velocity around a cylinder pile during scouring

VELOCITY AROUND A CYLINDER PILE DURING SCOURING PROCESS DUE TO TSUNAMI Kuswandi1, Radianta Triatmadja2 Istiarto3 1.

Ph.D Student Departmen of Civil and Enviromental Engineering Universitas Gadjah Mada. Indonesia [email protected] 2, 3 Department of Civil and Enviromental Engineering Universitas Gadjah Mada, Indonesia.

Abstract One of the reason that is relevant to the damage of buildings is tsunami scouring. In most cases, the scour formation around the buildings is due to the increasing velocity near the bottom of the structure. Tsunami may flow surrounding a building during run up and run down. This is different to that of river flows where the water move in the downstream direction. The other different is that the tsunami surge move significantly faster than that of river flow and hence creating a significant scour depth within a much shorter time. The research was aimed to observe the characteristic of flow velocity during tsunami run-up and run down as a prime variable affecting the scour depth surrounding a cylinder pile. The observation was conducted using a numerical model namely DualSPHysics which is an open source software. A dam break model was used to simulate tsunami surge in a numerical flume of size 20.7 x 0.4 x 1 m. The water depth at the upstream part of the flume was 0.5 m whilst the water depth in the downstream part of the flume that represents the coastal area was 0.2 m. The slope of the sea bottom was 1:20. The two parts of the flume were separated by a quick opening gate. A cylinder model sized 0.2 m was located 6 m downstream of the gate. The distance between the simulated water particles was 4.5 mm and hence the total amount of the water particles used in the simulation was 9 million. The result showed that during run-up, the flow in front of the building slowed down but the flow on both sides of the building increased. The flow characteristic during rundown was almost the same but of different direction and speed. Based on the characteristic of the flow surrounding the cylinder pile, the scouring depth and pattern may be approximated Key Words: run-down, tsunami, velocity, scour, DualSPHysics 1. INTRODUCTION Propagation of tsunami run up on inland may bring about damage and destruction to structures ata large scale. Triatmadja, (2000) mentioned that the level scale destruction caused by tsunami can be measured by the value of destruction level. The structures damage are caused by hydrodynamic force, impact forces by debris, fire, and scour that cause foundation failure (Yeh and Wenwen LI,2008).

Fig. 1. The abutment Lam Tengoh Aceh, instability caused scouring after Tsunami 2004

The scour and foundation failure on structures were massively found after Tsunami 2004 and Tsunami Japan 2011. For examples: the instability of the abutment of Lam Tengoh bridge Banda Aceh Indonesia due to tsunami 2004 and the scouring of Koizumi bridge pier during Tsunami japan 2011. The scour around structures has proved to be one of the damaging causes to coastal structures that may further create casualties. Many researches have dealt with the scouring process due to tsunami and aimed at clarifying the phenomena to support a better design for tsunami mitigation. 2. LITERATURE RIVIEW The process of local scour around cylinder due to tsunami is different than that on the river. Such scouring is caused by run up and run down of the tsunami even in relatively short duration. Yeh & Manson 2013 mentioned that the process of scouring caused by tsunami is shorter than the steady flow in the river. It only takes approximately 10 minutes. KATO, et al., (2000) also mentioned that the flow of tsunami is very different from the steady flow of the river so that the formation of scours around the building is approximately 15 minutes. When the flow passing through the structure, the main flow pattern changes around structure. The flow pattern around structure may cause the change of the erodible sediment bed and establish local scour around the structure. One of the parameters which influence the process and mechanism of local scour is the characteristic of the flow around a structure which normally is a combination of vortex systems. The vortex are horseshoe vortex, wake vortex system in the form of roller form vortex and spiral roller form vortex. (Figure. 2)

Fig. 2. Vortex around a cylinder Vortex system around a cylinder may consists of horseshoe-vortex, wake vortex and trailing vortex and associated down flow in front of the cylinder are basic mechanism of local scour (Breusers et al, 1977). The separation flow is extremely strong in front of and slowly decreased behind the cylinder. Wake vortex are created behind the cylinder which subsequently create larger flow Reynolds number (Yulistiyanto, 2009). The flow patterns around a cylinder due to tsunami are dominated by vortex system similar to those due to steady flow. Although they are similar but their generations are different Tonkin et al (2003) explained that the horseshoe vortex in steady flow is generated by the bottom layer whereas in tsunami the horseshoe vortex is generated by the plunging breaker and the associated overturn just before the cylinder. Tonkin et al also mentioned that flow bent around the cylinder and associated with the velocity shear may create the large coherent vortex in which can cause scouring. The physical model has been able to explain scouring process to some extent. However, detail of the flow patterns are difficult to observe. The numerical simulation is a useful method to explain the flow pattern nearby the cylinder pile during a tsunami attack although may have its own limitation. Smoothed Particle Hydrodynamics (SPH) is a powerful numerical method that is capable to obtain detail quantities such as pressures, velocities and free surface elevations during fluid-structure

interaction such as in tsunami attack. Monaghan (2005) developed SPH method based on integral interpolant equation. In SPH, the fundamental principle is to approximate any function A(r’) by

    A(r ) = ∫ A(r )W (r − r ' , h ) dr '

(1)

  Where h is the smoothing kernel and W (r − r ' , h ) is the weighting function or kernel DualSPHysics was developed from SPHysics an SPH method product. DualSPHysics utilized the new technology of the Graphics Processing Unit (GPUs), so that the runtimes of simulation significantly decreases. DualSPHysics require an NVidia CUDA-enabled GPU card installed on a personal computer. More information about DualSPHysics is available at www.dual.sphysics.org. 3. EXPERIMENT AND NUMERICAL SIMULATION SETUP The experiment was carried out in a flume of limited length to generate tsunami surge which run up on land. The length and the width of the flume was 20.7 m and 1.43 m respectively Kuswandi et al, 2016 (Figure 3a). The slope of the sea bottom was 1:20. A concrete cylinder was installed 6 m from the dam gate. The diameter of the cylinder was 0.2 m or b/B = 0.14. (b is the diameter of a cylinder and B is width of the flume). The tsunami was represented by a dam break surge by controlling both the upstream and downstream water depth. The velocity was measured using a deflection sensors that were installed at certain position along the downstream. The water depth was measured using wave height meter equipment. The water depth and the surge celerity were measured at the same time. The depth and shape of the scour around the cylinder was recorded using a laser scanner by measuring the elevation changes around the cylinder before and after each test. The measurement was conducted every 1 cm in both X and Y axis at a vertical accuracy of 1 mm. In the numerical simulation the initial distance between particles of fluids and boundaries was 4.5 mm and hence the total amount of the water particles used in the simulation was 9 million. The output of the simulation were velocity and water depth around cylinder during run up and rundown of the tsunami. (Figure 3.b). The simulation required a significantly long time for personal computer and in our case it was 4.5 days of computational time.

(Fig. 3a) (Fig. 3b) Fig. 3. Physical model setup and measurement grids for velocity and water depth measurement 4. RESULT AND DISCUSSION The numerical results of tsunami front propagation are shown in Figure 4. The surge started plunging at t = 0.2 s and breaking at t = 0.4 s after the gate was opened. When compare with Crespo (2008), the plunging and breaking phenomena seems to be slightly quicker. Such a quicker initial breaking condition could have been caused by the sloping bottom in the present experiment. At 0.5 seconds however, the plunging of the present surge diminished, whilst there were seems to be more plunging waves in Crespo’s results after even 0.8 s.

Fig. 4. Initial surge front to plunging breaker on shallow water The result of the numerical simulation is also compared with the physical of simulation. Figure 5 shows that the numerical velocity is smaller than the physical simulation which could have been the effect of viscosity and bottom friction.

Fig. 5. The comparison of velocities between experiment and numerical simulations After the impact with and through both sides of the cylinder, the main flow was changed and scoured the surrounding area of the cylinder. The surge front was reflected and created up flow and down flow in front the cylinder. Figure 6b indicates the reflecting flow that creates up flow for d i /d o ≥ 0,2 and down flow for d i /d o ≤ 0,15 at t = 0,5 s after the impact. This flow characters are different to that due to a steady flow where only down flow is created (Figure 6a).

(Fig. 6a) (Fig 6b) Fig. 6. Distribution of (a) velocity of uniform flow Yulistiyanto (2009) and (b) tsunami front surge during run up. The flow pattern around a cylinder during run up is different to that during run down. As the run up reached its maximum the water start to return off shore. The run up, run down and transition flow were approximately 0.22 %, 0.75 % and 0.03% of the total duration. Figure 7(a-b) shows the distribution of longitudinal velocity (to the X direction) across vertical and their respective water depths at the centerline section (cross section A-A of Figure 3b) and at the side section of cylinder (cross section B-B of Figure 3b). The down flow and up flow seem to be more dominant in front of the cylinder and at the center of the cylinder (cross section A-A) as the water was reflected back off shore direction. Along the side of the cylinder (cross section B-B), the down flow diminished. At three seconds after the impact the up flow and down flow velocities around the cylinder are no longer visible. During the run down phase the velocities at all cross sections are significantly slower than those during run up but the down flow was still observable. However, the maximum run down velocity was still higher than the critical velocity required for the initial motion of the bed material as indicated by the sediment movement during run down.

(Fig. 7a)

(Fig. 7b) Fig.7. Longitudinal Velocity (X direction) distributions across vertical and water depths (a) initial and end of run up at cross section A-A and at cross section B-B 2b of Fig 1b (b) initial and end of run down at cross section A-A at cross section B-B of Fig. 2b The separation flow was clearly occurred around the rear side of the cylinder during run up. As can be seen in Figure 8, the separation flow was very strong at t = 0.5 s after wave impact and was decreased gradually. The separation flow vanished after t= 2.0 s after wave impact.

Fig. 8. Comparison of flow separation between physical model results and numerical model results. Van Rijn (1984) mentioned that the bed load transport rate can be described sufficiently accurately by a dimensionless particle parameter (D * ) and a dimensionless transport of stage parameter (T * ). The transport of stage parameter equation can be written as equation 2.

T* =

(u '* )2 − (u*,crit )2

(u

)

2

*,crit

(2)

Where, u’ * is bed shear velocity, u *,crit is critical velocity of initial motion of the sediment. Based on Equation 2, the sediment transport appeared to be more along the cylinder side rather than in front and at the center of the cylinder (Figure 9). During run down, the sediment transport around cylinder may partly filled the local scour previously created during the run up with sediment.

(Fig 9a) (Fig 9b) Fig. 9. Stage of sediment transport during run up and run down The tsunami flow pattern affected the local scour pattern as indicated in Figure 10. As can be seen in the figure that the maximum scour occurred along both sides of the cylinder. At the front and the rear side of the cylinder, as the flow were significantly slower, not much sediment was transported. The tsunami used in this experiment was relatively short and may have not allowed for the full development of scour in front of and behind the cylinder. When enough time was available such as those in significantly longer tsunami, the erosion in front of and behind the cylinder could have been more developed due to the flow bent such as those in rivers.

Fig. 10 Final contour showing local scour around cylinder

5. CONCLUSION Three dimensional flow pattern of tsunami surge around cylinder pile has been simulated. The time history of the detail flow pattern can be portrayed using DualSphysics software. The agreement between the experiment and the numerical simulation during runup and run down and the flow around the cylinder are encouraging. The numerical simulation can partly explains the creation of local scour around a cylinder. Short duration of tsunami surge created a local scour around cylinder with maximum scour occurred along both sides of the cylinder, whilst the front and the rear scour were not fully developed. Acknowledgements The authors would like to express their gratitude’s to the Ministry of Research and Higher Education for providing PhD program scholarship for the first author and the Hydraulic and Hydrology laboratory, Research Center for Engineering Sciences, Universitas Gadjah Mada, Indonesia for supporting this research. References [1] Breusers, H., G. Nicollet & H.W. Shen, 1977. Local Scour around Cylindrical Piers. Journal of Hydraulic Research, 15(3). [2] Crespo, A.J.C, Geisteira - Gomez. M, Dalrymple R.A., (2007), Validation and Accuracy to Experiments Using Different Code Compiling Option (Benchmark Test Case5). Proceding SPHERIC 2nd International Workshop, May 23th-25nd, Universidad Poltenica de Madrid, Spain, pp 1-4 [3] Chanson, H. 2005. Applications of the Saint-Venant Equations and Method of Characteristics to the Dam Break Wave Problem. Hydraulic Model Reports pp-84. Department of Civil Engineering, the University of Queensland, Brisbane QLD 4072, Australia. ISBN No. 1864997966. [4] Chanson, H, Shin-ichi AOKI, Mamoru MARUYAMA, 2003. An Experimental Study of Tsunami Runup on Dry and Wet Horizontal Coastlines. Journal of Science of Tsunami Hazards, Volume 20, Number 5, page 278. [5] Istiarto, 2001. Flow Around a Cylinder in a Scoured Channel Bed, Lausanne: Departement De Genie Civil, Ecole Polytechnique Federale De Lausanne. [6] KATO. F., Shinji SATO., and Harry YEH., (2000): Large-Scale Experiment on Dynamis Response of Sand Bed Around a cylinder due to Tsunami. Journal of Coastal Engineering pp.1848-1859. [7] Kuswandi, Triatmadja. R., Istiarto., (2016), Simulation of Tsunami Run up and Scouring around a Cylinder Using a Flume of Limited Length, to be published [8] Raudkivi, A. J. & R. Ettema, 1983. Clear Water Scour At Cylinder Pier. Journal Hydraulic Engineering, 109(3), pp. 338-350 [9] Tonkin S, Yeh. H, Kato. F, and Sato. S (2003), Tsunami scour around a cylinder, Journal Fluid Mechanics Vol 496, pp 165-192 Cambridge University Press DOI: 10.1017/S00221120036402. [10] Triatmadja. R (2010): Tsunami Kejadian, Penjalaran, Daya Rusak dan Mitigasinya, Gadjah Mada University Press. Yogyakarta, Indonesia [11] Van. Rijn. (1984). Sediment Transport, Part I: Bed Load Transport. Journal of Hydraulic Engineering, Vol. 110, No. 10, October, 1984. ©ASCE, ISSN 0733-9429/84/0010-1431. Paper No. 19220. [12] YEH. H., WenWen LI. (2008): Tsunami Scour and Sedimentation, Proceedings of the Fourth Interntaional Conference on Scour and Erosion.K-7, pp. 95-106 [13] Yulistiyanto. B., (2009): Velocity Measurements on Flow around a Cylinder, Journal Dinamika Teknik Sipil, No. 2 Juli.