The effect of the intake head wall slope and the installation of a trash rack on the type and strength of vortices are studied experimentally. The strength within each vortex was determined by measuring its tangential velocity utilizing an Acoustic Doppler Velocimeter (ADV). Experiments were carried out with a projected intake and an intake with various head wall slopes, discharges and submerged depths. The results from the projected intake tests indicated that Type 6 vortex was present. However, the vortex strength and type reduced as the intake head wall slope increased up to the vertical position. In addition, increasing the intake head wall slope increased the vortex instability. All tests were repeated with a trash rack. A relationship was developed for vortex strength and the intake critical submerged depth based on the intake Froude number and the head wall slope. Moreover, a relationship has been found between vortex strength and type.
The efficiency of horizontal perforated and solid plates, installed on top of an intake for prevention of surface vortex formation, was experimentally investigated. The experiments were conducted on a horizontal intake of a reservoir. By changing the angle of the approach flow, submergence of the intake and the discharge, 36 air-core surface vortices with different strengths were generated. The strengths of these vortices were evaluated by measuring tangential velocities using an acoustic Doppler velocimeter. A relationship between the strength of the surface vortex and the necessary percentage of uniform opening of the perforated plate is presented. It was found that a perforated plate of width 1D and 1·5D length normal to the intake (D is the tunnel diameter) with 50% uniform opening and a solid plate of 1·5D width and 2D length eliminated all surface vortices at the intake. Pressure was also measured in a section immediately downstream of the intake by means of four transducers. By using the measured pressure values, the effect of anti-vortex plates on local loss at the intake was determined.
Carrier relaxation processes have been investigated in GaAs/AlGaAs v-groove quantum wires (QWRs) with a large subband separation (∆E ≃ 46 meV). Signatures of inhibited carrier relaxation mechanisms are seen in temperature-dependent photoluminescence (PL) and photoluminescenceexcitation (PLE) measurements; we observe strong emission from the first excited state of the QWR below ∼50 K. This is attributed to reduced inter-subband relaxation via phonon scattering between localized states. Theoretical calculations and experimental results indicate that the pinchoff regions, which provide additional two-dimensional confinement for the QWR structure, have a blocking effect on relaxation mechanisms for certain structures within the v-groove. Time-resolved PL measurements show that efficient carrier relaxation from excited QWR states into the ground state, occurs only at temperatures > ∼ 30 K. Values for the low temperature radiative lifetimes of the ground-and first excited-state excitons have been obtained (340 ps and 160 ps respectively), and their corresponding localization lengths along the wire estimated.
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