Dossier Océan et énergie - Énergie Thermique des Mers
Sommaire IOA News Letters
and the Marine Environment (U.S. Overview, Part 2 ):Mixing of Deep Ocean Water
Effluent Plumes in Open Ocean
Clark C.K. LIU
Department of Civil Engineering
University of Hawaii at Manoa
Honolulu, Hawaii, USA
Natural upwelling brings deep ocean water (DOW) to the ocean surface. The deep ocean water from depths of 300 meters and below is cold, rich in nutrients, and clean. While upwelling regions account for only 0.1 percent of the world's ocean surface, they yield roughly 44 percent of the fish catch (Roels et al. 1978).
Land-based mariculture using DOW pumped from the ocean depths into man-made ponds and enclosures has been in existence for a decade on an experimental basis (Roels et al 1978) and commercial basis at the Natural Energy Laboratory of Hawaii (Daniel 1984). The ability of the nutrient-rich DOW to enhance the growth of fish and other marine organisms has been proven by these ventures. However, the feasibility of using DOW in the open ocean has not been proven. The major engineering problems related to the commercial open ocean mariculture are the difficulty of bringing up the DOW to the surface and of containing it within an area of the open ocean without significant dilution (Liu and Sun 1990). Other problems exist in the fields of physical oceanography, marine biology and fishery.
Bringing up DOW cost effectively has been investigated by a research project supported by U.S. National Science Foundation (NSF) in which a wave-driven artificial upwelling device was developed and tested (Liu and Jin 1994; Chen, Liu and Guo 1994). Results of this research indicate that a single device with a buoy of 4.0 m in diameter with a tail pipe of 300 m in length and 1.2 m in diameter, can generate an upwelling flow of 0.95 m3/sec in random Hawaiian waves. Detailed studies on near-field mixing of DOW in the open ocean are still pending.
Many studies of near-field mixing were conducted in relation to the marine disposal of waste water and power generation cooling water (Mullenhoff et al. 1985). In these studies, the principal design objective was to achieve a higher degree of effluent dilution, contrary to the objective of an artificial upwelling and mixing (AUMIX) system. However, the basic principles are similar.
In a simple approach, asymptotic relations were formulated by dimensional analyses and verified with experimental data. These relations provide useful "order of magnitude" estimates (Fischer, et al. 1979). For example, these simple asymptotic relations indicate that buoyancy would increase effluent tracer dilution. Therefore, one possible way of reducing potential DOW effluent dilution is to design an inclined jet; the momentum flux and buoyancy flux for an inclined jet are usually not in the same direction and tend to counter balance their dilution effects(Roberts and Tom 1987).
In a more detailed approach, mathematical models of near-field mixing can be formulated based on the integral method and entrainment hypothesis. The integral method applies basic conservation equations in a control volume moving along the plume trajectory. The Lagrangian technique is often used together with the integral method. The entrainment hypothesis is that the rate of inflow of diluting water is proportional to the maximum mean velocity in the jet at the level of inflow. The entrainment may be induced in two different manners (Winiarski and Frick 1976; Frick 1981). Radial entrainment is induced by the component of the relative velocity parallel to the axis of the plume, and forced entrainment is induced by the component of the relative velocity perpendicular to the axis of the plume. Mathematical models following this approach include the popular model OUTPLM of the U.S. Environmental Protection Agency (Mullenhoff et al., 1985). Lee and Cheung (1990) formulated a generalized integral (Lagrangian) model for buoyant jets, which includes a three dimensional trajectory in cross currents. Their model is a generalization of a two-dimensional trajectory model developed earlier by Frick (1984).
The flow induced by a submerged jet is mostly turbulent. Mixing of tracers in a turbulent flow is difficult to analyze and was ignored by most of the previous studies. More recently, many experiments on submerged buoyant jets, considering more relevant mixing mechanisms, were conducted (Chen and Rodi 1976; Papanicolaou and List 1988). Results of these experiments provided a basis for the development of turbulent models for buoyant jets which do not use the entrainment hypothesis (Hossain and Rodi 1982; Prokhodko 1984; Martynenko and Korovkin 1994).
The physical and mathematical modeling conducted specifically for deep ocean water (DOW) discharge in a near-field is rather limited. The near-field mixing of discharge from ocean thermal energy conversion (OTEC) plants into a stagnant and stratified ocean were investigated by Jirka et al. (1977), Adams et al. (1979), and Coxe et al. (1981). Jirka et al. (1977) found that OTEC effluent will not recirculate in the near-field to create an OTEC operational problem. Studies on near field mixing of DOW in cross currents were conducted by Liu and Chen (1991).
In operating a wave-driven artificial upwelling and mixing (AUMIX) system, wave effects on mixing are important and must be carefully evaluated. Wave effects on near-field mixing were investigated only in a few past studies. Studies of wave effects on near-field mixing of a buoyant jet were conducted in shallow waters by Shuto and Ti (1974) and by Ger (1979). More recently, Chin (1987, 1988) studied near-field mixing of buoyant jets in an intermediate water with surface waves. Chin (1987) suggested that water depth does not play a significant role, and primary influence of the wave on mixing takes place in a region close to the nozzle of the jet. Since the DOW effluent of an AUMIX system is discharged into deep water, the effects of water depth to near-field mixing, especially under the action of waves, requires further investigation.
In his mathematical derivation, Chin (1988) ignored the local acceleration and the pressure terms. However, the local acceleration may be an important mechanism of the near-field mixing of a jet under the action of waves, especially in a region close to the nozzle of the effluent jet. In future research, the derivation of the integral momentum equations, especially the justification of ignoring the local acceleration, should be investigated.
It can be shown from the mass conservation principle that the dilution of a plume is the result of only the lateral entrainment which is produced by the difference of axial velocities of the plume and of the ambient water. However, in near-field mixing models formulated by Winiarski and Frick (1976), by Frick (1981) and by Chin (1988), the enforced entrainment which is produced by a difference of radial velocities was also included. Quantitative expression of the entrainment hypothesis needs to be investigated to further verify the governing equation and the relevancy of the forced entrainment in near-field mixing.