The color contours of the pressure at π π = 1 Γ 10' are presented in Figure 5.4. The pressure contour values seen in Figure 5.4 (a) represents the kinematic pressure π = π/πβ, where πβ is the constant mass density. Due to the block effect of the structures to the flow, there is a high-pressure region in front of the structure because of the energy conservation. Furthermore, due to the conservation of mass, a high-speed velocity region forms after the separation point above the structure. Hence, due to the Bernoulliβs principle, the high-speed velocity region creates a local negative pressure region around the right top edge of the structure as seen in Figure 5.4.
It can be observed that the intensities of both the positive and negative pressure regions reduces with increasing πΌ. This is due to the fact that the reducing blocking effect, associated with higher πΌ, causes lower velocity drop over the structure, hence resulting in weaken pressure regions due to the Bernoulliβs principle.
47
a) b)
c) d)
e)
Figure 5.4 Pressure contours for wall-mounted squares in tandem with gap ratios of: a) G/D = 2, b) G/D = 3, c) G/D = 6, d) G/D = 10 and e) G/D = 14
48
The contours of the horizontal velocities at π π = 1 Γ 10' are presented in Figure 5.5 for all πΌ.
The velocity at the front face of the structure is zero and propagates further upstream due to the blocking effect. After the separation point at the top left corner of the structure, the velocity is accelerated due to the conservation of mass. A shear layer is generated with a high-speed region above the structure and a recirculation region behind the structure. The area of the high-speed region reduces with decreasing πΌ. The negative velocity region downstream close to the bottom wall implies that recirculation takes place.
a)
b)
49
c)
d)
e)
Figure 5.5 Horizontal velocity contours at Re = 1 Γ 10' for varying πΌ: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
50
The contours of vertical velocity are presented in Figure 5.6 for all angles πΌ. There is an increase in the vertical velocity along the front face of the structure due to the conservation of mass, accelerating the mass above and past the structure to maintain conservation of energy.
The intensity of the high vertical velocity region around the top left corner reduces with increasing πΌ, as seen in Figure 5.6 (b) - (e). This is because the trapezoidal structures shift the direction of the flow more gradually. There is also a slight positive vertical velocity region at the rear face of the structure, indicating that a recirculation region forms behind the structures.
a)
b)
51
c)
d)
e)
Figure 5.6 Vertical velocity contours at Re = 1 Γ 10' for varying πΌ: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β°
and (e) 60Β°
52 5.5 Bed shear stress and Scour
In actual subsea environment, bed shear stress is closely related to the scour process, which is removal of sediment around the base of subsea structures. High absolute values of π πβ N close to the structure can indicate a significant scour process, which is a typical source for failures in subsea operations (Zhao et al.,2012). Therefore, the present study investigates the bed shear stress on the seabed surfaces adjacent to the structures with different configurations of πΌ.
Figure 5.7 shows the non-dimensional bed shear stress π πβ N, where πN is the undisturbed bed
shear stress, along the bottom wall surface. π πβ N stagnates towards zero close to the front faces of the structures which are located π₯/π· = (β0.5, β0.768, β1.077, β1.5, β1.732) for πΌ = (0Β°, 15Β°, 30Β°, 45Β°, 60Β°), respectively. Figure 5.7 (b) shows small negative π πβ N regimes in front of the structure for all πΌ due to the backflow and the adverse pressure gradient caused by the conservation of energy. It can be observed that these negative regimes have lower amplitudes and are shorter in streamwise direction with increasing πΌ. This behavior is due to the smooth flow transitions which is associated with larger πΌ. The bed shear stress has a large negative region behind the structures due to the recirculation motions for all configurations.
This negative region is also reduced with increasing πΌ, which is consistent with the declining recirculation length. It is also worth noting that the absolute minimal value of the bed shear stress also decreases with increasing πΌ and its location shifts closer to the structure as seen in Figure 5.7 (a).
a) b)
Figure 5.7 Bed shear stress along the bottom surface at Re = 1 Γ 10' for: a) the whole domain. b) the front face of the structures
53 5.6 Streamlines
To further investigate the flow around the five configurations with different πΌ, streamlines are plotted at π π = 1.19 Γ 10& in 5.9 and at π π = 1.0 Γ 10' in Figure 5.8 and Figure 5.10. There are three main recirculation motions around the square structure seen in Figure 5.8. The first vortex (1) forms because the fluid particle hits the structure, flows downward and reverses its direction due to the bottom wall. A large recirculation is generated by the shear layer separation after the front top edge of the square (2), which also induces a smaller one in the downward corner of the square (3). For both Reynolds numbers, with increasing πΌ, the length of the dominating vortex behind the structure decreases, seen in Figure 5.9 and Figure 5.10. In addition, as the flow tend to follow the angle of the front face after separation, the height of the recirculation reduces as the structure becomes flat. Furthermore, the increasing πΌ gradually suppresses the vortices (1) and (2) from 5.8 (a), of the structure because the flow tends to be attached to the structure surface as seen in Figure 5.8 (b).
a) b)
a)
1 2 3
Figure 5.8 Detailed stream lines for Re=1Γ106 for (a) Ξ±=0Β° and (b) Ξ±=60Β°
54 b)
c)
d)
e)
Figure 5.9 Stream lines at Re = 1.19 Γ 10& for varying Ξ±: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
55 a)
b)
c)
d)
e)
Figure 5.10 Stream lines at Re = 1 Γ 10' for varying Ξ±: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
56
6 Results and Discussion for Tandem Case
This chapter presents and discusses the results for the tandem case. The chapter presents the convergence study with respect to πΊ/π· and investigates the hydrodynamic coefficients, pressure distributions, streamlines and velocity profiles by varying the parameters of πΊ/π· and πΌ. All charts in this section are based on the data from Table 6.1, Table 6.2, Table 6.3 and Table 6.4. This chapter will use Square 1 and Square 2 when referring to the first and second square in tandem, respectively. Structure 1 and Structure 2 will similarly be used when referring to all trapezoidal configurations, including the square configuration.
Table 6.1 Results of convergence study for hydrodynamic coefficients for Square 1 and Square 2
Mesh πΊ/π· πΆSQ πΆTQ πΆSE πΆTE
57 6.1 Convergence
Convergence studies with all gap ratios, πΊ/π· = 2, 3, 6, 10 and 14, are performed with respect to hydrodynamic quantities for both squares in tandem to ensure mesh independence. First, 4 different mesh densities with πΊ/π· = 2 were constructed to establish converged results for the original mesh set up, which was found for the normal mesh at 54135 cells. Thereafter, three
πΌ πΆSQ πΆTQ πΆSE πΆTE
Table 6.2 Hydrodynamic coefficients for Structure 1 and Structure 2 with G/D = 6 for trapezoidal configurations using mesh number of 66855
Table 6.3 Hydrodynamic coefficients for Structure 1 and Structure 2 with πΊ/π· = 14 for trapezoidal configurations using mesh number of 89115
Table 6.4 Hydrodynamic coefficients for Structure 1 and Structure 2 with G/D = 10 for trapezoidal configurations using mesh number of 76395
58
meshes of different densities; coarse, normal and dense were constructed for each πΊ/π· as shown in Table 5.1 where all the normal meshes for square configurations were found to provide sufficient mesh resolution, with difference less than 5% for any hydrodynamic coefficient from its corresponding dense mesh. Figure 6.1 shows the variation of hydrodynamic coefficients with the grid numbers. The converged trapezoidal mesh configurations from the single structure case were combined with the converged mesh configuration with πΊ/π· β₯ 6 from Figure 6.1 to study the trapezoids in tandem as well. Investigations with πΊ/π· < 6 are not carried out for trapezoids in tandem because of the high skewness of the current mesh set up for trapezoidal geometries at low πΊ/π·.
a) b)
c) d)
Figure 6.1 Convergence study for wall-mounted squares in tandem with πΊ/π· = 2,3,6,10 and 14 for the hydrodynamic quantities: a) πΆSQ for Square1, b) πΆSE for Square2, c) πΆTQ for Square1 and d) πΆTE for Square2
59
6.2 Effects of πΊ/π· and πΌ on hydrodynamic quantities
Figure 6.2 presents the variation of the hydrodynamic quantities for the structures of same configuration at πΊ/π· at π π = 1.19 Γ 10&. The drag coefficient of Square 1 is increasing towards πΊ/π· = 6 however declining for higher πΊ/π· as seen in Figure 6.2 (a). The opposite is occurring for Square 2, seen in Figure 6.2 (b), where a distinct maximum absolute minimal value for the πΆSE is found at πΊ/π· = 6 and hence increasing linearly with the πΊ/π·. This relationship can be explained by the shielding effect, from Square 1, on Square 2 and the retarded vortex development between the squares which will be further explained in Chapter 6.3 and 6.5. The lift coefficients for Square1 is maximum with πΊ/π· = 3 and is monotonically decreasing with increasing πΊ/π·. Furthermore, πΆSQ and πΆTQ for Structure 1 decrease as πΌ becomes larger with πΊ/π· > 6 for all configurations. πΆTE is decreasing with the πΊ/π· for all structures and its variation tends to be flat out for higher πΊ/π·.
60
The variations of hydrodynamic quantities with πΌ = 0Β°, 15Β°, 30Β°, 45Β° and 60Β° are investigated with πΊ/π· = 6,10 and 14 and presented in Figure 6.2. Both πΆSQ and πΆTQ are decreasing, while πΆSE and πΆTE are increasing, with larger πΌ. The effects of the additional Structure2 are compared to the single structure case in Figure 6.2 (a) and (c). Here πΆSQ follows the same trend as the single structure case by declining with increasing Ξ±. Furthermore, the hydrodynamic effects of the gap ratios for the square configuration increase between 2 β€ πΊ/π· β€ 6 and the πΆSQ has a maximum deviation of 13.6% at πΊ/π· = 6 compared to the single square case. Moreover, it can be seen in Figure 6.2 (a) that the discrepancy of πΆS between the single square and πΊ/π· = 14 is 0.004%, which indicates that the tandem drag effects on Structure1 has disappeared.
a) b)
c) d)
Figure 6.2 Investigation of gap ratios πΊ/π· for different configurations of πΌ for the hydrodynamic quantities: a) πΆSQ for Square 1, b) πΆSE for Square 2, c) πΆTQ for Square 1 and d)
πΆTE for Square 2
61
However, Figure 6.3 (c) shows that the πΆTfor the single structure is most compatible with πΊ/π· = 10 while πΊ/π· = 14 have lower values of πΆT for all πΌ. This indicates that the lift effects of Structure2 on Structure1 not have fully disappeared. From Figure 6.2 (b), the absolute value of πΆSE slightly decreases with larger πΌ and has negative values for πΊ/π· = 6 and 10. However, with πΊ/π· = 14, the πΆSE becomes positive and increases with larger πΌ. From Figure 6.2 (d), it is clear that πΊ/π· = 6 have the largest values of πΆTE compared to the other πΊ/π·. πΆTE for πΊ/π· = 6 has its maximum value at πΌ = 45Β°, and slightly declines when πΌ = 60Β°. This is not the situation for πΊ/π· = 10 and 14, where the πΆTE is only slightly decreasing with larger πΌ.
a) b)
c) d)
Figure 6.3 Investigation of πΌ for gap ratios of πΊ/π· = 6,10,14 as well as for singular structures, for the hydrodynamic quantities: a) πΆS for Square 1, b) πΆS for Square 2, c) πΆT for Square 1 and d) πΆT for Square 2
62 6.3 Pressure distribution
The pressure distributions for all πΊ/π· = 2,3,6,10 and 14 for squares in tandem have been investigated and presented in Figure 6.4. It is worth to note that the pressure plotting scale for the tandem case has been adjusted from the single square case with the maximum positive pressure of 0.29 in front of Structure 1. Therefore, the pressure contour cases are not adequate for comparison.
6.3.1 Pressure distribution for squares in tandem
A large positive pressure region is observed at the front face of Square 1 due to the conservation of energy. Furthermore, for all πΊ/π·, two negative pressure zones are formed. One is formed in between the squares and the other one is formed behind Square 2. The negative pressure zone between the squares is associated with the vortex motions, seen from the streamline plots in Figure 6.5.2, and contributes most to the total drag force. The main contribution of the drag is from the pressure difference between the front and back face of the structures, which is the reason why πΆSE is negative for all the πΊ/π· for square configurations. Dai et al. (2017) has also observed that the pressure zone between the structures imposes a suction force on Square 2, hence results in negative πΆSE due to the pressure difference. For πΊ/π· = 14, πΆSE is almost zero and the pressures adjacent to Square 2 has reached equilibrium.
The strength of the negative pressure zone between the two square increases in the range of 2 < πΊ/π· < 6 and decreases in the range of 6 < πΊ/π· < 14 as indicated in Figure 6.4 (c) β (e), which can be associated with the drag coefficient variations in Square 1 and Square 2. The second pressure zone, forming at π₯/π·~5 behind Square 2 as seen in Figure 6.4 (a) and (b) disappears in Figure 6.4 (c) - (e). For πΊ/π· = 6, it seems that the two negative pressure zones consolidate and a negative pressure zone between the squares is formed, hence imposing the highest pressure on both structures. Moreover, the strength of the negative pressure zone decreases for πΊ/π· is 6 to 14 and seems to be less affected by Square 2.
63
a) b)
c) d)
e)
Figure 6.4 Pressure contours for wall-mounted squares in tandem with gap ratios of: a) πΊ/π· = 2, b) πΊ/π· = 3, c) πΊ/π· = 6, d) πΊ/π· = 10 and e) πΊ/π· = 14
64 6.3.2 Pressure distribution for trapezoids in tandem
Figure 6.5 shows the pressure contours of all trapezoidal configurations with πΊ/π· = 6, which is found to impose the most drag on the both squares in tandem. The pressure distribution at the front face of Structure 1 is, as for the single structure case, decreasing with increasing πΌ. The strength of the negative pressure zone between the structures is also decreasing for increasing πΌ and this trend is also observed for all configurations with πΊ/π· > 6 seen in Figure 6.5 and Figure 6.6. The low negative pressure zone forming behind Structure 2 is slightly increasing in size with increasing πΌ disparate to the negative pressure zone behind Structure1. This is because the pressure zone behind Structure 2 is depending on the flow velocity over Structure 2 which again is governed by the shielding effect from Structure1. Structure 2 will be subjected to higher flow velocities with reduced shielding effect from Structure1, hence creating a higher negative pressure zone because of Bernoulliβs principle.
The negative pressure region behind Structure 1 decreases in strength which is similar to πΊ/π· = 6 and πΊ/π· = 14. The increased negative pressure zone behind Structure 2, associated with increasing πΌ, is obvious for πΊ/π· = 10 as seen in Figure 6.6. Here, the strength of the negative pressure region is clearly increased behind Structure 2 seen in Figure 6.6 (d) and (e).
Figure 6.7 presents the pressure contours with all Ξ± configurations with πΊ/π· = 14. Here, the negative pressure region behind Structure 1 seems to behave identical to that of a single configuration at π π = 1 Γ 10'. This is further supported by the fact that the drag coefficient is
almost the same for the cases of single square and squares in tandem with πΊ/π· = 14. For Structure 2 with Ξ± > 0Β° the pressure on its front face shifts to be positive and becomes larger with increasing Ξ±, which leads to a positive drag on Structure 2 shown in
. This is because the vortex motions behind Structure 1 are different for each trapezoidal configuration, hence creating different downstream flows which Structure 2 is subjected to. It is observed in Figure 6.7 (e) that pressure contours behind Structure 2 is formed in a similar manner as Structure 1 since the flow profile it is subjected to has started to recover from the
πΌ πΆSQ πΆTQ πΆSE πΆTE
65
effects of Structure 1. This subsequently results in similar hydrodynamic quantities on Structure 2 to those on Structure 1 due to Bernoulliβs principle.
a) b)
c) d)
e)
Figure 6.5 Pressure contours for πΊ/π· = 6 for varying trapezoidal configurations Ξ±: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
66
a) b)
c) d)
e)
Figure 6.6 Pressure contours for πΊ/π· = 10 for varying trapezoidal configurations Ξ±: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
67
Figure 6.7 Pressure contours for πΊ/π· = 14 for varying trapezoidal configurations Ξ±: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
a) b)
c) d)
e)
68 6.4 Horizontal velocity contours
6.4.1 Horizontal velocity contours for squares in tandem
The horizontal velocity profiles for wall-mounted squares in tandem for πΊ/π· = 2,3,6,10 and 14 is presented in Figure 6.8. A high-speed region V1 is formed at (π₯/π·, π¦/π·) = (0,2) above the shear layer shown in Figure 6.8 (a). It is obvious that πΊ/π· = 6 have a squeezed V1, however, the strength of its core region is the largest of all gap ratios and can be seen with contours inside V1. Moreover, for πΊ/π· > 6 V1 is expanded with increasing πΊ/π· following Square 2. The low-speed region between the two squares are coherent with the vortex motions shown in Figure 6.11. For πΊ/π· = 2, 3 and 6 the core of the low-speed region is located close to the left top edge of Square 2. However, when πΊ/π· > 6 the core of the low-speed region is expanded and is attached to the bottom wall.
a)
b)
V1
69
c)
d)
e)
Figure 6.8 Horizontal velocity contours for wall-mounted squares in tandem with gap ratios of:
a) πΊ/π· = 2, b) πΊ/π· = 3, c) πΊ/π· = 6, d) πΊ/π· = 10 and e) πΊ/π· = 14
70
6.4.2 Horizontal velocity contours for trapezoids in tandem
The streamwise velocity contours for all Ξ± with πΊ/π· = 6 and 14 are presented in Figure 6.9 and Figure 6.10, respectively. The high-speed velocity core V1, present for πΊ/π· = 6, is visible by contours for πΌ β€ 30Β° in Figure 6.8 (a) β (c). However, as πΌ increases, the area of this region is decreasing and the maximum velocity in the core of the high-speed region also decreases.
a)
b)
71
c)
d)
e)
Figure 6.9 Horizontal velocity contours for πΊ/π· = 6 for varying trapezoidal configurations Ξ±:
(a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
72
a)
b)
c)
73
d)
e)
Figure 6.10 Horizontal velocity contours for πΊ/π· = 14 for varying trapezoidal configurations Ξ±: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
74 6.5 Streamlines
The streamlines for πΊ/π· = 2, 3, 6, 10 πππ 14 are plotted for squares in tandem, and vortexes are denoted Rx, where x is the number of the vortex explained in Figure 6.11. Moreover, the streamlines for all trapezoidal configurations have been plotted for πΊ/π· = 6 and 14.
6.5.1 Streamlines over tandem squares
The streamlines over the square configurations, presented in Figure 6.11, shows 7 visible vorticities whose appearances changes with different πΊ/π·. Figure 6.11(a), where πΊ/π· = 2 , shows that the small R1 at the front face of Square 1 is developed in the same manner as the single square case. However, the vorticities downstream of Square 1 is varying with πΊ/π·. The elongated R2 is located on top of the two tandem squares and induces R3 in a counter clockwise direction in the gap between the two squares. At the back surface of Square 2, there is a small counterclockwise R4 which is induced by the large downstream R5, similar to the single square case. Increasing the gap distance from πΊ/π· = 1 (a) to πΊ/π· = 2 (b), the R2 starts to suppress R3 and becomes a larger vortex attached to the bottom wall. R3 additionally induces two small vorticities: R6 and R7 in a counter clockwise direction.
Figure 6.11 Time-averaged streamlines over wall-mounted squares in tandem with gap ratios of: a) πΊ/π· = 2 , b) πΊ/π· = 3 and c) πΊ/π· = 6
Figure 6.12 shows the whole domain of interest with πΊ/π· = 2, 3, 6, 10 and 14 for the squares in tandem. It can be observed that R6 and R7 have been reduced significantly in size when increasing πΊ/π· to 10 and 14. The long downstream R5 behind Square 2 also decreases in size
R1 R2 R4
R6 R7
R5 R3
75
due to its position downstream of Square 1 with large πΊ/π·. This is because the turbulent shear layers above R2, formed at the left top corner edge at Square 1, no longer reaches past Square 2 and affects R5 as seen Figure 12 (d) and (e). This subsequently leads to a highly reduced velocity profile over Square 2 which can be linked to the pressure and velocity contours discussed in Chapter 6.3.1 and Chapter 6.4.1, respectively.
a)
b)
c)
d)
76 6.5.2 Streamlines over trapezoids in tandem
Figure 6.3 and Figure 6.4 shows the streamlines for all trapezoidal configurations in tandem with πΊ/π· of 6 and 14, respectively. With πΊ/π· = 6, the R6 and R7 are suppressed with increasing πΌ. R2 recirculates over the top of Structure2 when πΌ < 45Β° , however for larger πΌ, R2 seem to be shaped according to the gap shape between the two structures and is attached to its surrounding walls, as seen in Figure 6.3 (e).
The gap shape trend seems to be followed with πΊ/π· = 14 when πΌ is 0Β° and 15Β°, but there is a steady drop in R2Β΄s attachment to the front face of Structure 2 with increasing πΌ, as clearly seen in Figure 6.14 (d) and (e). This is obviously due to the reason that the reduced high-speed region above the structures for larger πΌ has not reach Structure 2. Finally, R5 is increasing in size in the streamwise direction with increasing πΌ, clearly seen in Figure 6.14.
a)
b) e)
Figure 6.12 Streamlines for wall-mounted squares in tandem with gap ratios of: a) πΊ/π· = 2, b) πΊ/π· = 3, c) πΊ/π· = 6, d) πΊ/π· = 10 and πΊ/π· = 14
77
c)
d)
e)
a)
b)
Figure 6.13 Streamlines for G/D = 6 for varying trapezoidal configurations Ξ±: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
78
c)
d)
e)
Figure 6.14 Streamlines for G/D = 14 for varying trapezoidal configurations Ξ±: (a) 0Β°, (b) 15Β°, (c) 30Β°, (d) 45Β° and (e) 60Β°
79
7 Conclusions
This chapter outlines the conclusions and main findings for the study on the single and tandem cases. It also discusses the possibilities of future work on the concept of square and trapezoidal structures in both single and tandem configurations.
This chapter outlines the conclusions and main findings for the study on the single and tandem cases. It also discusses the possibilities of future work on the concept of square and trapezoidal structures in both single and tandem configurations.