Air-water flows at hydraulic structures: Experimental investigations of interfacial characteristics and air-water mass transfer
Final Report Abstract
The aim of the DFG research project was to improve the prediction of air-water mass transfer processes in highly-aerated flows by comparing dissolved oxygen measurements with a numerical integration of the mass transfer equation. The major outcomes of the undertaken works can be summarized as follows: • Air-water mass transfer: Our current knowledge about liquid film coefficients and aeration efficiencies in hydraulic structures was partially derived from intrusive phasedetection probe measurements through an approximation of the specific interfacial area. The present work corrected for the shortcoming that the latter was incorrectly computed in the past. Simultaneous measurements of interfacial properties and dissolved oxygen levels were performed in a large-size stepped spillway model and the mass transfer equation was integrated in two dimensions, yielding an improved description of the mass exchange between the liquid and the gaseous phase. • Pseudo-instantaneous velocity measurements with dual-tip phasedetection probes: A novel signal processing technique (adaptive window cross-correlation (AWCC)) was developed during the project. The AWCC allows for a robust estimation of pseudoinstantaneous velocities in highly-aerated flows with dual-tip phase detection probes, thereby enabling an assessment of turbulence parameters. The AWCC was validated by stochastic modelling, showing that the method is accurate, although it was noticed that the uncertainty may increase with increasing velocity fluctuations. It is anticipated that the capability to measure velocity times series will open new opportunities in air-water flow research. • Non-intrusive measurements of air-water flow properties: Recent developments have allowed the non-intrusive estimation of airwater flow properties using high-speed video cameras or LIDAR technology. Based on the image gradient, a conditional averaging technique was developed for optical flow estimations in air-water flows. It was further established that LI- DAR technology is suitable to uncover turbulent air-water surface features, air-concentration distributions and surface velocities, highlighting the potential for future prototype applications.
Publications
- (2020) Turbulence and self-similarity in highly aerated shear flows: The stable hydraulic jump. International Journal of Multiphase Flow 129 103316
Kramer, M.; Valero, D.
(See online at https://doi.org/10.1016/j.ijmultiphaseflow.2020.103316) - (2018): Free-Surface Instabilities in High-Velocity Air-Water Flows down Stepped Chutes, 7th International Symposium on Hydraulic Structures, FH Aachen
M. Kramer and H. Chanson
(See online at https://doi.org/10.15142/T3XS8P) - (2018): Transition flow regime on stepped spillways: air-water flow characteristics and step-cavity fluctuations, Environmental Fluid Mechanics 18, pp. 947-965
M. Kramer and H. Chanson
(See online at https://doi.org/10.1007/s10652-018-9575-y) - (2019): Can we improve the characterisation of high-velocity air-water flows? Application of LI-DAR technology to stepped spillways. Journal of Hydraulic Research
M. Kramer, H. Chanson and S. Felder
(See online at https://doi.org/10.1080/00221686.2019.1581670) - (2019): Optical flow measurements in aerated spillway flows: Validation and discussion on sampling/ processing parameters, Experimental Thermal and Fluid Science 103, pp. 318-328
M. Kramer and H. Chanson
(See online at https://doi.org/10.1016/j.expthermflusci.2018.12.002) - (2019): Towards reliable turbulence estimations with phase-detection probes: An adaptive window cross-correlation technique. Experiments in Fluids 60:2
M. Kramer, D. Valero H. Chanson and D. Bung
(See online at https://doi.org/10.1007/s00348-018-2650-9)
