Physical Hydrography and Algal Bloom Transport in Hong Kong Waters

In sub-tropical coastal waters around Hong Kong, algal blooms and red tides are usually first sighted in the Mirs Bay, in the eastern waters of Hong Kong. A calibrated three-dimensional hydrodynamic model for the Pearl River Estuary （Delft3D） has been applied to the study of the physical hydrography of Hong Kong waters and its relationship with algal bloom transport patterns in the dry and wet seasons. The general 3D hydrodynamic circulation and salinity structure in the partially-mixed estuary are presented. Extensive numerical surface drogue tracking experiments are performed for algal blooms that are initiated in the Mirs Bay under different seasonal, wind and tidal conditions. The probability of bloom impact on the Victoria Harbour and nearby urban coastal waters is estimated. The computations show that： i） In the wet season （May - August）, algal blooms initiated in the Mirs Bay will move in a clockwise direction out of the bay, and be transported away from Hong Kong due to SW monsoon winds which drive the SW to NE coastal current; ii） In the dry season （November- April）, algal blooms initiated in the northeast Mirs Bay will move in an anti-clockwise direction and be carried away into southern waters due to the NE to SW coastal current driven by the NE monsoon winds; the bloom typically flows past the east edge of the Victoria Harbeur and nearby waters. Finally, the role of hydrodynamic transport in an important episodic event -- the spring 1998 massive red tide -- is quantitatively examined. It is shown that the strong NE to E wind during late March to early April, coupled with the diurnal tide at the beginning of April, significantly increased the probability of bloom transport into the Port Shelter and East Lamma Channel, resulting in the massive fish kill. The results provide a basis for risk assessment of harmful algal bloom （HAB） impact on urban coastal waters around the Victoria Habour.

Turbulence Modelling of A Lock-Release Oil Slick

The motion of a lock-release oil slick as an immiscible two-fluid gravity current is numerically studied by a finite dif ference algorithm based on the volume of fluid （VOF） method for the basic formulation and a rigid cover approximation for the open free surface. Detailed numerical simulation with careful model validation reveals the existence of turbulence and the adaptability of the renormalization group （RNG） k - ε model for the Reynolds-stress closure in the case of the oil slick. The time evolution and spatial distribution of the mean velocity, turbulence kinetic energy and turbulent viscosity are characterized. The mechanism for the transition from an initial gravity-inertial phase to a second gravity-vinous phase is shown to be the relaminarization effect of the initially highly turbulent slick. Compared well with known theoretical analyses and experimental observations, the turbulence modeling results in self-similar spreading laws in terms of the fact that the oil slick passes through the initial gravity-inertial phase with the front speed decreasing as t ^-1/3（where t is the time measured from lock release） and the second gravity-viscous phase with the front speed decreasing as t^-5/8.

Turbulent Evolution and Energetics of Lock-Release Gravity Currents

Turbulence modeling by use of the renormalization group （RNG） κ-ε model for Reynolds-stress closure is carried out to reveal the evolution dynamics for lock release gravity currents with the so-called slumping, inviscid and viscous phases. Field evolution of the turbulent current is investigated, and time transition of global energy balance is presented between the terms of potential energy, averaged kinetic energy, turbulent kinetic energy, turbulent dissipation and viscous dissipation. It is well illustrated that turbulent dissipation and viscous force are respectively dominant in the inviscid and viscous phases, while inertia effect accounts for the slumping.

THERMAL IMAGING STUDY OF SCALAR TRANSPORT IN SHALLOW WAKES

The thermal imaging technique relies on the usage of infrared signal to detect the temperature field.Using temperature as a flow tracer,thermography is used to investigate the scalar transport in the shallow-water wake generated by an emergent circular cylinder.Thermal imaging is demonstrated to be a good quantitative flow visualization technique for studying turbulent mixing phenomena in shallow waters.A key advantage of the thermal imaging method over other scalar measurement techniques,such as the Laser Induced Fluorescence（LIF）and Planar Concentration Analysis（PCA）methods,is that it involves a very simple experimental setup.The dispersion characteristics captured with this technique are found to be similar to past studies with traditional measurement techniques.