Station-keeping (SK) is indispensable in actual geostationary (GEO) satellite missions. Due to the luni-solar gravity perturbations, the inclination of a GEO satellite suffers the issues of secular ...drift and long-period oscillation. Current north-south (NS) SK strategies maintain the GEO satellite’s orbit with high accuracy but low fuel efficiency. In this work, an efficient high-accuracy NS-SK strategy is developed for the GEO satellites. First, an averaging method is employed to decrease the accumulation of the secular drift within a one-solar-day SK cycle, while the long-period oscillation caused by the solar gravity is damped to further improve the orbital accuracy using the impulse and finite-thrust propulsions. Second, we contribute a fuel-optimal cycle that reduces the fuel consumption and a fixed-interval cycle that executes SK control in fixed time interval every day to further enhance the proposed NS-SK strategy. Numerical simulations show that the improved strategy can achieve high-accuracy NS-SK with little fuel consumption. Moreover, results also demonstrate that the fixed-interval cycle can reach higher NS-SK accuracy while consuming less fuel.
Experiments were conducted to measure the onset of flow instability (OFI) (static flow instability) and dynamic unsteady flow in a compact heat sink consisting of 26 rectangular microchannels with ...300 μm width and 800 μm depth. The planform area is 5.0×1.53 cm2. Tests were performed with water and methanol as working fluids with mass fluxes of 20–1200 kg/m2s, inlet temperatures of 30, 50 and 70 °C and effective heating powers of 100–450 W. It was found that the onset of flow instability occurs at the outlet temperature of 93–96 °C, which was several degrees lower than the saturated temperature of 100 °C corresponding to the exit pressure. Fine bubbles were detected close to the channel exit, and they were moving out of the channel to form a unit circular bubble in the outlet plenum. Once the mass flux is lower than that of OFI, three types of oscillations were identified: large amplitude/long period oscillation superimposed with small amplitude/short period oscillation and small amplitude/short period oscillation. Thermal oscillations were always accompanying the above two oscillations. Observations through the microscope and physical explanations were given corresponding to the above oscillation flows. Similar tests were also performed with methanol as working fluid.
We have found that the current range is divided into two parts for the appearance of potential oscillation during methanol oxidation on polycrystalline platinum at 315 K when the methanol ...concentration is between 10 and 0.03 mol dm−3 (M). The current range for oscillation disappearance, named ΔI (od), increases with a decrease in the methanol concentration. At currents below the ΔI (od), the oscillation waveform is always a large-amplitude and long-period (sometimes of the order of an hour) oscillation, named oscillation L. At currents immediately above the ΔI (od), a small-amplitude and short-period oscillation, named oscillation S, is observed for 0.03 to 0.1 M methanol and oscillation L is for 0.3 to 10 M methanol. For the latter methanol concentrations, oscillation S also becomes observed at higher currents. Such a ΔI (od) has not been found for potential oscillations during formic acid or formaldehyde oxidation and, thus, the existence of the ΔI (od) is distinctive to potential oscillations during methanol oxidation.
Oceans and seas, which cover a large percentage of the Earth, interact with the atmosphere at their upper boundary. The generation, growth and breaking of wind waves are the most prominent physical ...processes that occur at water surface. In this study the transformation of waves groups in a medium water of coastal zone towards breaking point is investigated in two dimensional case. The governing equations are Navier–Stokes, continuity and Fractional VOF function equations. The fluid flow is assumed to be viscous and incompressible, and the turbulence effect has been considered in wave evolution using a two equation
k
–
ε
model. The DDAR–VOF method which is a kind of SLIC method defined by Moradi Moradi Larmaei M, Simulation of waves groups propagation and breaking in medium water depth using VOF method. M.Sc. thesis (Distinct). Iran: Amirkabir University of Technology; 2006 is used by employing a finite volume method based on the PISO algorithm. To validate the numerical model, different tests such as led-driven cavity, a simple translation test, rotation test, dam-break, turbulent pattern and plunging breaking are performed. These tests show that this model is a powerful and reliable tool to simulate free water surface in wave motion with breaking process. The numerical results obtained indicate the existence of the waves group inducing set-down in the form of long wave in a medium water depth due to passage of high waves group. Comparison of the results with the experimental data shows that the current modified model is able to predict this complicated phenomenon accurately.
We here investigate the frequency and intensity of oscillations in oceanographic data within intraseasonal time scales using spectral analysis of surface wind and wave time-series data collected at ...off-island weather stations or moored buoys around Taiwan. Data from marine weather stations were used to trace atmospheric conditions, while we used buoy data to examine sea states. The spectra and wavelet scalogram of the wind fields revealed oscillations with a period of around 20-33 days, and the energy density of the wind field at the off-island stations was stronger than that at the data buoy stations. However, the wavelet scalogram of the wave height measured at the buoy stations was stronger than its associated wind field. This long-period oscillation is consistent with the wavelet scalogram of the wind field calculated from the off-island weather stations. About 20-33 day oscillations exist within intraseasonal variations, which are closely linked to the atmospheric environment and to wind and ocean wave fields. Oscillations with a period of 5-10 days are a pronounced feature over northeastern Taiwan waters during the winter season and can be interpreted as the wave pattern following synoptic weather systems.PUBLICATION ABSTRACT
The long period oscillation (LPO) with high amplitude (1m) and period less than 5-minute was observed at Shimohaya Bay in Wakayama Prefecture in a storm. The sea wind gustiness and wave group ...observed at the offshore area had the nearly same period as the LPO. We had the numerical simulation of the LPO, assuming that the LPO was generated by only groupness effect of breaking waves. The results showed enough intensity of amplifying of long wave by topographic resonance (6-minute long wave was amplified more than 10 times in Shimohaya Bay). However, the high amplified point was not restricted in Shimohaya Bay by the simulation.
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