Propeller jet flow and its associated scour hole around open quay structures
Date of Issue2018-02-02
School of Civil and Environmental Engineering
This study presents the experimental results on the flow field of the confined propeller jet and its associated scour hole around an open-type quay. The experimental procedure was conducted in three phases. Experiment Phase I (EP-I) started with a simple case of plane boundary confined propeller jet and focused on velocity measurement. The objective was to investigate the influence of the clearance height (vertical distance between propeller axis and plane boundary) on the development of the 3-dimensional propeller jet. To this end, both the streamwise and transverse flow fields were measured by using particle image velocimetry (PIV) technique, in such a way the 3-dimensional characteristics of the propeller jet was reconstructed. Similar to a confined offset jet, the propeller jet also exhibits a wall attachment behavior when it is placed near a plane boundary. As a result, in contrast to its unconfined counterpart, the confined propeller jet features three regions, namely the free jet, impingement and wall jet regions. The result shows that the extent of each region varies under different clearance heights. The development of the mean flow and turbulence characteristics associated with varying clearance heights are compared to illustrate boundary effects in these regions. In the impingement region, the measured transverse flow fields provide new insights on the lateral motions induced by the impingement of the swirling jet. In the wall jet region, observations reveal that the jet behaves like a typical 3-dimensional wall jet and its axial velocity profiles show good agreement with the classical wall jet similarity function. Experiment Phase II (EP-II) shifted the focus to the scour measurement, in which the temporal and spatial developments of the scour hole around an open-type quay model have been investigated. The longitudinal distance between the propeller face and toe of the quay slope, i.e., toe clearance, was systematically varied to examine its effect on the scour hole development. The results reveal that the asymptotic centerline profile undergoes significant transitions with varying toe clearances, during which three toe clearance fields have been recognized, namely the near toe clearance, intermediate toe clearance and far toe clearance fields. Based on characteristics of the asymptotic scour hole, it may be inferred that two scouring mechanisms, namely jet diffusion and quay obstruction, govern the scouring process and dictate whether the scour hole forms in any one of these three fields. The influence of these two mechanisms on the development of the scour characteristic dimensions in each field was also investigated based on which quantitative demarcations amongst the fields were obtained. To further confirm the inference regarding the scouring mechanisms made in EP-II, the propeller jet flow within the developing scour hole was measured in Experiment Phase III (EP-III). The so obtained flow field data and its associated scour hole profile could, therefore, be connected directly, which provides a comprehensive understanding of the interaction between propeller jet flow and the erodible sand bed. By adopting a similar experimental configuration as that in EP-II experiments, EP-III was conducted at four selected toe clearances, which covered all three toe clearance fields defined in EP-II. The jet diffusion and quay obstruction mechanisms were visualized as the main jet flow and vortex in the form of vector and streamline plots. The relative importance of the main jet flow and vortex was discussed in the perspective of their contribution in shaping the final bedform in each toe clearance field. Moreover, turbulence intensity and near-bed Reynolds stress were analyzed for the selected case in the intermediate field, both of them exhibit a declining trend with the development of the scour hole. As the scouring process approaches the asymptotic phase, the bed shear stress (estimated by the near-bed Reynolds stress) falls below the critical shear stress across the entire scour hole, and its longitudinal variation exhibits a similar trend with that of the critical shear stress.