Shih-Chun Hsieh

Experimental study on mean velocity characteristics of flow over vertical drop

The characteristics of flows over a vertical drop were investigated experimentally using laser Doppler velocimetry for detailed quantitative velocity measurements, and a flow visualization technique for qualitative study of flow pattern. A range of velocity and depth of the subcritical approaching flows was tested to understand the flow structure at the regions of the falling jet, the sliding jet, and the energy dissipating pool. Using the measured velocity, four similarity profiles of the mean velocity at different locations were obtained: the jet velocity at the intersection of the falling jet and the sliding jet, the jet velocity along the free surface of the sliding jet, the maximum negative velocity and the mean horizontal velocity of the deflected wall jet in the pool.Variation trends of several important characteristic velocity and length scales of the deflected wall jet in the pool are also discussed

Characteristics of Recirculation Zone Structure behind an Impulsively Started Circular Cylinder

The characteristics of the recirculation zone structure behind an impulsively started circular cylinder for Reynolds number Re ranging from 500 to 2,000 and nondimensional time T ranging from 0.5 to 5.0 are investigated experimentally using particle image velocimetry (PIV) and flow visualization techniques. On the basis of the flow visualization pictures and velocity maps, both obtained by a reference frame with a moving coordinate system, the evolution of the recirculation zone structure is studied with special emphasis on the negative velocity subzone. The negative velocity subzone is enveloped by the boundary consisting of the zero-velocity points inside the recirculation zone, and the streamwise velocity profile inside and near the negative velocity subzone is characterized by a jetlike flow moving toward the cylinder. During the evolution of the recirculation zone, the representative dimensions of the vortical structure both in the streamwise and vertical directions, including the recirculation zone ...

Characteristics of Shear Layer Structure in Skimming Flow over a Vertical Drop Pool

The characteristics of shear layer structure between the sliding jet and the pool for skimming flows over a vertical drop pool were investigated experimentally, using flow visualization technique and high speed particle image velocimetry. Four series of experiments having different end sill ratios (h/H=0.12, 0.43, 0.71 and 1.0, where h=end sill height and H=drop height) with various approaching flow discharges were performed to measure the detailed quantitative velocity fields of the shear layer. The mean velocities and turbulence properties were obtained by ensemble averaging the repeated measurements. From the velocity profiles, it is found that the growth of the shear layer in the downward direction as the jet slides down the pool represents the momentum exchange. Analyzing the distribution of measured velocity, the similarity profile of the mean velocity at different cross sections along the shear layer was obtained. The proposed characteristic scales provided unique similarity profiles having promising regression coefficient. The selection of these characteristic scales is also discussed. Further, the spatial variations of mean velocity profiles, turbulence intensities, in-plane turbulent kinetic energy, and Reynolds shear stress were also elucidated in detail. The imperative observation is that the Reynolds shear stress dominates the major part along the shear layer as compared to the viscous shear stress. The study also provides an insight into the flow phenomena through the velocity and turbulent characteristics.

Velocity Fields in Near-Bottom and Boundary Layer Flows in Prebreaking Zone of a Solitary Wave Propagating over a 1:10 Slope

AbstractThe velocity characteristics of a solitary wave shoaling in the prebreaking zone and near the breaking point are investigated experimentally. The study focuses on the near-bottom and boundary layer flows on a 1:10 slope, with the incident wave steepness varying from 0.133 to 0.384. Both a flow visualization technique (FVT) with thin-layered dye as well as particle image velocimetry (PIV) with a high-speed camera were used. Results from FVT reveal that laminar boundary layer flow occurs not only in the prebreaking zone during the shoaling phases, but also in the postbreaking zone during the run-up and run-down phases. However, the laminar boundary layer disappears soon after breaking but before the run-up motion, and immediately after the flow separation followed by hydraulic jump during the later stage of the run-down motion. Results from the PIV measurement show that the maximum horizontal velocity appears under the wave crest and increases during the shoaling process. Flow reversal is observed a...

Characteristics of Vortex Shedding Process Induced by a Solitary Wave Propagating Over a Submerged Obstacle

The generation and evolution of shedding vortices induced by a solitary wave propagating over a two-dimensional submerged obstacle, a rectangular dike or a vertical plate, was investigated experimentally. The vortex shedding process was observed qualitatively using laser induced fluorescence (LIF) technique and particle tracing technique. The velocity fields were measured quantitatively using particle image velocimetry (PIV). Base on the results of the flow visualization, the comparison of the vortex shedding processes between rectangular dike and vertical plate was qualitatively made under the same wave condition. It is noted that the process of either rectangular model or vertical plate can be divided into four common phases, but the characteristics of shedding vortices are different in the same phase. In addition, the velocity similarity profiles for the formation of separated shear layer and the formation of vertical jet were also quantitatively compared between rectangular dike and vertical plate.Copyright

Characteristics of Boundary Layer Flow Induced by Solitary Wave Propagating over Horizontal Bottom

Experimental results on the flow characteristics of bottom boundary layer induced by a solitary wave propagating over a horizontal bottom are presented. Particle-trajectory flow visualization technique and high-speed particle image velocimetry (HSPIV) were used to elucidate detailed velocity fields underneath solitary waves with the ratios of wave height to water depth from 0.130 to 0.386. The results show that the velocity profiles can be classified into two classes with respect to the passage of the solitary wave-crest at the measuring section: ＂the pre-passing＂ and ＂post-passing phases＂. For the pre-passing phase, the velocity distributions can be deduced to a unique similarity profile with the use of unsteady free stream velocity and time-dependent boundary layer thickness as the characteristic velocity and length scales. On the other hand, the similarity profile for the flow reversal, acting like an unsteady wall jet, is obtained from the velocity distributions during the post-passing phase. The velocity deficit between the unsteady free stream velocity and the maximum negative velocity as well as the (time-dependent) thickness of reversal flow were identified as the characteristic velocity and length scales, respectively.

Discussion of “Periodic Oscillation Caused by a Flow over a Vertical Drop Pool” by Chang Lin, Shih-Chun Hsieh, Kai-Joe Kuo, and Kuang-An Chang

The authors considered in their experimental study bottom racks as the typical Alpine intake structure, and proposed novel design relations based on hydraulic experimentation. In their introduction, it is stated, “The present contribution aims at investigating experimentally the validity of the simplifying assumptions . . . at identifying the relevant dimensionless parameters, and at developing a physically based relationship allowing the correct design of bottom racks.” It may be considered a pity that no comparison with previous results was made, because the impact of the results would have been much larger then. The width of the test channel was relatively small with only 0.25 m, resulting in an extremely small intake structure. Toprounded bars were only tested, and the effect of the bar shape on the intake features was not further investigated. Are these bars really the current design basis? Or were these taken for experimental reasons? It is well known from the outflow characteristics of a tank that the authors’ outflow geometry would result in scale effects mainly due to fluid viscosity, given the extremely narrow bar openings of only 5 mm. Did the authors consider this limitation in their setup? It would be interesting to see their measurements under “static conditions,” as indicated at the bottom of page 19. The range of the approach flow Froude number is limited to within 1.2 Fo 2.05: Is this really the range encountered in hydraulic practice? What would happen if subcritical approach flow occurred? Another detail of considerable importance is the exact finish of the bottom rack end, such as shown, for example, in Fig. 6. If the concrete structure is made as shown in this plot, the down-flow would be considerably deflected by the impact onto the bar anchoring and lead to other results in terms of outflow as compared to an outflow not influenced by the concrete structure. Can the authors comment this effect? A result of this finish is the dramatic increase of sin in Fig. 7. It can easily be realized that the outflow behavior from a bottom rack therefore is divided into the uninfluenced upstream portion with a continuous decrease of sin x , and into the influenced downstream portion with a relatively sharp increase of sin . The lengths of these two reaches depend obviously on the entire opening length and the exact end geometry. Did the authors account for this end-effect in terms of the outflow features? The factor u=0.80 as obtained from Fig. 5 was derived from one experiment. What resulted for the other tests? This may be important, given the linear relationship with the outflow. Another shortcoming of the analysis is the neglect of the bottom slope. Previous observations indeed indicated that Eq. 6 applies within