Analysis of water surface fluctuations in open-channel turbulent flow with strip roughness by LES with VOF and PIV with two high-speed cameras.

Ichiro Fujita, Asahi Ohgami, Hideto Yoshimura, Kojiro Tani

Tuesday 30 june 2015

8:45 - 9:00h at Europe 1 & 2 (level 0)

Themes: (T) Water engineering, (ST) Computational methods

Parallel session: 4E. Engineering - Computational

It is commonly observed that a water surface, an interface between air and liquid phases, displays various features depending on flow conditions. Under static condition or a condition with very small velocity, the water surface becomes flat that may reflect a light like a mirror. With the increase of flow velocity, the water surface tends to subject to various effects mainly caused by turbulence-related pressure fluctuations near the interface. With further increase of velocity, the water surface may deform significantly with white caps which is usually observed in river floods in mountainous region. Although the above mentioned features can be visually observed easily, the characteristics of the surface fluctuations with respect to internal flow conditions have not been investigated in detail so far, mainly due to both the difficulty in conducting simultaneous measurements of water surface and internal velocity field and the difficulty of numerical simulation that allows deformations or fluctuations of water surface. The present research aims at overcoming the above difficulties by developing a sophisticated PIV system with two high speed cameras that enables a simultaneous surface and velocity measurements in a longitudinal vertical section. Moreover, a large eddy simulation model (LES) that incorporates a volume of fluid (VOF) technique for treating water surface fluctuations was developed. The boundary condition was given by an immersed boundary method. With these sets of experimental and numerical systems, water surface fluctuations in open-channel turbulent flow with strip roughness were investigated for the same hydraulic conditions in order to test the performance of the system. The results for hydraulic conditions under a water depth of six centimeters with the Froude number up to 0.6 and a relative roughness spacing of five and ten were compared together with the case of a smooth bed condition. After checking the reproducibility of the mean and statistical turbulent properties, advection characteristics of water surface fluctuations were examined by applying a cross-correlation analysis, demonstrating a different feature of rough flows compared with smooth flows. Furthermore, it was revealed from Pre-Multiplied Spectra that water surface fluctuations in the present condition were caused by bursting phenomena from near bottom.