The flux reconstruction (FR) method, a high-order accurate solution method for unstructured grid CFD, is known as a method with a smaller computational load than the traditional discontinuous Galerkin method. It also contains other discrete spectral element methods by properly selecting the flux correction function. The 1D and tensor-product FR method is intuitively easy to understand, but the general-purpose automatic computational grid generation also requires tetrahedral, prism, and pyramid elements, which are not yet in general use for high-order CFD due to the complexity of their formulation and implementation. In this study, we extended an in-house FR CFD to hybrid elements, verified the accuracy in benchmark problems, and confirmed its applicability to low Reynolds number separated flows around an airfoil. The detailed formula for the pyramid are rarely found and they are elaborated in this study. For all types of elements mentioned above, we confirmed that the ideal accuracies (mesh convergence of errors) are achieved. For the application to a low-Reynolds-number transitional flows around an airfoil, the results well agreed with other high-order CFD results with more degrees of freedom.

A wind tunnel test method was developed to investigate the flow around a rotating sphere without support interference. The method was developed using a 1-m magnetic suspension and balance system (MSBS) that can support the model through magnetic force. The sensing and control methods were applied to measure the Magnus force acting on a rotating sphere. In particular, the generation of the negative Magnus force was focused. The aerodynamic forces acting on a rotating sphere without support interference were obtained when the Reynolds number (Re) was approximately 6.5 × 10⁵ based on the diameter of the sphere model, and the spin parameter (SP) ranged from 0.6 to 0.0. In this test, the aerodynamic force evaluation had a large error due to the large fluctuations in the aerodynamic force. However, a Magnus effect was observed. Furthermore, a decrease in the positive Magnus force was observed in the range 0.5 < SP < 0.6.

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.

This study proposes and numerically analyzes a novel method for generating thermal drift using natural convection in a flat parallel plate channel. Unlike conventional approaches that rely on wavy walls and sinusoidal temperature distributions, the proposed method uses rectangular cavities and uniform temperature distributions. This structurally simple yet effective method offers enhanced practicality and industrial applicability. The analysis focuses on the formation of horizontal flows driven by natural convection. Rectangular cavities introduced in the lower plate have their vertical walls subjected to differential heating, generating efficient horizontal flows. Numerical simulations revealed that these flows arise from fluid circulation within the cavities, moving toward the heated wall relative to the cavity. Systematic investigations were conducted to examine the effects of cavity width and height, as well as throat width and height, on flow and heat transfer characteristics. Results showed that the ratio of flow rate to heat transfer could be optimized under specific geometric conditions. The formation, disappearance, and interaction of vortices with the channel’s upper wall were found to play a significant role in influencing these characteristics. The dimensionless results were converted into dimensional quantities based on realistic temperature differences and fluid properties, demonstrating the feasibility of applying this method to practical devices such as heat exchangers and thermal management systems. The use of natural convection eliminates the need for auxiliary power sources like pumps, enabling energy savings and cost reductions. This study highlights new possibilities for designing energy-efficient thermal-fluid systems that leverage natural convection. The findings are expected to contribute to the development of sustainable thermal management technologies and next-generation energy solutions.

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 excited-vibration 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 self-induced 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.

Expanding pipes with orifice plates are often utilized as silencers to reduce the noise in fluid machineries. However, intense aerodynamic tonal sound can be generated from flows through such expanding pipes. To clarify the generation mechanism and conditions for intense tonal sound, sound measurements and flow visualization were performed for a flow through an expanding pipe with two orifice plates, where three circular cavities were formed in the expanding pipe. The effects of the freestream Mach number and orifice radius on the flow and sound were analyzed. The variation in acoustic radiation with freestream Mach number demonstrated that the Strouhal number of the most intense tonal sound based on the cavity length changes discretely at particular freestream Mach numbers, where a shear layer mode number corresponding to the number of vortices in the cavity is varied. The tonal sound becomes intense owing to the coupling of the shear layer and circumferential acoustic modes. The rotational and stational acoustic modes were identified based on the circumferential phase distributions of pressure fluctuations in the expanding pipe, whereas in-phase pressure fluctuations occurred at a relative higher Mach number. The visualized vortical structures corresponded to these phase distributions of the pressure fluctuations. The sound measurements with different orifice radii showed that the tonal sound occurred at a lower Strouhal number with a larger orifice radius, which indicates weaker effects of the orifice plate on the vortex shedding in the expanding pipe. At a smaller orifice radius, acoustic radiation occurred at a higher Strouhal number owing to the flow acceleration through the orifices, which resulted in intensified sound propagation into the outside of the expanding pipe along the effects of the acoustic mode particularly at a high Mach number.

Suspensions play a vital role in various industrial products and medical applications, making it essential to understand their rheological properties. Blood is a suspension in which red blood cells (RBCs), the solid component, are dispersed in plasma, the liquid component. The motion of RBCs during blood flow varies based on the properties of the surrounding fluid and the RBCs themselves. These variations in motion modes are key contributors to the non-Newtonian behavior of blood, which is fundamental to its rheological properties. Plasma also exhibits non-Newtonian behavior influenced by its protein content. However, limited research has examined RBC motion mode changes under the assumption that plasma behaves as a non-Newtonian fluid. This study explored the impact of plasma’s non-Newtonian properties on the critical internal-to-external viscosity ratio, a key parameter affecting RBC motion. The findings confirmed that the critical viscosity ratio decreases with an increase in the power-law index. This indicates that the viscosity variations linked to transitions in RBC motion modes significantly influence hydrodynamic resistance and are closely associated with changes in local shear stress within blood flow.

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.

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 (L_s) 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 (d_f) 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.

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.