Large Drag Reduction Research Articles

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.

Self-adaptive turbulence eddy simulation of flow control for drag reduction around a square cylinder with an upstream rod

The use of an upstream circular rod to control the flow around a downstream square cylinder is found to efficiently reduce the drag of the square cylinder. The complex flow characteristics and mechanism of the rod-square system in tandem arrangement as a passive flow control method for various centre-to- centre gap ratios (L/D) are investigated by numerical simulation. Turbulence modelling of the rod-square cylinder system is challenging. To select a suitable turbulence modelling, the recently developed Self-Adaptive Turbulence Eddy Simulations (SATES), as well as IDDES (Improved Delayed Detached Eddy Simulation) and SBES (Stress Blended Eddy Simulation) are first assessed for single circular and single square cylinder flow test cases. The SATES method performs best among the three models for bluff body flow simulation and is thus used in the drag flow control study. For the rod-square flow control system, the Reynolds number is 34000 based on freestream velocity U0 and the edge length of the square cylinder D. The diameter of the control rod is kept constant at d/D = 0.18. The flow characteristics and mechanism are explored in detail, varying the rod and square cylinder distance of L/D = 1.5, 2.0, 2.5and 3.0. According to the absence or presence of vortex shedding from the upstream rod cylinder, two flow patterns (pattern I and pattern II) are observed. Large drag reduction is observed for all the simulation cases the maximum drag reduction is approximately 63% with L/D = 2.0, and the minimum drag reduction approximately 53% with L/D = 3.0. The secondary lock-frequency, which presents as pattern II, is caused by the excitation of upstream vortices. It is found that the side recirculation is associated with the wake recirculation length of the square cylinder, which is similar to the elongated cylinder in the flow pattern I. The flow mechanism of suppression of the vortices from the rod can be attributed to the effective diffusion of vorticity in the separating shear layer caused by the presence of the front wall of the square cylinder.

Aerodynamic impact of cycling postures on drafting in single paceline configurations

This paper presents finite-volume-based scale-adaptive simulations of single paceline configurations up to eight cyclists for three different postures. In cycling, drag reduction by drafting in pacelines is a key strategy to limit energy expenditure. The drag reductions of individual cyclists and of the paceline as a whole are determined by several factors, including cycling posture. To the best of our knowledge, a systematic study on the effect of cyclist posture on the drag in single pacelines has not yet been published in the scientific literature. In this study, drag reduction and flow field data were computed while validation was performed by wind tunnel measurements. The three investigated postures concern a road race dropped posture with either a large or small sagittal torso angle and a time trial posture. For the considered pacelines in which all cyclists have the same posture, the drag of a cyclist could be reduced by changing either the posture, or the position, or both. Changing posture can yield a maximum drag reduction of about 15% for the leading cyclist. The position in the paceline with minimum drag was the one but last, independent of the investigated cycling postures. For the pacelines containing eight cyclists, maximum drag reductions up to 63% were found. The largest drag reduction was 68%, obtained by riding in penultimate position in an eight cyclist paceline in dropped position with a small sagittal torso angle.

Active control of compressible channel flow up to Mab=3 using direct numerical simulations with spanwise velocity modulation at the walls

AbstractActive turbulence control has been pursued continuously for the last decades, striving for an altered, energetically more favorable flow. In this article, our focus is on a promising method inducing a spanwise wall movement in order to reduce turbulence intensity and hence friction drag, investigated by means of direct numerical simulation. This approach transforms a previously time dependent oscillatory wall motion into a static spatial modulation with prescribed wavelength in the streamwise direction [48]. Most procedures related to turbulence control including the present one have been overwhelmingly applied to incompressible flow. This work is different and novel to the effect, that this control method is applied to compressible, supersonic channel flow up to a bulk Mach number of . Due to substantial variations of viscosity, density, and temperature within the near‐wall region in supersonic flow, the impact of the control method is altered compared to solenoidal flow conditions. By creating a data set of different Mach‐/Reynolds numbers and control parameters, knowledge is gained in which way the effectiveness of oscillatory techniques and physical mechanisms change under the influence of compressibility. It is shown that the control method is able to effectively reduce turbulence levels and lead to large drag reduction levels in compressible supersonic flow. Variable property effects even enhance this behavior for the whole set of investigated parameters. Overall, the higher Mach number cases show a larger net power saving compared to the incompressible ones. Furthermore, we observe an increase of the optimum wavelength with increasing Mach number, which helps in guiding optimal implementations of such a control method.

Monolithic polymeric porous superhydrophobic material with pneumatic plastron stabilization for functionally durable drag reduction in blood-contacting biomedical applications

Superhydrophobic (SHP) surfaces can provide substantial reductions in flow drag forces and reduce blood damage in cardiovascular medical devices. However, strategies for functional durability are necessary, as many SHP surfaces have low durability under abrasion or strong fluid jetting or eventually lose their air plastron and slip-flow capabilities due to plastron gas dissolution, high fluid pressure, or fouling. Here, we present a functional material that extends the functional durability of superhydrophobic slip flow. Facile modification of a porous superhydrophobic polytetrafluoroethylene (PTFE, Teflon) foam produced suitable surface structures to enable fluid slip flow and resist protein fouling. Its monolithic nature offered abrasion durability, while its porosity allowed pressurized air to be supplied to resist fluid impalement and to replenish the air plastron lost to the fluid through dissolution. Active pore pressure control could resist high fluid pressures and turbulent flow conditions across a wide range of applied pressures. The pneumatically stabilized material yielded large drag reductions (up to 50%) even with protein fouling, as demonstrated from high-speed water jetting and closed loop pressure drop tests. Coupled with its high hemocompatibility and impaired protein adsorption, this easily fabricated material can be viable for incorporation into blood-contacting medical devices.

Effect of Counterflowing Jet on Supersonic Slender-Body Configurations: A Numerical Study

Previous studies have demonstrated that the use of counterflowing supersonic jets operating in the long penetration mode (LPM), directed from the nose of blunt bodies against a supersonic/hypersonic freestream, can greatly reduce the drag and heat loads on the vehicle. However, the benefits of LPM for slender bodies of revolution operating at low supersonic speeds, applicable to supersonic transport applications, remain largely unknown. This Paper conducts a systematic parametric study for the effects of a counterflowing jet on two slender body models (cone-cylinder and quartic body of revolution, respectively) operating at a freestream Mach number of 1.6, using time-accurate computations performed with an unstructured Navier–Stokes solver. Different ranges of jet flow rates, jet exit Mach numbers, nozzle exit diameters, and jet-to-base diameter ratios are examined systematically. The jet-to-base diameter ratio is determined to be the most important parameter to facilitate the establishment of the LPM regime. Depending on the geometry of the slender body, longer jet penetration (thus larger shock-standoff distance) associated with the LPM does not always result in larger drag reduction. Furthermore, current results indicate that inclusion of an LPM jet is likely to cause large oscillations of surface pressure and drag on these slender geometries.

Upstream actuation for bluff-body wake control driven by a genetically inspired optimization

The control of bluff-body wakes for reduced drag and enhanced stability has traditionally relied on the so-called direct-wake control approach. By the use of actuators or passive devices, one can manipulate the aerodynamic loads that act on the rear of the model. An alternative approach for the manipulation of the flow is to move the position of the actuator upstream, hence interacting with an easier-to-manipulate boundary layer. The present paper comprises a bluff-body flow study via large-eddy simulations to investigate the effectiveness of an upstream actuator (positioned at the leading edge) with regard to the manipulation of the wake dynamics and its aerodynamic loads. A rectangular cylinder with rounded leading edges, equipped with actuators positioned at the front curvatures, is simulated at $Re=40\,000$ . A genetic algorithm (GA) optimization is performed to find an effective actuation that minimizes drag. It is shown that the GA selects superharmonic frequencies of the natural vortex shedding. Hence, the induced disturbances, penetrating downstream in the wake, significantly reduce drag and lateral instability. A comparison with a side-recirculation-suppression approach is also presented, the latter case being worse in terms of reduced drag (only 8 % drag reduction achieved), despite the total suppression of the side recirculation bubble. In contrast, the GA optimized case contributes to a 20 % drag reduction with respect to the unactuated case. In addition, the large drag reduction is associated with a reduced shedding motion and an improved lateral stability.

Superhydrophobic Drag Reduction for Turbulent Flows in Open Water

Trapping air under water, superhydrophobic surfaces on boats have long been coveted for hydrodynamic drag reduction without bubble injection. Despite many reports of drag reduction in lab tests, such surfaces have not been successful in real-world tests on open water, at high Reynolds numbers. The authors develop hydrophobic microtrenches that provide a large slip, with a subtle detail that maximizes air retention. Replacing a portion of a motorboat's hull with the developed surface, they obtain large drag reduction under high-speed flows on the open sea. This has significant implications for energy efficiency in oceanic shipping.

Pathline analysis of traveling wavy blowing and suction control in turbulent pipe flow for drag reduction

Skin friction drag is much greater in turbulent flows as compared with that in laminar flows. It is well known that traveling wave control can be used to achieve a large drag reduction. In the present study, a direct numerical simulation of a turbulent pipe flow was performed to clarify the mechanism of the drag reduction caused by the traveling wave control. The flow induced by the control was evaluated using pathline analysis. Near the wall, a “closed flow” was formed, wherein the injected particles return to the wall owing to the suction flow. The random component of Reynolds shear stress was perfectly suppressed in the closed flow, which suggests that there was no turbulence. The controlled flow was categorized into four patterns, and each flow characteristic and drag reduction effect was discussed. When the closing rate is high, the drag decreases, while when the closing rate is low, i.e., when the injected particles are released into the main flow, the turbulence is maintained. If the thickness of the layer suppressing turbulence is insufficient, a significant effect in terms of the drag reduction cannot be expected. The large drag reduction owing to the traveling wave control can be attributed to the elimination of turbulence in the region near the wall.

On drag reduction scaling and sustainability bounds of superhydrophobic surfaces in high Reynolds number turbulent flows

The drag reduction characteristics and sustainability bounds of superhydrophobic (SH) surfaces in high Reynolds number turbulent flows are investigated using results from direct numerical simulation (DNS) and scaling-law analysis. The DNS studies were performed, using lattice Boltzmann methods, in turbulent channel flows at bulk Reynolds numbers of $Re_{b}=3600$ ($Re_{\unicode[STIX]{x1D70F}_{0}}\approx 222$) and $Re_{b}=7860$ ($Re_{\unicode[STIX]{x1D70F}_{0}}\approx 442$) with SH longitudinal microgrooves or SH aligned microposts on the walls. Surface microtexture geometrical parameters corresponding to microgroove widths or micropost spacings of $4\lesssim g^{+0}\lesssim 128$ in base flow wall units and solid fractions of $1/64\leqslant \unicode[STIX]{x1D719}_{s}\leqslant 1/2$ were investigated at interface protrusion angles of $\unicode[STIX]{x1D703}_{p}=0^{\circ }$ and $\unicode[STIX]{x1D703}_{p}=-30^{\circ }$. Analysis of the governing equations and DNS results shows that the magnitude of drag reduction is not only a function of the geometry and size of the surface microtexture in wall units, but also the Reynolds number of the base flow. A Reynolds number independent measure of drag reduction can be constructed by parameterizing the magnitude of drag reduction in terms of the friction coefficient of the base flow and the shift, $(B-B_{0})$, in the intercept of a logarithmic law representation of the mean velocity profile in the flow with SH walls compared to the base flow, where $(B-B_{0})$ is Reynolds number independent. The scaling laws for $(B-B_{0})$, in terms of the geometrical parameters of the surface microtexture in wall units, are presented for SH longitudinal microgrooves and aligned microposts. The same scaling laws are found to also apply to liquid-infused (LI) surfaces as long as the viscosity ratios are large, $N\equiv \unicode[STIX]{x1D707}_{o}/\unicode[STIX]{x1D707}_{i}\gtrsim 10$. These scaling laws, in conjunction with the parametrization of drag reduction in terms of $(B-B_{0})$, allow for a priori prediction of the magnitude of drag reduction with SH or LI surfaces in turbulent flow at any Reynolds number. For the most stable of these SH surface microtextures, namely, longitudinal microgrooves, the pressure stability bounds of the SH surface under the pressure loads of turbulent flow are investigated. It is shown that the pressure stability bounds of SH surfaces are also significantly curtailed with increasing Reynolds number of the flow. Using these scaling laws, the narrow range of SH surface geometrical parameters which can yield large drag reduction as well as sustainability in high Reynolds number turbulent flows is identified.

Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces

Superhydrophobic surfaces (SHSs) have the potential to achieve large drag reduction for internal and external flow applications. However, experiments have shown inconsistent results, with many studies reporting significantly reduced performance. Recently, it has been proposed that surfactants, ubiquitous in flow applications, could be responsible by creating adverse Marangoni stresses. However, testing this hypothesis is challenging. Careful experiments with purified water already show large interfacial stresses and, paradoxically, adding surfactants yields barely measurable drag increases. To test the surfactant hypothesis while controlling surfactant concentrations with precision higher than can be achieved experimentally, we perform simulations inclusive of surfactant kinetics. These reveal that surfactant-induced stresses are significant at extremely low concentrations, potentially yielding a no-slip boundary condition on the air-water interface (the "plastron") for surfactant concentrations below typical environmental values. These stresses decrease as the stream-wise distance between plastron stagnation points increases. We perform microchannel experiments with SHSs consisting of stream-wise parallel gratings, which confirm this numerical prediction, while showing near-plastron velocities significantly slower than standard surfactant-free predictions. In addition, we introduce an unsteady test of surfactant effects. When we rapidly remove the driving pressure following a loading phase, a backflow develops at the plastron, which can only be explained by surfactant gradients formed in the loading phase. This demonstrates the significance of surfactants in deteriorating drag reduction and thus the importance of including surfactant stresses in SHS models. Our time-dependent protocol can assess the impact of surfactants in SHS testing and guide future mitigating designs.

Drag reduction by means of dimpled surfaces in turbulent boundary layers

Direct force measurements and particle image velocimetry (PIV) were used to investigate the drag and flow structure caused by surfaces with patterns of shallow spherical dimples with rounded edges subject to turbulent boundary layers. Drag reduction of up to 4 % is found compared to a flat surface. The largest drag reduction was found at the highest tested Reynolds number of 40,000 (based on dimple diameter). A favorable trend promises further improvements at higher Reynolds numbers. PIV revealed the absence of significant separation inside the dimples but did show the existence of a converging/diverging flow in the upstream and downstream dimple half, respectively. This leads to the rejection of theories proposed by other authors concerning the mechanism responsible for drag reduction. Instead, a fundamental dependence on pattern orientation is observed. Furthermore, preliminary Reynolds-averaged Navier–Stokes (RANS) simulations have been compared with the PIV data. Although the large-scale mean flows show good agreement, the numerical simulation predicts no drag reduction. As the RANS approach is inherently incapable of resolving effects on the behavior of small-scale turbulence structure, the origin of drag reduction is attributed to effects on the small-scale turbulence, which is not resolved in the simulations. It is argued that dimples, when placed in well-designed patterns to create the necessary large-scale flow structure, lead to drag reduction by affecting the turbulent structures in the boundary layer, possibly in a way similar to spanwise oscillations of the wall.

0612 親水性ゲル壁の摩擦抵抗特性に関する研究

The flow resistance of flat plates with hydrophilic gel wall in water tunnel were measured. A test hydrophilic gel was produced by using agar, which was prepared with concentration of 0.75, 1.0, and 3.0%. Experiments were performed in the range of flow velocity U of 0.04 < U < 0.3 m/s. Drag reduction was obtained for all values of the concentration. The maximum drag reduction was approximately 9.7%. Throughout the measurement range, the larger drag reduction was obtained with the smaller agar concentration. As the flow velocity increased, the drag reduction decreased.

Study on the Drag Reducing Channel Fluids by Experiments and DNS Using Giesekus Model

Both experimental and numerical studies are simultaneously performed for fully developed water and surfactant solution channel flow. The comparison aims at the surfactant solution flow in experiment with mass concentration of 25 ppm at Re = 1000 and Giesekus model with Weissenberg numbers of 10 and 40 at Reτ = 150. Big differences are found between the experimental and DNS results by comparing the distributions of velocity fluctuations, Reynolds shear stress, and so on. Although large drag reduction appears in DNS, Giesekus model has some limitations in describing the fluid characteristics and viscoelasticity of the surfactant solution.

Giant Drag Reduction in Complex Fluid Drops on Rough Hydrophobic Surfaces

We describe a new spreading regime during the drop impact of model yield-stress fluids (Carbopol microgel solutions) on rough hydrophobic surfaces, in a range of parameters where classical Newtonian drops usually splash. For large surface roughness and high impact velocity, we observe that the maximal inertial spreading diameter of the drops can be as much as twice larger than on smooth surfaces in the same conditions, corresponding to apparent basal friction reductions of more than 80%. We interpret this large drag reduction using a simple energy balance model and a dynamic slip length that depends on both the surface roughness and the drop's dynamics.

Large Eddy Simulation of Flow Control Around a Cube Subjected to Momentum Injection

The concept of Momentum Injection (MI) through Moving Surface Boundary layer Control (MSBC) applied to a cubic structure is numerically studied using Large Eddy Simulation at a Reynolds number of 6.7×104. Two small rotating cylinders are used to add the momentum at the front vertical edges of the cube. Two configurations are studied with the yaw angle of 0° and 30°, respectively, with ratio of the rotation velocity of cylinders and the freestream velocity of 2. The results suggest that MI delays the boundary layer separation and reattachment, and thus reduces the drag. A drag reduction of about 6.2 % is observed in the 0° yaw angle case and about 44.1 % reduction in the 30° yaw angle case. In the case of 0° yaw angle, the main change of the flow field is the disappearance of the separation regions near the rotating cylinders and the wake region is slightly changed due to MI. In the 30° yaw angle case, the flow field is changed a lot. Large flow separations near one rotating cylinder and in the wake is significantly reduced, which results in the large drag reduction. Meanwhile, the yaw moment is increased about 50.5 %.

Turbulent drag reduction at moderate Reynolds numbers via spanwise velocity waves

AbstractThe question whether large turbulent drag reduction can be achieved at the high values of Re typical of applications is addressed. Answering such question, either by experiments or DNS, is obviously challenging. For DNS, the problem lies in the tremendous increase of the computational cost with Re, that has to be appreciated in view of the need of carrying out an entire parametric study at every Re, owing to the unknown location of the optimal forcing parameters.In this paper we limit ourselves to considering an open‐loop technique based on spanwise forcing, the streamwise‐traveling waves introduced by [1], and explore via Direct Numerical Simulations (DNS) how the drag reduction varies when the friction Reynolds number is increased from Reτ = 200 to Reτ = 2000. To achieve high Re while keeping the computational cost affordable, computational domains of reduced size are employed. We adopted special care to interpret results that are indeed still box‐size dependent, as well as strategies to compute the random errors and give the results an error bar.Our results indicate that still R = 0.29 can be obtained at Reτ = 2000 in the partial region of the parameter space studied. The maximum R is found to decrease as R ˜ Reτ−0.22 in the Reynolds range investigated. As most important outcome, we find that the sensitivity of R to Re becomes smaller when far from the low‐Re optimum parameters: in this region, we suggest R ˜ Reτ−0.08. (© 2012 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Drag reduction in turbulent boundary layers by spanwise traveling waves with wall deformation

The drag-reducing effect of a spanwise-traveling wave with wall deformation on a zero-pressure-gradient turbulent boundary layer over a flexible sheet was investigated. The test plate placed in the wind tunnel consisted of a flexible sheet section, where the traveling wave motion was generated by a vibration device with a crank via upstream and downstream smooth rigid wall sections. Streamwise and wall-normal velocity components were measured by single and cross hot-wire anemometers. Amplitude and frequency of the spanwise-traveling-wave motion were measured using two laser displacement sensors. The drag reduction ratio was estimated from the friction coefficients through the growth rate of the momentum thickness of the turbulent boundary layer over the flexible sheet section. A maximum drag reduction ratio of up to 13% was obtained. The relations between sheet displacement and streamwise and wall-normal velocity fluctuations were compared at a large (DR=8%) and at a very small (), which was almost the same as the no drag reduction case, taking into account the experimental uncertainty. For the larger drag reduction, a slow-velocity fluctuation with large amplitude appeared further away from the flexible sheet. Quadrant analysis of streamwise and wall-normal velocity fluctuations revealed that the drag reduction was attributed to a reduction in the sweep event and an increase in the negative contribution of the inward-interaction event to the Reynolds shear stress due to the sheet displacement fluctuation.

Microflow-based control of near-wall fluctuations for large viscous drag reduction

The stabilizing effect of microgroove surface morphology on viscous drag reduction was studied experimentally in the inlet region of a plane channel flow. The stabilization is thought to be due to the ability of a microgrooved surface pattern to suppress the velocity fluctuations in the spanwise direction on a restricted portion of the wetted surface, which prevents vorticity development at the wall and consequently across the entire flow field. This smart microflow control strategy, which works successfully only under very particular circumstances, was implemented in a microgroove-modified channel flow in which the front part has a microgrooved surface topology. The results of pressure drop measurements indicate that microgrooved surfaces can effectively stabilize laminar boundary layer development, leading to a significant reduction in the viscous drag. In the rear flat part of the microgroove-modified channel test section, a maximum drag reduction of \({\rm DR\simeq 35\%}\) was measured. This corresponds to an overall drag reduction of \({\rm DR\simeq 16\%}\) at a length Reynolds number of \(Re_x\simeq 10^6. \) The drag reduction effect persisted in a narrow range of flow velocities and for the reported experimental conditions corresponds to microgroove dimensions between 1.5 and 2.5 viscous length-scales.

Comparison of turbulence structures at large and small drag reduction ratios in turbulent boundary layer of surfactant solutions

In this study, turbulence structures of drag-reducing turbulent boundary layers in surfactant solutions at the momentum-thickness Reynolds number Re θ≃800 were compared at large and small drag reduction ratios and 34% at the solution temperature, T=30°C and 35°C. Turbulence statistics and structures were obtained by a two-component laser-Doppler velocimetry (LDV) and particle image velocimetry (PIV) systems for the streamwise and wall-normal (x–y) plane and the streamwise and spanwise (x–z) plane. The quadrant analysis of streamwise and wall-normal velocity fluctuations revealed that at , the positive contribution of sweep and ejection events to the Reynolds shear stress was comparable with the negative contribution of inward-interaction and outward-interaction events, so that the Reynolds shear stress was almost zero. At , the maximum positive contributions of quadrants II and IV and the maximum negative contributions of quadrants I and III were comparable with those of the corresponding water near the wall, but the region of positive contributions in quadrants II and IV became narrower and the region of negative contributions in quadrants I and III became wider, so that the Reynolds shear stress decreased. At , in high activity, the streamwise velocity fluctuations were smaller than those of water but were relatively large even near the center of the boundary layer, in addition to the near-wall region, while in low activity, the fluctuating velocity vectors were almost parallel to the wall. At , the scale of sweep and ejection events was much larger than that of water and the inclination angle of vortex packets at was much smaller than that of water. At , the spanwise turbulence fluctuation was strongly suppressed and vortical structures were not observed on the (x–z) plane, while at , several vortical structures, which seemed to correspond to the cross section of legs of hairpin vortices, were observed near the wall. Under the assumption of frozen turbulence, it was found that for both and 34%, the high- and low-speed regions were aligned laterally, although the low-speed regions at were obscured.