The purpose of the present study is to clarify the mechanism of drag reduction for a sphere with arc type dimples. The sphere has 328 dimples of different depths uniformly distributed on its surface. The present study measured the pressure and velocity distributions inside and between the dimples, and visualized the flow on the sphere surface by an oil film method. The results indicated that separation bubbles were generated inside the dimples and transformed a laminar boundary layer into a turbulent boundary layer. Compared to a smooth sphere, the critical Reynolds number decreased and the separation point shifted further downstream. Therefore, the drag coefficient of a dimpled sphere was smaller than that of a smooth sphere. The magnitude of the decrease in the critical Reynolds number was found to increase with dimple depth. However, the separation point shifted to the upstream side and the drag coefficient became larger in the super-critical Reynolds number region.
The objectives of this study were to measure the unsteady fluid force acting on the trunk of a swimmer using the ‘swimmer mannequin robot’ and to model the fluid forces based on the formulation of the swimming human simulation model SWUM, which was developed by the authors’ group. The swimmer mannequin robot consisted of a swimmer mannequin and a driving mechanism. The scale of the swimmer mannequin was 1/2 (half scale) and the three-dimensional shape of an athlete swimmer taking the gliding position was reproduced in detail. The driving mechanism could move the mannequin in the pitching, heaving and rolling motions. Using the swimmer mannequin robot, the trunk motions of four strokes (crawl, breast, back and butterfly) were reproduced, and the unsteady fluid forces acting on the mannequin were measured by the dynamometers installed in the robot. On the other hand, the swimmer mannequin was modeled using the swimming human simulation model SWUM. The fluid force coefficients in the model were identified so that the simulated fluid forces became as consistent as possible with the experimental ones for each stroke case. The identified coefficients were then unified into ones which can be used for all cases. It was found that the precision of the model almost did not decrease as a result of the unification. It was also found that the overall performance of the simulation using the determined fluid force coefficients to predict the time variation of the fluid forces was satisfactory.
A turbulent plane jet with a chemical reaction (A + B → R) in a liquid is investigated experimentally. The instantaneous concentrations of all species (A, B, and R) are measured simultaneously by using the light absorption spectrometric method and the mass conservation law. Statistics of the reactive scalar field are compared with those of the nonreactive scalar (frozen limit) and instantaneous reaction (equilibrium limit) fields. It is ascertained that, in comparison with the frozen limit, the mean concentrations of reactants A and B decrease while the mean concentration of product R increases in the downstream direction because of the chemical reaction. With regard to scalar fluctuations, it is also observed that in the region near the exit of the jet, the r.m.s. values of species A become larger than those in the frozen limit, whereas the r.m.s. values of species B become smaller than those in the frozen limit; the reverse situation is observed in the downstream and outer regions. Furthermore, the concentration correlations between species A and B have negative values. The segregation coefficients have values between 0 and -0.3 on the jet centerline and have minimum values at the location separated from the centerline in the cross-stream direction. The data obtained in the present study provide very useful and important information for modeling concentration correlation and the chemical source term in a turbulent reactive flow.
Dielectric barrier discharge plasma actuators (DBD-PA) and fiber Bragg grating (FBG) sensors have been investigated for active control of flow separation around a NACA0024 airfoil. Tangential jets were produced in the vicinity of the DBD-PA slightly aft of the leading edge of the airfoil. The flow separation control ability was evaluated at low Reynolds number in an open-circuit wind tunnel. Phase- and time-averaged velocity distributions around the airfoil were measured using Particle Image Velocimetry, and the flow separation control ability of the DBD-PA was assessed at 8°, 12° and 16° angle of attack. An FBG sensor was attached to a chord-wise cantilever near the trailing edge of the airfoil on the pressure surface. This was used to measure strain fluctuations in the cantilever root due to flow-induced oscillations. The feasibility of this system to detect flow separation was studied, where the standard deviations of strain fluctuations significantly increased when the flow was separated. This was utilized in an open-loop control system to detect flow separation by FBG then apply active control with the DBD-PA to reattach the flow.
Turbulence in the oscillatory flow in realistic model human central airways was measured by particle image velocimetry (PIV) to reveal the nature of turbulence in a lung. The transparent silicone model of multi-branching airways was fabricated from X-ray CT images by rapid prototyping. The multi-branching airways comprise trachea, and right and left bronchi, with airway diameters ranging from 14 to 2 mm, respectively. Experiments were performed for a Reynolds number from 1200 to 2200 and a Womersley number from 1.9 to 2.3 in the trachea. Results showed that spatial and temporal variations of turbulent intensity strongly depends on the airway geometry and on the phase of oscillatory flow, and that expiratory flow generates strong turbulence which explosively occurs in the entire cross-section especially in the right bronchi, whereas inspiratory flow generates relatively weak turbulence near the airway wall.
Many kinds of passive and active mixers have been studied using micromixing principles, and many of them mainly depend on molecular diffusion as the source of the mixing. Therefore, using a small diffusive distance is a primary method of improving in micromixing. The study of straight-flow micromixing utilizing a gas-liquid free interface in our laboratory confirmed that regional changes in flow direction near the bubble played a definitive role in rapid mixing. To achieve fast mixing using these two effects, we developed a new micromixing device that utilizes a thin liquid film between two static bubbles. The size of the bubbles was larger than that of the channel cross section in a Y-type straight-flow microchannel. In this study, we discuss the formation and behavior of bubbles or liquid films on the basis of the results of bubble forming experiments, as basic research toward developing the new device. We conducted the experiments using several test channels that had different sizes and locations of bubbles, and using CFD (computational fluid dynamics) simulation, we calculated the mixing performance in different designs of the mixing device. As a result, we successfully achieved the formation of static bubbles and thin liquid films under the condition of a low Reynolds number of 0.17. In addition to confirming that our new mixing device can be applied under that condition, we found that it has a stable capability for mixing.
Effects of the total gas flow rate on the water level in a diffuser pipe for a membrane bioreactor, the gas flow rate from each aeration hole and the bubble diameter are investigated. The diffuser has evenly positioned five aeration holes on the top and a larger hole on the bottom for introducing the liquid into the pipe. The gas flow rate from each aeration hole is measured by capturing generated bubbles. The water level and gas velocity inside the diffuser are computed by processing video images. The bubble diameter is calculated using the gas flow rate and the bubble generation frequency measured from the video images. The conclusions obtained are as follows: (1) the gas flow rate from the aeration hole depends on the water level inside the diffuser and becomes constant for all the holes as the total gas flow rate increases since the high total gas flow rate make the water level uniform, which results in a constant gas pressure in the diffuser, (2) the onset of slugging in the diffuser is well correlated in terms of the local gas velocity and the Mishima-Ishii's slugging model, (3) the increase in the total gas flow rate decreases the water level, causing suppression of the onset of slugging, (4) the diameter of aeration hole strongly affects the gas flow rate from each aeration hole and water level, and (5) the Davidson-Schuler correlation gives reasonable estimations of the bubble diameter, provided that the influence of slugging is not significant.
Plasma synthetic jet actuator (PSJA) is a flow control device which has structure that insulator is tucked with electrode pair. It generates electrohydrodynamic (EHD) effect and induces a flow. The experiment was held to investigate the effect of flow control using extremum seeking with PSJA placed on the surface of NACA0012 wing installed in the wind tunnel. Frequency of the input signal to PSJA is modulated to maximize the effect of PSJA in flow control. The wake velocity fluctuation is one of indexes on separation control effect. The wake velocity is minimized over the input frequency by employing extremum seeking. The seeking algorithm calculates the correlation of the modulation frequency and wake velocity fluctuation. The modulation signal frequency where the correlation changes from negative to positive minimizes the wake velocity fluctuation. To detect a local minimum of the wake velocity fluctuation by extremum seeking, it is necessary to change the modulation signal frequency with time. Sine and square waves change the modulation signal frequency to PSJA. The wind tunnel speed was changed as an external factor. The experimental results show that the modulation signal frequency can track the optimum value when the wind tunnel speed is changed. This paper shows that adaptive flow control to optimize the modulation signal frequency with PSJA using extremum seeking enables to suppress turbulence on the flow field of wings.
The objective of this study was to clarify the unsteady characteristics of the fluid forces acting on limbs during swimming. For this objective, an underwater robot arm was developed in this paper. The robot arm has five degrees-of-freedom in order to perform the various complicated limb motions during swimming. In addition, by changing the hand replica into the foot one, the robot also can perform the lower limb motions. The joint torques and the resultant thrust can be measured by the force sensors attached to the robot. In a circulating water tank, an experiment to measure the fluid forces was conducted for four swimming strokes of the upper and lower limbs. From the experiment, it was found that even the slight difference of the fluid forces between slightly different swimming motions can be quantified by the developed experimental system. In addition, it was suggested that ‘nipping’ the water by both lower limbs during the kick of the breaststroke almost does not affect thrust generation. The developed experimental system with the robot arm is useful not only for measuring the unsteady fluid forces, but also for flow visualization in future studies.
The objective of this study was to clarify the unsteady characteristics of the fluid force acting on limbs during swimming. For this objective, an underwater robot arm, which has five degrees-of-freedom in order to perform the various complicated limb motions during swimming, was developed. In the previous study, an experiment to measure the unsteady fluid force was conducted for four swimming strokes of the upper and lower limbs. In this paper, the unsteady fluid force model was firstly formulated. Second, the simulation of experimental conditions was conducted. Two fluid force coefficients, which are the parameters in the fluid force model, were identified using optimizing calculation, so that the discrepancies of the forces and moments between the experiment and simulation were minimized. In addition, fluid force models which are dependant only on the limbs’ shapes were determined. Good agreement between the experiment and simulation with the determined fluid force model indicated the validity of the determined model. The identified fluid model will be useful for mechanical analyses of various swimming motions in future studies.
In the piping system of power plants, pipe wall thinning caused by flow-accelerated corrosion (FAC), liquid droplet impingement (LDI) erosion, and cavitation Erosion (C/E), is a very serious problem because it leads to serious damage and eventual destruction of the piping system -. In this study, pipe wall thinning caused by FAC in the downstream of an orifice nozzle (flow meter) was examined. Experimental Analyses were performed to clarify the characteristics of FAC, its generation mechanism, and the prediction of the thinning and reduction of the pipe wall. The corrosion pattern on the pipe wall was also examined through an experimental simulation. This simulation clarified that the occurrence of thinning mainly depend on the amount of pressure fluctuation p' on the pipe wall. It was also found that the wall thinning rate can be estimated using p' and that the suppression of p' can be realized by replacing the orifice nozzle with a tapered one having an angle to the upstream.
The streamwise interfaces of an isolated turbulent region in a flat plate boundary layer are investigated by performing a wind tunnel experiment. A bimorph-type piezoceramic actuator is used to generate a trapezoidal turbulent region whose spanwise width is larger than those of a typical turbulent spot. The experimental results show that the traveling speeds of the leading and trailing ends of the turbulent region are almost same as those of the turbulent spots with an arrow-head shape. This result indicate that the spanwise width have little influence to the streamwise growth of an isolated turbulent region. It is also found that the traveling speed of the trailing end of the turbulent region exceeds that of local flow velocity in the near-wall region, which is η < 1.3, and vice versa. This result shows that a relaminarization is in progress in the near-wall region, where the turbulent fluid is moving across the trailing end turning laminar. The critical wall-normal distance of η = 1.3 corresponds to about 10 in the wall unit (y+), suggesting that the relaminarization process is taking place in the viscous sublayer.
Numerical study is conducted to investigate the fluid-structure interaction around a static circular cylinder in two and three dimensional simulations. The cylinder with an elastic surface is fixed in uniform flow at Reynolds number 1000. The objective is to investigate the influence of elastic surface on the fluid forces around the cylinder. The continuity equation, Navier-Stokes equations and Poisson equation are solved by Marker and cell method. The elastic wall is treated by a mass-spring-damper force model that is solved with fluid forces referred from the flow simulation. As a result, it is obtained that the elastic surface at the back side of cylinder (Model D) decreases the drag force on the cylinder.
Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with a spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. In this study, we improved PB-MTV in its processing speed by utilizing GPGPU (General Purpose Graphic Processing Unit) to increase sample number in measurements. We measured TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number as DNS data provided by Horiuti, and compared the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. We also measured TKE budget in a bubbly flow in the square duct to examine effects of bubbles on TKE budget. As a result, we found that (1) PB-MTV can accurately evaluate TKE budget in turbulent flows, (2) bubbles affect the production and diffusion rates of TKE and do not affect the dissipation rate so much, and (3) the model proposed by Troshko and Hassan can reasonably estimate the production rate of the bubble-induced pseudo turbulence.
Direct numerical simulations of a spectral method are performed to study the turbulent channel flow at the low-Reynolds numbers, where the quasi-laminar and turbulent regions simultaneously appear and form the stripe pattern, which is floated downstream with a constant bulk mean velocity. In the turbulent region, many quasi-streamwise vortical structures are followed by low-speed streaks upstream. In the quasi-laminar region, however, quasi-streamwise vortices are rarely observed, though vortex roll with weak streamwise vorticity is regularly found around the channel centre. The turbulent regions are fixed by subtracting their advection velocity from the mean flow. After being temporarily averaged, turbulence statistics are further averaged over the string direction. As a result, turbulent regions are found to be an interface between upwind high-speed fluids and downwind low-speed fluids, where the flow is redirected into the spanwise direction. We investigate the detailed flow structure of the striped turbulent region using the linear vorticity equations.