Skin Friction Drag Reduction Research Articles

Analysis of Onset of Transition Region on a Supersonic Wing

Abstract: Climate change is due to the sole effect of global warming, which is an outcome of Green House Gas (GHG) emissions. The aviation industry is considered to be one of the fastest-growing sources of these emissions. Mitigation of these emissions is a challenging task since it requires the entire world to work hand-in-hand. The International Civil Aviation Organisation (ICAO), an international governing body, has come up with a challenging target of net 50% reduction in global aviation emission by 2050, relative to the emission levels in 2005. To achieve this target, there must at least be 2% improvement in fuel efficiency every year. This 2% efficiency improvement is further split into Aircraft technology improvement (1.1%) and Airport infrastructure improvement (0.9%). This research aims to achieve the 2% improvement by aircraft technology improvement. Over the years, it was evidenced that mitigation of skin-friction drag on the wing would contribute to a majority of the fuel efficiency improvement. Hence, the primary aim is narrowed down to wing skin friction drag. The theory of boundary-layer has been studied thoroughly in this research work. Also, the interaction between the shock-waves and boundary-layer will be briefed in this paper as the baseline aircraft is supersonic. As it was understood as understood over the research that extension of the laminar boundary-layer favours the reduction of skin-friction drag, it is the primary objective of this research work. A Computational Fluid Dynamics (CFD) study on the of the wing aerodynamics has been conducted to enhance the performance of the aircraft and ANSYS Fluent has been used to serve this purpose. The results obtained from ANSYS fluent will be validated in one of the NASA’s open-source tool VSPAero which uses Vortex Lattice Method to model the flow. Turbulent Kinetic Energy technique is used to predict the onset of the transition of the flow. Initial study is performed on the 2D aerofoil and one the most optimum aerofoil is obtained, the study is continued in 3D to understand the effect of cross-flow instabilities. Free-stream velocity profile variation is another reliable technique in predicting the transition and will be discussed in the next volume of the paper. Finally, aerodynamic performance analysis will be performed to obtain the best wing configuration.

Characteristics of a dielectric barrier discharge plasma actuator driven by pulsed-DC high voltage

Dielectric barrier discharge using pulsed-DC high voltage (pulsed-DC DBD) have been proven to be capable of effectively reducing skin friction drag in turbulent boundary layers with limited power consumption, thus producing significant net power savings. In this work, the characteristics of pulsed-DC DBD, including power consumption, induced flow structure, thermal effect, and body force, are investigated sequentially. Both the power consumption and pressure waves produced by pulsed-DC DBD are similar to that of DBD using nanosecond pulses (ns-DBD), whereas the wall-bounded jet structure resembles that of DBD using sinusoidal high voltage (ac-DBD). A curved wall jet is induced at a small pulse width, which turns into a straight one due to the combined effect of momentum and thermal addition when the pulse width increases. With increasing pulse width, the induced body force goes up while the thermal effect weakens. Although pulse frequency has no impact on the wall-bounded jet topology, the body force increases with pulse frequency because of the enhanced energy entrainment. With these results, four parameters that affect the performance of pulsed-DC DBD are extracted, including the pulse leading edge, pulse width, frequency, and amplitude, which lays the foundation for the optimization of pulsed-DC DBD.

In-situ boundary layer transition detection on multi-segmental (a)synchronous morphing wings

This paper presents an experimental method to detect in-situ the location of transition on a multi-segmental trailing edge camber morphing wing during synchronous and asynchronous morphing. The wing consists of six independently morphing segments with two of the segments instrumented with eight embedded piezoelectric sensors distributed uniformly along the chord. Using suitable data processing, each of the sensors gives a signal that can be used to determine the state of the boundary layer (laminar, transitional, turbulent) at the location of that sensor. The results showed that synchronous morphing can substantially shift the location of transition, up to 20% of the chord length for angles of attack below 9°. Differences in the location of transition up to 5% are found between the near-root and near-tip segment. Using a dedicated data processing approach, the location of transition could be reconstructed in case of complex asynchronous morphing involving one to five segments. The results show a shift in the location of transition when morphing neighboring segments, but also show that non-neighboring segments have a minimal effect. This sensing method holds significant promise for online advanced morphing control to delay transition and thereby reducing skin friction drag.

Reduction of turbulent skin-friction drag by passively rotating discs

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Composite active drag control in turbulent channel flows

A composite drag control (CDC) combining the opposition (OC) and spanwise opposed wall-jet forcing (SOJF) methods is investigated. A maximum drag reduction of about 33% is obtained for CDC -- much higher than that produced by either individual method (namely, 19% for SOJF and 23% for OC). Flow analysis shows that CDC can take advantage of both OC and SOJF methods to better suppress drag producing near-wall turbulent structures --vortices and streaks. Our results suggest prospects of employing a composite control strategy for effective skin friction drag reduction, particularly at very high Reynolds numbers.

Reduction of surface friction drag in scramjet engine by boundary layer combustion

This study investigates the effects of boundary layer combustion on skin friction reduction in a model scramjet combustor. The 4-equation RANS model (Transition SST model) is employed as the turbulence model and one experimental case which involves supersonic turbulent boundary layer combustion is used for validating the numerical simulation method. Then a wall jet device which can be used to inject hydrogen into boundary layer is designed and added to the lower wall of the combustor in the model scramjet engine. The numerical results showed that the optimal ratio between the primary fuel, producing thrust, and the wall jet fuel, used for drag reduction, for getting the largest skin-friction drag reduction when installing the wall jet device in the combustor, is 3:1. The study of modifying the location of the wall jet device showed that setting the wall jet position too close to the upstream or downstream could not get the maximum overall drag reduction. The installation position in the flow path depends on the coupling of primary fuel and wall jet fuel combustion. In practical scramjet design, the distribution ratio of primary to wall jet fuel and the location of wall jet device should be weighed to improve skin-friction reduction efficiency and overall engine performance.

A review of turbulent skin-friction drag reduction by near-wall transverse forcing

The quest for reductions in fuel consumption and CO2 emissions in transport has been a powerful driving force for scientific research into methods that might underpin drag-reducing technologies for a variety of vehicular transport on roads, by rail, in the air, and on or in the water. In civil aviation, skin-friction drag accounts for around 50% of the total drag in cruise conditions, thus being a preferential target for research. With laminar conditions excluded, skin friction is intimately linked to the turbulence physics in the fluid layer closest to the skin. Hence, research into drag reduction has focused on methods to depress the turbulence activity near the surface. The most effective method of doing so is to exercise active control on the near-wall layer by subjecting the drag-producing flow in this layer to an unsteady and/or spatially varying cross-flow component, either by the action of transverse wall oscillations, by embedding rotating discs into the surface or by plasma-producing electrodes that accelerate the near-wall fluid in the transverse direction. In ideal conditions, drag-reduction margins of order of 50% can thereby be achieved. The present article provides a near-exhaustive review of research into the response of turbulent near-wall layers to the imposition of unsteady and wavy transverse motion. The review encompasses experiments, simulation, analysis and modelling, mainly in channel flows and boundary layers. It covers issues such as the drag-reduction margin in a variety of actuation scenarios and for a wide range of actuation parameters, the underlying physical phenomena that contribute to the interpretation of the origin of the drag reduction, the dependence of the drag reduction on the Reynolds number, passive control methods that are inspired by active control, and a forward look towards possible future research and practical realizations. The authors hope that this review, by far the most extensive of its kind for this subject, will be judged as a useful foundation for future research targeting friction-drag reduction.

Influence of riblet shapes on the occurrence of Kelvin–Helmholtz rollers

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Separation drag reduction through a spanwise oscillating pressure gradient

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Superhydrophobic drag reduction in high-speed towing tank

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Direct Numerical Simulations of Turbulent Flow Over Various Riblet Shapes in Minimal-Span Channels

Riblets reduce skin-friction drag until their viscous-scaled size becomes large enough for turbulence to approach the wall, leading to the breakdown of drag-reduction. In order to investigate inertial-flow mechanisms that are responsible for the breakdown, we employ the minimal-span channel concept for cost-efficient direct numerical simulation (DNS) of rough-wall flows (MacDonald et al. in J Fluid Mech 816: 5–42, 2017). This allows us to investigate six different riblet shapes and various viscous-scaled sizes for a total of 21 configurations. We verify that the small numerical domains capture all relevant physics by varying the box size and by comparing to reference data from full-span channel flow. Specifically, we find that, close to the wall in the spectral region occupied by drag-increasing Kelvin–Helmholtz rollers (Garcia-Mayoral and Jimenez in J Fluid Mech 678: 317–347, 2011), the energy-difference relative to smooth-wall flow is not affected by the narrow domain, even though these structures have large spanwise extents. This allows us to evaluate the influence of the Kelvin–Helmholtz instability by comparing fluctuations of wall-normal and streamwise velocity, pressure and a passive scalar over riblets of different shapes and viscous-scaled sizes to those over a smooth wall. We observe that triangular riblets with a tip angle $$\alpha =30^{\circ }$$ and blades appear to support the instability, whereas triangular riblets with $$\alpha =60^{\circ }$$ – $$90^{\circ }$$ and trapezoidal riblets with $$\alpha =30^{\circ }$$ show little to no evidence of Kelvin–Helmholtz rollers.

Machine-learning-based feedback control for drag reduction in a turbulent channel flow

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Characteristics of array of distributed synthetic jets and effect on turbulent boundary layer

An array of distributed round synthetic jets was used to control a fully developed turbulent boundary layer. The study focused on the related skin friction drag reduction and mechanisms involved. The control effects were analyzed by measuring the streamwise velocities using a hot-wire anemometer downstream of the array. A reduction in the skin friction was observed both in the regions downstream of the orifices and in the regions between two adjacent orifices. A statistical analysis with the variable-interval time-averaging (VITA) technique demonstrated a weakened bursting intensity with synthetic jet in the near-wall region. The streamwise vortices were lifted by the upwash effect caused by synthetic jet and induced less low-speed streaks. The control mechanism acted in a way to suppress the dynamic interaction between the streamwise vortices and low-speed streaks and to attenuate the turbulence production in the near-wall region. The forcing frequency was found to be a more relevant parameter when synthetic jet was applied in turbulent boundary layer flow control. A higher forcing frequency induced a higher reduction in the skin friction. The power spectral density and autocorrelation of the fluctuating velocities showed that the synthetic jets gradually decayed in the streamwise direction, having an effect as far as 34.5 times the displacement thickness that was on the trailing edge of the distributed synthetic jets array.

Heading control of a novel finless high-speed supercavitating vehicle with an internal oscillating pendulum

High-speed supercavitating vehicles are surrounded by a huge cavity of gas and only a small portion of the nose and the tail of the vehicle are in contact with the water which leads to a considerable reduction in skin friction drag and reaching very high speeds. High-speed supercavitating vehicles are usually controlled by the cavitator at the nose which controls the pitch and depth of the vehicle and the control surfaces or fins which control the roll and heading angle of the vehicle using the bank-to-turn maneuvering method. However, control surfaces have disadvantages such as the high drag force and ineffectiveness due to the supercavity. Therefore, the purpose of the present study is to eliminate the fins from high-speed supercavitating vehicles and propose a new bank-to-turn heading control of this novel finless high-speed supercavitating vehicle which is composed of the cavitator at the nose and an oscillating pendulum as the internal actuator. Sliding mode control as a robust method is used for the six-degrees-of-freedom model of this finless high-speed vehicle against exposed disturbances. Some design criteria for the design of the internal pendulum in this finless supercavitating vehicle are presented for the damping coefficient, pendulum mass, and radius.

3-dimensional particle image velocimetry based evaluation of turbulent skin-friction reduction by spanwise wall oscillation

The reduction of turbulent skin-friction drag and the response of vortical structures in a zero-pressure gradient, turbulent boundary layer subjected to spanwise wall oscillation is investigated using planar and tomographic particle image velocimetry (PIV). The experiments are conducted at a momentum based Reynolds number of 1000, while the range of spanwise oscillation amplitude and frequency is chosen around the optimum reported in previous studies. A high-resolution planar PIV measurement is employed to determine the drag reduction directly from wall shear measurements and to analyze the accompanying modifications in the turbulent vortical structures. Drag reduction of up to 15% is quantified, with variations following the trends reported in the literature. The analysis of the turbulence structure of the flow is made in terms of Reynolds shear stresses, turbulence production, and vortex visualization. A pronounced drop of turbulence production is observed up to a height of 100 wall units from the wall. The vorticity analysis, both in the streamwise wall-normal plane and in the volumetric results, indicates a reduction of vorticity fluctuations in the near-wall domain. A distortion of the hairpin-packet arrangement is hypothesized, suggesting that the drag-reduction mechanism lies in the inhibition of the hairpin auto-generation by the spanwise wall oscillations.

Influence of curvature on drag reduction by opposition control in turbulent flow along a thin cylinder

Opposition controlled fully developed turbulent flow along a thin cylinder is analyzed by means of direct numerical simulations. The influence of cylinder curvature on the skin-friction drag reduction effect by the classical opposition control (i.e., the radial velocity control) is investigated. The curvature of the cylinder affects the uncontrolled flow statistics; for instance, skin-friction coefficient increases while Reynolds shear stress (RSS) and turbulent intensity decrease. However, the control effect in the case of a small curvature is similar to that in channel flow. When the curvature is large, the maximum drag reduction rate decreased. However, the optimal location of the detection plane is the same as that in a flat plate. Further, the drag reduction effect is achieved even on a high detection plane where the drag increases in the flat plate. Although a difference in the drag reduction effect can be observed with a change in the curvature, its mechanism considered in this analysis based on the transport of the Reynolds stress is similar to that of the flat plate.

Turbulent drag reduction over curved walls

This work studies the effects of skin-friction drag reduction in a turbulent flow over a curved wall, with a view to understanding the relationship between the reduction of friction and changes to the total aerodynamic drag. Direct numerical simulations (DNS) are carried out for an incompressible turbulent flow in a channel where one wall has a small bump; two bump geometries are considered, that produce mildly separated and attached flows. Friction drag reduction is achieved by applying streamwise-travelling waves of spanwise velocity (StTW). The local friction reduction produced by the StTW is found to vary along the curved wall, leading to a global friction reduction that, for the cases studied, is up to 10\% larger than that obtained in the plane-wall case. Moreover, the modified skin friction induces non-negligible changes of pressure drag, which is favorably affected by StTW and globally reduces by up to 10\%. The net power saving, accounting for the power required to create the StTW, is positive and, for the cases studied, is one half larger than the net saving of the planar case. The study suggests that reducing friction at the surface of a body of complex shape induces further effects, a simplistic evaluation of which might lead to underestimating the total drag reduction.

Evaluation and Effect of Micro Bubble Lubrication in Drag Reduction

This paper aims to study the reduction of skin friction coefficient (Cf) of a flat plate by injecting micro bubbles onto the surface of the plate. Numerical simulation of flow of water past a flat plate was carried out at different Reynolds numbers. Skin friction coefficient (Cf) was found and compared with research papers and suitable empirical formula. The numerical solution was solved in ANSYS. Subsequently, micro bubbles were injected and the new Cf found. The velocity of injection of the bubbles was then varied. Drag reduction was observed for flat plate post micro bubble injection and injection velocity was varied, corresponding results were noted. The results agree well. This paper verifies effect of micro bubble injection in skin friction drag reduction computationally.

Phase-space dynamics of near-wall streaks in wall-bounded turbulence with spanwise oscillation

This work presents systematical investigations on the skin-friction drag reduction (DR) of turbulent channel flow subjected to spanwise wall oscillation using direct numerical simulation. Altogether 12 different oscillatory cases have been studied with a reference at Reτ = 200, varying the controlling parameters characterized by maximum wall velocity Wm+ and oscillation period T+. Some of the previously established facts have been reproduced by our analysis with a new focus on the phase-space dynamics of the near-wall streaks, on the basis of statistical data over entire oscillation periods and over phasewise variations. It is revealed that streamwise vortices are generated in the vicinity of oscillation walls, disrupting the formation of near-wall low-speed streaks. Although the overall turbulence is weakened, the Stokes layer is thicker within wall acceleration phases for larger Wm+, which causes the turbulence intensity to increase in the upper viscous sublayer. In addition, regarding the effect of T+, a long oscillation period promotes the formation of energetic near-wall structures, while for short T+, the streak-generation time scale preferentially restricts the growth of spanwise streaks. From a new vorticity-transport perspective of the Reynolds shear stress, our results further indicate that high drag-reducing phenomena are connected to the near-wall sweep events, and the shear stress variation is principally driven by the distortion of the spanwise transport of wall-normal vorticity, i.e., vortex tilting/stretching. The DR process is seen to be linked to the increase in enstrophy and turbulence-energy dissipation in the near-wall region.

Evaluation of Skin Friction Drag Reduction in the Turbulent Boundary Layer Using Riblets

A unique approach to evaluate the reduction of skin friction drag by riblets was applied to boundary layer profiles measured in wind tunnel experiments. The proposed approach emphasized the turbulent scales based on hot-wire anemometry data obtained at a sampling frequency of 20 kHz in the turbulent boundary layer to evaluate the skin friction drag reduction. Three-dimensional riblet surfaces were fabricated using aviation paint and were applied to a flat-plate model surface. The turbulent statistics, such as the turbulent scales and intensities, in the boundary layer were identified based on the freestream velocity data obtained from the hot-wire anemometry. Those turbulent statistics obtained for the riblet surface were compared to those obtained for a smooth flat plate without riblets. Results indicated that the riblet surface increased the integral scales and decreased the turbulence intensity, which indicated that the turbulent structure became favorable for reducing skin friction drag. The proposed method showed that the current three-dimensional riblet surface reduced skin friction drag by about 2.8% at a chord length of 67% downstream of the model’s leading edge and at a freestream velocity of 41.7 m/s (Mach 0.12). This result is consistent with that obtained by the momentum integration method based on the pitot-rake measurement, which provided a reference dataset of the boundary layer profile.