Drag Reduction Mechanism Research Articles

Optimization of drag-reduction microstructure parameters and study of the drag reduction mechanism in a rotating flow field

Fluid drag greatly lowers the efficiency and increases the energy consumption of underwater vehicles and devices working in similar environments. Therefore, drag reduction has become a major topic in fluids research. Inspired by the high drag-reduction effect of shark skin, this paper experimentally and numerically investigates the drag-reduction performance of a bionic shark skin microstructure with a triangular cross section. The structural parameters are optimized through numerical simulations. The microstructure reduces the drag by reducing the velocity gradient near the wall and changes the turbulent kinetic energy distribution in the flow field near the wall. Next, samples of microstructures were prepared using the template method. Experimental rheometer tests revealed a drag reduction rate of 14.29% on the microstructure surface under the set experimental conditions. Experiments and simulations have demonstrated the high drag-reduction effect of the microstructures within a rotating flow field. The developed method and theoretical basis for numerical simulations of rotating flow fields can be utilized in pump machinery such as magnetic levitation centrifugal flow pumps.

Calibration and Testing of Discrete Element Simulation Parameters for Ultrasonic Vibration-Cutter-Soil Interaction Model

This paper established an accurate discrete element for ultrasonic vibration-cutter-soil interaction model to study the interaction mechanism between the soil-engaging component and the soil. In order to reduce the interaction between calibration parameters and improve the calibration accuracy, it is proposed that the soil constitutive, contact parameters, and bonding parameters be calibrated by combining the soil repose angle experiment and the soil resistance experiment of ultrasonic vibration cutting. The study adopts the Hertz-Mindlin (no slip) contact model used in EDEM, to explore soil particle interactions. The central composite design is used to achieve systematic investigation. 3-factor 3-level orthogonal design experiment was employed using the coefficient of restitution, the coefficient of static friction, and the coefficient of rolling friction as key test factors and soil’s repose angle as the response index. Based on the Hertz-Mindlin with bonding contact model, Design-Expert 13.0 software was used to design the Plackett-Burman experiment, the steepest ascent, and the Box-Behnken experiment. With the maximum soil cutting resistance in ultrasonic vibration cutting experiment used as the response value, the adhesion parameters were optimized, and the optimal solution combination was obtained as: Normal Stiffness = 4.635 × 106 N/m, Shear Stiffness = 3.401 × 106 N/m, and Bonded Disk Radius = 2.57 mm. The optimal parameter combinations obtained from the calibration experiments were verified in two ways: ultrasonic vibration cutting and non-ultrasonic vibration cutting. The results showed that the errors between the simulation values and the actual values of the two comparative experiments were less than 5%, and the model calibrated for the three parameters can be used to study the drag reduction mechanism of ultrasonic vibration cutting in soil.

Drag Reduction Through Air-Trapping Discrete Grooves in Underwater Applications

Vehicles travelling underwater experience drag and the frictional drag costs 60% of the total energy consumption. Using trapped air serving as a lubricant is a promising way to reduce drag. The trapped air plays a significant role in drag reduction, and most failures in drag reduction are related to instability, escape, and dissolution of the trapped air. In this work, discrete grooves are employed to trap air and reduce drag. Through the analysis of the trapped air stability, the groove length and width are believed to be the main factors that influence the air escape and instability, and thus they are limited in this work to avoid these problems. The air dissolution is inevitable. The effective way to mitigate the air dissolution is to deepen the groove depth. The groove depth in this work varies from 0.5 mm to 4 mm. The numerical simulation is employed to analyze the flow field, reveal the drag reduction mechanism, and optimize the groove length. The experimental measurements are conducted to verify our design. The result confirms our design that the discrete grooves successfully avoid air escape and instability, mitigate air dissolution, and reduce drag. This work is meaningful for underwater vehicles to travel with low energy consumption and high speed.

Thrust generation by shark denticles

Direct numerical simulation is performed for flow separation over a bump in a turbulent channel. Comparisons are made between a smooth bump and one where the lee side is covered with replicas of shark denticles – dermal scales that consist of a slender base (the neck) and a wide top (the crown). As flow over the bump is under an adverse pressure gradient (APG), a reverse pore flow is formed in the porous cavity region underneath the crowns of the denticle array. Remarkable thrust is generated by the reverse pore flow as denticle necks accelerate the fluid passing between them in the upstream direction. Several geometrical features of shark denticles, including some that had not previously been considered hydrodynamically functional, are identified to form the two-layer denticle structure that enables and sustains the reverse pore flow and thrust generation. The reverse pore flow is activated by the APG before massive flow detachment. The results indicate a proactive, on-demand drag reduction mechanism that leverages and transforms the APG into a favourable outcome.

Ordinal pattern-based analysis of an opposition-controlled turbulent channel flow

We conduct a numerical study on the drag-reduction mechanism of an opposition- controlled turbulent channel flow from the viewpoint of a symbolic dynamics approach. The effect of the virtual wall formed by the opposition control is maximised at the location of the detection plane $y_d^+ \approx 10$ . At this wall-normal location, the local link strength of the self-loop of network nodes representing the negative correlation pattern between the streamwise and wall-normal velocity fluctuations is maximised in the uncontrolled flow. In the controlled case, the multiscale complexity–entropy causality plane and the spatial permutation entropy at $y_d^+ \approx 10$ indicate that the drag-reduction effect is attributed to the reduction of the region where streaks actively coalesce and separate and the suppression of the regeneration cycle in the region near the wall.

Experimental investigation on heated spheres entering water vertically at different temperatures

Our investigation of spheres entering water at high-temperature reveals that elevated temperatures modify traditional cavitation patterns and trigger novel fluid dynamic phenomena. Experimental analysis of the high-temperature sphere’s water entry process has identified four distinct cavitation morphologies: small cavities, complete cavities, dual cavities, and unstable cavities. These phenomena result from the sphere’s thermal effects altering the local flow dynamics around it, consequently impacting the hydrodynamic coefficients. Notably, thermal conditions cause the contact line from the sphere’s midpoint to transition to its tail, leading to transformations in cavity types. Furthermore, simulations employing the lattice Boltzmann method elucidate how unstable steam films formed on hot surfaces induce boundary slip, reducing pressure drag. This observation provides further insight into established mechanisms of fluid drag reduction. Our study deepens the understanding of how temperature influences water entry dynamics and offers new perspectives on reducing drag during the water entry process of objects.

Fluid-thermal-structural coupled investigation on rudder leading edge with porous opposing jet in high-speed flow

High-speed air rudders face extreme force/thermal environments, and the opposing jet can improve the thermal environment in the stationary point and leading edge. In order to investigate the effect and mechanism of porous opposing jet on the rudder leading edge, the numerical study is carried out by using SST k-ω turbulence model and a loose fluid-thermal-structural coupled approach. The drag reduction and thermal protection mechanism of porous jet with different pressure ratios (PRs) is comprehensively compared and analyzed. The obtained results show that the fluid-thermal-structural coupled approach is necessary for the precise aerodynamic heat prediction of air rudder. The lowest temperature regions distribute in the upstream of orifices due to the jet cooling effect and heat conduction in the solid structure. The PR is an important factor influencing the interaction between jets. The porous opposing jet can provide a good effect in both drag reduction and thermal protection, as the maximum temperature drops to about 40 % from no jet to porous jet. As PR increases, the maximum pressure and temperature also decrease to a larger extent. However, the most PR should be a balanced consideration among the flowfiled, thermal structure and coolant consume.

Influence of a dynamic gas film on underwater drag reduction

The gas film at the liquid–solid interface, induced by hydrophobic microstructure, can achieve a high-efficiency underwater drag reduction. However, previous studies have rarely considered the impact of changes in gas structure morphology on drag performance, especially under turbulent conditions. We conducted numerical simulations to examine the dynamic process of gas on a hydrophobic spanwise grooved surface under turbulent conditions. Our findings indicate that the morphology of the gas phase structure at the liquid–solid interface undergoes continuous alterations due to fluid action, resulting in a dynamic state of drag performance. In addition, the gas morphology that completely covers the groove surface will reduce the turbulent kinetic energy on the groove surface, resulting in a better drag reduction effect. In the flow velocity range of 10–20 m/s, the drag reduction effect of the superhydrophobic grooved surface increases with the flow velocity. Finally, we conducted experiments to validate the effectiveness of this result. A mechanism for underwater drag reduction was proposed based on these simulation results. This study offers a novel perspective on the phenomenon of underwater gas drag reduction, which could significantly influence its practical applications, especially under real working conditions.

Heat transfer enhancement and flow resistance reduction characteristics of non-uniform electrohydrodynamic conduction pumping in microchannels

In this study, the thermo-hydraulic performance of non-uniform electrohydrodynamic (EHD) conduction pumping in rectangular and straight microchannels was experimentally investigated. Electrodes are arranged in two ways: dimensions constant, spacing varies (Cases 1–3); both dimensions and spacing vary (Cases 4–6). The results show that EHD conduction pumping significantly enhances heat transfer performance, increasing the average Nusselt number (Nu) by 17.9 %-87 % compared to a smooth microchannel. As Re increases, the inertial force of the fluid dominates, diminishing EHD-induced vortices and limiting heat transfer enhancement. EHD conduction pumping enhances heat transfer by creating vortices that mimic solid pseudo-roughness microelements, increasing fluid disturbance and mixing near the wall. However, these vortices also impede the flow, resulting in a trade-off with increased pressure drop resistance. This paper innovatively adopts an arrangement of increasing electrode size (non-uniform), which effectively reduces the pressure drop resistance in the microchannel while maintaining the advantage of enhanced heat transfer. Cases 4, 5, and 6 exhibit significantly higher comprehensive heat transfer performance (η) than Case 0 (without electrodes), especially at low Re. While the inertial force at high Re suppresses vortex formation and heat transfer enhancement, η remains superior to Case 0. Notably, Case 5 maintains a minimum η of 1.18 even at Re = 1000. Furthermore, this paper proposes an equivalent slip drag reduction mechanism to elucidate the drag reduction effect of the increasing electrode size arrangement. Heat transfer augmentation and pressure drop reduction, as mentioned here in microchannels, are crucial for electronics cooling and microfluidic devices.

Optimization design of a windshield for a 12,000 TEU container ship based on a support vector regression surrogate model

This study presents the design of a windshield for a 12,000 TEU container ship using a multidisciplinary optimization system aimed at enhancing its aerodynamic performance. The framework integrates parametric modeling, design of experiment (DOE), numerical simulation, support vector regression (SVR)-based approximation, and a genetic algorithm (GA). Initially, the flow field around the ship is predicted using Reynolds-averaged Navier–Stokes (RANS) equations, with accuracy validated against computational fluid dynamics (CFD) and experimental fluid dynamics (EFD) results. The windshield's shape is then defined by three Bezier curves, and six design variables are selected based on these curves to ensure smooth deformation. Subsequently, 120 sample points are chosen within the design space using the DOE method, and an SVR-based surrogate model is established. Optimization is performed using the multi-island genetic algorithm to determine the windshield configuration. Validation tests confirm the robustness of the optimization process. On this basis, the drag reduction mechanism of the windshield is summarized and analyzed. Results demonstrate that the optimization system effectively enhances the aerodynamic performance of the container ship, achieving a maximum drag reduction rate of 20.67% under headwind conditions and significant improvement across wind angles from 0° to 60°.

Drag reduction mechanism based on the aerospike-jet composite approach under different incoming dynamic pressures

The drag experienced by the nose of an aircraft varies significantly under different incoming flow conditions. In order to reduce the aerodynamic force on the nose of the (hypersonic) supersonic vehicle, the numerical simulation approach has been used to analyze the drag reduction laws and mechanisms of a blunt-body vehicle with the aerospike-counterflowing jet composite approach under different incoming flow dynamic pressures. The effects of two environments with H = 0.1 km and H = 30 km, as well as the influence of different incoming Mach numbers on the drag reduction at H = 0.1 km were investigated. The research has shown that there exists a minimum overall drag value for the vehicle under different dynamic pressures, while the variation in the wall pressure is related to the actual operating conditions. The recirculation zones at the aerospike and in front of the blunt body are important flow structures that affect the drag reduction. When the jet total pressure ratio increases, the position of the recirculation zone at the head of the drag reduction rod moves away from the central axis of the model. When the incoming flow dynamic pressure is higher, the separation point of the recirculation zone in front of the blunt body is located further downstream, and the reattachment point is related to the incoming flow and jet conditions, aligning with the trend of variation of the position of the maximum pressure on the blunt body wall. As it can effectively push away the shock wave, the enhanced drag reduction effect of the jet is significant when the incoming flow dynamic pressure is higher. However, the high jet pressure resulting from high dynamic pressure needs to be considered in the actual design. Furthermore, the complex turbulent flow field formed by the high dynamic pressure incoming flow and the jet is worthy of further study by means of the large eddy simulation.

Aerodynamic development of the BYD SEAL – Part 2: Overview

BYD SEAL is a new developed A+ class sedan by BYD Auto, based on third generation high-performance and high-safety electric platform for EVs. SEAL gives full play to the advantages of electric vehicles in terms of space, handling, NVH, etc., while striving to reach the same level of fuel vehicles in terms of cost and recharge mileage. Therefore it has been dedicated to minimizing energy consumption from all components since its inception, and an important one is to reduce the aerodynamic drag effectively. However, the “ocean aesthetics” styling design based on the original concept model and platform with short overhangs must be kept on this car, which brings great challenge to aerodynamic development. An adaptable aerodynamic development process is set up and correlation between CFD simulation and wind tunnel test are established. This paper depicts the features of the flow field determined in the aerodynamic development process of the BYD SEAL, and describes the key factors in aerodynamic drag reduction as a full model change in detail. Effective rear roofline angle, tail design, front bumper corners, underbody, wheel, detail management such as sealing loss are studied and optimized by Powerflow and Star-CCM+ simulation softwares, and verified in wind tunnel tests of full-scale clay models and prototype vehicles. Design fusion and development tool/efficiency improvement around the vehicle are established, and the inherent drag reduction mechanism is revealed. These findings are of great significance to the aerodynamic optimization with diverse aesthetic and engineering requirements under the background of EV platformization.

Large-scale penetration model tests of bucket foundations of different structural types in clay

Clarifying penetration characteristics and accurately predicting penetration resistance of bucket foundations is crucial to ensure their smooth penetration. This study performed large-scale model tests on nearshore seven-compartment (SC) bucket foundations and novel deep-sea five-connected (FC) bucket foundations to systematically explore the penetration attitude, development of soil plugs, flow rates, bucket-soil interactions, and required suction of these two structures in clay. The good penetrability was identified for the new FC bucket foundation. The development of soil plugs due to bucket wall replacement as well as the mechanism of drag reduction of the inner wall and bucket end were revealed. The experiments demonstrated that the penetration flow rate was almost equal to the volume of bucket body entering the soil and accounted for approximately 95% of the total flow rate. A formula for calculating penetration resistance in clay that considers the effect of suction on the reduction in resistance of the inner wall and bucket end was proposed and validated by field tests. The research results improve the safety of bucket foundation penetration in clay and provide significant guidance for the installation of deep-sea FC bucket foundations.

Drag reduction performance of hydrophobic coatings with controllable wettability

Currently, an increasing number of studies focus on the applications of hydrophobic surfaces for underwater drag reduction. However, the drag reduction mechanisms of hydrophobic coatings with controllable wettability still lack systematic understanding. Herein, nano-SiO2 composite coatings with different contact angles were fabricated using air spraying techniques. The relationship between the hydrophobicity of these coatings and their drag reduction performance under different Reynolds numbers was further investigated. The results showed that the uniformed distributed air layer and low depinning force significantly contributed to drag reduction performance. The coating with a contact angle of 140° achieved a maximum reduction rate of 58 % at a Reynolds number of 750. However, the coatings with contact angles of 150° and 155° exhibited diminished drag reduction effects due to the uneven distribution of the air film. This study highlighted that the hydrophobicity of the coatings and the uniform microstructures are essential for sustaining effective drag reduction performance.

Design and Principles Analysis of Hydrofoil Appendages for Reducing Resistance of High-Speed Ships

To reduce the resistance of high-speed displacement ships with Froude numbers (Fr) between 0.4 and 0.5, this paper proposes the installation of hydrofoils at the bow and stern of the ship. Firstly, starting from the bow wave, this paper proposes the installation of a flat plate appendage at the free surface of the ship’s bow to suppress the height of the bow wave and thus reduce the hull resistance. Taking the DTMB 5415 ship model as the research object, CFD calculation results show that installing a flat plate appendage at the free surface of the ship’s bow can effectively suppress the height of the bow wave, and the total resistance reduction ratio can reach 6.49% when Fr = 0.45. Then, the flat plate appendage was improved to a hydrofoil appendage, further reducing the hull resistance. As a result, the total resistance reduction rate can reach 9.15% at Fr = 0.45. Following this, hydrofoil appendages were installed simultaneously at the bow and stern. The drag reduction effect and mechanism were studied, and the results show that the hydrofoils at the bow and stern have a good drag reduction effect. Suppressing the bow and stern waves and improving the flow field are the main reasons for the drag reduction. Finally, the drag reduction effect of the hydrofoil appendages was verified through experiments, demonstrating its excellent drag reduction effect when Fr = 0.4–0.5 and a maximum total resistance reduction ratio of 14.552%.

Numerical Simulation of Bionic Underwater Vehicle Morphology Drag Optimisation and Flow Field Noise Analysis

The study of aquatic organisms’ ectomorphology is important to understanding the mechanisms of efficient swimming and drag reduction in fish. The drag reduction mechanism in fish remains unknown yet is needed for optimising the efficiency of bionic fish. It is thus crucial to conduct drag tests and analyses. In this paper, an optimal dolphin morphological model is constructed taking the beakless porpoise as the research object. A numerical simulation of the dolphin body model is carried out for different combinations of pitch angle and speed adopting computational fluid dynamics, and the flow field noise of the dolphin body model is solved for different speeds using the FW-H equation. When the dolphin model is oriented horizontally, the differential pressure drag accounts for approximately 20–25% of the total drag as airspeed increases. As both the pitch angle and airspeed increase, the differential pressure drag and friction drag decrease with increasing airspeed. Moreover, the acoustic energy is mainly concentrated at low frequencies for both the dolphin and Bluefin-21 models. The dolphin body model has better noise performance than the Bluefin-21 model at the same speed. The optimisation of the external morphology of the bionic underwater submarine and the analysis of the shape drag are thus important for revealing the drag reduction mechanism, reducing noise in the flow field and provide guidance for research on bionic fish.

Solid–fluid phase transition characteristics of loess and its drag reduction mechanism

Solid–fluid phase transition characteristics of loess and its drag reduction mechanism

Flow-thermal coupled investigation on hypersonic spike-jet with channel

As for blunt-body hypersonic vehicles, the extreme drag and aero-heating pose great challenges to the conceptual design. Different from the conventional configurations, a blunt body slotted with a channel is proposed in this paper. The channel is further combined with a jet on the shoulder to reduce drag and aero-heating. Simultaneously, based on the position of the separation point and vortex edge, a modified effective body approach is developed to accurately investigate the drag reduction mechanism. Specifically, the more slender the modified effective body is, the more obvious the drag reduction is. The simulation results show that the spike-channel-jet can push the shear layer on the spike away from the wall. In this way, the position of shear reattachment point moves downstream, which can enlarge the recirculation zone and weaken the shock-shock interaction. Additionally, the reattachment shock is pushed far away from the blunt body, which is also weakened. For drag, net power is introduced to evaluate the practical value of spike-channel-jet for drag reduction. Compared with a simple spike (spike length L/D = 1) of the same size, the spike-channel-jet can reduce the total drag by 24.68 %, the peak pressure coefficient of the blunt body by 38.54 %, the peak temperature of the blunt body by 65.75 %, and the peak temperature of the spike by 10.63 %. The maximum net power can reach 13.30 kW. Considering the fluid-thermal interaction, the findings reveal the influence of the spike length and jet mass flow rate on aerodynamic performance, by modified effective body further analyze the drag reduction mechanism with the jet, and validate the different reasons for the peak temperature and pressure of the blunt body.

Investigation on drag reduction on rotating blade surfaces with microtextures.

To enhance the aerodynamic performance of aero engine blades, simulations and experiments regarding microtextures to reduce the flow loss on the blade surfaces were carried out. First, based on the axisymmetric characteristics of the impeller, a new simulation method was proposed to determine the aerodynamic parameters of the blade model through the comparison of flow field characteristics and simulation results. Second, the placement position and geometrical parameters (height, width, and spacing) of microtextures with lower energy loss were determined by our simulation of microtextures on the blade surface, and the drag reduction mechanism was analyzed. Triangular ribs with a height of 0.2 mm, a width of 0.3 mm, and a spacing of 0.2 mm exhibited the best drag reduction, reducing the energy loss coefficient and drag by 1.45% and 1.31% for a single blade, respectively. Finally, the blades with the optimal microtexture parameters were tested in the wind tunnel. The experimental results showed that the microtexture decreased energy loss by 3.7% for a single blade under 57° angle of attack and 136.24 m/s, which was favorable regarding the drag reduction performance of the impeller with 45 blades.

Drag Reduction Mechanism of Array Microslit-Opposed Jets in the Passivation Leading Edge of High-Speed Aircrafts

Drag Reduction Mechanism of Array Microslit-Opposed Jets in the Passivation Leading Edge of High-Speed Aircrafts