Skin Friction Drag Reduction Research Articles

Bidirectional Underwater Drag Reduction on Bionic Flounder Two-Tier Structural Surfaces.

Engineering marvels found throughout the exclusive structural features of biological surfaces have given rise to the progressive development of skin friction drag reduction. However, despite many previous works reporting forward drag reduction where the bio-inspired surface features are aligned with the flow direction, it is still challenging to achieve bidirectional drag reduction for non-morphable surface structures. Inspired by the flounder ctenoid scales characterized by tilted, millimeter-sized oval fins embedded with sub-millimeter spikes, we fabricate a bionic flounder two-tier structural surface (BFTSS) that can remarkably reduce the forward skin friction drag by ηdr = 19%. Even in the backwards direction, where the flow is completely against the tilting direction of surface structures, BFTSS still exhibits a considerable drag reduction of ηdr = 4.2%. Experiments and numerical simulations reveal that this unique bidirectional drag reduction is attributed to synergistic effects of the two-tier structures of BFTSS. The array of oval fins can distort the boundary layer flow and mitigate the viscous shear, whilst the microscale spikes act to promote the flow separation to relieve the pressure gradient in the viscous sublayer. Notably, the pressure gradient relief effect of microscale spikes remains invariant to the flow direction and is responsible for the backward drag reduction as well. The bidirectional drag reduction of BFTSS can be extensively applied in minimizing the energy consumption of ships and underwater vessels, as well as in pipeline transport.

Comparative Study on the Effect of Leading Edge Protuberance of Different Shapes on the Aerodynamic Performance of Two Distinct Airfoils

This study investigated the effect of leading-edge protuberances on the aerodynamic performance of two distinct airfoils with low Reynold’s number (Re): E216 and SG6043. Three protuberance shapes, namely sinusoidal, slot, and triangular, were considered. The amplitudes (A) of protuberances considered were 0.03c, 0.06c, and 0.11c, and the wavelengths (W) were 0.11c, 0.21c, and 0.43c, where c is the chord of the airfoil. The numerical and experimental analyses were performed in the angle of attack (AoA) range of 0° to +20° at and Re of 105. The numerical investigation was performed using the commercial computational fluid dynamics package ANSYS FLUENT. The SST k-ɷ model was used to simulate turbulent flow. The experimental force measurements were conducted using a highly sensitive three-component force balance in a subsonic wind tunnel facility. The flow physics was analyzed using vorticity contours in streamwise and spanwise slices and static pressure distribution contours. The smoke flow visualization technique was used to observe flow streamlines, boundary layer separation, and reattachment over the airfoil surface. The result indicated that the triangular and slot protuberances were the most beneficial for improving poststall lift and reducing skin friction drag. The operating mechanism involved a shift in pressure distribution due to leading-edge alterations and flow energization by secondary flow emanating from the protuberances.

Optimization of spatial-temporal distribution of a body force for reducing skin friction drag in a fully developed channel flow

Optimization of spatial-temporal distribution of a body force for reducing skin friction drag in a fully developed channel flow

Numerical investigation of the drag reduction effect in turbulent channel flow by superhydrophobic grooved surfaces

Abstract Superhydrophobic surfaces (SHSs) are considered to be a promising technology for achieving skin-friction drag reduction. Development of more efficient techniques for simulating the turbulent boundary layer on SHSs continues to be a subject of interest. In this study, numerical simulations were carried out to capture near-wall behaviours due to the effect of the SHS on wall-bounded flows. To achieve this, high- to intermediate-fidelity turbulence models including Reynolds-averaged Navier–Stokes, detached eddy simulation and large eddy simulation were utilized. With regard to slip conditions, the well-known Navier slip velocity method was used over the SHS. For validating the numerical solutions, the slip velocity and skin friction over the SHS were compared with the experimental output. Results showed that the velocity profile and Reynolds stresses on the SHS were comparable to the reported results. Then, the developed models were further extended to investigate the drag reduction effect of SHSs with rectangular grooves. The subsequent results showed that the combination of superhydrophobicity and rectangular grooves led to a better performance with a maximum drag reduction of 46.1%. This is due to the surface slip caused by the SHS and the secondary vortex effect created by the grooves. Our results revealed that Reynolds stresses of the slippery grooved surface were higher than those of the case in which a shear-free condition was employed for the grooved surface. More importantly, the numerical results indicate the previous assumption of the shear-free condition is inaccurate for the geometrically simplified grooved SHSs. Therefore, geometry modifications rather than an overly simplified shear-free boundary condition should be applied in computational fluid dynamics simulations for SHSs with grooves or other complex structures.

Influence of Micro-Blowing Technique Hole Parameters on Drag Reduction of Civil Aircraft Engine Nacelle: A Computational Study

The numerical parametric analysis conducted to analyze the impact of micro-blowing technique (MBT) hole-parameters are quite few at the present stage. The main aim of this research paper is to analyze the effect of micro blowing flow rate and its different hole-parameters on the skin friction drag reduction of an aircraft engine nacelle operating at cruise conditions. The primary tasks are focused to understand the behavior of the flow characteristics at the vicinity of the micro-porous holes by means of different types of MBT configurations. The interaction between main-stream flow and the micro-channel flow is numerically solved by using the Reynolds average Navier-Stokes equation and the k-omega SST is used to model the turbulent flow at the vicinity of the wall region. The hole-pattern is kept aligned in a single-row channel and the shape of the hole cross-section is kept straight to obtain an overall simplicity of the simulation model. The influences of the micro blowing technique are quite clearly seen from the simulation results, as there is a significant reduction in the velocity gradient between the solid engine nacelle surface and all the MBT configurations. The porous engine nacelle surface with zero blowing velocity is capable to reduce the skin friction drag by 7.045 % than of its solid surface, implying that the presence of the micro-porous holes possesses low effective surface roughness, and it is an effective method to manipulate the turbulent boundary layer. The optimum skin friction drag reduction is observed when the geometrical characteristics of the holes possess small diameter and high aspect ratio.

Skin-friction drag reduction by local porous uniform blowing in spatially developing compressible turbulent boundary layers

This paper investigates the effects of local porous uniform blowing on the skin-friction drag reduction in the spatially developing compressible turbulent boundary layers through direct numerical simulations. Under the influence of uniform blowing, the skin-friction drag decreases drastically within the controlled regions, and the effect extends downstream. The drag reduction rate exceeds 10% for the uniform blowing velocities considered herein. The intrinsic mechanism of decreased skin-friction drag is further explored, in terms of the various contributions to this quantity, the Reynolds shear stress budget, and the multi-scale distributions and transfer of enhanced turbulent motions. The numerical results for the identity of Renard and Deck [“A theoretical decomposition of mean skin friction generation into physical phenomena across the boundary layer,” J. Fluid Mech. 790, 339–367 (2016)] indicate that the decreased skin-friction drag is reflected in a negative contribution in the streamwise developing direction. The enhanced turbulent motion is investigated through the Reynolds shear stress budget, and uniform blowing is found to strengthen the magnitudes of all budget terms. We conclude that uniform blowing promotes the energy cascade process in the near-wall region, promoting the appearance of smaller turbulent structures and the formation of large outer scales. Similar skin-friction drag reduction in the subsonic case is also observed in a supersonic case. The upstream transition process is delayed, which leads to a relatively larger skin-friction drag reduction rate in a supersonic case. Finally, we investigate the porosity effects on the skin-friction drag and conclude that the blowing flux is a decisive factor.

Direct numerical simulation of turbulent pipe flow by large-scale control using buoyancy force

Skin-friction drag reduction control for a fully developed turbulent pipe flow is performed using direct numerical simulations. Heating and cooling of pipe wall is homogeneous in the streamwise direction and varies in the azimuthal direction and buoyancy force induces the flow to decrease the skin-friction drag. The control parameters are the amplitude of buoyancy force and wavenumber of heating and cooling in the azimuthal direction. There are two different mechanisms of skin-friction drag reduction: stable stratification and large-scale vortical structure. In the former, the buoyancy force attenuates the turbulence directly. In the latter, the buoyancy force creates wall jets along the pipe wall, and a large-scale streamwise vortex is formed. The vortex increases the coherent contribution to the skin-friction coefficient, but it reduces the random contribution indirectly.

Feature-based analysis of a turbulent boundary layer under spanwise wall oscillation

Spanwise wall oscillations alter the organization of low-speed streaks and ejections in turbulent boundary layers, eventually leading to skin friction drag reduction. Such flow regimes are represented by pointwise statistics or spatial correlation. This work attempts to quantify the systematic distortions of the dominant turbulent structures by feature-analysis, intended to overcome the dispersion observed in pointwise statistics and correlation functions. Furthermore, data from tomographic particle image velocimetry are employed to clarify the mechanism that inhibits hairpin auto-generation, as described in Kempaiah et al. [“3-dimensional particle image velocimetry based evaluation of turbulent skin-friction reduction by spanwise wall oscillation,” Phys. Fluids 32(8), 085111 (2020)]. Based on the instantaneous distribution of Reynolds stresses, a specific spatial template is defined for low-speed streaks and flow ejections. Events corresponding to this template are collected and parametrized with their occurrence, geometrical properties (length and orientation), and dynamics (intensity). The approach is compared with most practiced statistical analysis to explain the significance of the features extracted by the detection algorithm in relation to the drag reduction mechanism. Data comparing stationary and oscillating wall in a drag-reducing regime (A+osc = 100, T+osc = 100) are investigated in the near-wall region (y+ < 100). Ejections and low-speed streaks systematically exhibit a positive pitch, supporting the hypothesis that only the rear region, close to the wall, is affected by the wall motion. A side-tilt of elongated ejection events is observed past the phase of maximum oscillation velocity, which is hypothesized to inhibit hairpin auto-generation. The latter indicates a phase dependence of the side-tilt in the oscillating regime. The results also indicate that low-speed streaks and ejection events are reduced by approximately 10% and 15%, respectively, compared with the stationary wall, further consolidating the mechanism of rapid lateral distortion being responsible for the different organizations of the turbulent structures in the near-wall region.

Darcy-Forchheimer natural convection flow along a complex roughened surface

The influence of complex roughened surface on the Darcy-Forchheimer natural convection flow is investigated numerically. A natural convection flow within porous medium occurs in a variety of engineering applications that include solar power collectors that are used to control the pollutant spread, cooling of nuclear reactors, and geothermal engineering. Recent research has focused on reducing skin friction drag induced by viscous forces acting on a rigid body surface as it moves through a fluid by considering roughened surface or mass transfer (mass injection or suction) at the surface. The purpose of this study is to investigate how much skin friction drag reduces by considering complex roughened surface in Darcy-Forchheimer natural convection flow. A mathematical model is obtained in the form of nonlinear PDEs by transferring surface roughness effect from boundary to governing equations and made dimensionless through suitable scaling variables. To obtain a numerical solution for resulting nonlinear PDEs, a finite difference scheme is applied. The results reveal that skin friction drag reduces by making roughened surface a complex surface and further reduces by adding the combined effects of Darcy and Forchheimer numbers also total heat transfer rate enhances by increasing the values of Darcy as well as Forchheimer numbers. These results seem novel findings in this study.

Dependence of skin-friction reduction on the geometric parameters of blowing jet array

Unsteady wall-normal blowing through one array of streamwise slits has been recently demonstrated to reduce substantially skin-friction drag in a turbulent boundary layer [X. Q. Cheng et al., “Skin-friction reduction using periodic blowing through streamwise slits,” J. Fluid Mech. 920, (2021)]. This work aims to understand the dependence of skin-friction drag reduction (DR) on the parameters of the slit array, including the slit width b, length l, and number N or actuation width. The control performance exhibits a dependence on the three parameters in terms of DR, downstream drag recovery, and net-energy saving. It has been found for the first time that DR achieved under high-frequency unsteady blowing may exceed that under steady blowing given the same blowing mass flow rate. Physical mechanisms behind are discussed. The scaling of the drag reduction is also examined empirically, providing valuable insight into the flow physics behind control.

Drag reduction using velocity control in Taylor–Couette flows

Direct numerical simulation of Taylor–Couette flow subject to opposition control is investigated at Reynolds number (Re) of 3000. The idea is to impose exact opposite velocities of the detection plane at the walls to counteract near-wall stream-wise vortices. In this study, various velocity control strategies, namely wall-normal, axial, combined and blowing only, have been investigated from the viewpoint of skin-friction drag reduction. Further, the effects of skipping spatial points in azimuthal and axial directions and in time have been investigated from a drag reduction point of view. Based on the emergence of a virtual wall that hinders the vertical transport of momentum (i.e. on reduction of Reynolds shear stress production as well as sweep ejection events), flow physics has been explained via statistical analysis of fluctuations, Reynolds shear stresses, and near-wall coherent structures. The spatial density of near-wall vortical structures shows a marked reduction, followed by quadrant contribution analysis of Reynolds shear stresses reveals a decrease in ejection and sweep events, leading to reduced production of Reynolds shear stresses and skin-friction drag.

Experimental investigation and reduced-order modeling of plasma jets in a turbulent boundary layer for skin-friction drag reduction

Stereo particle imaging velocimetry measurements and reduced-order modeling are combined to provide a full picture of the interaction of plasma jets with a turbulent boundary layer (TBL). Three working modes of the plasma actuator are investigated, corresponding to a unidirectional jet (mode A), a steady crashing jet (mode B), and a spanwise oscillating jet (mode C). The results show that in mode C, a periodical alteration of two opposite wall jets can only be achieved at a low modulation frequency of 20 Hz. As the frequency increases to 100 Hz, the two unsteady wall jets collide in the middle, producing a meandering vertical jet column. In the cross-flow TBL, mode A induces a single streamwise vortex, which grows in size within the plasma actuation zone and decays rapidly in strength after propagating beyond. As a comparison, modes B and C produce a counter-rotating vortex pair during the interaction. The skin-friction drag variations within the plasma actuation zone are dominated by the cross-stream momentum transportation of streamwise vortices. In the vortex upwash zone where a strong shear is present, high levels of turbulent kinetic energy are produced. Physically, the spanwise shaking and vertical jumping of plasma jet heads contribute noticeably to turbulent fluctuation. Experimental evidence supports the simplification of a streamwise momentum equation into a nonlinear transportation–diffusion equation, resulting in a reduced-order streamwise vortex transportation model. Detailed comparison with the experimental data shows that this model is able to give a reasonable prediction of the cross-stream flow patterns and streamwise velocity variations within minutes.

Dimples for Skin-Friction Drag Reduction: Status and Perspectives

Dimples are small concavities imprinted on a flat surface, known to affect heat transfer and also flow separation and aerodynamic drag on bluff bodies when acting as a standard roughness. Recently, dimples have been proposed as a roughness pattern that is capable of reducing the turbulent drag of a flat plate by providing a reduction of skin friction that compensates the dimple-induced pressure drag and leads to a global benefit. The question whether dimples do actually work to reduce friction drag is still unsettled. In this paper, we provide a comprehensive review of the available information, touching upon the many parameters that characterize the problem. A number of reasons that contribute to explaining the contrasting literature information are discussed. We also provide guidelines for future studies by highlighting key methodological steps required for a meaningful comparison between a flat and dimpled surface in view of drag reduction.

Modification of a turbulent boundary layer by circular cavities

It is shown how well-chosen perforations in a wall flow can locally reduce skin friction drag by modifying the generation of bursts in the boundary layer. For this purpose, a detailed hot wire boundary layer experimental investigation of the flow past a perforated plate, complemented with large eddy simulations, is carried out and compared to the smooth case. The perforated plate is obtained with an array of flush-mounted circular cavities. These cavities are disposed in a periodic staggered arrangement. For the three tested flow velocities, the momentum thickness-based Reynolds number varies from Reθ=1830 to 3380 and the cavity diameter and spacing in wall units, respectively, from d+=130 to 250 and L+=587 to 1075, the latter being identical in both spanwise and streamwise directions. The mean velocity profiles evidence a thickening of the viscous sublayer and a decrease in the friction velocity as compared to the smooth wall case. The application of the Variable Interval Time Averaging technique highlights an upward shift of the bursts from the wall and an attenuation of the average burst intensity and duration. Spanwise measurements evidence an overall bursts attenuation despite the lack of spanwise uniformity. The three-dimensional (3D) mean flow topology arising from the large eddy simulations provides evidence for the qualitative similarities between the current setup and the spanwise wall oscillations.

Drag reduction on a transonic airfoil

Flow control for turbulent skin-friction drag reduction is applied to a transonic airfoil to improve its aerodynamic performance. The study is based on direct numerical simulations (with up to 1.8 billion cells) of the compressible turbulent flow around a supercritical airfoil, at Reynolds and Mach numbers of $Re_\infty = 3 \times 10^{5}$ and $M_\infty =0.7$ . Control via spanwise forcing is applied over a fraction of the suction side of the airfoil. Besides locally reducing friction, the control modifies the shock wave and significantly improves the aerodynamic efficiency of the airfoil by increasing lift and decreasing drag. Hence, the airfoil can achieve the required lift at a lower angle of attack and with a lower drag. Estimates at the aircraft level indicate that substantial savings are possible; when control is active, its energy cost becomes negligible thanks to the small application area. We suggest that skin-friction drag reduction should be considered not only as a goal, but also as a tool to improve the global aerodynamics of complex flows.

Drag reduction in wall-bounded turbulence by synthetic jet sheets

A turbulent drag-reduction method employing synthetic jet sheets in a turbulent channel flow is investigated by direct numerical simulations. The jet sheets are wall-parallel and produced by periodic blowing and suction from pairs of thin slots aligned with the main streamwise flow. By varying the slot height and the jet-sheet angle with respect to the spanwise direction, drag-reduction margins between $10\,\%$ and $30\,\%$ are obtained for jet-sheet angles between $45 ^{\circ }$ and $75 ^{\circ }$ , while a drag increase of almost 100 % is computed when the jet sheets are spanwise-oriented. When global skin-friction drag reduction occurs, the wall-shear stress near the jet-sheet exits increases during suction and decreases during blowing, while the velocity fluctuations weaken during suction and intensify during blowing. The global drag-reduction effect is produced by a finite counter flow induced by the nonlinear interaction between the jet-sheet flow and the main flow, although the turbulent intensity and Reynolds shear stresses increase. The power spent to generate the jet sheets is computed by modelling numerically the actuator underneath the channel flow as a piston oscillating sinusoidally along the spanwise direction in a round-shaped cavity from which the fluid is released into the channel through the cavity exits. A power balance leads to the computation of the efficiency of the actuator system, quantifying the portion of the piston power that is lost as internal power fluxes and heat transfer through the cavity walls. For the tested configurations, the power consumed by the piston to generate the jet sheets is larger than the power saved thanks to the drag reduction.

Modification of energetic modes for transitional flow control

We introduce a mechanism of using collocated serpentine shaped plasma actuators for controlling the flow via an input electrical signal to either advance or delay transition by amplification or annihilation of energetic modes of turbulent structures. A wall resolved implicit large eddy simulation is conducted to examine the process of turbulent flow control due to this mechanism. Collocation allows for selective superposition of different energetic modes, which can either subtract or add energy to the baseline flow resulting in turbulent streak manipulation. Unlike most flow control methods that use either a localized large amplitude forcing or a low amplitude distributed forcing, this mechanism uses a localized low amplitude forcing to cause reduction in skin friction of more than 53% across the plate. This is achieved by manipulating strength as well as the spacing between the low-speed turbulent streaks, which are ubiquitous in a turbulent flow field. Reduction in skin friction drag can result in decreasing fuel consumption and in turn reducing pollution.

A General Model for the Longevity of Super-Hydrophobic Surfaces in Under-Saturated, Stationary Liquid

Abstract We perform a numerical study of the longevity of a super-hydrophobic surface (SHS) in under-saturated, stationary liquid. We numerically solve the spatial-temporal evolution of the gas concentration in the liquid, the time-variation of mass flux of gas out of the plastron, as well as the time required for the gas in the plastron to be fully dissolved (i.e., the plastron lifetime). We find that the profiles of gas concentration at different times are self-similar, and the mass flux reduces with time (t) at a rate of 1/t0.5. In addition, we examine the impact of texture parameters, including pitch, gas fraction, texture height, and advancing contact angle, on the diffusion process. Our results show that both plastron lifetime and diffusion length increase with increasing the gas fraction or increasing the texture height and are independent of the advancing contact angle and pitch. We propose simple analytical models for plastron lifetime and diffusion length. We show that the model has a fair agreement with the experimental data reported in the literature, and can predict the longevity for SHS with various texture geometries, texture sizes, and under different degrees of under-saturations. Our models could guide the design of long-life SHS for underwater applications such as reducing skin-friction drag and preventing biofouling.

Numerical investigations of the slot blowing technique on the hypersonic vehicle for drag reduction

In this paper, the slot blowing flow-control technique is numerically studied for the hypersonic vehicle, aiming for skin friction drag reduction. Firstly, the influence patterns of the slot blowing parameters on friction drag reduction are analyzed based on a flat-plate boundary layer flow. It is concluded that there is an optimal blowing Mach number to optimize the drag reduction effect with other parameters unchanged, while as the increase of blowing pressure ratio, the skin friction drag reduction effect first increases and then remains almost invariant. On this basis, a slot blowing methodology is designed for a typical lifting-body hypersonic vehicle with a 10 mm slot installed on the middle of the windward side. Under the typical freestream condition of Mach 15 and 60 km altitude, numerical results indicate that the slot blowing with the blowing maximum Mach number 2 and blowing pressure ratio 1 could achieve a remarkable control effect of increasing the lift-drag ratio by about 10.55%. After that, the parametric study of slot blowing for the lifting-body vehicle reveals that when the slot height is increased from 10 mm to 20 mm, the lift-to-drag ratio rises slightly after paying the price of doubling the blowing mass flow rate, which indicates that increasing the slot height does not obtain a more efficient result. As for the effect of the slot position along the streamwise, moving the slot position forward could receive a higher total drag reduction rate with less blowing mass flow rate while also leads to a slight decrease of lift. Thus, the practical application of the slot blowing technique needs to consider the variations of the drag and lift comprehensively.

Analysis of the Turbulent Boundary Layer and Skin-Friction Drag Reduction of a Flat Plate by using the Micro-Blowing Technique

Analysis of the Turbulent Boundary Layer and Skin-Friction Drag Reduction of a Flat Plate by using the Micro-Blowing Technique