Maximum Drag Research Articles

Effects of varied roughness coverage area on drag in a turbulent boundary layer using numerical simulations

Turbulent boundary layer (TBL) over various area coverages of rough surfaces have been studied via computational fluid dynamics using the steady Reynolds Averaged Navier–Stokes (RANS) technique. The rough surfaces are modelled by randomly distributed hemispheres with a 1 mm radius, covering 5%, 10%, 20%, 30%, 40%, and 50% of the wall surface area designed to represent a typical three-dimensional roughness and also replicate the biofouling growth of a ship’s hull. The results of this study show the relationships between the roughness area coverage and the skin friction drag within a fully rough regime. The flow experiences maximum drag and the highest equivalent sand grain roughness height at 30% roughness area coverage. However, beyond this area coverage value, the drag gradually decreases. Further analysis indicates that the 30% roughness area coverage corresponds to a roughness frontal density of λf=0.15. Recent reports indicate that when λf≲0.15, the pressure drag to total drag ratio increases with increased roughness frontal density.

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.

Bio-based synthesis of a sustainable Nano drag reducing agent for sprinkler irrigation systems

High-grade silica Nano particles were extracted from rice husk using a straightforward thermochemical method. The specifications of the isolated Nano particles were verified using a variety of material characterization techniques. Siloxane and silanol groups were notably visible in the spectra obtained using Fourier transform infrared spectroscopy. Scanning Electron Microscopy images revealed main Nano particles alongside secondary Micro particles. The size of the particles varied from 14.56 to 33.72 nm. The drag reduction experiments were took place in a facility that uses a forced closed loop. The extracted Nano silica was mixed with faucet water at a weight concentration of 50 to 400 mg/l to reduce drag. Records of pressure loss were obtained along a 186 cm carbon steel tube with internal diameters of 1.6 and 2.7 cm and variable flow rates of solutions at a comfortable temperature of 25 °C. The friction factor values were found to be around Blasuis asymptote for pure water but they were found to be near maximum drag reduction asymptotes when using the Nano material. A maximal drag reduction of nearly 68 % was achieved by using 400 ppm of Nano silica. It was observed that there is a crucial Reynolds number around 96000 that should not be surpassed since any additional increase results in a drop in drag reduction. Furthermore, a relationship between wall shear stress and velocity of the fluid was established.

Flow past two rotationally oscillating cylinders

Wake interaction of two rotationally oscillating cylinders in side-by-side configuration is studied experimentally at a Reynolds number of 150. Five spacing ratios, $T/D$ (ratio of centre-to-centre spacing to cylinder diameter), are considered, namely, 1.4, 1.8, 2.5, 4.0 and 7.5. Both in-phase and antiphase forcing are investigated. Oscillation amplitude is varied from ${\rm \pi} /8$ to ${\rm \pi}$ , and forcing frequency, $FR$ (ratio of the oscillation frequency to the vortex-shedding frequency of a stationary cylinder) is varied from 0 to 5. The experimental investigation is done using laser-induced fluorescence, hot film anemometry and particle-image velocimetry (PIV). The interaction between the two cylinders under forcing results in new wake modes and vortex structures and a comprehensive study from the wake visualisations is conducted. Quantitative results are presented in terms of streamwise and cross-stream mean velocity profiles, centreline velocity recovery, peak velocity deficit, wake width, fluctuation intensity, circulation, vorticity contours and drag coefficient. The magnitude of streamwise velocity deficit and cross-stream velocity variation is strongly affected by the presence of second cylinder. The recirculation region behind the cylinders is found to extend further downstream with increase in the forcing. Scaling analysis is carried out to express the peak velocity deficit variation with forcing. It is observed that the relative strength of the vortices shed from inner and outer shear layers depends on the phase of oscillation. An experimental set-up for direct force measurement is designed and the drag force acting on the oscillating cylinders assembly is directly measured and the effect of forcing on the variation of ${C}_{d}$ is studied. An estimate for drag coefficient is also made from the PIV data following a detailed control volume analysis. It is observed that the set of forcing parameters that correspond to maximum and minimum drag also yield extrema in the values of circulation and fluctuation intensity.

On the Benefits of Active Aerodynamics on Energy Recuperation in Hybrid and Fully Electric Vehicles

In track-oriented road cars with electric powertrains, the ability to recuperate energy during track driving is significantly affected by the frequent interventions of the antilock braking system (ABS), which usually severely limits the regenerative torque level because of functional safety considerations. In high-performance vehicles, when controlling an active rear wing to maximize brake regeneration, it is unclear whether it is preferable to maximize drag by positioning the wing into its stall position, to maximize downforce, or to impose an intermediate aerodynamic setup. To maximize energy recuperation during braking from high speeds, this paper presents a novel integrated open-loop strategy to control: (i) the orientation of an active rear wing; (ii) the front-to-total brake force distribution; and (iii) the blending between regenerative and friction braking. For the case study wing and vehicle setup, the results show that the optimal wing positions for maximum regeneration and maximum deceleration coincide for most of the vehicle operating envelope. In fact, the wing position that maximizes drag by causing stall brings up to 37% increased energy recuperation over a passive wing during a braking maneuver from 300 km/h to 50 km/h by preventing the ABS intervention, despite achieving higher deceleration and a 2% shorter stopping distance. Furthermore, the maximum drag position also reduces the longitudinal tire slip power losses, which, for example, results in a 0.4% recuperated energy increase when braking from 300 km/h to 50 km/h in high tire–road friction conditions at a deceleration close to the limit of the vehicle with passive aerodynamics, i.e., without ABS interventions.

Drag reduction study of naturally occurring oscillating axial flow induced by helical corrugated surface in Taylor–Couette flow

This study investigates drag reduction capability of naturally occurring-oscillating axial secondary flow (ASF) induced by helical-corrugated surface in Taylor–Couette flow (TCFHelical) for three values of pitch to wavelength ratios (P* = 1, 2, and 3) and amplitude to wavelength ratio(A*) of 0.25. As reported in Razzak et al. [“Numerical study of Taylor Couette flow with longitudinal corrugated surface,” Phys. Fluids 32(5), 053606 (2020)], emergence of naturally occurring-oscillating ASF induced by longitudinal-corrugated surface in TCF (TCFLongitudinal) and increasing trend on its magnitude with Reynolds number (Re) results in the occurrence of drag reduction. This has motivated us to study the possibility of enhancing drag reduction by maintaining a consistently increasing trend with Re in the magnitude of naturally occurring-oscillating ASF induced by the helical-corrugated surface on the stationary outer cylinder in TCF. From flow structures, steady ASF with non-zero mean is observed at Re = 60, which suppresses the strength of azimuthal vorticities for Re > 85, and contributed to the occurrence of drag reduction. As Re is increased to 100, 90, and 85 for P* = 1, 2, and 3, respectively, the formation of periodic oscillating ASF with non-zero mean and its increasing trend in magnitude with Re suppresses azimuthal vorticities further, which contributes to the maximum drag reduction of 13%. For Re > 165, 145, and 140 for P* = 1, 2, and 3, respectively, non-periodic oscillating ASF is observed, and its magnitude remains nearly unchanged or decreases slightly with Re, which results in the suppression effect of azimuthal vortices to be weaker. This results in the decrease in the drag reduction. Oscillating ASF observed in TCFHelical is found to occur at earlier Re, and it is stronger than that of TCFLongitudinal, which contributes to the occurrence of higher drag reduction in TCFHelical.

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 study on the wake control of a square cylinder mounted with dual rigid/flexible splitter plates in the subcritical regime

The wake control of a bluff body is vital to the safety of structures in the field of coastal and ocean engineering. The quantitative statistics wake control and flow field in the subcritical regime are relatively unexplored. The purpose of this research is to find an optimum arrangement for the wake control of a square cylinder mounted by dual flexible/rigid splitter plates and uncover the physical mechanisms. The Reynolds number based on the side length of the square cylinder is 4.2×103≤Re≤1.0×104. Splitter plates with two thicknesses were adopted, which represents the flexible and rigid splitters, respectively. The splitter length was considered over the range l/L=1∼2 with an increment of Δl/L=0.25. The force exerting on the bluff body and the instantaneous velocity field were measured by the triaxial load cell and particle image velocimetry techniques. The results indicate the modulation of the flapping splitter as well as the splitting effect of the tip vortices determine the lift suppression and the gap flow and streamlined trailing play a vital role in drag reduction. The maximum drag reduction and lift suppression reach ηmax=58.3% and 90.6%, respectively. The flexible splitter outperforms its rigid counterpart, especially at high Reynolds numbers, which is beneficial for those structures in marine engineering. Given the hydrodynamic characteristics and sensitivity to incoming flow conditions, the optimum arrangement is the flexible splitter configuration with splitter length of l/L=1.25∼1.5.

Innovative Morphing UAV Design and Manufacture

In this study, an unmanned aerial vehicle (UAV) with passive pre-flight and active in-flight morphing capability was designed and manufactured. First of all, conceptual design work was done. Wing and tail, which are the main carriers, were selected to ensure maximum liftt, minimum drag and stability of the UAV. Liquid fuel engines were preferred due to their high power and airtime. The engine, which enables the controlled and uncontrolled flight of the UAV, has been run-in to make it safer and more efficient before being used in real-time flights. Profiles were selected by analyzing the tail set consisting of the rudder and the elevator. The nose cone of the unmanned aerial vehicle was produced by improving the aerodynamic performance. In the aircraft geometry, the passive morphing mechanism, which is performed once before the flight, and the active morphing mechanism, which is performed continuously during the flight, are manufactured using servo motors. This improved the flight performance and made it possible to fly in some unfavorable conditions. The most basic superior feature of the manufactured UAV from the existing UAVs is its ability to morphing.

Study on the Synthesis and Properties of a New Salt-Resistant Drag Reducer

Summary To solve the problem of poor salt resistance of conventional drag reducers, a hydrophobic associative polymer drag reducer was prepared by inverse emulsion polymerization with acrylamide (AM), methacrylic acid (MAA), 2-acrylamide-2-methylpropane sulfonic acid (AMPS), and hexadecyl dimethyl allyl ammonium chloride (C16DMAAC) as the main monomers. The synthetic product was confirmed as the target product by infrared (IR), nuclear magnetic resonance (NMR), and mass spectrometry (MS). The viscosity-average molecular weight of the prepared drag reducer is 1100×104 g/mol. The pipeline friction results show that the drag reducer has good friction reduction and salt resistance. When the concentration in clean water is 0.06%, the maximum friction reduction rate is 71.1%. When the salinity is 5×104 mg/L, the calcium ion concentration is 2000 mg/L, and the suspended solid content is 500 mg/L, the maximum friction reduction rate is 68.9% when the concentration of the drag reducer is 0.06%. Salt water will not significantly lower the friction reduction rate. If the concentration of the drag reducer is increased to 0.08%, the maximum drag reduction rate will reach 73.8%. The microrheological test results of the friction reducer solution show that, at 0.2% concentration, there is no network structure between friction reducer molecules, which is consistent with Newtonian fluids possessing a certain viscosity. The elasticity index (EI) of the drag reducer solution is basically unchanged over time, maintaining good friction reduction and sand-carrying performance during the shearing process of large displacement pumping.

Numerical analysis of the buoy design to extract the effective kinetic wave energy

In many countries, sea wave energy is becoming more widely recognized as a significant and hopeful resource. The objective of this research is to evaluate the impact of alterations in buoy shape on the efficacy of the wave model by utilizing Computational Fluid Dynamics (CFD) to assess modifications in the buoy's dynamic behavior. The buoy's behavior is simulated using ANSYS Fluent, the volume of fluid approach, the open channel flow module, and 5th order Stokes wave models. To further investigate the wave sensitivity in challenging circumstances, Stokes waves were applied in the shallow domain at high wave steepness. The validation process consisted of a comparative analysis between the practical and mathematical results. The outcomes of a numerical fluid dynamical computation were compared, and laboratory observations were modified. For various configurations of underwater buoyancy, the drag and lift characteristics of numerical results were analyzed. The aim of this study was to identify the optimum buoy shape. The selection of this shape was based on several factors, of which the most significant were the least entropy, as well as the maximum lift and drag coefficient. Ideal shape was determined to be a spherical shape (shape B).

Aerodynamic Drag Improvements on a Simplified Heavy Vehicle using Three-sided Plain and Notched Base Flaps

An experimental investigation has been undertaken to determine the effects of plain and notched base flaps on the drag performance of simplified tractor-trailer combination without any intermediate gap, Generalized European Transport System (GETS). Both plain and notched base flaps are rigid, made up of three identical flaps whose length is equal to the width of the GETS model, and not angled inward or outward. The experiments examined three-sided flap configurations corresponding to various combinations of seven heights of the plain part (from 10 to 40 mm in steps of 5 mm), four notch amplitudes (from 2.5 to 10 mm in steps of 2.5 mm), and five notch wavelengths (from 10 to 50 mm in steps of 10 mm). It is shown that the drag performance of the plain flap at zero yaw highly depends on the height of the plain flap. The maximum drag reduction occurs for e/w=0.1 yielding a drag reduction of 1.9% when compared to the GETS model without flap (baseline GETS). It was shown that the time-averaged drag coefficient increased slightly until a maximum was reached at e/w=0.3 but then decreased slightly with increasing e/w. Under zero yaw angle conditions, GETS model with a notched base flap, e10-a05.0-λ20, gives the lowest drag. The addition of this base flap to the GETS model resulted in a 2.8% drag reduction. This notched base flap was shown to be more effective not only at reducing <CD> under yawed flow conditions tested but also at reducing time-averaged side coefficient under yawed flow conditions tested, compared to the e10-a00.0-λ00 flap.

Wake control of a square cylinder: Impinging the vortex cores with dual flexible splitter plates

Numerous works have been conducted to study the wake control of a bluff body and more efficient strategies are still what we seek for. This paper studies the complex fluid–structure interaction (FSI) problem of a square cylinder mounted with dual flexible splitter plates from a new perspective: impinging the vortex core with flapping plate tip. The Reynolds number (Re) based on the side length of the square cylinder is fixed at 332.6. The splitter plates are attached at both sides of the square cylinder. Structural parameters, such as plate length and bending stiffness, are considered to achieve an optimal arrangement of this configuration. Compared to the traditional single splitter plate arrangement, only a short splitter length is needed for this dual splitter plates configuration. Flow visualization indicates that the drag reduction is associated with the increase of base pressure and the lift suppression is related to the inhabitation of the interaction of the shear layer and split effect of the tip vortices. The dimensionless bending stiffness plays a vital role in determining the behavior of the splitter plate. Two vibration modes, i.e., off-center vibration and asymmetry vibration modes are captured and the up and down splitter plates tend to vibration in-phase. The Lissajous trajectories of the free tip mainly exhibits arc-shape and sickle-shape. Given the performance of the square cylinder-dual splitter plates configuration, splitter length of l/L=1.5 is an optimal configuration, which is less affected by flexural rigidity. The maximum drag reduction could reach 23.1% and the lift force exerting on this configuration does not increase significantly, which is beneficial for the structural stability.

Numerical simulation of subsonic and transonic water entry with compressibility effect considered

The purpose of this study is to analyze the subsonic and transonic water entry of the axisymmetric body with compressibility effect considered. The water-entry processes of axisymmetric body are numerically investigated, with Tait state equation and energy equation used to describe the compressibility of the liquid. Reasonable agreement has been obtained between the numerical results and the experimental measurements of trajectory, velocity and acceleration. The cavity evolution and dynamic characteristics of subsonic cases are investigated for vertical and oblique water-entry. The results show that the maximum drag coefficient of the oblique water entry is lower, and it is affected by a large moment which causes pitch angle and trajectory inclination angle to change significantly. In the oblique transonic conditions, the effect of compressibility of the liquid on the dynamic flow characteristics is analyzed. On this basis, the influence of entry angle on the hydrodynamic characteristics is investigated. It is concluded that the water-entry angle has a more significant influence on the trajectory characteristics.

Research on Adaptive Flexible Wing with Variable Trailing Edge

The trailing edge deformable wing is one of the main development directions of future aircraft design. The technology of active camber of wing trailing edge can significantly improve the aerodynamic performance of aircraft. In this paper, the motor drives the parabolic crankshaft and the distributed crankshaft drives the flexible skin of the trailing edge of the wing, which can realize the deflection of the trailing edge of the wing at a large angle of 0~30 degrees. At the same time, the surface of the wing is continuous and smooth. Moreover, the aerodynamic simulation results of the deformed wing show that the trailing edge deflection is beneficial to increase the lift. It can increase the maximum lift coefficient by 2 times and the maximum lift drag ratio by 18%. Adaptive flexible wing technology is applied to rocket sleds with side wings. This technology can increase the downward pressure of the side wing of the rocket block and improve the operation safety.

Development and drag-reducing performance of a water-soluble polymer coating

Adding a small amount of polymers can achieve significant drag reduction effects. However, for external flows, the common homogeneous mixing and diffusing injection methods are not feasible. As an alternative, the present work developed a novel water-soluble polymer composite coating. The coating made use of the film-forming property of polyvinyl alcohol with polyethyleneoxide (PEO, a well-known drag reduction polymer) incorporated into it. When the coating dissolved, PEO continuously dispersed into the external flow. The surface characteristics of the water-soluble polymer coating were characterized. Drag reduction tests were conducted using a gravity circulation system. The coating exhibited a maximum drag reduction rate (DR) of 7% in the coating section and 27% in the downstream section. The larger percentage and the greater molecular weight of PEO not only promoted polymer drag reduction but also increased the surface roughness. Competition between effects of drag reduction and surface roughness led to complex effects in the coating section. It was also found that partial coating could induce significant drag reduction effects. The optimal length ratio of coated to total surface was related to the polymer characteristics and the speed of the main flow. The 1/4 and 1/2 coating resulted in a maximum DR of approximately 7% (Re = 27 523) in the test plate section for coatings with 10 000 wppm PEO, while the 1/4 coating had a maximum DR of approximately 9% (Re = 11 468) for coatings with 20 000 wppm PEO. These results indicated that such drag-reducing composite polymer coatings have great potential to be applied in underwater equipment.

Drag reduction and optimization on a sphere with the effect of Lorentz force

The flow of a weakly conductive fluid (i.e. seawater) can be controlled by Lorentz force, which holds promising applications in drag reduction. In this paper, the drag reduction on a stationary sphere in the weakly conductive fluid with the Lorentz force on the sphere surface are numerically investigated at Re = 300. The relations among the vortex structures, the hydrodynamic forces, and the effects of drag reduction are discussed before and after the application of the Lorentz force. The results indicate that the fluid near the surface of the sphere is accelerated with the application of Lorentz force. Therefore, the flow separation on the rear surface of the sphere is suppressed, and the periodic shedding mode of hairpin vortices is replaced by the steady double-thread wake structure. Meanwhile, the increase of momentum near the sphere improves the capability to overcome the adverse pressure gradient. Therefore, the pressure on the leeward side of the sphere increases, and then the effect of drag reduction is achieved. Moreover, the effects of drag reduction are distinct with the different locations (θm) of the applied Lorentz force, which reaches the maximum drag reduction rate together with the maximum drag reduction efficiency for θm = 90°.

Numerical simulation of drag reduction effect on the surface of bionic fish-scales

With the development of marine transportation and underwater navigation technology, fluid drag reduction has become an international research hotspot as an important technical means to save energy and reduce environmental pollution. It has been found that the body surface structures of many organisms have unique drag-reduction properties. Therefore, it is feasible and important to replicate the morphological features of the body surface to the material surface. In this research, the structural features of biomimetic fish scales were summarized and extracted by observing the laminar arrangement features and morphology features of the surface scales of aquatic fish. Then, the arrangement features and morphology of fish-scales were abstracted into the oblique groove structure, and a three-dimensional model of the bionic fish-scale with a fan-shaped structure was constructed. The surface flow field of the bionic fish-scale was numerically simulated by COMSOL Multiphysics to revealing the mechanism of resistance reduction on the surface of the bionic fish-scale. The results indicate that the maximum drag reduction rate of the bionic fish scale surface is 8.40% compared with the smooth surface at a water speed of 0.6 m/s.

Development and performance of a gelatin-based bio-polysaccharide drag reduction coating

The secreting drag reduction mucus in fish epiderm inspires the manufacturing of five gelatin–polysaccharide drag reduction coatings. First, a mixed solution composed of the gelatin and bio-polysaccharides [guar gum, xanthan gum, locust bean gum, tragacanth gum, or acacia gum] was poured into rectangular grooved polymethyl methacrylate (PMMA) plates, and bionic coatings were obtained after curing. Then, the surface characteristics of the coatings were characterized, and the internal micro-/nanoscale three dimensional (3D) net structures provided releasing access for the polysaccharide molecules. Importantly, a parametric study focusing on the gelatin and polysaccharide proportion affected the drag reduction of the coatings in a turbulent channel flow. Based on a smooth PMMA plate without a coating as a reference, the five developed coatings exhibited considerable drag-reducing effects with the corresponding maximum drag reduction rates that all exceeded 20%. There are three drag reduction mechanisms (polymer drag reduction, slip phenomenon, and wall flexibility) and one drag increase mechanism (surface roughness). Increasing the gelatin proportion affects the release rate of the drag-reducing agents, surface flexibility, and surface slip properties. Meanwhile, increasing the polysaccharide proportion promotes the release of polysaccharides, but increases the surface roughness. Thus, the effects of gelatin and polysaccharide are complicated due to competition between these mechanisms. Future works should focus on clarifying the complex mechanisms to improve the drag reduction efficiency of the gelatin-based bio-polysaccharide coatings. These biomimetic drag-reducing coatings could be further applied to underwater equipment.

Hydrodynamic investigations of a cross-domain vehicle with the capability of high-speed cruising on water surface

With the growing demands for marine resource exploitation and marine defense security deployment, unmanned surface vehicles (USVs) and autonomous underwater vehicles (AUVs) are of great value in both civil and military fields. However, both USVs and AUVs can only operate in a single scenario, which is difficult for complex cross-domain missions. Here, we propose a cross-domain vehicle (CDV) with both high-speed mode as USV and diving mode as AUV. To obtain the capability of high-speed cruising on the water surface, two sets of well-tailored hydrofoils are employed for drag reduction by adjusting the attitudes of CDV. Numerical investigations are carried out to evaluate the hydrodynamic characteristics of the CDV at different velocities. Moreover, prototype tests are conducted in open water, demonstrating its technical feasibility and validating the numerical methods. The results demonstrate a maximum drag reduction of 43% and a significant improvement in longitudinal stability with elaborate design of the hydrofoils. The mechanism of the drag reduction is revealed by flow field analysis, which is a joint effect resulting from the inhibition of wave-making as well as the reduction of the wetted surface area. The current work has significant value for future improvement of CDVs.