NCR DAYS 2018
|
Delft, February 8-9
Book of abstracts
NCR DAYS 2018 The future river
Celebrating 20 years NCR
Future
The
River
Ymkje Huismans, Koen D. Berends, Iris Niesten, Erik Mosselman (eds.) NCR publication 42-2018
* Corresponding author
email address: posthumusrik@gmail.com
Turbulence in scour holes of sharp bends
Rik Posthumusa*, Bart Vermeulena, Kathelijne Wijnberga
aUniversity of Twente, Department of Water Engineering and Management, Faculty of Engineering Technology,
P.O. Box 217, 7500 AE, Enschede, the Netherlands
Keywords — Scour holes, Turbulence
Introduction
Many deep and stable scour holes were re-cently observed in sharp bends along the Ma-hakam River in Indonesia (Vermeulen et al., 2015). Understanding where they may devel-op, how deep they can become and why they remain stable is important for the safety of bur-ied infrastructure (Sawatsky et. al., 1998, Beltaos et. al., 2011) and buildings near the river banks (Klingeman et. al., 1984, Huismans et. al., 2016). However, the current knowledge is insufficient to explain the cause of the ob-served characteristics in scour holes of sharp bends (MacVicar and Rennie, 2012, Ver-meulen et al., 2015).
A detailed study of the hydrodynamics in scour holes may elucidate the physical pro-cesses that govern the dynamics of scour holes. Here, we explore the possibility to quan-tify terms in the momentum balance from in-situ field measurements (Niesten, 2016) and compare them with the results of a hydrody-namic model (Vermeulen et al., 2015).
Figure 1. Location of transects in sharp bend where flow velocities were measured.
Methods
Detailed flow velocity measurements, collected in one of the sharp bends with a scour hole in the Mahakam River, are used for a quantitative study. The scour hole in this sharp bend is
rep-resentative for other scour holes found in the river (Vermeulen, 2014). The data was collect-ed with an Acoustic Doppler Current Profiler at seven transects around the scour hole (Fig. 1). The data is processed in such way, that most terms in the Reynolds Averaged Navier Stokes equations could be determined. We analyzed the terms in the balances, and determined the relative contribution of accelerations, pressure gradient and the divergence of the Reynolds stresses.
We compared these field based results, with the results obtained from a three-dimensional finite element model (Vermeulen et al., 2014). The model results are evaluated in such way that the terms in the momentum balance can be studied along the whole bend. Therefore, the curved coordinate system is transformed into rectangular coordinates, which makes it easier to compare the processes at different locations along the bend. The results from the hydrodynamic model were calibrated with the data from the flow velocity measurements and show good agreement (Vermeulen et. al., 2015) (Fig. 2).
Figure 2. Streamwise flow velocity from measurements and model simulations in transects 3 and 4.
Results
A comparison between the terms in the mo-mentum balance along the bend, reveals, as expected, a large influence of the scour hole. In the streamwise and transverse momentum balance, the dominant terms upstream and downstream of the scour hole are the accelera-tion and pressure gradient. In the scour hole, the turbulent stress gradient increases and reaches the same values as the other two terms. In the vertical momentum balance, the NCR DAYS 2018: The Future River. Deltares
pressure gradient and the turbulent stress gra-dient show a huge increase in the scour hole and become more than 6 times larger than the acceleration (Fig. 3).
A detailed evaluation of the terms in the ver-tical momentum balance reveals that the large increase of the turbulent stress divergence is mostly caused by an increase in the vertical normal stress. Because there is no vertical flow at the water surface and the river bed, the normal vertical stress also vanishes. The larg-est variances are found around mid-depth in the deepest part of the scour hole. This may explain why the normal stress gradient in verti-cal direction shows such a huge increase (Fig. 4).
Figure 4. Peak of vertical normal stress around mid-depth in the deepest part of the scour hole.
Discussion
Although the model results show good agree-ment with the field measureagree-ments, there are some differences. The hydrodynamic model underestimates the vertical flow velocity, and predicts larger turbulent stresses in the scour hole, compared to the measured ones. How-ever, the detection of vertical stresses from ADCP is still experimental, and the uncertainty involved in these measurements is unclear.
Conclusions
A study of how the terms in the momentum balance change near a sharp bend with a scour hole reveals that the turbulent stress di-vergence strongly increases in the scour hole and becomes a significant term in the vertical momentum balance. First, this suggests that turbulent stresses are important to consider in improving the understanding of scour hole for-mation and stability. Furthermore, this result confirms that advanced turbulent models (such as LES) are needed to reproduce the flow through scour holes. This also highlights the need to develop new techniques to measure turbulent stresses in the field, to understand the dynamics of complex three-dimensional flow.
References
Beltaos, S., Carter, T. & Prowse, T., 2011. Morphology and genesis of deep scour holes in the Mackenzie Delta. Canadian Journal of Civil Engineering, Volume 38, pp. 638-649, doi: 10.1139/L11-034.
Huismans, Y., Van, G., O'Mahoney, T. & Wiersma, A., 2016. Scour hole development in river beds with mixed sand-clay-peat stratigraphy. Oxford, CRC Press. MacVicar, B. J. & Rennie, C. D., 2012. Flow and
turbulence redistribution in a straight artificial pool. Water resources research, Volume 48, pp. W02503, doi: 10.1029/2010WR009374.
Niesten, I., 2016. Deviations from the hydrostatic pressure distribution retrieved from ADCP velocity data, Wageningen: Wageningen University.
Sawatsky, L. F., Bender, M. J. & Long, D., 1998. Pipeline exposure at river crossings: causes and cures. Canada, ASME.
Vermeulen, B., 2014. Rivers running deep. Wageningen: ISBN: 978-94-6257-206-5.
Vermeulen, B., Hoitink, A. J. F. & Labeur, R. J., 2015. Flow structure caused by a local cross-sectional area increase. Journal of Geophysical Research: Earth Surface, Issue 120, pp. 1771-1783, doi: 10.1002/2014JF003334.
.
Figure 3. Hydrodynamic processes along scour hole in vertical momentum balance
SESSION IA MORPHO- AND HYDRODYNAMIC PROCESSES
