Drag Reduction Effect Research Articles

Polymer-dominant drag reduction in turbulent channel flow over a superhydrophobic surface

In this study, we focused on the integration of a flexible polymer (polyacrylamide) and a (randomly patterned) superhydrophobic surface in a large-scale turbulent channel flow rig to investigate their combined drag reduction effectiveness. Experimental results indicate that, prior to degradation, polyacrylamide (at a 100-ppm concentration) and superhydrophobic surfaces individually manifest drag reductions of 35% and 7%, respectively. However, when combined, the influence of polymer additives remained consistent, with the introduction of superhydrophobic surfaces yielding negligible differences. A clear predominance was evidenced in our facility looking at realistic pressure for applications, with polymer additives overshadowing the impact of superhydrophobic surfaces.

Influence of the slip boundary on square cylinders with lattice Boltzmann method

In this investigation, two-dimensional flow past square cylinders with slip boundary have been studied with the lattice Boltzmann method. Three modes, which are a single cylinder, an oblique cylinder, and side-by-side cylinders, are investigated with Reynolds numbers from 25 to 200 while relative slip length ranges from 0 to 0.1. It can be concluded that both the flow state and the slip boundary have a great effect on the drag reduction rate. For a single square cylinder, drag forces decrease with larger slip length while the Strouhal number is almost constant. For an oblique cylinder, the slip length also has effects on the stability of the flow except the drag and lift forces. Vortex separation delays with a slip wall of oblique cylinder. For side-by-side cylinders, the jet between two cylinders is not conducive to the drag reduction rate of the slip boundary. Moreover, the application of slip boundary may also lead to additional drag force as vortex separation intensifies, which is extremely different from simple channel flows. Studies have shown that the slip boundary does not always reduce the drag in some complex flow fields. It can be concluded that the drag reduction effect of slip boundaries is more effective in uniform flow.

A Numerical Study on the Influence of Transverse Grooves on the Aerodynamic Performance of Micro Air Vehicles Airfoils

Micro Air Vehicles (MAVs) airfoils usually operate at low Reynolds number conditions, where viscous drag will consume a large amount of propulsion power. Due to the small dimensions, many drag reduction methods have failed, resulting in limited current research. To develop an effective method of reducing viscous drag, transverse grooves were placed on the surface of MAVs airfoils in this study, and a numerical investigation was implemented to uncover the corresponding flow control law as well as the mechanism. Research has shown that transverse grooves have an impact on the drag and lift of airfoils. For drag, properly sized transverse grooves have the effect of reducing drag, but under high adverse pressure gradients or when the continuous arrangement of grooves is excessive, the optimal drag reduction effect achieved by the grooves is weakened, and even the drag increases due to the significant increase in pressure difference. In severe cases, it may also cause strong flow separation, which is not conducive to MAV flight. For lift, the boundary vortex in the groove has the ability to reduce the static pressure near the groove. However, high adverse pressure gradients or too many grooves will thicken the boundary layer and increase the blockage effect, resulting in a large static pressure on the grooved side of the airfoil (with an increase in drag). From the perspective of circulation, the static pressure changes on the suction and pressure surfaces have opposite effects on lift. Considering the comprehensive aerodynamic performance of the airfoil, we designed a high lift-to-drag ratio airfoil with grooves, which increased the lift-to-drag ratio by 33.747% compared to the smooth airfoil. Based on the conclusions, we proposed preliminary design criteria for grooved airfoils, providing guidance for subsequent research and applications.

Mesoscopic analysis of drag reduction performance of bionic furrow opener based on the discrete element method.

In order to study the dynamic interface mechanical behavior between soil and agricultural machinery and reveal the causes of tillage resistance, three kinds of bionic furrow opener were designed according to the characteristics of earthworm head surface curve, using the discrete element method to simulate and analyze the process of the furrow openers. The results showed that the order of ditching resistance from large to small is traditional opener, bionic corrugated opener, bionic ridgeline opener, bionic composite opener. With the same ditching speed, the drag reduction effect of the three bionic openers increases with the increase of the ditching depth. During the process of increasing the depth from 30 mm to 60 mm and 90 mm, the ditching resistance of the traditional opener increased from 11.56 N to 28.32 N and 48.61 N as well as the maximum drag reduction ratio increased from 5.58% to 7.20% and 8.93% for the bionic composite opener. With the same ditching depth, the bionic composite opener reached the highest drag reduction rate of all bionic openers when the speed is 100 mm/s, the value is 9.08%. The width of the ditch of the three bionic openers is smaller than that of the traditional opener. Bionic corrugated opener can improve the ditch height and reduce the ditch width,the corrugated structure creates a gap between the surface of the core and the particles, reducing the number of contact and contact area of the particles. The number of contact particles of the three bionic openers is smaller than that of the traditional opener. The bionic composite opener has the smallest force field and the soil disturbance caused by the core share surface is small, the soil is evenly distributed along the core surface. The discrete element simulation shows that the bionic opener can effectively reduce the ditching resistance and improve the quality of ditching, which provides a theoretical basis for subsequent research and optimization.

Influence of fluorine incorporation into waterborne acrylic coatings on resulting hydrophobic, thermal stability and drag- reduction properties

Fluorinated polyacrylate copolymers containing various amounts of 1H,1H,2H,2H-perfluorooctyl methacrylate (FA) were synthesized via a miniemulsion polymerization process. The characteristics of latex particles and films were investigated using transmission electron microscopy, dynamic light scattering, attenuated total reflectance Fourier transform infrared spectroscopy, 1H and 19F nuclear magnetic resonance spectroscopy, atomic force microscopy, energy dispersive spectroscopy, water contact angle measurements, and thermogravimetric analysis. The results showed that particle size increased almost linearly with FA content, and the surface hydrophobicity of the films increased with FA content up to 14.6 wt% and then plateaued. As the FA content increased, the surface roughness of the films initially increased and then decreased. The thermal stability of fluorinated polyacrylate was significantly enhanced when a high amount of FA was incorporated into the polymer chains. The fluorinated acrylic copolymer coating also exhibited a significant drag reduction effect, with the maximum drag reduction ratio reaching 12% at a water flow velocity of 0.8 m/s.

Research on the impact of air-blowing on aerodynamic drag reduction and wake characteristics of a high-speed maglev train

The maglev train fills the speed gap between ground transportation and airplanes. However, the increasing train speed results in greater energy losses due to increased aerodynamic drag, impeding the green and sustainable development of high-speed railways. This study employs the numerical simulation method to explore the effects of installing air-blowing slots on the surface of the TR08 maglev train's tail car and blowing air along three different directions at two different speeds on drag reduction and the characteristics of the wake flow field. Among them, only blowing air along the streamwise direction at speeds of 12 m/s (X12) and 24 m/s (X24) shows drag reduction effects of 2.06% and 6.53%, respectively. However, considering the energy efficiency, only X12 achieves a net energy saving of 58.96%, while the energy consumption by blowing air and saving by reducing drag in X24 are roughly balanced. Air-blowing reduces the aerodynamic lift of the tail car (Cl), with blowing air along the perpendicular direction at 24 m/s (Z24) reducing the Cl by 99.57%. Additionally, Z24 reduces the maximum velocity value of the train-induced air flow (Usmax¯) by 61.91%. The research findings provide new insight and data support for the development of blowing/suction drag reduction techniques.

Experimental study on the synergic effects of surfactants and nanoparticles on the seepage of water injected into coal seams

To study the synergic effects of surfactants and nanoparticles on the seepage of water injected into coal seams, we prepared three chip models with complex cracks using glass etching technology based on the real conformation of cracks in coal mass and set up a visual experimental platform for microscopic seepage. Based on these models, we carried out a series of seepage experiments on gas expelling with different solutions and investigated the effects of surfactants, nanoparticles, and flow velocities on seepage characteristics. The results showed that all tested anionic, non-ionic, and cationic surfactant solutions could reduce flow resistance, showing a downward trend with the rise of the mass fractions of these surfactants. The combination of nanoparticles with these surfactant solutions further reduced flow resistance, showing the best drag reduction effects with hydrophobic nanoparticles at all tested flow velocities. Moreover, all three tested surfactants could displace gas and reduce gas residue in the crack networks. The higher the mass fraction of the surfactant solution is, the more significant the gas displacement effect. Furthermore, the gas displacement effect of the surfactant solution is positively proportional to the flow velocity. The higher the flow velocity is, the more significant the gas displacement effect.

Drag reduction effect of distributed roughness on the transitional flow state using direct numerical simulation

We analyzed the details of Tollmien–Schlichting wave transitions on a sandy rough surface through direct numerical simulations by resolving each roughness level using the volume penalization and zonal methods. Although wall roughness has generally been discussed in terms of increased drag in fully developed turbulent flows, we found that the sandy roughness on a flat plate suppresses the growth of both the turbulent kinetic energy and friction drag coefficient during the laminar-turbulent transitional flow state. The roughness size is on the microscale order, assuming similarity based on the Reynolds number and considering the case of the surface of a transonic airplane. Therefore, we call such three-dimensional and complex-shaped roughness with this effect distributed microroughness (DMR). On the sandy rough surface, the Tollmien–Schlichting wave collapses into a three-dimensional flow structure in the initial stage; however, the turbulent kinetic energy remains small. Generally, the turbulent kinetic energy of the subsequent three-dimensional turbulent component increases in correlation with the size of the primary T-S vortex. On the rough surface, the T-S roll vortex collapses before it maximally grows due to the spanwise disturbance caused by the three-dimensionality of the sandy roughness shape, resulting in the three-dimensional turbulence associated with the roll remaining small. Near the wall surface, the velocity fluctuates due to the distributed roughness, and the statistical peaks of the kinetic energy budget tend to be away from a wall. Although the details of the mechanism by which changes in the energy balance cause a decrease in the productive turbulent energy have not been elucidated, we will clarify the changes due to roughness parameters in a concurrent study.

Effects of ammonia combustion on skin friction characteristics for supersonic flow

Boundary layer combustion is an effective method to reduce friction in scramjet combustors. In existing research on drag-reducing fuels, hydrocarbons pose challenges related to carbon deposition and emissions. Meanwhile, hydrogen presents significant difficulties in storage and transportation. For the first time, this paper explores ammonia as a drag-reducing alternative. We employed the Reynolds-averaged Navier-Stokes (RANS) method to investigate the characteristics of drag reduction by ammonia combustion in supersonic flow. Numerical results indicate that the local drag reduction by ammonia combustion is better than hydrocarbon and worse than hydrogen compared to existing research. Ammonia exhibits superior potential for drag reduction. However, the overall drag reduction at room ammonia injection temperature is limited due to the significant distance between the ammonia self-ignition point and the injection port. To address this challenge, we proposed two optimization strategies: increasing the ammonia injection temperature and increasing the inlet ammonia decomposition ratio. As the ammonia injection temperature increases, the self-ignition position moves towards the inlet, and the combustion drag reduction region expands upstream. Interestingly, a higher ammonia inlet decomposition ratio does not necessarily lead to a better drag reduction effect. In fact, an excessively high decomposition ratio can inhibit the combustion drag reduction effect. The most effective drag reduction occurs when the ammonia inlet mass fraction is 50%, resulting in a 42.8% reduction in integrated drag compared to the case without NH3 injection. Overall, this paper offers critical insights into the utilization of ammonia combustion in scramjets to reduce drag.

Numerical study on the effect of pantograph fairing on aerodynamic performance at various train heights

The pantograph is one of the primary sources of aerodynamic drag and noise during the operation of a high-speed train. To investigate the impact of the pantograph fairing on the aerodynamic performance at various train heights, six computational models are established, and the Improved Delayed Detached Eddy Simulation method is employed. The results indicate that the installation of the fairing can effectively reduce the aerodynamic drag coefficient of the high-speed train and pantograph at different train heights. As the train height increases, the drag reduction effect of the fairing enhances when the sinking platform coexists. The overall drag reduction rate of up to 4.30% can be achieved at a train height of 4000 mm. However, the larger reduction rate of 58.32% in the pantograph's drag coefficient at a train height of 3700 mm indicates that the fairing could make a greater effect on the pantograph at a smaller train height. The reduction in train height and pantograph sinking height leads to an increase in drag coefficient on the tail car, and the implementation of the fairing further amplifies the drag. This work offers a reference for optimizing the aerodynamic performance of high-speed train.

Secondary vortex drag reduction and heat transfer enhancement of nanofluids in hierarchical microchannels applied to thermal management of electronic components

In order to improve the heat dissipation performance of electronic components, two hierarchical microchannel structures with curve and straight corners are proposed, and semi-circular and trapezoidal secondary flow structures are introduced into the structure with the best performance. The effects of coolant (Fe3O4-water nanofluid), layered structure and secondary flow structure on the comprehensive performance of MCHS were studied by numerical simulation. Results showed that deionized water is more suitable for the heat dissipation of microchannels at this scale. The hierarchical structure has the effect of enhancing heat transfer, and Nu can be increased by up to 67.32 %. The introduction of secondary flow structure has drag reduction effect in a certain Reynolds number range. The hierarchical MCHS with semi-circular secondary flow structure has drag reduction effect in the Reynolds number range of Re = 200–400, and the pressure drop can be reduced by up to 1.89 %. The hierarchical MCHS with trapezoidal secondary flow structure always has a drag reduction effect in the range of Reynolds number Re = 200–600, and the maximum pressure drop can be reduced by 2.46 %, showing excellent drag reduction ability. In addition, the introduction of trapezoidal secondary flow in MCHS increases the Nusselt number by 9.70 %, the comprehensive performance index reaches 1.10, and the cooling performance and comprehensive performance are optimal. The combination of hierarchical structure and secondary flow structure has excellent drag reduction and heat transfer enhancement effects, and has broad application prospects in the field of thermal management of electronic devices.

Research on drag reduction performance of sliding plate of rice direct seeding machine based on non-smooth structure of loach surface

Sliding plate has the problems of large sliding resistance and serious soil adhesion. Loach moves freely and flexibly in mud, has highly efficient lubrication and drag reduction effects. The sliding plate of rice direct seeding machine was selected as the research object and loach as the bionic prototype. The macroscopic and microscopic structure characteristics of loach were observed, the body surface of loach was covered by scales, which had a ridged non-smooth structure. The simulation analysis of the drag reduction performance of the non-smooth structure was carried out, the maximum drag reduction rate was 2.55% at the speed of 1 m/s. A bionic sliding plate of rice direct-seeding machine was constructed based on the non-smooth structure of loach body surface, and its working performance was simulated and analyzed. The results of orthogonal test show that the order of primary and secondary factors of bionic structure parameters affecting drag reduction rate was ribbed spacing > ribbed width > ribbed height. The optimal parameter combination was ribbing height 4 mm, ribbing width 4.5 mm, ribbing spacing, and the optimal drag reduction rate was 4.21%. The results of this study can provide theoretical support for bionic design of soil-engaging components in wet and soft paddy field.

Drag reduction using bionic groove surface for underwater vehicles.

Introduction: The reduction of drag is a crucial concern within the shipping industry as it directly influences energy consumption. This study addresses this issue by proposing a novel approach inspired by the unique ridge structure found on killer whale skin. The objective is to develop a non-smooth surface drag reduction method that can effectively decrease drag and improve energy efficiency for ships. Methods: The study introduces a technique involving the creation of transverse bionic groove surfaces modeled after the killer whale skin's ridge structure. These grooves are aligned perpendicular to the flow direction and are intended to modify the behavior of turbulent boundary layer flows that form around the ship's hull. Numerical simulations are employed using the Shear Stress Transport k-ω model to analyze the effects of the proposed groove surface across a wide range of flow conditions. The research investigates the impact of various parameters, such as the width-to-depth ratio (λ/A), groove depth, and inlet velocity, on the drag reduction performance of the bionic groove surface. Results: The study reveals several key findings. Optimal shape parameters for the bionic groove surface are determined, enabling the most effective drag reduction. The numerical simulations demonstrate that the proposed groove surface yields notable drag reduction benefits within the velocity range of 2∼12m/s. Specifically, the friction drag reduction ratio is measured at 26.91%, and the total drag reduction ratio at 9.63%. These reductions signify a substantial decrease in the forces opposing the ship's movement through water, leading to enhanced energy efficiency. Discussion: Comparative analysis is conducted between the performance of the bionic groove surface and that of a smooth surface. This investigation involves the examination of velocity gradient, streamwise mean velocity, and turbulent intensity. The results indicate that the bionic groove structure effectively mitigates viscous stress and Reynolds stress, which in turn reduces friction drag. This reduction in drag is attributed to the alteration in flow behavior induced by the non-smooth surface. Conclusion: The study proposes a novel approach for drag reduction in the shipping industry by emulating the ridge structure of killer whale skin. The transverse bionic groove surface, aligned perpendicular to flow direction, demonstrates promising drag reduction outcomes across diverse flow conditions. Through systematic numerical simulations and analysis of key parameters, the research provides insights into the drag reduction mechanism and identifies optimal design parameters for the groove surface. The potential for significant energy savings and improved fuel efficiency in maritime transportation underscores the practical significance of this research.

Numerical study on the effect of airfoil attack angle and height on the resistance reduction of the planing trimaran

The planing trimaran has unique rapidity and stability. In this paper, airfoils are added to the planing trimaran to provide additional aerodynamic and hydrodynamic lift, achieving the purpose of reducing the resistance of the planing trimaran. By solving Reynold-Averaged Navier-Stokes (RANS) equations and combining SST k-Omega turbulence model with Volume of Fraction (VOF) method, a numerical simulation method is established to analyze the motion characteristics of the planing trimaran. The attack angle and the height of airfoils are mainly studied in this paper. NACA4404 and a special airfoil named 2# airfoil are selected as the research objects. The range of attack angle is 0–8°. The drag reduction effect of the airfoil does not change linearly with the increase of the attack angle in the high speed range. NACA4404 and 2# airfoil have the best drag reduction effect at 2°and 4°respectively. The range of airfoil height is 0.8h–1h (“h” is the wing height from baseline). For NACA4404, lower wing height is beneficial to reduce the resistance, especially in the low speed range. The 2# airfoil has poor performance in the low speed range after reducing the height, and can increase the resistance by up to 16.3%.

Study on high-speed water entry process of non-rotating vehicles

The non-rotating trans-media vehicle can better adapt to the hypersonic flight conditions, but new problems in the hydrodynamic characteristics and motion stability arise during the water entry process. With the VOF multiphase model, Realizable k-ε turbulence model, and Schnerr-Sauer cavitation model, this paper establishes a simulation method of coupling the movement of non-roating body into water with multiphase flow fields based on overset grid technology, and the accuracy of the established model is verified. The simulation calculation of the oblique water entry process of the vehicle with different ratio of short axis to long axis in cross section is carried out. And the results show that although the ratio of short axis to long axis in cross section does not change the cavitation morphology, it affects the hydrodynamic and kinematic characteristics of the vehicle by influencing the relative position of the cavitation. As the ratio of short axis to long axis decreases, the motion stability of the vehicle increases gradually. However, if the ratio of short axis to long axis is too small, the drag reduction effect will be weakened, and the structure will suffer a bigger load.

Investigation of drag reduction mechanism for blunt bodies with plasma spikes

The two main problems in hypersonic vehicles are wave drag and aerodynamic heating. This work proposes a novel method for drag reduction by combining spikes with the plasma synthetic jet actuator (PSJA). Numerical simulations are performed to better understand the drag reduction mechanism with an incoming flow at Mach 6. The results suggest that the flow field is affected primarily by the diffracted wave and synthetic jet. The maximum drag reduction reaches 47.7% with the plasma spike compared with the opposing jet. A better drag reduction effect is achieved when increasing the energy density of the PSJA, while the propagation velocity of the diffracted wave remains constant. A wider and faster jet is obtained with a larger PSJA orifice diameter. However, the control time of the jet shortens. A mode conversion occurs when the orifice diameter is 1 mm. Furthermore, the maximum drag reduction rate increases from 37.6% to 49.0% when the length diameter ratio (L/D) increases from 0.5 to 1.5. The effect of spike length on drag reduction decreases gradually at greater lengths.

Experimental Investigation into the Drag Performance of Chevron-Shaped Protrusions in Wall-Bounded Turbulence

Chevron-shaped protrusions have been proposed in the literature for turbulent skin friction reduction. However, there is no consensus on the performance of this passive flow control technique; both an increase and a decrease in drag have been observed in previous studies. There is also no experimental evidence to support the working mechanism behind the drag reduction effect that has been postulated in the literature. In this study, direct force measurements were used to replicate experiments from the literature and, in addition, were used to test new array configurations to characterise the effect of individual design parameters on drag performance. A total of 23 different protrusion configurations were investigated in a turbulent boundary layer flow. In addition to the integral force measurements, particle image velocimetry was used to measure wall-parallel velocity fields in order to extract the statistical sizing and energy of the near-wall cycle turbulence. All configurations increased the drag between 2% and 10% for a friction Reynolds number of 1700. The drag reduction reported in the literature could not be replicated; however, these findings agreed with an experimental and numerical study that reported drag increase. The trend observed in the low-speed streak spacing from the PIV experiments was consistent with that observed in the balance data. Nevertheless, no evidence was found to support the working mechanism proposed in the literature. These results cast doubt on the proposed drag reduction potential of chevron-shaped protrusions. In the authors’ view, the results of this study strengthen previous conclusions regarding their minor increase in drag. Future studies to further approach a consensus are proposed.

Drag Reduction by Fish-Scale Inspired Transverse Asymmetric Triangular Riblets: Modelling, Preliminary Experimental Analysis and Potential for Fouling Control.

The natural surfaces of many plants and animals provide examples of textures and structures that remain clean despite the presence of environmental fouling contaminants. A biomimetic approach to deciphering the mechanisms used by nature will facilitate the development and application of fouling-resistant surfaces to a range of engineering challenges. This study investigated the mechanism underlying the drag reduction phenomenon that was shown to be responsible for fouling resistance for underwater surfaces. For this purpose, a novel fish-scale-inspired microstructure was shown to exhibit a drag reduction effect similar to that of its natural replica. The primary mechanism through which this occurs is a delayed transition to turbulence. To investigate this mechanism, a Large Eddy simulation was performed at several Reynolds numbers (Re). This analysis demonstrated a peak drag reduction performance of 6.7% at Re = 1750. The numerical data were then experimentally validated through pressure drop measurements performed by means of a custom-built micro-channel. In this case, a peak drag reduction of 4.8% was obtained at Re = 1000. These results suggest a relative agreement between the experimental and numerical data. Taken together, this study advocates that, for the analyzed conditions, drag reduction occurs at low Reynolds numbers. Nonetheless, once flow conditions become more turbulent, the decline in drag reduction performance becomes apparent.

Combined immersed boundary and discrete unified gas kinetic scheme for the motion of an autonomous underwater vehicle model with slip over a solid-liquid interface

To improve the endurance and speed of an autonomous underwater vehicle (AUV), we consider adopting the slip over a solid-liquid interface, which has the effect of drag reduction. However, the simulation of motion of an AUV with the slip remains a challenge due to the fluid-structure interaction and multiscale problems involving the moving boundary. To solve these problems, the present work proposes a novel and competitive mesoscopic method. Firstly, the Discrete Unified Gas Kinetic Scheme (DUGKS) coupled with the immersed boundary method is improved by correcting the boundary force, avoiding any additional computational time or implementation effort. Its effectiveness and accuracy are validated by two numerical tests involving the moving boundary. Furthermore, a fluid-solid interaction force is introduced to this method to model the slip condition due to its easy and effective implementation. We perform the experiment of motion of an AUV model in water to further validate the proposed method, which shows that it can predict acceptable results. Then, the method is applied to study the effect of the slip on the motion of the AUV model. The greater slip induced by the fluid-solid interaction force has a greater effect on reducing the drag and improving the velocity. It is indicated that increasing the fluid-solid interaction force can increase the amplitude and decrease the oscillation frequency of the drag curve. Overall, the slip can improve the endurance and speed of the AUV model.

Drag-reduction effect of staggered superhydrophobic surfaces in a turbulent channel flow

In this study, direct numerical simulations of turbulent channel flow with superhydrophobic surfaces (SHS) were performed. The staggered SHS pattern alternating the free-shear and no-slip regions was designed to be robust to variations in the main flow direction. A constant-pressure gradient condition was imposed, and the Reynolds number was set to Reτ=180 and 395. The bulk mean velocity in the SHS case increased compared with the no-slip case, and the effect was enhanced by increasing the size of the free-shear region lb+. The main reason for the increase in bulk mean velocity was the increase in slip velocity (or slip length). If the size of the free-shear region was small, the drag-reduction rate was found to be proportional to the slip velocity. However, the turbulent contribution to the bulk mean velocity remained almost unchanged in the no-slip case owing to the negative coherent component of the Reynolds shear stress.