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Turbulence : in scour holes of sharp bends : Study about hydrodynamic processes in a scour hole of a sharp bend in the Mahakam River, Indonesia

Posthumus, Rik (2017) Turbulence : in scour holes of sharp bends : Study about hydrodynamic processes in a scour hole of a sharp bend in the Mahakam River, Indonesia.

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Abstract:Vermeulen et. al (2014) found many deep and stable scour holes in sharp bends along the Mahakam River in Indonesia. Understanding where they may develop, how deep they can become and why they remain stable is important for the safety of buried infrastructure and buildings near the river banks. However, the current knowledge is unable to explain the cause of the observed characteristics in scour holes of sharp bends. Therefore, we studied three hydrodynamic processes, which are in balance with each other and related to the scouring of the river bed. Previous studies also payed attention to some of these processes, but did not investigate how they relate to each other. For our quantitative study, we used field measurements obtained in a sharp bend of the Mahakam River. The scour hole in this sharp bend is representative for other scour holes found in the river. The data was collected with an Acoustic Doppler Current Profiler at seven transects around the scour hole. We processed the collected data in such way, that we could calculate the terms in the Reynolds Averaged Navier Stokes equations. In this set of three momentum balances, we find the three hydrodynamic processes of interest: accelerations, pressure gradients and turbulent stress gradients. Vermeulen (2014) used the field measurements as input for a simulation of the processes in the sharp bend with a hydrodynamic model. We used the model output as second dataset in our study. We processed the data in such way that we could evaluate the hydrodynamic processes along the whole bend. Therefore, we transformed the curved coordinate system into rectangular coordinates, which made it easier to compare the processes at different locations along the bend. A comparison between the two datasets revealed that the hydrodynamic model underestimates the plunging pattern of the flow, and overestimates the turbulent stresses in the scour hole. In our study about the dominant hydrodynamic processes in scour holes of sharp bends, we first checked if we could assume an inviscid flow as suggested by Vermeulen et al. (2015) and Niesten (2016). They based their suggestion on the observation of non-hydrostatic pressure and vertical flow accelerations in scour holes. Therefore, Niesten proposed a method to obtain the non-hydrostatic pressure from such flow accelerations. However, she did not include turbulent stresses in her equation. We used the field measurements and the model output to validate her equation and showed that the underlying assumptions fail. The normal turbulent stress must be included in the equation to get a reliable approximation of the non-hydrostatic pressure head. Next, we showed how the acceleration, pressure gradient and turbulent stress gradient behave along the sharp bend and how they change in the scour hole. In the streamwise and transverse momentum balance, the dominant terms upstream and downstream of the scour hole are the acceleration and the pressure gradient. In the scour hole, the turbulent stress gradient increases and reaches the same values as the other two processes. In the vertical momentum balance, the pressure gradient and the turbulent stress gradient show a huge increase in the scour hole and become more than 6 times larger than the acceleration. Therefore, we concluded that the assumption of inviscid flow fails in the scour hole. A detailed evaluation of the processes in the vertical momentum balance revealed that the large increase of the turbulent stress is mainly covered by the vertical normal stress. This term origins from the variance in vertical flow velocity. Because there is no vertical flow at the water surface and the river bed, the velocity variance is also zero there. The largest variances were found around mid-depth in the deepest part of the scour hole. Therefore, we found large vertical gradients of the normal stress in the scour hole. This may explain why the normal stress shows such a huge increase. In the discussion, we showed that the choices we made in processing the data, did only have small impact on our results. On the other hand, the differences between the field measurements and the model output may be more important. The underestimation of the vertical flow velocities and the overestimation of the turbulent stresses may have caused an invalid rejection of the inviscid flow assumption in the streamwise and transverse momentum balance. In the vertical momentum balance, the turbulent stress gradient remains dominant, even when we take the differences between the field measurements and the model output into account. Therefore, we suggested that the large increase of the turbulent stresses in the scour hole is important to understand the processes related to scouring of the river bed. However, we recommend to study possible methods to make a reliable approximation of the velocity variances from field measurements and use these values to calibrate the hydrodynamic model again. If the hydrodynamic model gives better values of the turbulent stresses, our findings will probably somewhat change, but they will also be more useful in increasing our understanding of the processes related to scour holes in sharp bends.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/73808
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