The behavior of vortex street in the wake behind a circular cylinder under controlled excitation was investigated with the techniques both of hot-wire and of flow visualization at the Reynolds numbers lower than 160 in the pure Kármán vortex street range. The response frequencies of the velocity fluctuation indicated the non-linear interaction among the disturbances of the frequency of primary vortex street, of the most amplified frequency in the wake and of the forcing frequency. When the wake flow is forced with the disturbance of the intermediate frequency lower than that of the primary vortex street, the velocity fluctuation at far downstream location had two modes of the frequency caused by the non-linear effect. One of them led to merging of two vortices in the primary vortex street, and the other led to merging of three vortices, i. e., merging of two vortices followed by joining of another vortex afterward. The width of the wake was rapidly increased by the vortex merging.
A method for detecting flaws and local fiber concentrations in CFRP composites by using Joule effect is presented. The method involves passing an electric current through CFRP specimen and to probe the surface distribution of temperature for irregularities. The current path is disturbed due to the presence of any discontinuities and the steep gradient of electrical potential is produced in their neighborhood. Then the Joule heat is generated and the temperature becomes different from that of the far-field. Therefore, defects can be detected by evaluating irregularities in surface temperature distribution in the current carrying specimen. The closed form solution of the Joule heat generation around an elliptic hole in an infinite plate with orthotropy has been obtained utilizing complex variable method. It is shown that the anisotropy of the material brings about the considerable change in the generation of Joule heat around the hole. Finite element analysis procedures have also been for mulated to obtain the transient temperature responses of rectangular orthotropic plates containing various kinds of defects. A simple experimental verification has been done using cholesteric liquid crystals. The results compare very fabourably with both the analytical solutions and numerical ones. The present technique provides a useful method for detecting flaw-type defects, local fiber concentrations and the fiber directions.
In this paper three types of gust generators are tested in a wind-tunnel to examine their characteristics. The generators which were investigated are the flag method, turbulent-flow-glid method and their combined method. The gust velocities generated by these methods are measured using a hot wire anemometer of X type, and the characteristics of the power spectra are examined for several parameters such as wind velocity, the size and thickness of a flag. To estimate a gust model for controller designs, discrete models based on an ARMA process are first identified by the AIC method. A continuous model with a lower dimension is then derived by selecting the coefficients of a rational function. These selections are based on the quasi-Newton method which minimizes the square sum of errors at some discrete frequencies. It is found that the gust generators considered here can furnish one with a convenient mean for wind-tunnel experiments. In particular, the estimated gust model may be expected to provide a simple mathematical model for designing controllers.