3D JET FLOW BEHAVIOUR AROUND A PILE IN A SCOUR HOLE

Transcription

1 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt 911 3D JET FLOW BEHAVIOUR AROUND A PILE IN A SCOUR HOLE Aye YÜKSEL OZAN * and Yalçın YÜKSEL ** Yildiz Technical University, Civil Engineering Department, Besiktas, Istanbul, Turkey * Dr., ayuksel@yildiz.edu.tr ** Prof. Dr., yuksel@yildiz.edu.tr ABSTRACT Nowadays, ships can berth and unberth under their own power with increasing ships maneuvering capability and high power engines. So, propeller jets causes erosion near quay walls and around the piers and this causes serious damages to harbor structures. For this reason, to obtain the jet velocity distribution besides the pile and quay walls becomes very important problem for the designers. In this study, the circular jet was used to simulate the propeller jet and the velocity distributions of a submerged circular jet flow around a pile in an equilibrium scour hole were investigated experimentally. In the research densimetric Froude Number, Fr d =13.68 were considered. Keywords: Jet, Pile, Local scour, ADV. Nomenclature: d 0 : Jet exit diameter D: Pile diameter h: Water depth above the bed Fr d ( Fr d = U 0 gd 50 ): Densimetric Froude Number Re( Re = U0d0 ν ) : Reynolds number, based on the nozzle jet exit velocity and jet exit diameter S mak : Maximum scour depth in front of the pile at the upstream side in equilibrium conditions u, v, w: Velocity components in x, y and z directions, respectively U 0 : Average jet exit velocity x: Horizontal distance from the jet exit y: Vertical distance from the wall ν: Viscosity

2 912 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt INTRODUCTION The propeller jet has received a great deal of attention in the last few decades due to its importance as a reason of the erosion at the harbor basin as well as the berthing structures. One of the reasons for this erosion is installation of higher-powered engines to ships in recent years. Furthermore, ships nowadays are often equipped with bow thrusters, which functions to increase navigability and reduce the need of tug assistance. (Yüksel et al. [12]) A rotating ship propeller generates a turbulent jet, with axial, radial and tangential velocity components. (Dargahi [4]) When a pile inserted in the flow area, important diffreneces appears in flow behaviour. The flow around a pile is very complex and this complexity becomes more and more difficult with a scour hole around the pile. By this way, investigation of the velocity distributions close to the mooring structures caused by the ships propeller has become very important research area for the engineers. Graf and Istiarto [5] studied on flow area around a cylindir in a scour hole experimentally. Sarker [9] was performed experiments on flow characteristics around a cylindir under current and wave effect. Kurniawan et al. [7] studied the flow area in ascour hole caused by a submerged jet experimentally. Chin et al. [3] investigated the mechanism and the characteristic parameters affecting local scour around a vertical pile caused by turbulent round wall jets flow in their experiments. Karim and Ali [6] studied modelling 3D flow area around a pile. Ali and Karim [1] investigated the prediction of difference turbulence models about flow area in a scour hole caused by submerged jet flow. Dargahi [4] focused on the propeller jet flow and estimation the rate of erosion caused by the jet numerically. The objective of the present study was investigation of the jet flow behavior around a pile in a scour hole experimentally. The clear water scour condition was considered and the experiments were performed in equilibrium conditions in scour hole. Acoustic Doppler Velocimeter (ADV) was used to determine the velocity distributions. As water approaches a pile, it is forced to seperate and pass around the pile. The flow phenomena are complex due to the presence of a boundary layer as well as an adverse pressure gradient set up by the pile. Consequently, the mechanism of the local scouring process is complicated by 3D flow patterns, such as horseshoe vortex, downward current (downflow), and bed shear distribution around the pile. (Yen et al. [11]) The schematic of the flow and scour around a pile is presented in Figure 1.

3 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt 913 Figure 1. Flow area around a pile and scouring. (Van Rijn, [10]) Balachandar et al. [2] described the state of scour research with these words: A comprehensive understanding of the scour mechanism remain elusive because of the complex nature of the process. The coupling between the shape of the eroded bed profile and the hydrodynamics characteristics of the jet flow increases the complexity.. Most previous studies have been experimental in nature and any attempts to theoretically model the scouring process have been semi-emprical at best. Furthermore, many of the suggested relationships provide a wide range of scour prediction for similar flow conditions. EXPERIMENTAL STUDY The experiments were conducted in the Hydraulic and Coastal Engineering Laboratory of the Civil Engineering Department in Yildiz Technical University. The experimental set-up includes a tank with length of 3 m, width of 0.65 m and height of 1.25 m (Figure 2). Both sides of flume were made of glass at test section. A pump supplied the water jet and the flow rate was controlled by a gate-valve. Discharge was measured by an electronic flow meter. In order to determine the sidewall effects in the tank, potassium permanganate was injected for flow visualization. The water height was 0.40 m above the bed and fixed. The nozzle diameter was d 0 =0.022 m and the pile diameter was D=0.048 m. Densimetric Froude number was considered as Fr d =13.68 (Re=43000, Q=45 lt/min) in the experiments. The Acoustic Doppler Velocimeter (ADV) with a 5 cm downlooking and sidelooking probes was used to measure the instantaneous three-dimensional velocity components

4 914 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt at a rate of 25 Hz (Figure 3). The seeding material (10-50 grams per cubic meter) supplied by NORTEK was used in the experiments. Experiments were performed on the sand bed between false floors in the tank. The characteristics of the sand were given in the folowing; d 50 =1.28 mm, d 60 =1.43 mm, d 90 =1.89 mm and σ=1.57. Figure 2. Experimental set up Figure 3. The parts of Acoustic Doppler Velocimeter (NDV Operations Manual [8])

5 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt 915 RESULTS In the experiments, velocity measurements were performed in several sections along scour profile. The measurement scheme was given in Figure 4. The jet exit was replaced X=8d 0 further away from the pile at the upstream section. Figure 4. Velocity measurement scheme The non-dimensional maximum scour depth results of Yüksel et al. [12] and Chin et al. [3] were presented with maximum scour depth was obtained from this study in Figure 5. Ali and Karim [1] investigated three-dimensional flow behaviour around a pile in a scour hole for a channel flow. They indicated that the Gaussian distribution gives a good agreement with the experimental results of equlibrium scour profile. The scoured bed profile was examined in terms of Gaussian profile. It was found that the scour and hill profiles showed good agreement in equilibrium conditions. The Gaussian distributions plotted for scour profile were presented in Figure 6 and 7.

6 916 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt Figure 5. Non-dimensional maximum scour depth (Yüksel et al. [12]) Figure 6. The Gaussian distributions for the equilibrium scour area

7 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt 917 Figure 7. The Gaussian distributions for the equilibrium crest area The non-dimensional streamwise velocity distributions at the upstream side of the pile were presented in Figure 8. It was seen that the velocity decreased from the jet exit to the pile and it had maximum 0.7U 0 value at point x=6d 0. Negative velocities appears from point x=4d 0 close to the bed. The measurements coudn t be performed close to the pile at upstream and downstream directions because of the ADV geometry. So, even the downflow in front fo the pile and secondary flow behind the pile were observed, they couldn t be measured. The non-dimensional vertical velocity distributions at the upstream side of the pile were presented in Figure 9. It had positive and negative values along the vertical line and had maximum value of 0.025U 0 at point x=6d 0. Figure 8. Non dimensional streamwise velocity distributions at the upstream side of the pile

8 918 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt Figure 9. Non dimensional vertical velocity distributions at the upstream side of the pile The non-dimensional streamwise velocity distributions at the downstream side of the pile were presented in Figure 10. It had maximum velocity of 0.25U 0 at x=2d 0. The measurements showed that the velocity decreased from the pile to the crest. It had maximum value of 0.15U 0 close to the crest. Figure 10. Non dimensional streamwise velocity distributions at the downstream side of the pile

9 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt 919 The non-dimensional vertical velocity distributions at the downstream side of the pile were presented in Figure 11. The downflow was measured (negative values) from the pile to the point of x=4d 0. The downflow occured under the original bed level and it had maximum 0.05U 0. Additional, the maximum upflow velocity had a value of 0.075U 0. It was obvious that the vertical velocity had almost three times bigger values at the downstream side of the pile than the values at the upstream side of the pile. Figure 11. Non dimensional vertical velocity distributions at the downstream side of the pile CONCLUSION In this study, the experiments were performed to understand the scour mechanism around a pile under three dimensional jet flow effect. The results were given for equilibrium conditions as follows: 1. It was seen that the streamwise velocity decreased from the jet exit to the pile at the upstream side of the pile and from the pile to the crest at the downstream of the pile. 2. Equlibrium scour hole profile had good agreement with the Gaussian distribution as Karim and Ali [8] defined for channel flows.

10 920 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt 3. The maximum streamwise velocity were determined as 0.7U 0 at x=6d 0 at the upstream side of the pile. 4. Negative streamwise velocity values were found from point x=4d 0 to the pile at the upstream side of the pile. 5. The vertical velocity values had positive and negative values around zero and had maximum value of 0.025U 0 value at the upstream side of the pile. 6. The streamwise velocity had maximum value of 0.25U 0 at x=2d 0 for the downstream side of the pile. 7. The streamwise velocity had maximum value of 0.15U 0 close to the crest of the profile. 8. It was obvious that the vertical velocity had almost three times bigger values at the downstream side of the pile than the values at the upstream side of the pile. 9. When the velocity profiles examined, it was seen that the jet flow behaved like free jet at the upstream side of the pile and like wall jet at the downstream of the pile. ACKNOWLEDGEMENT The authors are thankful for the financial support of The Research Fund of Yildiz Technical University. REFERENCES 1. Ali, K. H. M. and Karim, O. (2002), Simulation of Flow Around Piers, Journal of Hydraulic Research, Vol. 40 (2), pp Balachandar, R., Kells, J. and Thissen, R. (2000), The effect of tailwater depth on the dynamics of local scour, Can. J. Civil Eng., Vol. 27, pp Chin, C.O., Chiew, Y.M., Lim, S.Y. and Lim, F.H. (1996), Jet Scour Around Vertical Pile, Journal of Waterway, Port, Coastal and Ocean Engineering, Vol. 122 (2), pp Dargahi, B. (2003), Three Dimensional Modelling of Ship-Induced Flow and Erosion, Water & Maritime Engineering, Vol. 156, pp Graf., W. H. and Istıarto, I., Flow Pattern in the Scour Hole Around a Cylinder, Journal of Hydraulic Research, Vol. 40 (1), pp Karim, O. A. and Ali, K. H. M. (2000), Prediction of Flow Patterns in Local Scour Holes caused by turbulent water jets, Journal Hydr. Res., Vol. 38 (4), pp Kurniawan A., Altınakar, M. S. and Graf, W. H. (2001), Flow Pattern of an Eroding Jet, pp , XXIX IAHR Congress Procedings, Beijing, China. 8. Nortek 10 MHz Velocimeter, Operations Manual, N

11 Thirteenth International Water Technology Conference, IWTC , Hurghada, Egypt Sarker, A. (1998), Flow Measurement Around Scoured Bridge Piers Using Acoustic-Doppler Velocimeter (ADV), Flow Measurement and Instrumentation, Vol. 9, pp van Rijn, L. C. (1998), Principles of Coastal Morphology, Aqua Publications, Amsterdam, Netharlands. 11. Yen, C.L., Lai, J.S. and Chang, W.Y. (2001), Modelling of 3D Flow and Scouring around Circular Piers, Proc. Natl. Sci. Conc. ROC (A), Vol. 25, No. 1, pp Yüksel, A., Çelikolu, Y., Çevik, Y., Yüksel, Y. (2005), Jet Scour Around Vertical Piles and Pile Groups, Ocean Engineering, Vol. 32, Issues 3-4, March, pp