A novel measure for long-term sediment reduction inspired by dragonfly wings

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 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.

In these equations, G_k represents the generation of turbulence kinetic energy due to the mean velocity gradients. G_b is the generation of turbulence kinetic energy due to buoyancy. The constants have the following meanings: C_1ε = 1.44, C₂ = 1.9, σ_k = 1, σ_ε = 1.2.

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 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).

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.

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.

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.

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.

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 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.