Cationic surfactants dissolved with counter-ion suppliers in water form drag reducing worm-like micelles. The drag reducing micelles deform vortices in turbulent flow, which decrease turbulent intensity. The relationship between a characteristic time scale in turbulent flow and that of surfactant solutions such as relaxation time is a key to understand the phenomena. Therefore, we observed how the relaxation time of each drag reducing surfactant solution affected the vortex deformation in a two-dimensional (2D) flow. Subsequently, the effects of the relaxation time on turbulent statistics was discussed.

The turbulent friction drag of Newtonian fluid flow can be reduced dramatically by adding small amount of polymers or surfactants. Such drag-reducing (DR) effect of these non-Newtonian solutions has been utilized in some practical applications such as oil pipeline, district heating and cooling systems, etc. While the DR effect has been studied well in simple wall turbulence, such as turbulent plane channel and straight pipe flows, few studies have focused on the flows of these non-Newtonian solutions in more complex geometries where the mean flow is three dimensional and massive separations and/or significant secondary motions take place. In the present study, we experimentally investigate a surfactant-solution pipe flow over a semicircular orifice. A 500 ppm solution of a commercial drag-reducing additive (LSP-01M) was used as the working fluid, and we at first confirmed the DR effect of this solution in a turbulent straight pipe flow. Then, the flow in a pipe with a semicircular orifice was investigated based on stereoscopic PIV measurements. The focus is particularly placed on how the secondary flow patterns and turbulence intensities downstream the orifice is affected by the addition of surfactant in comparison with the Newtonian fluid (water) case.

Hydraulic jump of viscoplastic fluids in a two-dimensional open channel was numerically analyzed. The numerical simulation was performed with an open-source CFD software OpenFOAM. The viscoplasticity is described using Herschel-Bulkley model as a constitutive equation. Flows in a two-dimensional open channel with a dam. In viscoplastic flows, the magnitude of stress exceeds the yield stress and the apparent viscosity dramatically decreases with increasing the flow rate. As a result, the height of hydraulic jump notably appears at high flow rates.

The fiber orientation of a cellulose nanofiber (CNF) suspension in a plane channel with an abrupt contraction flow was investigated using flow-induced birefringence (FIB) measurement. The velocity profiles were also measured in order to associate the flow field with the fiber orientation field. In this study, 4:1 abrupt contraction channels with contraction ratios of 2, 4 and 8, and aspect ratios of 1 and 2 were tested in the experiments. 0.5 wt% CNF was used as a test fluid and how the fiber orientation developed near the centerline after the abrupt contraction was examined. As a result, the birefringence increased dramatically near the contraction and the birefringence decreased gradually after the abrupt contraction and reached a constant value. Furthermore, it was found that the distance where the birefringence reaches a constant value does not affect by contraction ratio, aspect ratio and flow rate.

This study focuses on the parameters of the constitutive equation proposed by Lipscomb et al. and the fiber orientation behavior predicted by the F-T-S model which is added a slip factor to Folgar-Tucker isotropic rotary diffusion (IRD) model. For comparing numerical and experimental data, the step shear experiments are conducted to fiber suspension. The fiber orientation was visualized by a microscope attached to a rotational rheometer. The visualization can observe the azimuth angle of fibers, but it lacks the polar angle. Hence, we converted the orientation tensor to a spatial two-dimensional orientation tensor, i.e. the polar angle is eliminated. The model prediction fitted to the experimental data to determine the parameter values. Then, the values were compared with some equations suggested by the other researchers. It is found that the predicted orientation tensor is almost matched to experimental ones, but the mismatch on shear viscosity is enlarged with the increasing fiber volume fraction of suspensions. The model parameter values depend on the material coefficient as the fiber volume fraction. One parameter (interaction parameter of the F-T-S model is agreed with the equation by Phan-Thien et al. However, the other parameters disagreed with some equations.

The dispersed particles make alignments in the sheared direction under unsteady shear flows when the fluid relaxation time is sufficiently long. The alignment changes the effective viscoelasticity because a corded-like pattern of particles functions as a macro-spring effect and reduction of particle interactions. Those experimental findings regarding the effective viscoelasticity of non-Newtonian fluids with spherical particles are found by means of ultrasonic spinning rheometry. Further numerical tests using a simple toy model assuming dispersed particles combined by spring forces considering yield stresses were conducted to clarify factors caused by the particle alignment under unsteady shear flows.

Time variation of the effective viscosity in the separating water-in-oil mixture was evaluated by ultrasonic spinning rheometry (USR) combining ultrasonic velocity profiler (UVP) and equation of motion. We obtained the results similar to the theoretical formula that the effective viscosity and pseudoplasticity decrease as the diameter and volume fraction of the water droplets decrease with time. On the other hand, the effective viscosity was estimated to be higher than the value estimated by theory at the initial time. One of the considerable factors is the increase in the effective volume fraction due to the accumulation of the droplets, and this requires further experiments for the evidence.

A system in which oil phase droplets containing water phase droplets are dispersed in the water phase is called W/O/W double emulsions (DE). DEs are produced with a variety of materials, which is able to contain beneficial components. Therefore, DEs are used in many industrial and medical appreciation. Several types of flow-focusing devices were proposed to produce uniform DEs in size. The flow-focusing techniques are promising, however, not many studies focused on stability of the DEs production in terms of flow characteristics. In this study, we have categorized flow regimes that produce uniform DEs on Capillary number mapping. The uniform DEs produced in the device were injected in a capillary tube to observe its migration behavior in a flow. Flow rates affect the stable migration of DEs in the flow.

Turbulent/non-turbulent intermittent structure in the outer layer was investigated experimentally with the aid of momentum transport equation with 5 component decomposition of the fluctuating velocity components. Considerations can be connected to significantly reduce of wake strength in the law of the wake and turbulent diffusion of the kinematic energy equation. Evaluate of some leading components of the momentum transport around turbulent/non-turbulent interface provide motions contributing to entrainment and pressure gradient effect.

In this study, the effect of the shear layer existing between large-scale motions (LSMs) on bursting phenomenon in turbulent boundary layer (TBL) was investigated by wind tunnel experiment. The TBL was generated on the flat plate made of Bakelite with a zero pressure gradient. The Reynolds number based on friction velocity given by Re_τ = u_τδ/ν is about 1260. A self-made multi-point hot-wire probe consisting of a hot-wire Rake and a single I-type hot-wire probe was adopted. The hot-wire Rake was used to extract the LSMs from the fluctuation velocity distribution in the log-region in TBL. And the shear layer was considered to exist between the positive large-scale motion (pLSM) and the negative large-scale motion (nLSM). The bursting phenomenon was detected by VITA technique from the fluctuation velocity measured by the single I-type hot-wire probe installed under the center of the Rake in the buffer layer in the TBL. To evaluate the effect of the shear layer on bursting phenomenon, the bursting frequency under several different conditions was calculated. It was found that the bursting frequency has increased 20% with the existence of the shear layer. A further investigation showed that these increased bursting phenomena were related to the leg of the hairpin vortex.

Experimental study on the effect of LEBU device was made for a two-dimensional turbulent channel flow. The local wall shear stress, mean velocity and turbulent intensity profiles were measured at the downstream positions of LEBU device located at the center of the channel width. The wall shear stress measured by sublayer plate is reduced due to LEBU device by about 10% and the logarithmic mean velocity seemes to be shifted upward to the standard profile for drag reduction case.

In this study, localized structures appearing in transition to turbulence are numerically investigated in rectangularduct flow. It is observed that streamwise localized turbulent puffs are skewed for the specific values of the Reynolds number and the aspect ratio. The organized cross flow arises in the averaged flow field as a consequence of the symmetry breaking of turbulent puffs with respect to the duct midplane for the critical aspect ratio. The streamwise vorticity fields are examined to find out that there emerges clear skew in the arrangement of vortical structures if the aspect ratio exceeds the critical value.

To understand turbulent flows over permeable roughness, PIV measurements are performed for turbulent channel flows over rib-roughened porous media. The porous ribs are mounted on the porous media with the same spacing w as the rib height k, w/k=1. Flows over impermeable roughness cases are also measured for comparison. It is confirmed that flow patterns below the ribs are changed by the permeability. Between the impermeable roughness elements, the recirculation bubbles are observed being isolated from the core flow. They move downstream and downward as the permeability increases and those in the highest permeability case disappear. This tendency is qualitatively the same as that for the previously reported permeable k-type roughness case, w/k=9. The magnitude of turbulent kinetic energy for d-type roughness becomes small as the permeability increases, while the opposite tendency is observed in k-type roughness cases. Further discussions on turbulence generated by the roughness are carried out by the two-point correlation analysis of the wall-normal fluctuating velocity. Although turbulence above the impermeable d-type roughness is isolated from the recirculation region below the ribs, it penetrates strongly into the square cavity between the ribs in the permeable roughness case. It is also seen that turbulence generated by permeable ribs tends to expand isotropically regardless of the roughness geometry.

The propagation of the pressure fluctuations in turbulent channel flows is analyzed using direct numerical simulation up to Reynolds number of 2000, based on the channel half-depth h, friction velocity u^τ, and kinematic viscosity ν. It was found that the convection velocity of the pressure field is consistent with the local mean velocity above the wall-normal height of y⁺ ≈ 20 (y⁺ = yu_τ/ν). Below y⁺ ≈ 20, the convection velocity is invariant and equals the local mean velocity at y⁺ ≈ 20.

In this study, direct numerical simulations of turbulent channel flow with a porous wall have been performed to investigate the effect of the permeability on the heat and momentum transfer. The porous wall is anisotropic, consisting of many small holes in the wall-normal direction, which are introduced by the immersed boundary method. The simulation has been carried out under the constant bulk mean velocity and bulk mean temperature at the bulk Reynolds number Re_b = 3550 and the Prandtl number Pr = 1. There are two cases in this study. The one is a flow with a chamber outside of the porous wall, and the other one has no chamber. By arranging the chamber, the heat transfer is significantly enhanced compared to the no-chamber case. In this case, we have found that large-scale spanwise roll structures, leading to the dissimilar heat transfer enhancement, emerge due to the generation of blowing and suction on the porous wall.

Lagrangian trajectories of small particles in a fully developed turbulent state are studied in a rectangular duct based on thermal counterflow. Small particles made of solid hydrogen are visualized by high-speed camera and their trajectories are recorded. The liquid helium temperature was set to two temperatures, 1.90 K and 2.00 K, and the applied heat flux was 800 W/m2. In order to studying the influence of the wall on the flow field, a total of five visualized cross sections were set up at different distances from the wall. In this research, the original algorithm of image processing and particle tracking was developed. At each visualized cross section, Particle Tracking Velocimetry (PTV) was used to measure the instantaneous velocity in fluids using tracer particle. And we statistics the velocity distribution of the tracer particles to study the wall influence on the flow field.

Direct numerical simulation has been performed for turbulent plane Couette flow to achieve the unprecedented state with stronger turbulence than a usual turbulent state found by Watanabe et al. (2015) by introducing spatiotemporally localized body force mimicking blowing and suction control in experiments. It is confirmed that the introduced body force leads to the unprecedented state sustaining even after forcing, significantly enhancing heat and momentum transfer. The present numerical simulation suggests the experimental achievement of the unprecedented turbulence for novel strategy of heat transfer enhancement.

The stably-stratified turbulence with mean shear is investigated by direct numerical simulation, and the applicability of a rapid distortion theory (RDT) is examined. In this turbulence, the effects of stratification and shear are most significant at large scales, but the stratification is effective down to the Ozmidov scale, and the mean-shear is effective down to the Corrsin scale. Since the stratification and mean shear have linear effects on the governing equations, the RDT, which is a linear theory, applies well down to the Ozmidov or Corrsin scale. The DNS results also show that the mean shear excites the large-scale motion and suppresses the small-scale vertical motion, leading to a better applicability of RDT even at small scales.

In previous research, it has been reported that wing with sinusoidal leading edge has the effectiveness of separation control. The wing models used in previous studies were, however, limited to single wing and it wasn’t practical for actual devices with multiple wings. Then, we studied the effect of sinusoidal leading edge in the case of tandem arranged wings. To investigate the effect of wake from forward wing, we measured the fluid dynamic force on backward wing with load cells and visualized the flow around backward wing by the smoke wire technique. The experiment was conducted with a wind tunnel. The performance of the backward wing with linear leading edge was improved when the vertical gap between forward wing and backward wing, y/c, was in the range of -0.125≦y/c≦0.25. From visualization results, it was found that the improvement was caused because the wake from forward wing controlled the stall of backward wing. On the other hand, the backward wing with sinusoidal leading edge maintained the effectiveness of stall control regardless of vertical gap, presence or absence of forward wing. Therefore, the wing with sinusoidal leading edge is effective as separation control technique for devises having multiple wings like wind power generator.

We investigate the effects of pressure gradient by a periodic flow on the separation and lift of an oscillating wing. The periodic flow is a freestream velocity changing sinusoidally in the mainstream direction. We conduct an experiment on the oscillating wing using a water tunnel at Reynolds number 3800. Velocity fields are obtained by particle image velocimetry (PIV), and lift coefficient acting on the wing is directly measured using a load cell. The lift coefficient is greater in freestream acceleration than its deceleration. The velocity data from PIV shows that the separated flow becomes attached due to the negative pressure gradient in the range of freestream acceleration, and the evolution of separated flow occurs near the trailing edge due to the positive pressure gradient in the range of freestream deceleration. A numerical simulation is carried out to investigate the velocity fields on the wing surface in more detail. It is found that the flow separation occurs near the leading edge as well as the trailing edge during freestream deceleration. These separated flows probably affect the lift coefficient of the oscillating wing.

The purpose of this study is to clarify the effects of periodic disturbance with amplitude or frequency modulation on the turbulence characteristics in the wake of flat parallel plates. The periodicity of the wake without reverse flow like the flat plate wake is sensitive to disturbance. We are trying to control the turbulence characteristics of the wake using various disturbance method. In this study, the amplitude or frequency of the periodic disturbance signal based on the prominent frequency in the wake was modulated. As a result, it was found that the spectrum distribution and turbulence characteristics of the wake change on the modulation frequency.

In the present study we investigate the relationship between various aerodynamic characteristics and attack angle α for FP (a flat plate), NACA0015 and iNACA0015 (the NACA0015 placed back to front) by numerical analysis and water-tank experiment at Re = 100 – 800. Then, we reveal the effects of α upon various aerodynamic characteristics such as the lift coefficient C_L, drag coefficient C_D and the lift-to-drag ratio C_L/C_D. In order to discuss these revealed α effects, we further visualize the flow around the airfoils in terms of the velocity vector, vorticity and the Q value at α = 0 – 30 deg., based on PIV analyses by water-tank experiment. Such results suggest that FP is similar not with NACA0015 but with iNACA0015 from an aerodynamic point of view.

In order to develop a vibration-power generator using magnetostrictive material and flow-induced vibration, experiments were performed in a wind tunnel for prisms with several span lengths. We focus on transverse vortex-induced vibration for a circular cylinder, and low and high-sped galloping vibrations for rectangular prisms with a side ratio of D/H = 0.2 and 1 (where D is the depth of the prism in the flow direction and H is the height of the prism). The maximum power generation of a short prism is larger than that of a long prism because the short prism has a high characteristics frequency. However, the short prism begins to generate power from a high wind velocity. The cap of prisms has an influence on the characteristics frequency of the test model. The power for a circular cylinder with a span length of 200 mm becomes (P_rms)_max = 1.8 mW, which is larger than the other test models.

In recent years, there has been increasing interest in the flow field around elastic airfoils that perform unsteady motion, and many studies have been conducted using experimental and numerical approaches. There are many reports on the relationship between the wake structure of the unsteady airfoils and the dynamic fluid force acting on them. On the other hand, the dynamic behaviors of micro vortex structure and the growth process of the vortex structure have not been clarified sufficiently. The purpose of our present study is to clarify the growth process of the vortex structure in the vicinity of the wall. Especially, the authors focus on the movement of the wall and defined the as the temporal change in the spatial gradient in order to evaluate the elastic deformation quantitatively. The authors perform the numerical simulation of the flow field around a heaving elastic airfoil by using ANSYS CFX 17.0 and investigate the dynamic behaviors of the vorticity in vicinity of the wall.

The purpose of the present study is to investigate the relationship between unsteady drag and flow field for a flat plate during the pitch-oscillating motion. In this study, the angle of attack of the flat plate was alternately changed from 70° to 110° . The flow fields were visualized by using a smoke wire method with a high-speed camera. The roll-up was clearly observed by pitch-oscillation in the shear layer from the edge in comparison with the result under static condition. It is concluded that the drag can be enhanced by the generating vortices due to the pitch-oscillating motion.

The aim of this study was to investigate the vortex structure and the maximum vortex scale in the far wake of a two-dimensional NACA0012 airfoil. In order to estimate the maximum vortices scale, Large-Eddy Simulation (LES) for far wake of the airfoil was performed and integral scale of turbulence length for each of the three-dimension by using the cross-correlation coefficient of the flow velocity at two points in the wake was calculated. The condition of the flow around the airfoil was set to a Reynolds number of 4.0×10⁵ and an angle of attack of 9 degrees. The analysis area is 7C (C: chord length of the airfoil) in the flow direction from the trailing edge of the airfoil, 3C in the vertical direction of the flow direction and the grid resolution of (x⁺, y⁺, z⁺) = (40, 4, 10). The wake grid has a size that can capture eddies of 1mm in both the x and y directions. The vortex structure was observed by visualizing the second invariant of the gradient tensors. From this result, wake structures with various scales from the trailing edge to 2C or 3C had become longer in the flow direction at downstream were observed. The integral length was (x, y, z) = (0.056C, 0.025C, 0.017C) at 2C from the trailing edge and (x, y) = (0.083C, 0.039C) at 6C from it. Their aspect ratio on x/y of integral length was almost 2. As a result, the vortex structure generated from the airfoil was a structure that extended in the flow direction and integral length was twice long in the flow direction of the vertical direction.

The drag force acting on the disk depends on the presence or absence of holes. For example, it is known that the drag coefficient increases when a donut-shaped hole is made in a disk and the hole is enlarged. Therefore, in this study, we created a disk model with 0, 3, 4, and 6 holes, and investigated the effects of the number of holes and aperture ratio on fluid drag. In the experiment, the model was freely dropped in water, and the drag coefficient was derived from the force balance. We also report the results of applying dye to the model and visualizing the flow around the model.

This study investigated the effect of longitudinal vortices arrangement on the turbulent diffusion. Interaction of longitudinal vortices may cause an increase in the diffusion effect. We used two vortex generators (VG). One is susceptible to interaction on longitudinal vortices and the other is not. VG was mounted on upstream of contraction of wind tunnel. Longitudinal vortices in the contraction area were stretched and the vorticity increased. CO₂ gas was injected into wind tunnel from one point source. To evaluate diffusion, CO₂ concentration was measured in the downstream region. As a result, the concentration distribution of CO₂ in the downstream direction was clarified, and the difference in the turbulence diffusion effect of two VG was clarified.

For the promotion of heat transfer, various measures have been taken to induce stirring flow until now. It was confirmed that the dimple can restrain the pressure loss to the same level as the bare tube and obtain the heat transfer effect equal to or more than that of the fin⁽¹⁾. However, it is hardly mentioned what sort of flow style was formed by the dimples and so what influence was brought out on the main flow. In addition to this question, many studies have dealt with typical circular dimples and arrangements of multiple dimples to adapt to the actual machine⁽²⁾⁽³⁾, so the performance in the cases of other shape and single dimple have not been clarified yet. The purpose of this study is expose the mixing flow characteristics generated by simplex dimple of different shape was evaluated. In this paper, it described the external mean flow around the dimples, which is circular, elliptical and teardrop shape, in the three-dimensional flow field by PIV measurement⁽⁴⁾. As a result, with respect to the Reynolds stresses u’v’, the circular and elliptical dimples had distributed from the center of dimple to the backward. The teardrop-shaped dimple had a generated region extending backward from the trailing edge near the sidewall. The Second invariant of velocity gradient Q value was distributed along the trailing edge of the dimple. In the elliptical, it was also confirmed at the leading edge.

In this study, the pressure field of the flow past a simplified vehicle body with a wheelhouse and a tire is estimated from the velocity data measured by particle image velocimetry (PIV). The pressure field is estimated by solving the Poisson equation for pressure. The three-dimensional velocity field is obtained by reconstructing the time-averaged velocity distributions of multiple cross-sections taken from two orthogonal directions measured by two-dimensional PIV measurements. The experiment is carried out in a water tunnel at a Reynolds number of 12500. The pressure field is first validated by comparing it to a numerical solution. Then, the cause of the longitudinal vortices which have a significant influence on the aerodynamic performance of vehicles is discussed based on the estimated pressure field. The estimated pressure is higher at the front of the tire and lower at the sides of the tire. This result suggests that the longitudinal vortices are caused by the pressure difference around the tire.

Numerical simulation is performed of the flow around a sphere descending in a vertically stratified fluid. We focused here on the axisymmetric flow under relatively strong stratification, and found that a vorticity region is periodically discharged from the sphere, with a sign opposite to the usual vortex discharged in the fluid of uniform density. A Lagrangian analysis along the movement of a fluid particle shows that such a vorticity is generated baroclinically, by the horizontal density gradient across the nearly vertical isopycnal surface.

In order to improve the heat transfer performance of industrial applications, impinging jets are used. A single impinging jet produces a high heat transfer rate around an impinging position on an impinging wall, while the heat transfer performance decays increasing distance from the impinging position. Thus, in order to overcome the shortcoming of single impinging jet such as the occurrence of both inhomogeneous heat distribution on the wall and the narrow heating area, multiple impinging jets in which multiple jets are arranged are used. However, multiple impinging jets have a problem that heat transfer characteristics become unevenness because adjacent jets form a complicated flow field. In this paper, to reduce the unevenness, a oscillation control in which nozzle is moved along the row direction at a sin cycle is proposed. Then, using DNS (Direct Numerical Simulation) and DMD (Dynamic Mode Decomposition), we investigate the effects of oscillation control on the dynamic flow structure of multiple impinging jets with varying oscillation frequency and its amplitude. As a result, the uniformity of the velocity distribution near the wall surface is improved by performing oscillation control.

The particle-based method for computational fluid dynamics is one of the powerful methods to simulate the multi-phase flows because of the advantages such as the sharp interface tracking and conservation of the fluid. Thus, the particle method has been introduced for engineering design. However, comparing with the grid-based methods, it generally takes long CPU time to obtain solutions, especially for multiphysics problems. Therefore, we need to improve or modify the particle-based method in this point of view. In the present study, we focus on a heat transfer problem between a impinging jet and a wall. Concretely, the purpose of this study is to construct a heat transfer model calculated from the flow field information. By applying the empirical formulation of local heat transfer coefficient, we developed a new model to estimate the heat transfer of collision liquid jet cooling. The developed model consists of following estimations; the local heat transfer coefficient model on the wall surface, the characteristic velocity and length scales of the Reynolds and Nusselt numbers of computational particles near the stagnation point. The present model was validated by comparing with the correlation equation of the impinging jet for the heat transfer coefficient. In addition, the effect of each modification was discussed in terms of the estimation for heat transfer coefficient. Comparing the accuracy of the correlation equation with the developed model, the deviation is less than ten percent. Therefore, the proposed model showed reasonable agreement with the experiment.

The fluid flow characteristics of offset jet are investigated experimentally. The offset jet is produced by the flow of air which issue from the end of a long parallel channel. The exit Reynolds number Re is changed in 2.0 ×10³ to 7.0 ×10³, and the offset ratio H/h (H : step height, h : channel height) is changed in 1.0, 2.0, 3.0, 4.0 and 5.0. When the position of the reattachment point is nondimensionalized by the step height, it moves to the upstream direction as the offset ratio increases in the laminar region. However, the reattachment length is approximately the same in the flow for H/h ≥ 2.0. The wall static pressure coefficient distributions and flow patterns change greatly between H/h =1.0 and H/h =2.0 in the laminar region, but those in the turbulent region hardly change even if the offset ratio is different.

A jet in cross flow is one of typical phenomena in fluid engineering and many studies about this flow field have been already reported. On the other hand, a sweeping jet produced by a fluidic oscillator has a high frequency and a large amplitude and is attracted attention from the viewpoint of an active flow control in recent years. However, the studies on sweeping jet in cross flow have not been understood sufficiently and the detailed flow structures and vortex structures have not clarified. The purpose of the present study is to clarify the detailed flow structures in the sweeping jet in cross flow by using a numerical simulation. Especially, the authors focus on the flow structures and its dynamic behaviors formed by an interaction between the sweeping jet and main flow near the outlet of the sweeping jet.

Synthetic jets are produced by periodic ejection and suction of fluid from an orifice induced by movement of a diaphragm inside a cavity, and a synthetic jet actuator is a useful tool for active flow control. The synthetic jet actuators are low operating power, zero-net-mass-flux and very compact devices which have demonstrated their capability in modifying the subsonic flow characteristics for boundary layer flow control. In the present study, a smoke flow visualization was used to reveal the characteristics of the vortical structures produced by a synthetic jet in a crossflow. A significant difference was observed in the flow field for the synthetic jet issuing into the crossflow by changing dimensionless stroke length. The deformation of the vortex ring in the boundary layer promotes at high dimensionless stroke length. The flow structure generated by the interaction between the synthetic jet and the crossflow was affected by the dimensionless stroke length.

A jet flow is one of the most basic fluid flow used in industrial field and used for heating, cooling and mixing process. The properties and characteristics of diffusion and mixing of the jet flow strongly depend on the large-scale vortex structures formed in the near-field. Thus, an appropriate control method of a turbulent jet flow is important research topic in fluid mechanics to enhance diffusion and mixing efficiency. Recently, attention has been paid to blooming jet realized with superimposition of axial and helical excitations on the inlet velocity profile, and detailed characteristics of control parameters have been clarified. We demonstrated that the blooming jet also can be realized by a mechanical precessing nozzle with axisymmetric excitation. However, the detail structure of the blooming jet has not been clarified. To improve these performances, it is also indispensable to elucidate the details of the jet diffusion. In this study, we analyzed the detail flow characteristics of the blooming jet using dynamic mode decomposition (DMD) method. It was revealed that the effect of both the axial excitation and the nozzle precession extends to the downstream.

A synthetic jet has a zero-net mass flow rate at the nozzle outlet but generates a net mass flow rate downstream. Considering the momentum conservation, we supposed that the synthetic jet produces the time-averaged thrust. Furthermore, the thrust induced by the synthetic jet would be unsteady. In order to use the synthetic jet as a thrust control, we experimentally and numerically investigated the unsteady thrust characteristics for the synthetic jet. For a pulsating flow generator, a reciprocating piston operated by a motor with an inverter was used. A torque induced by the synthetic jet was measured by a torque sensor and the thrust was evaluated by the torque based on the moment equilibrium. The unsteady flow around the chamber was calculated by using commercial numerical code and the unsteady thrust was evaluated by integrating the jet direction force acting on the chamber surface. The unsteady thrusts induced by the synthetic jet were confirmed in the experiment and the CFD. In the CFD, the two peaks and the one peak were observed during the blow and suction conditions, respectively. On the other hand, in the experiment, we could not observe two peaks during the blow condition. The discrepancy may be caused by fluid resistance due to the vibration of the thruster.

An experimental study is conducted to investigate the flow characteristics of round jet which surrounded by multiple round sub jets at Reynolds number, Re (= U_cld₁/ν, U_cl ≈ 8.0 m/s, d₁: diameter of main nozzle) = 1.21×10⁴. Number of sub nozzles was varied between 2 to 10. The mean and fluctuating velocities were measured using cross-wire probe with constant-temperature type hot-wire anemometer. The length of potential core for the main round jet was x_c /d₁ ≈ 5, without influence the number of sub nozzles. Also, the centerline velocity decayed along x^-1/2 at 5 < x/d₁ ≲ 20, after the potential core region. At x/d₁ ≳ 20, the centerline velocity decayed along x^-1, similar to the single round jet.