Fluid Drag Research Articles

Synthesis of zinc hydroxyl acetate nanosheet-assembled honeycomb-like and flower-like structures by chemical bath deposition method for the construction of hydrophobic surfaces

Layered zinc hydroxyl acetate (LZHA) nanosheet-assembled honeycomb-like and flower-like structures were synthesized by a chemical bath deposition method using aqueous zinc acetate solution as the bath solution and a porous ZnO coating as the substrate. The morphology of LZHA architectures growing on the surface of the substrate depends on the type of LZHA nucleation. The homogeneous nucleation facilitates the adsorption of crumpled LZHA nanosheets generated in the solution onto the substrate surface, whereas heterogeneous nucleation promotes the formation of flat LZHA nanosheets on the substrate surface. The crumpled and flat LZHA nanosheets serving as the seeds undergo cyclic “growth-division” processes to evolve into LZHA nanosheet-assembled honeycomb-like and flower-like structures, respectively. The hierarchical architecture features of the LZHA honeycombs and flowers can be topologically inherited without any crack and collapse when converted into ZnO by calcination. The LZHA honeycomb-like and Ag nanoparticle modified ZnO flower-like structures were characterized by a scanning electron microscope, field-emission scanning electron microscope, energy dispersive X-ray spectrometer and X-ray diffractometer. The synthesized ZnO honeycomb-like and flower-like structures show excellent hydrophobic properties owning to the appropriate roughness provided by the hierarchical structures. In particular, the surface assembled by ZnO honeycombs derived from their LZHA precursors with deposition time of 4 h shows a higher water contact angle of 162.3°. In addition, the Ag nanoparticle modification on the petals of ZnO flowers results in the formation of multi-scale rough surfaces, and thus increases the water contact angle. The synthesized superhydrophobic surfaces exhibit promising applications in fields such as anti-icing, anti-corrosion and reducing fluidic drag.

A multilayered homogeneous hydrodynamic cloak realized in a free fluid flow

In recent years, hydrodynamic invisibility cloaks have attracted significant attention from the scientific and engineering communities due to their potential in applications such as fluid flow manipulation, drag reduction, submarine stealth, and biomedical engineering. However, cloaks based on transformation mapping theory typically require porous medium flow, limiting practical implementation. To address this, we draw inspiration from Hele-Shaw flow and develop a homogeneous hydrodynamic cloak composed of multiple layers in free fluid flow at low Reynolds numbers (Re≪1). Through structural optimization, we construct a cloaking shell with ten concentric layers of alternating heights, achieving near-perfect cloaking as demonstrated by simulations and experiments. This non-porous cloak design offers valuable insights into the rapidly advancing field of hydrodynamic metamaterials and enables methods for controlling fluid flow at the microscale, with promising applications in microfluidics.

Contribution of rolling resistance to the drag coefficient of spheres freely rolling on a rough inclined surface

The drag coefficient (CD) of a sphere freely rolling without slipping on a rough plane is presented in this study. Increasing panel roughness has been found to increase CD, although lubrication theory predicts that the larger gap imposed by the rougher panel should yield a smaller drag. We propose that this increase in drag is due to the effects of rolling resistance, which increases with panel roughness. The total drag on a sphere is decomposed into fluid drag and drag due to rolling resistance, where the fluid drag is predicted using a combined analytical–numerical approach. It is shown that rolling resistance can be modeled as a resistive torque opposing the sphere motion, generated by the offset contact normal force from the sphere center plane. This coefficient of rolling resistance (μr) can be predicted using the root mean square roughness (Rq) of the panel. Additionally, μr is observed to increase with sphere down-slope velocity and an empirical relationship between μr, Rq, and non-dimensional velocity (U∗) is given. A comparison of the drag predicted by the proposed model with measured data indicates good agreement for all the four panels considered. Consistent with previous literature, a non-linear relationship between μr, Rq, and U∗ is proposed. Although increasing panel roughness leads to a smaller fluid drag due to the larger gap imposed by rougher panels, the drag due to rolling resistance increases more rapidly. This leads to an increase in total drag with increase in the panel roughness. Additionally, increasing panel roughness is observed to have a significant effect on the sphere wake, leading to irregular wake shedding and increase in the Strouhal number.

In-capillary magnetic separation and enrichment coupled with laser-induced fluorescence for rapid determination of biomolecules

Biomolecules are indispensable in the life activities and metabolism of various organisms, and their highly sensitive detection is of great significance for disease diagnosis and food safety. Due to the complexity of the matrix and the low concentration of the target, separation and enrichment of biomolecules prior to detection are necessary and time-consuming. Herein, a novel approach combining in-capillary magnetic separation and enrichment with laser-induced fluorescence detection method was proposed for qualitative and quantitative analysis of biomolecules. Two different sequence aptamers modified with magnetic nanoparticles and labeled with fluorescence were used as capture probes and signal probes to form sandwich structures with target molecules for detection. Under the combined influence of magnetic field force and fluid drag force, the integration of separation, enrichment and on-line detection were finished within 5min. Under optimal conditions, the developed method provided a low limit of detection as 2.7 fmol/L, 0.57 nmol/L, 3 CFU/mL for DNA, thrombin and Staphylococcus aureus, respectively. This proposed method offers a promising platform for the sensitive and rapid determination of biomolecules. It holds significant potential for future applications in fields of environmental and biochemical analysis.

Examining Pipe–Borehole Wall Contact and Pullback Loads for Horizontal Directional Drilling

Pipeline pullback load is a crucial basis for drill rig selection and pipeline strength design. This paper presents a new pullback load calculation model from the perspective of pipe–borehole wall contact. The pipe–borehole wall contact analysis includes the distribution of contact pressure and the relationship between the external load and compressive displacement. The friction force between the pipe and the borehole wall was calculated based on the pipe–borehole wall contact analysis and adhesion theory without depending on the empirical friction coefficient. The effects of the eccentricity were also considered when calculating the fluid drag force. Through case studies, we verified the applicability of the model and discussed the possible reasons for the errors between the theoretical and field-measured results. This study can provide a helpful tool for analyzing the pipe–borehole wall contact and pullback loads for horizontal directional drilling.

Irreversibility analysis of CNT‐enhanced Williamson nanofluid flow in a stretching cylinder

AbstractThis study presents a novel exploration into the intricate interplay of slanted magnetohydrodynamic and radiative fluxes within a Williamson fluid incorporating single‐walled carbon nanotubes (SWCNTs) and multi‐walled carbon nanotubes (MWCNTs) in a water‐based system. Utilizing an exponentially stretching cylinder model, the research holistically integrates entropy production, encompassing thermal radiation, porosity, buoyant forces, and heat source/sink constraints within the governing equations. To solve the resulting governing equations with boundary conditions, the shooting technique is employed to convert them into initial value problems. Subsequently, the Runge–Kutta–Fehlberg 4–5th order method is used to obtain numerical solutions. Unveiling unprecedented insights, the investigation reveals distinctive parameter dependencies governing fluid behavior. It delineates how variations in different factors distinctly impact fluid velocity, thermal profiles, entropy generation, Bejan number, drag force, and heat transfer rates across different nanofluid compositions. Furthermore, the study highlights the transformative impact of nanocomposites on altering the thermal properties of the fluid and orchestrating nanofluid mobility. The results show that MWCNTs have an enhanced entropy generation rate when compared to SWCNTs. These findings bear substantial promise for real‐world applications involving water‐based nanofluids, shedding light on their manipulation and optimization in various technological domains.

Study on the regulation of droplet motion behavior on superhydrophobic surfaces

Wettability properties of the surface is essential for controlling the dynamic sliding of droplets, thereby facilitating surface self-cleaning and drag reduction. This study makes use of chemical etching and surface modification techniques to prepare superhydrophobic surfaces with micro-nano pore structures on copper. This research examines and evaluates the chemical composition, surface morphology, and hydrophobic characteristics of superhydrophobic surfaces, also investigates the exhibited excellent superhydrophobic properties, a positive correlation observed between the concentration of the modifier solution and the surface’s hydrophobic strength. Droplets move in an accelerated fashion on superhydrophobic surfaces, and the total resistance to droplet sliding decreases as the surface hydrophobicity increases and the wall tilt angle decreases. During the motion of droplets, the fluctuation range of the receding angle is considerably greater than that of the advancing angle, thereby rendering the receding angle as the primary determinant factor for droplet movement, and as the hydrophobicity of the surface improves, the fluctuation extent of the droplet’s receding angle experiences a significant diminution. Droplet motion on superhydrophobic surfaces comprises both slipping and rolling behaviors, the increased hydrophobicity of the surface and the larger tilt angle facilitate droplet movement in a slipping mode on the surface, within the experimental parameters, the proportion of rolling distance to slipping distance varies from 35 % to 15 %. This study has revealed innovative regulatory approaches for modulating the moving behavior of liquids on microstructured surfaces, potentially enabling the development of superior functional surfaces for fluid drag reduction or droplet manipulation in the future.

Study of the flow characteristics of the dense bed and its enhancing effect on coal particles separation by CFD-DEM applied in Teeter bed separator

ABSTRACT Teeter bed separator (TBS) plays an important role in coal and mineral fines beneficiation. It is believed that a favorable dense concentrated bed is the key to ensuring the effective particle separation. This paper mainly presents the investigation of the flow characteristics of the dense bed and its enhancing effect on particle fines separation by CFD-DEM to reveal the complex separation behavior of TBS. First, the enhancing effect of the dense bed for separating real fine coal is experimentally studied. The specific simulation for particle motion is further verified by the literature experiment. The simulated dynamic behavior of the mismatch-prone particles reveals that the dense bed in the concentration range of 20–47% could promote particle separation by density. The fact is that a certain velocity difference is generated between mismatch-prone particles, and the smaller heavier particles move faster than the larger lighter particles. It is especially beneficial for separating fine coal material in TBS. The enhancing mechanism of the dense bed is further revealed by analyzing the mismatch-prone particle forces of collision and fluid drag force. Comprehensively, the appropriate bed concentration of 35–45% is suggested for the actual TBS operation.

Influencing heat transfer and drag in magneto-hydrodynamic non-Newtonian fluids with polymer additives: A novel theoretical approach

This article examines the impact of polymers on boundary layer flow and heat transfer in non-Newtonian fluids over a magnetized stretching surface. Using the Oldroyd-B model, Reiner-Philippoff fluid stress deformation, and Maxwell’s equations, the study explores polymer behavior in a magnetic field. Similarity transformations are applied to derive the governing equations, which are solved numerically. The effects of polymers and magnetic fields on drag and heat transfer are illustrated graphically. The analysis highlights that introducing polymers increases skin drag and the Nusselt number while reducing the magnetic flux coefficient. Higher polymer concentrations enhance drag and heat transfer. The magnetic field significantly improves heat transfer and reduces skin drag. The research offers valuable insights into the optimal use of polymer additives to modify heat transfer and drag in non-Newtonian fluid flow, contributing to advancements in fluid dynamics and thermal management technologies.

Fluid‐Driven Particle Migration and Its Impact on Hydraulic Transmissivity of Stressed Filled Fractures

AbstractThe accurate assessment of hydraulic transmissivity in rock fractures filled with particles is not only a scientific challenge but also a critical need for various industrial applications. However, the intricate dynamics of particle erosion and pore clogging that govern transmissivity evolution remain largely unexplored. In this study, we experimentally examine the fluid‐driven particle migration behavior in filled fractures and its consequent impact on fracture transmissivity under various hydraulic gradients, normal stresses, and fracture apertures. We find that escalating hydraulic gradients not only intensify particle erosion through amplified fluid drag forces and hydro‐mechanical coupling effects but also lead to an increase in the size of migrating particles, thereby augmenting pore clogging. The dynamics of erosion and clogging define four distinct migration phases within the filled fractures. Variations in normal stress and initial fracture aperture significantly alter the particle arrangement and the soil structure stability within the fractures, thereby modulating the progress of particle migration in response to hydraulic gradients. The pattern of particle migration in filled fractures dictates the development of the internal pore structure and normal deformation, ultimately affecting fracture transmissivity. We propose an empirical expression to encapsulate the comprehensive evolution of fracture transmissivity across different particle migration patterns. Our research advances the understanding of fluid‐driven particle migration within filled fractures and provides a practical tool for the precise determination of hydraulic properties of fractured rocks amidst complex geological settings.

On the fluid drag reduction in scallop surface.

In the field of biomimetics, the tiny riblet structures inspired by shark skin have been extensively studied for their drag reduction properties in turbulent flows. Here, we show that the ridged surface texture of another swimming creature in the ocean, i.e., the scallops, also has some friction drag reduction effect. In this study, we investigated the potential drag reduction effects of scallop shell textures using computational fluid dynamics simulations. Specifically, we constructed a conceptual model featuring an undulating surface pattern on a conical shell geometry that mimics scallop. Simulations modeled turbulent fluid flows over the model inserted at different orientations relative to the flow direction. The results demonstrate appreciable friction drag reduction generated by the ribbed hierarchical structures encasing the scallop, while partial pressure drag reduction exhibits dependence on alignment of scallop to the predominant flow direction. Theoretical mechanisms based on classic drag reduction theory in turbulence was established to explain the drag reduction phenomena. Given the analogous working environments of scallops and seafaring vessels, these findings may shed light on the biomimetic design of surface textures to enhance maritime engineering applications. Besides, this work elucidates an additional evolutionary example of fluid drag reduction, expanding the biological repertoire of swimming species.

A Review of Weak Gel Fracturing Fluids for Deep Shale Gas Reservoirs.

Low-viscosity slickwater fracturing fluids are a crucial technology for the commercial development of shallow shale gas. However, in deep shale gas formations with high pressure, a higher sand concentration is required to support fractures. Linear gel fracturing fluids and crosslinked gel fracturing fluids have a strong sand-carrying capacity, but the drag reduction effect is poor, and it needs to be pre-prepared to decrease the fracturing cost. Slick water fracturing fluids have a strong drag reduction effect and low cost, but their sand-carrying capacity is poor and the fracturing fluid sand ratio is low. The research and development of viscous slick water fracturing fluids solves this problem. It can be switched on-line between a low-viscosity slick water fracturing fluid and high-viscosity weak gel fracturing fluid, which significantly reduces the cost of single-well fracturing. A polyacrylamide drag reducer is the core additive of slick water fracturing fluids. By adjusting its concentration, the control of the on-line viscosity of fracturing fluid can be realized, that is, 'low viscosity for drag reduction, high viscosity for sand-carrying'. Therefore, this article introduces the research and application status of a linear gel fracturing fluid, crosslinked gel fracturing fluid, and slick water fracturing fluid for deep shale gas reservoirs, and focuses on the research status of a viscous slick water fracturing fluid and viscosity-controllable polyacrylamide drag reducer, with the aim of providing valuable insights for the research on water-based fracturing fluids in the stimulation of deep shale gas reservoirs.

Contact-angle hysteresis provides resistance to drainage of liquid-infused surfaces in turbulent flows

Lubricated textured surfaces immersed in liquid flows offer tremendous potential for reducing fluid drag, enhancing heat and mass transfer, and preventing fouling. According to current design rules, the lubricant must chemically match the surface to remain robustly trapped within the texture. However, achieving such chemical compatibility poses a significant challenge for large-scale flow systems, as it demands advanced surface treatments or severely limits the range of viable lubricants. In addition, chemically tuned surfaces often degrade over time in harsh environments. Here, we demonstrate that a lubricant-infused surface (LIS) can resist drainage in the presence of external shear flow without requiring chemical compatibility. Surfaces featuring longitudinal grooves can retain up to 50% of partially wetting lubricants in fully developed turbulent flows. The retention relies on contact-angle hysteresis, where triple-phase contact lines are pinned to substrate heterogeneities, creating capillary resistance that prevents lubricant depletion. We develop an analytical model to predict the maximum length of pinned lubricant droplets in microgrooves. This model, validated through a combination of experiments and numerical simulations, can be used to design chemistry-free LISs for applications where the external environment is continuously flowing. Our findings open up new possibilities for using functional surfaces to control transport processes in large systems. Published by the American Physical Society 2024

Fully coupled discrete element method for graded particles transport in pipes

Hydraulic conveying of graded particles is much more complex than that of uniform particles but is not fully understood. A fully coupled computational fluid dynamics–discrete element method model is established for the hydraulic conveying of graded particles, which integrally considers the particle–fluid and particle–particle interactions and turbulence modulation from particles. The proposed model accounts for the stochastic motion of particles by the discrete random walk method and applies the diffusion averaging algorithm to obtain particle concentration in arbitrary cells for smooth and cell-independent data fields on unfavorable cells (fluid cell size ≤ particle size). The particle–fluid drag force is applied to slurry flows through densely packed particle beds due to the consideration of porosity modification. The proposed model well performs in simulating the hydraulic conveying of dense graded particles. Dynamics in slurry mixtures of bi-disperse particles are investigated regarding different particle size compositions. The results show obvious stratification between coarse and fine particles, e.g., fine particles settling at the pipe bottom elevate the coarse particles and form a “lubrication layer” with high velocity. The torque caused by particle–particle/wall contact is greater than the torque caused by the fluid. The pipe cross section is divided into four regions according to the particle angular velocity. The effect of particle concentration on liquid motion is small because the difference in local particle concentration is relatively small, but the maximum pressure drop corresponds to a critical particle size composition.

Particle distributions in Lamb wave based acoustofluidics

Acoustic streaming enabled by a Lamb wave resonator (LWR) is efficient for particle trapping and enrichment in microfluidic channels. However, because Lamb waves combine the features of bulk acoustic waves and surface acoustic waves, the resulting acoustic streaming in the LWR occurs in multiple planes, and the particle flow behavior in this acoustofluidic system is largely unknown. Reported here are numerical simulations and laboratory experiments conducted to investigate the boundary conditions for particle motion inside a microvortex induced by an LWR. Upon dynamic capture, the particles’ trajectories become orbital paths within an acoustic vortex. The suspended particles encounter two distinct acoustic phenomena, i.e., the drag force resulting from acoustic streaming and the acoustic radiation force, which exert forces in various directions on the particles. When the acoustic radiation force and the fluid drag force are dominant for large and small particles in a mixed solution, respectively, the large particles reside within the vortex while the small particles remain at its periphery. Conversely, when the acoustic radiation force is dominant for both types of particles, the distribution pattern is reversed.

Fluid Slip and Drag Reduction on Liquid-Infused Surfaces under High Static Pressure.

Liquid-infused surfaces (LIS) have been shown to reduce the huge frictional drag affecting microfluidic flow and are expected to be more robust than superhydrophobic surfaces when exposed to external pressure as the lubricant in LIS is incompressible. Here, we investigate the effect of applying static pressure on the effective slip length measured on Teflon wrinkled surfaces infused with silicone oil through pressure measurements in microfluidic devices. The effect of static pressure on LIS was found to depend on air content in the flowing water: for degassed water, the average effective slip length was beff = 2.16 ± 0.90 μm, irrespective of applied pressure. In gassed water, the average effective slip length was beff = 4.32 ± 1.06 μm at zero applied pressure, decreased by 55% to 2.37 ± 0.90 μm when the pressure was increased to 50 kPa, and then remained constant up to 200 kPa. The result is due to nanobubbles present on LIS, which are compressed or partially dissolved under pressure, and the effect is more evident when the size and portion of surface nanobubbles are higher. In contrast, on superhydrophobic wrinkles, the decline in beff was more sensitive to applied pressure, with beff = 6.8 ± 1.4 μm at 0 kPa and, on average, beff = -1 ± 3 μm for pressures higher than 50 kPa for both gassed and degassed water. Large fluctuations in the experimental measurements were observed on superhydrophobic wrinkles, suggesting the nucleation of large bubbles on the surface. The same pressure increase did not affect the flow on smooth substrates, on which gas nanobubbles were not observed. Contrary to expectations, we observed that drag reduction in LIS is affected by applied pressure, which we conclude is because, in a similar manner to superhydrophobic surfaces, they lose the interfacial gas, which lubricates the flow.

Effects of electrostatic interaction on clustering and collision of bidispersed inertial particles in homogeneous and isotropic turbulence

In sandstorms and thunderclouds, turbulence-induced collisions between solid particles and ice crystals lead to inevitable triboelectrification. The charge segregation is usually size dependent, with small particles charged negatively and large particles charged positively. In this work, we perform numerical simulations to study the influence of charge segregation on the dynamics of bidispersed inertial particles in turbulence. Direct numerical simulations of homogeneous isotropic turbulence are performed with the Taylor Reynolds number ${Re}_{\lambda }=147.5$ , while particles are subjected to both electrostatic interactions and fluid drag, with Stokes numbers of 1 and 10 for small and large particles, respectively. Coulomb repulsion/attraction is shown to effectively inhibit/enhance particle clustering within a short range. Besides, the mean relative velocity between same-size particles is found to rise as the particle charge increases because of the exclusion of low-velocity pairs, while the relative velocity between different-size particles is almost unaffected, emphasizing the dominant roles of differential inertia. The mean Coulomb-turbulence parameter, ${Ct}_0$ , is then defined to characterize the competition between the Coulomb potential energy and the mean relative kinetic energy. In addition, a model is proposed to quantify the rate at which charged particles approach each other and to capture the transition of the particle relative motion from the turbulence-dominated regime to the electrostatic-dominated regime. Finally, the probability distribution function of the approach rate between particle pairs is examined, and its dependence on the Coulomb force is further discussed using the extended Coulomb-turbulence parameter.

How particle shape affects granular segregation in industrial and geophysical flows

Industrial and environmental granular flows commonly exhibit a phenomenon known as "granular segregation," in which grains separate according to physical characteristics (size, shape, density), interfering with industrial applications (cement mixing, medicine, and food production) and fundamentally altering the behavior of geophysical flows (landslides, debris flows, pyroclastic flows, riverbeds). While size-induced segregation has been well studied, the role of grain shape has not. Here we conduct numerical experiments to investigate how grain shape affects granular segregation in dry and wet flows. To isolate the former, we compare dry, bidisperse mixtures of spheres alone with mixtures of spheres and cubes in a rotating drum. Results show that while segregation level generally increases with particle size ratio, the presence of cubes decreases segregation levels compared to cases with only spheres. Further, we find differences in the segregation level depending on which shape makes up each size class, reflecting differences in mobility when smaller grains are cubic or spherical. We find similar dynamics in simulations of a shear-driven coupled fluid-granular flow (e.g., a simulated riverbed), demonstrating that this phenomenon is not unique to rotating drums; however, in contrast to the dry system, we find that the segregation level increases in the presence of cubic grains, and fluid drag effects can qualitatively change segregation trends. Our findings demonstrate competing shape-induced segregation patterns in wet and dry flows that are independent from grain size controls, with implications for many industrial and geophysical processes.

Aggregation of slightly buoyant microplastics in 3D vortex flows

Abstract. Although the movement and aggregation of microplastics at the ocean surface have been well studied, less is known about the subsurface. Within the Maxey–Riley framework governing the movement of small, rigid spheres with high drag in fluid, the aggregation of buoyant particles is encouraged in vorticity-dominated regions. We explore this process in an idealized model that is qualitatively reminiscent of a 3D eddy with an azimuthal and overturning circulation. In the axially symmetric state, buoyant spherical particles that do not accumulate at the top boundary are attracted to a loop consisting of periodic orbits. Such a loop exists when drag on the particle is sufficiently strong. For small, slightly buoyant particles, this loop is located close to the periodic fluid parcel trajectory. If the symmetric flow is perturbed by a symmetry-breaking disturbance, additional attractors for small, rigid, slightly buoyant particles may arise near periodic orbits of fluid parcels within the resonance zones created by the disturbance. Disturbances with periodic or quasiperiodic time dependence may produce even more attractors, with a shape and location that recurs periodically. However, not all such loops attract, and rigid particles released in the vicinity of one loop may instead be attracted to a nearby attractor. Examples are presented along with mappings of the respective basins of attraction.

Steady Flow Over a Finite Patch of Submerged Flexible Vegetation

AbstractAn immersed boundary‐finite element with soft‐body dynamics has been implemented to study steady flow over a finite patch of submerged flexible aquatic vegetation. The flow structure interaction model can resolve the flow interactions with flexible vegetation, and hence the reconfiguration of vegetation blades to ambient flow. Flow dynamics strongly depend on two dimensionless parameters, namely vegetation density and Cauchy number (defined as the ratio of the fluid drag force to the elastic force). Five different flow patterns have been identified based on vegetation density and Cauchy number, including the limited reach, swaying, “monami” A, “monami” B with slow moving interfacial wave, and prone. The “monami” B pattern occurred at high vegetation density and is different from “monami” A, in which the passage of Kelvin‐Helmholtz billows strongly affects the vegetation interface. With soft‐body dynamics, blade‐to‐blade interactions can also be resolved. At high vegetation density, the hydrodynamic interactions play an important role in blade‐to‐blade interactions, where adjacent vegetation blades interact via the interstitial fluid pressure. At low vegetation density, direct contacts among vegetation blades play important roles in preventing unphysical penetration of vegetation blades.