Sound radiation from a rectangular cylinder, the cross-sectional aspect ratio (AR) of which varied from 0.3 to 4, was studied experimentally at Reynolds numbers of 7.5 × 103 to 1.4 × 105 and Mach numbers below 0.16. In addition to measurements of the lift-dipole sound, detailed flow fields around the rectangular cylinder were examined by means of PIV to better understand the dependence of sound radiation on the AR and Reynolds number as well as the vortex-cylinder interaction associated with sound generation. The mean square sound pressure was proportional to U6∞ St2, with almost the same factor of proportionality for aspect ratios larger than 0.5 in the Reynolds number range examined in spite of the fact that the spanwise correlation length and magnitude of the lift fluctuations were highly dependent on the AR. When vortex shedding at the trailing-edge was locked in phase with the leading-edge-generated vortices, which occurred for AR > 3, the shed vortices were quite two-dimensional, and correspondingly, the radiated sound exhibited a very sharp spectral peak like a line spectrum; the SPL was as strong as that for AR = 0.75, for which the maximum SPL was observed. Reynolds number effects on both SPL and the Strouhal number were weak except around the aspect ratio where reattachment of the separated leading-edge shear layer occurred.
This paper investigates the Earth escape for spacecraft in a Sun-Earth halo orbit. The escape trajectory consists of first ejecting to the unstable manifold associated with the halo orbit, then coasting along the manifold until encountering the Moon, and finally performing lunar-gravity-assisted escape. The first intersection of the manifold tube and Moon’s orbit results in four intersection points. These four manifold-guided encounters have different relative velocities (v∞) to the Moon; therefore, the corresponding lunar swingbys can result in different levels of characteristic energy (C3) with respect to the Earth. To further exploit these manifold-guided lunar encounters, subsequent swingbys utilizing solar perturbation are considered. A graphical method is introduced to reveal the theoretical upper limits of the C3 achieved by double and multiple swingbys. The numerically solved Sun-perturbed Moon-to-Moon transfers indicate that a second lunar swingby can efficiently increase C3. Compared to the direct low-energy escape along the manifold, applying a portion of the lunar swingbys before escape is shown to be more advantageous for deep-space mission design.
Nanyang Technological University has built and successfully launched the VELOX-PII and VELOX-I cubesats. Both of the satellites carry a low-power consumption in-house-developed attitude determination and control subsystem (ADCS). The ADCS uses an 8-bit microcontroller. However, in-orbit propagation data indicates that the ADCS suffers high orbit propagation error due to the finite word length of the processor. High orbit propagation error could lead to a higher attitude determination error that affects the satellite’s pointing accuracy. This paper presents a pre-processed orbit parameter method to minimize the orbit propagation and attitude determination errors. The two-line-element (TLE) data is propagated to the nearest seven significant digits of a selected time epoch. The time epoch selection is required to fulfill the single precision floating point conversion constraint. The orbit propagator performance has been evaluated for modified orbital parameters in both Geocentric Celestial and true equator mean equinox reference frames, direct input of NORAD TLE, and SGP4 methods using the VELOX-I and FORMOSAT-3/Cosmic Global Positioning System data. The results show a reduction in propagation error on the order of one to two magnitudes. In addition, the method proposed has the lowest attitude determination and target pointing error when compared with different TLE input methods.
Acoustic waves generated from an impingement of the exhaust jet on the flame deflector is one of the causes of the payload vibration during the liftoff of a launch vehicle. To understand this acoustic phenomenon, the authors conducted experiments using a correctly expanded jet impinging on an inclined flat plate, and observed an acoustic wave propagating from the jet impingement region in a direction approximately perpendicular to the plate. The results of the wavelet analysis indicated that this acoustic phenomenon occurs intermittently. In this study, a flow phenomenon related to this intermittent acoustic phenomenon is extracted from schlieren visualization movies and wall surface pressure signals using a conditional sampling analysis. The extracted flow phenomenon is similar to the Mach wave of a free jet. With the nozzle-plate distance at which the OASPL of this acoustic phenomenon is the maximum, the flow phenomenon extracted is also similar, but some minor differences appear. These differences are expected to be a key to understand the change in OASPL when varying the nozzle-plate distance.
The objective of this study is to ascertain the steady-state aerodynamic characteristics of a thin circular arc airfoil with a large-camber by comparing its camber with various cambers at Reynolds numbers ranging from 1×103 to 10×103, similar to that of insect flight. A large camber of over 20% chord is seen in the elytra of beetles (Coleoptera), many of which fly by opening the elytra. A low-pressure wind tunnel that we uniquely designed was used to measure the very small forces and moment acting on the wing. As a result, a high maximum lift coefficient was obtained for a large-camber circular arc airfoil, even at Reynolds numbers less than 3×103. In addition, the pitching moment of the airfoil was found to be zero in a wide range of angles of attack when the moment center was set to the appropriate position. The large-camber circular arc airfoil will be useful in designing very-low Reynolds number aircraft such as insect-sized micro-air vehicles (MAVs).
The prediction method for heat flux in the pseudo-shock region is improved based on data measurements taken during ramjet-mode experiments. Combustion experiments using a ramjet-mode combustor are conducted to investigate heat flux in the pseudo-shock region. The heat flux is measured in the region with and without combustion. The heat transfer coefficient is modified using the Stanton number and specific heat under combustion. Using modified coefficients, the modified heat transfer coefficient ratios show the same relation to the pressure ratios as those without combustion. The ratio of heat transfer coefficients is calculated using a single equation. The calculation method can predict the experimental results of heat flux in the pseudo-shock region with combustion in the divergent duct.