INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 6308 (Print), ISSN 0976 6308 (Print) ISSN 0976 6316(Online) Volume 5, Issue 11, November (2014), pp. 44-56 IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2014): 7.9290 (Calculated by GISI) www.jifactor.com IJCIET IAEME USE OF DOWNSTREAM-FACING AEROFOIL-SHAPED BRIDGE PIERS TO REDUCE LOCAL SCOUR a, d Adnan Ismael, b Mustafa Gunal, c Hamid Hussein a College of Civil Engineering, Gaziantep University, Gaziantep, Turkey b Prof., College of Civil Engineering, Gaziantep University, Gaziantep, c Asst. Prof., Engineering Technical College, Mosul, Iraq d Asst. Lecturer, Technical Institute, Mosul, Iraq ABSTRACT The current study is to provide a new method to reduce scour depth in front of bridge pier. The idea of this method is dependent on the change the orientation of an aerofoil pier so that it faces downstream rather than upstream according to the direction of flow (named after here as downstream-facing aerofoil-shaped pier). The downflow deflected away from the front of the downstream-facing aerofoil-shaped pier and the vortex becomes small and does not affect the pier. In this study three piers (upstream-facing aerofoil, downstream-facing aerofoil and circular)were tested under live-bed condition with flow intensity of 0.058m 3 /s for duration 5 hours. The velocity field measurements were obtained using an Acoustic Doppler Velocimeter (ADV). The results showed that, the downstream-facing aerofoil-shaped pier reduces local scour around the pier. The reduction of the scour hole volume was about 87% compared with circular pier and the maximum depth of scour reduced 59% compared with upstream-facing aerofoil and 68% compared with circular pier. Changing the orientation of an aerofoil pier so that it faces downstream rather than upstream according to the direction of flow is an effective countermeasure for reducing local scour depth. The present experimental study shows that downstream-facing aerofoil design can reduce scour depth, thereby reduces the potential need for countermeasures. Keywords: Hydrualic Structure, Scour Reduction, Local Scour, Aerofoil Shaped Pier. I. INTRODUCTION A common reason of bridge failures is local scour around bridge foundations such as piers and abutments. Local scour erodes the soil around the piers and reduces the lateral capacity of the foundations Rambabu et al.(2003). Local scour is the engineering term for erosion of the soil surrounding an obstruction caused by flowing water. 44
It has been long established that the basic mechanism causing local scour at bridge piers is the down flow at the upstream face of the pier and subsequent formation of vortices at the base of pier Muzzammil et al. (2004). There are many parameters that affect the flow pattern and the process of scour around bridge piers. These include the size, cohesion, and grading of the bed material, depth of flow, size and shape of the bridge pier, flow constriction, flow velocity, and geometry of the bed. Other factors that influence scour that are the result of significant flood events include floating debris and accumulation and buildup of debris. Yanmaz and Altinbilek (1991) conducted experiments to study the development of scour hole at bridge piers in uniform sands under clear water conditions. The duration time of the experiments were kept constant for around 6 hours, during which the final equilibrium scour was not achieved. They concluded that the shape of the scour hole remains almost unchanged with time. The observed average side slope of the scour hole was 33º which was approximately the angle of repose of the used sand. Dargahi (1990) presented an extensive study containing the temporal evolution of the main flow features around circular bridge piers placed in a fixed sediment bed. He also visualized the flow field using air bubbles. Additionally the bed shear stresses were measured with a Preston tube. Dargahi observed the simultaneous presence of various horseshoe vortices upstream of the leading pier front. Muzzammil and Gangadhariah (2003) investigated experimentally that the primary horseshoe vortex formed in front of a cylindrical pier which is the prime agent responsible for scour during the entire process of scouring. A simple and effective method was employed to obtain the time-averaged characteristics of the vortex in terms of parameters relating variables of flow, pier and the channel bed. An expression for the maximum equilibrium scour depth was also developed from the vortex velocity distribution inside scour hole. The resulting scour prediction equation was found to give better results compared to the results of well-known predictor models when applied to model scour data. Dey and Raikar (2007) presented the outcome of an experimental study on the turbulent horseshoe vortex flow within the developing intermediate stages and equilibrium scour holes at a circular pier and equilibrium scour holes at a square pier was measured by an acoustic Doppler velocimeter. The contours of the time-averaged velocities, turbulence intensities, and Reynolds stresses at different azimuthally planes 0 0, 45 0 and 90 0 were presented. The bed-shear stresses were determined from the Reynolds stress distributions. The vorticity contours and circulations were computed. It was observed that the flow and turbulence intensities in the horseshoe vortex flow in a developing scour hole are reasonably similar. Scour countermeasures can be basically divided into two groups: armoring countermeasures and flow altering countermeasures. The main idea behind flow altering countermeasures is to minimize the strength of the down flow and subsequently horseshoe vortexes, which are the main causes of pier scour. In contrast the principle of armoring countermeasures is to provide a protection layer acts as a resistant layer to hydraulic shear stress and therefore provides protection to the more erodible materials underneath. A comprehensive review of flow-altering countermeasures by Tafarojnoruz et al. (2010a) shows that although several types of flow-altering countermeasures were already investigated and proposed in the literature, some of them exhibit low efficiency in terms of scour depth reduction or suffer from serious problems for practical purposes. Tafarojnoruz et al. (2012) evaluated experimentally the performance of six different types of flow-altering countermeasures against pier scour. They found some countermeasures, which were recommended as highly efficient in the literature; do not perform well under test conditions Chen et al. (2012) examined the use of a hooked collar for reducing local scour around a bridge pier. The efficiency of collars was studied through experiments and compared with an 45
unprotected pier. The velocity field measurements were obtained using an Acoustic Doppler Velocimeter. Results showed that a hooked-collar diameter of (1.25b) has effectiveness similar to a collar diameter of (4.0b) used by Zarrati et al. (1999)where b is the pier width. With hooked collar installed at the bed level, there was no sign of scouring and horseshoe vortex at the upstream face of the pier. In contrast, with unprotected pier, the down flow and turbulent kinetic energy were reduced under the effects of the hooked collar. This study concentrated on use of downstream-facing aerofoil-shaped pier to reduce local scour. The main objective of this paper is to determine the effect of downstream-facing aerofoilshaped pier to reduce the local scour around aerofoil shaped pier. II. LOCAL SCOUR Local scour around bridge piers is the result of acceleration of the flow and formation of vortices around the piers. As the flow is interrupted a strong pressure field decreasing with the depth is formed in front of the obstruction. If the pressure field is strong enough it causes a threedimensional separation of the boundary layer. It drives the approaching flow downwards and a recalculating primary vortex is formed on the upstream side of the pier. As the flow passes by the bridge the vortices wrap around the sides of the piers in the shape of a horseshoe and continue downstream as shown in Figure(1). This is called a horseshoe vortex Rambabu et al.(2003). Fig.1: The horseshoe vortex is created at the upstream side of the pier and wraps around it The primary vortex system in front of the pier is the main scour force and it removes bed material from the base of the pier. Since the rate at which material is carried away from the area is greater than the transport rate into it, a scour hole is formed around the pier. Local scour is caused by an acceleration of flow and resulting vortices induced by obstructions to the flow.as the scour hole gets deeper the strength of the vortices at the base of the pier decrease and eventually a state of equilibrium is reached. For a clear-water situation that happens when the shear stress from the vortices equals the critical stress for the sediment particles, and no more bed material is scoured. For a live-bed situation equilibrium is reached when the amount of sediment inflow equals the amount of outflow from the scour hole. In that situation the scour depth fluctuates but the average depth remains constant. The horseshoe vortices act mainly on the upstream face and the sides of a pier, whereas on the downstream side of the pier another type of vortices dominates, called wake vortices. They are formed along the surface of the pier, then detached from both sides and continue downstream as 46
shown in Figure(2). The wake vortices together with the accelerated side flow and the upward flow behind the pier causes the downstream scouring. The wake vortices are generated by the pier itself and their size and strength depend mainly on the velocity of the flow and the pier size and shape. The strength of a wake vortex declines rapidly downstream of a pier and therefore there is often a deposit of material downstream of piers Richardson and Davis (2001). Fig. 2: Wake vortices are formed at the surface of the pier and continue downstream III. EXPERIMENTAL WORK The experiments were carried out in the Hydraulic Laboratory of Civil Engineering Department of Gaziantep University. The flume is 12 m long, 0.8m width and 0.9 m depth as shown in Figure (3) with glass sides and steel bottom. The test section was made with a ramp which is located at the beginning and the end of the section. The test section is 3 m long and 0.2 m depth as shown in Figure(3). Fig. 3: Schematic layout of the flume 47
The test section was filled with sediment of median particle size =1.45 mm and standard deviation, =3.16 [σ g = (d84 / d16) 0.5 ] with the specific gravity of 2.65, the sieve analysis of the sand is given in Figure(4). = =. =3.16 Fig. 4: Grain size distributions Flume discharge was measured by a magnetic flow meter installed in the pipe system before the inlet of channel. The scour hole and the elevation of the bed was measured by laser meter, the instrument mounted on a manually moving carriage sliding on rails on the top of the flume wall. Three piers circular, upstream-facing aerofoil and downstream-facing aerofoil-shaped piers as shown in Figure(5) were tested. Fig. 5: Three tested piers 48
The present study is perhaps the first experimental work to change the orientation of an aerofoil pier according to flow direction. Breusers and J.A. Raudkivi(1991) showed that scour depth increases for an upwardly widening pier and decreases for an upwardly narrowing pier. Experiments were performed under a live-bed water scour regime. The discharge was measured as 0.058 m 3 /s with 0.125 m flow depth. Initial bed elevations were taken randomly to check the leveling of the test section by using laser meter. The flume was first filled with water until desirable flow depth to avoid undesirable scour. Inlet flow to the flume was then gradually increased until the desired discharge, and the temporal variation of scour was monitored. The scour depth was measured under an intense light. The progress of scour depth was observed 5 hours. At the end of each test, the pump was shut down and the water was slowly drained without disturbing the scour topography. The test section was then allowed to dry and frozen by pouring glue material (varnish). V. ACOUSTIC DOPPLER VELOCIMETER MEASUREMENTS During each experiment, the velocity is periodically monitored using ADV. Stream velocity is measured upstream of the pier using the ADV in the center of the flume. The ADV probe was then positioned above the scour hole and velocities were recorded for a period of 120 seconds. The sample period of 120 seconds was chosen to ensure that sufficient flow variations were captured. VI. VELOCITY MEASUREMENT COORDINATES Velocities were measured over half the width of the flume due to the symmetrical shape of the pier and the centered alignment of the pier within the flume. Velocities were recorded in interval from X= -28 cm to X= 24 cm for each Y coordinate from Y=0 cm (center line of channel) to Y=10 cm for upstream-facing aerofoil-shaped pier as in Figure(6). The Z coordinates at the bed level is zero. Fig. 6: Velocity measurement coordinates 49
Velocities were recorded for the downstream-facing aerofoil and circular piers using a similar methodology as was done for the upstream-facing pier. VII. SCOUR HOLE DIMENSIONS Dimensions of the scour holes for each run of experiments were measured. The top width of scour in the transverse direction, distance from upstream face to front outer edge of hole, and depth at upstream face were compared for each of the three bridge piers as shown Table (1). Table 1: Scour hole dimensions from physical modeling Scour hole dimension Downstream-facing aerofoil-shaped pier Upstream-facing aerofoil-shaped pier Circular pier Top Scour Hole Width (m) 0.24 0.44 0.50 Distance from Upstream face to front outer edge of hole (m) 0.04 0.14 0.18 Depths at Upstream face (m) 0.035 0.085 0.11 VIII. RESULT AND DISCUSSION Experimental results of the model piers within cohesion less bedding material will be compared and discussed in this section. The results are a comparison of scour and sediment scour hole depths for the circular and upstream-facing aerofoil piers were quite similar which was expected due to the identical shape on the upstream side of the pier as illustrated in Figure (7). A 68 % reduction in scour hole depth of the downstream-facing aerofoil pier was observed as compared to the circular pier and 59% reduction as compared to the upstream-facing aerofoil pier. The effect of the horseshoe vortex was reduced due to the downflow being deflected away from the base of downstream aerofoil pier. The development of the scour hole around the pier perimeter is therefore strongly influenced by this down flow. In addition to a reduction of the scour depth, the rate of scouring is also reduced considerably as in Figure(7). Reduction in the rate of scouring can reduce the risk of pier failure when the duration of floods is short Melville and Chiew(1999). 50
Fig. 7: Time evolution in scour depth measured at the upstream face of the piers Figure (8) shows the comparison of longitudinal velocity (Vx) upstream the three bridge piers. It observed that fluctuation of turbulent flow for downstream-facing aerofoil pier was less than for circular and upstream-facing aerofoil piers due to activity of horseshoe vortex is more than at the upstream side of circular and upstream-facing aerofoil piers as compared to downstream-facing aerofoil pier. Therefore, more scour observed at circular and upstream-facing aerofoil shaped piers than downstream-facing aerofoil shaped pier. Fig. 8: Comparison of tangential velocity (Vx) upstream side of three piers at x=4 cm, y=0 cm and z=-3 cm 51
Figure (9) shows the scour pattern around the circular, upstream facing aerofoil and downstream-facing aerofoil piers. The downstream-facing aerofoil pier minimizes the scour depth, producing a little scour in the front and on the sides of the pier because the effect of the horseshoe vortex was reduced due to the downflow being deflected away from the base of downstream-facing aerofoil bridge the pier. Fig. 9: Scour pattern around A) Circular pier B) Upstream-facing aerofoil-shaped pier C) Downstream-facing aerofoil-shaped pier 52
The percentage reduction in distance from the upstream face of the pier to the upstream front outer edge of the hole as compared to the circular pier was 22% for the upstream-facing aerofoil pier and 78% for the downstream-facing aerofoil pier as in Figure(10) and Figure(11) demonstrate the top scour holes of three piers. The top scour hole width of downstream-facing aerofoil shaped pier was 52 % less than the circular pier and 12 % less than the upstream-facing aerofoil pier because the effect of the horseshoe vortex was reduced due to the downflow being deflected away from the base of downstream-facing aerofoil pier. Fig. 10: Longitudinal scours holes of three bridge piers Fig. 11: Transverse scour holes of three bridge piers 53
Figure(12) shows a strong horseshoe vortex was detected in the upstream reach of circular and upstream-facing aerofoil piers positioned at the base of piers, but the effect of horseshoe vortex reduced (not so visible) due to the down flow being deflected away from the base of downstreamfacing aerofoil pier. It is seen from Figure(12) that, a strong wake vortex was visible and leading into scour hole in the downstream reach of circular and downstream-facing aerofoil piers. Fig.12: Time average velocity vectors of A)Upstream-facing aerofoil B)Downstream-facing aero foil C) Circular pier 54
IX. CONCLUSION Changing the orientation of an aerofoil pier so that it faces downstream rather than upstream is an effective for reducing scour. The present study is perhaps the first experimental work to change the orientation of an aerofoil pier so that it faces downstream rather than upstream according to the direction of flow, with the new method there was a very little sign of downflow and horseshoe vortex at the upstream side of pier. Downstream-facing aerofoil-shaped pier reduces scour depth and volume, length of scour hole and scour hole width more than upstream-facing aerofoil and circular bridge piers. The present experimental study shows that downstream-facing aerofoil design can reduce scour depth, thereby reduces the potential need for countermeasures. The performance of aerofoil pier improved by locating it as downstream-facing aerofoil pier to the flow. Notation b = Pier width (m). D = Pier diameter (m). d 50 = Median particle size (mm). d 16 = Grain size for which 16% by weight of the sediment is finer; d 84 = Grain size for which 84% by weight of the sediment is finer; = (d 84 /d 16 ) 0.5 Geometric standard deviation of the grain size distribution. REFERENCES [1] Breusers, H.N.C., and Raudkivi, A.J. (1991). Scouring.A.A. Balkema, Rotterdam/Brookfield, Netherlands. ISBN 90 6191 983 5. [2] Chen et al. (2012). Experimental investigation of the Flow Field around a Bridge Pier with Hooked collar." ICSE6 Paris - August 27-31A. [3] Dargahi, B. (1990). Controlling mechanism of local scouring. J. Hydraul. Eng., 116 (10): 1197-1214. [4] Dey, S., and Raikar, R. (2007). Characteristics of Horseshoe Vortex in Developing Scour Holes. J. Hydraul. Eng., 133(4): 399-413. [5] Melville, B.W. and Chiew, Y.M. (1999). Time scale for local scour at bridge piers. Journal of Hydraulic Engineering, ASCE, 125(1): 59-65. [6] Muzzammil, M. and Gangadhariah, T. (2003). The mean characteristics of horseshoe vortex at a cylindrical pier. J. Hydraul. Res., 41(3), pp. 285-297. [7] Muzzammil, M., Gangadhariaiah, T., and Gupta, A.K. (2004). An experimental investigation of a horseshoe vortex induced by a bridge pier. Water Management Journal, Proceeding of the Institution of Civil Engineers, Thomas Telford Journals, London, 157(2): 109-119. [8] Richardson, E.V. & Davis, S.R. (2001) Evaluating Scour At Bridges Fourth Edition, U.S. Department of Transportation, Federal Highway Administration, and Washington DC, USA. [9] Rambabu, M., Rao, S.N. and Sundar, V. (2003). Current-induced scour around a vertical pile in cohesive soil. In: Ocean Engineering, Elsevier Science B.V., Amsterdam, The Netherlands, Vol. 30, Issue 7, pp 893-920. 55
[10] Tafarojnoruz, A., Gaudio, R., and Dey, S. (2010a). Flow-altering countermeasures against scour at bridge piers: review. J. Hydraul. Res., 48(4), 441 452. [11] Tafarojnoruz et al. (2012). Evaluation of Flow-Altering Countermeasures against Bridge Pier Scour, J. Hydraul. Eng., 138:297-305. Yanmaz, A.M., and Altinbilek H.D. (1991). Study of Time-dependent local scour around bridge piers. J. Hydraul. Eng., 117 (10): 1247-1268. [12] Imad H. Obead and Riyadh Hamad, Experiments to Study the Effect of Dissipation Blocks upon Energy of Flow Downstream the Compound Weirs, International Journal of Civil Engineering & Technology (IJCIET), Volume 5, Issue 3, 2014, pp. 32-49, ISSN Print: 0976 6308, ISSN Online: 0976 6316. [13] H J Surendra and Paresh Chandra Deka, Effects of Statistical Properties of Dataset in Predicting Performance of Various Artificial Intelligence Techniques for Urban Water Consumption Time Series, International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 2, 2012, pp. 426-436, ISSN Print: 0976 6308, ISSN Online: 0976 6316. [14] Prof. P.T. Nimbalkar and Vipin Chandra, Estimation of Bridge Pier Scour for Clear Water & Live Bed Scour Condition, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 3, 2013, pp. 92-97, ISSN Print: 0976 6308, ISSN Online: 0976 6316. 56