This study aims to clarify particle adhesion characteristics on riblet surface using direct numerical simulations and experiments. Numerical simulations of particle-laden, fully developed turbulent channel flow with rectangular riblets were performed to investigate the influence of particle adhesion on drag reduction. A one-way coupling method was employed, in which the flow affects the particles but the particles do not affect the flow. The spatial distribution of particle adhesion on the riblet surfaces was evaluated. Most particles adhered to riblet crests and the upper half of the sidewalls owing to tumbling motion within the grooves. Outdoor and laboratory experiments also revealed nonuniform particle accumulation on the riblet groove floor. The riblet shape was then modified based on the particle adhesion distribution, and a series of numerical simulations with different shapes was performed. The results reveal a decrease in the skin-friction drag reduction effect compared with the initial riblet configuration.
This study proposes a novel Mass-Preserving Particle Level Set method with Kernel function correction (mpPLS/KC) method that achieves both mass conservation and interface shape reconstruction in multiphase flow simulations. While the conventional PLS/KC method exhibits high shape reconstruction accuracy, it suffers from mass conservation issues. Therefore, a global mass conservation process was added to address this limitation. The proposed method converts minute mass errors occurring throughout the computational domain into offset values for the level set function, performing mass correction while maintaining the distance function properties. Through this efficient algorithm, mass conservation errors were successfully reduced to below 1.0 & times;10-14 (machine epsilon error level) in all benchmark tests. This represents a significant improvement compared to the mass errors of the conventional PLS/KC method (10(-3) to 10(-6) order). Regarding interface shape reconstruction, the method demonstrated high-accuracy tracking of complex interface shapes involving large deformations in Zalesak's three-dimensional rigid body rotation test and LeVeque's three-dimensional deformation test. The computational time increase due to the addition of the mass conservation process was approximately 4-6%, which is within a practically acceptable range. For practical validation, the proposed method was applied to three engineering problems: dam-break flow, dam-break flow with obstacles, and tsunami flow through coastal forests. In all cases, the results showed good agreement with experimental data. This study demonstrates that simultaneous achievement of extremely high mass conservation and excellent shape reconstruction, which was previously difficult, is now possible. The method is expected to advance as a practical multiphase flow analysis tool for various engineering fields including ocean engineering and fluid machinery.
The effect of nozzle spacing on the flow characteristics of triple rectangular free jets was studied experimentally and numerically. The nozzle spacing ratio S/d(e), where d(e) is the equivalent diameter of a rectangular nozzle with an aspect ratio of 2, was varied from 2.06 to 4.13. The Reynolds number based on the nozzle exit velocity and the equivalent diameter was 1.5 & times; 10(4). The mean and fluctuating velocities were measured using a constant-temperature hot-wire anemometer with an X-type probe. Three-dimensional numerical simulations of triple rectangular free jets were also performed using Open FOAM 5.0, and the numerical results depicted the vortical structures in the jets. Triple rectangular free jets with a small nozzle spacing ratio, specifically S/d(e) = 2.06, merge in the vicinity of the nozzle. The mean velocity and the turbulent kinetic energy near the jet centerline become higher than those with a large nozzle spacing ratio. An axis-switching phenomenon occurs after the jets merge, and the whole cross-sectional shape of the triple rectangular free jets changes to vertically long elliptical free jet. Farther downstream, the elliptical free jet becomes round. The flow characteristics of each of the triple rectangular free jets with a large nozzle spacing ratio, specifically S/d(e )= 4.13, are similar to those of a single rectangular free jet. Downstream, the streamwise mean velocities of triple rectangular free jets become identical for the dimensionless coordinates of x/S and y/S.
Surface tension modeling plays a significant role in diffuse-interface simulations of liquid-gas two-phase flows. However, widely used surface tension formulations in phase-field models often produce unphysical force even flat interfaces under equilibrium conditions, leading to spurious velocity fields and degraded numerical fidelity. The objective of this study is to identify an optimal combination of surface tension force formulation and discretization schemes that eliminates such unphysical force and velocity while maintaining computational locality. Several surface tension formulations are systematically examined using three-dimensional flat liquid films and spherical droplets under equilibrium conditions. Special attention is given to curvature-based formulations, which theoretically satisfy the requirement that the surface tension force vanishes at flat interfaces. Although these formulations can be written in mathematically equivalent forms, their numerical behavior differs significantly after discretization. The distributions of surface tension forces are analyzed to clarify how discretization errors arise and how they depend on the choice of formulation. Furthermore, the impact of finite difference schemes on computational locality and numerical accuracy is investigated. It is shown that evaluating the gradient of the order parameter using an isotropic difference scheme while computing the curvature term with a central difference scheme provides an effective balance between accuracy and locality, significantly reducing communication overhead in parallel computations. The proposed formulation and discretization strategy are further validated through phase-field lattice Boltzmann simulations of a three-dimensional static droplet. The results demonstrate a substantial reduction in spurious velocity around the droplet compared with conventional formulations. These findings provide practical and physically consistent guidelines for surface tension modeling in phase-field lattice Boltzmann simulations, particularly for large-scale and parallel computations of multiphase flows.
This study presents numerical simulations of magnetorheological (MR) fluids under a wide range of shear rates using the Euler-Lagrange hybrid scheme. In a confined space between plane-parallel walls with gravity neglected, under a constant magnetic field and particle volume fraction, the formation of magnetic particle microstructures and the shear-thinning behavior of the apparent viscosity of the MR fluid are investigated. As the shear rate increases, the microstructures formed by the magnetic particles transition from chain-like to sheet-like formations around a Mason number (Mn) of 1. Beyond a certain shear threshold, these structures begin to aggregate toward the stationary wall, while particles move away from the moving wall. For Mn = 30, hydrodynamic two-phase separation is observed with respect to the vertical direction of the walls: a particle-free region appears on the moving wall side, while an aggregate structure without interparticle contact forms on the stationary wall side. With further increase in shear rate to Mn = 100, the upper-layer particles of the aggregates detach and disperse. The two-phase separation observed at Mn = 30 is induced by magnetic and hydrodynamic interactions, and the aggregation toward the stationary wall is attributed to the constraint of particle rotation by applied magnetic field. These findings indicate that the presence or absence of particle rotation influences the hydrodynamic lift forces acting on the particles. The ratio of the magnetic torque induced by the applied magnetic field to the hydrodynamic torque is as a key factor governing the microstructure of magnetic particles in MR fluids.
In this study, we investigated the mechanism of drag reduction in turbulent flow in an annular pipe subjected to large-scale buoyancy-driven control through direct numerical simulations. The control was achieved by imposing alternating heating and cooling along the outer wall, which induces a buoyancy force that generates large-scale vortical motion. Two radius ratios ( = 0.3 and 0.5) were examined at a bulk Reynolds number of 5600. A maximum reduction in skin friction of 20.1% was obtained at = 0.3 with a Richardson number Ri = 1.25 x 10-2 and azimuthal wavenumber ohm= 3. The results of an analysis of the flow fields revealed that the drag was reduced in the formation of two pairs of large-scale counter-rotating vortices and the establishment of stable thermal stratification. A three-component decomposition of the Reynolds shear stress and an annular Fukagata-Iwamoto-Kasagi (FIK) identity analysis indicated that the reduction primarily resulted from the suppression of the turbulent component near the outer wall. These findings demonstrate that buoyancy-induced large-scale control is an effective and environmentally sustainable strategy for reducing frictional drag in wall-bounded turbulent flows.
We developed a numerical method for simulating a stationary droplet with a high density ratio on a solid surface using an improved lattice Boltzmann method (LBM) for incompressible two-phase flows. This method uses the phase-field model and wettability of a solid surface is determined from the wetting boundary condition based on a solid surface free energy density. We first reviewed existing solid surface free energy densities from both physical and numerical viewpoints. We then proposed a new one as a cosine power form, which was derived by improving drawbacks in the existing forms. We next implemented a new wetting boundary condition in the LBM based on the proposed density function form. Using the developed method, we conducted three-dimensional simulations of a stationary droplet on a flat solid surface and investigated the range of static contact angles theta that can be computed with the numerical stability. As a result, we successfully demonstrated stable simulations over a wide range of 0, including the superhydrophobic (theta> 150 degrees) and superhydrophilic (theta< 30 degrees) surfaces. Furthermore, the simulated contact angles showed good agreement with the theoretical values predicted by Young's equation within the range of 30 degrees less than or similar to theta less than or similar to 150 degrees, confirming the validity of the proposed approach.
The endplate for a small H-Darrieus vertical axis wind turbine is optimized using a global optimization method with three-dimensional computational fluid dynamics (CFD) simulations. This study focuses on the design of a three-bladed wind turbine characterized by a diameter of 0.2 m, a height of 0.2 m, and an operational tip-speed ratio of 0.7. The design process is conducted in the following two phases. First, CFD simulations are performed on various endplate topologies to identify a potential one. Second, an aerodynamic shape optimization is performed on the potential topology using the framework of the surrogate model-based global optimization approach. The performance of wind turbines is evaluated using unsteady Reynolds-averaged Navier-Stokes simulations. The potential design obtained in the first phase (investigation of various topologies) is a three-pronged endplate whose outlines are defined by quadratic curves. The optimized design obtained in the second phase (shape optimization) covers more around the leading edges and less around the trailing edges of the blade airfoils than the baseline design obtained in the first phase. The optimal endplate generates low-pressure regions around the leading edge when the position of the blade is at a rotational angle of 0 degrees. This enables a larger thrust force to be generated, thereby achieving an additional 4% improvement in performance compared with the baseline design. This performance improvement is also reproduced in wind tunnel experimental evaluations.
As skin-friction drag in wall turbulence has a considerable environmental impact, a feedback control technique is applied to provide the drag reduction effect. Specifically, opposition control, a feedback control technique, is used, which involves applying blowing and suction from the wall to cancel out turbulent vortical structures in the region near the wall. Opposition control typically involves the use of detected velocity immediately as the control input. However, in the present study, the near-wall velocity is predicted by an orbit-instability-based forecasting method based on deterministic chaos in low-Reynoldsnumber flow. We performed direct numerical simulations of turbulent channel flows to investigate the drag reduction performance. The maximum drag reduction rate was found to be comparable to that of the original opposition control. Although the drag reduction rate decreased slightly with increasing prediction time in the orbit-instability-based forecasting method, a positive drag reduction rate was achieved even when the prediction time was more than 100 times the time resolution of direct numerical simulation.
The effect of wettability on lamella behavior during droplet impingement on flat surfaces is investigated by detailed numerical simulations with different contact angle parameter sets. The coupled level set and volume of fluid method (CLSVOF) incorporated with a dynamic contact angle model and the contact angle imposing method is implemented to account for the wetting dynamics of the droplet on the surface. The numerical framework is extensively validated by comparing the droplet impingement behavior with the experimental results, qualitatively and quantitatively. The results show that an increase in the difference between the advancing contact angle and the equilibrium contact angle can trigger repulsion of the lamella on the surface, which can result in subsequent cusp generation and rim destabilization. Similarly, when the difference between the advancing contact angle and the equilibrium contact angle is kept constant, rim destabilization can also be triggered solely by increasing the equilibrium contact angle. Such destabilization contributes to enhanced ligament generation. Additionally, increasing the equilibrium contact angle significantly enhances rim destabilization by increasing the advancing contact angle on surfaces with equilibrium contact angles below 90 degrees. The differences in lamella breakup behavior due to the variation in surface wettability are attributed to the dominant effect of wettability on the splashing angle during impingement. The findings in this work would help to elucidate the effect of wettability on impinging droplets and thereby offer guidance for designing surfaces that enable the manipulation of droplet behavior during impingement.
This study presents a numerical investigation of the hydrodynamic behavior of a quayside wave energy recovery system composed of a rectangular floating buoy oscillating near a rigid vertical dike in intermediate water depth. The main objective is to determine the dimensionless hydrodynamic coefficients (added mass, radiation damping, and excitation force) within the framework of linear potential theory, while explicitly analyzing the influence of key geometric parameters such as the floater's draft, width, and proximity to the dike. The modeling is based on the Boundary Element Method (BEM), enabling accurate simulation of wave diffraction and radiation phenomena induced by the floater's oscillations. To ensure consistency with the assumptions of the model, the analysis is carried out in the low-frequency regime, which also establishes the range of validity of the results. Numerical results show good agreement with existing experimental and analytical data, validating the proposed approach. The study reveals that the proximity of the dike amplifies wave interferences, significantly increasing radiation damping and excitation forces, especially for intermediate wavelengths. Additionally, increasing the floater's draft reduces the hydrodynamic coefficients and shifts their peak values toward longer wavelengths. Conversely, a wider floater enhances added mass, damping, and excitation force due to increased fluid interaction. These findings highlight the critical role of geometric configurations and environmental parameters in optimizing the efficiency of wave energy converters integrated into coastal infrastructures, aiming to enhance both energy harvesting performance and shoreline protection.
Single string cavitation (SC) and a swirling flow around it may occur in a fuel injector of diesel engines at the moment of low needle lift, if the tip of the needle valve is close to the nozzle entrance. The swirling flow in a nozzle of the injector induces a hollow-cone spray with a very large spray angle and enhances spray atomization, which reduces the emission of particulate matters. However, the characteristics of the complicated gas-liquid two-phase swirling flow in a nozzle and its effect on spray angle have not been clarified yet. In this study, a simple one-dimensional model is developed to simulate the swirling flow, based on the mass and momentum conservation equations for the annular swirling liquid film around a single SC inside a nozzle and the hollow-cone liquid film in stagnant air. In order to obtain the boundary condition at the entrance and exit of a nozzle and to verify the validity of the swirling film flow model, we conduct a high-speed visualization of a single SC in a transparent mini-sac nozzle by visible light with refractive index matching for diesel as well as a high-speed Xray phase contrast imaging of a hollow-cone spray to measure spray angle and the thickness of the hollow-cone liquid film along the axis. Then, we carry out a number of one-dimensional simulations to investigate the effects of the axial and azimuthal velocity components of the swirling liquid film flow in the nozzle on spray cone angle. As a result, we obtain the following conclusions. (1) The proposed one-dimensional model on the swirling film flow with single SC can quantitatively predict the motion of the swirling film flow within a nozzle and the hollow-cone spray. (2) We derived a correlation on spray angle based on the axial and azimuthal velocity components in the nozzles, and verified its validity.
The forces acting on a delta wing are predominantly influenced by local flow structures, specifically leading-edge vortices (LEVs). This study applies colour-coded particle image velocimetry (PIV) to quantify wake dynamics and lift generation for a delta wing at varying angles of attack in wind tunnel experiments. Key contributions include an application of the novel colour-PIV technique through comparison with theoretical models and previous experimental data, and an in-depth analysis of vortex dynamics. Results reveal that LEVs exhibit maximum stability and strength at an angle of attack of 30 degrees, corresponding to peak lift generation. These findings provide insights into the aerodynamic characteristics of delta wings and demonstrate the advantages of colour-PIV in experimental fluid mechanics.
Single string cavitation (SC) and a swirling flow occur in a nozzle of diesel fuel injector under low needle lift conditions, when the distance ZN between needle tip and nozzle entrance is short. The swirling flow induces a hollow-cone spray and enhances the atomization of fuel spray, by which the emission of particulate matter is reduced. Previous studies have shown that spray angle alpha can be predicted by the ratio of the azimuthal velocity v theta to the axial velocity vz in the nozzle. In order to evaluate v theta and vz, we have to predict the circulation Gamma en in a nozzle and the diameter Dsc of single SC. However, empirical correlations for Gamma en and Dsc have not been established yet. In this study, five transparent single-hole nozzles with different sac inlet widths and needle lifts were designed to change and measure the circulation Gamma sac at the sac upstream of the nozzle by particle image velocimetry (PIV). We estimate Gamma en using one-dimensional simulation model and measure Dsc by image analysis. As a result, we obtained the following conclusions. (1) Circulations Gamma sac in the sacs with various needle lifts and inflow velocities are proportional to the local inlet velocity vin at the sac inlet. (2) The diameter Dsc of single string cavitation can be simply evaluated using Gamma sac in the sac. (3) The circulation Gamma en at the entrance of the nozzle can be evaluated by the correlation based on Wsac, ZN, D and vN. (4) By using the correlations on Gamma en and Dsc, we can evaluate v theta, vz, and the resulting spray angle alpha.
Surge is a well-known destructive flow instability observed in systems that include compressors. When a system encounters the surge, flow rate and pressure fluctuate drastically, significantly reducing performance. Moreover, the surge causes an unstable operation and structural damage to the system. Therefore, a deeper understanding of the surge is essential for achieving better performance and higher reliability in compression systems. This study aimed to elucidate the unsteady flow phenomena during the mild surge cycle, using experimental and numerical analyses. Specifically, we focused on a transonic centrifugal compressor with a vaneless diffuser used in vehicle turbochargers. In the experiment, compressor performance and surge characteristics were investigated using unsteady measurements of pressure and velocity in a test piping system where the compressor was installed. Additionally, a large-scale detached eddy simulation (DES) was conducted on the supercomputer FUGAKU using a computational grid of 900 million cells for the entire piping system. In the simulation, a throttle valve was represented as an outflow boundary condition to control the flow rate. By adjusting the throttle valve parameter to gradually reduce the flow rate, the simulation successfully captured a mild surge characterized by large oscillations in flow rate and pressure, without reverse flow throughout the system. Comparison of time variations in wall static pressure upstream and downstream of the compressor between the experimental and numerical analyses demonstrated that the dominant low-frequency mode excited by the mild surge was accurately reproduced by the DES. Furthermore, detailed flow structures were visualized at key phases during one surge cycle. During the mild surge, a recirculation near the impeller blade tip periodically enlarged and shrank with flow rate fluctuations. The recirculation was initially generated by a spiral-type tip leakage vortex breakdown and drastically evolved by blade stalls near the impeller tip as the flow rate decreased. In the process of the flow rate reduction, rotating stall cells in the vaneless diffuser emerged around pressure rise peak and rotated at 15-20 percent of the impeller rotational speed.
The impact of output voltage waveform variations on the induced flow velocity and driving efficiency of plasma actuators (PAs) was investigated by measuring the velocity distribution, power consumption, and thrust. When PAs were driven using a simple power supply unit with a semiconductor power device, the capacitance of the PA varied with noted differences in the total spanwise overlapped electrode length (Ls) at which dielectric barrier discharge occurred, affecting the induction flow efficiency based on the applied voltage waveform. For a PA with 100 mm spanwise overlapped electrodes, the output voltage waveform could be adjusted from a “bumpy” to a “sinusoidal” form by incorporating a variable reactor. This reactor modified the impedance on the secondary side of the power supply circuit by adjusting the air gap (df) of the EI core, which regulates inductance. Applying a sinusoidal voltage waveform is preferable for efficient PA operation in cases in which a power supply is used in conjunction with a simple electrical circuit. However, even when a slightly distorted waveform was used, such as a “quasi-sine wave,” instead of a pure sinusoidal voltage, the drive efficiency remained largely unaffected. In contrast, waveforms with high-voltage gradients near zero crossings such as “wavy” and “bumpy” led to reduced efficiency.
The effects of oscillating mean spanwise pressure gradient (OMSPG) are imposed on unstably stratified turbulent channel flow under a constant mean temperature gradient. First, an unstably stratified turbulent channel flow without OMSPG is performed under a constant mean vertical temperature gradient. The mean temperature gradient and gravity acceleration are imposed simultaneously in the same direction. Our results show that with an increase in the buoyancy, the skin friction coefficient first decreases, and then bounces back to increase, which non-monotonic behavior is related to the interaction between near-wall turbulence and thermal plumes generated by the buoyancy. Then, the effects of OMSPG are investigated and it is found that imposing OMSPG disrupts the generated thermal plumes, and hence decreases the effects of buoyancy, which increases the skin friction coefficient. In contrast, a decrease in heat transfer and Nusselt number is observed compared to cases without imposing OMSPG.
In this paper, we investigate how data quality improvement affects the performance of salinity forecasting in Hichirippu Lagoon, Hokkaido, Japan. A Long Short-Term Memory (LSTM) network, a type of recurrent neural network, was used for salinity forecasting. Due to the limited availability of field observation data, conventional forecasts use data collected from an Automated Meteorological Data Acquisition System which is 10 km from the lagoon. It is theoretically possible to improve data quality by predicting the parameters to be used; however, little research has been conducted on the effect of data quality improvement on the performance of salinity forecasting. This prompted us to focus on water elevation as a key feature, and to conduct data assimilation analysis using the Kalman filter finite element method (KF-FEM), which combines the Kalman filter with the finite element method. KF-FEM considers climate change information, such as rainfall, by using observation data for the calculations. KF-FEM cannot be carried out in the absence of field data, so a surrogate model based on deep learning was used instead. Finally, we conducted salinity forecasting using the model, DA-LSTM (data assimilation LSTM), trained with data assimilation results. To evaluate the effect of data quality improvement, we compared the forecast results of DA-LSTM and conventional LSTM. As a result, DA-LSTM showed improved forecast performance when salinity was 30 [g/L] or below. An overall performance comparison revealed the impact of data quality improvement to be limited, but it showed the potential to contribute to improved accuracy under specific conditions. This is an element that should be considered in future research.
The aim of this study is to investigate the effect of weak but prolonged mean flow accelerations, such as those observed in wind tunnel blockage acceleration, on free-stream turbulence. Specifically, this research aims to validate a model previously developed based on the k-epsilon model. To test this model, the study focuses on scenarios where the turbulence under acceleration is steady and isotropic, since the model suggests that this type of acceleration has no effect on the turbulent kinetic energy. To examine this suggestion, the turbulence within a periodic box was analyzed using large-eddy simulation (LES) based on the conventional Smagorinsky model framework. The numerical analysis is based on a method that conserves velocity fluctuation intensities. The results show that while high rate of acceleration deviates turbulent kinetic energy, low rate acceleration has hardly any effect on turbulent kinetic energy, enstrophy, pressure fluctuation, relative pressure fluctuation intensity, and higher-order statistics of a velocity fluctuation. These results validate the accuracy of the model proposed in the previous studies. These results were obtained by focusing on differences in Reynolds numbers and the spatial scale of the forcing.
While numerous studies have examined self-induced vibrations in jets, the focus has predominantly been on continuous jets. Reports on self-induced vibrations in synthetic jets, which are characterized by their excitedvibration frequencies, are limited. This paper presents an experimental investigation of self-induced vibrations in two-dimensional synthetic jets interacting with a flat plate target downstream of the exit slot. Using a speaker as the synthetic-jet actuator, velocity measurements were conducted with a hot-wire anemometer, and flow visualization was achieved using the smoke-wire method and particle image velocimetry (PIV) under typical conditions. This study examined the influence of the target plate's position and shape on the flow characteristics of synthetic jets at various dimensionless frequencies. Results were compared with those from continuous jets to discuss the conditions and mechanisms of self-induced vibration. Key findings include the occurrence of selfinduced vibrations in both continuous and synthetic jets when a target plate is positioned downstream of the slot, with the vibration frequency being approximately proportional to the flow velocity and inversely proportional to the slot-plate distance; however, the frequency of self-induced vibrations is largely independent of the excitation frequency that generates the synthetic jets. The self-induced vibration frequency of the continuous jet is lower than that of the synthetic jets under identical geometric and flow-velocity conditions. Additionally, reducing the target plate thickness induces edge tone in continuous jets but suppresses self-induced vibrations in synthetic jets. This suggests that the self-induced vibrations in the synthetic jet examined in this study are not edge-tone phenomena but are more similar to the flip-flop phenomenon.
