Picosecond CARS spectroscopy and phased-locked schlieren imaging are used to measure time-resolved temperature and visualize compression waves in a diffuse filament, nanosecond pulsed discharge sustained between a pair of spherical electrodes in nitrogen and air at P=100 torr. The discharge generates stable plasma with high specific energy loading (up to ∼0.5 eV/molecule), and with spatial dimensions sufficiently large to enable laser diagnostic studies. The results demonstrate that significant temperature rise, up to ΔT∼200 K, occurs both in nitrogen and in air, on the time scale shorter than the acoustic time scale. The characteristic time for the rapid temperature rise in air, ∼100 ns, is significantly shorter compared to that in nitrogen, ∼1 µs. In air, a second significant temperature rise, up to ΔT∼350 K, occurs on a time scale of ∼100-500 µs. This “slow” temperature rise is almost entirely missing in nitrogen. Phase-locked schlieren images demonstrate a near cylindrical shape compression wave formed around the discharge filament, both in nitrogen and in air. An additional, near spherical shape compression wave is formed near the cathode, due to significant energy release in the cathode layer of the discharge. The compression waves, caused by rapid localized heating quantified by the present measurements, are similar to the ones produced by a surface nanosecond pulse discharge in atmospheric air used for high-speed flow control, where comparable temperature rise was detected previously.
A multi-phase AC arc plasma has been applied in the glass melting technology as a promising heat source. The electrode erosion of the multi-phase arc is one of the most important issues to be solved. In the present work, the high-speed camera system with appropriate band-pass filters was applied to visually observe the dynamic behavior of the electrode metal vapor in the arc on millisecond time scale. By using this system, the evaporation of 2%-thoriated tungsten electrode was visually observed at the anodic period in the case of the 12-phase arc, while electrode evaporation is negligible in the 2-phase arc. High-speed observation of the oxygen entrainment in the arc was also carried out. Results indicated the ambient oxygen is entrained into the arc more easily with increasing the number of the phases. As a result, the electrode evaporation becomes more severe with larger number of the phases.
In order to design hybrid rocket engines, we developed a numerical prediction method which describes the internal ballistics, thus the solid fuel regression rate. In order to obtain a fast code, simple but efficient models have been implemented. The fluid dynamics and the thermal heat conduction in the solid fuel have longer characteristic times than chemical phenomena, hence time-dependency is required. The regression rate is evaluated by solving the energy-balance equation at the solid fuel surface. The code validation is made by comparing calculation results with open literature experimental data. This comparison confirmed that the numerical estimation of time- and space-averaged regression rates has the same order of magnitude as the experimental data of time- and space-averaged regression rates. However the dependency of regression rate on oxidizer mass flux differs between the calculations and the experiments. This difference could be mainly due to the use of only convective heat transfer between flame and fuel surface. Nevertheless, by considering also the radiative heat transfer, it is possible to obtain a higher accuracy in the calculation of the regression rate dependency on oxidizer mass flux.
Designing high-performance air intakes is of crucial importance for the successful operation of hypersonic scramjet propulsion. This paper investigates the performance of axisymmetric intakes obtained by applying two shortening methods, namely, leading-edge truncation and stunting (longitudinal contraction), to the full Busemann intake in inviscid and viscous flowfields. The primary aim is to identify the key design factors and underlying flow physics in order to achieve the optimum performance with minimum weight by striking the balance between viscous and shock losses. The effects of intake shortening on performance are similar for the two methods for moderate length reduction (25%), conducing to considerable reduction in intake weight. Even reduced to half length, truncated intakes can produce reasonable compression (60% of the full Busemann intake) with the original level of total pressure recovery maintained in viscous flowfields. Stunted intakes, on the other hand, can enhance compression with considerable total pressure penalty, eventually leading to intake unstart at a certain point due to the emergence of Mach reflection, which makes this method potentially useful in situations where locally high pressure and temperature are desired near the centreline at the cost of total pressure recovery.
Experiments were carried out to visualize mixing and reaction of two water-based solutions inside the droplets generated in the microchannels on a rotating disk. The microchannels were composed of a Y-junction to bring the two solutions into contact, a flow-focusing junction to form water-in-oil droplets, and a U-shaped channel to further enhance the droplet mixing. In the centrifugal microfluidics, plug-type droplets of 582-820 µm in length were formed at rotational speeds in the range 400-500 rpm. The mixing efficiency can be highly enhanced to reach as much as 85% for the lower rotational speed where smaller droplets are formed due to lower dispersed-phase flow velocity.
The purpose of the research is to develop the method of designing and manufacturing a sensitivity-adjustable three component force balance utilized in a wind tunnel experiment. This paper describes the characteristics of the force balance and its application to wind tunnel testing. In this study, an oval-shape force balance is newly introduced and the characteristics are investigated by FEM analysis and force balance calibration. Also, the validity of the force balance is investigated in a wind tunnel experiment. As a result, the sensitivity of the oval-shape force balance can be adjusted by changing the aspect ratio and aerodynamic forces can be measured in a wind tunnel experiment using it. In conclusion, we can develop the sensitivity-adjustable force balance by applying oval shape and the force balance can be used in a wind tunnel experiment. It is recommended that the strain gauge position and its paste procedure be still improved.
A deconvolution method is presented for correcting the smoothing effect caused by vortex wandering in velocity measurements of wingtip vortices. Flowfield velocity data are acquired using SPIV in an open-jet wind tunnel at several streamwise locations. A parametric Wiener filter that incorporates an estimate of the noise-to-signal ratio and the joint probability density function of the vortex motion is applied to correct for the wandering using a Fourier-domain deconvolution approach. In order to assess the accuracy of the correction, the instantaneous vector fields obtained from SPIV are shifted to the mean vortex center, and then averaged to yield the uncorrupted vortex flowfield. The unshifted mean flowfield data is then corrected by deconvolution and is then compared with the uncorrupted vortex flowfield. The results show that the deconvolution technique accurately corrects for the vortex wandering.
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