Drag Reduction Research Articles

Review on flow separation control: effects of excitation frequency and momentum coefficient

This paper presents a comprehensive review of the studies and research conducted on flow separation control on lifting surfaces. In this paper, two critical parameters, namely, the momentum coefficient and excitation frequency, that significantly impact flow separation control are analyzed in detail. Through a comprehensive literature review, experimental and numerical studies are examined in order to gain insight into the underlying mechanisms of momentum injection and excitation frequency on the shear layer and wake dynamics and to quantify the impact of these parameters on the flow separation control. Effective flow control on lifting surfaces can modify the streamlines and pressure distribution, thereby increasing their aerodynamic efficiency. This paper focuses on the control of flow separation on airfoils, with particular attention paid to the benefits of such control, including lift enhancement, drag reduction, aerodynamic efficiency enhancement, performance enhancement, and other important features. This paper presents a review of studies that have employed blowing actuators, as well as zero net mass flux, plasma, and acoustic actuators, in order to provide an appropriate historical context for recent developments. The findings of this review paper will contribute to a better understanding of the optimal conditions for efficient flow separation control on lifting surfaces using unsteady excitation, which can have significant implications for improving the performance and efficiency of various aerodynamic applications. This paper aims to elucidate and emphasize the positive and negative aspects of existing research, while also suggesting new interesting areas for future investigation.

Viscosity Reduction and Drag Reduction Performance Analysis of Bionic Excavator Buckets Based on Discrete Element Method

With the aiming of solving problems with the existing ordinary excavator buckets used in the process of operations (such as heavy digging resistance, ease of adhesion, and others), seven types of bionic buckets and a prototype bucket were designed, based on the contractile-state curve of the earthworm head and the contour curve of the pangolin claw toe. The digging processes of the buckets were simulated using the discrete element method. The results show that, compared with the prototype buckets, all seven types of bionic buckets have significant drag reduction effects at the same digging depth, and the drag reduction effects increase with the decrease of digging speed. Among them, the composite bionic bucket-3 has the highest drag reduction rate, of 14.469% when the digging speed is 2 rad/s. At the same digging speed, different buckets disturb the soil particles to different degrees, and the bionic buckets disturb the soil more significantly compared with the prototype buckets. By conducting contact force field analysis for the buckets, it was shown that the bionic corrugated structure brings the bucket surface into incomplete contact with the soil particles, where the contact is on small areas or even on points, so that the relative velocity between the soil and the shovel body increases under the same driving force, which reduces the excavation resistance. This study provides a theoretical and design basis.

Experimental study on heat transfer and resistance of round tube with tube sheet under ultrasonic action

AbstractEnergy is vital to the survival and development of human beings. In the era of ‘carbon neutrality’, all walks of life around the world are upgrading their technological capabilities to reduce carbon emissions. As a new type of active strengthening technology to improve the comprehensive performance of heat exchangers, ultrasonic technology has attracted the attention of more and more scholars. From the actual installation situation, a set of ultrasonic enhanced heat transfer test device with tube sheet round tube was built. After experimental research and data analysis, the influence of ultrasonic sound intensity, frequency, and position on heat transfer, flow resistance, and comprehensive performance of heat exchange tube under different working conditions and without ultrasonic wave is studied. The results show that the ultrasonic enhanced heat transfer and drag reduction capacity increases with the decrease of ultrasonic frequency, and under the action of ultrasonic waves of different frequencies, the critical sound intensity of its enhanced heat transfer and drag reduction performance is different. The drag reduction performance of the ultrasonic heat exchange tube is increased by 18.41%, the heat transfer performance is increased by 36.53%, and the overall performance is increased by 20.57%.

Optimizing aerodynamics: Numerical flow analysis for drag and lift reduction in SUV designs

Optimizing aerodynamics: Numerical flow analysis for drag and lift reduction in SUV designs

Preparation of adaptive bifunctional reconfigurable polymers and their sand carrying and drag reduction behaviour

AbstractIn order to solve the problem of drag reduction at the front end of shale fractures and sand carrying at the tail end with increased viscosity, the molecular dynamics simulation (MD) method was used to design polymer molecules and simulate the steric resistance, interaction potential energy, mean square displacement, and radial distribution function of the polymer. The polymer AM‐AMPS‐LMA‐DiC12AM (ASLC12) with better solubility, diffusion, and resistance reduction potential was obtained and synthesized. By scanning electron microscope (SEM) and viscoelastic analysis, ASLC12 has a stable mesh structure, good viscoelasticity, and shear resistance, and the mesh structure formed by it is in a dynamic equilibrium state of fracture‐reorganization under shear. We then analyzed the drag reduction, sand carrying, and salt resistance of ASLC12. When the concentration of ASLC12 is 0.09%, the sand‐carrying requirement is satisfied. When the concentration is 0.05%, the drag reduction rate can reach 74.1%, and the resistance reduction rate of ASLC12 in salt ion solution can still reach more than 62%. This shows that the polymer ASLC12 has better sand carrying, drag reduction, and salt resistance.

Modelling of filtered drag force for clustered particle-laden flows based on interface-resolved simulation data

Interface-resolved direct numerical simulations (DNS) of clustered settling suspensions in a periodic domain are performed to study the filtered drag force for clustered particle-laden flows. Our results show that, for the homogeneous system, the filtered drag is independent of the filter size, whereas for the clustered particle-laden flows, the averaged drag becomes smaller than the homogeneous drag at the filter size above 4 particle diameters. The drag reduction saturates at the filter size being comparable to the cluster size in the horizontal direction in our simulations. A new correlation is proposed to account for the mesoscale effect on the filtered drag force by using drift velocity and variance of the solid volume fraction, based on the modification of existing subgrid drag models for the inhomogeneous system. The existing models for the drift velocity and the variance of the solid volume fraction are assessed using our DNS data. A new model for the drift velocity and the variance of the solid volume fraction is proposed, based on the combination and modification of the previous models. All mesoscale models considered can predict well the filtered drag with comparable accuracy, and are superior to the homogeneous drag model for the clustered system. Our models with the same parameter values obtained from the large-scale system can also predict well the filtered drag for smaller computational domain sizes.

Location and scales of drag reduction in turbulent pipe flow with wall oscillations at low Reynolds number

Location and scales of drag reduction in turbulent pipe flow with wall oscillations at low Reynolds number

Factors Controlling Superrotation in a Terrestrial Aquaplanet

Abstract Extending previous work with a dry model, this study investigates the sensitivity of superrotation to the location/strength of baroclinic eddies in an idealized moist aquaplanet GCM with terrestrial rotation rate and planetary radius. A suite of fixed-SST experiments is performed in which the extratropical SST gradient is flattened poleward of some specified latitude. Consistent with the dry simulations, transition to superrotation is found as this reference latitude moves near the subtropics. The superrotation is dependent on the equatorial acceleration due to interactions between equatorial Kelvin waves and subtropical Rossby waves, but is strongly enhanced by a reduction in drag by the baroclinic eddies on the subtropical upper troposphere. The reduction in the extratropical drag and the strength of superrotation depend on the strength and structure of the Hadley cell, and hence on convective closure. The transition to strong superrotation is aided by a positive feedback that cannot occur when a strong Hadley cell drag limits the equatorial vertical shear and upper-troposphere equatorial westerlies.

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.

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.

Impact of annular nanosecond plasma actuators on drag reduction in transonic flow

During the last few decades, plasma actuators have emerged as promising devices for aerodynamic flow control. This study focuses on the use of nanosecond plasma actuators for such purposes. A thermal phenomenological model is employed to simulate the effects of these actuators. The propagation of shock waves and their interactions for two specific geometries of plasma actuators, linear and annular plasma synthetic jet actuators, are examined here. A comparative analysis of the performance of these two configurations is presented. Furthermore, the geometric characteristics and temperature model are analyzed to provide insights that can be applied to practical problems. The influence of the actuators on a projectile in the transonic flow is also investigated. The results of the present study show that actuators placed in the conical and cylindrical regions of the object do not contribute to drag reduction. Conversely, actuators positioned at the boat-tail and base of the object effectively reduce drag. This drag reduction is primarily attributed to thermal disturbances in the separation area. Additionally, it is observed that the effects of shock waves and their interaction with stationary waves around the projectile are negligible in terms of drag force reduction.

Study on characteristics and prediction of the pressure drag of the swept strut under supersonic wide-range conditions

In this paper, the numerical simulation of the swept strut ramjet combustor was carried out under wide-range conditions (Ma3 = 1.8 ∼ 5.0), and the pressure drag characteristics of the swept strut were discussed. The results show that the pressure drag characteristics of the swept strut are related to the Mach number of the combustor inlet and the swept angle of the strut. The decreased boundary of the strut pressure drag coefficient gradually advances with the decrease of the Mach number of the combustor inlet. When Ma3 = 1.8 ∼ 2.0, the pressure drag reduction boundary is α = 15°. When Ma3 = 2.2 ∼ 2.8, the pressure drag reduction boundary is α = 45°. When Ma3 = 3.0 ∼ 4.0, the pressure drag reduction boundary is α = 60°. When Ma3 = 5.0, the pressure drag reduction boundary is α = 65°. In addition, with the decrease of the Mach number of the combustor inlet, the pressure drag reduction performance benefit brought by increasing the swept angle of the strut will gradually increase. Furthermore, a pressure drag coefficient prediction model suitable for wide-range conditions and multiple configurations of swept struts was proposed based on deep learning. The prediction model consists of two parts in series, which includes the prediction model of the surface pressure coefficient of the swept strut based on multilayer perceptron (MLP) and the prediction model of the pressure drag coefficient of the swept strut based on convolutional neural network (CNN). To improve the prediction accuracy of the MLP model, new training samples were added based on the ensemble-based uncertainty quantification, and the improved MLP model was obtained by retraining. The results show that both the two prediction models have high prediction accuracy under the effect of multiple complex flow characteristics on the strut. The results of this study are helpful to provide a reference for the aerodynamic drag reduction design of the strut in the wide-speed supersonic combustor.

A bio-inspired liquid-like smooth copolymer coating with superior biofouling resistance and drag reduction

A bio-inspired liquid-like smooth copolymer coating with superior biofouling resistance and drag reduction

Experimental investigation on transitional flow of supercritical hydrocarbon fuel within straight channels of Printed Circuit Heat Exchangers for hypersonic thermal management systems

The onboard supercritical CO2 closed Brayton cycle (SCBC) has established an effective thermal management pathway for hypersonic vehicles operating at high Mach numbers. A Printed Circuit Heat Exchanger (PCHE) is employed within the cycle system to facilitate heat transfer between fuel and carbon dioxide, with its thermo-hydraulic characteristics significantly impacting both power generation and propulsion performance. Given the insufficient research on the flow and heat transfer of hydrocarbon fuels in PCHEs, this study aims to experimentally address the gaps in understanding transition flow for supercritical hydrocarbon fuels, particularly within semi-circular microchannels. We conducted experiments on the flow transitions using a specially designed test piece with a diameter of 1 mm, replicating the PCHE channel characteristics. The experimental Reynolds number range from 300 to 8300 spans laminar, transition, and turbulent flow regimes. Friction factors for n-decane are obtained under adiabatic and heated conditions. The results validate the nomenclature of the quasi-turbulent region and reveal that the critical Reynolds numbers for the transitional, quasi-turbulent and hydraulic smooth turbulent regions under adiabatic conditions are 2200, 2440 and 2980, respectively. A reduction in friction drag attributed to boundary layer viscosity is observed to alter the friction factor under heating conditions, while heating influences both the onset and termination of the transition region, but does not affect its width. Classical correlations are compared with the experimental results, and new empirical correlations for both adiabatic and non-adiabatic conditions are developed, showing superior predictive accuracy compared to existing models. This provides a reliable tool for the engineering design of heat exchangers in the onboard thermal manage systems.

Drag Reduction of Truck and Trailer Combination with Different Passive Flow Control Methods

In this study, drag force and surface pressure measurements were conducted on a 1/32 scaled truck-trailer combination model. The experimental tests were carried out between the ranges of 312×103- 844×103 Reynolds Numbers in a suction type wind tunnel. The aerodynamic drag coefficient (CD) and distribution of pressure coefficient (CP) were experimentally determined on the truck and trailer combination. The regions where has big pressure coefficients were determined on the truck-trailer by using flow visualizations. The aerodynamic structure of truck-trailer combination models was improved by passive flow control methods on 4 different models. By using newly designed spoiler on the model 1, drag coefficient was reduced 10.01 %. On the model 2, adding trailer rear extension with a spoiler, the reduction was obtained as 11.35 %. For the model 3 which is obtained adding side skirt to model 2, the improvement reached 18.85 %. The model 4 was composed of model 2 and a bellow between the truck and trailer. The drag force improvement was obtained as 22.80 % for model 4.

Dynamics and heat transfer characteristics on tube side and shell side of micro-tube compact air-to-air heat exchanger

Applied to the design and selection of compact micro-tube heat exchangers for aviation, an experimental study on the flow and heat transfer performance on both the tube side and shell side of micro-tube heat exchangers was conducted based on the Wilson plot method. The results demonstrated that, although the fluid on the tube side entered the hydraulically smooth region earlier at Reh = 3000, the enhancement in the heat transfer performance and drag reduction characteristics was primarily manifested on the shell side. Compared with traditional empirical correlations, the shell-side Nusselt number showed a maximum enhancement of 47.99 %, and the flow resistance was reduced by a maximum of 59.84 %. Furthermore, a new evaluation index was employed to verify the performance of the heat exchanger. The results indicated that the heat exchanger exhibited advantages in designs requiring low flow resistance, compactness, and lightweight compactness.

Optimizing Bionic Blades for Multi-blade Centrifugal Fans: Asymmetric Thickness Inspired by Carangiform Fish

Multi-blade centrifugal fans find wide application across various fields, with their internal airflow exhibiting complex turbulent behavior in three dimensions. Historically, blade optimization relied on constant thickness airfoils, limiting the effectiveness of optimization efforts. However, marine organisms have developed airfoil structures with highly efficient drag reduction, offering a novel approach to optimizing multi-blade centrifugal fans. This study proposes an airfoil optimization design method utilizing variable-thickness airfoils to maintain consistent pressure surface leading-edge parameters. By integrating the Non-dominated Sorting Genetic Algorithm II with biomimetic optimization design, performance improvements are achieved. The study constructs an asymmetric bionic blade using a Bezier curve to fit the mean camber line of the blade. Experimental testing validates the optimized fan's performance, demonstrating the effectiveness of the proposed design approach in reducing the unsteady interaction between the impeller and the volute tongue. This reduction significantly diminishes sound pressure fluctuations on the blade surface. Notably, at the maximum volume flow rate, the optimized fan featuring the asymmetric bionic blade exhibits a remarkable enhancement, with a 10.5% increase in volume flow rate and a notable 1.7 dB reduction in noise compared to the original fan configuration.

The Impact of a Non-Uniform and Transient Magnetic Field on Natural Convective Jeffrey Fluid Flowing over a Heated Porous Plate: A Comparitive Study

Examining the effects of varying transient magnetic fields and their orientations on boundary layers aids in understanding the flow in complex surface geometries, turbulence control, drag reduction, aircraft and ship design, groundwater management, and the study of construction and coastal engineering. The study introduces a novel dynamics of the boundary layer of a natural convective Jeffrey fluid flowing over an erect porous moving plate under the influence a non-uniform and transient magnetic field M ˜ that grows exponentially over time with α as an accelerating parameter. The governing partial differential equations (PDEs) were non-dimensionalized using similarity variables and then solved analytically using the perturbation technique. The impact of various thermosphysical properties including chemical reactions, thermal source, Jeffrey fluid parameter and plate permeability on the fluid flow were presented graphically utilizing finite difference approximation(FDA) technique in MATLAB ODE15s solver. The study highlighted a notable decrease in shear stress (skin friction) by introducing M ˜ to the vertical plate results in the reduction of momentum boundary layer, accompanied by an increment in thermal boundary layer. Also, the results shows that increasing the magnitude of α leads to decrease the velocity and increase the temperature distribution curve. Additionally, the concentration profile was also found to decrease with stronger chemical reactions. Furthermore, key parameters like heat and mass flux were estimated and discussed through comparison tables. Understanding the effects of unsteady and externally applied increasing magnetic field M ˜ on viscous fluid flow have potential applications in biomedical engineering, such as the design of magnetic drug delivery systems and the analysis of blood flow in human body.

Study on vortex structure and hydrodynamic characteristics of water-exit projectile with shoulder ventilation

During the vertical motion of a water-exit projectile, the gas stored inside is expelled from the shoulder under the effect of pressure differential, forming a ventilated cavity covering the surface of the projectile. This paper explores the vortex structure characteristics of the surface ventilated cavity through numerical simulation and summarizes its impact on the hydrodynamic characteristics of the water-exit projectile. It was found that the end of the shoulder ventilated cavity presents a periodic vortex shedding form. And the existence of the shoulder ventilated cavity can reduce the vertical resistance of the projectile, improve the vertical movement speed, and play a role in load reduction and drag reduction. Additionally, the lateral displacement of the projectile at the moment of water exit decreases by approximately 45.2%, and the deviation angle reduces by about 55.6%, significantly improving the stabilization of its water exit posture. This indicates that the presence of the shoulder ventilated cavity can effectively enhance the hydrodynamic performance of the projectile.

Sustainable drag reduction in Taylor-Couette flow using riblet superhydrophobic surfaces

Superhydrophobic surfaces (SHSs) have been proven effective in reducing frictional drag force in various flow conditions. However, at high flow speeds, the air plastron on these surfaces collapses, leading to a decline in their effectiveness. In this study, we investigated the frictional drag forces of various SHSs and their combination with surface patterns across a wide range of flow conditions (5.00×102 < Re < 1.12×105) by using an facile coating method. Our experiments involved incorporating a superhydrophobic coating on the inner cylinder of a custom-made Taylor-Couette apparatus, integrated with a rheometer to measure torque applied on the inner rotor as a function of rotational speed. As part of our research, we calculate the effective slip length to assess the drag reduction performance of coatings, revealing an effective slip length of around 63 µm on a flat SHS. Furthermore, we explore the combined effect of superhydrophobic coatings and triangular-shaped riblets on drag reduction in Taylor-Couette flow, comparing the performance of these surfaces based on the riblet’s sharpness and the Reynolds number. Our experimental results show a reduction in measured torque of up to 24 % and 48 % on a V-grooved SHS in laminar and turbulent flow, respectively. Longevity tests confirm that the designed surfaces maintain their superhydrophobicity and drag reduction performance under turbulent flow conditions. Overall, this work introduces a passive drag reduction strategy through surface design, which substantially mitigates the frictional drag force and demonstrating considerable potential for enhanced performance and increased efficiency of Taylor-Couette systems.