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BALE RahulGraduate School of System Informatics / Department of System InformaticsAssociate Professor
Research activity information
■ Award- Jul. 2021 The Platform for Advanced Scientific Computing (https://pasc21.pasc-conference.org/program/awarded-contributions/), Best Paper, Simulation of Droplet Dispersion in COVID-19 Type Pandemics on Fugaku
- May 2021 The Japan Society for Computational Engineering and Science, Best Graphics award, Dispersion of cough droplets in an indoor environment
- Mar. 2021 RIKEN, RIKEN EIHO (Significant Achievement) Award, Prediction and countermeasure for droplet/Aerosol infection in indoor environments for the fight against COVID19
- This study analysed the mechanism of unsteady negative pressure around a square cylinder during forced oscillation in the rocking or the first order beam-bending mode, using an advanced large eddy simulation solver on the hierarchical Cartesian grid system using the constraint immersed boundary method for arbitrary moving boundary shape. The study also investigated an onset of negative damping force in relation to the reduced velocity and the oscillation amplitude. The results showed that in the beam-bending mode, the phase of pressure generally advanced more for the reduced velocity defined by the cylinder width, wind velocity and oscillation frequency compared to that of the rocking mode, and the transition to negative damping force occurring at a lower reduced velocity. The findings also indicate that the generation of negative pressure was influenced by timing and location of the vortex formation in the wake of 3D square cylinder.2024, Journal of Physics: Conference Series, 2647(24) (24)International conference proceedings
- Elsevier BV, Nov. 2023, Computers & Fluids, 266, 106047 - 106047, English[Refereed]Scientific journal
- Elsevier BV, Oct. 2023, Heliyon, 9(10) (10), e20540 - e20540, English[Refereed]Scientific journal
- Elsevier BV, Sep. 2023, International Journal of Mechanical Sciences, 253, 108401 - 108401, English[Refereed]Scientific journal
- As evidenced by the worldwide pandemic, respiratory infectious diseases and their airborne transmission must be studied to safeguard public health. This study focuses on the emission and transport of speech-generated droplets, which can pose risk of infection depending on the loudness of the speech, its duration and the initial angle of exhalation. We have numerically investigated the transport of these droplets into the human respiratory tract by way of a natural breathing cycle in order to predict the infection probability of three strains of SARS-CoV-2 on a person who is listening at a one-meter distance. Numerical methods were used to set the boundary conditions of the speaking and breathing models and large eddy simulation (LES) was used for the unsteady simulation of approximately 10 breathing cycles. Four different mouth angles when speaking were contrasted to evaluate real conditions of human communication and the possibility of infection. Breathed virions were counted using two different approaches: the breathing zone of influence and direction deposition on the tissue. Our results show that infection probability drastically changes based on the mouth angle and the breathing zone of influence overpredicts the inhalation risk in all cases. We conclude that to portray real conditions, the probability of infection should be based on direct tissue deposition results to avoid overprediction and that several mouth angles must be considered in future analyses.Public Library of Science (PLoS), Mar. 2023, PLOS Computational Biology, 19(3) (3), e1010972 - e1010972, English[Refereed]Scientific journal
- During the SARS-CoV-2 pandemic, the need to optimize ventilation in the hospital environment to lower the cross-infection risk of health-care workers has become evident. Local ventilation has been employed to enhance mechanical ventilation in in-patient recovery rooms. The present study applies computational fluid dynamics to analyse existing indoor air distribution of a hospital room as well as the respiratory droplets generated by a coughing patient. A local ventilation system in the form of a dental vacuum equipment was integrated and its location and design were considered. Additionally to this vacuum equipment, a push–pull system was also considered to compare performance through decrease in particle count in the breathing zone. Results show that although the alternative techniques reduce particles in the breathing zone, performance variation is greatly dependent on location. A push–pull system, which covers a wider area, is recommended.2023, Environmental Science and Engineering, 2127 - 2136International conference proceedings
- This study was designed to develop a computational fluid dynamics (CFD) method for unsteady analysis of a series of ski jump movements with attitude changes, and to analyse the aerodynamic characteristics of an expert jumper over the entire ski jump movement. Two ski jumpers participated in this study. A sensor-based motion capture suit was used to capture the jumper's posture during the actual ski jump. A three-dimensional computer graphics animation was created by superimposing the joint angles obtained from the motion measurements of the 3D shape of the athlete. The unsteady aerodynamic forces acting on the ski jumper, from the takeoff to the landing, were then calculated using CFD. A time-varying spatially uniform flow was specified as the inflow boundary condition of the computational domain. The results indicated that both the lift and drag forces of the expert jumper increase rapidly during the initial flight when the jumper's posture changes drastically. Thereafter, drag force decreased considerably, but the decrease in the lift force was less drastic. Later in the flight phase, the lift force acting on the expert jumper increased, and throughout the flight phase, the lift-drag ratio of the expert jumper remained higher than that of the unskilled jumper.ROUTLEDGE JOURNALS, TAYLOR & FRANCIS LTD, Dec. 2022, Sports Biomechanics, English[Refereed]Scientific journal
- The dose-response model has been widely used for quantifying the risk of infection of airborne diseases like COVID-19. The model has been used in the room-average analysis of infection risk and analysis using passive scalars as a proxy for aerosol transport. However, it has not been employed for risk estimation in numerical simulations of droplet dispersion. In this work, we develop a framework for the evaluation of the probability of infection in droplet dispersion simulations using the dose-response model. We introduce a version of the model that can incorporate the higher transmissibility of variant strains of SARS-CoV2 and the effect of vaccination in evaluating the probability of infection. Numerical simulations of droplet dispersion during speech are carried out to investigate the infection risk over space and time using the model. The advantage of droplet dispersion simulations for risk evaluation is demonstrated through the analysis of the effect of ambient wind, humidity on infection risk, and through a comparison with risk evaluation based on passive scalars as a proxy for aerosol transport.Dec. 2022, Scientific Reports, 12(1) (1)Scientific journal
- We propose a monolithic fluid-structure interaction (FSI) method that uses the cell-centered finite volume formulation in the Eulerian description, Lagrangian marker particles allocated in the solid region, and the incompressible mixture formulation. In the proposed method, we compute all the basic equations and spatial derivatives, except the solid constitutive equations, on an Eulerian mesh to avoid neighboring particle search. Although full Eulerian methods that use a Cartesian mesh are attractive for FSI problems that require large-scale computing and include complex geometries and the large deformation of the solid, they cannot avoid the numerical dissipation of the interfaces or internal variables of the solid caused by their advection. This computational problem has been a barrier to the industrial application of full Eulerian mesh methods. In the numerical examples, we confirmed that the proposed method retains sharp interfaces, such as the corners of a square solid, and yields more accurate numerical results for the deformation, energy, and incompressibility of a solid in fluid than our conventional Eulerian FSI method.WILEY, Mar. 2022, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, 123(5) (5), 1294 - 1328, EnglishScientific journal
- Transient flow and particle deposition in the respiratory tract: RANS versus LES comparisonDue to the worldwide pandemic, estimation of inhaled dose through in silico models has become crucial. The present study compares airflow distribution and particle deposition inside the respiratory tract using RANS and LES modelling. RANS approach underestimated particle deposition because it only solves the mean flow, affecting particle path.2022, 17th International Conference on Indoor Air Quality and Climate, INDOOR AIR 2022International conference proceedings
- Japan Society of Mechanical Engineers, 2021, Mechanical Engineering Letters, 7, 21 - 00136Scientific journal
- 2021, CoRR, abs/2104.07738A one-sided direct forcing immersed boundary method using moving least squares.Scientific journal
- ACM, 2021, PASC '21: Platform for Advanced Scientific Computing Conference(PASC), 4 - 11International conference proceedings
- The constraint-based immersed boundary (cIB) method has been shown to be accurate between low and moderate Reynolds number (Re) flows when the immersed body constraint is imposed as a volumetric constraint force. When the IB is modelled as a zero-thickness interface, where it is no longer possible to model a volumetric constraint force, we found that cIB is not able to produce accurate results. The main source of inaccuracies in the cIB method is the distribution of the pressure field around the IB surface. An IB surface results in a jump in the pressure field across the IB. Evaluation of the discrete gradient of pressure close to the IB leads to a pressure gradient that does not satisfy the Neumann boundary condition for pressure at the IB. Furthermore, a non-zero discrete pressure gradient on the IB results in spurious flow at grid points close to the IB. We present a novel numerical formulation which adapts the cIB formulation for `zero-thickness' immersed bodies. To impose the Neumann boundary condition on pressure on the IB more accurately, we introduce an additional body force to the momentum equation. A WENO based stencil penalization technique is used to define the new force term. Due to the more accurate imposition on the Neumann pressure boundary condition on the IB, spurious flow is reduced and the accuracy of no penetration velocity boundary condition on the IB is improved. (C) 2020 Elsevier Ltd. All rights reserved.PERGAMON-ELSEVIER SCIENCE LTD, Mar. 2020, COMPUTERS & FLUIDS, 200, EnglishScientific journal
- The IC engine combustion simulation using hierarchical cartesian mesh frameworkThe IC engine is investigated numerically by Cartesian mesh system. The Building Cube Method is adopted to generate the mesh partition for complicated engine geometry and parallel computation. The fully compressible flow solver by Roe scheme and 5th order MUSCL is used to calculate the flow field with high pressure and temperature differences. The species transport equations are solved with 11 species of combustion in this framework. The chemical reaction of combustion is conducted by equilibrium solver of Cantera module, which is used for evaluate the equilibrium state of the reacting flow, and merged with the flow solver and G-equation flame front treatment. In order to simulate the engine motion, the geometry and engine moving piston is calculated by Immersed Boundary Method. The flow field of velocity, density and flame front due to the combustion and engine motion is shown in the results. The validation is done by the Rapid Compression Machine simulation with the comparison of experimental data. The flame front shapes and flame propagation speed of this framework are well consistent with the experimental results.2020, Proceedings of the 6th European Conference on Computational Mechanics: Solids, Structures and Coupled Problems, ECCM 2018 and 7th European Conference on Computational Fluid Dynamics, ECFD 2018, 2032 - 2043International conference proceedings
- Numerically modelling the multi-physics phenomenon of combustion is challenging as it involves fluid flow, chemical reaction, phase change, energy release, etc. Combining numerical models for all these phenomena into a single solver ensuring scalability and performance is a daunting task. Based on the hierarchical meshing technique building cube method (BCM) we present a numerical framework for modelling internal combustion engines. The framework efficiently combines a fully compressible flow solver, chemical reaction and combustion model, a particle-in-cell based liquid spray model, and an immersed boundary method for geometry treatment. The flow, temperature fields and the transport of reacting species an all speed Roe scheme is adopted discretization of the advective flux. The solver is coupled with the equilibrium chemical reaction library CANTERA to model combustion. The parcel model-based particle-source-in-cell (PSI-cell) method is adopted for modelling liquid fuel spray and its evaporation. Validation of the numerical framework is carried out by using experimental data of a model internal combustion engine known as the Rapid Compression Machine (RCM). Evaluation of the framework with strong scaling analysis shows good scalability.ACM, 2020, Proceedings of the Platform for Advanced Scientific Computing Conference, PASC 2020, 15 - 10International conference proceedings
- In vehicle aerodynamics analysis considering actual traveling conditions, it is known that the flow around the rotating wheel interferes with the wake of the wheel arch, the flow ejected from the engine room, and the flow under the floor. This significantly affects the aerodynamic drag value. In particular, the fuel consumption measurement method by the World Wide Harmonized Light Vehicle Test Procedure has been carried out since FY 2018 in Japan. A calculation condition on computational fluid dynamics is required to be accompanied with wheel rotation. However, it is not easy to carry out simulations with wheel rotation for a full-vehicle model using a general-purpose method with an unstructured grid or voxel lattice. This method has difficulty in performing calculations faster and easier. On the other hand, the authors proposed a method for complex geometries using a hierarchical Cartesian grid and a topology-independent immersed boundary method, making it possible to handle "dirty" CAD data directly. Several research results for full-vehicle aerodynamics have been reported with high parallel computing efficiency and high practicality. In this study, this method is applied to vehicle aerodynamics analysis with wheel rotation, and is compared with experimental values measured in a wind tunnel with a moving belt facility. Full-vehicle aerodynamics analysis was performed on six configurations, and the difference in the aerodynamics forces was predicted. The grid resolution was about 6 mm, the number of computational grids was about 120 million, and the calculation time for an unsteady flow of 2 s took about 15 h in stationary cases and about 39 h in full-rotation cases. As a result, the drag value could be predicted within a range of ΔCdA ± 0.015. In addition, a difference in the flow field around the rotating wheel was observed owing to the difference in the numerical configuration of the rotation condition.Apr. 2019, SAE Technical Papers, 2019-April(April) (April)International conference proceedings
- We propose a full Eulerian incompressible solid-fluid interaction scheme capable of achieving high parallel efficiency and easily generating meshes for complex solid geometries. While good scalability of a full Eulerian solid-fluid interaction formulation has been reported by Sugiyama et al, their analysis was carried out using uniform Cartesian mesh and an artificial compressibility method. Typically, it is more challenging to achieve good scalability for hierarchical Cartesian meshes and a fully incompressible formulation. In addition, the conventional full Eulerian methods require a large computational cost to resolve complex solid geometries due to the usage of uniform Cartesian meshes. In an attempt to overcome the aforementioned issues, we employ the building-cube method, where the computational domain is divided into cubic regions called cubes. Each cube is divided at equal intervals, the same number of cubes is assigned to each core, and the spatial loop processing is executed for each cube. The numerical method is verified by computing five numerical examples. In the weak scaling test, the parallel efficiency at 32768 cores with 32 cores as a reference is 93.6%. In the strong scaling test, the parallel efficiency at 32768 cores with 128 cores as a reference is 70.2%.WILEY, Jan. 2019, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, 117(2) (2), 221 - 248, EnglishScientific journal
- Writing high-performance solvers for engineering applications is a delicate task. These codes are often developed on an application to application basis, highly optimized to solve a certain problem. Here, we present our work on developing a general simulation framework for efficient computation of time-resolved approximations of complex industrial flow problems—Complex Unified Building cube method (CUBE). To address the challenges of emerging, modern supercomputers, suitable data structures and communication patterns are developed and incorporated into CUBE. We use a Cartesian grid together with various immersed boundary (IB) methods to accurately capture moving, complex geometries. The asymmetric workload of the IB is balanced by a predictive dynamic load balancer, and a multithreaded halo exchange algorithm is employed to efficiently overlap communication with computations. Our work also concerns efficient methods for handling the large amount of data produced by large-scale flow simulations, such as scalable parallel I/O, data compression, and in-situ processing.2019, Int. J. High Perform. Comput. Appl., 33(4) (4), 678 - 698Scientific journal
- Transient growth of perturbations by a linear non-modal evolution is studied here in a stably stratified bounded Couette flow. The density stratification is linear. Classical inviscid stability theory states that a parallel shear flow is stable to exponentially growing disturbances if the Richardson number (Ri) is greater than 1/4 everywhere in the flow. Experiments and numerical simulations at higher Ri show however that algebraically growing disturbances can lead to transient amplification. The complexity of a stably stratified shear flow stems from its ability to combine this transient amplification with propagating internal gravity waves (IGWs). The optimal perturbations associated with maximum energy amplification are numerically obtained at intermediate Reynolds numbers. It is shown that in this wall-bounded flow, the three-dimensional optimal perturbations are oblique, unlike in unstratified flow. A partitioning of energy into kinetic and potential helps in understanding the exchange of energies and how it modifies the transient growth. We show that the apportionment between potential and kinetic energy depends, in an interesting manner, on the Richardson number, and on time, as the transient growth proceeds from an optimal perturbation. The oft-quoted stabilizing role of stratification is also probed in the non-diffusive limit in the context of disturbance energy amplification.IOP PUBLISHING LTD, Feb. 2018, FLUID DYNAMICS RESEARCH, 50(1) (1), EnglishScientific journal
- Japan Society of Civil Engineers, 2018, Journal of Japan Society of Civil Engineers, Ser. A2 (Applied Mechanics (AM)), 74(2) (2), I_253 - I_263Scientific journal
- What wavelengths do undulatory swimmers use during propulsion? In this work we find that a wide range of body/caudal fin (BCF) swimmers, from larval zebrafish and herring to fully-grown eels, use specific wavelength (ratio of wavelength to tail amplitude of undulation) values that fall within a relatively narrow range. The possible emergence of this constraint is interrogated using numerical simulations of fluid-structure interaction. Based on these, it was found that there is an optimal specific wavelength (OSW) that maximizes the swimming speed and thrust generated by an undulatory swimmer. The observed values of specific wavelength for BCF animals are relatively close to this OSW. The mechanisms underlying the maximum propulsive thrust for BCF swimmers are quantified and are found to be consistent with the mechanisms hypothesized in prior work. The adherence to an optimal value of specific wavelength in most natural hydrodynamic propulsors gives rise to empirical design criteria for man-made propulsors.PUBLIC LIBRARY SCIENCE, Jun. 2017, PLOS ONE, 12(6) (6), EnglishScientific journal
- BCM-LES analysis of turbulent flows over and within actual urban canopyThis study discusses the analysis of turbulent flows over and within urban canopy by using BCM-LES technique. BCM is the building Cube Method which uses the very fine Cartesian grid with octree pattern. For the numerical model, the urban canopy has been constructed by the actual buildings in the city. This study recognizes the development of urban boundary layer over urban canopy and investigates the vertical wind profile. Also, this study examines the coherent structures of turbulence formed at the edge of urban canopy and clarifies the occurrence of wind gust at the specified part. Finally the data base of the turbulent boundary layer is explained.2017, 9th Asia Pacific Conference on Wind Engineering, APCWE 2017International conference proceedings
- Some groups of fish have evolved to generate propulsion using undulatory elongated fins while maintaining a relatively rigid body. The fins run along the body axis and can be dorsal, ventral, dorsoventral pairs or left-right pairs. These fish are termed as median/paired fin (MPF) swimmers. The movement of these groups of fish was studied in an influential series of papers by Lighthill and Blake. In this work, we revisit this problem by performing direct numerical simulations. We interrogate two issues. First, we investigate and explain a key morphological feature, which is the diagonal fin insertion found in many MPF swimmers such as the knifefish. Not only are these results of biological relevance, but these are also useful in engineering to design bioinspired highly maneuverable underwater vehicles. Second, we investigate whether there is a mechanical advantage in the form of reduced cost of transport (COT) (energy spent per unit distance traveled) for not undulating the entire body. We find that a rigid body attached to an undulating fin leads to a reduced COT.TAYLOR & FRANCIS LTD, 2017, EUROPEAN JOURNAL OF COMPUTATIONAL MECHANICS, 26(1-2) (1-2), 31 - 43, EnglishScientific journal
- Natural convection is commonly used as a means of heat transfer in many practical products because it is highly cost-effective. The development of simulation tools for this type of application is generally accompanied by several critical issues, including high-temperature differences, rapid turnaround demand, and complex geometries. Under conditions of natural convection with high-temperature differences, the density of the medium is variable but the flow speed is low. Therefore, a compressible solver, i.e., Roe scheme developed by P.L. Roe in 1981, must be combined with a preconditioning method that can make the Roe scheme available at low speeds to allow the above situation to be addressed. The building cube method is adopted to make our method suitable for massive parallelization systems, which can reduce the calculation and turnaround times immensely. An immersed boundary method for compressible flows combined with a fast, easy to implement, and robust interpolation method is developed to handle flows with complex immersed geometries. The results show that the program described here is suitable for application to product design and analysis because of its wide applicability to natural convection with high-temperature differences, its capacity to handle complex geometries, and its feasibility for use in massive parallelization systems. (C) 2016 Elsevier Ltd. All rights reserved.PERGAMON-ELSEVIER SCIENCE LTD, Jul. 2016, INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER, 75, 52 - 58, EnglishScientific journal
- Jun. 2016, Proc. of the 8th International Colloquium on Bluff Body Aerodynamics and Applications, English[Refereed]International conference proceedings
- In parallel computing load balancing is an essential component of any efficient and scalable simulation code. Static data decomposition methods have proven to work well for symmetric workloads. But, in today’s multiphysics simulations, with asymmetric workloads, this imbalance prevents good scalability on future generation of parallel architectures. We present our work on developing a general dynamic load balancing framework for multiphysics simulations on hierarchical Cartesian meshes. Using a weighted dual graph based workload estimation and constrained multilevel graph partitioning, the required runtime for industrial applications could be reduced by 40% of the runtime, running on the K computer.Springer, 2016, Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics), 10164 LNCS, 13 - 23International conference proceedings
- We investigate analytically the short-time response of disturbances in a density-varying Couette flow without viscous and diffusive effects. The complete inviscid problem is also solved as an initial value problem with a density perturbation. We show that the kinetic energy of the disturbances grows algebraically at early times, contrary to the well-known algebraic decay at time tending to infinity. This growth can persist for arbitrarily long times in response to sharp enough initial perturbations. The simplest in our three-stage study is a model problem forced by a buoyancy perturbation in the absence of background stratification. A linear growth with time is obtained in the vertical velocity component. This model provides an analogy between the transient mechanism of kinetic energy growth in a two-dimensional density-varying flow and the lift-up mechanism of the three-dimensional constant density flow. Next we consider weak stable background stratification. Interestingly, the lowest order solution here is the same as that of the model flow. Our final study shows that a strong background stratification results in a sub-linear growth with time of the perturbation. A framework is thus presented where two-dimensional streamwise disturbances can lead to large transient amplification, unlike in constant density flow where three dimensions are required.ROYAL SOC, Sep. 2015, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, 471(2181) (2181), EnglishScientific journal
- Examples of animals evolving similar traits despite the absence of that trait in the last common ancestor, such as the wing and camera-type lens eye in vertebrates and invertebrates, are called cases of convergent evolution. Instances of convergent evolution of locomotory patterns that quantitatively agree with the mechanically optimal solution are very rare. Here, we show that, with respect to a very diverse group of aquatic animals, a mechanically optimal method of swimming with elongated fins has evolved independently at least eight times in both vertebrate and invertebrate swimmers across three different phyla. Specifically, if we take the length of an undulation along an animal's fin during swimming and divide it by the mean amplitude of undulations along the fin length, the result is consistently around twenty. We call this value the optimal specific wavelength (OSW). We show that the OSW maximizes the force generated by the body, which also maximizes swimming speed. We hypothesize a mechanical basis for this optimality and suggest reasons for its repeated emergence through evolution.PUBLIC LIBRARY SCIENCE, Apr. 2015, PLOS BIOLOGY, 13(4) (4), EnglishScientific journal
- For nearly a century, researchers have tried to understand the swimming of aquatic animals in terms of a balance between the forward thrust from swimming movements and drag on the body. Prior approaches have failed to provide a separation of these two forces for undulatory swimmers such as lamprey and eels, where most parts of the body are simultaneously generating drag and thrust. We nonetheless show that this separation is possible, and delineate its fundamental basis in undulatory swimmers. Our approach unifies a vast diversity of undulatory aquatic animals (anguilliform, sub-carangiform, gymnotiform, bal-istiform, rajiform) and provides design principles for highly agile bioinspired underwater vehicles. This approach has practical utility within biology as well as engineering. It is a predictive tool for use in understanding the role of the mechanics of movement in the evolutionary emergence of morphological features relating to locomotion. For example, we demonstrate that the drag-thrust separation framework helps to predict the observed height of the ribbon fin of electric knifefish, a diverse group of neotropical fish which are an important model system in sensory neurobiology. We also show how drag-thrust separation leads to models that can predict the swimming velocity of an organism or a robotic vehicle.NATURE PUBLISHING GROUP, Dec. 2014, SCIENTIFIC REPORTS, 4, EnglishScientific journal
- Nearly eighty years ago, Gray reported that the drag power experienced by a dolphin was larger than the estimated muscle power - this is termed as Gray's paradox. We provide a fluid mechanical perspective of this paradox. The viewpoint that swimmers necessarily spend muscle energy to overcome drag in the direction of swimming needs revision. For example, in undulatory swimming most of the muscle energy is directly expended to generate lateral undulations of the body, and the drag power is balanced not by the muscle power but by the thrust power. Depending on drag model utilized, the drag power may be greater than muscle power without being paradoxical.NATURE PUBLISHING GROUP, Jul. 2014, SCIENTIFIC REPORTS, 4, EnglishScientific journal
- Which animals use their energy better during movement? One metric to answer this question is the energy cost per unit distance per unit weight. Prior data show that this metric decreases with mass, which is considered to imply that massive animals are more efficient. Although useful, this metric also implies that two dynamically equivalent animals of different sizes will not be considered equally efficient. We resolve this longstanding issue by first determining the scaling of energy cost per unit distance traveled. The scale is found to be M-2/3 or M-1/2, where M is the animal mass. Second, we introduce an energy-consumption coefficient (C-E) defined as energy per unit distance traveled divided by this scale. C-E is a measure of efficiency of swimming and flying, analogous to how drag coefficient quantifies aerodynamic drag on vehicles. Derivation of the energy-cost scale reveals that the assumption that undulatory swimmers spend energy to overcome drag in the direction of swimming is inappropriate. We derive allometric scalings that capture trends in data of swimming and flying animals over 10-20 orders of magnitude by mass. The energy-consumption coefficient reveals that swimmers beyond a critical mass, and most fliers are almost equally efficient as if they are dynamically equivalent; increasingly massive animals are not more efficient according to the proposed metric. Distinct allometric scalings are discovered for large and small swimmers. Flying animals are found to require relatively more energy compared with swimmers.NATL ACAD SCIENCES, May 2014, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 111(21) (21), 7517 - 7521, EnglishScientific journal
- While wake structures of many forms of swimming and flying are well characterized, the wake generated by a freely swimming undulating fin has not yet been analyzed. These elongated fins allow fish to achieve enhanced agility exemplified by the forward, backward and vertical swimming capabilities of knifefish, and also have potential applications in the design of more maneuverable underwater vehicles. We present the flow structure of an undulating robotic fin model using particle image velocimetry to measure fluid velocity fields in the wake. We supplement the experimental robotic work with high-fidelity computational fluid dynamics, simulating the hydrodynamics of both a virtual fish, whose fin kinematics and fin plus body morphology are measured from a freely swimming knifefish, and a virtual rendering of our robot. Our results indicate that a series of linked vortex tubes is shed off the long edge of the fin as the undulatory wave travels lengthwise along the fin. A jet at an oblique angle to the fin is associated with the successive vortex tubes, propelling the fish forward. The vortex structure bears similarity to the linked vortex ring structure trailing the oscillating caudal fin of a carangiform swimmer, though the vortex rings are distorted because of the undulatory kinematics of the elongated fin.COMPANY BIOLOGISTS LTD, Jan. 2014, JOURNAL OF EXPERIMENTAL BIOLOGY, 217(2) (2), 201 - 213, EnglishScientific journal
- Simulating the electric field-driven motion of rigid or deformable bodies in fluid media requires the solution of coupled equations of electrodynamics and hydrodynamics. In this work, we present a numerical method for treating such equations of electrohydrodynamics in an immersed body framework. In our approach, the electric field and fluid equations are solved on an Eulerian grid, and the immersed structures are modeled by meshless collections of Lagrangian nodes that move freely through the background Eulerian grid. Fluid-structure interaction is handled by an efficient distributed Lagrange multiplier approach, whereas the body force induced by the electric field is calculated using the Maxwell stress tensor. In addition, we adopt an adaptive mesh refinement (AMR) approach to discretizing the equations that permits us to resolve localized electric field gradients and fluid boundary layers with relatively low computational cost. Using this framework, we address a broad range of problems, including the dielectrophoretic motion of particles in microfluidic channels, three-dimensional nanowire assembly, and the effects of rotating electric fields to orient particles and to separate cells using their dielectric properties in a lab-on-a-chip device. We also simulate the phenomenon of electrolocation, whereby an animal uses distortions of a self-generated electric field to locate objects. Specifically, we perform simulations of a black ghost knifefish that tracks and captures prey using electrolocation. Although the proposed tracking algorithm is not intended to correspond to the physiological tracking mechanisms used by the real knifefish, extensions of this algorithm could be used to develop artificial "electrosense" for underwater vehicles. To our knowledge, these dynamic simulations of electrolocation are the first of their kind. (C) 2013 Elsevier Inc. All rights reserved.ACADEMIC PRESS INC ELSEVIER SCIENCE, 2014, J. Comput. Phys., 256, 88 - 108, EnglishScientific journal
- Many problems of interest in biological fluid mechanics involve interactions between fluids and solids that require the coupled solution of momentum equations for both the fluid and the solid. In this work, we develop a mathematical framework and an adaptive numerical method for such fluid-structure interaction (FSI) problems in which the structure may be rigid, deforming, or elastic. We employ an immersed boundary (IB) formulation of the problem that permits us to avoid body conforming discretizations and to use fast Cartesian grid solvers. Rigidity and deformational kinematic constraints are imposed using a formulation based on distributed Lagrange multipliers, and a conventional IB method is used to describe the elasticity of the immersed body. We use Cartesian grid adaptive mesh refinement (AMR) to discretize the equations of motion and thereby obtain a solution methodology that efficiently captures thin boundary layers at fluid-solid interfaces as well as flow structures shed from such interfaces. This adaptive methodology is validated for several benchmark problems in two and three spatial dimensions. In addition, we use this scheme to simulate free swimming, including the maneuvering of a two-dimensional model eel and a three-dimensional model of the weakly electric black ghost knifefish. © 2013 Elsevier Inc.2013, J. Comput. Phys., 250, 446 - 476Scientific journal
- CAMBRIDGE UNIV PRESS, Apr. 2011, JOURNAL OF FLUID MECHANICS, 673, 603 - 605, EnglishScientific journal
- The phrases 'transient growth' and 'non-normality' have become common parlance in fluid mechanics nowadays. We present these ideas with a simple two-dimensional system, to enable the reader to look for transient growth, as a trigger for nonlinear behaviour to set in, in a variety of situations probably having nothing to do with fluid mechanics. The article is aimed at undergraduate students of science, engineering, finance, etc., and the material is based completely on the excellent books of Trefethen and Embree, and Schmid and Henningson [1,2]. © 2010 Indian Academy of Sciences.2010, Resonance, 15(5) (5), 441 - 457Scientific journal
- 東京 : 日本流体力学会, Apr. 2023, ながれ : 日本流体力学会誌 = Nagare : journal of Japan Society of Fluid Mechanics, 42(2) (2), 91 - 94, Japaneseスキージャンプにおける離陸から着地までの一連の競技動作を考慮した空力シミュレーション—Numerical Analysis of Aerodynamic Forces Acting on a Ski Jumper from Takeoff to Landing—第36回数値流体力学シンポジウム特集 ; 特集 注目研究in CFD36
- 日本計算工学会, Jun. 2021, 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science, 26, 883 - 885, JapaneseThe numerical simulation of spread of droplets in lecture room
- 日本計算工学会, Jun. 2021, 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science, 26, 902 - 904, EnglishDispersion and evaporation of airborne respiratory droplets
- 日本流体力学会, Apr. 2020, ながれ : 日本流体力学会誌 = Nagare : journal of Japan Society of Fluid Mechanics, 39(2) (2), 88 - 91, JapaneseAerodynamics simulation framework for Ski-jumping take-off and its application to various jumpers
- The Japan Society of Mechanical Engineers, 2020, The Proceedings of Mechanical Engineering Congress, Japan, 2020, J05112, Japanese
- 日本計算工学会, May 2019, 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science, 24, 3p, EnglishAnalysis of underwater undulatory swimming using self-propulsion simulations
- The Japan Society of Mechanical Engineers, 2019, The Proceedings of the Symposium on sports and human dynamics, 2019, B - 2, Japanese
The ski jumper rapidly changes the body posture from crouching to flight posture during take off. In this study, the relationship between flight distance and take-off motion, in terms of trunk angle of attack and lift-drag ratio, was investigated through aerodynamics simulations that reproduce the take-off motion dynamically. The trunk angle of attack was extracted from two-dimensional image in the sagittal plane of the take-off motion. From the results of the numerical simulation it was found that athletes with higher lift-drag ratio at the initial take-off posture had longer flight distance and the frontal pressure on these athletes was relatively lower.
- 日本計算工学会, Jun. 2018, 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science, 23, 6p, EnglishThe four-stroke IC engine simulation using hierarchical Cartesian mesh framework by CUBE
- 日本計算工学会, May 2017, 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science, 22, 3p, EnglishA stencil penalty approach for improving accuracy of constraint immersed boundary method
- 日本計算工学会, May 2016, 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science, 21, 4p, EnglishAn immersed boundary framework for large scale simulations with moving structures
- Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Early-Career Scientists, Grant-in-Aid for Early-Career Scientists, Institute of Physical and Chemical Research, 01 Apr. 2020 - 31 Mar. 2022CFD simulations of fluid dynamics in sports science for better performance of athletes