Using the resolvent analysis, we investigate how the near-wall mode primarily responsible for the friction drag is amplified or suppressed depending on the shape of the mean velocity profile of a turbulent channel flow. Following the recent finding by Kuhnen et al. (2018), who modified the mean velocity profile to be flatter and attained¨ significant drag reduction, we introduce two types of artificially flattened turbulent mean velocity profiles: one is based on the turbulent viscosity model proposed by Reynolds and Tiederman (1967), and the other is based on the mean velocity profile of laminar flow. A special care is taken so that both the bulk and friction Reynolds numbers are unchanged, whereby only the effect of change in the mean velocity profile can be studied. These mean velocity profiles are used as the base flow in the resolvent analysis, and the response of the wavenumber-frequency mode corresponding to the near-wall coherent structure is assessed via the change in the singular value (i.e., amplification rate). The flatness of the modified mean velocity profiles is quantified by three different measures. In general, the flatter mean velocity profiles are found to result in significant suppression of near-wall mode. Further, increasing the mean velocity gradient in the very vicinity of the wall is found to have a significant importance for the suppression of near-wall mode through mitigation of the critical layer.

Compared with conventional high-specific-speed axial flow pump, better cavitation performance and compact size have been achieved in contra-rotating axial flow pump, where the rear rotor is employed additionally to the front rotor to convert the swirling flow to the pressure rise. Meanwhile, significantly deteriorated performance has also been observed at well off-design flow rates with design rotational speed. The rotational speed control (RSC) of front and rear rotors has been experimentally proved to be effective to enhance the performance. However, thorough investigations are necessary to find the optimum rotational speeds of rotors. It may be done by computational fluid dynamics (CFD) simulations, whereas it is time-consuming to cover the wide ranges of rotational speeds. Therefore, in the present paper, a fast and effective performance prediction model is established by considering radial equilibrium condition, conservation of rothalpy and mass, empirical deviation angle, bladerows interaction and empirical losses. Experimental and CFD results are employed to validate the proposed prediction model. It is found that the proposed model shows good enough accuracy in predicting performances of contra-rotating axial flow pump under RSC in broad flow rate range. Furthermore, an energy saving application of the proposed model is also illustrated for two typical system resistances. Compared with the traditional valve control under the design rotational speed operation, the RSC method can well modify the pump head to satisfy the system resistance at wide flow rate range with the significantly improved energy performance.

The aerodynamic performance of the multi-rotor drone under the wall proximity has been investigated by experiment and numerical simulation. The propeller airflow along with the wall deflects toward the wall due to the Coanda effect, and it yields a negative impact on the aerodynamic performance. The present study aims to reveal the link between the propeller thrust and the propeller airflow under different wall proximity conditions. The deflection of the flow is confirmed by the flow visualization, and the wall pressure exhibits the signature of flow attachment both in the profiles of the mean and the fluctuation. The force measurement indicates that the degradation of the thrust is significant enough to affect the stability of the drone body. A possible reason of the decrease in thrust is found in the streamwise velocity distribution. The velocity distributions obtained by the numerical simulation indicate that the swirling motion is significantly suppressed due to the wall proximity effect. Moreover, the pressure distribution on the propeller surface explains the decrease of the thrust. The magnitude of the pressure difference becomes smaller when the propeller blade approaches very close to the wall.

A wind tunnel experiment was performed to investigate the requirements for providing a suitable environment for easily creating a localized turbulent region in a laminar boundary layer. A combination of a short-duration jet and suctions was used to prepare a potentially unstable region upstream, and another jet was ejected downstream against the region at various timings and different relative spanwise locations. Based on the results, the combination of the potentially unstable region and the short-duration jet promoted the transition to turbulence only under limited conditions, whereas in most cases, it worked negatively. However, the turbulent spot generation was enhanced when the downstream jet was used at the timing and location that enlarged the low-velocity area created upstream. Moreover, the locally disturbed region generated by the combination of the potentially unstable region and the short-duration jet did not directly grow into a turbulent spot; rather, the turbulent spot grew in the region following the disturbed region.

External forcing on a wing-tip vortex can affect its instability, and therefore an optimal perturbation can improve the aerodynamic performance of the wing. The present study examined the unsteadiness of the wing-tip vortex under periodic wing-tip vibration, and revealed its effect on the aerodynamic performance of the wing. A 3D-printed vibrating wing-tip model was prepared, which was driven by a sheet-type piezo actuator. Phase-averaged stereo particle image velocimetry (PIV) measurements clarified that the averaged position of the vortex depends on the phase of the wing-tip vibration, and the vortex shifted further from the wing as the actuation frequency increased. The phase-averaged velocity distributions indicate that the velocity deficit inside the vortex is significantly enhanced near the end of the downstroke of the wing-tip motion. The wing-tip vortex is weakened in the mid-upstroke, and its impact depends on the actuation frequency. This is because the motion of the wing is in the same direction as the flow rolling up from the pressure side, which prevents the formation of the vortex. In the mid-upstroke phase, the turbulence quantities, e.g., the turbulent kinetic energy and the Reynolds shear stress, are significantly suppressed; these effects depend monotonically on the actuation frequency. These arguments are supported by time-resolved recordings of the flow and the wing motion. The force measurements reveal that the vibration of the wing-tip brings a positive effect on the lift-to-drag ratio.

The characteristics of a fine-bubble plume passing through two tandem cylinders are investigated. Fine-bubbles with a mean diameter of 0.055 mm are released by water electrolysis from electrodes placed at the bottom of a tank. They induce an upward water flow around them with rising due to buoyancy. Orthogonally to the axis of such fine–bubble plume, two cylinders with a diameter D of 30 mm are arranged in tandem. The distance between the cylinders, L, ranges between 1.5D and 3D. This study visualizes the bubbles and water flow around the cylinders. It also measures the bubble velocity distribution. The experiments reveal the water and bubble shear layers originating at the sides of the lower cylinder and make clear their behavior around the upper cylinder. This study elucidates the effects of L on the characteristics of the fine-bubble plume, such as the stagnant bubbly flow and the bubbly wake around the cylinders.

Air lubrication systems have gained considerable popularity as a promising drag reduction technology in recent years. However, numerical simulations of intermediate-sized bubbles are quite challenging because of the numerical diffusion of the conventional method and the high deformability of the bubbles. This hinders the study of the physical mechanisms involved in a variety of phenomena in such types of bubbles, such as the bubble–liquid interaction effect, high bubble deformation, and flow in the liquid film generated above the bubble. In this study, a solver, viz. interIsoFoam of OpenFOAM, which is directly captured by the improved volume of fluid method, was applied to solve the gas–liquid interface problem. We established the numerical procedure by dividing it into three stages and validating the accuracy of the given solver to minimize numerical errors such as smearing the volume fractions. The numerical results for variables such as the bubble shape, the skin friction of the liquid film, and the instantaneous momentum flux display trends similar to those observed in the experiments. The calculated bubble shows a high skin friction in the secondary flow, which corresponds to the distribution of streamwise vortices in the secondary flow.

This paper presents the development of a statistical emulator to estimate free-surface heights with less computational time than a particle method. Particle methods can simulate free-surface flow problems by solving Navier-Stokes and continuity equations, but they require more computational time as the number of particles becomes greater in computational domains. Accordingly, it is not pragmatic to conduct statistical analysis of free-surface problems with respect to a variety of initial conditions by particle methods. In the place of the simulation methods, statistical emulators can estimate predictive values in these problems with less computational time. In this study, we apply a Gaussian process for designing a statistical emulator of the Explicit Moving Particle Simulation (EMPS) method and predict free-surface heights in dam break problems. Once it is developed based on a dataset made from only one simulation run of a dam break problem, the Gaussian process emulator is able to approximate these heights in other dam break problems. By measuring the coefficient of determination, root mean squared error, and mean absolute error, we evaluate the accuracy of emulated free-surface heights in dam break problems where the shapes of water columns are distinct from the original shape at the initial condition. We alter the initial lengths in the x-direction and the initial heights in the z-direction remaining the same initial width in the y-direction. Consequently, in terms of the computational speed and the accuracy, it is demonstrated that we can adopt the Gaussian process emulator as a replacement of the EMPS simulator especially when free-surface flow analysis is repeatedly conducted with different initial conditions.

An algorithm to optimize texture shape under constant load conditions and a shape update equation of the design variables are proposed. The tribological properties are improved by machining grooves and holes, termed“ texture ”, on frictional surfaces that are lubricated by fluid. Improvement of tribological properties, such as the friction coefficient, is likely to lead to a reduction in energy loss and extension of machine life, resulting in major economic benefits. Because tribological properties depend on the shape of the texture, the focus of this study was on the dimensional shape of the texture. Most countermeasures to this problem involve size optimization rather than shape optimization. Conventionally, when the effect of the texture shape on the friction coefficient is evaluated experimentally, the load is kept constant. However, when the texture shape is changed as part of the analysis, the pressure field changes. The load, which is the integrated value of the pressure, also changes. Therefore, it is difficult to evaluate the friction coefficient accurately. At this time, the load can be kept constant by adjusting the basic oil film thickness, which is the distance between the frictional surfaces. This occurs naturally in real-world situations. In general, when the adjoint variable method is applied to determine the texture shape, constraint conditions are included in the Lagrange function. But, in this study, the constant load condition, i.e., the constraint condition, was simply added to keep the initial load, because it is difficult to calculate the gradient of the constraint condition with respect to the design variable. Considering the above, the purpose of this study was to find an appropriate oil film thickness for a texture by shape optimization and to reduce the friction coefficient by adding an algorithm that keeps the load constant by varying the basic oil film thickness. In addition, the shape update equation for the design variable was improved, and results based on the present method were compared with those based on the steepest descent and the conjugate gradient methods. This was achieved by replacing the interpretation of the update equation using the steepest descent method with a differential equation and by applying the differential to the step length of the design variable in the Taylor expansion equation of the design variable. By improving the shape update equation, a lower performance function was obtained. Texture shape optimization was performed by the adjoint variable method using the Reynolds equation as the governing equation. The performance function is defined by the frictional force, and the friction coefficient is optimized at the same time by keeping the load constant. FreeFEM++ was used to calculate the optimal shape.