Paola Gualtieri

Turbulence-based models for gas transfer analysis with channel shape factor influence

Gas transfer through surface water of streams is an effective process for the environmental quality of the aquatic ecosystem. Several theoretical approaches have been proposed to estimate gas transfer rate. This paper is devoted to present a turbulence-based model and to compare it with other 3 turbulence-based modeling frameworks that provide an estimation of gas-transfer coefficient KL at the air-water interface. These models were derived for the reaeration process. In this paper, they have been verified both for reaeration and volatilization using experimental data collected in a laboratory rectangular flume and in a circular sewer reach. These data refer to oxygen absorption and cyclohexane volatilization, respectively. Comparison of results for oxygen shows that the tested models exhibit an average absolute difference between their results and the experimental data ranging from 12.5% and 25.6%. Also, the scaling analysis of the experimental data support both small-eddy based models and the model proposed by the authors. Moreover, volatilization results show that the process is also affected by a channel shape factor, which was, finally, quantified.

Use of conventional flow resistance equations and a model for the Nikuradse roughness in vegetated flows at high submergence

Abstract This study examines the problem of flow resistance due to rigid vegetation in open channel flow. The reliability of the conventional flow resistance equations (i.e. Keulegan, Manning and Chézy-Bazin) for vegetated flows at high submergence, i.e. h/k >5, (where h = flow depth and k = vegetation height) is assessed. Several modern flow resistance equations based on a two-layer approach are examined, showing that they transform into the conventional equations at high submergences. To compare the conventional flow resistance equations at high submergences, an experimental methodology is proposed and applied to the experimental data reported in the literature and collected for this study. The results demonstrate the reliability of the Keulegan equation in predicting the flow resistance. Based on the obtained results, a model to evaluate the Nikuradse equivalent sand-grain roughness, kN, starting from the vegetation height and density, is proposed and tested.

On Evaluating Flow Resistance of Rigid Vegetation Using Classic Hydraulic Roughness at High Submergence Levels: An Experimental Work

Vegetation resistance is generally evaluated using drag coefficient CD related to friction factor f; however, some authors examined a possibility of employing classic hydraulic roughness coefficients (i.e., Nikuradse’s or Strickler’s) to calculate vegetation resistance in case of high submergence (h/k > 5, in which h is flow height and k represents vegetation height). In order to compare conventional roughness at high submergence levels, an experimental methodology was developed, focused, in particular, on fully submerged and rigid vegetation, for different hydraulic conditions and varying non-dimensional vegetation density.

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

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

Complementary Experimental Methods ForMeasurements Of Air Entrainment In VerticalDropshafts

Vertical dropshaft is an important element in hydraulic structures since it easily allows covering a vertical jump. In the present paper the problem faced is to quantify the phenomenon of air entrainment in vertical shafts, two methods are presented for the measurement of entrained air flow rate. The anemometric method is semi-classical, while the volumetric method is fully innovative. A deep analysis has been carried out concerning the principles on which the two methods are based and they are shown to be complementary methods. A comparison is also provided between the two methodologies.

Boundary Layer Intermittency Model

At the present time, the impact of free stream turbulence on fully turbulent boundary layer flow has been examined in numerous experimental, analytical and computational studies. Many of these studies [1]..[12] deal with the effects of the free stream turbulence on the main statistical turbulence quantities (for example mean velocities, turbulent shear stress, turbulent kinetic energy): in particular some workers have studied Length Scales, Skewness and Kurtosis, which will be used in this paper.