Maximum Drag Reduction Research Articles

Analysis of the Drag Reduction Performance and Rheological Properties of Drag-Reducing Additives.

In the practical application of hydraulic rotating machinery, it is essential to thoroughly explore drag reduction and rheological characteristics of drag-reducing additives to optimize machinery efficiency and reduce equipment consumption. This paper combines simulation and experimental approaches to investigate the drag-reduction performance and rheological properties of drag-reducing additives. Numerical simulations are initially conducted to investigate the shear-thinning properties of drag-reducing fluid and explore variations in drag-reduction rate. Turbulent phenomena characteristics are described by analyzing turbulent statistical quantities. Subsequently, the rheological behaviors of polyethylene oxide (PEO), cetyltrimethyl ammonium chloride (CTAC), and their mixed solutions under different conditions are scrutinized using a rotational rheometer. The findings indicate that the drag reduction effect amplifies as the rheological index n and characteristic time λ decrease. The numerical simulations show a maximum drag reduction rate of 20.18%. In rheological experiments, a three-stage viscosity variation is observed in single drag-reducing additives: shear thickening, shear thinning, and eventual stabilization. Composite drag-reducing additives significantly reduce the apparent viscosity at low shear rates, thereby strengthening the shear resistance of the system.

Achieving travelling wave drag reduction by micro piezoelectric actuator

In order to improve the endurance mileage and the sailing speed of micro underwater vehicles, it is critical to reduce the drag of underwater devices. However, the existing underwater drag reducers are not suitable for micro underwater device due to its large size and complex structure. Inspired by the adaptive deformation of fish to achieve traveling wave drag reduction when the flow field changes, a micro piezo-actuated traveling wave drag reducer (PTWDR, the size of 30 mm × 28 mm × 0.75 mm), which can be embedded into the wall of the micro vehicle and hardly increasing the volume and weight of the vehicle, is proposed in this work to realize active drag reduction. The mechanism of active disturbing the turbulent boundary layer by piezoelectric micro-amplitude traveling waves to achieve drag reduction has been revealed. The drag reduction performance control method, which can meet different drag reduction requirements, is given. The experimental results of frictional drag and total drag indicate that the designed micro PTWDR is feasible to reduce the drag. The maximum local drag reduction rate of 47 % can be achieved at resonance frequency, which can improve the maneuverability of micro vehicles. While when the low frequency is applied, it cannot only obtain a certain drag reduction rate in a large drag reduction area, but also realize net energy saving. This work provides a novel and effective theoretical and technical reference for realizing active drag reduction and net energy saving, which can be applied for micro underwater vehicles.

Experimental investigations on the effects of longitudinal groove fabric for drag reductions on a track cyclist

The previous study has shown that the longitudinal groove fabric can reduce drag forces on a circular cylinder by forcing a drag crisis (Zheng et al. 2021). In this study, the effects of the longitudinal groove fabric are investigated on a full-scale track cycling mannequin. The force measurement results show that the longitudinal groove fabric on the upper arms can achieve a maximum drag reduction of about 7% at a flow speed of 17 m/s, and its control effects depend on flow speeds. Large-scale particle image velocimetry measurements further show that the drag reductions on the upper arm are characterized by diminished streamwise velocity deficits. The control effects also vary on different spanwise locations of the arm, where the flow behaves distinctively. The measurements also reveal the distinct flow dynamics at different heights, i.e., wake interactions and swirling motions, showing the complexity of reducing drag forces from a track cyclist.

Drag reduction design and experiments for the chisel-shaped shovel tip

To address the issue of high resistance encountered by traditional chisel-shaped shovel tips during tillage, this study drew inspiration from the micro V-shaped structures found in shark skin. Using laser cladding technology, a V-shaped wear-resistant coating was applied to the front surface of the shovel, with different drag-reducing V-shaped structures achieved by controlling the coating overlap ratio H (including 20%, 40%, and 60%). Additionally, the rear surface of the shovel tip was designed to mimic the V-shaped morphology of shark skin, proportionally amplified, and given a certain backward tilt angle θ to further reduce resistance. Through the discrete element simulation experiments while maintaining θ at 0°, it was found that the shovel tip achieved the best drag reduction effect when H was 40%. Based on this, the study varied the values of θ (including 0°, 1°, 3°, and 5°) while keeping H at 40%. Discrete element simulation experiments were conducted at depths of 250mm, 275mm, and 300mm to analyze the disturbance effect, fragmentation effect, and resistance of the shovel tip. Considering all factors, the shovel tip with θ of 5° was selected as the optimal choice. Finally, a soil trench experiment was conducted to verify the performance of the V-shaped shovel tip with H of 40% and θ of 5°, as well as the chisel-shaped shovel tip, in tillage operations. The experimental results showed good agreement with the simulation results, and the designed V-shaped shovel tip achieved a maximum drag reduction of 12.87%. This design provides valuable references for the structural optimization of subsoiler, contributing to the improvement of their performance and efficiency.

Numerical simulations of the flow past a perforated plate enclosed by a filament using the Immersed Boundary Method

The influence of the central hole on dynamics of the coupled system including a perforated plate and a filament was numerically investigated using the immersed boundary method (IBM). Dynamics of the system were first investigated across various length ratios of filament to plate Lr and hole sizes Dh at the Reynolds number Re=100. When Dh≤0.1, the IBM was proven to be unusable due to the area leakage brought about by the smoothed approximation of the Dirac delta function. When Dh>0.1, the calculation results were reliable and showed that the key values were not significantly affected by Dh. Compared to a bare plate, the system exhibits a maximum drag reduction of about 30% at Lr=2.0, regardless of Dh. In contrast to a closed system, a hole facilitated a more harmonious filament flap by allowing fluid exchange, thereby preventing local oscillation of the filament. Additionally, the multiple frequency dominant (MFD) mode was observed and its distribution in Lr-Dh space was also presented. Finally, the simulations under various Re were conducted. The effect of Dh on the system drag was still weak regardless of Re. However, the filament motion changed from periodic to chaotic as Re increased.

Numerical Analysis of the Drag Reduction Performance of a Double M-Ship Boat With Stepped Planning-Air Coupling

Abstract In this paper, analyze the influence of the stepped planning structure on the drag performance by observing waveform diagrams at the stern of the double M-ship and water–air and pressure distribution diagrams at the bottom of the ship. This study uses the combined stepped planning-air drag reduction technology to improve the sailing characteristics of the double M-ship. Research findings: The stepped planning contributes to a reduction in bottom pressure, enhances water–air distribution, and augments the amplitude of hull movement. Within the design speed range, the maximum drag reduction rate achieved by the stepped planning is 7.574%. However, this enhancement comes at the expense of increased viscous pressure resistance, which becomes the predominant resistance when sailing at full speed; Injecting air at the stepped planning can effectively reduce the viscous pressure resistance increased by the stepped planning. The combined drag reduction technology of stepped planning and air successfully realizes the total drag reduction at the double-M ship's high speed. The total resistance experienced when air is injected at the stepped planning is reduced by up to 20.981% compared to the original hull.

Drag reduction of the turbulent boundary layer over divergent sawtooth riblets surface with a superhydrophobic coat

Energy conservation and environmental protection have become pivotal components of the green economy. In recent years, underwater drag reduction technology has garnered significant interest. This study discusses a novel composite surface that combines divergent riblets with a superhydrophobic coating (D-rib&SHS) to enhance the drag reduction rate. Alongside this new surface, a riblet surface with a superhydrophobic coating (rib&SHS, without yaw angles) and a smooth surface are used as comparison groups. The turbulent boundary layer flow of these three surfaces is measured using a two-dimensional particle image velocimetry system. The results indicated that the maximum drag reduction rate of D-rib&SHS is approximately 27% higher than that of rib&SHS, and the drag reduction range is increased to Reθ≈4100 compared to rib&SHS (Reθ≈2200). Using correlation algorithms, it observed that the spacing between low-speed streaks over D-rib&SHS is larger than that of rib&SHS and the smooth surface. This finding suggested that the spacing between the hairpin vortex legs of D-rib&SHS is wider. The increased spacing between the hairpin vortex legs reduces the likelihood of vortex head formation between the two quasi-streamwise vortices, ultimately suppressing the auto-generation of hairpin vortices. Consequently, the development of hairpin vortex packets over D-rib&SHS is also inhibited. These phenomena observed over D-rib&SHS can be attributed to the combined effects of velocity slip on the superhydrophobic coating and the secondary flow over the divergent riblets near the wall. In addition, unlike divergent riblets that are not covered with a superhydrophobic coating, where the drag reduction effect is more pronounced with a yaw angle of 30° compared to 10°, D-rib&SHS with a 10° yaw angle demonstrated a superior drag reduction effect compared to D-rib&SHS with a 30° yaw angle. This innovative composite surface enhances the drag reduction effect and expands the drag reduction Reynolds number range, offering a new approach to mitigating drag in turbulent boundary layer flows.

Drag reduction in a spanwise-infinite boat-tailed body through multiple transverse grooves

Appropriately-designed spanwise-extruded small grooves, oriented transverse to the freestream flow, may delay flow separation in adverse pressure gradients by generating local and stable recirculation zones. In this study, we explore the potential of multiple grooves to reduce the drag on a boat-tailed bluff body characterized by vortex shedding. Numerical simulations reveal that the presence of two consecutive transverse grooves results in a maximum boat-tail drag reduction of 23.2%, further decreasing the drag by 15% when compared to the performance of the same boat-tailed bluff body with a single groove. This flow-control mechanism proves effective when the flow reattachment takes place downstream of both grooves throughout the entire vortex-shedding cycle, leading to the formation of two distinct local recirculation zones within the grooves. Lower frictional losses are observed along the streamline bounding the recirculation within the grooves as compared to the friction losses caused by the no-slip boundary conditions along the lateral walls of a plain boat tail (i.e., without grooves). Consequently, the boundary layer exhibits higher momentum downstream of the grooves, resulting in a delayed flow separation and an improved reduction in body drag for the boat tail.

An experimental investigation and flow-system simulation about the influencing of silica–magnesium oxide nano-mixture on enhancing the rheological properties of Iraqi crude oil

This study aims to investigate the effects of introducing a 50/50 mixture of silica and magnesium oxide nanoparticles (SNP + MgONP) to the viscosity of Al-Ahdab crude oil (Iraq) at varied concentrations and temperatures. It is observed that the viscosity value drops from 38.49 to 7.8 cP. The highest degree of viscosity reduction is measured to be 56.91% at the maximum temperature of 50 °C and the greatest concentration of 0.4 wt% SM4. The Bingham model can be used to classify the behavior of the crude oil before the Nano-mixture is added. The liquid behavior grew closer to Newtonian behavior once the Nano-mixture was added. Along with a rise in plastic and effective viscosity values, the yield stress value decreases as the concentration of the Nano-mixture increases. The numerical data demonstrate that when the volume proportion of nanoparticles increases, the pressure distribution decreases. Furthermore, as the nanoparticle volume fraction increases, the drag decrease would also increase. SM4 obtains a maximum drag reduction of 53.17%. It is discovered that the sample SM4 has a maximum flow rate increase of 2.408%. Because they reduce the viscosity of crude oil, nanoparticles also reduce the friction factor ratio.

Experimental Study on the Performance of Frictional Drag Reducer with Low Gravity Solids

Reducing energy consumption during drilling operations is beneficial to both the environment and economy. Frictional drag reducers (FDR) are widely used to reduce the energy loss caused by turbulent flow. FDR plays an important role in flow lines as they can reduce the frictional pressure drop effectively, and benefit the selection of circulating fluid and pump. However, several factors can influence the performance of FDR, including fluid additives and incorporated solids, such as drill solids. Thus, the main objective of this paper is to study the influence of low gravity solids (LGS) on the performance of the FDR. This paper is mainly based on experimental study. The experimental work contains two parts: rheology characterization and flow loop tests. Rheology characterization tests were performed to calculate the flow consistency index (K) and flow behavior index (n). Flow loop experiments were conducted for two geometry (0.457 inch and 0.797 inch diameter). Xanthan gum was used as a fractional drag reducer. Bentonite and quartz sand were added as low gravity solids. Three designed water-based mud systems are tested for drag reduction efficiency of Xanthan gum. Flow rate of the mud varied from 3 gpm to 16 gpm. Concentration of Xanthan ranged from 0.1 lbm/bbl to 0.6 lbm/bbl. Low weight solids were added with weight percentage of 0.5%, 1%, 2% and 2.5%. The result shows that xanthan gum is an efficient drag reducer for adequate reasons. Firstly, even at al low concentration, xanthan gum shows high resistance to degradation. Secondly, the maximum drag reduction with xanthan gum is up to 70.54% with a concentration of 0.6 lbm/bbl. However, the existence of different low gravity solids influence the efficiency of xanthan gum in different styles. Experiment results indicate that the higher the weight percentage of bentonite, the lower the drag reduction effectiveness. While with the increasing concentration of quartz sand, the drag reduction does not show an intense change. This study intents to give an instructive guidance on usage of frictional drag reducers in drilling mud system design. Removal of low gravity solids from the mud is difficult, which pose a danger to the drilling fluid. By understanding the effectiveness of FDR, we can reduce energy consumption when irremovable low gravity solids exist. FDR can be used for modifying the mud contents to develop a lower pressure gradient under turbulent flow condition. In the same scenario, adding FDR can suppress turbulent at a constant pressure gradient but with a higher flow rate.

Turbulent drag reduction in compressible flows using streamwise traveling waves

Skin-friction drag reduction (DR) in supersonic turbulent channel flows using streamwise traveling waves of spanwise velocity (STWSV) has been studied using direct numerical simulations. In the present study, simulations are carried out for different phase speeds of the upstream and downstream traveling waves for a bulk Mach number Mab=1.5 and bulk Reynolds number Reb = 3000. The efficacy of the control has been investigated for other bulk Reynolds numbers as well. The Stokes layer generated by the control affects the near-wall statistics primarily by modifying the coherent structures, thereby either reducing or aggravating the skin-friction drag. For Reb = 3000 and Mab=1.5, a maximum drag reduction (DRmax) of 49% is achieved, primarily due to suppression of sweeps and ejections on application of control. The efficacy of skin-friction drag reduction varies in a small range when Reb is varied from 3000 to 6000; however, the maximum drag reduction is observed for Reb = 4000, owing to a drastic decrease in sweep events. Quadrant analysis reveals that for drag reduction (DR) cases, STWSV reduce magnitude of streamwise u′ and wall-normal w′ velocity fluctuations magnitude and stacks them around zero. In contrast, w′ is increased for drag increase (DI) cases. Skin-friction decomposition using Fukagata, Iwamoto and Kasagi (FIK) identity suggests that the control alters the turbulent component significantly. The behavior of the control is also studied at Mab = 0.3 and 2.5, and it is observed that the maximum drag reduction obtained decreases for high bulk Mach numbers.

Spatial discretization effects in spanwise forcing for turbulent drag reduction

Wall-based spanwise forcing has been experimentally used with success by Auteri et al. (Phys. Fluids, vol. 22, 2010, 115103) to obtain large reductions of turbulent skin-friction drag and considerable energy savings in a pipe flow. The spatial distribution of the azimuthal wall velocity used in the experiment was not continuous, but piecewise constant. The present study is a numerical replica of the experiment, based on a set of direct numerical simulations (DNS); its goal is the identification of the effects of spatially discrete forcing, as opposed to the idealized sinusoidal forcing considered in the majority of numerical studies. Regardless of the discretization, with DNS the maximum drag reduction is found to be larger: the flow easily reaches complete relaminarization, whereas the experiment was capped at 33 % drag reduction. However, the key result stems from the observation that, for the piecewise-constant forcing, the apparent irregularities of the experimental data appear in the simulation data too. They derive from the rich harmonic content of the discontinuous travelling wave, which alters the drag reduction of the sinusoidal forcing. A detailed understanding of the contribution of each harmonic reveals that, whenever for example technological limitations constrain one to work far from the optimal forcing parameters, a discrete forcing may perform very differently from the corresponding ideal sinusoid, and in principle can outperform it. However, care should be exercised in comparison, as discrete and continuous forcing have different energy requirements.

Performance enhancement of a Box-Wing airliner with the application of riblets

The rapid growth of the commercial aviation sector in recent years, as well as the ambitious emission reduction targets, necessitate the investigation of novel methods to improve the aerodynamic efficiency of future airliners. With increasing passenger demand and evolving industry requirements, innovative designs, like the Box-Wing aircraft configuration, and flow control techniques, such as riblets, are essential to enhance efficiency, reduce fuel consumption and emissions, and meet future aviation needs. In this work, the performance enhancement of a novel Box-Wing airliner with the application of riblets is investigated through CFD modeling. The riblets are small, streamwise grooves aligned with the airflow, which when applied correctly, can reduce the turbulent skin friction drag. The riblets installed on the aircraft are modeled through a dedicated surrogate model, based on the cross-section area of their groove. In this study, both optimal size riblets, as well as constant size riblets, have been examined, assessing the performance degradation associated with the practical application limitations. The results show that the riblets can improve the aircraft’s aerodynamic characteristics, with a maximum drag reduction of 60 drag counts, as well as the overall flight performance, providing a maximum increase of 6.4% in payload and 13.3% in range.

Macroscopic and stable gas film obtained by superhydrophobic step and its drag reduction performance

Drag reduction technology has a promising application in marine fields and has drawn much interest in scientific fields. Superhydrophobic surface has been proven to be effective in drag reduction due to thin film of gas adsorbed on surface because of its low friction drag and large slip length. Here, macroscopic and stable gas film was observed when water flowed over superhydrophobic surface with step without additional gas injection under laminar flow and turbulent flow. Superhydrophobic surface was prepared with contact angle more than 150° and roll-off angle nearly 0°. Macroscopic gas film could maintain under laminar flow and turbulent flow and keep up to 80% after 1 h water flowing with optimized parameters of step, showing different morphological deformations under different velocities and Reynolds numbers. Compared with untreated hydrophilic surface, superhydrophobic surface with step exhibited good drag reduction performance with maximum drag reduction rate 20% under laminar flow and turbulent flow, after optimizing of height of step and distance between steps. Mechanisms of gas film drag reduction were the ultra-low skin friction drag force between liquid–gas interface, large slip length on liquid–gas interface, and flexible gas film surface acted like compliant wall. Gas film of millimeter scale was much larger than thickness of boundary layer and reduced turbulence intensity near wall. This work provides a new way to obtain macroscopic gas film and analyze liquid–gas interface.

Silicon gels with sustainable self-replenishment: Fluid drag reduction through viscosity dependency

Slippery liquid infused surfaces (SLISs) provide excellent dynamic lubrication properties by storing the lubricant internally to form a homogeneous liquid layer to insulate external ambient fluids. However, the kinetic instability caused by lubricant depletion gradually degrades the surface slippery performance. Herein, inspired by skin gland secretion system, silicon gels with sustainable self-replenishment were prepared using polydimethylsiloxane (PDMS) and commercially available dimethyl silicone oils. By carefully tuning the kinematic viscosity and content of infused oil, the surfaces of corresponding silicon gels demonstrated significant syneresis behavior, and the lubricating oils released from the surface were still detected after continuous wiping for more than 120 days. More importantly, when the ratio of working fluid to infused oil was 6.8, the maximum drag reduction of silicon gel in Couette flow could reach up to about 16%, while the surfaces remained long-lasting water repellency under the continuous shear action of encapsulated water. Self-replenishable silicon gels exhibiting strong kinetic stability were expected to provide valuable design insights for robust slippery interfaces.

An analysis of drag reduction using spanwise forcing on rough walls

Spanwise opposed wall-jet forcing has been shown to reduce the skin-friction drag of wall-bounded turbulent flows by suppressing the near-wall turbulent motion (Yao et al., 2018). In the present work, the response of this drag reduction mechanism to the presence of surface roughness is studied. To this end, direct numerical simulations of flow in smooth and rough plane channels at a matched friction Reynolds number (Reτ=180) are carried out. The roughness is generated by distributing discrete roughness elements at two different values of frontal solidity, namely 0.021 and 0.045. The roughness elements height is k+=18 (＋indicates viscous scaling). By varying the forcing amplitude A+, it is shown that, when other forcing parameters are fixed, maximum drag reduction in a rough channel is achieved at a considerably larger A+ than in a smooth channel. Notably, the strength of the wall-jet at which the maximum drag reduction is achieved, is comparable for the smooth and both rough cases. Additionally, it is observed that the maximum drag reduction values attained in rough channels are smaller compared to those observed in smooth channels, for both lower and higher frontal solidity cases — 2.4% and 2%, respectively. The reduced drag reduction potential originates from two observations; firstly, drag on roughness elements, which is dominated by pressure drag, is not reduced by the method in question. Secondly, the drag on channel floor is reduced to a lesser extent when roughness elements are present. The latter can be attributed to the observation that, while in all cases the forcing suppresses streamwise random turbulent fluctuations and shear stress, this suppression is less pronounced when roughness elements are present.

Bayesian-Optimized Riblet Surface Design for Turbulent Drag Reduction via Design-by-Morphing With Large Eddy Simulation

Abstract A computational approach is presented for optimizing new riblet surface designs in turbulent channel flow for drag reduction, utilizing design-by-morphing (DbM), large Eddy simulation (LES), and Bayesian optimization (BO). The design space is generated using DbM to include a variety of novel riblet surface designs, which are then evaluated using LES to determine their drag-reducing capabilities. The riblet surface geometry and configuration are optimized for maximum drag reduction using the mixed-variable Bayesian optimization (MixMOBO) algorithm. A total of 125 optimization epochs are carried out, resulting in the identification of three optimal riblet surface designs that are comparable to or better than the reference drag reduction rate of 8%. The Bayesian-optimized designs commonly suggest riblet sizes of around 15 wall units, relatively large spacing compared to conventional designs, and spiky tips with notches for the riblets. Our overall optimization process is conducted within a reasonable physical time frame with up to 12-core parallel computing and can be practical for fluid engineering optimization problems that require high-fidelity computational design before materialization.

Experimental Study on Two-Phase (Solid-Liquid) Flows of Ground Wheat Straw in Inclined Pipes

Highlights Knife milled wheat straw-water slurry flow through inclined pipes at various inclinations is experimentally studied. Slurry concentration and pipe inclination affect the onset velocity of drag reduction and the percentage of drag reduction. The critical mass concentration of maximum drag reduction is a vital function of velocity and is independent of the pipe angle. Downhill wheat straw slurry flows exhibit the most significant reduction in drag and the smallest onset velocities. Wheat straw slurry flows moving uphill exhibit nonmonotonic variations in onset velocity and drag reduction. Abstract. Long-distance pipeline hydro-transport of lignocellulosic biomass for industrial-scale biofuel production at levels comparable to conventional oil refineries presents an economically and logistically viable alternative to fossil fuels. There is very limited understanding on the behavior of the transportation of agricultural biomass slurries in an inclined pipeline. This research is focused on a laboratory-scale investigation of 6.4 mm nominal particle length (d50 = 4.85 mm) knife-milled wheat straw-water suspensions' uphill and downhill flows for a range of pipe inclination and saturated mass concentrations. The range of pipe inclination and saturated mass concentration was -7° to +21° and 5%-30%, respectively. The inclined test section was 29 m long with a 50 mm inside diameter of a closed pipeline loop. The accuracy of the measurements was verified by calibrating the inclined pipe section with fine sand (d50 = 0.103 mm)-aqueous slurries and comparing the results with established correlations. Most wheat straw-aqueous suspensions in the inclined flows showed the characteristics of the plug flow and the transition flow regions together for saturated mass concentration, Cm = 5%-30% and the entire flow range (0.5-4.7 m s-1), with a clear dependence of both the onset velocity of drag reduction (vOD) and drag reduction (%DR) on the pipe inclination as well as the slurry concentration and the critical concentration of maximum drag reduction (Cm)cr on a specific range of suspension velocity. Because of the accelerating effect of gravity, downhill slurry flows had the lowest vOD and the highest %DR at every Cm and pipe inclination with a maximum drag reduction of 25.53% at vm = 4.5 m s-1 and Cm = 25%. Uphill flows demonstrated some nonmonotonic changes in vOD and %DR, which need more experimental data for us to reach a firm conclusion. The research outcomes could help design and operate a long-distance integrated pipeline network for biomass transportation to produce biofuels on a large scale. Keywords: Biomass transport, Drag reduction, Flow regions, Frictional pressure drop, Pipe inclination, Wheat straw slurry.

Active control for a low-drag Ahmed vehicle model based on steady blowing

An experimental investigation is conducted on the active drag reduction (DR) of an Ahmed body with a rear slant angle of 35°, corresponding to the low-drag regime, using single and combined actuations at the Reynolds number Re = 1.7 × 105. Five different actuations, produced by steady blowing jets, are deployed independently around the edges of the rear slant surface and vertical base, achieving the maximum DR of 1%–9%. An artificial intelligence control system based on ant colony algorithm is used for finding near-optimal control laws of the combined jets. With both DR and control power input considered in the cost function, the maximum DR obtained reaches 18%, though the corresponding control efficiency η (≡ES/EI, where ES and EI are the saved power due to DR and the total input power of the actuations, respectively) is only 0.13. However, η may go up greatly, climbing to 5.8, given a 3% sacrifice of DR. Extensive flow measurements are conducted, with and without control, to understand the flow physics and mechanisms under the control of individual and combined actuations. A linear regression model is established to describe the correlation between the control efficiency and parameters under the combined actuations.

Numerical simulation study on the drag reduction characteristics of grooves-microbubbles coupling surfaces

Surface drag reduction technologies can significantly reduce the resistance during ship navigation, enhancing speed, efficiency and adaptability under various operating conditions. This paper uses numerical simulation technology to analyze the drag reduction characteristics of grooved and grooves-microbubbles coupling surface, focusing on the effects of groove width, gas flow rate, and liquid flow velocity on the drag reduction performance. The research results indicate that the grooved surface is suitable for full surface drag reduction at velocity below 3 m/s with a maximum drag reduction rate of 4.02%. Microbubbles can greatly improve the drag reduction effect of the grooved surface, and the drag reduction effect of the coupling surface gradually increases with the gas flow rate increases. The maximum drag reduction rate can reach 89.86% at the gas inlet velocity of 1 m/s. The liquid flow velocity has a significant impact on the drag reduction. In both the groove model and the coupling model, the drag reduction rate initially rises and then declines with the liquid flow velocity increases.