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We examined the flow-field characteristics of free jets issued from three types of jet exits opened on a convex curved surface for aspect ratios of 16, 12, and 9.6 and height h of 9, 12, and 15 mm, respectively. In addition, we examined the heat transfer characteristics of the impinging jet, the shear stress inside a half-cylinder, and the flow-field near the impinging jet region through flow visualization in cases where each jet impinges vertically onto the inner part of the half-cylinder. The impinging distance ratios H/h between the jet exit and its inside for height h were 4, 3, and 2.4; and two semicircular plates spaced at a value of W were installed parallel to both long sides of the exit. The Reynolds number of the jet based on the mean bulk velocity at the jet exit and height h was approximately 11,700, except in cases where the flow visualization was made using smoke, at which the Reynolds number was approximately 5800. The flow-field characteristics of the free jets were similar to those of a two-dimensional jet. The flow-field in the region surrounded by the inside of a half-cylinder and two semicircular plates can be classified into three flow patterns by changing the space, namely W/H. This revealed that the rate of increase in heat transfer coefficient became 20% larger than that of the jet operating power by controlling the flow-field using two semicircular plates; in this case, the impinging jet flow vibrated the surface at the center of the jet. It was also clarified that even when the local heat transfer coefficient changed when mounting semicircular plates, the time-averaged shear stress on the inside of the half-cylinder did not change within the impinging jet region.

Cavitation sometimes occurs serious problems in hydraulic system. The problems become especially obvious in water hydraulic system because of higher saturated vapor pressure of water comparing of that of oil. This study is focused on cavitation phenomenon in a spool valve for Aqua Drive System. A spool valve model was made of transparent acrylic resin for visualization of cavitation phenomenon. Vibration and noise from the model were measured with changing the cavitation number. As a result, the vibration and noise are enhanced for the smaller cavitation number and the larger opening gap of a control port. It is, however, noticed that vibration and noise become large for some specific condition regardless of large cavitation number. It is revealed that these phenomena depend on cavitation type from visualization results.

Small hydropower development is one of the best way for enhancement of energy security and good solution of global environmental issues. A new submerged impulse turbine has been developing for generation by replacing a pressure reducing valve in drinking water pipe lines. This study has been especially focused on torque loss on the turbine, but it is not fully understood the reason why the loss is occurred. The torque loss is serious problem for a submerged turbine performance. Thus the torque loss on the runner and a disk was measured. This study, therefore, focuses on the loss mechanisms and aims to reveal key parameters for the loss reduction. We measured and compared to the torque loss between a simple disk and the submerged impulse runner experimentally. In addition, we aimed to reduce the agitation loss with the runner blade.

This study is focused on performance of a developed submerged impulse turbine, which is suitable for small hydropower generation in drinking water pipelines around mountainous region. The turbine works as a pressure reducing valve and can be generated electricity. Loss for submerged operation of the runner should be clarified to estimate accurately the turbine performance. The performance tests were conducted with changing the casing pressure, supplying flow rate and number of nozzles. As a result, turbine performance for submerged operation drops down comparing that for non-submerged operation. The water jet issuing from the nozzle occurrs cavitation. The cavitation degradates turbine performance. The cavitation phenomenon can be suppressed by increase the casing pressure. It is also revealed that the number of nozzles is important for the turbine performance. This is because of that the input energy from the nozzle is increased with the number of nozzles, on the other hand disc friction and mixing loss on the runner is only depended on the runner rotation speed.

The dynamics of flapping motion of a rectangular jet under acoustic excitation is studied experimentally by means of hot-wire measurement and flow visualization with smoke method. The excitation sufficiently enables“phase-lock”, which permitted us to extract the organized wave motion from a background field of finite turbulent fluctuations. The mean and fluctuation velocity are investigated and focused on the excitation frequency and the Reynolds number. As the excitation frequency decreases, it was found that the jet flapping and the jet spread were enhanced. The excitation with sub-harmonic frequency has significant effects on the rectangular jet behavior. The maximum value of the periodic velocity fluctuation strongly depends on the excitation frequency.

This paper examines the effect of sound on the disturbance in the shear layer of a round jet with at a range of sound frequencies, from 200 Hz to 1 000 Hz at intervals of 100 Hz using pure tones and recordings of machines operating at different frequencies. Hot-wire velocity measurements, FFT analysis and flow visualization were used to reveal the shear layer structure. Since round jets are used in industrial applications, their environments are generally noisy, thus it is important to understand how the frequency of background sounds affect shear layer disturbance. When the frequency was varied the maximum disturbance was observed at 500 Hz, which coincides with the predominant frequency of a free jet. Spectral components in harmonics arose via nonlinear interaction as the disturbance moved downstream, and transition from laminar flow to turbulent flow progressed through vortex pairing and collapse. Pure tone and machine generated sound exhibited similar characteristics.

An experimental investigation has been carried out for the flow-induced sound in a grooved circular pipe. The length of the pipe is 500 mm, the inner diameter is 23 mm and the groove pitch is kept at 8 mm. The velocity in the pipe is varied from 6 m/s to 66 m/s. In the region of the velocity smaller than 30 m/s, the frequency of the sound generated in the pipe increases in the shape of stairs with increase of the velocity, and then resonant sound of the pipe generates from the pipe. When the velocity becomes large, the frequency of the sound increases linearly to the velocity. The shape of change of the frequency can be explained from characteristics of a cavity flow and lock-in phenomenon by the resonant frequency of the pipe.

The flow field around a circular cylinder elastically suspended with a cantilever-type plate spring in the jet impingement region was visualized to investigate the mechanism of the impingement heat transfer. The impingement distance H was kept constant at 3 or 5 times as large as the jet slot width, h = 15 mm.The Reynolds number was fixed at 10,000, or 5000 in the case of flow visualization. The self-induced periodic swing motion of the cylinder across the jet axis was caused by the interaction between the jet and the elastically suspended cylinder. It was found that this swing motion has direct effects on the flow and heat transfer characteristics of the stagnation region. The ensemble-averaged values of the flow velocity and its fluctuations depended on the cylinder diameter and the impingement distance. The local Nusselt number in the case of H/h = 3 with the oscillating cylinder of the smallest diameter D = 4 mm was increased to 1.15 times as large as that without the cylinder. The interesting patterns of the intermittency function defined with a certain threshold level of turbulence intensity were obtained under the above experimental conditions. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 313–330, 2001

One of the circular cylinders with several diameters was elastically suspended with springs in the jet impingement region between a jet slot and a heat transfer target plate. The slot-to-plate distance H was kept constant to be 3 or 5 times of the slot width, h=15mm. The Reynolds number was fixed at 10000, being exceptionally 5000 in the case of flow visualization. Periodical swinging motion of the cylinder across the jet was observed because of the cylinder vibration self-induced with the jet flwo. It was found that this vibration has direct effects on the characteristics of the flow and heat transfer around the stagnation region. The ensemble-averaged values of velocity and its fluctuation depend on the cylinder diameter and the impinging distance. The local Nusselt number in the case of H/h=3 with the vibrating cylinder of the smallest diameter D=4mm was increased up to 1.15 times of the Nusselt number without the cylinder. The interesting patterns of intermittency function defined together with threshold levels of turbulence intensity were observed in the above case.

The self-sustained oscillation is known to be excited by an axisymmetric jet impinging upon the sharp-leading edge of a ring. When a finite length of pipe, instead of a ring, is placed in a straight line with the jet axis, longitudinal resonant modes of the pipe are produced under a certain flow and geometerical conditions. In this paper, the influence of the inserted screen on the self-sustained oscillation and resonance of the pipe column oscillation are studied when a screen is placed between the nozzle exit and the ring or the pipe. Two patterns of the feedback loop are formed by the interaction between the vortical structure in the shear layer and the screen, or by the interaction between the former and the ring or the pipe. In the case of the pipe there are three frequency modes, namely oscillations in the two feedback loops and a resonant mode of the pipe. The resonance of the pipe is excited or suppressed according to the relationship among the three modes. When the screen is placed near the nozzle exit, the screen prevents the self-sustained oscillation and the resonance of the pipe dose not appear.

This paper is conerned with the self-excited oscillation in a shear layer of an axisymmetric jet when a fine screen is placed normal to the axis of the jet. The diameter of the nozzle exit is 28 mm and the exit velocity is 10.4 m/s corresponding to Reynolds number 1.99×104. Screens with three different mesh sizes are employed. In the experiment the screen is moved in the direction of the jet axis. The shear layer starts to oscillate periodically and simultaneously a pure tone is generated due to the impingement of the jet upon the screen. The frequency varies in the same way as in the jetring system. It is recognized that the configuration of the peaks in the velocity spectrum reflects that of the peaks in the sound spectrum. After passing through the screen, the periodic oscillation in the velocity remains at the location twice the nozzle diameter away from the screen.

Self-excited oscillation of an axisymmetric jet impinging upon a ring is investigated experimentally. The discreat vortices impinge upon the downstream sharp edge of the ring, in turn producing an upstream influence, thereby closing the loop for sustaining oscillations, as confirmed by the form of phase analysis of velocity and pressure signals. The fundamental frequency component exhibits an overall phase difference of (2nπ) +1/2 (n=0, 1, 2) between the nozzle lip and edge. Furthermore, the fundamental component propagates with phase speed, which is about 0.52∼0.59Uo. Spectral analysis of velocity fluctuation reveals multiple frequency components having substantial amplitudes relative to that of the fundamental frequency component ; using coherence analysis it is shown that the lowest frequency component can interact with the fundamental component either to reinforce itself or to produce an additional frequency component.