Double diffusive effluent discharges

Supervisors: NTU: A/P Adrian Law; Stanford: Prof Steve Monismith, Prof Jeff Koseff

Introduction

The phenomenon of double diffusion has an important impact on the mass transfer processes in the ocean, and has been investigated extensively in the area of oceanography. A question that has not been resolved in a satisfactory manner is the effect of turbulence. At present, it has been confirmed that double diffusion would be significant in slow intrusion that is laminar and with very low Reynolds number, but once turbulence is present it is believed that turbulence mixing would dominate rendering the mixing by double diffusion insignificant.

However, qualitative experiments in Law et al. (2003) have shown that the double diffusive effect remains substantial in the presence of weak turbulence when the stability ratio, R, is near 1.0, and hence it may still play a significant role in the transport of salinity and pollutant towards the seabed in effluent discharges of prototype engineering scale. The finding has considerable implications on the environmental impact assessment of wastewater discharges, such as the combined thermal and salty effluents from desalination.

Objectives

The proposed study will focus on the mixing behaviour of double diffusive discharges in a stagnant environment over a range of Reynolds number from laminar to turbulence to examine in details the turbulence effect. It will also cover a range of stability ratios from approximately 0.5 to 5.0, which allows a better representation of the variation of the prototype conditions. Specific objectives are to

• Investigate the relative importance between the mixing caused by shear turbulence and double diffusion, and perform quantitative measurements to define the interplay processes
• Develop a conceptual model on the observations of mass transport for both salinity and temperature in the experiments,
• Perform DNS numerical simulations for simplified configuration to confirm the conceptual understanding
• Relate the findings to the environmental impact assessment of wastewater discharges into the sea

Scope

Organization and Training

The subject of double diffusion has been a focus topic of research in the Stanford’s Environmental Fluid Mechanics group. At the same time, there has been an ongoing effort in CEE, NTU to improve the engineering assessment on the mixing of effluent discharges. Hence, this proposed study complements the research interests on both sides. In fact, cooperative work in this area has already been begun between

the principal investigators, and a journal paper has been accepted for publication in the ASCE Journal of Hydraulic Engineering.

It is anticipated that the student will initially spend time at Stanford on courses as well as equipping himself with knowledge in the subject of double diffusion, and subsequently carry out bulk of the numerical and experimental work at NTU.

References

Law, A.W.K., Ho, W.F., and Monismith, S., 2003. “Double diffusive effect on desalination discharges.” To appear in Journal of Hydraulic Engineering, ASCE.