Experimental and theoretical investigations of the high speed jets impinge on a wedge-shaped edge have been conducted. Sonic circular and square jets of the same hydraulic diameter are examined in the present study. Edge-tones produced due to a wedge-shaped edge having different angles, specifically, 10°, 20°, 60° and 180°, have been investigated. Experiments reveal that several instability modes exist simultaneously in the microphone-captured acoustic signal. The minimum breadth is defined as the minimum distance of the edge from the nozzle exist for the first tone to be generated. Experiments showed that at small edge angle, the minimum breadth is approximately half of its equivalent two-dimension case. Semi-empirical frequency formula for the dominant tone is proposed based on the present experimental data for small edge angles. The theoretical results based on the vortex-sheet model proposed by Tam and Ahuja(1) showed that both helical and axisymmetric stability modes exist during jet impingement. The dominant tone usually has helical stability mode which is the well known edge-tone. Other tones are known as impinging-tones in the literatures and have axisymmetric stability modes. Finally, it has been shown that the experimentally obtained mean Strouhal numbers for helical and axisymmetric modes show good agreement with the Strouhal number of the least dispersive wave of the same mode calculated at various Mach numbers.
Cavitation instability in the turbo-pump can be divided into two different stages: the local instability such as the rotating cavitation and the system instability such as the cavitation surge. In our study, a numerical analysis of cavitating flow in a two-dimensional cascade, which represents the tip region in an inducer of liquid-fuel rocket engine, was conducted to find an indicator of the onset of cavitation surge. The response characteristic of cavitating flow field to three types of inflow boundary conditions was examined: steady inflow, inflow with sinusoidal fluctuation and inflow considering the characteristic of piping system. The last one is modeled by the unsteady Bernoulli's equation. Various cavitation modes that cause local instability and system instability were reproduced by considering flow rate variation in piping system. It is found that the specific mode, which is hidden behind the fluctuation of rotating cavities, is to be developed to the cavitation surge. The frequency of the surge was confirmed to be in the experimentally observed range and the ratio of the surge frequency to the propagating frequency, which is generated by rotating cavitaion, is almost constant while inflow changes.
The measurement error when calculating the bubble volume by using image analysis was estimated by taking a single bubble of known volume (0.02 × 10-6-2.00 × 10-6 m3, or the volume equivalent diameter of 3.37-15.63 mm) and using a stereo visualization system with a high-speed camera. Previous studies calculated the bubble volume from images observed by the one-way photography method using the short-axis rotation of the bubble shape, which was assumed to have short-axis symmetry for approximation of the bubble shape to a spheroid. This study clarified the measurement accuracy for bubble volumes using one- and two-way image analysis methods. The results showed that the one-way method tended to underestimate the bubble volume when using the long-axis rotation, which approximates the bubble depth diameter by using the short-axis diameter, and overestimate the volume when using the short-axis rotation, which approximates the bubble depth diameter by the long-axis diameter. The measurement accuracy of the calculated bubble volume degraded for large bubble volumes. A one-way volume measurement method using a correlation equation was developed, and its accuracy was equivalent to the measurement accuracy of the two-way method using stereo visualization.
A plain water jet is applied for the removal of plasma sprayed Al2O3 and ZrO2 coatings on stainless steel (JIS SUS304) substrates. The water jet impinges perpendicularly to the surface of the specimen with a constant traverse rate of VT = 150 or 300 mm/min. The jet pressure (Jp = 70-300 MPa) and the standoff distance (Ds = 5-30 mm) are systematically varied. The flow and erosive properties of the water jet are experimentally investigated to avoid unwanted erosion of the substrate. Particle image velocimetry (PIV) analysis is also applied, but only for lower jet pressure (less than 10 MPa) conditions. The results show that coating removal is significantly dependent on the material properties, especially the adhesion strength and porosity. Coating removal is successfully achieved within the present experimental conditions. The reported data suggest that the water jet process is a useful method for coating removal, although further verification and understanding of the fracture mechanisms are required.
We performed direct numerical simulations (DNSs) of turbulent channel flow for surfactant solutions with the purpose in developing a modified large eddy simulation (LES) method based on the knowledge obtained from such DNSs that include drag reduction effects. Our results indicate that, by a variable viscosity-dependent normalization of the spatial scale, the profiles and the peak positions of the turbulence statistics approximate those of a Newtonian fluid. The implication is that by such normalizations we can investigate universal characteristics of the turbulent structures. Further, we performed LESs of turbulent channel flow and confirmed the viability of the proposed method. We obtained velocity profiles and the wall friction coefficient that correlate well with DNS results. Consequently, we are now able to predict the wall turbulence modulation for surfactant solutions with the LES method by estimating the spatial scale of the turbulent flow filtered with a viscosity-dependent correction function.
The discharge characteristics and bubble dynamics of argon multiple bubble jets, in which a streamer is generated by a DC pulsed discharge, have been experimentally clarified through visualization analysis. The decolorization characteristics of a methylene blue solution and the properties of a treated solution using multiple bubbles jets, including argon, oxygen and air, have also been clarified. There is complex streamer behavior in a bubble in which streamer propagation is along the interface of an issuing bubble. O and OH radicals during streamer discharge in Ar and O2 bubbles were detected clearly near the interface of a breaking up bubble by spectroscopic measurement. Finally, the decolorization efficiency by O and OH radicals and ozone in a pulsed discharge multiple bubble jets are estimated.
This paper presents some modifications of the gamma-based model developed by K.M. Shyue (J. Comput. Phys., 1998, 142, 208-242). In deriving process of the original model, pressure equilibrium and an isentropic condition are not involved explicitly. Therefore, to adopt these physical conditions, we improved evolution equations for problem-dependent material quantities (e.g. ratio of specific heat). As a result, a non-conservative term associated with the pressure relaxation was added to the equations. The modifications have no effect on an oscillation-free property of the original model. Furthermore, because division by a volume fraction is not included in the present model, an inherent property of mixture-type models is maintained. To capture contact discontinuity sharply, we utilized an approximate Riemann solver HLLC with a third-order MUSCL interpolation. Finally, we carried out one-dimensional numerical tests in order to evaluate the influence on accuracy. Numerical results show that the modified model has good capability for simulating low Mach number flow problems.
The laminar-turbulent transition of a mixing layer induced by oscillating flat plates at an exit of a two-dimensional nozzle was experimentally investigated. The mixing layer was formed between the jet, which issued from the nozzle and the surrounding quiescent fluid. The plates oscillated vertically in relation to the mean flow. The oscillation frequency 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. In the oscillating state, the same phenomenon as in the natural transition process appeared more upstream. In the early stage within the nonlinear region, the growth of fluctuating velocity weakened and the Reynolds shear stress component decreased. The decrease in their production rate due to the expansion of the mixing layer contributed to the weakening and decrease. The probability of the streamwise and normal fluctuating velocity components taking the same sign increased or decreased in accordance with the increase and decrease of the Reynolds shear stress component. The randomness factor, which had been proposed by Sato, appeared to be a reasonable indicator of the present transition process, especially in the process in which the periodic velocity fluctuation became irregular. However, this factor certainly indicated the same value at two streamwise positions.
The authors numerically investigate the forced-oscillation-frequency responses on flow and thermal characteristics of the thermal convection in a cubic cavity heated from below in the gravitational field, concerning global quantities such as spatially-averaged kinetic energy and so on. The authors assume incompressible fluid with a Prandtl number Pr = 7.1 (water) and a Rayleigh number Ra = 1.0×104. The direction of a forced sinusoidal oscillation is parallel to that of the terrestrial gravity. The authors focus their special attention on the amplitude effects of the forced oscillation upon kinetic energy, the optimum frequency for response and flow structure. The obtained results are as follows. Even at large acceleration amplitudes, the optimum frequency exists. At the optimum frequency, periodicity is exact, while the wave form is far from sinusoidal at a non-dimensional acceleration amplitude η ≥ 1. A boundary exists at η = 1.5, where both the increasing rate of a spatially-averaged kinetic energy and the flow structure change abruptly, and where the optimum frequency attains the minimum. This abrupt change in the flow structure can be explained by an indicial response.
The main steam stop valve (MSV) in thermal or nuclear power plants is liable to undergo excitation by sound having a specific frequency. The purposes of this article are to clarify the mechanism of the sound generation in the MSV using numerical analysis and to propose a suppression method for the sound generation. The analysis results were validated experimentally using a 1/5-scale model of a typical MSV. The analysis results showed good agreement with the experimental data. From the analysis results, it was clarified that the sound of the MSV was generated by the hole-tone in the opening and the acoustic resonance in the vertical direction of the MSV. A method was proposed to install triangular or slanted tabs at the inlet seat of the MSV in order to prevent formation of vortex rings in the opening which leads to the hole-tone. Effects of the proposed method were tested experimentally. Eight triangular tabs having a height of 15% of the inner diameter in the seat reduced the peak value of the sound pressure by 60%. Eight slanted tabs having a height of 7.5% of the inner diameter in the seat suppressed generation of the hole-tone in the MSV. These results showed that the slanted tabs are effective in suppression of the hole-tone compared with the triangular tabs.
Three-dimensional numerical simulation of the flow around a leading edge of a flat plate when vortices with their axes normal to the flat plate surface is carried out to investigate the leading-edge receptivity to the vortical disturbances. It is shown that the vertical vortices outside the boundary layer are not titled and deformed, contradicting to what was reported in previous studies. It is revealed that the streamwise structures, which are dominant in the boundary layer, are formed due to the velocity field induced by the wall-normal vortices outside the boundary layer. It also is found that neighboring pair of vortices side-by-side in the spanwise direction are connected to each other very close to the wall.