This research focuses on analyzing the human characteristics of pair assembly work in a microgravity environment. To obtain the necessary data, two groups of test subjects participated in an experiment in a pseudomicrogravity environment, and how such factors as work allotment, sizes of assembled objects, and conditions of parts influence the work efficiency were studied. The results show when the assembled object is smaller that it is easier to change the object’s position and that workers can move it to an appropriate site and work easily. If the assembly object is attached to an object with very large mass, the work allotment does not result in a difference in work time. As for positioning the objects, step-saving processes can shorten the work time. For instance, if a part can be put so that both ends automatically touch the proper spots, time can be shortened.
The aerodynamic characteristics of a hemi-circular wing-in-ground are studied analytically. The exact expression of a spanwise optimum normal force distribution and the minimum induced drag are obtained from Trefftz plane flow field analysis, where Söhngen’s inversion integral formula and the elliptic integrals and Jacobian elliptic functions are used. The minimum induced drag obtained in the present study is coincident with that of Mamada and Ando.
Nonlinear optimal state feedback control is designed for the deployment and retrieval of a tethered satellite in the orbital plane. The mass and flexibility of the tether are neglected, and the controlled system is expressed by a bilinear state equation. To stabilize the system asymptotically, the receding horizon control is applied with the terminal state constrained to be zero. As a practical approach to the problem, the time-dependent penalty function method is employed to achieve the terminal constraint asymptotically, and an algorithm is derived so that an increase of the terminal penalty will cause no numerical difficulty. The effectiveness and robustness of the present approach are confirmed in numerical simulations.
This paper applies a sliding mode controller for suppressing two-dimensional incompressible flow flutter problems in which aerodynamic uncertainties and structural nonlinearity are incorporated. To consider the aerodynamic uncertainties, the steady aerodynamic theory is regarded for the controller design, although the quasi-steady aerodynamic theory is employed in numerical simulation. The structural nonlinearity is included about pitch motion to make a limit cycle oscillation appearing in flutter phenomenon. The control inputs are leading and trailing edge control surfaces. At a velocity of 20m/s, the sliding mode controller is designed to suppress the initial response within 1 second while the uncontrolled wing shows limit cycle oscillation. Furthermore, the sliding mode controller shows robust characteristics in a wide range of the flow velocity. These results indicate that the sliding mode controller is effective for suppressing the flutter problems that have aerodynamic uncertainties and structural nonlinearity.
As pointing requirements of astronomy satellites become more demanding, a conventional momentum-exchange scheme for attitude-pointing control is sometimes not enough to satisfy stringent stability requirements. This paper presents a new attitude control system for improving the attitude-pointing stability of astronomy satellites/and earth-observation satellites. In our proposed control system, an actuator system is composed of wheel-rotor drive motors and magnetic torquers driven by a newly devised “cooperative drive law.” The pointing stability is improved by torque control of the actuator system so that the total torque acting on the satellite body is minimized with the use of disturbance observers and in a way that shortcomings of the actuators are compensated for by each other. In the torque control, attitude control and momentum management are both treated consistently. In this paper, the mathematical formulation of our proposed scheme is presented, and the results of numerical simulations are also presented to demonstrate the stability improvement.
The stability of compressible three-dimensional boundary layers to stationary disturbances is examined on the basis of the linear stability theory. Comparisons of stability characteristics are made between the subsonic and supersonic boundary layers at the edge Mach numbers 0.2 and 2.0, respectively. The result shows that the boundary layer becomes unstable to stationary three-dimensional modes when the cross-flow velocity exceeds a rather small threshold of less than 1% of the external flow velocity. Important to note that the critical Reynolds number for stationary modes does not strongly depend on the Mach number. It is also found that the wavelength of the most amplified stationary three-dimensional mode is four or five times the boundary-layer thickness, not depending on the magnitude of cross-flow velocity both for the subsonic and supersonic flows.
In-plane/out-of-plane librations of a tethered system in elliptical orbits are investigated. It is aimed to clarify the fundamental characteristics of libration of the tethered system subjected to orbital motion and atmospheric drag. Periodic solutions and their stability of a simplified 3-DOF model are analyzed, in which in-plane/out-of-plane librations and longitudinal elongation of the tether are considered. It is shown that the librations become asymptotically stable or unstable because of atmospheric drag. The effects of system parameters on stability are analyzed, and it is shown that the property of longitudinal elongation of the tether determines the nature of stability, that is, asymptotically stable or unstable. It is also shown that the property of atmospheric drag determines only the degree of stability. Results of direct numerical simulations show the validity of the results of stability analyses. Physical interpretation of the phenomena is also clearly shown.
To evaluate coherent structures in the dimension of time and scale, a definition of wavelet multiresolution autocorrelation based on the discrete wavelet transform is first developed. Then a new identification technique that combines the wavelet multiresolution analysis combined with the wavelet multiresolution autocorrelation analysis is proposed. By analyzing u- and v-components of fluctuating velocity, the coherent structure and its scales can be identified when larger local amplitude fluctuation and stronger autocorrelation appear at the same wavelet level. For a turbulent jet at a downstream distance of x/d=6, the coherent structures with frequency 39Hz can be deduced about times 0.29, 0.53, 0.6, and 0.67s. This also represents the passing of eddies through the shear layer and concentration of the energy of the flow at these instants.
This paper describes the experimental and numerical studies of a laboratory model of the low-power nitrogen arcjet thruster that was developed to provide thruster performance data to validate numerical results. The arcjet thruster was operated by using a nozzle 1.0mm in constrictor diameter. Thrust and input power were measured for various arc currents and nitrogen mass flow rates. The operation was done at power levels ranging from 156W to 540W and nitrogen mass flow rates from 5mg/s to 30mg/s. Typical specific impulse obtained in the experiment was 188s at 542W. Numerical simulation was conducted by using the physical model of a thermochemical nonequilibrium gaseous flow, a two-temperature model consisting of heavy particle and electron temperatures. The flowfield equations were numerically solved by combining with the Maxwell’s equation and the generalized Ohm’s law. It is shown that the predicted thruster performance is higher than the experimental data for the specific impulse, and the possible causes for this trend are discussed.