Numerical Simulation Of Turbulent Flow Over Complicated Boundaries In Rivers

The physical movements in the natural riverway, the hydraulic and hydro-power and shipping engineering are all turbulent flows under complex boundary. Such as: flow on the sand wave riverbed in the riverway, flow on the rough riverbed surface, flow around sand bars, and flow on the riverbed covered by vegetation, etc. in which the complex boundaries of flow on the sand wave riverbed and the flow on the rough riverbed surface especially on the riverbed with changeable roughness exist very widely. It significantly changes the structure of water flow movement near the bottom, and then plays important roles in discharge capacity of the riverway, bed-load transport salad change, suspended load transport and the adjustment of bed surface shape. This paper selects the two typical complex boundaries of sand wave riverbed and riverbed with changeable roughness, analyzes the characteristics of the water flow movement, further deepens the understanding of typical flow on the complex boundaries, and provides the reliable technical base for the planning, design and construction of hydraulic and hydro-power engineering and shipping engineering, etc. The main conclusions of this paper are as follows:(1) Numerical model of flow movement on the complex boundary in the riverway has been established. The Reynolds stress model is adopted to close the flow control equation. In the numerical model, unified wall functions are adopted to reflect the impact of sands forming the boundary of the riverbed on the water flow movement. In the calculation of water flow movement on the sand wave, relative roughness, ripple steepness and relative wave height are chosen to reflect the geometric characteristics of sand wave. In the calculation of flow movement on the bed surface with changeable roughness, different roughnesses are adopted to describe the changes of roughness of bed surface. Validation calculation showed that the experimental values of the mean velocity on the sand wave bed and the mean shear stress are basically same to the calculated values, which means this model precisely described the major characteristics of the flow movement on sand wave bed.(2) A systematic study was made of the ripple resistance coefficient and the sand-grain resistance coefficient. The sand-grain resistance on sand wave bed is surface resistance, whose coefficient is correlated to Reynolds, relative roughness and ripple steepness, and the coefficient is larger than the sand-grain resistance coefficient under the same conditions. Sand wave resistance on the sand wave riverbed belongs to physical resistance; ripple resistance coefficient on sand wave bed is closely correlated to relative height and steepness of ripples. Given the same wave height of ripples, the ripple resistance coefficient rises with the increment of sand wave steepness. In calculation of flow movement on the ripple, the wavy wall function was introduced to reflect the impact of sand gain roughness upon the flow. When the steepness of ripples is less than 0.07, the bed resistance is mainly in form of sand-grain resistance; if the steepness of ripples is more than 0.07, the bed resistance is mainly in form of ripple resistance. Increase in ripple steepness results in greater sand-grain resistance. (3) A comparison was made between the turbulent diffusion coefficient and the sediment diffusion coefficient. On a flat bed, the sediment diffusion coefficient and the turbulent diffusion coefficient on vertical distribution are almost identical, while different on the sand wave bed; and the vertical position where sediment diffusion coefficient reaches the maximum is higher than that of turbulent diffusion coefficient; but when far away from the wave wall, the sediment diffusion coefficient is basically the same as the turbulent diffusion coefficient.(4) A systematic study was made of the flow characteristics on the bed under uniform roughness, to conclude that after change in roughness, there was correlation between development in inner boundary, adjustment in bed shear stress and flow velocity to the change in inlet flow Reynolds and in roughness. The study concluded that the inner boundary developed toward the downstream.1) Gradual increase in thickness of the inner boundary was followed by increase in vertical distance; 2) For different inlet flow Reynolds, the inner boundary developed in almost the same trend, but development along the bed tended to slow down with increment in Reynolds; 3) In case of sudden bed change from the hydraulic rough zone to the hydraulic smooth zone, the inner boundary developed much more swiftly. Sudden change in roughness results in such change in the bed shear stress as that 1) in downstream of the sudden change point of the bed roughness, there was sudden change in the bed shear stress until new balance was achieved after flow for a certain distance; 2) as the increment in the inlet flow Reynolds, sudden changed value of the bed shear stress was increased, and the recovery length correspondingly reduced.3) In case of abrupt bed change from the hydraulic smooth zone to the hydraulic rough zone, there was more noticeable sudden change in the bed shear stress and shorter recovery length for the shear stress. Change in roughness was followed by such main change in flow velocity as 1) for different values of the inlet flow Reynolds, flow velocity change caused by abrupt roughness change was distributed along the vertical bed, until to achieve balance in the downstream; 2) there was slight increment in outer boundary caused by development of inner boundary; 3) In case of sudden bed change from the hydraulic rough zone to hydraulic smooth zones, there was increment in flow velocity of underneath part close to the bed (in which case the flow velocity value is more rapidly balanced vertically along the bed); otherwise when the bed abruptly changes from the hydraulic smooth zone to the hydraulic rough zone, there was decrease in flow velocity of underneath part close to the bed.(5) A relatively systematic study was made of the flow characteristics in case of successive change in bed roughness. Since there was complex successive change in bed roughness, for the sake of analysis of the flow characteristics after successive change in bed roughness, two factors were taken into consideration, namely the roughness change zone length and the roughness change forms. In case of successive change in bed roughness, there was such change in inner boundary in such forms as 1) in case of short roughness change zone length, the inner boundary developed swiftly. Increase in the roughness change zone length caused the inner boundary to develop in the same orientation as in case of the uniform roughness change; 2) after the second sudden change point of roughness, there was new development in the inner boundary. The bed shear stress changed in such major ways as 1) in case of relatively short roughness change zone length, there was no sudden change in the bed shear stress; only when the roughness change zone length increased to a certain point (such a length is usually shorter than the recovery length of the bed shear stress in case of uniform roughness abrupt change), would the bed shear stress achieve fresh balance in the zone concerned.2) With increase in roughness change zone length, the recovery length of the bed shear stress would be lengthened first and then reduced until to reach the recovery length of the bed shear stress in case of uniform roughness abrupt change.3) Compared with bed change from the hydraulic smooth zone to the hydraulic transition zone and then to the hydraulic smooth zone again, there was larger sudden change in the bed shear stress value but shorter recovery length in case of bed change from the hydraulic smooth zone to the hydraulic rough zone and then to the hydraulic smooth zone again; In case of bed change from the hydraulic smooth zone to the hydraulic transition zone and then to the rough zone, or in case of bed change from the hydraulic rough zone to the hydraulic smooth zone and then to the rough zone again, there was relatively limited change in the bed shear stress and recovery length.(6) Tentative analysis was made of the flow movement caused by combined action of the bed elevation change and bed roughness change. In case of coarsening bed sand, there would be corresponding decrease in bed elevation; otherwise when the bed sand is under refinery, there would be corresponding increase in bed elevation. Under combined action of the bed elevation change and bed roughness change, the flow was mainly under impact from the bed elevation change.