Transverse and longitudinal waves in steady flows

Problematic issue

The aim is to study the transverse and longitudinal free surface waves (seiche or seiching phenomenon) induced by vortex shedding behind emerged  or slightly immersed macro-roughnesses in laterally confined flows. Macro-roughnesses represent tree or house models.

Scientific issues

• Quantification of the seiche phenomenon (transverse and longitudinal oscillations of the free surface)  for emerged macro-roughness elements (D/h<1, with D = time averaged water depth and h = macro-roughness height) and/or slightly submerged elements (1<D/h<1.5).
•  Influence of key dimensionless numbers: submergence D/h, Froude number based on water height, Reynolds and Strouhal numbers based on the macro-roughness elemnt width.
• Influence of the streamwise flow non-uniformity  on the seiche phenomenon.
• Influence of the bed roughness (smooth bottom vs. rough bottom) on the seiche  phenomenon
• Influence of the free surface oscillations on the time-averaged velocity and on turbulence statistics.

Methodology

The experiments are conducted in the 18m x 3m channel of Irstea, but using only one third of the width. The cross-section is rectangular, the side walls are made of glass. For the bottom, different hydraulic roughnesses are studied:

• Tree models (wooden cylinders 1 cm in diameter) placed on a smooth bottom (glass)
• Tree models (wooden cylinders 1 cm in diameter) placed on a rough bottom (dense artificial grass). Tree density: 81 trees/m², with a staggered distribution.
• House models (6.4 cm side PVC cubes) placed on a rough bottom (dense artificial grass). Density of houses: 49 houses/m².

Water level fluctuations are measured using ultrasonic sensors.

These flows were also numerically modelled with 2D and 3D industrial or research codes.

Channel 18m x 1 m, with (1) cylindrical roughness elements (tree models) emerged on a rough bottom (meadow model) and (2) cubic roughness elements (house models) emerged on a rough bottom (meadow model).

Research projects

Projet ANR FlowRes (2015-2018). ‘Prédire les écoulements dans les plaines d’inondation dont l’occupation du sol varie, lors de crues extrêmes’.

Partners

• École Nationale Polytechnique d’Alger (ENPA)
• Laboratoire d’Hydraulique Saint-Venant (LHSV)

PhD theses and post-doctorate

•  PhD of Meriem Chétibi (January 2016 – December 2019) entitled : ‘Turbulent flows in non-prismatic open-channels’ (co-directors : S. Benmamar, ENPA; S. Proust, INRAE)
• PhD of  Marina Oukacine  (January 2016 – Juin 2019): ‘Experimental and numerical study of flows through emerged and slightly submerged macro-roughness elements’.  Co-directors:  N. Goutal (EDF), et S. Proust.
•  Post-doctorate of  Marc Chatelain (March 2017 –  august 2018) entitled ‘3D LES (Large Eddy Simulation) of flooplain flows in the presence of emergent and slightly submerged macro-roughness elements’. Supervised by S. Proust.

Example of result

Longitudinal profiles of the standard deviation of water level fluctuations σ  (normalised by the time-averaged flow depth D) and of the submergence D/h, where h = height of a macro-roughness element (Chetibi et al. 2019)

Publications

• Chatelain, M., and S. Proust (2021).  Open-channel flows through emergent rigid vegetation: Effects of bed roughness and shallowness on the flow structure and surface waves.  Physics of Fluids, American Institute
  of Physics, 2021, 33 (10), pp.106602. DOI:10.1063/5.0063288. hal-03369211
• Chetibi, M., Proust, S., Benmamar, S. (2019). Transverse surface waves in steady uniform and non-uniform flows through emergent and weakly submerged square cylinders. Journal of Hydraulic Research, DOI: 10.1080/00221686.2019.1647885
• Dupuis, V., S. Proust, C. Berni, and A. Paquier (2016). Combined effects of bed friction and emergent cylinder drag in open channel flow, Environmental Fluid Mechanics, vol. 16, n° 6, p. 1173-1193, DOI: 10.1007/s10652-016-9471-2.