The pulse widths of light from sonoluminescing air bubbles in various solutions were measured by a time-correlated single photon counting (TC/SPC) method to investigate the effect of density on sonoluminescence characteristics. The measured values of the pulse width were about 150∼190 ps. No appreciable difference in the pulse width and the bubble behavior for the sonoluminescing air bubbles in various solutions was found even though they have quite different density values varied from 1000 kg/m3 to 1800 kg/m3.
Effects of electrolytes on bubble coalescence are investigated experimentally from fluid dynamic point of view. Collisions of a bubble with free surface in two kinds of liquids, namely, ultrapure water and sodium chloride solution, are observed using a high speed camera. It is found that the repetitive number of bubble bounces on a free surface in electrolyte solution is larger than that in ultrapure water when the equivalent bubble radii are the same. This result qualitatively shows that electrolyte works to prevent bubbles from coalescing. It is also revealed that the bubbles can coalesce if their Weber numbers, which are based on the radius of curvature of the front side of the bubble and the approach velocity just before the collision with the free surface, are less than a critical Weber number. Furthermore, the critical Weber number decreases with the increase in the concentration of the electrolyte solutions despite the fluid dynamic characteristics, such as bubble rise velocity and bubble shape, are not affected by the addition of the electrolytes. It is concluded that Weber number is the most important parameter to specify the effects of electrolytes on bubble coalescence.
A new method for measurement of evaporation coefficient using sound resonance experiment is proposed on the basis of a theory of molecular gas dynamics, by which the evaporation coefficient is expressed as a function of the amplitude of standing sound wave between a planar sound source and a vapor-liquid interface facing against it. To demonstrate the applicability of this method, we carried out test experiments under the condition of neither evaporation nor condensation for several initial pressures, 30, 50, 80 and 101 kPa, at room temperature. In the experiments, we measure the amplitude of standing wave with a resonant frequency generated in a cylindrical space filled with air and closed by liquid water. We utilize the second harmonics component excited by the nonlinearity of sound to determine the evaporation coefficient, thereby eliminating the electromagnetic noises from measured signals. We find that the amplitude of the second harmonics at sound resonance decreases with the decrease in the initial pressure.
An experimental investigation to obtain the three-dimensional flow structure of the synthetic jet in low Reynolds number cross flow was conducted for the purpose of examining the effective method of vortex generation for flow control. Jet orifice diameter was 0.5 mm, and Reynolds number of the cross flow based on channel height was 650. The phase-averaged three-dimensional flow structure of synthetic jet and a continuous jet for the comparison with the synthetic jet were measured by scanning stereoscopic PIV. It was found that the synthetic jet generated the hairpin vortex. Although the shape of the longitudinal vortex pair of synthetic jet in cross flow (SJCF) is similar to that of continuous jet in cross flow (JICF), the maximum vorticity of SJCF remains higher than that in JICF in the downstream.
Flow visualization, particle image velocimetry (PIV), and fluctuating static pressure measurements were carried out simultaneously in a turbulent shear flow separated from a sharp-edged fence to estimate the probable location of the sound source that is closely related to vortex motion and aerodynamic sound generation. The measurement of the fluctuating static pressure is an effective way to detect vortices from pressure drops and the sound source term of the dilatation theory proposed by Ribner, which is a double time derivative of the fluctuating static pressure. Therefore, in this study, attempts were made to find the relationship between variations in the waveforms of the fluctuating static pressure, sound source term, and vortex motions obtained from the flow visualization and PIV measurements. From the results, it was revealed that pressure drops deduced from the waveforms agreed with the visualized vortices, and the waveform of the sound source term was almost the same as that of the aerodynamic sound. The results suggest that this simultaneous measurement method is useful for determining the mechanism of aerodynamic sound generation.
This study is devoted to measure the aerodynamic forces acting on two railway trains, one of which is a high-speed train at 300km/h maximum operation speed, and the other is a conventional train at the operating speed 100km/h. The three-dimensional train shapes have been modeled as detailed as possible including the inter-car, the upper cavity for pantograph, and the bogie systems. The aerodynamic forces on each vehicle of the trains have been measured in the subsonic wind tunnel with 4m×3m test section of Korea Aerospace Research Institute at Daejeon, Korea. The aerodynamic forces and moments of the train models have been plotted for various yaw angles and the characteristics of the aerodynamic coefficients has been discussed relating to the experimental conditions.
The liquid jet impingement with phase change heat transfer has long been an attractive method of cooling especially in steelmaking process and heat treatment in metals. The current study focuses on making detailed measurements of the stagnation-point heat transfer as a jet impinges on the rough metal surfaces at high temperature nominally up to 900°C. The local heat flux measurements are introduced by a novel experimental technique in which test block assemblies with cartridge heaters and thermocouples are used to measure the heat flux distribution on the surface of hot steel plate as a function of heat flux gauge. The effects of surface roughness on the stagnation-point heat transfer were investigated for well-characterized four rough surfaces with root-mean-square average roughness heights ranging from 40 to 80 µm. The results show that surface protrusions on rough surface can penetrate the thermal sublayer in the stagnation point and thus increase the heat transfer. The heat transfer enhancement mechanism on roughened surface can be investigated by the different boiling regimes.
A L2F (Laser 2-Focus velocimeter) was applied for the measurements of the velocity and size of droplets in diesel fuel sprays. The micro-scale probe of the L2F has an advantage in avoiding the multiple scattering from droplets in a dense region of fuel sprays. A data sampling rate of 15MHz has been achieved in the L2F system for detecting almost all of the droplets which passed through the measurement probe. Diesel fuel was injected into the atmosphere by using a common rail injector. Measurement positions were located along the spray axis at 10, 15, 20, 25, and 30 mm from the nozzle exit. Measurement result showed that the velocity and size of droplets decreased and the number density of droplets increased along the spray axis. It was clearly shown that the mass flow rate in the spray was highest near the spray tip and was lower inside the spray.
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