In this paper, a model-based control design method (i.e., composite controlled Lagrangian method) for a two-link flexible manipulator is proposed. A two-link flexible manipulator is an underactuated Euler-Lagrange system, meaning that there is no control input acting directly on the flexible variables. In general, it is hard to apply the controlled Lagrangian method to this kind of system. But if we take into account the larger stiffness of the links, it is easy to see that the dynamics of this kind of Euler-Lagrange system express two-time-scale characteristics, which means the total Euler-Lagrange system can be considered as a two-time-scale system and can be decomposed into two subsystems: a slow subsystem describing the rigid motion and a fast subsystem describing the flexible vibration. For this two-time-scale Euler-Lagrange system, we explore a new control design idea; that is, an energy-based two-time-scale control design. First, we show a new way to separate the time-scale by using coordinate transformation according to the rigid and flexible modes, which is different from other multi-scale methods such as the singular perturbation approach applied to controlling flexible manipulators, where the separation of time-scale is completed by assuming that the small perturbation parameter is equal to zero. After completing the energy shaping and damping injection for two Euler-Lagrange subsystems of different time-scales, a composite controller is obtained following the idea of composite control. Both the simulation and experimental results are presented to show the effectiveness of this control design method.
The fundamental aerodynamic characteristics of a paraglider’s canopy are investigated in wind tunnel experiments using an inflatable cell model designed to represent the dynamic behaviors of each cell comprising the canopy. At attack angles greater than a few degrees, the cell model inflates fully. To characterize its aerodynamic characteristics, we focus our attention on the flow around the inflated cell model at the plane of symmetry of the model. The cross-sectional profile of the inflated cell model, streamline pattern, internal air pressure and external surface pressure distribution are measured at various attack angles in order to identify the function of air intake and to obtain the lift and drag coefficients of the airfoil with an open air intake. The results reveal the mechanism of how the cell inflates into a stable wing shape and bears the buckling force caused by the cables suspending a pay load.
We have developed a new wind turbine system that consists of a diffuser shroud with a broad-ring brim at the exit periphery and a wind turbine inside it. The brimmed-diffuser shroud plays the role of a device for collecting and accelerating the approaching wind. Emphasis is placed on positioning the brim at the exit of the diffuser shroud. Namely, the brim generates a very low-pressure region in the exit neighborhood of the diffuser by strong vortex formation and draws more mass flow to the wind turbine inside the diffuser shroud. To obtain a higher power output of the shrouded wind turbine, we have examined the optimal form for the brimmed diffuser, such as the diffuser open angle, brim height, hub ratio, centerbody length, inlet shroud shape and so on. As a result, a shrouded wind turbine equipped with a brimmed diffuser has been developed, and demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about five compared to a standard (bare) wind turbine.
Four different circulation controlled airfoils have been numerically simulated. The baseline airfoil was a 17% thick supercritical airfoil. Different blowing rates have been examined by adjusting the slot height and blowing velocity. A number of turbulence models were employed, these were: Spalart-Allmaras, standard κ–ε, realizable κ–ε, SST κ–ω and Reynolds stress model. The results from the numerical simulations were compared with experimental data at zero angle of attack. The solutions indicated that at momentum coefficients, Cμ=0.1 or greater, all isotropic turbulence models failed to capture the physics of the circulation control problem. The Reynolds stress model captured successfully the physics at Cμ=0.1. At greater values of momentum coefficient, the Reynolds stress model also failed to predict the experimentally measured lift coefficients because the jet remained attached to the surface of the airfoil. The Spalart-Allmaras model consistently predicted the right trend for lift variation with Cμ in all cases tested.
The objectives of this study are to assess the accuracy of CFD codes, investigate the effect of turbulence models as applied to the flow around high-lift devices, and increase the knowledge for computing this kind of flow. CFD validation is conducted for a two-element airfoil and compared with a wind tunnel test to predict the aerodynamic forces, including the maximum lift and the stall angle. Sensitivity to grid density and influence of the grid extent are investigated. Four RANS codes with the same turbulence model are tested and computational results are compared with each other. Three commonly used turbulence models implemented in a CFD code are applied to investigate the effects of turbulence models for this kind of high-lift flow.
In this study, the accuracy of structured and unstructured mesh CFD codes in simulating the flow around a three-element high-lift configuration (slat, main wing, and flap) is assessed, and mesh dependency and effect of turbulence models are studied. In the first part of the study, the results of two structured mesh CFD codes and an unstructured mesh CFD code using the same turbulence model are compared and discussed. A mesh refinement approach is also used to examine the dependency of numerical accuracy on the density of unstructured meshes. By properly distributing mesh points in an unstructured mesh, the detail in flow physics is obtained. It is also shown that the quantitative prediction of the vortex in the slat cove is an important factor that affects accuracy. In the second part of the study, three turbulence models are compared using one of the structured mesh CFD codes. All turbulence models can produce similar flowfields. However, several differences are seen in the separated regions, especially in the slat cove. Commonly used turbulence models, Spalart-Allmaras model and Menter’s SST model, produce similar aerodynamic forces at lower angles of attack. At higher angles of attack, the SST model gives better results for the present computations. It is found that the maximum lift and the angle at which the stall occurs are very sensitive to the turbulence model.
Atomic oxygen wall recombination coefficients at a high-temperature regime, more than 1300 K, and in a low static pressure environment are measured using the actinometric method for SiC, highly catalytic and SiO2 materials. For this purpose, the inductively coupled plasma of O2 with argon, as an actinometer, is employed. Measured values are compared with the numerical prediction obtained using the Kurotaki model. Agreement with the Kurotaki model is reasonable at least qualitatively.