The dynamic behaviors of coherent structures in the near field of a plane jet are extensively studied by hot-wire measurements. The instability evolution for both streamwise and transverse velocity fluctuations are investigated simultaneously. The shear stresses contributed by the individual instabilities are detected by using the cross-spectrum technique. Experimental results show that the jet spreading is governed by the vortex interaction mechanism, and the evolution of the instabilities for the two velocity components is found to perform distinct patterns. The characteristics for the instability evolution cannot be adequately identified from only streamwise velocity fluctuations. The vortex formation and merging locations should be determined from the saturation condition of the transverse velocity fluctuations v′( f ). Results also indicate that the instabilities cannot only absorb energy from the mean flow, but also transfer their energy to the subharmonics, or even back to the mean flow. The variations of these instability shear stresses are associated with the resulting formation and merging processes of the coherent structures. The subharmonic evolution model proposed by Ho [in Proceedings of the Symposium on Numerical and Physical Aspects of Aerodynamic Flow, edited by T. Cebeci (Springer, New York, 1982), p. 521] is then further demonstrated and extended from the energy transfer characteristics between the instability waves. Finally, the near-field acoustic properties are investigated to verify that the interaction processes of the coherent structures are the dominant mechanism for the production of sound.

The development of the mode arrangement in the developing region of an acoustically excited plane jet is extensively studied using hot-wire measurements. Two single-wire probes are located within the two shear layers of the jet to detect the flow structure patterns simultaneously. The jet is excited at its fundamental frequency in either varicose mode (m=0) or sinuous mode (m=1) condition. Experimental results show that different excitation mode conditions lead to different spreading rates and velocity fluctuation distributions in the developing flow field. The frequency spectral distributions for the m=1 excitation exhibit double peaks of fa and fb, rather than a single subharmonic f0/2 for the m=0 excitation. These three components constitute a relationship of (fa+fb)/2=f0/2. The mode switching phenomenon is found to be prominent under the m=0 case, while less pronounced for the m=1 case. The mode development is mainly governed by the evolution of the primary instabilities in the jet shear layers.

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Coherent structure dynamics in the developing region of a plane jet under acoustic excitation is studied experimentally by means of hot-wire measurements and smoke-wire flow visualization. The mean and fluctuation velocity properties, such as mean and fluctuation intensities in both streamwise and transverse directions, turbulent shear stress variations, and the turbulent energy production and energy convection phenomena, are carefully investigated. The results indicate that flow property behavior is closely related to vortex formation and the merging processes, which are the dominant mechanisms governing flow behavior in the developing region of a plane jet.

The development and the application of a cost-effective differential global positioningsystem for remotely piloted vehicles are presented. With the aid of an aerial-photographysystem, i.e., digital camera and charge-coupled device camera, the low-cost global positioning system receivers (Garmin SvyII and XL-25) can perform valuableand highaccuracy positioning and navigation measurements, which are competitive with the precise (but more expensive) Ashtech Z-12 receiver. Flight-test results reveal that the low-cost payload (XL-25 system) developed can reach 10 m in spatial accuracy of the static ight trajectory and 0.17 m/s and 1 deg in accuracy for ight velocity and heading angle. The heading angle and ight-path angle can also achieve 1-3 deg rms accuracy level. Relative deviations of the rms accuracy values between XL-25 and Z-12, such as the kinematic, three-dimensional position and the ight velocity, are found to be below 2.6 m and 0.5 m/s, respectively. Nomenclature a = semimajor axis of the ellipsoid C = speed of light d ion ; d trop = ionospheric and tropospheric delay effect dT = offset of the receiver clock dt = offset of the satellite clock d½ = global positioning system (GPS) ephemeris error and selective availability effect error e = geometric attening ratio P = pseudorange between the GPS satellite and the receiver O P u = rover station's corrected pseudorange R = rotation parameter for coordinate transformation S = scale parameter for coordinate transformation V n ; V e ; V d = local north, east, and down velocity of the navigation coordinate system V t = navigation velocity of the ight vehicle X; Y; Z = world geodetic system 84 (WGS-84) coordinate system X m ; Y m ; Z m = gravity center of the Taiwan datum 67 (TWD-67) coordinate system X u ; Y u ; Z u = position of the GPS receiver antenna 1 P = GPS pseudorange correction item for differential GPS processing 1X; 1Y; 1Z = translation parameter for coordinate transformation "= residual error (such as multipath effect) ½ = true geometric range from GPS satellite to receiver's antenna ½ m ; ½ P = meridian and prime radius of curvature Á;¸; h = latitude, longitude, and altitude of the geodetic ellipsoidal coordinate system P Á; P = geodetic latitude and longitude rate