Drag Reduction Techniques Research Articles

Numerical Analysis of Small Caliber Spinning Projectile with Variety Boattail Configurations

Range extension has received great interest in the field of artillery and spinning projectiles. Reducing base drag is an effective way for rang extension, because it is a major component in transonic and supersonic Mach numbers. Boattailing is considered as one of the most implemented techniques to reduce base drag. This work uses variety of passive drag reduction techniques over M33 .50cal projectile. Multiple design variations on projectile boattail are considered. This includes varying boattail angle, use of riblets, and use of arced boattail. Computations are conducted using computational fluid dynamics by solving RANS equations. Simulations are performed over Mach numbers from 0.6 to 2.5. The aim of the simulations is to calculate most affecting aerodynamic forces and moments which include drag force, lift force, Magnus force, Magnus moment, overturning moment, and spin damping moment. Then these coefficients are compared for all design variations. Results are validated by comparing with experimental and numerical results from the literature. Furthermore, a mesh independence study is done for verification. It was found that every configuration has good performance at a certain speed range with regard to zero yaw drag and significant yaw drag reduction for riblets boattail for low supersonic speeds.

A Review on Drag Reduction and Stability Optimization Techniques for Small Satellite Launch Vehicles

A Review on Drag Reduction and Stability Optimization Techniques for Small Satellite Launch Vehicles

Drag-Mitigating Dynamic Flight Path Design and Sensitivity Analysis for an Ultra-Long Tether Underwater Kite

Abstract This paper presents a computational study of an underwater kite operating in environments requiring tethers exceeding a kilometer in length, referred to herein as ultralong tether (ULT) applications. Leveraging a detailed dynamic model of the kite and tether, we study the relationship between path shape and tether drag at varying tether lengths in order to develop meaningful insights as to the operation of systems that require ultralong tethers in order to reach viable flow resources. An initial study of a ULT kite application operating in a uniform flow field is presented, demonstrating that with an appropriately designed flight path, the kite is able to suppress the motion of the majority of the tether in order to achieve an order of magnitude greater power output than can be achieved with a straight tether, which is the typical assumption used in a performance estimation. Tether drag mitigation is characterized through the use of a novel metric termed effective tether length, which characterizes the total length of tether engaged in drag production. It is shown that a high-performance path shape can reduce the effect of tether drag by over 50%. This performance is shown to be comparable to the multi-airborne wind energy system (MAWES) proposed by Leuthold et al. (2017, “Induction in Optimal Control of Multiple-Kite Airborne Wind Energy Systems,” IFAC-PapersOnLine, 50(1), pp. 153–158; 2018, “Operational Regions of a Multi-Kite Awe System,” 2018 European Control Conference (ECC), IEEE, Limassol, Cyprus, June 13–15, pp. 52–57), which suppresses tether motion through mechanical design rather than merely though careful path selection and control. This initial study in a uniform flow field is followed by two sensitivity studies: one that assesses performance in realistic environments where the flow magnitude is a function of altitude above the seabed and a second that assesses the impact of tether drag reduction techniques. It will be shown that by careful selection of path shape, site, and tether design, a single kite in a ULT marine application can achieve performance rivaling that of the MAWES without the extra required mechanical complexity.

The Impact of Using Dimpled Surfaces on Ahmed Body on The Flow Characteristics

Vehicle aerodynamics which directly affects fuel consumption values in vehicles has great importance in terms of vehicle stability, acceleration ability, vehicle noise, and environmental pollution. Ahmed car model is one of the most important generic models, so this popular car model is widely used in the literature on fluid mechanics and vehicle aerodynamics studies. This study investigates the effect of a passive drag reduction technique which is known as dimple surface, on Ahmed car model with a 35° slant angle. The flow control methods were applied numerically also these methods were compared with similar works. Analysis made in this study were conducted with 2.4 million cell numbers and the maximum drag reduction is 3.5 % with optimum dimple surfaces. Moreover, the applicability and manufacturability factors were also evaluated in terms of implementing the dimpling modification to today’s current road vehicles.

Fabrication of a scalable slippery surface via novel sprayable breath figure technique for sustainable drag reduction and anti-biofouling in marine environments

The maritime industry has been seeking efficient solutions to combat hydrodynamic friction and biofouling, which increase operational costs and environmental issues. To address these concerns, we developed a novel sprayable breath figure (sBF) method to create a scalable and cost-effective lubricant-infused surface (LIS) for reducing frictional drag forces on marine vehicles. The proposed sBF method facilitated the rapid production of multilayered porous polymer films with micron-scale spherical cavities. It can be applied to large and curved surfaces, thus overcoming the limitations of traditional breath figure methods. Further surface treatment of oxygen plasma etching followed by polydimethylsiloxane (PDMS) brush grafting was employed to optimize the interfacial slip and lubricant retention of the slippery surface coating. The PDMS brush-grafted slippery surfaces demonstrated significant drag reduction under turbulent flow conditions. This was confirmed by direct velocity field measurements, showing nonzero slip velocity unlike conventional no-slip surfaces. The same surfaces also exhibited remarkable anti-biofouling performance, severely resisting marine biofouling in real-sea field tests over 50 days. The proposed sBF method was successfully applied to large-area and curved submerged bodies, and achieved a noticeable drag reduction under high-speed turbulent flows. This study is a critical step toward the practical implementation of this technology in the marine industry.

Experimental investigation of blade-shaped riblets for drag reduction on UAV applications

This study presents an experimental investigation of blade-shaped riblets for drag reduction in unmanned aerial vehicle (UAV) applications. UAVs have gained significant attention since they can perform various missions, including surveillance, reconnaissance, and package delivery. However, their aerodynamic performance, specifically the high drag associated with their exposed surfaces, remains a key challenge for enhancing their efficiency and extending their flight endurance. To address this issue, riblet geometries are proposed as a potential solution, which can reduce the turbulent skin friction drag by up to 8%. The experimental investigation involves wind tunnel testing of blade-shaped riblets, with various spacing-to-height (s/h) ratios and constant groove cross-sectional area (Ag). The riblets are designed for application on the wing, empennage, and fuselage surfaces of a UAV. The investigations are performed on a flat plate for various flow conditions, including different freestream velocities, to evaluate the drag reduction effectiveness of the riblet configuration. The drag force is measured using a force balance system and flow visualization techniques are employed to assess the position where the boundary layer has transitioned to fully turbulent. The results demonstrate the drag-reducing effect of blade-shaped and trapezoidal riblets and the different performances observed for the various s/h ratios. The cases with s/h=1 result in the smallest drag coefficients, while the cases with s/h=2 have significantly increased drag values, compared to the smooth flat plate, due to the increased wetted surface area. These findings highlight the potential of riblets as an effective drag-reduction technique for UAV applications, enabling increased endurance and/or enhanced payload capacity.

Surrogate‐based optimization for active drag reduction of turbulent boundary layer flows

AbstractTwo surrogate‐based optimization strategies using support vector regression (SVR) and Gaussian process regression (GPR) as surrogates are investigated to guide the design of actuation parameters for active drag reduction techniques in turbulent boundary layer flows encountered at civil airplanes in cruise flight and high‐speed trains. As an approximation, the turbulent flow over a flat plate subjected to spanwise traveling transversal sinusoidal surface waves is simulated by wall‐resolved large‐eddy simulations (LESs). These simulation data are used to model the dependence of the objective drag reduction on the actuation parameters, that is, the optimization variables. In this work, the previous purely exploitative approach of SVR‐based ridgeline optimization is extended to GPR‐based Bayesian optimization to further automate the simulation‐driven tuning of the actuation parameters.

Superior shear-stable slippery surface of porous carbon nanospheres (PCN)-oleogel

Frictional drag reduction (DR) is of great importance in marine environments as it has the potential to significantly reduce fuel consumption, leading to both economic and environmental benefits. While various conventional DR techniques exist, many of them are energy-inefficient and require external intervention to function effectively in real marine conditions. Unlike conventional methods, bioinspired surfaces offer cost-effective and sustainable DR. However, these surfaces suffer from (i) poor durability of superhydrophobic surfaces, (ii) limited scalability, (iii) substrate dependency of coatings, and (iv) fast depletion of impregnated lubricant oil of liquid infused surfaces (LIS). In this work, we proposed a double-layered LIS oleogel surface embedded with re-entrant surface (RES) morphology by adopting spin/spray-assisted deposition processes. The RES structure, achieved by depositing porous carbon nanospheres (PCN) on the oleogel surface, effectively stabilized and replenished a large amount of lubricated oil, ensuring long-lasting lubrication. The developed PCN-oleogel surface could retain its slippery property despite exposure to harsh physical abrasion, and various chemically contaminated aqueous solutions. The developed PCN-oleogel exhibited long-term lubrication performance and shear-stable durability under high-speed, high-pressure conditions up to 15 ms−1 — such shear-stable lubrication in extremely rare in literature. Conclusively, the proposed PCN-oleogel surface is envisioned to have a great potential in achieving sustainable underwater DR in harsh marine environments.

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

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

Turbulent Drag Reduction by Streamwise Traveling Waves of Wall-Normal Forcing

We review some fundamentals of turbulent drag reduction and the turbulent drag reduction techniques using streamwise traveling waves of blowing/suction from the wall and wall deformation. For both types of streamwise traveling wave controls, their significant drag reduction capabilities have been well confirmed by direct numerical simulation at relatively low Reynolds numbers. The drag reduction mechanisms by these streamwise traveling waves are considered to be the combination of direct effects due to pumping and indirect effects of the attenuation of velocity fluctuations due to reduced receptivity. Prediction of their drag reduction capabilities at higher Reynolds numbers and attempts at experimental validation are also intensively ongoing toward their practical implementation.

An adjoint-based drag reduction technique for unsteady flows

A framework based on a continuous adjoint-based analysis of steady and unsteady flows to predict and control the drag force due to surface morphing is presented. By establishing a relation between perturbations in the body shape and in the boundary condition on a certain geometry, we derive an analytical expression of the sensitivity to changes in the geometry of the body and its relation to the sensitivity to the perturbation of the boundary conditions. The methodology is evaluated on the incompressible flow around a cylinder for steady and unsteady flows. A reduction of the drag coefficient was obtained and investigated by several surface deformations, conducted in the direction of the sensitivity vector field obtained by solving the derived adjoint problem. In unsteady flows, the sensitivity field is computed by integrating the unsteady adjoint problem backward in time from the unsteady flow solution. Two different types of deformations based on the calculated sensitivity were applied: time-averaged deformation and time-dependent. Attempting the latter, a deformation at each time step, did not yield the same satisfactory results as the time-averaged in terms of expected drag reduction. We provide a theoretical reasoning for the difference between both methodologies, together with an insight into the physics of the sensitivity vector field distribution relating it to the drag force sources.

Saving energy in turbulent flows with unsteady pumping

Viscous dissipation causes significant energy losses in fluid flows; in ducts, laminar flows provide the minimum resistance to the motion, whereas turbulence substantially increases the friction at the wall and the consequent energy requirements for pumping. Great effort is currently being devoted to find new strategies to reduce the energy losses induced by turbulence. Here we propose a simple and novel drag-reduction technique which achieves substantial energy savings in internal flows. Our approach consists in driving the flow with a temporally intermittent pumping, unlike the common practice of a constant pumping. We alternate “pump on” phases where the flow accelerates, and “pump off” phases where the flow decays freely. The flow cyclically enters a quasi-laminar state during the acceleration, and transitions to a more classic turbulent state during the deceleration. Our numerical results demonstrate that important energy savings can be achieved by simply modulating the power injection into the system over time. The physical understanding of this process can help the industry in reducing the waste of energy, creating economical benefits and preserving the environment by reducing harmful emissions.

Shark Skin—An Inspiration for the Development of a Novel and Simple Biomimetic Turbulent Drag Reduction Topology

In this study, a novel but simple biomimetic turbulent drag reduction topology is proposed, inspired by the special structure of shark skin. Two effective, shark skin-inspired, ribletted surfaces were designed, their topologies were optimized, and their excellent drag reduction performances were verified by large eddy simulation. The designed riblets showed higher turbulent drag reduction behavior, e.g., 21.45% at Re = 40,459, compared with other experimental and simulated reports. The effects of the riblets on the behavior of the fluid flow in pipes are discussed, as well as the mechanisms of fluid drag in turbulent flow and riblet drag reduction. Riblets of various dimensions were analyzed and the nature of fluid flow over the effective shark skin surface is illustrated. By setting up the effective ribletted surface on structure’s surface, the shark skin-inspired, biomimetic, ribletted surface effectively reduced friction resistance without external energy support. This method is therefore regarded as the most promising drag reduction technique.

Computational Parametric Study and Live Firing Validation of Base-Bleed Exit Configuration

Base bleed, along with boattailing, is widely confirmed to be an efficient technique for drag reduction of projectiles. It is believed that the flow pattern at the projectile base, hence the level of drag reduction, is dependent on the base-bleed exit configuration design. It was also shown that bleeding from annular slots overperformed that from a single central orifice. However, central orifices are required for practical issues and may not be completely eliminated. The effect of simultaneous bleeding from the central orifice and annular slots was not adequately explored before. In addition, the impact of annular slot design on level of drag reduction was not thoroughly clarified in the available open literature. The objective of the present study was to systematically address these two gaps. Following parameterization of the bleed exit configuration, a set of three-dimensional computational simulations are conducted to find the design that yields the maximum drag reduction. A set of live firing experiments are conducted to assess the validity of simulation results. Based on parametric study results, the impact of bleed exit configuration on base flow pattern, base pressure, and total drag coefficient is explained. The optimized bleed configuration was found to yield a further drag reduction up to 5% compared to the baseline configuration, including a single central orifice.

Experimental and numerical investigation of squat submarines hydrodynamic performances

This paper empirically examines the hydrodynamic performances of squat submarines under the resistance and wave tests beside numerical investigation of pressure drag reduction techniques. Despite vast information about the operation of the streamlined fluid vessels, there is not much information about the geometries and hydrodynamic behaviors of squat vessels with L/D ratios below four. This study experimentally investigates the impacts of various relative depths and flow inclinations, intending to find drag, heave, and sway forces at the velocities of 0.5, 1.0, 1.5, 2.0, and 2.5-m/s. A one-tenth scaled model of a squat submarine is examined under the resistance and wave train scenarios as the captive model in the towing tank experiments via 110 unique testing scenarios. Then, different pressure drag reduction techniques on the full-scale submarine are evaluated numerically at one-phase fluid by developing stern geometric optimization, nose rod devices, water corridors, and surface patterns at the bow and stern. Results present drag forces increase up to 150% at the 0.5D level for the resistance and wave tests due to the influences of the free surface water. The drag coefficients at the fully submerged depth of 6D differ by up to 13.3% between the numerical and experimental evaluations.

CFD analysis of base drag reduction using indent modification in suddenly expanded nozzle

The total drag is the combination of skin friction, wave and base region drags. With all moving projectiles, rockets, missiles, and launch vehicles, the base pressure at the base or back face is a common concern. Base drag increase in unexpectedly bigger flows due to a sudden expansion of duct size at exhaust. At the duct’s outflow, the base pressure is vacuum or sub-atmospheric. To lower the base drag, the vacuum must be increased to near atmospheric pressure to reduce total drag. Base drag is undesirable since it accounts for a large portion of the total drag. The reduction in base drag benefits the space and defense programmers greatly. This is highly responsible to major losses of energy as a result of drag. Out of the different types of drag, base drag, commonly known as wake drag is responsible for increased drag in case of high velocity flow through the nozzles. Henceforth it is imperative to have drag reduction techniques. There are active or passive methods used to reduce base drag. The proper recovery of base pressure can be done by optimum indent geometry and position optimization. In this numerical analysis, different geometric shape and position indents are used on the exhaust duct so at to reduce base drag. It was found from the analysis that the outlet velocity radically alters from the inlet to outlet. Furthermore, the magnitude of velocity was high for the nozzle with a circular indent when compared with other indent geometries in the analysis.

Simulation Study on the Screening of Hydrophobic Surface Materials for Pipeline Drag Reduction Based on Adsorption Properties.

Hydrophobic surface drag reduction techniques are effective in reducing the frictional resistance of fluids, the adsorption of liquid molecules on hydrophobic surfaces can reflect the resistance to fluid flow through such solid surfaces. Based on molecular simulation technology, we investigate the adsorption characteristics of water molecules on hydrophobic surfaces to achieve rapid screening of hydrophobic materials in fire-fighting water supply systems. The Monte Carlo method was used to simulate the adsorption process of polymers and to analyze the effects of temperature and fixed adsorption quantity. Contact angle tests were also done to verify polymer hydrophobicity. The isothermal adsorption heat, water molecule distribution, and energy distribution were studied by molecular mechanics and molecular dynamics methods. Then, adsorption localization simulations and electrostatic potential distributions were used to predict possible adsorption sites on hydrophobic surfaces and single-molecule chains. Finally, the interaction energy, diffusion coefficient, and free volume were investigated to explain the adsorption mechanism at the molecular level. Simulation results show that, overall, PTFE was more hydrophobic and PES was more hydrophilic and at 298 K, the number of adsorbed water molecules was ranked as follows: PTFE < PVDF < PVC < PMMA < PPS < CSM < BD-HDI < BD-MDI < BD-TDI < PES. Furthermore, PTFE, PVDF, PVC, PES, and PPS have more stable adsorption configurations on the (0 -1 0) surface. According to the findings, hydrogen bonding dominates the interaction between water molecules and hydrophilic polymers, whereas π-π interactions increase water molecules' diffusion resistance in polymers with benzene rings. In addition, PES contains many sulfone groups and ether bonds, which disorganize the chain arrangement to provide more free volume, whereas the water adsorption rate of PTFE is reduced because its molecular chains are less convoluted and more organized.

Riblet Drag Reduction Modeling and Simulation

One of the most interesting passive drag reduction techniques is based on the use of riblets or streamwise grooved surfaces. Detailed flow features inside the grooves can be numerically detected only by Direct Numerical Simulations (DNS), still unfeasible for high Reynolds numbers and complex flows. Many papers report the DNS of flows on microgrooved surfaces providing fundamental details on the drag reduction devices, but all are limited to plate or channel flows far from engineering Reynolds numbers. The numerical simulation of riblets and other drag reduction devices at very high Reynolds numbers is difficult to perform due to the riblet dimensions (microns in aeronautical applications). To overcome these difficulties, some models for riblet simulation have been developed in recent years, due to the data provided by DNS, experiments, and theoretical analyses. In all these models, the drag reduction is modeled rather than effectively captured; however, the analysis of some nonlocal effects on practical aeronautical configurations with riblets, requires their adoption. In this paper, the capabilities of these models in predicting riblets’ performance and some interesting features of the riblets’ effect on form drag and shock waves are shown. Two models are discussed and compared showing their respective advantages and limitations and providing possible enhancements. A comparison between the two models in terms of accuracy and convergence is discussed, and two new formulae are proposed to improve one of these models. Finally, a review of the results obtained by the two models is provided showing their capabilities in the analysis of the riblet effect on complex configurations.

Superhydrophobic surfaces to reduce form drag in turbulent separated flows

The drag force acting on a body moving in a fluid has two components, friction drag due to fluid viscosity and form drag due to flow separation behind the body. When present, form drag is usually the most significant between the two, and in many applications, streamlining efficiently reduces or prevents flow separation. As studied here, when the operating fluid is water, a promising technique for form drag reduction is to modify the walls of the body with superhydrophobic surfaces. These surfaces entrap gas bubbles in their asperities, avoiding the direct contact of the liquid with the wall. Superhydrophobic surfaces have been vastly studied for reducing friction drag. We show they are also effective in reducing flow separation in turbulent flow and therefore in reducing the form drag. Their conceptual effectiveness is demonstrated by performing direct numerical simulations of turbulent flow over a bluff body, represented by a bump inside a channel, which is modified with different superhydrophobic surfaces. The approach shown here contributes to new and powerful techniques for drag reduction on bluff bodies.

Drag reduction of high-speed trains via low-density gas injection

Aerodynamic drag reduction is crucial for the development of high-speed trains. This study considers the principle of supercavitation torpedo drag reduction and conducts wind tunnel tests and improved delayed detached eddy simulation numerical simulations to develop a drag reduction technique using low-density gas injection on the surface of a high-speed train. Models for jetting gases with different densities on the surface of the high-speed train were employed, and the aerodynamic drag, frictional resistance, pressure resistance, and flow field structure were compared and analyzed. The results demonstrated that jetting of low-density gas reduced the drag of the train, whereas air and high-density gas increased the drag. The jetting gas reduced the frictional resistance of the train to a certain extent and low-density gas injection significantly reduced the frictional resistance. However, the pressure resistance coefficient increased with an increase in the density of the jetting gas, resulting in increased drag. The density of the gas medium in the boundary layer significantly decreased when low-density gas was jetted and the effect of reducing the friction resistance was superior. The pressure resistance between the front and rear of the jet port increased with an increase in the gas density, thereby increasing the drag on the train.