An underexpanded jet is one of supersonic jets. The jet becomes underexpanded when the pressure ratio exceeds the critical value across the convergent nozzle discharging it. The jet is not uniform because of the presence of the expansion wave, the compression wave and the shock wave formed in it. In the jet boundary, many vortices are generated by the shearing stress between the supersonic flow of jet and still atmosphere. The interaction of vortices and jet is closely related to the high frequency sound radiating from such flow field. The high frequency sound is called Screech Tone. This Screech Tone has been studied extensively and its relationship to the vortex has been discussed. However, measurement methods are still not established for vortex moving in the vicinity of the jet boundary. In this study, the vortex moving in the vicinity of the jet was experimentally examined. The device consisting of a laser and an optical sensor was applied to measure the vortex. The vortex was measured at several points around the jet and frequency and time differences between points were investigated. The flow field was visualized using optical methods. And the high frequency sound emitted from the flow field is measured using a microphone.

An underexpanded jet perpendicularly impinging on a flat plate was experimentally studied in this study. The flow field radiates noise including screech, and its frequency and sound pressure are varied against the nozzle pressure ratio and the nozzle-plate spacing. A feedback loop in the flow field is known to describe the screech phenomena well, and disturbances moving along the jet boundary play an important role in the feedback loop. This study focused on contribution of the disturbance velocity to the sound pressure variation of the screech. The jet issued from a circular convergent nozzle with a diameter D. Sound pressure measurement and visualization were carried out at different nozzle-plate spacings l/D. The sound pressure measurement showed the screech due to the jet impingement was very weak in a certain range of l/D. The Schlieren photographs described the disturbance did not develop in such range. The disturbance velocity is one of the parameters for the screech phenomena. The disturbance velocity was estimated from frequency characteristics of the sound pressure. Characteristics of the disturbance velocity changed in the weak screech range of l/D, where there are disturbances with different velocities. This denotes the change of the disturbance velocity characteristics attenuated the sound pressure of the screech.

A jet radially spreading like a disc from a small gap between two circular tubes placed face to face with each other was experimentally analysed in this study. The jet is underexpanded and impinges on a cylindrical inner wall concentric with the circular tubes. Typical cellular structure appears in the jet and a detached shock wave forms near the wall. As this flow pattern is distributed over the circumferential direction, visualization by the shadowgraph method captured them as some ring-shaped shock waves. They indicate the nodes of cell sstructure and a detached shock. The diameter of the shock ring increased as the nozzle pressure ratio increased, and the ring of the node gradually approached that of the detached shock. At a certain nozzle pressure ratio, the detached shock ring jumped in the upstream cell. These shock rings oscillated and amplitude of the oscillation was varied with the pressure ratio. Strain of the wall surface was measured at nozzle pressure ratios ranging from 2.0 to 4.8. The wall surface is deformed by the pressure acting on the wall. The strain histories were analyzed using FFT, and multiple dominant frequencies were observed. These were classified by frequency as high, middle and low bands. The middle-band frequency increased gradually and dropped suddenly at certain nozzle pressure ratios, where the jump of the detached shock occurred. Furthermore, the middle-band frequency was in good agreement with the integer multiple of the dominant frequency in the low band. This means the low-band frequency is the fundamental frequency of the pressure acting on the wall. Therefore, the pressure oscillation on the wall has multiple modes and the mode change synchronizes with the flow pattern change such as the detached shock jump.

Analyzes of leachate from several landfill sites in Fukushima Prefecture have shown that most of the sites have radioactive cesium concentrations as low as a few becquerel, but in some disposal sites it showed values exceeding 10 Bq/kg. In order to clarify the behavior of seepage water flowing inside the waste disposal site under the conditions that do not contain radioactive materials, therefore, numerical simulations were conducted using a system that simply simulates a structure of landfill materials accumulated in layers in the vertical direction. The effect of porosity in landfills on seepage water behavior was investigated. When the porosity is large, there are large spaces in the landfill, and it takes time for seepage water to fill the spaces. As a result, it was found that the time required for seepage water to move to the bottom of the waste disposal site was longer than when the porosity was small. The prospect of qualitatively evaluating the tendency of movement behavior of seepage water by simulation was obtained.

It was said that the lower limit of the critical Reynolds number was 2300 as a result of experiments on flow in a circular pipe. It was investigated whether this could be reproduced by CFD. When the center part of fluid phase with a radius of 40% was continuously disturbed strongly, no turbulence occurred near the wall under Reynolds number 2300, and turbulence occurred near the wall over Reynolds number 2400. The results were consistent with the experimental results.

Particle migration in microfluidic devices has received considerable attention because of its applications in medicine, chemistry, and engineering, including diagnostics, particle separation, and cell sorting. In this study, we observe inertial migration of soft particles in a straight tube at Re ranging from 100 to 1000. Uniform double emulsions as soft particles are produced by a flow focusing technique in a micro fluidic device. Mechanical properties of the double emulsions were measured prior to migration experiments. Equilibrium radial positions of the double emulsions were measured at several Re. Re affected the distance from the tube center and numbers of the equilibrium radial position. In Re < 250 range, a single equilibrium radial position was observed, which gradually shifted toward the channel wall. Two equilibrium positions were observed at Re ranging from 300 to 500. In Re > 500 range, a single equilibrium radial position closer to the center was observed. These different flow regimes and transitions were influenced by particle deformability.

Flow characteristics of a hydrogel with different butylene glycol amounts are investigated by the stress-ramp and creep tests. We estimated the viscoelastic parameter and determined a creep compliance J_ramp using least-squares fitting experimental data measured by stress-ramp test before the yield. Our findings show that J_ramp varies with some deformation phases, which are in good agreement with the results of the creep test. From these results, the dashpot has an impact on the condition of the yield behavior.

The relationship between the modulations of mean velocity and wall-frictional drag in high Reynolds-number Taylor-Couette turbulence by surfactant additive is experimentally investigated. Due to the viscoelasticity of a dilute surfactant solution, mean azimuthal velocity shows a non-monotonic profile in the radial direction accompanied by the drag enhancement for the co-rotation of the cylinders while such profile is not observed for the counter-rotation where the drag reduction occurs. In the non-monotonic velocity profile, a flow structure like solid-body rotation exists in a bulk region. Thus, the cause of the drag enhancement can be explained by considering that an additional angular momentum transfer is required to sustain this solid-body-like rotational flow although the generation mechanism of the mean flow structure is to be explored.

Under pressure-oscillating field, a tiny air bubble which expands/contracts alternately shows various kinds of shape in viscoelastic fluid depending on the degree of pressure-oscillation. In this study, we evaluated effects of viscoelasticity and local flow on the tiny bubble shape under large pressure-oscillating field in the viscoelastic fluid such as CMC liquids. In the case of forming a unique shape, it is found that the elastic dominates at gas/liquid interface of a tiny bubble.

The motion of single bubbles rising in hydrophobically modified alkali-soluble emulsion polymer (HASE) solutions is experimentally examined. In this study, HASE material is neutralized with sodium carbonate (Na₂CO₃), and 1.46 wt% and 1.36 wt% HASE solutions with pH = 9 are prepared. These solutions are viscoelastic fluids with weak non-Newtonian properties. When the equivalent volume diameter of the bubble exceeds a certain threshold value, characteristic phenomena that the gas-liquid interface, which was in a steady state, suddenly fluctuates can be observed for both HASE solutions. Once the bubble interface fluctuates, the bubble rise trajectory is non-linearly changed, and the rise velocity is increased because the bubble rise motion is accelerated by fluctuation. Characteristic interfacial phenomena do not always arise, and various fluctuation patterns are observed. The bubble rise motion observed in this study are unique phenomena that occur at the interface only when using HASE material neutralized Na₂CO₃.

A two-dimensional shear flow of a viscoelastic fluid over a wavy wall was numerically analyzed by the finite difference method. The upper convected Maxwell (UCM) model was used as the constitutive equation, and the unsteady flow problem was solved numerically using the stream function-vorticity method. Variables are discretized using a staggered grid with uniform mesh width. The volume penalization method was used to represent the moving upper wall and the fixed lower wavy wall that induces a vorticity perturbation. The flow in an infinitely long channel is computed using periodic boundary conditions in the flow direction. The results show that the vorticity intensifies over the wavy surface with positive and negative vorticity stripes, at which extreme wall-normal velocity fluctuations are also observed. At relatively high Deborah numbers, the lower ends of these extreme fluctuation regions are close to the critical line associated with a change of type of the governing partial differential equations in a Couette flow. In contrast, the upper ends of the extreme fluctuation region are close to the critical layer of the shear flow, where the flow velocity coincides with the viscoelastic Alfvén wave speed.

Relaminarization process of an accelerated turbulent boundary layer was analyzed using neural network. The neural network model was constructed and trained from the streamwise fluctuating velocity data with the aid of a hot-wire anemometer. Keras, a deep learning library in python, was used to build the model. Two types of data were used for the training data: turbulent and laminar. From the created model, the predicted probability of turbulent or laminar flow was obtained for the streamwise and wall-normal fluctuating velocities in the whole region within the relaminarization process. From the predicted probabilities, the relaminarization process was examined. The predicted probability of turbulence for both the streamwise and wall-normal fluctuating velocities decreased downstream. During the relaminarization process, range of the predicted probability of turbulence differed between streamwise and wall-normal components. The difference in directional components between the training and the test data cause a difference in range and ambiguity of the predicted probability. The predicted probability was able to extract the characteristics of the turbulence within the minimum eddy. The distribution of the predicted probability of turbulence for the streamwise fluctuating velocity differed from that of Normalized compression distance of streamwise fluctuating velocity whose reference position was within turbulent region.

It is known that wall pressure fluctuations in a turbulent boundary layer are closely related to the turbulent flow structures. In this study, a two-rows 60ch microphone array has been developed to evaluate the wall pressure fluctuations and its spatial gradients. The measurements have been conducted in a turbulent boundary layer. The probability density functions (PDFs) of the wall pressure fluctuations exhibit intermittent characteristics and clear departure from the Gaussian distribution. In the PDFs of the wall pressure fluctuation gradient, the spanwise gradient indicates the symmetric profile as it is expected from the geometrical symmetry of the flow. On the other hand, although the streamwise gradient is considered to show the negative skewness because of the effect of the mean velocity gradient on the fluctuations of the spanwise vorticity flux, the PDFs of the present data show rather symmetric distributions.

An information analysis using permutation entropy of was applied to a relaminarizing sink flow turbulent boundary layer. The turbulent boundary layer was relaminarized by an acceleration because of the contraction of the flow area. It has been shown by the present authors that laminar-turbulent transition processes in the free-shear flow, the permutation entropy was effective for a measure of the transition progress. Velocity fluctuations were measured by a hot-wire anemometer. From the time series of the fluctuations, the permutation pattern of the data increase or decrease was discriminated, then the entropy based on the probability of the pattern was obtained. The permutation entropy did not decrease with the progress of the relaminarization. Therefore, it is hard to be an appropriate measure of the progress. It increased at the outer edge of the boundary layer. The two factors contributed it. One was an increase of number of permutation patterns, the other was an equalization of the permutation pattern probabilities. Both factors were affected by the intermittent fluctuations there. When the number of extracted data matched the Kolmogorov timescale, the permutation entropy took a large constant value near the wall in the boundary layer because of extremely large number of the extracted data.

This paper aims to introduce one of the parameters which contribute to heat transfer enhancement of graphene nanofluid to seek the optimal heat transfer system. Numerical analysis has been performed on the turbulent heat transfer of the nanofluid in a horizonal stainless steel tube which is subjected to a constant heat flux at its over surface. To model turbulent flow, the finite volume method with realizable k-εmodel and Wolfstein model is employed to calculate the momentum, continuity, energy conservation, and turbulent equation of continuous phase in two-dimension domains （Axially symmetric flow）. To assure its reliability, wall functions are compared with the result of numerical analysis, and it gained its reliability. Lagrangian two-phase model is employed to solve the mass, momentum, energy conservation of dispersed phase and model the physical interaction between two phases. In this model, phase interaction occurs to predict the influence of one phase upon the other phase within the two-phase simulation. This paper mainly focuses on this phase interaction as parameter which contributes to heat transfer enhancement since many other papers on this topic use a single-phase flow model and fail to focus on this. The results of the two-phase model are compared with those of the single-phase model and experimental results. Finally, some results reveal changed property, such as velocity distribution, turbulent kinetic energy distribution and it is found that this change influences on heat transfer enhancement. Moreover, the insight that nanoparticle gathering is the factor of this change is also obtained.

We have conducted direct numerical simulations for turbulent heat and momentum transfer in plane channel flow with fins which are streamwise-slit flat plates. The slit fins are introduced by the immersed boundary method. All simulations were conducted at the friction Reynolds number Re_τ = 200 and the Plandtl number Pr = 1. It is found that the slit fins lead to dissimilar heat transfer enhancement. As the thickness of the fins is increased, heat and momentum transfer are simultaneously enhanced, and significant dissimilarity between heat and momentum transfer is observed. The dissimilar heat transfer enhancement is attributed to large-scale roll structures.

In this study, we report first results of streamwise spectrum analysis at fully developed turbulent pipe flow up to Re_τ=20000 using laser Doppler velocimetry (LDV). The distribution of Pre-Multiplied Spectra (PMS) measured by LDV were nice agreement with previous ones measured by hot wire. We focus on very-large-scale-motion (VLSM) and large-scale-motion (LSM) which have different sizes with radius R scale away from the wall, and discuss the change of energydominant structures from the overlap region to the wake region. The dominant structure in the overlap region is VLSM, and it changes to LSM in the wake region. The peak value of PMS for VLSM; P_V-peak increases with Reynolds number with the relation of P_V-peak=0.137ln(Re_τ)-0.140. On the other hands, the peak value of PMS for LSM is independent with Reynolds number at wake region.

An oil-cooling method is often used for electric vehicle motors, in which oil is directly sprayed onto motor coil ends. Computational fluid dynamics (CFD) is utilized in the research and development processes of the oil-cooling system, but the accuracy has not been quantitatively verified because of the high Prandtl number flow with the free surface of the oil. Another reason is that the coil end, in which copper wires are bundled, has some complicated structures and many narrow gaps, making it difficult to confirm and comprehend the oil flow quantitatively. In this study, acrylic rectangular columns with gaps imitating oil-cooling motor coil end were created, and visualization experiments of how dropped oil flows on the side of columns and through the gaps were conducted. The simulations using the particle-based and finite volume methods were performed and compared with the experiments. On the side of the columns, the particle-base method well reproduced oil flow, while there was no flow in the finite volume method. In the gaps, both CFD methods reproduced the same qualitative flow. In the gap of the upper column near the dropping point, the flow simulated by both CFD methods were faster than those in the experiment.

A numerical simulation was performed in order to investigate the relationship of the pressure fluctuation period on the surface of a circular cylinder and the velocity fluctuation period of the circular cylinder wake in the Karman vortex shedding. The used calculation method is a vortex method. The lift coefficient was calculated in quest of the pressure coefficient on the surface of a circular cylinder, and it determined for the fluctuation period of the lift by time progress. The velocity of a wake was found with the velocity probes prepared behind the circular cylinder. The numerical simulation was performed about Reynolds number 500, 5000, and 50000. Those relationships were clarified by comparing the fluctuation period of a lift with the velocity fluctuation period of wake. The pressure fluctuation waveform was a waveform similar to a sign wave. The velocity fluctuation waveform was a peculiar waveform. As for waveform analysis, direct measurement of the wavelength was performed. As a result of analysis comparison, the period of lift fluctuation and the period of velocity fluctuation are not in agreement, when the most. However, it was found in the particular place that it is in agreement. It was proved that position picking of a hot wire probe carried out by previous study is proper.

In this study, steady drag force measurements and time-resolved schlieren visualization were conducted for subsonic compressible flows around a rectangular cylinder with a side length ratio from 0.5 to 1.2. The effect of the compressibility on the critical geometry was evaluated based on the drag coefficient using a low-density wind tunnel in the Mach number range from 0.1 to 0.6 at the Reynolds number of O(10³). In addition, the relationship between the change in the drag coefficient and the flow field was discussed. The side length ratio that the drag coefficient takes the maximum was confirmed to change with the Mach numbers. Remarkably, the peak drag coefficient at the Mach number of 0.6 appears at the side length ratio of 1.0, even though the side length ratio of the critical geometry at the incompressible flow is approximately 0.65. However, the sharp peak in the drag coefficient which is observed in the previous incompressible studies could not be reproduced even at the Mach number of 0.2. This is considered to be due to the lack of the spanwise length of the model or the Reynolds number effects. The schlieren images showed the influence of the Mach number on the location of the vortex formation, and the location moves downstream as the Mach number increases. The change in the flow fields behind rectangular cylinders causes a change in the back pressure, and it might result in a sudden change in the drag coefficient.

When In volleyball, the air flow around the ball causes the ball to change direction. Comparing various balls, the ball bends, shakes, and falls differently. This is because the surface finish of the ball, the groove depth of the panel, and the panel shape change not only the surface flow but also the AC pattern, resulting in different aerodynamic characteristics for each ball. Therefore, ball performance research can lead to the development of the game, as knowledge of the ball's characteristics can give rise to new tactics. The serve is said to be the most offensive play because the player can raise the toss at his or her own timing. In this study, we focused on the jump serve, which has a high scoring rate on the serve, and obtained data on the flight trajectory, differences in aerodynamic characteristics, and force applied to the ball from groove measurements, ejection experiments, and fluid force measurements to obtain data for improving technique and coaching ability. In addition, to clarify the air flow around the ball, the separation point was visualized by 2D-PIV measurement. From the results obtained, it was confirmed that the ball lands in front of the surface as the number of rotations increases, but that it does not land in the court unless more rotations are applied depending on the ball, indicating that the surface and panel shapes have a significant impact on the flying trajectory.

Volleyball is a sport that begins with the serve, so effective service is essential to winning matches. The trajectory of the ball changes in a complex manner under the influence of fluid forces that depend on the speed, number of revolutions, and shape of the frame. Therefore, the purpose of this study was to understand the aerodynamic characteristics of the ball and to propose an ideal serving method by measuring fluid forces, measuring flight trajectory, and analyzing particle image velocimetry. In wind tunnel experiments, the fluid forces on the ball were measured at wind speeds ranging from 4 to 28 m/s. The results showed that the fluid force on the ball strongly depends on the type of ball and the orientation of the panel. In the flight trajectory measurement, the flight trajectory of the ball was measured by a high-speed camera and its speed was controlled by an ejector. The landing position of the ball was found to be strongly influenced by the shape of the panel; PIV measurements revealed the detachment of the ball. The air hole also moved the position of the detachment backward. The influence of panel shape shown by the hydrodynamic force measurement was consistent with the results of the trajectory analysis, revealing the importance of panel shape in serving.

The objective of this study is to apply the Global Luminescent Oil-Film Skin-Friction (GLOF) meter, which optically observes the skin friction vectors on the object surface, to reveal the fluid structure around the wheelhouse. The image analysis results show characteristic longitudinal vortices on the side surface of the front wheel and the wheelhouse. In contrast, the separation line at the rear end of the side near the rear wheelhouse is curved significantly upstream, and the separation area expands in the lower half of the rear end, probably due to the suction flow generated between the wheel and the wheelhouse.

Dimples on the surface of a golf ball are one example of a superior technique for reducing drag from flow. In this study, multiple grooves were processed on the back of the wing model, and the drag reduction effect was verified by wind tunnel experiments and two-dimensional CFD analysis. Experimental results confirmed a significant drag reduction effect. In addition, it was found that the value of the angle of attack that generates the maximum lift is shifted to the high angle of attack side by about 5° compared to the non-machined wing. In the CFD analysis, three representative turbulence models were used to reproduce the experimental results. Although the qualitative tendency of the angle-of-attack dependence of drag and lift could be reproduced, a high-precision solution showing the quantitative effect of grooving could not be obtained.

In this study, the mechanism of heat transfer enhancement near the reattachment point behind a backward facing step is experimentally investigated at a Reynolds number of 5400. The unsteady fluctuations of the velocity and temperature were simultaneously measured by using 2 dimensions and 3 components (2D-3C) Particle Image Velocimetry (PIV) at x/H = 4.5 and the high-speed infrared (IR) thermograph on the heated wall. In order to discuss the thermal and flow structure near the reattachment point, 3 components of the velocity and the heat transfer coefficient were calculated. The spatial distribution of the flow and thermal fluctuations was reconstructed by using proper orthogonal decomposition (POD) analysis. As a result, the characteristic structure of the flow and thermal fluctuations appeared for the lower number of POD modes. These downwash and upwash flow in the transverse direction corresponded to the increasing and decreasing of the heat transfer coefficients at x/H of 4.5.

The flow and heat transfer characteristics were investigated by numerically simulating backstep flow in a three-dimensional duct with periodic disturbance to the inlet velocity at moderate Reynolds numbers (Re = 625, 700, and 1000). The periodic disturbance was applied to the approximate equation for a well-developed flow by varying the relative amplitude and forcing frequency. The results show that the reattachment position moves upstream as the disturbance increases. Furthermore, as the Reynolds number increased, the amount of change in the reattachment position due to the disturbance became smaller. This change in the reattachment position was more dependent on the forcing frequency than on the relative amplitude, because the meander spacing of the mainstream narrowed as the forcing frequency increased. The spatiotemporally averaged Nusselt number at each Reynolds number increased by 27.2% when a disturbance was applied, compared to the case where no disturbance was applied. This is due to the fact that the higher the disturbance, the more active the heat exchange becomes not only in the flow direction but also in the height direction, resulting in a higher rate of rise. As with the reattachment position, the amount of change became smaller as the Reynolds number increased. These results suggest that the use of the disturbance can be expected to enhance heat transfer at the lower wall.

This paper describes the flow characteristics of air injected from a slit nozzle in a cylindrical container. The numerical simulation was performed by the large-eddy simulation (LES) using ANSYS Fluent 19.0 based on the finite-volume method. The non-dimensional nozzle offset distance was set to y_n/D = 0.4, where D is the diameter of the cylindrical container. The Reynolds number of the plane jet flow was settled at 2,000. The velocity profiles by the numerical simulation agreed with the experimental results. The jet reached near the bottom of container, and the jet flowed out from the left side of container. Although the flow in the cylindrical container showed a three-dimensional turbulent flow structure, the mean velocity field was generated symmetrically with respect to the central cross-section of the container perpendicular to the slit nozzle. The large recirculating flow around the z-axis occurred near the central cross-section of the container by the jet. In addition, the weak recirculated flows around the y-axis were observed near the upper side wall in the container.

Two flat plates wake without reverse flow is very unstable and is affected by extremal disturbances.In this study，visualization of two flat plates wake was conducted to clarify the effect of periodic disturbances.The periodic disturbance is generated by vibrating the rubber wall. It was found that there are various flow patterns in the vortex formation process and the vortex breakdown process，and these flow patterns change greatly depending on the disturbance frequency.

We investigated the phase difference and frequency ratio effect on the thrust and lift coefficient of oscillating foil in a periodic flow. The periodic flow is the freestream oscillating in the streamwise direction. The frequency ratio is the ratio of foil frequency to freestream one. The thrust and lift coefficients were obtained using OpenFOAM v8 under the conditions of Reynolds number of 4066 and frequency ratio of 1, 2, and 3. We found that the thrust increases with increasing frequency ratio. Furthermore, the effect of phase difference on lift coefficient is relatively small at the frequency ratio of 2 and 3 under the present condition. We also found the correlation between forces and wake velocity.

A two-dimensional unsteady numerical analysis of a corrugated airfoil was performed to clarify the variation of flow around the airfoil as a function of leading-edge angle. A corrugated airfoil with five crests was made to follow the outline of NACA4412, and three types of airfoils with leading edge angles of 25° and 35° were selected as the target airfoils. The Reynolds number was set to 1.0 × 10⁴. Comparison of NACA4412 and the three types of corrugated blades showed that the lift coefficient was highest for NACA4412 at angles of attack of 4° or less and was significant for the corrugated blades at angles of attack higher than that. On the other hand, the drag coefficient was the lowest for NACA4412 at all angles of attack. Comparison of corrugated blades with different leading-edge angles revealed that at low angles of attack of 6° or less, the circulating flow in the first valley has a greater effect on aerodynamic characteristics than that in the second valley. On the other hand, at high angles of attack of 10° or more, flow cancellation was observed from the leading edge to the first crest of the upper surface of the wing, which is one of the reasons for the improved wing characteristics.

Butterfly generates unsteady dynamic fluid force with a pair of vortex rings. In order to elucidate the flight mechanism of butterflies, it is necessary not only to clarify the vortex rings formed around the wings but also elucidate the characteristics of the unsteady fluid force generated by the flapping motion. By applying the momentum theory to the vortex rings formed during flapping motion, it is possible to estimate the force generated by flapping motion. In this study, we reconstruct a pair of vortex rings formed by flapping motion based on a small number of visualization results and elucidate dynamic fluid force characteristics. A part of estimation result of Lift estimated by the rough shape vortex rings generated by downstroke and upstroke do not match with the direct measurement result. But not only the peak value of estimation result of Lift is near with the direct measurement result quantitatively but also the value of estimation result between upstroke is a small negative as well as the direct measurement result. Although there is a phase difference between the estimation result and direct measurement result, there is a qualitative and quantitative agreement between estimation results and direct measurement result. From those results, butterfly generate the unsteady dynamic Lift between downstroke, especially at butterfly spread wing maximum (t/T = 0.40). After that, the unsteady dynamic Lift decrease and become 0 at upstroke.

The Fluidic Oscillator can permanently generate a high-velocity oscillating jet (Sweeping jet) and give a large fluctuation to the flow field without driving parts. Therefore, it is attracting attention as a new active flow control device. However, the flow characteristics that directly contribute to flow separation control have not been sufficiently clarified, and their optimization and practical application have not progressed. Therefore, in order to clarify the details of the flow characteristics that contribute to the flow separation control, it is necessary to clarify the flow structure generated by the sweeping jet that blows into the main flow from the wall surface．In this study, we clarify the growth process of the vortex structure generated by the interaction of the sweeping jet, which blows out from the wall at 30 degrees to the main flow, by numerical simulation. It is conducted to understand the complex and large-scale flow structure and its generation process generated by a sweeping jet in the main flow. The vortex structure forms one flow tube at z/d < 4.5 and splits into two at z/d > 4.5. Shear layer vortices with the same frequency as the oscillation frequency are generated in the stream tube, and after splitting into two, it is generated alternately to the left and right. After that, at z/d > 6.0, the shear layer vortex begins to collapse due to wall interference, and at z/d > 7.5 it transforms into discrete vortices.

A jet in crossflow formed by the interaction between the jet and a main flow is a typical unsteady flow. Jet in crossflow have been studied using experimental and numerical approaches. In particular, a sweeping jet in crossflow has recently attracted attention from the viewpoint of active flow separation control. A sweeping jet, which is an oscillating jet with high frequency and large amplitude, energy can be injected widely into the flow separation area. However, the flow fields formed by the interaction between the sweeping jet and the main flow and their flow mechanisms are poorly understood, it has not been put to practical use. Furthermore, the flow structure produced by the interaction between a sweeping jet and a main flow with different behaviors is very complex and difficult to understand. Therefore, it is extremely important to elucidate the flow structure generated by the interaction between a sweeping jet and a main flow. In this study, the detailed vortex structure generated by the interaction between the sweeping jet and the main flow was quantitatively examined using stereo PIV. The vortex structure behaves differently depending on the velocity ratio. The larger velocity ratio, a sweeping jet extends to the wake and develops near the wall.

Side jets are generated when the jet density is smaller than that of the ambient gas. It has been suggested that the side jets are related to self-excited oscillations due to the local absolute instability that occurs when the density ratio is less than 0.7, and vorticity generation due to baroclinic torque caused by the cross product of density gradient and pressure gradient. In addition, it has been suggested that a side jet is also formed by a pair of streamwise vortices with different rotation directions. First of all, in order to confirm previous hypotheses, we have tried the formation of side jets in a helium gas jet by a numerical simulation using ANSYS Fluent. As a result, it is shown that a side jet is formed between a streamwise vortex pair.

The purpose of this study is to control the jet diffusion by using the deforming nozzle. The shape of polypropylene nozzle can change from square shape into the cruciform shape or the octagonal shape smoothly. The velocity measurement of the jet flow was performed using X-type hot wire sensors and the constant temperature anemometer. The mean flow properties of the jet flow, such as the velocity distribution and the half value width were obtained. Moreover, to catch the flow feature of the jet near the nozzle the flow visualization was conducted by using the Laser Light Sheet and the high speed video camera. As a result, it has been found that the diffusion of the jet issuing from the deforming nozzle is promoted, comparing with the case of the square nozzle jet

The effects of velocity fluctuations of upper flow on turbulent structure of atmospheric boundary layer were experimentally investigated. The atmospheric boundary layer was reproduced in a wind tunnel. In order to produce various velocity fluctuations in the upper flow, three kinds of turbulence grids were set to the entrance of the test section. The streamwise and spanwise velocities were simultaneously measured by using hot-wires anemometer. Turbulence statistics were estimated. The vertical profiles of Reynolds shear stress, Ruw, for M25 and M50 decrease as z/L increase, on the other hands, those for M75 increase. The ratio of the integral length scale, λ_(GT)/λ_(ABT), for M75 were higher than those for M25, M50 at 0.33≦z/L ≦0.67 in the upper region of the atmospheric boundary layer. The joint probability density functions of u and w for M75 show the strong negative correlation in the upper region of the atmospheric boundary layer. It is suggested that the effects on the turbulent structure of the atmospheric boundary layer depends on the scale of velocity fluctuations of the upper flow.

A Kalman vortex street is generated in the wake of a circular cylinder placed in the flow. Visualization experiments have shown that the formation process of a Karman vortex street begins with the separated of a velocity boundary layer from the surface of the cylinder, the growth of minute disturbances in the separated velocity shear layer, and the transition to turbulence, followed by the gathering of multiple disturbances to form a vortex about the size of the cylinder's diameter. In this study, a hot-wire anemometer, which has a proven track record in measuring velocity boundary layers such as flat plates, is used to measure the velocity in the boundary layer. The hot-wire anemometer used was a self-made one, with a C-shaped sensor section to avoid disturbing the flow, and the sensor was inserted from the direction perpendicular to the flow. As a result, the formation process of the Karman vortex street in this case began to be disturbed by instability when the separated shear layer was about x/D=0.5. At x/D=2.5, a disturbance with a Strouhal number of about 0.2, which corresponds to the Kalman vortex street, occurred and grew downstream. The results show that the growth scenario is similar to that reported in previous visualization experiments.

The large flow separation in a steam control valve is a problematic phenomenon because it causes unstable conditions such as oscillation and the noise. However, there are many challenges in predicting flow separation by CFD simulation, and it is difficult to estimate the conditions under which unstable behavior occurs. Therefore, the unsteady simulation using the correction function which works to correct the turbulence source term in the separated region was performed in this research. Also, the air test using the quasi-3D model of the steam control valve was conducted in order to quantitatively evaluate the CFD simulation result. As a result, the simulation result shows the good agreement with the test result about the oil flow and the pressure amplitude. It was suggested the simulation method using the correction function is effective for the prediction of the flow separation.

Separation control by plasma actuator is performed in flow around car front bumper to improve electric economy by reducing drag. The evaluation of flow separation control was performed using numerical simulation by OpenFOAM. A front bumper corner model was made in order to reproduce the actual model used in the wind tunnel experiments. Plasma actuator was installed in two different ways, common flow up type (CFU) and common flow down type (CFD) on the surface of front bumper corner model. The effect of separation control in PA both installations was analyzed in the distributions of streamwise velocity u in the various X-Y planes. The vorticity distribution in Y-Z plane showed the same tendency as the experimental results that the streamwise vortices created by the induced flow setting decreased as the flow downstream. From the results of Q-criterion, CFD case archived the development of streamwise vortices for the downstream direction, more than the case of in CFU. It provides insight into the understanding of the phenomenon of the separation control effect. In the evaluation using momentum thickness, CFD had the greatest separation control effect in the central area.

Dual bell nozzle has a structure that combines two bell nozzles with different expansion ratios, consisting of a base nozzle with a low expansion ratio and an extension nozzle with a high expansion ratio. In this paper, axisymmetric two-dimensional unsteady numerical calculations were performed using two types of dual bell nozzles. There was a difference whether there is a step at inflection point of the nozzle, and the conditions for controlling separation inside the nozzle and creating a stable flow were investigated. The step size is 0.01 times the throat radius. Grid points of the nozzle were 800 in the axial direction and 200 in the radial direction. The static pressure at the outlet was set to 3kPa, and the total pressure at the inlet was linearly increased from 3kPa to 600kPa in 0.3 second. Injection of nozzle inflection point with step were set to no injection, 50 kPa and 100 kPa. The working fluid was ideal gas air. The calculation results were arranged by the nozzle pressure ratio (NPR), and the thrust coefficient and the wall pressure were compared. As a result, it was found that adding a step to the inflection point of the dual bell nozzle has little effect on the nozzle performance. In addition, the separation point inside the nozzle could be controlled by the injection from the nozzle inflection point.

In the initial region of a round jet, there are periodic vortex rings, which merge to form a large-scale vortex ring that subsequently collapses and becomes turbulent. This study focuses on the merging of vortex rings in a round jet, and aims to clarify the conditions for the regular merging of vortex rings. In order to investigate the merging process of vortex rings by acoustic excitation, visualization experiments were conducted to the jet with various Reynolds numbers by applying disturbances. As a result, it is found that there is the range of Strouhal number in which the merging of 4 vortex rings occurs. The reason is discussed and it is found there is suitable distance between vortex rings.

The time-resolved three-dimensional velocity measurement of an underexpanded jet is performed using time-resolved microphone and non-time-resolved particle image velocimetry (PIV) measurements. Flapping screech jets at the nozzle pressure ratio (NPR) of 2.97 was measured and analyzed using the proposed approach. It was found that the dominant azimuthal mode at the present case is flapping mode. The screech tones are strongly associated with the first two proper orthogonal decomposition modes of the dominant azimuthal modes of the microphone data. The azimuthal characteristics of these modes are helical mode, and the rotation direction is opposite. These findings are qualitatively consistent with the previous results. The flapping structure rotates over a wide range of temporal scales, with random changes in direction of rotation.

Water with free surface inside the water tank was pressurized and ejected as a water jet from nozzle with abrupt contraction flow path attached to the bottom of the tank. When the height of the free surface of water makes low into some height from the bottom of the tank, a peculiar air column is generated in the nozzle, a phenomenon occurs in which the droplets are periodically aligned outward in the radial direction and are rapidly splashed from the jet due to the pressure difference from the atmospheric pressure. In order to investigate how water swirling near the nozzle inlet affects this droplet splashing phenomenon, The bottom of the tank was designed to affect the flow of water, and the flow of water was analyzed using visualization photography. As a result, it was found that the circumferential velocity has a great influence on the droplet splashing phenomenon under low pressure. When the circumferential velocity increases, it affects the shape of the air column in the large tube of nozzle, and the shape makes it easier for air bubbles to be generated in the small tube of nozzle. It was found that these processes facilitated the occurrence of the droplet splashing phenomenon.

One of the problems with multirotors is that when flying near a wall, such as during infrastructure inspections, the companion flow passing through the rotor blades is blown back by the ground or wall surface, causing instability in the flight attitude of the aircraft⁽¹⁾⁽²⁾. As a previous study on this phenomenon, as part of the development of a quadrotor capable of autonomous flight even in complex narrow spaces, we focused on a small quadrotor with four rotors among multi-rotors and investigated the effect on aerodynamic characteristics when hovering near a wall, and found that even at heights outside the ground effect, the direction and position of sidewalls Ground effect The results showed that even at heights outside the ground effect, the direction and position of the sidewalls increased the thrust of the wall-side rotor due to the blowback effect, causing the aircraft to tilt (roll) at some positions. In this study, to clarify the effect of the wake interference of quad rotor on the induced flow, we investigated the tip vortex trajectry and the induced flow velocity in the high thrust and rolling region(posture tilted backward against the wall) using a high-speed camera, a hot-wire anemometer, respectively. We show some of reasons to increase the thurust of wall side roters.

Drones are used in various situations, and they should be operated with a high level of safety. Drones should be stable under disturbances such as sudden strong winds and turbulence. Although the effect of aerodynamic disturbance is usually considered as a simplified wind model, it is desirable to take the characteristics of turbulence into account. For this purpose, we will clarify the effect of wind disturbance generated by active grid turbulence on the position and attitude of the hovering quad-rotor drone. The power spectrum of the velocity fluctuation generated by the active grid exhibits its slope of -5/3. With the present flight system consisting of an external depth camera and a quad-rotor drone with Blackbox functionality, the drone can hover at a target position in the wind tunnel and the time-series flight data can be obtained. It is found that the roll control signal is negatively correlated with the acceleration during hovering in the wind.

In this study, the attenuation and frequency characteristics of the Karman vortex passing through the wire mesh was investigated. This experiment was conducted by generating the Karman vortex from a cylinder in an acrylic duct of the blowdown type wind tunnel and passing through the wire mesh at the duct exit. The cylinder with a diameter of 6mm was used to generate the Karman vortex, and the Reynolds number was 3000 based on the flow velocity toward the cylinder and the cylinder diameter. The wake of the Karman vortex passing through the wire mesh was measured by hot-wire anemometer, and was visualized by smoke wire method. As a result, it was found that the smaller the wire diameter and open area ratio of the wire mesh, the velocity and turbulence intensity distributions ware more uniform. And, the dominant frequency of the Karman vortex was confirmed that even in the wire mesh that showed uniformity and in the wire mesh that appeared to have collapsed in the photograph by frequency analysis.