Development of localized disturbances generated by an oscillating point source in compressible boundary layers with a zero pressure gradient at Mach numbers from 0.2 to 2.0 is studied theoretically on the basis of the linear stability theory. The method of complex characteristics recently proposed by Itoh as an extension of Whitham’s kinematic wave theory, is applied to describe wave propagation from the oscillating source. The analysis demonstrates distinct differences in the development of localized disturbances between the subsonic and supersonic boundary layers. Importantly, maximum growth occurs away from the midspan in supersonic boundary layers, while it occurs at the midspan in subsonic boundary layers.
This paper describes the performance characteristics of a 20 kW-class arc-heated wind tunnel and the investigation of material catalysis as an application of this facility. First of all, stagnation heat fluxes and pressures are measured in high-enthalpy air and nitrogen arc jets that are generated by a constrictor-type arc heater. Total enthalpies of the arc jets are estimated based on Pope’s theory. The total enthalpies are 7.5–22 MJ/kg for nitrogen and 13–19 MJ/kg for air. The operation envelope of this arc-heated wind tunnel and the applicable conditions for reentry simulation or the thermal protection tests are discussed based on the experimental results. Finally, as an application of this arc-heated wind tunnel, evaluation tests of wall catalysis have been attempted by the use of two flat-faced cylindrical models equipped with two kinds of specimens of catalytic material. The evaluation tests are composed of heat flux measurements and spectroscopic measurements. It is found that there is a certain difference in obtained heat fluxes between the two catalytic materials. Emission spectra from a shock layer formed in front of the model are measured, and wall catalysis is discussed based on the spectra measurements. The results show the existence of catalytic wall effects.
A wavelet-based vector compression technique was proposed as a new technique of PIV post-processing for efficiently eliminating erroneous vectors and reducing physical storage. To determine the effect of the choice of wavelet bases, the velocity vector field obtained from standard PIV images was compressed using some known wavelet functions, such as Daubechies, Coifman and Baylkin families. It was found that the lower-order wavelet bases provide good compression performance because they have good physical localization, which in turn, increases energy compaction. When being applied to the PIV result of a jet, the correction of erroneous vectors can be realized by increasing the compression ratio to 16.
An extension of classical theories for a two-dimensional thin airfoil oscillating in an incompressible flow is achieved by assuming arbitrary and constant convection velocity of the wake vortices. The well-known Theodorsen function is replaced by different functions composed of the Theodorsen function and a new function. Their vector properties, which depend on the magnitude of the convection velocity of the wake vortices, are presented. The extended results include classical theories as special cases.
A method to control flow separation on a square cylinder has been investigated. The method incorporates the use of small tabs that were deployed in highly separated regions on the sides of the cylinder. The effectiveness of the tabs was investigated mainly at M=0.56, with limited data also taken at M=0.71. The tabs proved to be very effective in reducing drag and suppressing force fluctuations on the cylinder. The largest drag reduction, amounting to almost 35%, was achieved at M=0.56. Pressure measurements on the cylinder surface revealed that large pressure increases were created across the tabs and on the cylinder’s rear surface, leading to the observation of a large reduction in drag. Based on the experimental observations, a simplified flow model to explain these phenomena is proposed in which the tab effects are attributed mainly to the obstruction of the reverse flow in the separated region, creating pressure reduction upstream of the tab and pressure increase downstream of it.
An experimental study on the pulse detonation engine (PDE) is conducted using hydrogen-air mixtures. Several key issues for PDE development, including valve operation, injection, mixing, filling, cycle repetition, ignition timing, DDT distance and propagation of detonation/quasi-detonation, are investigated. The fuel and oxidizer are injected into the PDE from opposite sidewall directions so as to be well mixed by collision of the two jets. A good agreement is obtained between the calculated and measured mixing ratios, indicating the occurrence of nearly instant mixing. Before the detonation velocity has reached the CJ value, it was found that the wave propagation velocity at the PDE exit increases with the increase in diameter, length and blockage ratio of the Shchelkin wire, and initial pressure. The PDE performance acquired was a specific impulse of about 2000 s, which was measured from the pressure history at the head end of the PDE.
The cycle of an ideal pulse detonation engine (PDE) was theoretically analyzed. A PDE was modeled as a straight tube, one end of which was closed and the other end open. A detonation wave was ignited at the closed end and simultaneously started to propagate toward the open end. When the detonation wave broke out from the open end, a rarefaction wave started to propagate from the open end toward the closed end. We analytically obtained a functional form of the thrust-density history, showing a plateau followed by decay. Using the obtained history of the thrust density, we formulated some PDE performance parameters such as impulse density per cycle.
The concept of applying semi-active control systems to vibration isolation and shock attenuation is proposed. In this paper, a semi-active isolator for enhancing the pointing performance of optical equipment on-board satellites and its characteristics are first introduced and then a generally applicable semi-active control law is proposed for vibration isolation. The isolation performance of the semi-active control law proposed in this study is evaluated through numerical simulations. The performance of the system with the semi-active control law proposed in this study is compared with those of a passive system and a semi-active system with a previously proposed semiactive control law. Next, to investigate whether or not the isolator is also effective for attenuating shock that is induced by rocket lift-off and pyrodevice ignition for inter-stage separation, we also performed numerical simulations and evaluated its effectiveness.
Extended lift and moment, based on an extended Theodorsen function for arbitrary and constant convection velocity of wake vortices, are converted to aerodynamic work. Pure pitching oscillations, pure translational oscillations and their combinations are considered. Decrease in convection velocity of wake vortices yields an increase in aerodynamic work, thus destabilizing airfoil oscillations. Some of the available wind-tunnel data support the present theoretical predictions.