CFD investigations of OXYFLUX device, an innovative wave pump technology for artificial downwelling of surface water

Abstract

No other environmental variable of such ecological importance to estuarine and coastal marine ecosystems around the world has changed so drastically, in such a short period of time, as dissolved oxygen in coastal waters. The prevalent methods for counteracting anoxic sea events are indirect measures which aim to cutdown anthropic loads introduced in river and marine environments. To date, no direct approaches, like artificial devices have been investigated except the WEBAP and OXYFLUX devices. The present paper adopts a numerical approach to the analysis of the pumped surface water as well as the analysis of the dynamic response of the OXYFLUX device. By means of a CFD-RANS code and through the application of overset grid method, the 1/16 OXYFLUX model's dynamic response and pumping performance are evaluated. The appropriate grid is selected after an extensive sensitivity analysis carried out on 9 different grids. The CFD model is validated by comparing numerical and physical results of heave decay test, heave response, and surface water discharge under the action of regular waves. The extensive comparison with experimental results shows consistently accurate predictions. The main findings of the study show that nonlinear effects remarkable reduce the dynamic behaviour of the OXYFLUX and generate an unexpected second harmonic for pitch response intensifying the overtopping discharge also for small waves caused by the summer's low intensity winds.

Review of Artificial Downwelling for Mitigating Hypoxia in Coastal Waters

“…Antonini et al (2016) conducted numerical experiments simulating the OXYFLUX pumping capacity for artificial downwelling by using a CFD-RANS code and an overset grid method. A series of simulations were performed to investigate nonlinear effects due to interactions between waves and the OXYFLUX model and the performance of OXYFLUX in regular waves [61]. The CFD model showed good agreement with the measured data of the heave decay test under the action of regular waves; the error of the heave decay period was 3.7%.…”

“…The CFD model showed good agreement with the measured data of the heave decay test under the action of regular waves; the error of the heave decay period was 3.7%. Moreover, the simulation results showed that "nonlinear effects remarkably reduce the dynamic behavior of the OXYFLUX and generate an unexpected second harmonic for pitch response intensifying the overtopping discharge also for small waves caused by the summer's low-intensity winds" [61]. The design of the bottom stabilizing ring increases the OXYFLUX pumping capacity with the increase of the viscous dissipation, reduces the amplifier response of the floater, and induces a phase shift between the incoming wave and the heave motion, which facilitating the overtopping.…”

On the Oxygenation Efficacy of Geoengineered Artificial Downwelling in the Estuarine Ecosystem

“…In open coastal environment, AD provides a likely solution for hypoxia mitigation. Techniques such as wind power units (Stigebrandt & Gustafsson, 2007), wave overtopping (Antonini et al, 2012(Antonini et al, , 2016, and tidal pumping (Xiao et al, 2018) have been suggested as feasible ways to generate downwelling. All these approaches likely become commercially available in the future.…”

Experimental Study on the Performance of an Innovative Tide-Induced Device for Artificial Downwelling

“…Besides, it was estimated that 750,000 m 3 of surface water, corresponding to 5200 kg of DO, could be brought down to the bottom water of the Northern Adriatic Sea coast by using a wave-induced downwelling device entitled OXYFLUX [13]. Subsequently, a numerical analysis on the device using Computational Fluid Dynamics (CFD) was proposed to predict the pumping performance, the findings of which showed consistently accurate predictions with experimental results [14]. A tidal pump could pump surface oxygen-rich water to a depth of 15.36 m below the pycnocline at a flow rate of 0.4 m 3 /s in the Changjiang Estuary [9].…”