The Effect of Support Structure on Flow Patterns ...

Arab J Sci Eng DOI 10.1007/s13369-015-1604-2

RESEARCH ARTICLE - CIVIL ENGINEERING

The Effect of Support Structure on Flow Patterns Around T-Shape Spur Dike in 90◦ Bend Channel M. Vaghefi · A. Ahmadi · B. Faraji

Received: 16 September 2014 / Accepted: 4 February 2015 © King Fahd University of Petroleum and Minerals 2015

Abstract Totally, spur dike is an economical way to preserve morphology of rivers. Numerical methods can be useful for evaluating hydraulic parameters for spur dike because of their reduced simulation time, while experimental ones take a long time and need lots of tools to simulate any models. In this paper, flow patterns around a T-shape spur dike and a support structure, which is located upstream of the Tshape spur dike, is analyzed in 90◦ bend channel by Flow-3D model. The numerical and laboratory data are compared in longitudinal section to verify numerical model. The results show very good correspondence between numerical and laboratory data. After verification numerical model, a support structure has been installed upstream of the T-shape spur dike with 3, 5, 7 and 9 times distance longer than the length of the T-shape spur dike. The support structure altered flow patterns and hydraulic parameters such as power of secondary flow and separation zone in all of sections. By increasing support structure distance from 3L up to 9L, the power of secondary flow around main spur dike decreases by 40–120 % and the length of separation zone increases from 0.8 to 2.5 times bigger than the length of T-shape spur dike. Keywords Numerical model · Spur dike · Distance of support structure · Bend channel

M. Vaghefi (B) Department of Civil Engineering, Persian Gulf University, Bushehr, Iran e-mail: [email protected] A. Ahmadi Water Civil Engineering, Ferdosi University, Mashhad, Iran B. Faraji Water Civil Engineering, Razi University, Kermanshah, Iran

1 Introduction Generally, river morphology varied in period, so it is necessary to control riparian and bed of rivers. Spur dikes protect riverbank from erosion, create stable pools for aquatic habitat, and trap suspended sediment in backwater zones by redirecting main flow. The main purpose of spur dike is to prevent riverbanks from scouring because some types of flow patterns are combined together which cause scouring sediment in bend channels. Therefore, it is essential to be familiar with the flow pattern around these kinds of hydraulic structures. Many experiments were conducted by scientists in laboratories and in numerical methods [1,2]. Rajaratnam [1] analyzed shear stress around groin, experimental results showed maximum shear stress occurred on nose of spur dikes. Hydraulic parameters, separation zone, pressure gradient and vortex have been experimented by Maccoy et al. [3] for series of two dikes. Fazli et al. [4] studied the scour pattern and, to a lesser extent, the flow pattern around spur dikes located in a bend. Vaghefi et al. [5,6] have studied flow patterns and sediments scour phenomena around T-shape spur dikes. They are varied in length, wing, shape of spur dikes and Froude numbers. Subsequently, effects of them were measured experimentally. Finally, they suggested some equations to predict local sediment scour around single T-shape spur dikes. Masjedi et al. [7,8] examined T-shape spur dikes that are located in 45◦ and 60◦ in outer bank of 180◦ bends. Nevertheless, the number of numerical and physical studies is limited. Gill [9] tried to analyze sediment scour depths around spur dikes in laboratories. The results indicated that equivalent scouring depths strongly depended on the size of materials and the fluid depth of the channel upstream. Therefore, maximum scouring occurred when sand beds commenced to move upstream. Soliman [10] used 2D simulation were used to determine morphology of Nile channel around

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groin. Both numerical and experimental simulation used to determine flow patterns around spur dikes in meandering channels by Giri et al. [11]. Olsen et al. [12] used a numerical method to analyze flow patterns in meandering channels, and main purpose of their study was to examine effects of the bed surface roughness on flow characteristics. Zhange et al. [13] utilized experimental and CFD methods to identify turbulent flow in bend with clear water. Numerical models used the k − ε turbulent model and results showed good correspondence between physical and numerical data. Acharya and Duan [14] carried out a three-dimensional numerical investigation of turbulent flow pattern around series of straight spur dikes located in a straight route with live and fixed bed, using Flow-3D software. They used k − ε turbulence model for modeling and compared the results with experimental results. Chen et al. [15] used the compressive volume of fluid (VOF) method to develop the models for water and suspended sediment in a 90◦ bend flume with non-submerged spur dyke at different angles. They simulated local scouring, deposition and resuspension and analyzed the processes of adsorption and desorption of pollutants on suspended sediment. Fang et al. [16] studied the turbulent flow past a series of groins in a shallow, open channel by large-eddy simulation (LES). They obtained the time-averaged velocities and turbulence intensities at the water surface using particle image velocimetry (PIV) to validate the LES model. Their model results showed that a rectangular-headed groin generates higher turbulence intensities and larger vortices than a round-headed groin. Ghiassi and Abbasnia [17] used numerical model to simulate flow and bed deformation around hydraulic structures. They introduced the new sedimentation formula about vorticity effects on local scouring around bridge pier and a groin. Vaghefi et al. [18] used SSIIM numerical model to simulate effects of T-shape spur dike submergence ratio on the water surface profile in 90◦ channel bend. Due to fewer researches done on flow patterns around support structures, especially those located in bend channels. In this paper, effects of distance of support structures on flow patterns around T-shape spur dikes have been studied to analyze hydraulic parameters and flow characteristics.

2 Materials and Methods 2.1 Governing Equations in Numerical Models According to many of numerical methods, Flow-3D software uses momentum and continuity equations. Equation (1) shows continuity relation that is relinquishing compressibility of fluid and Cartesian coordinate directions (x, y, z). Momentum formulas are shown from Eqs. 2–4.

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∂ (ρ) ∂ ∂ RsoR ∂ + (u A x )+ (v A y )+ (w A z ) = ∂t ∂x ∂y ∂z ρ ∂u 1 1 ∂p ∂u ∂u ∂u u Ax + + v Ay + w Az =− ∂t VF ∂x ∂y ∂z ρ ∂x +G x + f x 1 ∂u ∂u ∂u ∂v 1 ∂p + + v Ay + w Az u Ax =− ∂t VF ∂x ∂y ∂z ρ ∂y +G y + f y ∂w 1 1 ∂p ∂u ∂u ∂u u Ax + + v Ay + w Az =− ∂t VF ∂x ∂y ∂z ρ ∂z +G z + f z

VF

(1)

(2)

(3)

(4)

In these equations, (u, v, w) are velocity components in (x, y, z) directions, and (A x , A y , A z ) are fractional areas opened to flow in subsidiary directions, (G x , G y , G z ) are gravitational force in the subsidiary directions. F is a part of the fluid volume that includes 1 and 0.1 is used when the cell is combined with fluid and zero is used in vacant conditions. By solving Eq. 5, the amount of F will be obtained shown below. 1 ∂F + ∂t VF

∂ ∂ ∂u (Fu A x )+ (Fv A y )+ (Fw A z ) = 0.0 ∂x ∂y ∂z (5)

2.2 Boundary Conditions and Meshing Figure 1 shows a schematic view of computational field of the numerical model. Generally, each mesh block included six boundaries; discharge conditions have been used for inlet boundary, and in the outlet, all of the solid surface and water surface have been used for specific pressure, wall law and symmetry boundary conditions, respectively. Form this figure, it can be seen that four mesh blocks have been created to simulate the model. Because of the Cartesian meshing system, two straight mesh blocks were used for upstream and downstream channel, and two mesh blocks, named mesh blocks 3 and 4, were used in the bend channel and around the T-shape spur dike. Mesh block 4 is created to increase precision of the numerical results due to complex geometry of the model around the spur dike. Non-uniform meshing in the depth and width of the channel is another parameter, which is used to increase simulation accuracy and reduce simulation time. To tell truth, we used lots of meshes and analyzed them to get higher accuracy between numerical and experimental data. Finally, the numbers of mesh blocks and mesh sizes have been selected as below because the error of between this cell size and finer cell size became constant (Table 1).

Arab J Sci Eng Fig. 1 view of computational mesh blocks

Table 1 Number of cells in each mesh blocks Mesh block 1 Mesh block 2 Mesh block 3 Mesh block 4 X direction 130

40

310

170

40

130

310

170

Z direction 35

35

35

35

Y direction

2.3 Physical Model Laboratory data, used in numerical model, were at the Hydraulic Laboratory of Tarbiat Modares University, Tehran [5,6]. The main channel consisted of 7.1 m long upstream and 5.2 m long downstream straight reaches. A 90◦ bend channel located between two straight reaches. The channel included rectangular cross section that is 0.6 m in width, 0.7 m in height and 2.5 m radius of bend to the centerline. The bed and sides of the channel were made of glass that was supported by a metal frame. The channel has 0.001 slope and ratio between radius and channel wide equals four. Uniform sediment with a median size d50 = 1.28 mm and standard deviation σ = dd84 = 1.3 were used, and the length of 16

the channel was completely covered by 0.35 m thickness. d84 and d16 stand for sediment diameters, which are finer than 84 and 16 % of bed materials, respectively. However, sediment bed materials are fixed by special glue to simulate flow patterns. Many simulations are done to make the same situations between laboratory and numerical surface roughness. Finally, 0.00128 surface roughness was selected to create good accuracy. Spur dike was made of Plexiglas with one centimeter thickness. Measurement of discharge was performed by a calibrated orifice, set in the supply pipe. Flow depths were measured by a digital point gage with an accuracy of ±0.1 mm. A sluice gate was located at the end of the main channel to control the flow depth.

3 Results and Discussions Accuracy of the numerical results is the most important parameter of each simulation. Furthermore, each numerical model should be verified with laboratory data, so in this article, many laboratory data are compared with numerical model. Figure 2 shows regression between physical and

Fig. 2 Comparison between numerical and laboratory data at section in: a 40◦ , b 42.5◦

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Fig. 3 View of geometry of numerical and physical case studies field

Fig. 4 View of streamline at cross section in a 40◦ , b 44.5◦ and c 46.5◦ for single T-shape model

laboratory velocity in plan section to 95 % of fluid depth at 40◦ and 42.5◦ sections. Results show that high regression exists in 40◦ section because of its distance to support structure and T-shape spur dike. The amount of regression number decreased by approaching to T-shape spur dike. These differences were caused due to the complex turbulent situation or errors of laboratory data. However, Flow-3D model can simulate such models with high precision. In this paper, the effect of a support structure on flow patterns around T-shape spur dike in a 90◦ bend channel was analyzed by flow-3D model. For this purpose, a support structure is made with nine centimeters in length (L) and one centimeter thickness (t), located upstream of a T-shape spur dike. Figure 3 shows geometry of the numerical and physical case studies fields. In this study, the distance of support structure is five times bigger than the length of the T-shape spur dike (L) which is upstream.

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3.1 Streamlines in Cross Section Totally, streamline direction and velocity distribution of each section can help us to analyze flow pattern around the T-shape spur dike with high accuracy. Figures 4, 5, 6, 7 and 8 show velocity distributions in vertical direction (Wz) in cross sections in 40◦ , 44.5◦ and 46.5◦ . According to results, it can be understood that streamline direction goes to water surface where velocity contour is positive, and streamline patterns moves to bed of channel in which velocity contour is negative. Figures 4a, 5a, 6a, 7a and 8a show that flow patterns do not vary in single T-shape model, but the amount of velocity decreases in outer wall and increases in inner bank as distance of support structure and T-shape spur dike increases. Separation zone that is created around both structures cause negative area in third part of channel width in outer wall of channel. In the 44.5◦ cross section, Figs. 4b, 5b, 6b, 7b and 8b show that down flow streamline pattern occur between wing of T-shape spur dike and outer wall. Thus, this kind of

Arab J Sci Eng

Fig. 5 View of streamline at cross section in a 40◦ , b 44.5◦ and c 46.5◦ for 3L model



flow pattern has a negative effect on bed stability. Streamline flow patterns created up flow pattern between the wing of the T-shape spur dike and the outer wall of the channel for 3L and 5L model, but streamline direction moved in parallel of the channel width for 7L model. Down flow pattern occurred where the distance of support structure and the T-shape spur dike was nine times bigger than the distance of the wing of the T-shape spur dike. Because of the influence of the main flow on the flow pattern between both structures, a small anticlockwise vortex existed at top of the fluid depth. Figures 4c, 5c, 6c, 7c and 8c are samples of cross section view for 46.5◦ of the channel which is located downstream of the T-shape spur dike. As result demonstrates, down flow pattern exists only in a model

with 5L distance. This condition is useful for stability of channel bed materials; however, other models cause up flow patterns in this cross section. 3.2 The Power of Secondary Flow The power of secondary flow was one of the hydraulic parameters that were investigated by Shukry [19]. In this paper, from Eq. (6), the power of secondary flow was obtained by dividing lateral kinetic energy to total kenotic energy by each section in the 90◦ bend channel. S=

K latral (vr )2 + (wz )2 = K total (Uθ )2 + (vr )2 + (wz )2

(6)

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Fig. 9 The power of secondary flow at 90◦ bend channel

In this equation, K Latral is lateral kinetic energy, K Total is total kinetic energy in each section, u θ , vr and wz are tangential, radial and vertical velocity components, respectively. Figure 9 indicated that if a single T-shape spur dike is located in bend channel, then the power of secondary flow will increase in section with 2.5 times bigger than the distance from upstream of spur dike exponentially to the wing of the T-shape spur dike. After the spur dike position, the power of secondary flow plunges rapidly from section with 5 times bigger than distance with downstream of T-shape spur dike because channel the active width increases after this section. One of the purposes of spur dike is that it can reduce the power of secondary flow in sensitive sections of channel, for example, in this study, the support structure decreases the amount of the power of secondary flow about 50 % around the T-shape spur dike. This phenomenon can preserve bed and banks of channels from scouring. As the result shows, peak point of power of secondary flow occurred at first structure. 3.3 The Position of Maximum Velocity To sum up, the position of maximum velocity varied in every fluid depth and any section of channel. At the beginning of the bend channel, maximum velocity occurred near the inner

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wall of the channel because of existence of gradient energy and after that non-uniform cross section of water surface profile. By approaching downstream of bend, due to the effect of centrifugal force on flow patterns, the position of maximum velocity moved to the outer wall bank. It is obvious that shear stress has a direct relationship with amount of velocity. As a consequence, by increasing velocity of each section, shear stress increases as well as the amount of sediment scouring channel expand in channel. It can be seen that high velocity, created at vicinity of wing of the spur dike, also exist in 7L and 9L model near the bed of the channel. The support structure moved maximum velocity position to nose of itself by reducing the active width of the channel bed, so that amount of velocity around T-shape spur dike decreased. Figure 10 indicates that the position of the support structure, which is created around the T-shape spur dike, tended to the middle of the channel width, thus it means that support structures can decrease amount of shear stress at vicinity of T-shape spur dikes. The main point of Fig. 10 is that the maximum velocity position is the same in all of the models at upstream and downstream of channel. On the other hand, the main flow existed downstream of channel, goes to outer wall, near water surface, but moves to inner wall, near bed of channels. This mechanism directly depends on the position of the maximum velocity.

Arab J Sci Eng

Fig. 10 View of maximum velocity position at plan section with a 1 cm and b 11.5 cm away from bed

Figure 11a, b indicate the amount of maximum velocity in each cross section of the whole bend channel at 1 and 11.5 cm away from bed. As results shown in Fig. 11a, the maximum velocity around the T-shape spur dike occurred in a single T-shape model; however, the support structure decreases this amount up to 12 % which demonstrates appropriate act of support structures. Because the support structure first moved at maximum velocity position around the T-shape spur dike and after that decreased the amount of it. Due to separation zone, which is created in the middle of both structures, the active width of channel decreased, and after that the amount of velocity in 20◦ –40◦ cross section increased. Conversely, it can be mentioned that each support structure model has different characteristics on flow pattern and to select the best of them, it is necessary to consider all of the results together.

3.4 Separation Zone The main characteristic of flow patterns around T-shape spur dikes and support structures is separation area, which is indicated in plan section 10 and 95 % of fluid depth distance from bed in Figs. 12 and 13. Figure 12 shows separation zone path for the entire model. The width of separation zone in 5L model is larger than other support structure distances, but the length of separation zone in a model with 9L distance is higher than the others models. It should be considered that by increasing the dimensions of separation zone, it cannot be concluded that model is acceptable, because in this model by increasing the distance of the support structure two vortexes with opposite rotation were created in the middle of both structures. A clockwise vortex can be harmful for stability

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Fig. 11 Amount of maximum velocity at plan section with a 1 cm and b 11.5 cm away from bed

Fig. 12 separation zone path at plan section with a 1 cm and b 11.5 cm away from bed

of channels. Here L is length of T-shape spur dike. As the results show, the length of separation zone (Ls) is related to support structure distance to T-shape spur dike. Thus, if support structure distance increases, then length of separation zone will increase linearly. It should also be mentioned that length of separation zone is eight times bigger than length of Spur dikes for single T-shape models. Indeed, the length of separation zone will decrease near water surface. In 9L model, the length of separation zone increases more than 2.5 times bigger than single T-shape spur dike separation’s length. As Fig. 13 show, if the support structure located at higher distance to T-shape spur dike, the width of separation zone (ws) changed nonlinearly. The width of separation zone increases in 5L model, but after that this amount decreases gradually.

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4 Conclusion Spur dike are important structures in river engineering, so it is necessary to be familiar with them. In this paper, in verification of Flow-3D model, acceptable correspondence between numerical and laboratory data obtained which showed good ability of numerical model. Different locations of support structure have been simulated and results show that Support structure decreased the amount of the power of secondary flow 50 % around the T-shape spur dike and the peak point of the power of secondary flow occurred around it. Support structure decreased the amount of maximum velocity up to 12 % around the T-shape spur dike and increased separation zone so that the maximum width of separation zone occurred in model with 5L distance. If the distances of support struc-

Arab J Sci Eng

Fig. 13 Variation of width and length of separation zone at plan section with a 1 cm and b 11.5 cm away from bed

ture increase from T-shape spur dike, then width and length of separation zone changed nonlinearly and linearly, respectively. Just in model with 5L distance, down flow patterns were created at downstream of T-shape spur dike. Totally, it can be concluded if bed of channels is covered with moveable materials, then models with 5L distance can protect more efficiently than other models.

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