#### Year

2012

#### Degree Name

Doctor of Philosophy

#### Department

School of Civil, Mining and Environmental Engineering

#### Recommended Citation

Gamage, Nadeesha Chamini Dharmasiri Keembiye Liyana, Macro turbulent characteristics over two-dimensional bed forms and its skin friction and form drag separation, Doctor of Philosophy thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2012. https://ro.uow.edu.au/theses/3807

#### Abstract

Turbulence plays an important role in the transport of momentum, heat and mass in the fluid flow. Investigations into the turbulence in open channels by previous researchers have demonstrated the requirement and importance of understanding the turbulence structures due to irregularities in rough bed flows. Flow resistance created by these irregularities, being a fundamental problem to hydraulic engineers, increases the need to carry out a thorough investigation on this requirement. Despite of the numerous works undertaken in this area, the correct mechanism of flow resistance is yet to be discovered as well. Therefore this research attempts to address these research gaps with respect to turbulent structures and flow resistance mechanism on rough bed flows and explain the fundamental hydrodynamic mechanisms behind them.

With a very detailed mathematical approach, this study critically evaluates the contribution of skin friction and form drag to total resistance, in order to investigate the underlying mechanism of flow resistance in open channels and pipelines. Different from Einstein’s method, this study extends the principle of boundary shear stress summation developed by Yang and Tan (2005, 2008) in a mobile bed to the fixed boundary. This results in the development of a mathematical equation, which explicitly calculates the friction factor in all the regions, irrespective of flow conditions. Therefore this study unravels the hydrodynamic mechanism of flow resistance, thus providing a very important theoretical platform to hydraulic engineers in their friction factor calculations.

Unlike the pipe flow where only the Reynolds Number and the Relative Roughness Height act as particularly useful parameters to determine the total friction, in rough bed open channel flow with either bed forms or vegetation, the spatial configuration also plays s very important role in the determination of flow resistance. Therefore the developed analytical model is then extended by incorporating the "Roughness Element Concentration" as a means of explaining the flow resistance. This has resulted in the development of a new "roughness length scale" based on the concept of "hydraulic radius", which we termed as "Roughness Element Concentration Related Hydraulic Radius". This length scale incorporates the concentration of roughness element presents in the flow and explains how the volume of flow separation region changes even with the slight change in roughness element concentration. Also this length scale clearly illustrates that, after complete separation occurs between the roughness elements at a particular concentration, further increase in concentration results in the reduction of the volume of flow separation, thus reducing the friction factor. Therefore this study accurately calculates at which concentration the friction factor is at maximum with respect to strip roughness. This approach will be very important in many practical hydraulic works.

Based on the newly developed roughness length scale, this study gives a new definition for the "Friction Velocity" as well as the "Reynolds Number". This friction velocity performs far better than the other definitions as a factor for normalising velocity and turbulent profiles over 2D bed forms.

Through a comprehensive experimental program, by evaluating the influence of bottom roughness on the main flow over a series of two dimensional asymmetric dunes, this study discovers the development of Turbulent Boundary Layer (TBL) along the flow over the dune covered surface. Based on the investigated phenomena on streamline compression, flow acceleration and the internal boundary layer structure, this study classifies the flow region into three distinctive regions. This provides a new classification for the flow regions over two dimensional bed forms, thus presenting a very useful insight into turbulent structures over rough boundary.

Also, by undertaking a comprehensive quadrant analysis for the experimental data, this study establishes a direct relationship between the bursting phenomena and the free surface, providing a very innovative and novel outcome to the Hydraulic Engineering knowledge base. By emphirically proving the existence of kolks and boils, it has discovered that, these macro turbulent structures originate at the flow separation region, just upstream to the reattachment point and thus travel upwards at an angle until it reaches the free surface. At the free surface they contribute to the surface renewal, which should be further quantified and mathematically explained. The experimental investigations of these macro turbulent characteristics make substantial contributions in Applied Hydraulic Engineering, thus being significantly useful for practising engineers.

**Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.**