Relevant issues in fluid mechanics related to water resources and chemical engineering

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Relevant issues in fluid mechanics related to water resources and chemical engineering

Rafik Absi

To cite this version:

Rafik Absi. Relevant issues in fluid mechanics related to water resources and chemical engineering . 5th International Symposium on Aqua Science and Water Resource, ISASWR’17 Fukuoka Japan 8-11 August 2017, Fukuoka University Aug 2017, Fukuoka Japan. pp.49. �hal-01573577�

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5th International Symposium on Aqua Science and Water Resource, ISASWR'17 Fukuoka Japan 8-11 August 2017

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INVITED LECTURE

Relevant issues in fluid mechanics related to water resources and chemical engineering

Rafik ABSI

EBI, Ecole de Biologie Industrielle, Cergy-Pontoise, France.

E-mail : [email protected]

Fluid mechanics is involved in many practical problems related to both environmental and industrial situations. On the one hand, the aim of fluid mechanics in civil engineering is to provide adequate water services such as the supply of potable water, sewerage, drainage, dams, irrigation, river and sea (flood) defences … On the other hand, a knowledge of fluid mechanics is essential in chemical engineering because most of chemical-processing operations are conducted in the fluid phase. Examples : biochemical, chemical, energy, fermentation, materials, petroleum, polymer, food, cosmetics, pharmaceuticals and waste-processing industries.

We will present the following key issues of fluid mechanics related to both hydraulic and chemical engineering which are about accurate prediction of : (1) velocity profiles in turbulent flows, (2) concentration profiles in fluid-solid two phase flows.

In both hydraulic and chemical engineering, we need accurate modeling of the flow in pipes, ducts and channels. The concept of boundary layer (Ludwig Prandtl, 1904) has allowed importnat theoretical progress in fluid mechanics. Turbulent flows are significantly affected by the presence of walls. Successful predictions of turbulence models used for wall-bounded turbulent flows depend on accurate description of the flow in the near-wall region. On the one hand, we will see how it is possible to improve the prediction of near-wall mean streamwise velocity profile U + by solving the momentum equation. An eddy viscosity formulation based on a near-wall turbulent kinetic energy function (Absi, 2008) and the van Driest mixing length equation (van Driest, 1956) is used (Absi 2009). On the other hand, an ordinary differential equation (ODE) for velocity distribution in open channel flows is presented based on an analysis of the Reynolds-averaged Navier–Stokes equations and a log-wake modified eddy viscosity distribution. This proposed equation allows to predict the velocity-dip-phenomenon, i.e. the maximum velocity below the free surface (Absi 2011) and shows therefore a potential interest for the prediction of flow rates in sewers.

Fluid-solid two phase flows are involved in both hydraulic and chemical engineering, in sediment transport in rivers and channels (important since sediments reduce dams reservoir capacity), pneumatic conveying of raw materials involved in the manufacture of food, cosmetic or pharmaceutical products, fluidisation, … In particle-laden flows, the coefficient of turbulent diffusion of suspended particles is related to the eddy viscosity, through the turbulent Schmidt number. The classification is based on the volumetric particle concentration and Stokes number St which is the ratio between the particle timecale and the integral turbulence timescale or turnover time of large eddy. Therefore, a large value of St corresponds to a weak sedimentation process and therefore to a more uniform concentration profile. At the opposite, a small value of St corresponds to a strong concentration gradient. We will present an interesting case of suspended sediments over wave ripples where concentration profiles, in semi-log plots, are upward convex for fine sand and upward concave for coarse sand. We will first show the importance of sediment diffusivity with convective transfer which is different from the sediment diffusivity associated to turbulent flux. It is possible to interpret concentration profiles

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thanks to a relation between second derivative of the logarithm of concentration and derivative of sediment diffusivity (Absi, 2010, 2011).

References :

[1] Absi R. (2008) “Analytical solutions for the modeled k-equation”, Journal of Applied Mechanics, Transactions of the ASME, American Society of Mechanical Engineers, 75(4), 044501.

[2] Absi R. (2009) “A simple eddy viscosity formulation for turbulent boundary layers near smooth walls”, Comptes Rendus Mecanique, Académie des Sciences, Elsevier, 337, 158- 165.

[3] Absi R. (2011) “An ordinary differential equation for velocity distribution and dip phenomenon in open-channel flows”, Journal of Hydraulic Research, IAHR, Taylor &

Francis, 49(1), 82-89.

[4] Absi R. (2010) “Concentration profiles for fine and coarse sediments suspended by waves over ripples: An analytical study with the 1-DV gradient diffusion model”, Advances in Water Resources, Elsevier, 33(4), 411-418.

[5] Absi R. (2011) “Engineering modelling of wave-related suspended sediment transport over ripples”, Coastal Sediments '11, 7th International Symposium on Coastal Engineering and Science of Coastal Sediment Processes, May 2-6, 2011, Miami, Florida, USA., edited by Julie D Rosati, Ping Wang and Tiffany M Roberts, World Scientific Publishing, pp. 1096 1108.

[6] Absi R., Marchandon S., Lavarde M. (2011) “Turbulent diffusion of suspended particles:

analysis of the turbulent Schmidt number”, Defect and Diffusion Forum, Trans Tech Publications, Vols. 312-315, pp 794-799.

[7] El Gharbi N., Absi R., Benzaoui A., Bennacer R., (2011) “An improved near wall treatment for turbulent channel flows”, International Journal of Computational Fluid Dynamics, Taylor & Francis, 25(1), 41-46.

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