Committee on Fluid Mechanics

Fluid mechanics within hydraulic research investigates transport and mixing in turbulent flows. Flow boundaries are often irregular, and shaped by the flow itself. They are characterised by large roughnesses, which lead to flow separation and free shear layers within the flow. The investigations include stratified flows and flows in rotating fluids, where at larger scales the rotation of the earth becomes relevant.
The basic difficulty in fluid mechanics in the context of hydraulic engineering is the transition between the microscopic scale, described by the Navier-Stokes equations, and the large scale of engineering applications. Because of the difficulty in making this transition, fluid mechanics within hydraulic engineering is replete with unsolved problems. Some of the important examples include:

back to top

Turbulence: The problem of turbulent flows, linking the instantaneous flow characteristics to the flow forcing functions and boundary conditions, is unsolved. Most current treatments are empirical and statistical. The concept of coherent structures is an alternative approach, which limits itself to the description of typical instantaneous motions within the turbulent flow. Stability analysis for low Reynolds number non-turbulent flows need to be conducted to learn more about flow breakdown and turbulence generation. The stability of high Reynolds number flows ought to be analysed to learn more about the formation of coherent structures and secondary flows that significantly affect engineering systems. Turbulence onset, generation, and maintenance in stratified or rotating flow systems is another complex problem area with a surprising variety of flow phenomena. The theories of chaos and fractals, although not a focal point of hydraulic research, may have the implication that flows described by non-linear equations are not entirely predictable.

Two-Phase Laminar or Turbulent Flows: The formulation of the dynamic equations for practically all two-phase dispersed systems is empirical. The true dynamics remains a mystery. A key hydraulic engineering problem is sediment transport. The elements of suspended load and bed load transport in turbulent flow need further investigation. Present hydraulic "theories" are at great odds with available data. Even seemingly well behaved laminar flow systems show unexpected behaviour. For example a suspension in a settling tank can exhibit the formation of surprising wave-like fronts. High concentration mixtures that show non-Newtonian fluid behaviour, including slurries, debris flows, mud slides, represent another difficulty. This question is related to rheology. Due to their thermodynamic complexities, gas-water mixture flows, caused by air entrainment at high-velocity in hydraulic structures or by cavitating flows, pose yet further problems. Finally, low Reynolds number porous media flow (Darcy flow) is still waiting for satisfactory predictive explanation of its macroscopic properties, such as hydraulic conductivity.

Transport Phenomena: Transport processes for any materials contained in the water flow are incompletely understood as well. At present, the mixing of buoyant mass or momentum injections into a river or coastal current, of great engineering importance, is not well-understood. Concepts such as large-scale eddy diffusivity or hydrodynamic dispersion cannot be rigorously related to the actual flow or solid matrix properties. Empiricism prevails. Advances are urgently needed to provide the tools for the solution of modern hydraulic engineering problems - which are increasingly devoted to the prediction of the transport and deposition of materials in the natural or engineered environment. Double-diffusion is another phenomenon which has not gained needed attention.

Interface Problems: The transition from the microscopic to the macroscopic becomes especially severe and intractable at system boundaries, the so-called interfaces. The air-water interface on the surface of a water body remains enigmatic, especially concerning the generation, growth, and instabilities of wind waves. Similarly, the water-sediment interface at a stream bed separating turbulent water flow from the behaviour of granular media has not been successfully described. A systematic approach that reconciles the large-scale macroscopic techniques to describe processes away from the interface with the microscopic processes directly at the interface is needed. Shear waves and internal waves are another interfacial problem which needs attention.

Interdisciplinary Problems: Finally, it must be stressed that increasingly many hydraulic problems transcend a purely mechanical approach. This is especially true for transport processes. They involve, for example, the intricate interaction of fluid mechanical transport with physical, chemical, or biological transformations. Entirely new disciplines, such as "physico-chemical hydrodynamics", are being developed and require the close co-operation between the fluid mechanist and other engineering scientists.

back to top

Leadership Team

Chair

Prof. George Constantinescu University of Iowa, Dept. Civil and Environmental Engineering, IIHR - Hydraulics and Engineering, 100 Stanley Hydraulics Lab, IA 52242-1585 Iowa City Tel: +1 319 384 0630 Fax: +1 319 335 5238 E-mail: sconstan@engineering.uiowa.edu

• Third International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering (3rd ISUD), EPFL, Lausanne, Switzerland, September 9 - 11, 2002 – Conference Report
• 3rd International Symposium on Environmental Hydraulics (ISEH), December 5-8, 2001, Arizona, USA Conference Report
• 2nd International Conference on Waste Water Discharges, September 16-20, 2002, Istanbul, Turkey Conference Report
• 2nd Int. Conference on Hydrodynamics ICHD - 96, Hong Kong, December 16-19, 1996. From Prof. A. Chwang, Chairman LOC (ICHD-96), Dept. of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong.
• 4th Int. Symposium on Fluid-Structure Interactions, Aeroelasticity, and Flow-Induced Vibration and Noise, Dallas, USA, November 16-21, 1997. From M.P. Païdoussis, Dept of Mechanical Engineering, McGill University, 817 Sherbrooke Street West, Montreal QC, Canada

Committee Communications

• Minutes of the Fluid Mechanics Committee Meeting held on 25, August 2003, at the Aristotle University of Thessaloniki, Greece, on the occasion of the 30th IAHR Biennial Congress, August 24-29, 2003.

back to top