A three-dimensional Euler code for cascade flow calculation which is accurate and robust with good convergence property has been developed to investigate unsteady flow phenomena such as rotating stall or surge. The time-dependent Euler solvers provide a single approach for subsonic, transonic and supersonic flows, and have inherent shock-capturing capability. Consequently, they can provide important information on flow field such as shock location and pressure distribution. Moreover, they form a very essential part of Navier-Stokes solvers. In this research, the code is developed based upon a non-MUSCL-type TVD scheme using Roe's approximate Riemann solver. During this development, an insight which we consider to be quite important in the numerical simulations has been obtained. There exists ambiguity in the derivation of “Roe's averaging” on density. Our research has revealed that inappropriate use of this averaged density is not only analytically incorrect, but can have detrimental effects on flow calculation based upon Roe's approximate Riemann solver, especially when this averaged density is used in conjunction with unphysical eigen vectors of the Jacobian matrix of flux. The reason why this occures is fully discussed. The example for such a case is illustrated. Also, in order to show the quality of the developed code, the result of a transonic compressor rotor cascade at its design point is presented.
The instabilities of confined, supersonic, double shear layer flow are examined in order to develop a mixing enhancement scheme in two-dimensional flow configuration. It is shown that flow confinement is essential to produce the skew-symmetric mode of instability wave which is responsible for rapid mixing in the subsequent nonlinear evolution stage. The linear stability analysis provides a design tool with which suitable flow configuration can be selected. Direct numerical simulation demonstrates that hydrogen gas slowly issued in between supersonic air streams menders, accelerates to supersonic speed and mixes with the air quickly without shock generation, thus leading to explosive combustion. The underlying new physics are explored.
The cross coupling effects of a helicopter rotor in low advance ratio are experimentally studied using the Dynamic Running Test Facility which was built at National Defense Academy recently. Design features and basic performances of the facility are briefly described. Flapping motions of the model rotor blade are precisely measured together with its thrust and torque characteristics at various combinations of test parameters. It is clarified that nonlinear behavior of the cross coupling effects on the advance ratio are attributed to the phase shift properties of blade flapping motions due to nonuniformity of induced velocity distributions on the rotor disc. Usefulness of a dynamic running test is also ascertained for accurate measurements of aerodynamic and dynamic phenomena of a rotorcraft in low speed flight.
This paper discusses the application of precise clock synchronization to satellite-based navigation (positioning) and mobile communication and mentions its usefulness. The synchronization of the clock carried on a mobile radio station with the precise clock installed in a ground radio station is accomplished precisely by the bidirectional communication between these stations, because the greater part of radio propagation delay errors are canceled in the calculation of the clock offset between the two stations. The use of the clock offset for the clock synchronized navigation and communication will bring us the reduction of the number of satellites in orbit and the optional communication between mobile and ground stations, and will produce the efficient integration of wide area navigation and mobile communication.
This paper first describes a modeling method for the uncertainty of aerodynamic coefficients in a linear state-space aircraft model. Second, a design method of robust flight control systems based on the model is also presented. The proposed method is applied to the structure of the uncertainty for the μ-analysis and has the following characteristics: 1) the variations of the altitude and the velocity are explicitly treated as real repeated scalar perturbations since all dimensional derivatives are the function of the altitude and the velocity, and as a result 2) a gain-scheduled controller using the dynamic pressure compensation which satisfies the robust performance for a comparatively wider flight envelope can be designed. Simulation study is carried out to indicate that the proposed method is less conservative and satisfies the robust performance for a wider flight envelope than those of the H∞ design and the μ-synthesis where all aerodynamic coefficients are assumed to be individually perturbed.
In this paper, buckling and postbuckling of composite laminates with a delamination are studied by numerical analysis. Embedded delamination is a kind of damage being frequently observed in composite laminates and may significantly reduce the load carrying capacity due to the buckling of sublamination and the propagation of the delamination. A three dimensional nonlinear finite element program is developed to simulate the above mentioned problem. A special attention is given to the effects of the local asymmetry due to delamination on the behavior of buckling and postbuckling of the laminate under compressive load. Analytical results show the obvious difference between the critical loads obtained from the present analysis and those from the two dimensional finite element analysis which cannot consider the effects of the local asymmetry.
The effect of the diffusive-thermal instability on the burning velocity of cellular flames is numerically studied. The burning velocity of cellular flames is large compared with that of plane flames and increases as the Lewis number becomes lower. When the Lewis number is unity, rise of the burning velocity is equal to that of the area of a flame surface. On the other hand, when the Lewis number is lower than unity, the former is larger than the latter. The local burning velocity increases/decreases at the convex/concave flame surface toward the unburned gas for low Lewis numbers. The increase of the local burning velocity at the convex surpasses the decrease at the concave. Thus, the ratio of the burning velocity to the surface area increases as intensity of the diffusive-thermal instability becomes greater.
Numerical flutter simulation of a high-aspect-ratio transport type of wing in transonic region is presented. The aeroelastic responses of the wing are obtained by integrating compressible thin-layer Navier-Stokes (N-S) equations coupled with the equations of motion of the wing structure. The Yee-Harten implicit TVD scheme and the Wilson θ method are employed to integrate N-S equations and structural ones, respectively. The flutter boundaries are obtained at the angle of attack of 2 degrees and compared with experimental results. The flutter characteristics of this wing model are investigated and discussed. Furthermore, Limit cycle oscillations are found in the unstable region and the mechanism is clarified.