Setting the second orifice nozzle after the first orifice nozzle with a resonance room, the jet causes a periodic resonance oscillation. In this study, the resonance frequency dependence on the jet velocity and the volume of resonance room were presented. Furthermore, in order to enhance the flow fluctuation at the nozzle exit, a notched orifice nozzle was used as the second one and a comparison of the flow characteristics with those of the resonance orifice nozzle is also performed. The velocity measurements of jets from the resonance notched nozzle by a constant temperature hot-wire anemometer indicate that the turbulent intensity at the nozzle exit increased by about 74% compared with that of the resonance orifice jet. It is also found that the resonance frequency decreases with increasing the volume of resonance room and that the mixing characteristics are improved in the downstream.
This work describes the performance of a high-order Large Eddy Simulation (LES) turbulent model related to the prediction of active control of flow separation over a wall mounted hump. The baseline conditions with no flow control, steady suction, and oscillatory blowing and suction flow control cases are considered for this study. The computed results are assessed to the results of experiment conducted by NASA which was one of the test cases in their 2004 CFD Validation on Synthetic Jets and Turbulent Separation Control Workshop. The main aim of this work is to have adequate understanding of the effects of leading parameters in controlling the flow separation, and to facilitate assessment of the mechanism of separation control. Therefore emphasis is put on the turbulence modeling related laminar-turbulent transition phenomena, unsteady flow separation phenomena, etc. A high order LES model developed by the first author is used to carry out 3-D unsteady turbulent viscous flow analysis by considering the whole model configuration.
In order to understand the aspect of the mutual interference flow from two circular cylinders, the visual observation experiment was performed. The experiment was performed by making three kinds of distance ratios (L/d=1.5, 2.5 and 5.5), and seven kinds of arrangement angles (α=0, 15, 30, 45, 60, 75 and 90 degrees) into experiment parameters. The Reynolds number was about 640. As the result of experiment, even if the distance ratio was the same, the vortex shedding characteristics changed with arrangement angles. The mutual interference will become remarkable if the distance ratio is small. In the arrangement angle, 30 degrees and 45 degrees are carrying out mutual interference most. Even when a forced in-line oscillation was performed under the conditions in which two circular cylinders are carrying out mutual interference, it was found that a lock-in phenomenon occurs. The vortex shedding characteristic was obtained.
The promotion of mixing and diffusion in an axisymmetric jet is tried by using multiple prominences: six vortex generators (VGs). The experimental instrument consists of the wind tunnel and the test section. The square jet is issued at the exit of wind tunnel. The circular skimmer of 30mm diameter is placed at the inlet of the test section in order to eliminate boundary layer developed on the inner wall of the wind tunnel contraction. A VG has a right-angled triangle-shaped chip, and it is installed in an angle of 30 degrees from the flow direction. VGs are mounted around the exit of the skimmer and their motions can be controlled independently by computer programming. VGs can impose the disturbances on the turbulent jet. In this study, two moving-modes are tested beside the stationary mode: in the first mode, all the VGs are oscillated radially in phase (axisymmetric mode); and in the second mode, three VGs in the opposite side are put in and out to the main flow region alternately (alternative mode). Next, the experiment in a heated axisymmetric jet is performed where heat is considered as a passive scalar. Simultaneous measurements of the velocity and temperature are performed using a composite probe of I-type Hot-wire and I-type Cold-wire. The results show the axisymmetric mode is the most effective for the promotion of the heat transport in the near-field region of the axisymmetric jet exit.
The wind-tunnel test was conducted to investigate the structural characteristics of wind fields behind an inclined flat panel in turbulence logarithmic layer, which mimics a photovoltaic panel in the atmospheric surface layer. The active turbulence grid was installed at the front of the test section to generate the fully developed logarithmic layer downstream consists of a turbulent flow which has similar characteristics to the atmospheric surface layer. The tilt angle of the panel was set to 20 °; this angle is typical in real power stations and yields flow separations behind the panel. The time-series of velocity vectors measured with a dynamic PIV technique showed that the flow separations lead to two kinds of turbulence generation process: one is high-frequency fluctuations with the strong wind shear just below the separation bubbles and the other is low-frequency fluctuations with the flapping of the separation bubbles.
Based on the results from direct numerical simulation (DNS), the effects of initial conditions (i.e. the Reynolds numbers and the mean velocity profile at the jet exit) on coherent structure (CS) and flow evolution of turbulent plane jets are analysed. The strength, scale and pattern of CS are observed using the vorticity data and the vortex eduction schemes based on the low-pressure isosurfaces and Q-criterion. The effects of large scale CS on the decay of mean velocity, turbulent fluctuation and the mean momentum transportation are also clarified in this study. The results suggest that CS in the turbulent plane jets are mainly of two types: spanwise and streamwise vortices, whose patterns are line and helix, and in the transition process CS will act the main role, which is on the higher energy level compared with the non-coherent structure, and beneficial for the mean momentum transport. The decrease of Re and the setting of parabola velocity profile at jet exit will enhance the strength of CS.
In the piping system of power plants, the pipe wall thinning caused by flow accelerated corrosion (FAC), liquid droplet impingement (LDI) erosion, and cavitation erosion (C/E), are very serious problem because they lead to serious damage and 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 and numerical analyses were carried out to clarify the characteristics of FAC, generation mechanism, and to propose of a new governing parameter, the prediction of the thinning, and reduction method. The corrosion pattern on the pipe wall was also examined by 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 or using a downstream pipe with a smaller diameter than that for the orifice.
Jet flow phenomenon is one of the most important basic flows in the thermo and fluid mechanics because it includes the free and wall bounded turbulent shear flows and largescale vortex structures. In addition, it is used widely in the field of industry for heating, cooling, mixing, diffusion, propulsion, and material processing. By the way, in order to improve the mixing and diffusion characteristics between the jet and surroundings there are many trials, for example, by using of various non-circular nozzles. In this study, the flow characteristics, mixing and diffusion properties, of the submerged free jet from the petal-shaped special nozzle with 6 petals are examined experimentally. It is expected that 6 petal-shaped free jet will have a good mixing and diffusion characteristics because of a large wetting perimeter, and forming of large vortex structures.
In order to evaluate the performance of water jet, radial density distribution or liquid fraction distribution of jet is one of the most important parameters. However, no measurements have been carried out do for due to the very high speed (order of several hundred meters per sec) of water jet. The authors have developed newly measurement system of liquid fraction distribution of high speed water jet using Laser Schrielen method. Laser beam is expanded to parallel beam and crosses the water jet. Then the beam is collected by lens and expanded again through pin hole and recorded by digital camera. From the two-dimensional recorded image, liquid fraction distribution in radial direction of water jet is reconstructed by tomographic method. The experiments were carried out using high speed water jet of industrial use, the nozzle diameter is 1.7 mm, and pressure at outlet of nozzle is 0.1-20 MPa. Radial distributions of liquid fraction were successfully obtained at various positions form nozzle exit. Numerical simulation of water jet was also carried out using homogeneous droplet flow models. The predicted density distributions reasonably agree with experimental data.
A microscale sensor for an accurate measurement of wall shear stress is developed using the microelectromechanical system fabrication technique. The sensor uses a thin metal film whose temperature is kept constant using a constant temperature circuit, and the wall shear stress is determined by measuring the heat flux from this film. The circuit consists of a Wheatstone bridge and an integrated circuit. For sensor calibration, an instrument is built using a circular cylinder and a flat plate. When the cylinder is rotated above the flat plate, a flow with a constant shear rate is produced on the flat plate. A calibration test is conducted under several conditions of the wall shear stress. Results of the calibration tests show that the output voltage of the constant temperature circuit increases with the wall shear stress. Moreover, the square of the output voltage is proportional to the cube root of the wall shear stress, as predicted theoretically.
The dependence of the scalar statistics on the Schmidt number is investigated experimentally in a liquid axisymmetric jet. In this experiment, the concentration of two diffusing dyes and temperature are measured. As diffusing dyes, Acid Blue 9 and Direct Blue 86 are chosen whose Schmidt numbers are about 600 and 3,800, respectively. The Prandtl number of temperature is about 7. Experimental results show that the power spectra of the concentration fluctuation for two different diffusing dyes show some differences each other in the higher frequency region. Further, the power spectrum of the temperature fluctuation obeys -5/3 power law only in the lower frequency region than those observed in the power spectra of the concentration fluctuation. Moreover, the probability density function of the concentration fluctuation of the scalar is skewed more negatively as Schmidt number becomes higher.
The relationship between the circulation of cyclic vortex rings, which are generated cyclically by a pulsating jet, and jet pulsation conditions are investigated. First, a method for estimating the circulation of cyclic vortex rings is established based on experimental results. In this method, the formation time, i.e., the time it takes for the cyclic vortex rings to form completely, can be estimated using the self-induced velocity of the vortex rings and the convection velocity of the shear layer of the jet. If the total circulation generated by the jet up to the formation time is assumed to be acquired within the vortex core, then the circulation of cyclic vortex rings can be estimated using the slug flow model. The self-induced velocity of cyclic vortex rings and the convection velocity of the shear layer can be estimated based on the jet pulsation conditions. According to these estimation results, there exists a specific condition under which the circulation of the vortex rings becomes maximum.
Large Eddy Simulations are used to study passive flow control for drag reduction of simplified ground vehicle. Add on devices in form of short cylinders are used for formation of streaks in the streamwise direction leading to the separation delay. The result of the present numerical simulations are compared with the experimental data showing good agreement. The two-stage flow control mechanism is analyzed from the LES results. It was found in agreement with the previous experimental observations that the counter-rotating vortices behind the impinging devices influence the separation only indirectly through the longitudinal vortices further downstream.
Steam-water mixed sprays can remove thin polymer or metal films on substrates. However, the detailed mechanism remains unclear. One possible mechanism is the temperature change in the film due to the mixed spray because the film and substrate typically have different linear expansion coefficients. Here, we control the initial temperature of aluminum films deposited on glass using a Peltier device and apply steam-water and air-water mixed sprays to examine the film removal performance. We also measure the surface temperature change using a thin thermocouple. Steam-water mixed spray can remove the film, but air-water mixed spray cannot, although comparable temperature changes occur in both methods. A long-duration irradiation test reveals that the steam-water mixed spray can remove a relatively thick film despite an almost constant surface temperature. Thus, surface temperature changes may have little effect on film removal by steam-water mixed sprays.
This paper deals with the computational study of water droplets evaporation in the turbulent round gas jet. The Eulerian two-fluid and tracking Lagrangian models are employed for spray computation wherein the gas turbulence models using the second moment closure model. In the present study it is examined the influence of main parameters of two-phase flow, such as droplet diameter and droplet mass fraction, on the droplet evaporation process and flow structure in the gas-droplet jet. The axial gas velocity declines more slowly with a growth of droplets mass concentration in comparison with a one-phase jet. The increase in mass fraction of droplets results in contraction of two-phase gas-droplet jet. The dispersed phase acts to decrease the gas turbulence level, this influence becoming more pronounced with increase in the droplet mass concentration and initial size.
The laminar-turbulent transition of a boundary layer induced by jet flow injection in the inlet region of a circular pipe was experimentally investigated with hot-wire anemometer. The jet flow was periodically inserted radially from a small hole in the inlet region into the pipe flow. The threshold value of the jet flow rate to generate the turbulent patch was then obtained. The threshold value decreased and saturated finally with the increase in jet flow duration. The normalized jet flow duration when the threshold value was saturated increased with the increase in Reynolds number, contrary to the developed region. The normalized threshold flow rate tended to vary with the Reynolds number among three regions. All tendencies were different from those of the developed region. With the increase in jet flow rate beyond the threshold value, the duration of the turbulent patch increased, though the fluctuating velocity within the patch did not.
The stirring and mixing of a passive scalar by grid-generated turbulence in the presence of a mean scalar gradient is studied in three dimensions by DNS (Direct Numerical Simulation). Using top-end high fidelity computer simulations, we calculate and compare the effects of various fractal and regular grids on scalar transfer and turbulent diffusion efficiencies. We demonstrate the existence of a new mechanism present in turbulent flows generated by multiscale/fractal objects and which has its origin in the multiscale/fractal space-scale structure of such turbulent flow generators. As a result of this space-scale unfolding (SSU) mechanism, fractal grids can enhance scalar transfer and turbulent diffusion by one order of magnitude while at the same time reduce pressure drop by half. The presence of this SSU mechanism when turbulence is generated by fractal grids means that the spatial distribution of length-scales unfolds onto the streamwise extent of the flow and gives rise to a variety of wake-meeting distances downstream. This SSU mechanism must be playing a decisive role in environmental, atmospheric, ocean and river transport processes wherever turbulence originates from multiscale/fractal objects such as trees, forests, mountains, rocky river beds and coral reefs. It also ushers in the new concept of fractal design of turbulence which may hold the power of setting entirely new mixing and cooling industrial standards.
Experiments were performed to investigate the effect of a line of roughness elements on a flat-plate boundary layer transition. Each of 11 roughness elements was a cylinder 2 mm in both diameter d and height k, forming a row with an interval of a 22 mm in the line of elements in the spanwise direction. Wedge-shaped turbulent regions ("turbulence wedges") developed downstream from the respective roughness elements. Farther downstream, two adjacent wedges merged and a two-dimensional turbulent boundary layer was then formed. Mean and fluctuating velocities, Reynolds shear stress and turbulent energy production rates were measured by hot wire anemometers inside and outside the wedge regions. The relationship between the fluctuating velocity and the turbulent energy production rates was examined.
Research and development of various renewable energies have been promoting after Fukushima nuclear power plant accident. Among the renewable energy sources, hydropower is geographically promising source in Japan. This study has focused on development of undershot opened cross-flow turbine which is applicable extremely low head hydro power. Especially, it is rapid and shallow stream such as one of agricultural water ways in mountain area. A model of a cross-flow turbine was constructed in similar shape of Banki turbine and examined experimentally. The authors investigated the effects of the blade thickness and number with changing an impinging flow thickness on power performance. The power coefficient is drastically changed by difference of the impinging flow thickness. The maximum power coefficient, CPmax, becomes higher as the impinging flow thickness is thinner. It was evaluated by flow visualization around the runner for revealing mechanism of power production.
The aim of this investigation is to develop an open type cross-flow runner for eco-friendly pico-hydraulic turbine utilizing an extra-low head waterfall. A relative distance between the runner and a waterfall is one of the important factors for a stable generation. It is often necessary to adjust precisely the relative position of the runner every time when flow rate changes. We therefore focused on a flow direction control method with a convex shaped channel. The runner performance was evaluated in two channels which having different curvatures and in various flow rate. As a result, it was found that the control method is applicable for flow direction control. The power coefficient is higher for the channel which has smaller curvature. The method we found is very simple, inexpensive and can also avoid obstructing of water-bone debris at the inlet of channel. It is superior to other control methods from practical point of view for small hydro turbine.
Cross-flow over a smooth circular cylinder was studied experimentally in a wind tunnel at Reynolds numbers in the range of 2-3.2 X 105. A discrete transition indicating the initial transition from the sub-critical to the critical states is noticed. This transition leads to a pre-critical state so-called. The flow state features two separation bubbles situated symmetrically and intermittently on the two sides of the circular cylinder. The results obtained from the pressure measurements and the MEMS thermal tuft sensors on the cylinder surface reveal a trend that the separation bubbles tend to move upstream as the Reynolds number get higher. Moreover, for the cases of the circular cylinder roughened by textile materials, it is found that this transition process still holds, but takes place at lower Reynolds numbers.
In the present study, a new method measuring a thin liquid film in a small and curved channel by using an Optical Fiber Probing (OFP) is proposed. The OFP has highly spatial resolution due to its sensing-tip of 6-μm diameter. Hence, it can be highly adaptive for measuring a thickness of a micrometer-order and wavy liquid film. We simultaneously measured a thickness and maximum amplitude of a thin and wavy liquid film by using the OFP and a high-speed video camera. The OFP detects liquid film surfaces based on a difference in refractive indices between the gas and liquid phases. A fraction of liquid phase (integrated time while the sensing tip is positioned in the liquid film) is calculated from the output signals of the OFP at every fixed position, and is compared with a wave height obtained from the visualization. Calibrating the experimental results via a numerical simulation, we found that the fixed position of the OFP corresponded with the average thickness of a liquid film when the fraction of the liquid phase was 0.52. We demonstrated effectiveness of the OFP measurement through applying this method to an annular liquid-film flow inside a small two-fluid nozzle that is impossible to visualize. As a result, the average thickness and amplitude of the liquid film in the nozzle were 61 μm and 28 μm, respectively. Our new method possesses high measurement accuracy and appropriateness satisfactorily for practical measurement.
The laminar-turbulent transition of a mixing layer excited by oscillating flat plates at an exit of a two-dimensional nozzle was experimentally investigated. The mixing layer was formed between the jet issued from the nozzle and the surrounding quiescent fluid. The plates oscillated vertically in relation to the mean flow. Upper and lower flat plates oscillated anti-symmetrically. The oscillation frequency, 5 Hz, was two orders of magnitude smaller than the fundamental frequency of the velocity fluctuation. Mean and fluctuating velocity components in the streamwise and normal directions were measured by hot-wire anemometers. The results were compared with the previous case in which the plates oscillated symmetrically. Anti-symmetrical oscillation promoted the expansion of the mixing layer and promoted the disappearance of the potential core more than symmetrical oscillation. The fluctuations from time and phase averages in the anti-symmetrical oscillation were larger than those in the symmetrical oscillation. The contribution of periodic fluctuation disappeared downstream in the symmetrical oscillation but persisted longer in the anti-symmetrical oscillation.
The present study attempts to clarify flow characteristics in a synthetic jet by a discrete vortex method, particularly focusing on the streamwise mean velocity along the centerline of jets. The onset condition of the synthetic jet was simulated reasonably and the streamwise mean velocity distributions along the centerline of jets agree quantitatively well with the experimental results. For the synthetic jet, the acceleration region, where the streamwise velocity is increasing partially, is caused by the decrease of the negative velocity induced from the wall circulations and the reductions of the decreasing rate of the velocity induced from the shedding circulations.
Wall interference of Jet-blast is investigated. Jet-blast form airplane is simulated by using a sonic nozzle at a laboratory scale. In this study, far-field velocity distributions are measured by using X-type hot wire anemometer at four measurement planes. Understanding the velocity distributions in faraway downstream is very important when we concern the problems of jet-blast. We also intend to utilize these data develop new turbulence model on CFD. As a result, HWHM, aspect ratio and spreading rate of wall-jet are consistent with published data [1-6]. Distributions of Reynolds stress are analyzed from the data obtained. Interesting property of w distribution is found, that is, peak of w moves outwards on lateral direction as X/D increased. On the other hand, w'w' distribute around the center of the wall-jet.
The current research is aimed at experimental investigation of round and plane submerged macro- and microjets. The measurements were carried out by two methods: Particle Image Velocimetry (PIV) and measurements with the use of hot-wire anemometer. At the first stage there were the experiments on flow visualization in microjets by means of the PIV system. The axisymmetrical microjets were formed with the help of round metal channels with diameter d=500-8000 μm and length of 100 d. The plane channel was formed by two glass plates of 16x70 mm; its height was b =600 μm. The range of Reynolds numbers was Re=Ud/ν=200-6000. It is accepted that submerged jets are significantly unstable, therefore, the Re numbers of laminar-turbulent transition are not higher than 10. However, according to visualization, the jet stays laminar even at Re numbers of several hundreds. This is typical both to microjets and macrojets. At the next stage, according to visualizations and anemometer measurements, the coordinate of laminarturbulent transition L in the submerged jets was determined at variations of Re number. Our experimental data was compared with data of other authors. The length of the laminar zone in the round jets is twice as large as in the plane jets. Our experiments for the plane jet agree with data of Gau C. et. al. 2009 (nozzle height is 50-360 μm). Data for the round jets of different authors diverge significantly.