This paper studies the controller design problem for linear time-invariant systems that have on-off type actuators. Although phase-plane analysis, the describing function approach and pulse-modulation technologies have been studied in the classical literature, such approaches do not permit the direct application of optimal controller design techniques including non-linear effects. In this paper, we propose the application of input-state linearization theory to the general class of single on-off input plants. The linearized space resulting from this method allows the application of any linear controller design tool. The method is also extended to be applicable to on-off controllers with a dead-band. To demonstrate the effectiveness of this approach, some numerical studies are presented including the lunar landing problem, and the results are evaluated and compared with numerical optimal solutions.
A new numerical method using a hybrid (explicit/implicit) scheme with the matrix-free Lower-Upper Symmetric Gauss-Sidel (LU-SGS) method has been developed to compute unsteady viscous flows on highly stiff grids, where the ratio of the maximal to the minimal cell spacing is large. A dual-time stepping method is used to reduce unsteady residuals. Results show that the present method has advantage over conventional explicit and implicit schemes. The CPU time has been reduced by 69% and 57% compared with the 2nd-order explicit scheme and the full implicit scheme, respectively, in the case of a rather low Reynolds number flow. On the other hand, at a practical Reynolds number of 5.54×107, the CPU time becomes about 30 times shorter than that of the explicit scheme.
The experimental study has been conducted to estimate the performance of a cryogenic wind tunnel by using flashing liquid nitrogen and the location of reattachment at the downstream of the backward-facing step. Total and static pressure and temperature of the flow in the test section are measured to calculate flow the Mach number and Reynolds number. Flow visualization by color-schlieren method was also performed. Liquefied nitrogen at a pressure of 700kPa produced the uniform flow, of which the Mach number and total pressure ranges were 0.08 to 0.8, and 20 to 200kPa, respectively. The Reynolds number based on the step height (5mm) ranged from 0.15×105 to 0.44×105, and the duration of stable operation was limited to 0.4–0.8 seconds in our configuration. The results indicated that the prediction by FFT analysis to the reattachment location over the backward-facing step was good agreement with that of their static pressure distributions and visualization results.
We have developed a shrouded wind turbine with a brimmed-diffuser. To realize higher power augmentation of it, we have investigated the optimum parameters of the diffuser angle and hub ratio, and the optimum shapes of the inlet shroud and center body. As a result, the new wind turbine has demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 5 compared with a bare wind turbine.
Six Sigma is the management strategy developed by Motorola to reduce defects in products. Design for Six Sigma (DFSS) is a methodology for determining the values of the design parameters, which maximize the performance of some system without tightening the material, manufacturing or environmental tolerances. This paper presents the implementation of DFSS for redesign of the LE-7 engine. Uncertainties with design parameters and operational conditions are considered in evaluating thrust performance, thrust chamber life, turbo-pump cavitation, and combustion stability. Traditional deterministic optimization results and probabilistic optimization results are compared. It is found that robustness of rocket engine is not always consistent with the extension of thrust chamber life.
In order to control a reentry trajectory of a vertical landing rocket, an opposing jet system is experimentally tested at supersonic speeds. Experiments are conducted at Mach number 4.0 in the supersonic wind tunnel of ISAS. Supersonic nozzles of the exit Mach number 2.4 are installed on a blunt nose of a rocket model. The most significant drag reduction due to the jet-spike effect is obtained when the jet is injected from the stagnation point of the body in the opposite direction to the freestream. Various types of the nozzle arrangement are investigated and the method to increase the L/D is discussed. It is found that the off-axis jet is effective both to increase the L/D and to enable a vehicle to be trimmed at an intended attack angle.
The control of drag and lift oscillation around a two-dimensional elliptical airfoil in a transonic regime is numerically investigated in the present study. In our laboratory, experimental researches on the control of both drag and lift oscillation around a circular cylinder, a square cylinder and an elliptical airfoil have been carried out, using small tabs. The objective of the present research is to make clear the detailed mechanism of reduction in drag and lift oscillation about the elliptical airfoil. As a result, it was found that small tabs make a virtual shape, and that the tabs push separation vortices, which cause the base drag, away from the airfoil. In addition, streamlines over the tabs are not largely curved in the wake. Therefore, the strength of shock waves produced on the airfoil is weakened, leading to a reduction in lift oscillation.