthermore, the resilience-based disaster prevention concept should be highlighted for its outstanding ability in preparedness, response, and recovery when threatened by unknown disasters. [ABSTRACT FROM AUTHOR]

Flow-structure interaction has raised significant attention among geotechnical community because it is important for prevention of high-speed flowing geo-disasters. Physical modeling serves as a fundamental tool with which to reveal the disaster-causing mechanism, but such modeling is challenging to perform especially in 1 g model tests because of the complex rheology of flows, transient nature of impact processes and strong nonlinear response of structures. The author thus presents a critical review of the state-of-the-art practices of modeling flow-structure interaction using the centrifuge and discusses several key issues. Hierarchical scaling is useful to guarantee flow similarity in microscale and macroscale, while the scaling of the Brazil nut effect and particle–fluid interaction still questionable in current stage. Flow similarity and structure similarity are equally crucial in the modeling of the flow–structure interaction, which indicates that a coupled modeling method should be encouraged. The Coriolis effect complicates flow regimes and exaggerates or lessens the impact effect of simulated flowing disasters, and it is thus important to investigate the mechanism and magnitude of such an influence. To address the above issues and facilitate in establishing basic scaling laws, robust numerical tools are recommended for experiment design and exploring fundamental mechanisms. [ABSTRACT FROM AUTHOR]

• The flow-induced snap-through dynamics of side-by-side buckled flexible filaments in a uniform flow were investigated using the penalty immersed boundary method. • We systematically examined the effects of gap distance and bending rigidity on the filament's motion and energy harvesting performance. • A phase transition from out-of-phase to in-phase motion was observed at low gap distances and bending rigidities. • The elastic energy was concentrated at the rear part of the filaments and increased with higher gap distances and bending rigidities. The flow-induced snap-through dynamics of side-by-side buckled flexible filaments in a uniform flow were investigated using the penalty immersed boundary method. We systematically examined the effects of gap distance and bending rigidity on the filament's motion and energy harvesting performance. Four distinct modes were observed as we varied the aforementioned parameters: the dual equilibrium mode, the dual streamwise oscillation mode, the single snap-through oscillation (STO) mode, and the dual STO mode. We analyzed the corresponding wake patterns associated with each of these modes. A phase transition from out-of-phase to in-phase motion was observed at low gap distances and bending rigidities. At lower gap distances, the interaction between two filaments was pronounced, leading to a co-motion of the filaments and enhanced elastic energy. This interaction decreased as the gap distance increased. Filaments in the 2STO mode exhibited significantly higher strain energy compared to the other modes. The elastic energy was concentrated at the rear part of the filaments and increased with higher gap distances and bending rigidities. Considering both energy harvesting efficiency and space utilization rate, a gap distance equal to 1 exhibited the best performance. [ABSTRACT FROM AUTHOR]

The immersed boundary method (IBM) is often adopted to simulate various flow systems. The two key steps in IBM, namely the force distribution and velocity interpolation steps, are both performed in the immersed boundary layer region. As a result, the flow velocity in this area becomes less reliable, and the boundary velocity calculated from the local flow is less accurate. To enhance the boundary velocity accuracy, here we propose the counter-extrapolation method (CEM), which estimates the boundary velocity using the flow velocity outside of the immersed boundary layer, and thus effectively improved the accuracy in the calculated velocity. Several benchmark tests, which involve flat and curved boundaries and steady and unsteady flows, are conducted; and CEM is compared to other IBM variants recently proposed in the literature. Our results show remarkable accuracy improvements in flow velocity and flow-structure interaction from CEM compared to other methods. [ABSTRACT FROM AUTHOR]

The Coriolis effect in centrifuge modeling of flow-structure interaction has not been well understood, and thus this paper presents numerical simulations to address this issue. The results indicate that the Coriolis acceleration obviously regulated the flow-structure interaction pattern. The maximum total impact force was amplified by approximately 2 times, and the force was distributed along the entire barrier surface when changing the Coriolis acceleration direction from acting at the slope to acting away from the slope. Reducing the flow velocity by 30%-40% decreased the amplification ratio by approximately 14%-29%. The alteration of the microcontact condition and the energy consumption of the dry granular flow was the main influence mechanism of the Coriolis acceleration on the flow's impact behavior. The influence of the Coriolis effect on the viscous flow impact was completely different from that exerted on frictional flows. This discrepancy resulted from the intrinsic flow mobility determined by the material characteristics. Some practical discussions about centrifuge modeling of flow-structure interaction are made as well as some suggestions for future work. [ABSTRACT FROM AUTHOR]

This work is motivated by a longstanding interest in the long time behavior of flow‐structure interaction (FSI) PDE dynamics. We consider a linearized compressible flow structure interaction (FSI) PDE model with a view of analyzing the stability properties of both the compressible flow and plate solution components. In our earlier work, we gave an answer in the affirmative to question of uniform stability for finite energy solutions of said compressible flow‐structure system, by means of a "frequency domain" approach. However, the frequency domain method of proof in that work is not "robust" (insofar as we can see), when one wishes to study longtime behavior of solutions of compressible flow‐structure PDE models, which track the appearance of the ambient state onto the boundary interface. Nor is a frequency domain approach in this earlier work availing when one wishes to consider the dynamics, in long time, of solutions to physically relevant nonlinear versions of the compressible flow‐structure PDE system under present consideration (e.g., the Navier–Stokes nonlinearity in the PDE flow component or a nonlinearity of Berger/Von Karman type in the plate equation). Accordingly, in the present work, we operate in the time domain by way of obtaining the necessary energy estimates, which culminate in an alternative proof for the uniform stability of finite energy compressible flow‐structure solutions. Since there is a need to avoid steady states in our stability analysis, as a prerequisite result, we also show here that zero is an eigenvalue for the generators of flow‐structure systems, whether the material derivative term be absent or present. Moreover, we provide a clean characterization of the (one dimensional) zero eigenspace, with or without material derivative, under an appropriate assumption on the underlying ambient vector field. [ABSTRACT FROM AUTHOR]

Rapid granular flows are one of the most catastrophic geo-disasters frequently encountered in mountainous areas. The baffle structure has been demonstrated to be an effective measure for decreasing the destructivity of such geo-disasters. In this paper, a flow–baffle interaction model based on the 3D discrete element method is adopted to assess the baffle performance, hoping to facilitate the optimal design of baffles. A multiple-indicator-based framework, which covers three aspects and six metrics, is proposed and used to thoroughly and quantitatively assess the energy dissipation capacity, deposition regulation function, and failure potential of the baffle structure considering the particle size and baffle shape effect. Results indicate that the particle size significantly affects the baffle performance, and several linear relationships are proposed to account for the effect of the particle size, which may serve to improve engineering structural design. The square baffle performs better than the triangular baffle even though they have identical transverse blockage. Investigation of the patterns of the force chain distribution in granular flows confirms that the flow–baffle interaction is controlled by the evolution of force chains. The particle size and baffle shape effect can be explained by the difference in stability of arches that form during flow–baffle interaction. In addition, the quantification of energy loss due to inelastic contact between particles and baffles reveals that enhanced particle–particle interaction is the dominant energy dissipation mechanism, accounting for more than 80–90% of the total energy loss. [ABSTRACT FROM AUTHOR]

In the context of flow-induced vibration, the component of the hydrodynamic coefficient in-phase with the velocity of an oscillating body, Cv, can be termed “positive excitation” or “negative damping” if Cv > 0. While this empirical approach is of long standing in the literature it does not account for distinct physical mechanisms that can be associated with fluid excitation and fluid damping. In this work, we decompose the total hydrodynamic force into a drag component aligned with the time-dependent vector of the relative velocity of a cylinder oscillating transversely with respect to a free stream and a lift component normal to the drag component. The drag and lift components are calculated from laboratory measurements of the components of the hydrodynamic force in the streamwise and cross-stream directions combined with simultaneous measurements of the displacement of an elastically mounted rigid circular cylinder undergoing vortex-induced vibration. It is shown that the drag component only does negative work on the oscillating cylinder, i.e., it is a purely damping force as expected from theoretical considerations. In contrast to this the lift component mostly does positive work on an oscillating cylinder, i.e., it is the sole component providing fluid excitation. In addition, the new excitation (lift) coefficient, CL displays the same scaling as the linear theory predicts for the traditional excitation coefficient, Cv, even though CL is two orders of magnitude higher than Cv. More importantly, while Cv depends on the mechanical properties of the hydro-elastic system, according to linear theory, we provide here evidence that CL depends solely on fluid-dynamical parameters. Finally, an effective drag is calculated that represents the dissipation of energy within the fluid, and it is found that the effective drag is not equal to the mean value of the drag component. The effective drag provides complementary information that characterizes the state of the wake flow. Its variation suggests that the wake can dissipate the kinetic energy most vigorously at the end of the initial branch.

We present an image-guided computational approach for inversely determining in vivo material properties of fish fins and simulating flow-structure interactions (FSI) of fin deformations based on a highly realistic hybrid membrane-beam structure. This approach is established by coupling an imaged-based reconstruction, a genetic-algorithm (GA)-based optimization, a finite-element-method (FEM)-based computational structural dynamics model and an immersed-boundary-method (IBM)-based computational fluid dynamics (CFD) solver. An inverse-problem procedure is developed to determine material properties from prescribed kinematic motions obtained from high-speed images. The procedure is validated through two tests including a flexible pitching plate and a shell-beam structured flexible plate in heaving motion. The FSI model (forward problem) is validated through two benchmark tests including flow-induced vibration of a flexible beam attached to a fixed cylinder and a flexible pitching plate in a uniform flow. This integrated method is then applied to the FSI analysis of propulsion of a rainbow trout caudal fin with a specific focus on the fin material properties, fin deformations, hydrodynamic performances and flow structures. We demonstrate that, by using reconstructed kinematics and deformation obtained from the high-speed videos, the non-uniform material properties of the fin can be determined through the inverse problem procedure. A fully-coupled FSI simulation is then carried out based on the outcome of the inverse problem. The results have shown the feasibility of the present integrated approach in accurately modeling and quantitatively evaluating flexible-fin kinematics and hydrodynamics in swimming in terms of both chordwise and spanwise deformations, thrust and lateral forces, and vortex dynamics. [ABSTRACT FROM AUTHOR]

The induced flow and the dynamics of flexible plates under heaving characterized by impulsive acceleration patterns Π (t) were experimentally inspected for various Cauchy numbers C a and fractions τ a of the time under acceleration with respect to the period of oscillations T. Simultaneous measurements of the plate motion along their span and the surrounding flow were obtained with particle tracking velocimetry (PTV) and particle image velocimetry (PIV). The Π (t) function was imposed at the root of a low-aspect-ratio plate vertically submerged in a quiescent water tank. Results show that C a and τ a significantly modulated the plate dynamics as well as the induced flow and coherent motions. We developed a simple model to estimate the velocity difference Δ u between the root and tip of the plates; it is based on the damped harmonic oscillator and exhibits good performance across the fifteen scenarios inspected (three C a by five τ a). In general, the plate dynamics and flow exhibited three distinctive patterns characterized by flapping-like motions, translation-like motions and another intermediate state. The type of motion was induced by the relative instant at the reverse direction within the forced Π (t) -heaving within one of three characteristic stages; two of them depending on the sign of Δ u and the other when Δ u → 0 , i.e., in the limit of pure translation. [ABSTRACT FROM AUTHOR]

A flow–particle interaction solver was developed in this study. For the basic flow solver, an improved divergence-free-condition compensated coupled (IDFC2) framework was employed to predict the velocity and pressure field. In order to model the effect of solid particles, the differentially interpolated direct forcing immersed boundary (DIIB) method was incorporated with the IDFC2 framework, while the equation of motion was solved to predict the displacement, rotation and velocity of the particle. The hydrodynamic force and torque which appeared in the equations of motion were directly evaluated by fluid velocity and pressure, so as to eliminate the instability problem of the density ratio close to 1. In order to effectively evaluate the drag/lift forces acting on the particle, an interpolated kernel function was introduced. The present results will be compared with the benchmark solutions to validate the present flow–particle interaction solver.

The cross-flow over a surface-mounted elastic plate and its vibratory response are studied as a fundamental two-dimensional configuration to gain physical insight into the interaction of viscous flow with flexible structures. The governing equations are numerically solved on a deforming mesh using an arbitrary Lagrangian-Eulerian finite-element method. The turbulent flow is resolved using the unsteady Reynolds-averaged Navier–Stokes equations at a Reynolds number of 2.5×104 based on the plate height. The material properties of the plate are selected so that the structural frequency is close to the frequency of vortex shedding from the free edge of a rigid plate, which is studied initially as the reference case. The results show that the plate tip oscillates back and forth in response to unsteady fluid loading at twice the frequency of vortex shedding, which is attributable to the sequential formation of a primary vortex from the free edge and a secondary vortex near the base of the plate. The effects of the plate elasticity and density on the structural response are considered, and results are compiled in terms of the reduced velocity U* and the density ratio ρ*. The standard deviation of tip displacement increases with reduced velocity in the range 7.1⩽U*⩽18.4, irrespective of whether the elasticity or the density of the plate is varied. However, the average deflection of the plate in the streamwise direction displays different scaling with U* and ρ*, but scales almost linearly with the Cauchy number ∼U*2/ρ*. Interestingly, the synchronization between plate motion and vortex shedding ceases at U*=18.4, and the excitation mechanism in the latter case resembles flutter instability, rather than vortex-induced vibration found at lower U*.

In the context of dam breaks, tsunami, and flash floods, it is paramount to quantify the time-history of forces by the rapidly transient flow to vertical structures and the characteristics of the induced flow patterns. To resemble on-land tsunami-induced flow, a free-surface-piercing structure is exposed to long leading depression waves in a tsunami flume where long waves run up and down a 1:40 smooth and impermeable sloping beach after its generation by a volume-driven wave maker. The structure and its surrounding were monitored with force transducers, pressure gauges and cameras. Preparatory steady-state experiments were accomplished to determine the drag force coefficient of the square cylinder at various water depths. The flow during wave run-up and draw-down acting on the structure resulted in distinct flow pattern which were characteristic for the type of flow-structure interaction. Besides bow wave propagating upstream, a standing or partially-standing wave was observed in front of the structure together with a wake formation downstream, while a von Kármán vortex street developed during the deceleration phase of the flow motion and during draw-down. Force measurements indicated a sudden increase in the stream-wise total force starting with the arrival of the flow front during initial run-up. Lateral velocities showed significant oscillations in correlation with the von Kármán vortex street development. A comparison of the total measured base force with the analytically-calculated share of the drag force revealed that forces were prevailingly drag-dominated.

In this study, experimental investigation is performed into modification to aerodynamic characteristics by distortion and vibration of planar wings with various Young's modulus as well as flow fields around them. Motions of the wings are captured with a high-speed camera. Flows around them are evaluated with phase locked PIV technique. As a result, bending vibration of the wings is observed almost all over the angles of attack while twisting is observed around the angles giving maximum lift force. The twisting vibration causes larger aerodynamic forces and delay in the stall. It is found that amplitude of the twisting determines magnitude of variations in aerodynamic forces. Flow field measurement shows the twisting vibration generates periodic vortices shedding from the leading and trailing edges. They reduce the separation bubble on the wing, in particular, around the middle span in spite of localization of the twisting in the vicinity of the wing tip. [ABSTRACT FROM AUTHOR]

Aeronautical studies have shown that subtle changes in aerofoil shape substantially alter aerodynamic forces during fixed-wing flight. The link between shape and performance for flapping locomotion involves distinct mechanisms associated with the complex flows and unsteady motions of an air- or hydro-foil. Here, we use an evolutionary scheme to modify the cross-sectional shape and iteratively improve the speed of three-dimensional printed heaving foils in forward flight. In this algorithmicexperimental method, 'genes' are mathematical parameters that define the shape, 'breeding' is the combination of genes from parent wings to form a daughter, and a wing's measured speed is its 'fitness' that dictates its likelihood of breeding. Repeated over many generations, this process automatically discovers a fastest foil whose crosssection resembles a slender teardrop. We conduct an analysis that uses the larger population to identify what features of this shape are most critical, implicating slenderness, location of maximum thickness and fore-aft asymmetries in edge sharpness or bluntness. This analysis also reveals a tendency towards extremely thin and cusp-like trailing edges. These findings demonstrate artificial evolution in laboratory experiments as a successful strategy for tailoring shape to improve propulsive performance. Such a method could be used in related optimization problems, such as tuning kinematics or flexibility for flapping propulsion, and for flow-structure interactions more generally. [ABSTRACT FROM AUTHOR]

The influence of end and inflow conditions on the vortex-induced vibrations of a cylinder is studied on the basis of wind tunnel experiments, combining measurements of the body displacement and flow velocity in its wake. The cylinder of length-to-diameter aspect ratio 5.5 has one end exposed to the current. It is elastically mounted in the cross-flow direction, with low damping. The structure to displaced fluid mass ratio is close to 1000. The behavior of the system is explored over a range of values of the reduced velocity, U ⋆ , defined as the inverse of the oscillator natural frequency, non-dimensionalized by the inflow velocity (U ∞) and the cylinder diameter (D), typically between 4 and 8, at a Reynolds number of the order of 1 0 4 , based on U ∞ and D. Three end conditions are examined: two free-end conditions with either a flat or a hemispherical shape, and a condition where a fixed plate is placed close to the flat end. Three inflow states are considered: a laminar condition and two grid-generated turbulent conditions of different turbulence intensity levels (up to 10%) and comparable integral length scales, around 30% of D. Vibrations characterized by bell-shaped evolutions of their magnitude with U ⋆ develop, under flow-body synchronization (lock-in), in all conditions, with peak amplitudes ranging from 4% to 12% of D. The free end causes a shift of the response bell-shaped curve towards higher U ⋆ values. The shift may be such that there is no overlap between the vibration domains in the end-plate and free-end conditions. The passage from laminar to turbulent inflow conditions is found to attenuate this shift. A clear reduction of vibration amplitude is observed when the end plate is removed, and the amplitude is further reduced when the flat end is replaced by the hemispherical one. On the other hand, turbulent inflows induce a global enhancement of the vibration amplitudes, which may reach + 45 % relative to the laminar stream condition. The end/inflow conditions are also shown to impact the regularity of the structural responses. [ABSTRACT FROM AUTHOR]

Vortex-induced oscillations offer a potential means to harness hydrokinetic energy even at low current speeds. In this study, we consider a novel converter where a cylinder undergoes angular oscillations with respect to a pivot point, in contrast to most previous configurations, where the cylinder undergoes flow-induced oscillations transversely to the incident free stream. We formulate a theoretical model to deal with the coupling of the hydrodynamics and the structural dynamics, and we numerically solve the resulting nonlinear equation of cylinder motion in order to assess the performance of the energy converter. The hydrodynamical model utilizes a novel approach where the fluid forces acting on the oscillating cylinder are split into components acting along and normal to the instantaneous relative velocity between the moving cylinder and the free stream. Contour plots illustrate the effects of the main design parameters (in dimensionless form) on the angular response of the cylinder and the energy efficiency of the converter. Peak efficiencies of approximately 20% can be attained by optimal selection of the main design parameters. Guidelines on the sizing of actual converters are discussed.

Blistering occurs when a thin solid layer locally separates from an underlying substrate through cracking of a bulk material, delamination of a composite material, or peeling of a membrane adhered to the substrate by a thin layer of viscous fluid. In this last scenario, the expansion of the newly formed blister by fluid injection occurs via a displacement flow, which peels apart the adhered surfaces through a two-way interaction between flow and deformation. Such blisters are prone to fluid and solid mechanical instabilities. If the injected fluid is less viscous than the fluid already occupying the gap, patterns of short and stubby fingers form on the propagating fluid interface. This process is regulated by membrane compliance, which if increased delays the onset of fingering to higher flow rates and reduces finger amplitude. Suppression is mediated by the locally tapered geometry of the blister near the fluid interface, which is imposed by the underlying blistering flow. Buckling/wrinkling instabilities of the delaminated layer arise for sufficiently thin membranes and can interact with the fluid mechanical fingering instability. [ABSTRACT FROM AUTHOR]

In this paper a new finite-volume non-hydrostatic and shock-capturing three-dimensional model for the simulation of wave-structure interaction and hydrodynamic phenomena (wave refraction, diffraction, shoaling and breaking) is proposed. The model is based on an integral formulation of the Navier-Stokes equations which are solved on a time dependent coordinate system: a coordinate transformation maps the varying coordinates in the physical domain to a uniform transformed space. The equations of motion are discretized by means of a finite-volume shock-capturing numerical procedure based on high order WENO reconstructions. The solution procedure for the equations of motion uses a third order accurate Runge-Kutta (SSPRK) fractional-step method and applies a pressure corrector formulation in order to obtain a divergence-free velocity field at each stage. The proposed model is validated against several benchmark test cases. [ABSTRACT FROM AUTHOR]

We present a strongly-coupled immersed-boundary method for flow–structure interaction problems involving thin deforming bodies. The method is stable for arbitrary choices of solid-to-fluid mass ratios and for large body motions. As with many strongly-coupled immersed-boundary methods, our method requires the solution of a nonlinear algebraic system at each time step. The system is solved through iteration, where the iterates are obtained by linearizing the system and performing a block-LU factorization. This restricts all iterations to small-dimensional subsystems that scale with the number of discretization points on the immersed surface, rather than on the entire flow domain. Moreover, the iteration procedure we propose does not involve heuristic regularization parameters, and has converged in a small number of iterations for all problems we have considered. We derive our method for general deforming surfaces, and verify the method with two-dimensional test problems of geometrically nonlinear flags undergoing large amplitude flapping behavior. [ABSTRACT FROM AUTHOR]

Computer simulations of steady plus low-amplitude oscillatory flow about a circular cylinder are reported at a fixed Reynolds number of 150 based on the steady component. The conventional Keleugan–Carpenter number based on the oscillatory component is fixed at π /5. The oscillation frequency is varied so as to study a wide spectrum of flows where inertial forces dominate at one end and viscous drag forces at the other as a function of the modified Keleugan–Carpenter number. The hydrodynamic force on the cylinder in-line with the flow direction is represented by Morison's equation and an extended version with three terms. The drag and inertia coefficients in Morison's equation are determined by least-squares fits to data directly computed from integration of skin friction and pressure distributions around the periphery of the cylinder. The root-mean-square value of the residue of reconstructed minus directly-computed forces varies between 2 and 41% depending on the flow parameters. Comparable results can be obtained with a semi-theoretical approach using inviscid inertia and quasi-steady viscous drag terms. Physical explanations for the variation of the force coefficients are provided and implications for pertinent flow–structure interactions are discussed. [ABSTRACT FROM AUTHOR]

We previously described an analytical model regarding how human falsetto voice is produced upon interaction between the respiratory airflow and vocal fold motion. This theory highlights the role of an unsteady flow effect―or specifically, convective acceleration of wall motion-induced flow―in inducing a Hopf bifurcation or aerodynamic flutter of vocal folds, reminiscent of falsetto voice production. Importantly, the mucosal wave motion and glottal closure of the vocal folds―typically observed in human modal voice production but absent in falsetto vocalization of high voice pitch―are dispensable in this analytical model. Thus, given its rigorous applicability to high-pitched vocalization, our model may function as a universal physical mechanism underlying the vocalization of not only humans but also other diverse vertebrate animals that share a basic anatomical design. Here we show that the relationship between the vocal frequency and animal body size and mass, derived from the present model, captures the actual features reported elsewhere, thus suggesting that the allometric scaling of animal vocalization is explained theoretically. Moreover, the critical biomechanical conditions that induce the vocalization are rewritten to highlight an intriguing consequence from our model that the voice pitch can be controlled simply and extensively with the mechanical tension in vocal membranes. Furthermore, several dimensionless numbers that characterize the aerodynamic flutter are introduced to shed light on the physical essence of the possible universal mechanism underlying vertebrate animal vocalization: whether the animal prefers to falsetto or modal vocalization is determined by its communication frequency.

Urban flood flow characteristics are usually computed using two-dimensional numerical models. How such modelling can be implemented in dense urban areas with obstacles is investigated in this paper. A strategy for representing the effect of urban obstacles in various flow conditions is proposed. The comparison between the available laboratory measurements and the model results show that if the water depth is high enough and the flow remains subcritical, two-dimensional modelling with constant eddy viscosity represents the effect of the obstacles on the flow distribution accurately, even with a coarse mesh. In contrast, if the water depth is low and/or the flow becomes supercritical, the description of the flow is not sufficient and it generates errors in the flow distribution at the crossroads. [ABSTRACT FROM PUBLISHER]

Oscillation of the vocal folds makes a sound source of the human voiced sound. Understanding of the oscillation mechanism, which is a complex flow-structure interaction problem in the airway, is crucial for considering clinical diagnosis of voice disorders. However, details of the oscillation mechanism are still unclear partly because, from a fluid mechanical viewpoint, the effect of oscillation of the vocal fold wall during the phonation on airflow behaviors remains elusive. In the present study, flow characteristics in a sinusoidally-oscillating constriction mimicking the vocal fold were investigated by numerical and experimental approaches. The numerical analyses focused in particular on the effect of constriction oscillation on flow separation demonstrated that the flow separation point moves continuously and periodically in a frequency-dependent manner. In the experimental study, an apparatus was newly designed, with a view to detect the oscillation-induced movement of the flow separation point, to enable detailed measurement of pressure distribution along the constriction with an interval of 2 mm that is synchronized with measurement of constriction displacement. Although movement of the separation point as seen in the numerical analyses was not detected by this limiting resolution of the apparatus, we obtained pressure-width relations that is partly contrary to the numerical results but is presumably dependent on the inlet boundary condition. These findings indicate that appropriate evaluations of separation point and inlet boundary conditions are key factors to characterize the flow in oscillating constriction, which is crucial for better understanding of the vocal fold mechanics.

The vocal folds in the larynx experience a self-excited oscillation with a wavelike motion during speech owing to interaction with respiratory airflow. The mechanism of the onset of the oscillation remains elusive partly because of compound effects of laryngeal muscles, although its better understanding has clinical significance in determining the ease with which phonation can be achieved. Approaches to the mechanism using a mechanical vocal fold model are useful because it allows investigating the roles of interested parameters in isolation. Here, we designed a mechanical vocal fold model made of a pair of rubber sheets. A key feature of the experimental setup is that it enables observations of high-speed deformation of the oscillating vocal fold model, together with pressure evaluations while changing separately isolated parameters associated with the laryngeal muscle functions. The observations of the oscillation onset demonstrated a gradually developed wavelike oscillation that spreads out over the rubber sheets. The magnitude of the motion is restricted by either increase in rubber restoring force or reduction in flow path width, each of the effects mimics the actual laryngeal muscle functions and reduces, in the experimental results, the threshold upstream pressure that induces the onset of the self-excitation. Thus, the present study highlights close association between degrees of oscillation, flow-tissue interaction, and threshold pressure required for the onset.

We give a survey of recent results on flow-structure interactions modeled by a modified wave equation coupled at an interface with equations of nonlinear elasticity. Both subsonic and supersonic flow velocities are considered. The focus of the discussion here is on the interesting mathematical aspects of physical phenomena occurring in aeroelasticity, such as flutter and divergence. This leads to a partial differential equation treatment of issues such as well-posedness of finite energy solutions, and long-time (asymptotic) behavior. The latter includes theory of asymptotic stability, convergence to equilibria, and to global attracting sets. We complete the discussion with several well known observations and conjectures based on experimental/numerical studies. [ABSTRACT FROM AUTHOR]

Asymptotic-in-time feedback control of a panel interacting with an inviscid, subsonic flow is considered. The classical model [E. Dowell, AIAA, 5 (1967), pp. 1857-1862] is given by a clamped nonlinear plate strongly coupled to a convected wave equation on the half space. In the absence of imposed energy dissipation the plate dynamics converge to a compact and finite dimensional set [I. Chueshov, I. Lasiecka, and J. T. Webster, Comm. Partial Differential Equations, 39 (2014), pp. 1965-1997]. With a sufficiently large velocity feedback control on the structure we show that the full flow-plate system exhibits strong convergence to the stationary set in the natural energy topology. To accomplish this task, a novel decomposition of the nonlinear plate dynamics is utilized: a smooth component (globally bounded in a higher topology) and a uniformly exponentially decaying component. Our result implies that flutter (a periodic or chaotic end behavior) can be eliminated (in subsonic flows) with sufficient frictional damping in the structure. While such a result has been proved in the past for regularized plate models (with rotational inertia terms or thermal considerations [I. Chueshov and I. Lasiecka, Springer Mongr. Math., Springer-Verlag, Berlin, 2010; I. Lasiecka and J. T. Webster, Comm. Pure Appl. Math., 13 (2014), pp. 1935-1969; I. Ryzhkova, J. Math. Anal. Appl., 294 (2004), pp. 462-481; I. Ryzhkova, Z. Angew. Math. Phys., 58 (2007), pp. 246-261], this is the first treatment which does not incorporate smoothing effects for the structure. [ABSTRACT FROM AUTHOR]

A flexible inverted flag immersed in a Poiseuille flow with heated walls was numerically modeled to investigate the dynamics of the flag and its effect on convective heat transfer. An immersed boundary method was used to analyze the interaction between the fluid and the inverted flag. This inverted flag readily becomes self-oscillating because of its configuration, in which the leading edge is free to move and the trailing edge is clamped. The inverted flag has three dynamic modes according to the characteristics of the surrounding fluid and the flag flexibility: deflected, flapping, and straight. In the flapping mode, nearly 6 pairs of vortical structures are generated in the wake of the inverted flag, which include counter vortical structures formed near the walls as well as structures generated by the interaction between the flag and the surrounding fluid. These vortical structures affect the thermal boundary layer near the walls and the temperature field in a manner that enhances the heat transfer performance of the channel flow. [ABSTRACT FROM AUTHOR]

The phenomenon of lock-in in vortex-induced vibration of a circular cylinder is investigated in the laminar flow regime (20≼Re≼100). Direct time integration (DTI) and linear stability analysis (LSA) of the governing equations are carried out via a stabilized finite element method. Using the metrics that have been proposed in earlier studies, the lock-in regime is identified from the results of DTI. The LSA yields the eigenmodes of the coupled fluid-structure system, the associated frequencies (FLSA) and the stability of the steady state. A linearly unstable system, in the absence of nonlinear effects, achieves large oscillation amplitude at sufficiently large times. However, the nonlinear terms saturate the response of the system to a limit cycle. For subcritical Re, the occurrence of lock-in coincides with the linear instability of the fluid-structure system. The critical Re is the Reynolds number beyond which vortex shedding ensues for a stationary cylinder. For supercritical Re, even though the aeroelastic system is unstable for all reduced velocities (U*) lock-in occurs only for a finite range of U*. We present a method to estimate the time beyond which the nonlinear effects are expected to be significant. It is observed that much of the growth in the amplitude of cylinder oscillation takes place in the linear regime. The response of the cylinder at the end of the linear regime is found to depend on the energy ratio, Er, of the unstable eigenmode. Er is defined as the fraction of the total energy of the eigenmode that is associated with the kinetic and potential energy of the structure. DTI initiated from eigenmodes that are linearly unstable and whose energy ratio is above a certain threshold value lead to lock-in. Interestingly, during lock-in, the oscillation frequency of the fluid-structure system drifts from FLSA towards a value that is closer to the natural frequency of the oscillator in vacuum (FN). In the event of more than one eigenmode being linearly unstable, we investigate which one is responsible for lock-in. The concept of phase angle between the cylinder displacement and lift is extended for an eigenmode. The phase angle controls the direction of energy transfer between the fluid and the structure. For zero structural damping, if the phase angle of all unstable eigenmodes is less than 90°, the phase angle obtained via DTI evolves to a value that is close to 0°. If, on the other hand, the phase angle of any unstable eigenmode is more than 90°, it settles to 180°, approximately in the limit cycle. A new approach towards classification of modes is presented. The eigenvalues are tracked over a wide range of U* while keeping Re and mass ratio (m*) fixed. In general, for large values of m*, the eigenmodes corresponding to the two leading eigenvalues exhibit a decoupled behaviour with respect to U*. They are classified as the fluid and elastic modes. However, for relatively low m* such a classification is not possible. The two leading modes are coupled and are referred to as fluid-elastic modes. The regime of such occurrence is shown on the Re-m* parameter space. [ABSTRACT FROM AUTHOR]