The sediment accumulation in drainage pipes has long been recognized as a significant concern in the environmental field. This study addresses sediment accumulation in drainage pipes by introducing an innovative bioinspired approach using various shapes and angles of plates for long-term sediment reduction. Through experiments and numerical simulations, the velocity field, the turbulent kinetic energy, the head loss, and the dynamic pressure distribution in the pipeline with plates are analyzed. Results demonstrate significant increases in local velocity, dynamic pressure, and turbulence energy due to the presence of plates. The sediment reduction performance shows a positive correlation with the angle for folded plates and a non-linear relation with curvature for curved plates. Notably, the superior performance of folded plates is attributed to their exceptional ability to induce vortex formation. The head loss due to sediment reduction measures increases linearly as the angle and the curvature increase. Furthermore, the intentional induction of strong eddies and high shear flow using the undulating topography created by the locally installed folding plates in the pipeline was the main cause of sediment reduction. This novel approach holds promise for more efficient and sustainable sediment reduction in drainage systems.

  • A novel measure for long-term sediment reduction inspired by dragonfly wings has been proposed.

  • A 25° folded plate is considered a favorable choice for this sediment reduction measure.

  • The main reason for sediment reduction is to induce strong eddies and high shear flow intentionally by using undulating topography.

In the urban drainage system, the flow of drainage pipes contains a large number of particles that tend to form sediment. The low and uneven velocity of the pipe flow exacerbates the phenomenon of sediment accumulation at the bottom of the pipe. Hydrodynamic and environmental problems are brought on by sediment settling and accumulation in combined sewer systems. In a qualified sewer, the sediment undergoes dynamic changes throughout the day due to fluctuations in flow rate dynamics, leading to infrequent sediment accumulation and constrained microbial activity (Ren et al. 2022). However, the actual operating conditions of the drainage system are harsh, and troublesome sediment can build up over a period. The sediment is strongly correlated with the subtle dynamic changes in the velocity and flow rates at the inlets and outlets (Di et al. 2023). Sediment deposition occurs during dry periods and when storm flows are receding, which depends upon local structural and hydraulic conditions for a given sewer (Ashley & Crabtree 1992). These sediments are concentrated in hard-to-flush-out locations like flat and inverted pipe slopes, where even at large flows, the velocity is incredibly low (Zuo et al. 2019). Sediment reduces the flow transport capacity of the pipeline, leading to more frequent overflows upstream. Sediments are resuspended from the bottom of the pipe during heavy rainfall and discharged into the environment, causing serious pollution. In most cases, the deterioration of the sewer is predominantly attributed to the concrete corrosion resulting from sulfides, whose production is closely associated with organic components in sediments (Pikaar et al. 2014). Siltation of pipes is one of the major factors contributing to poor drainage and urban flooding, which must take cost-effective measures to address the silting.

Since the development of modern sewer systems in the middle of the 19th century, the control and cleaning of sediments deposited in pipes have been and remain crucial aspects of sewer maintenance and operation. To control or reduce the environmental pollution problems and safety risks caused by drainage pipe deposits, various desilting methods have been proposed (Dinkelacker 1992). Nevertheless, despite the availability of numerous sediment reduction measures and extensive experience, each measure has its limitations in certain scenarios. Chemical treatments would not be ideal as they may damage pipe and channel materials and are relatively expensive (Westin & Rasmuson 2005). The high-pressure jet method is commonly employed due to its effectiveness in removing various types of debris and deposits. However, it is an expensive approach and may result in additional economic damage to the pipe surface during the flushing process. On the other hand, traditional mechanical methods appear to be less cost-effective and feasible and may even pose safety risks to workers. Furthermore, existing hydraulic and mechanical-based methods for pipeline sediment control necessitate the installation of mechanical devices within the pipeline or at the curb, which require long-term operation and maintenance (Bong et al. 2013). Additionally, such measures may potentially give rise to safety concerns related to flood control within the drainage system.

During the last decades, hydraulic flushing, a preferred technique as a control concept both to reduce hydraulic restriction problems and prevent pollution, has been widely promoted and applied for its cost-effective advantages in the removal of sewer deposits (Pisano et al. 2003; Bong et al. 2016). The main part of the previous work analyzed the sensitivity of flushing ‘clean’ performance to flushing hydraulic parameters, the characteristics of sewer geometry, and accumulated sediment (Shahsavari et al. 2017). Evidently, enhancing cleaning efficiency can be achieved by reducing the cohesive strength of sediment before flushing. However, the currently prevalent physical flushing methods are unable to completely disperse and diminish the cohesiveness of sewer deposits. This limitation arises from the fact that hydraulic power does not directly impact extracellular polymeric substances (EPSs) and microorganisms, which play a significant role in sediment cohesion. In view of this, much research has been carried out to improve flushing efficiency by reducing the cohesive strength of the sediment. Meng et al. (2020) demonstrated that ultrasound reduced cohesive strength by lowering bound-EPS concentration and disrupting tryptophan protein structure in sewer sediment, leading to smaller particle sizes and reduced agglomerate stability. Liu & Xu (2022) have proposed the addition of pipe resonance technology before the instantaneous drainage process of the traditional drainage pipe hydraulic flushing technology to enhance the desilting capacity of the pipe.

Controlling sediment deposition and accumulation in drainage pipes is imperative for mitigating the discharge of pollutants during rainy days and enhancing the operational safety of the drainage system. In the context of constructing urban water environments, it is crucial to identify a viable solution for the proper removal of sediment from the drainage system. Consequently, proposing more rational and cost-effective measures to reduce silt and effectively address the issues arising from gravity sewer deposits will yield positive outcomes for the optimal functioning of the sewer system and environmental preservation.

Previous studies have primarily focused on remediation efforts targeting sediment that has already accumulated over time, with limited attention given to preventative strategies at the source of siltation. Even in environments with minimal wind, dragonflies possess the ability to hover due to the generation of vortices resulting from the airflow passing through the grooves on their wings. This phenomenon leads to an acceleration of the airflow above these grooves, creating a pressure differential between the upper and lower surfaces of the wings, ultimately generating lift. In light of this, we propose an approach to reduce sediment inspired by the vortices observed when fluids traverse the uneven surface grooves of dragonfly wings. Unlike previous studies, our approach involves the implementation of a folding plate that modifies the hydraulic characteristics of water flow, inducing both local and global vortices that effectively dislodge fouling and prevent the initial attachment of contaminants. This measure is particularly significant, as once scale forms on the inner walls of pipes, its removal becomes considerably more challenging. Furthermore, we investigate the mechanism underlying this sediment reduction measure and analyze the appropriate style and angle of baffles to ensure practicality and feasibility. These findings serve as a valuable reference for the design of future measures aimed at reducing sediment accumulation in pipelines.

The experimental system for this research is installed at the hydraulic laboratory in Zhengzhou University. The experimental setup and arrangement are shown in Figure 1. The acrylic plate has been polished on both sides to ensure proper adhesion to the pipe wall. It extends from the outlet side of the pipeline to a specific position within the experimental section and is securely fixed in place. The length and diameter of the pipes with a slope of 0.3% are 10 and 0.3 m, respectively. The presence of multiple openings at the top of the pipe facilitates the convenient installation of folded plates and enables the measurement of velocity. This experiment is a 5° folded plate working condition (case no. P3 in Table 1). The entire set of plates is 3.3-m long and 5-mm thick, with a 0.5-m long flat plate at the front and rear and a continuously undulating folded plate in the middle inclined at 5° to the axis. To alleviate the influence of fluctuating water flow on experimental accuracy, the acrylic plate is positioned in the pipe at a distance of 4.7 m from the water inlet. During the experiment, the water level in the water supply tower is kept constant, and the flow valve is adjusted to maintain an inlet water flow rate of 35 L/s. Adequate distance and time are available to establish a steady flow.
Table 1

List of sediment reduction plate conditions

Sediment reduction plate case no. P#Angle of folded plates θ (°)Minimum curvature of curved plates K1 (1/mm)Maximum curvature of curved plates K2 (1/mm)Length of horizontal plates L (m)Height of the undulations of the folded plates H (m)
3.2 
0.05 
10 0.05 
15 0.05 
20 0.05 
25 0.05 
30 0.05 
0.0001 0.0009 0.05 
10 0.0004 0.0038 0.05 
11 0.0009 0.0087 0.05 
12 0.0015 0.0161 0.05 
13 0.0022 0.0271 0.05 
14 0.0029 0.0406 0.05 
Sediment reduction plate case no. P#Angle of folded plates θ (°)Minimum curvature of curved plates K1 (1/mm)Maximum curvature of curved plates K2 (1/mm)Length of horizontal plates L (m)Height of the undulations of the folded plates H (m)
3.2 
0.05 
10 0.05 
15 0.05 
20 0.05 
25 0.05 
30 0.05 
0.0001 0.0009 0.05 
10 0.0004 0.0038 0.05 
11 0.0009 0.0087 0.05 
12 0.0015 0.0161 0.05 
13 0.0022 0.0271 0.05 
14 0.0029 0.0406 0.05 
Figure 1

Schematic diagram of the experimental setup.

Figure 1

Schematic diagram of the experimental setup.

Close modal
Six sections of the pipeline are selected and equipped with a set of measurement devices to monitor the flow transport parameters during the experiment. The monitoring sections were located upstream (S−1.8, S−0.8) and downstream (S+0.486, S+1.057, S+3.5, S+4.5) of the middle of the pipeline. The vertical heights of the measurement points are 10 mm apart on each monitored section. The velocity was measured with an accuracy of 0.5% by an acoustic Doppler velocimeter (ADV) positioned at the central location at each of the six identified sections (see Figure 2). To measure the water level in the pipe flow, a needle level gauge with a measurement accuracy of 0.1 mm is utilized.
Figure 2

Monitored sections with installed measurement equipment.

Figure 2

Monitored sections with installed measurement equipment.

Close modal

The pipe with sediment reduction measures has been simulated, and 13 other operating conditions were added. Dense tetrahedral meshes are adopted in the vicinity of the plates and pipe walls, whereas hexahedral meshes are employed in the majority of the flow domain. The accuracy and effectiveness of numerical simulation directly depend on meshing. Thus, we tested a range of mesh volumes with cell counts ranging from 1,135,000 to 4,548,000. In Section 4.1, the sensitivity of the simulation result to mesh size will be discussed.

Governing equations

Drainage pipes are typically designed for non-full flow conditions to address factors such as the removal of harmful gases and the accommodation of variations in flow volume. Under these conditions, the upper boundary of the flow is in direct contact with the atmosphere, and the flow within the pipe is primarily governed by gravity, resembling open channel flow. The Navier–Stokes equations for unsteady, incompressible flow were solved in this study. The equations representing the conservation laws of mass and momentum are (Davidson 2015):
formula
(1)
formula
(2)
where ui represents velocity component; x and t denote spatial and temporal coordinates; p stands static pressure; ρ indicates density; μ and μtsignifies dynamic viscosity and turbulent viscosity; ρgi represents gravitational body force; and Fi denotes external body forces.
A new transport equation for ε and a new formulation for turbulent viscosity are both included in the realizable k–ε model. Certain mathematical requirements that cannot be achieved in either the Standard kε or RNG kε models are satisfied by the realizable kε model and can be used to predict moderately strong vortices flows. The modeled transport equations for k and ε in the realizable kε model are (Shih et al. 1995):
formula
(3)
and
formula
(4)
where , , and

In these equations, Gk represents the generation of turbulence kinetic energy due to the mean velocity gradients. Gb is the generation of turbulence kinetic energy due to buoyancy. The constants have the following meanings: C1ε = 1.44, C2 = 1.9, σk = 1, σε = 1.2.

The turbulent viscosity has the same form as in other kε model:
formula
(5)
However, Cμ is no longer constant and is a function of the mean strain and rotation rates, the angular velocity of the system rotation, and the turbulent fields. It is computed from:
formula
(6)
To predict the air–water interface, the Volume of Fluid (VOF) model is chosen. The VOF model is based on the calculation of volume fractions and allows the localization of the interface between the two immiscible fluids, which can simulate complex surface deformations. This approach entails the solution of a transport equation for the volume fraction function α, as defined by Equation (7), employing interface-capturing algorithms. The VOF model solves a unified set of flow equations, where the density and dynamic viscosity values at the interface are determined using the volume fraction values (α) at the interface, as described in Equations (8) and (9), respectively (Hirt & Nichols 1981). The value of the volume fraction function is equal to 1 in cells filled with water and 0 in cells filled with air. The interface is positioned within cells with volume fraction values ranging from 0 to 1. The free surface is precisely identified at points where the volume fraction function equals 0.5, allowing for the mixing of water and air within a single cell:
formula
(7)
formula
(8)
formula
(9)

Boundary conditions

A total of 14 working conditions corresponding to the experiments were set up. The parameters of the sediment reduction plates are listed in Table 1.

The boundary conditions are illustrated in Figure 3. The wall boundary condition is set to a no-slip wall condition, and the velocity distribution near the wall is calculated using the Scalable Wall Functions. The coupled algorithm was used for the pressure–velocity coupling. The transport equations were discretized using a second-order upwind scheme. The residuals of the various parameters contributing to the convergence criterion were set at 10−5. In addition, the difference in mass flow rate between the inlet and outlet and the velocity at one point were monitored simultaneously during the calculation. At steady state, it was determined that the calculation had converged when the mass flow rate value approached zero and the velocity at that point no longer varied.
Figure 3

Schematic diagram of the model and mesh. (a) Simplified numerical model, (b) mesh grid of the pipe inlet, (c) mesh grid of the pipe wall, (d) mesh grid of the folded plate.

Figure 3

Schematic diagram of the model and mesh. (a) Simplified numerical model, (b) mesh grid of the pipe inlet, (c) mesh grid of the pipe wall, (d) mesh grid of the folded plate.

Close modal

The sensitivity of the grid size

The pipe was simulated using the realizable kε model with four grid densities, namely 1,135,060; 2,343,321; 3,743,293; and 4,548,139, to explore the sensitivity of the numerical results to the grid size. Figure 4 shows the vertical profiles of the velocity at the S+0.485 section in the median plane. By carefully determining the appropriate grid size, computational resources can be conserved while still ensuring reliable results. The velocities obtained from meshes 1 and 2 display significantly greater differences than those obtained from meshes 3 and 4. The latter pair yields similar results and exhibits a better agreement with the experimental results. When the number of grid cells reaches 3,743,293, increasing the number of grid cells does not affect the computational results. Consequently, the subsequent calculation utilizes a mesh size that corresponds to the mesh number of 3,743,293. The relative deviation can serve as a metric for assessing the disparity between the numerical and experimental results. Under the condition of a mesh number of 3,743,293, the velocity at the S+0.485 section in the median plane exhibits a maximum relative deviation of 11.4% and an average relative deviation of 4.5%.
Figure 4

Vertical profiles of the velocity at the S+0.485 section in the median plane.

Figure 4

Vertical profiles of the velocity at the S+0.485 section in the median plane.

Close modal

Model validity

To confirm the accuracy of the numerical simulation, the water surface and the flow patterns acquired from the simulation are compared with experimental results. Figure 5 shows the comparison of the free water surface height in the centerline of the pipe with 5° folded plates under a discharge of 35 L/s between the experimental data and the numerical simulation data. The good agreement shows that the change of the water surface line is consistent, and the average relative error is 4.46%. Firstly, due to the existence of sediment reduction measures, the upstream water is choked, and the water level drops along the pipe. Hydraulic jumps and falls occur continuously in the grooves and ridges of the folded plate. Because only a small amount of water flows down from the plate, the water level downstream of the folded plate gradually stabilizes.
Figure 5

Comparison of experimental and numerical results of the free water surface height.

Figure 5

Comparison of experimental and numerical results of the free water surface height.

Close modal
The comparison of the simulated and measured flow velocities at the six measured parts is shown in Figure 6. The simulation results demonstrate a good agreement with the experimental results. In the median plane, the velocity at the six measured sections shows an average relative deviation of 5.23, 6.87, 4.54, 6.95, 2.47, and 3.14%, respectively. Simultaneously, the maximum relative deviations are 14.76, 15.24, 11.43, 11.26, 4.97, and 5.58%, respectively. Therefore, the numerical model can satisfactorily predict the flow field of the pipe in the case of installing sediment reduction measures.
Figure 6

Comparison of experimental and numerical results for the velocity of six monitored sections. (a–f) Velocity of S−1.8, Sv0.8, S+0.486, S+1.057, S+3.5, S+4.5 sections.

Figure 6

Comparison of experimental and numerical results for the velocity of six monitored sections. (a–f) Velocity of S−1.8, Sv0.8, S+0.486, S+1.057, S+3.5, S+4.5 sections.

Close modal

The velocity field

The sediment layer is unstable due to the interaction between the sediment particles and the flow in the sewer pipes. The minimal velocity necessary to remove particles from the sediment deposits at the pipe's bottom is known as the incipient velocity (Rabinovich & Kalman 2009).

Figure 7 shows the velocity contours under the various measures. It is apparent that the inclusion of various types and angles of plates in the pipe has a discernible effect on the velocity increase in the lower section of the pipe. With a gradual escalation in both angle and curvature, the effect of the increase in velocity becomes more significant. This phenomenon arises due to the sudden obstruction of the flow by the plates, resulting in a reduction in the cross-sectional area and the formation of vortices in the grooves of the plates. When the main flow passes through the vortex area at the groove, the direction of the vortex is the same as that of the main flow, and the vortex transfers its rotational momentum to the flow, resulting in an increase in velocity.
Figure 7

The velocity contours in the median plane: (a–n) Case numbers: P1–P14.

Figure 7

The velocity contours in the median plane: (a–n) Case numbers: P1–P14.

Close modal
To gain a clearer understanding of the impact of different sediment reduction measures on velocity near the pipe bottom, the velocity on a line below the plate at a distance of 5 mm from the bottom of the pipe is taken, as shown in Figure 8. It is seen that the addition of a flat plate does not have a significant effect on the increase of velocity in the pipe. The increase of velocity at the bottom from the installation of low-angle folded plates in the pipe is related to the undulatory topography. At ridges, the velocity exhibits an augmentation of approximately 1.5 times, whereas at grooves, the effect of the heightened velocity diminishes to such an extent that the velocity in the central region of the groove is lower than it would have been in the absence of any implemented measures. The velocity curve, concavely and convexly, corresponds to the shape of the plates, whereas with large-angle folded plates, the increase in the velocity at the bottom of the pipe cannot be clearly defined based on the location.
Figure 8

Velocity at 0.005 m from the bottom of the pipe in the median plane.

Figure 8

Velocity at 0.005 m from the bottom of the pipe in the median plane.

Close modal
It can be seen from Figure 9 that plates with large angles and curvatures have a smaller groove-to-ridge span and form separated vortices within the grooves. The continuous increase in velocity at the bottom of the pipe below the large-angle folded plate can be attributed to the persistent formation of large vortices resulting from the undulatory topography. With an increasing angle of the folded plate, the velocity increment becomes more significant, even in the grooves underneath the plates, where velocity exceeds that without any measures. The velocity curves for the folded plates with angles of 25° and 30° show a closely similar trend, suggesting that increasing the angle beyond 25° may yield diminishing returns in terms of its impact on the velocity. The velocity after installing low-curvature curved plates follows a similar pattern to that of the small-angle folded plates. However, the high-curvature curved plates did not achieve a better velocity increase compared to the low-curvature ones. In contrast to folded plates, curved plates possess a smoother surface and are less susceptible to vortex generation. Even in the grooves of highly curved plates, as depicted in Figure 9(f), the absence of continuous large vortices is evident. This fundamental rationale underlies the relatively inferior performance of curved plates in enhancing velocity as compared to folded plates.
Figure 9

Vector of the velocities in the y direction and z direction for the median plane: (a–f) Case numbers: P4, P6, P8, P10, P 12, P14.

Figure 9

Vector of the velocities in the y direction and z direction for the median plane: (a–f) Case numbers: P4, P6, P8, P10, P 12, P14.

Close modal

The sediment reduction measures exert a substantial influence, not solely confined to the precise location of the plates but also extending to the immediate surrounding vicinity. When the flow reaches the leading edge of the sediment reduction plate, it becomes stratified completely by the presence of the plates. Deceleration of the water flow occurs at the concave groove above the plate, and the necessity of jumping over the ridge results in an elevation of the upstream water level on the plate. With the rise in water level, there is a subsequent decrease in velocity in front of the plates. Notwithstanding the decrease, the velocity at the bottom of the pipe downstream of the folded plates still displays a perceptible augmentation compared to the scenario where folded plates are absent. However, the velocity of the pipe bottom downstream of the high-curvature curved plates is greatly affected by the phenomenon of hydraulic falls, which impede the flow and lead to low velocity near the end of the sediment reduction plates.

The turbulent kinetic energy

When turbulent flow encounters certain unstable factors, such as changes in velocity or flow direction, it can give rise to a sudden escalation in turbulent intensity and the emergence of a turbulent burst. The occurrence of this phenomenon is prevalent in flows characterized by higher velocities and is notably inclined to take place when irregular obstructions are present within the flow. Both sweep and ejection events are major contributors to turbulent bursting, as they exert forces that cause sediment particles to roll, slide, or maintain suspension in water (Nakagawa & Nezu 1981; Khan et al. 2017). Cleaver & Yates (1973) stated that particle removal is strongly enhanced when the mean flow adjacent to the surface is turbulent. Entrainment of particles from the surface of sediment deposits, influenced by flow turbulence that creates fluctuating shear stress, occurs when the lifting forces overcome the holding forces.

Figure 10 shows turbulent kinetic energy (TKE) contours in the median plane. It is evident that the impact of the plate on the magnitude of TKE and the extent of its influence are positively correlated with the angle. It can be seen from Figure 11(a), 11(b), 11(e), and 11(f) that the TKE is greater mainly in the region below the plate and also relatively greater at the free water surface at grooves and ridges of small-angle and low-curvature plates. The region distanced from the plates and situated close to the bottom of the pipe exhibits a notable reduction in TKE, reaching its minimum value. Figure 11(c), 11(d), 11(g), and 11(h) presents that TKE is greater at the grooves and ridges of both the large-angle folded and curved plates, but the region of greater TKE at the grooves and ridges of the curved plate is smaller. Due to this, large vortices can be generated in both the upper and lower grooves of the large-angle folded plate, whereas only small localized vortices can be generated in these regions of the high-curvature curved plate, as depicted in Figure 9(c) and 9(f). And in turbulent flows, the vortex core is the region where TKE is most concentrated.
Figure 10

TKE contours in the median plane: (a–n) Case numbers: P1–P14.

Figure 10

TKE contours in the median plane: (a–n) Case numbers: P1–P14.

Close modal
Figure 11

TKE contours of cross-sections. (a, c, e, g) The sections at the ridges beneath the plates of Case numbers: P5, P8, P11, and P14. (b, d, f, h) The sections at the intermediate region of grooves beneath the plates of Case numbers: P5, P8, P11, and P14.

Figure 11

TKE contours of cross-sections. (a, c, e, g) The sections at the ridges beneath the plates of Case numbers: P5, P8, P11, and P14. (b, d, f, h) The sections at the intermediate region of grooves beneath the plates of Case numbers: P5, P8, P11, and P14.

Close modal

This indicates that the flow near plates or pipe walls experiences a significant wall effect, which leads to higher shear stress and increased turbulence intensity. The reason for this result is that the presence of a solid boundary disrupts the flow and creates frictional forces that affect the flow motion. In the vicinity of plates or pipe walls, the flow is constrained, and the velocity decreases due to the no-slip condition. This change in the velocity gradient results in higher shear stress and increasing turbulence intensity. The flow near the wall becomes more chaotic and turbulent with the formation of smaller-scale vortices and eddies. The TKE is higher at the free surface due to the fluctuation of the water surface, which is caused by successive hydraulic jumps and drops above the plate. The interaction between wind and the water surface can lead to an amplification of turbulence. Away from the plates or pipe walls, the flow gradually transitions to a more stable and less turbulent state. The momentum exchange becomes more uniform, and the turbulence intensity decreases as the distance from the wall increases.

Dynamic pressure

Particles may separate from the surface of sediment deposits during hydro-transport as a result of hydrodynamic drag forces or updrafts brought on by turbulent bursts, depending on the particle size and flow conditions, which are described as wall shear stress (Phillips 1980). Dynamic pressure is used to describe the pressure of a flow in motion and is closely related to velocity, which increases as the square of the velocity increases. Thus, as demonstrated in Figure 12, the contours of dynamic pressure bear a resemblance to the contours of velocity in Figure 7, with high velocity yielding correspondingly high dynamic pressure. As the flow passes through the pipe, the dynamic pressure acts as a hydrodynamic drag force on the deposits in the pipe. However, the sediment is often highly adherent, necessitating the application of high hydrodynamic drag forces for its removal.
Figure 12

Dynamic pressure contours in the median plane: (a–n) Case numbers: P1–P14.

Figure 12

Dynamic pressure contours in the median plane: (a–n) Case numbers: P1–P14.

Close modal

The hydrodynamic drag forces exerted by water on sediment are directly proportional to the dynamic pressure. Consequently, an increase in dynamic pressure facilitates the removal of sediment. Besides, the rate of sediment erosion is influenced by dynamic pressure. A higher dynamic pressure leads to an increased erosion rate of the sediment, thereby reducing sediment accumulation and attachment.

Head loss

While focusing on the benefits brought by sediment reduction measures, it is also important to consider head losses. It represents an energy loss and primarily occurs when there are changes in fluid velocity and the cross-section of the pipe (Nam et al. 2013; Pradhan et al. 2022). In the fully developed turbulent region, large-scale eddies extract energy from the main flow, which is then transferred to smaller-scale eddies through their interactions. Eventually, due to fluid viscosity, the small-scale eddies dissipate, converting mechanical energy into thermal energy within the fluid. In order to analyze the head loss of various sediment reduction measures, the turbulent dissipation rate contours in the median plane are shown in Figure 13.
Figure 13

Turbulent dissipation rate contours in the median plane. (a–n) Case numbers: P1–P14.

Figure 13

Turbulent dissipation rate contours in the median plane. (a–n) Case numbers: P1–P14.

Close modal

The peaks of TKE dissipation rate are observed at the leading edge of sediment reduction plates and in the grooves beneath, with their range expanding as angles and curvature increase. Furthermore, near the downstream position, peak values are discerned within the grooves above plates with large angles and high curvature, as well as in the zones where hydraulic jumps and falls. The local maxima of the turbulent dissipation rate primarily stem from substantial values of the velocity gradient. This can be attributed to the persistent, long-term recirculating flow patterns within grooves above the plate, successive hydraulic jumps and falls, and the strong vortices within the grooves beneath the plate.

The undulating topography of the plate leads to the formation of strong separating vortices in the grooves beneath the plate, where a sharp increase in the velocity gradient triggers strong shearing between fluid molecules. Simultaneously, the main flow continuously transports strong turbulence downstream, intensifying downstream turbulence and resulting in continual adjustments in velocity distribution. These factors contribute to increased energy dissipation within the fluid, leading to greater head losses. The inclusion of a flat transition section in the sediment reduction plate design serves to prevent abrupt contractions in flow cross-sections, thus mitigating increased turbulence and reducing local head loss. The folded plate region is the primary site of energy dissipation, while the frictional head loss along the pipe is negligible when compared to the local head loss. The head loss in this study is determined by subtracting the total head at the upstream control section S−1.8 from the total head at the downstream control section S+4.5. Figure 14 shows the variation of head loss at 35 L/s flow for 14 plate conditions. As the angle and curvature increase, the head loss resulting from sediment reduction techniques increases for a given flow rate. It is evident from the results in Figure 14 that the head loss due to small-angle and curvature folded plates is relatively small, whereas the head loss has reached about 10 cm with large-angle and curvature folded plates.
Figure 14

Head loss of 14 plate conditions.

Figure 14

Head loss of 14 plate conditions.

Close modal

Recommended installation angle and type

The bending degree of the plate should not be too small to satisfy the need to induce strong vortices and high shear flows to block the generation of siltation. At the same time, considering the influence of the shape and angle of the plates on the cost and the subsequent loss of the water head, it is not appropriate to choose plates with too large curvature or angle. Due to the increased consumption of human and material resources associated with larger angles and curvatures, our study focused solely on working conditions involving folded plates ranging from 0° to 30°. As depicted in Figure 8, when the angle increases to approximately 25°, further increasing the angle to enhance the velocity proves to be less effective. Therefore, the optimum installation angle and type should be determined.

Furthermore, taking into account the characteristics of the actual sewage system, it is observed that heavier particles in the sewage tend to settle in the area preceding the plates. However, a small quantity of lighter suspended particles still passes over the plates. When employing large-angle plates, as shown in Figure 9, two flow separation points emerge near the grooves of the folded plates. These points are situated at the top of the slope and the subsequent slope, respectively. A portion of the flow adheres to the upstream slope, generating a separating vortex within the groove, while another portion attaches itself behind the second separation point, propelling sediment particles toward the subsequent crest. Notably, significant vortices form within the grooves, capable of agitating the settled particles and re-suspending them into the effluent, subsequently carrying them downstream.

In a turbulent burst, the size and intensity of the vortex increase rapidly, and the energy of the flow is briefly concentrated in a local area, forming a region of high turbulent intensity. As shown previously in Figure 9, the lower velocity in the recess of the plate creates a large velocity gradient above the grooves. The comparatively low local velocity at the burst point close to the surface has a strong correlation with the occurrence of ejections. As the jet direction is at an angle to the sediment layer, particles are ejected to a certain height and begin to move in suspension. While suspended particles may fall due to gravity, the ‘ejection cushion’ formed by multiple ejections occurring at the same time may block the falling and allow the particles to move further (Mao 2003). The downstream slope of the plate depressions will eject water toward the water surface, forming Kolk-boils, which were named by Matthes (1947). The fluid pressure along the Kolk-boils jets is low enough to create a negative pressure, causing the sediment particles to be suspended due to the rising position and the constant and rapid increase in velocity.

In practical applications, the utilization of folded plates is highly recommended to establish an appropriate and rational angle based on the specific conditions within the pipeline. This approach ensures the maintenance of a relative balance between normal operation, cost-effectiveness, and energy loss.

In this study, a novel measure for long-term sediment reduction inspired by dragonfly wings has been proposed. The hydraulic characteristics of various sediment reduction measures installed in pipelines were investigated using a combined approach of experiments and numerical simulation. The following conclusions can be drawn:

  • (1)

    The influence of plate types and bending degrees, particularly folded and curved plates, on sediment reduction performance has been elucidated. Notably, folded plates demonstrated superior vortex-inducing capabilities compared to curved plates. Moreover, the correlation between plate angle, curvature, and sediment reduction performance has been established, emphasizing the relationship between these parameters.

  • (2)

    Detailed analysis reveals the intricate hydraulic behaviors induced by various plate configurations. The enhancement of bottom velocity by small-angle and low-curvature plates is highly sensitive to the specific locations of undulating topography, whereas large-angle folded plates are less influenced by these particular topographic features. High-curvature curved plates do not significantly improve velocity compared to low-curvature curved plates. Furthermore, the constraints imposed near plates or pipe walls cause velocity gradients, resulting in higher shear stress and turbulence intensity.

  • (3)

    The study highlights the crucial role of increased dynamic pressure, turbulent bursts, and intentional induction of eddies and high shear flows in preventing sediment accumulation. These factors work collectively to influence flow dynamics and effectively reduce sediment deposition.

  • (4)

    Energy dissipation analysis emphasizes the sites of significant dissipation within the pipeline. This includes continuous hydraulic jumps and falls above the plates and viscous dissipation in the grooves below, contributing to the overall energy loss. The energy loss associated with adopting a proper and reasonable plate style and angle is acceptable.

In actual operation, plates can be made of materials such as stainless steel. This sediment reduction measure does not require the application of additional forces. It relies solely on the movement of the flow itself through the measure to passively induce vortex generation, effectively preventing sediment accumulation. The sediment reduction performance of different types and angles of vertically mounted plates and diagonally mounted plates will be investigated in future research.

This work was supported by the [National Key R & D Program of China] (Grant number [2022YFC3801000]), the [Open Research Fund of MWR Key Laboratory of Lower Yellow River Channel and Estuary Regulation] (Grant number [LYRCER202202]), the [Program for Innovative Research Team (in Science and Technology) in University of Henan Province] (Grant number [23IRTSTHN004]), the [Key Scientific Research Projects of Colleges and Universities in Henan Province] (Grant number [22A570009]), the [China Postdoctoral Science Foundation funded project] (Grant number [2022M712904]), the [Scientific and Technological Research Program of Henan Province] (Grant number [232102321099]), the [Open Research Fund of Key Laboratory of Water-saving Irrigation Engineering of the Ministry of Agriculture and Rural Affairs (MARA)] (Grant number [FIRI2021020201]), the [Yellow River Laboratory (Zhengzhou University) first-class project special fund project] (Grant numbers [YRL221R11] and [YRL22YL03]), the [Special Scientific Research Project of Yellow River Water Resources Protection Institute] (Grant number [KYY-KYZX-2022-01]).

Z.L. was involved in conceptualization, validation, formal analysis, funding acquisition, writing – review and editing. B.W. was involved in conceptualization, software, data curation, writing - original draft, writing – review and editing. F.W. was involved in writing – review and editing, resources, funding acquisition. B.S. was involved in writing – review and editing, resources, project administration. S.Z. was involved in writing – review and editing, project administration.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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