Discussion of “Combined Effects of Wind and Streamflow on Gas-Liquid Transfer Rate” by Zhiyong Duan, James L. Martin, William H. McAnally, and Richard L. Stockstill

Abstract

In their interesting and timely contribution, the authors proposed atheoretical model to predict the wind-streamflow–driven gas-liquidtransfer rate. When wind blows over the stream surface, turbulenceis generated at both the air-water interface and the water-bed inter-face, which is the drivingforce of the surface renewal movement ofwater parcels. The total surface renewal rate is considered as thesum of the effective surface renewal rates driven by the turbulencegenerated at these two interfaces. This model correlates the gas-liquid transfer rate with the dynamic fluid parameters such as windspeed and stream velocity.The discussers would like to highlight some points raised in thepaper. First, the authors considered at the air-water interface a tur-bulent boundary layer formed by a viscous sublayer (VBL) and anunderlying turbulent layer (TL). This structure of layers is assumedto control the gas-transfer process. Also, this structure controls themomentum transfer.However, a more detailed analysis is needed to distinguish be-tween momentum transfer and mass transfer (Gualtieri and PulciDoria2008).Farfromtheinterface,intheturbulentboundarylayer,both momentum and mass transport are dominated by turbulentmotions, which provide full vertical mixing. Thus, the main bodyof the liquid phase is assumed to bewell mixed with the gas profilepracticallyuniformatthebulkconcentration.IntheTL,momentumand mass-transport processes can be related to the turbulent eddyviscosity ν