Friction Drag Reduction Research Articles

Friction drag reduction of Taylor–Couette flow over air-filled microgrooves

Reducing drag under high turbulence is a critical but challenging issue that has engendered great concern. This study utilizes hydrophilic tips in superhydrophobic (SHP) grooves to enhance the stability of plastron, which results in a considerable drag reduction ( $DR$ ) up to 62 %, at Reynolds number ( $Re$ ) reaching $2.79 \times 10^{4}$ . The effect of the spacing width $w$ of the microgrooves on both $DR$ and flow structures is investigated. Experimental results demonstrate that $DR$ increases as either microgroove spacing $w$ or $Re$ increases. The velocity fields obtained using particle image velocimetry indicate that the air-filled SHP grooves induce a considerable wall slip. This slip significantly weakens the intensity of Taylor rolls, reduces local momentum transport, and consequently lowers drag. This phenomenon becomes more pronounced with increasing $w$ . Furthermore, to quantify the multiscale relationship between global response and geometrical as well as driving parameters, $DR\sim (w, \phi _s, Re)$ , a theoretical model is established based on angular momentum defect theory and magnitude estimate. It is demonstrated that a decrease in the surface solid fraction can reduce wall shear, and an increase in the groove width can weaken turbulence kinetic energy production, rendering enhanced slip and drag reduction. This research has implications for designing and optimizing turbulent-drag-reducing surfaces in various engineering applications, such as transportation and marine engineering.

Experimental investigation on transitional flow of supercritical hydrocarbon fuel within straight channels of Printed Circuit Heat Exchangers for hypersonic thermal management systems

The onboard supercritical CO2 closed Brayton cycle (SCBC) has established an effective thermal management pathway for hypersonic vehicles operating at high Mach numbers. A Printed Circuit Heat Exchanger (PCHE) is employed within the cycle system to facilitate heat transfer between fuel and carbon dioxide, with its thermo-hydraulic characteristics significantly impacting both power generation and propulsion performance. Given the insufficient research on the flow and heat transfer of hydrocarbon fuels in PCHEs, this study aims to experimentally address the gaps in understanding transition flow for supercritical hydrocarbon fuels, particularly within semi-circular microchannels. We conducted experiments on the flow transitions using a specially designed test piece with a diameter of 1 mm, replicating the PCHE channel characteristics. The experimental Reynolds number range from 300 to 8300 spans laminar, transition, and turbulent flow regimes. Friction factors for n-decane are obtained under adiabatic and heated conditions. The results validate the nomenclature of the quasi-turbulent region and reveal that the critical Reynolds numbers for the transitional, quasi-turbulent and hydraulic smooth turbulent regions under adiabatic conditions are 2200, 2440 and 2980, respectively. A reduction in friction drag attributed to boundary layer viscosity is observed to alter the friction factor under heating conditions, while heating influences both the onset and termination of the transition region, but does not affect its width. Classical correlations are compared with the experimental results, and new empirical correlations for both adiabatic and non-adiabatic conditions are developed, showing superior predictive accuracy compared to existing models. This provides a reliable tool for the engineering design of heat exchangers in the onboard thermal manage systems.

Hybrid lattice Boltzmann method for turbulent nonideal compressible fluid dynamics

The development and application of a compressible hybrid lattice Boltzmann method to high Mach number supercritical and dense gas flows are presented. Dense gases, especially in Organic Rankine Cycle turbines, exhibit nonclassical phenomena that offer the possibility of enhancing turbine efficiency by reducing friction drag and boundary layer separation. The proposed numerical framework addresses the limitations of conventional lattice Boltzmann method in handling highly compressible flows by integrating a finite-volume scheme for the total energy alongside a nonideal gas equation of state supplemented by a transport coefficient model. Validations are performed using a shock tube and a three-dimensional Taylor–Green vortex flow. The capability to capture nonclassical shock behaviors and compressible turbulence is demonstrated. Our study gives the first analysis of a turbulent Taylor–Green vortex flow in a dense Bethe–Zel'dovich–Thompson gas and provides comparisons with perfect gas flow at equivalent Mach numbers. The results highlight differences associated with dense gas effects and contribute to a broader understanding of nonideal fluid dynamics in engineering applications.

Direct numerical simulation of skin friction drag reduction on supersonic turbulent boundary layers with micro-blowing

The flow control mechanism and skin friction drag reduction characteristics of micro-blowing on a Ma2.25 supersonic turbulent boundary layers are investigated through direct numerical simulations, and the effects of blowing intensity and micro-hole arrangement on turbulent structure and skin friction drag in the local control region and downstream region are considered. The results show that the skin friction drag decreases remarkably in the control region under the influence of micro-blowing, and a certain drag reduction can still be maintained in the downstream region. The drag reduction performance in the control region is jointly determined by blowing intensity and micro-hole arrangement. The drag reduction performance of the staggered arrangement is 5.7% and 11.1% higher than that of the inline arrangement at blowing intensities of 0.2% and 0.5%, respectively. However, it is found that the drag reduction in the downstream region is only determined by the blowing intensity and almost independent of the micro-hole arrangement. The effect of micro-blowing on turbulent structures is quantitatively characterized by energy spectrum analysis, and it shows that the streamwise scales of the near-wall streaks are significantly reduced under the influence of micro-blowing. In addition, the compressibility of fluids and the local reverse transfer in the strong expansion region are significantly improved under the influence of micro-blowing. These effects should be considered when performing Large Eddy Simulation modeling of supersonic turbulent boundary layers with micro-blowing.

Rainbow Imaging Of 3-D Microbubble Distribution Clustered Inside A Turbulent Boundary Layer

Microbubbles dispersed in a turbulent boundary layer spatially developing along a flat plate is investigated experimentally. The aim of study is to find out visually how microbubbles interact with turbulent eddies to achieve frictional drag reduction on a wall. For stream Reynolds number up to 3×104 , microbubble suspended at Stokes number smaller than 10 − 3 is measured. Stream-wise vortices in the boundary layer were visualized by flake optics, of which spacing expands with injection of microbubbles. 3-D microbubble distributions were measured by a color-coded volumetric illumination applied to the boundary layer. The result showed preferentially accumulated microbubble clouds to low-speed streaks close to viscous sublayer and to hair-pin vortices in the buffer layer.

Corrigendum to “Airfoil friction drag reduction based on grid-type and super-dense array plasma actuators”

Figure (b) of the paper [Plasma Sci. Technol. 26 (2024) 025503] has been corrected. This modification does not affect the result presented in the paper.

Computational analysis of air bubble-induced frictional drag reduction on ship hulls

About 60% of marine vessels’ power is consumed to overcome friction resistance between the hull and water. Air lubrication can effectively reduce this resistance and lower fuel consumption, and consequently emissions. This study aims to analyze the use of a gas-injected liquid lubrication system (GILLS) to reduce friction resistance in a real-world scenario. A 3D computational fluid dynamics model is adopted to analyse how a full-scale ship (the Sea Transport Solutions Designed Catamaran ROPAX ferry) with a length of 44.9 m and a width of 16.5 m is affected by its speed and draught. The computational model is based on a volume of fluid model using the k-ꞷ shear stress transport turbulence model. Results show that at a 1.5 m draught and 20 knots cruising speed, injecting 0.05 kg/s of compressed air into each GILLS unit reduces friction resistance by 10.45%. A hybrid model of natural air suction and force-compressed air shows a friction resistance reduction of 10.41%, which is a promising solution with less required external power. The proposed technique offers improved fuel efficiency and can help to meet environmental regulations without engine modifications.

Hydrodynamic Function of the Slimy and Scaly Surfaces of Teleost Fishes.

The scales and skin mucus of bony fishes are both proposed to have a role in beneficially modifying the hydrodynamics of water flow over the body surface. However, it has been challenging to provide direct experimental evidence that tests how mucus and fish scales change the boundary layer in part due to the difficulties in working with live animal tissue and difficulty directly imaging the boundary layer. In this manuscript, we use direct imaging and flow tracking within the boundary layer to compare boundary layer dynamics over surfaces of fish skin with mucus, without mucus, and a flat control surface. Our direct measurements of boundary layer flows for these three different conditions are repeated for two different species, bluegill sunfish (Lepomis macrochirus) and blue tilapia (Oreochromis aureus). Our goals are to understand if mucus and scales reduce drag, shed light on mechanisms underlying drag reduction, compare these results between species, and evaluate the relative contributions to hydrodynamic function for both mucus and scales. We use our measurements of boundary layer flow to calculate shear stress (proportional to friction drag), and we find that mucus reduces drag overall by reducing the velocity gradient near the skin surface. Both bluegill and tilapia showed similar patterns of surface velocity reduction. We also note that scales alone do not appear to reduce drag, but that mucus may reduce friction drag up to 50% compared to scaled surfaces without mucus or flat controls.

On the fluid drag reduction in scallop surface.

In the field of biomimetics, the tiny riblet structures inspired by shark skin have been extensively studied for their drag reduction properties in turbulent flows. Here, we show that the ridged surface texture of another swimming creature in the ocean, i.e., the scallops, also has some friction drag reduction effect. In this study, we investigated the potential drag reduction effects of scallop shell textures using computational fluid dynamics simulations. Specifically, we constructed a conceptual model featuring an undulating surface pattern on a conical shell geometry that mimics scallop. Simulations modeled turbulent fluid flows over the model inserted at different orientations relative to the flow direction. The results demonstrate appreciable friction drag reduction generated by the ribbed hierarchical structures encasing the scallop, while partial pressure drag reduction exhibits dependence on alignment of scallop to the predominant flow direction. Theoretical mechanisms based on classic drag reduction theory in turbulence was established to explain the drag reduction phenomena. Given the analogous working environments of scallops and seafaring vessels, these findings may shed light on the biomimetic design of surface textures to enhance maritime engineering applications. Besides, this work elucidates an additional evolutionary example of fluid drag reduction, expanding the biological repertoire of swimming species.

Interplay of roughness and wettability in microchannel fluid flows—Elucidating hydrodynamic details assisted by deep learning

Under microconfinement, the complex interaction between surface roughness and fluid slippage yields unexpected variations in friction factor and drag reduction. These variations arise from the combined effects of roughness and hydrophobic interactions of the surface with the hydrodynamic field. Our study investigates alterations in frictional characteristics within long microchannels, considering fluid slippage, hydraulic diameter, and roughness. This exploration holds promise for precise drag reduction control applications for lab-on-a-chip and small-scale devices. To address computational limitations in analyzing diverse hydrodynamic conditions, we employ an artificial neural network prediction model, validated with experimental and numerical results. Contrary to the macroscopic conclusions obtained from the Moody chart, our findings indicate that fluid slippage, apart from surface roughness, significantly influences the friction factor. The interdependencies of friction factor on the flow and fluid parameters are thoroughly studied toward the proposition of a new slip-modified constricted flow friction factor formula, predicting friction in microchannels with combined roughness and hydrophobicity effects. This combined numerical and machine-learning approach presents a noteworthy counterpart to the moody chart at microscales offering the potential for a unified continuum-based description to include interfacial effects.

Turbulent drag reduction using dolphin-inspired near-wall ultrasonic microvibrations

The skin-friction drag generated by wall-bounded turbulent flows can potentially be reduced by a wall-parallel oscillatory motion. Inspired by microvibrations and the high sensitivity of dolphin skin, we examine whether wall-normal undulating motion actuated by longitudinal micro-ultrasonic waves (LMUWs) with ultrasonic-frequency oscillations and micro-size amplitudes significantly alters the multi-eddy motion on the surface, thereby reducing skin-friction drag. Simulations of the LMUW-induced turbulent flows are performed in an open channel at a Reynolds number of 1.24 × 106 for three motion modes, i.e., two traveling waves (downstream and upstream) in the streamwise direction and a standing wave. It is verified that the wall-normal turbulent fluctuations are remarkedly altered within the viscous sublayer of the turbulent boundary layer, resulting in a reduced velocity gradient. This leads to lower or even extinguished friction drag, which is strongly associated with the LMUW-excitation mode. Informed and validated by numerical results, we further derived a theoretical model for the dynamic boundary layer. This model is based on Fourier series expressions of the velocities and is used to elucidate the underlying mechanisms in association with the LMUW-excited turbulent flow and active friction drag reduction. The results indicate that upstream traveling waves enable 100% friction drag reduction, while downstream traveling waves are capable of overcoming the trade-off between friction and pressure drag, accomplishing 100% total drag reduction. This study thus provides a novel active and controllable method for turbulent drag reduction.

Drag reduction study based on boundary layer combustion on single expansion ramp nozzle

In hypersonic flight, frictional drag is an important factor affecting the overall engine performance. The wall frictional drag analysis and drag reduction technology of a unilateral expanding tail nozzle under typical working conditions are carried out by means of two-dimensional Reynolds-averaged N-S equations and SST k − ω turbulence model. The effects of hydrogen injection and combustion with different mass flow rates under different jet positions on the wall friction resistance are analyzed and compared, and the results show that, under the appropriate opening position, a better drag reduction effect can be obtained by injecting hydrogen with a mainstream flow rate of about 2.5%, which can achieve a reduction of 43.7%.

Manipulation of a turbulent boundary layer using sinusoidal riblets

We investigate experimentally the effects of micro-grooves on the development of a zero pressure gradient turbulent boundary layer at two different values of the friction Reynolds number. We consider both the well-known streamwise aligned riblets as well as wavy riblets, characterized by a sinusoidal pattern in the mean flow direction. Previous investigations by the authors showed that sinusoidal riblets yield larger values of drag reduction with respect to the streamwise aligned ones. We perform new particle image velocimetry experiments on wall-parallel planes to get insights into the effect of the sinusoidal shape on the near-wall organisation of the boundary layer and the structures responsible for the friction drag reduction and the turbulence generation. Conditional averages, aimed at identifying the topology of the low-speed streaks in the turbulent boundary layer, reveal that the flow is highly susceptible to wall manipulation. This is particularly evident in the cases that are associated with greater values of drag reduction. The results suggest a fragmentation and/or weakening of the streaks in the sinusoidal cases, that is triggered by the larger values of the wall-normal vorticity found at the streaks’ edges. The results are also confirmed by applying the variable interval spatial averaging events eduction technique. The turbulent kinetic energy budget also shows that the sinusoidal geometry significantly attenuates the turbulence production, hence supporting the idea of the manipulation of the turbulence regeneration cycle.

Numerical Investigations of Outer-Layer Turbulent Boundary Layer Control for Drag Reduction Through Micro Fluidic-Jet Actuators

Drag reduction through turbulent boundary layer control (TBLC) is an essential way to develop green aviation technologies. Compared with traditional approaches for drag reduction, turbulence drag reduction is a relatively new technology, particularly for skin friction drag reduction, and it is becoming a hotspot problem worldwide. This paper focuses on the research of micro fluidic-jet actuators used for outer-layer boundary layer control with high-performance computing (HPC). This study aims to reduce turbulent drag by reshaping the flow structure within the turbulent boundary layer. To ensure the calculation accuracy of the core region and reduce the consumption of computing resources, a zonal LES/RANS strategy and WMLES method are proposed to simulate the effects of fluidic-actuators for outer-layer boundary control, in which high-performance computing has to be involved. The studies are performed on the classical zero-gradient turbulent flat plate cases, in which three different control strategies named “W-control,” “V-control,” and “VW-control” are used and compared to study the effects of drag reduction under a low Reynolds number at Reτ = 470 and a higher Reynolds number at Reτ = 4700. The mechanism for drag reduction is analysed via a pre-multiplied spectral method and a parallel dynamic mode decomposition (DMD) method. The results show that the present approach can effectively simulate the outer-layer turbulent boundary control where the “V-control” with the fluidic-jet actuator array behaves well to achieve an average drag reduction (DR) rate of more than 5% for the high Reynolds number case of the flat plate boundary layer. The high Reynolds shear stress and turbulent kinetic energy distribution in the boundary layer region show an obvious uplift under the effects of actuators, which is the main mechanism for drag reduction.

Airfoil friction drag reduction based on grid-type and super-dense array plasma actuators

To improve the cruise flight performance of aircraft, two new configurations of plasma actuators (grid-type and super-dense array) were investigated to reduce the turbulent skin friction drag of a low-speed airfoil. The induced jet characteristics of the two actuators in quiescent air were diagnosed with high-speed particle image velocimetry (PIV), and their drag reduction efficiencies were examined under different operating conditions in a wind tunnel. The results showed that the grid-type plasma actuator was capable of producing a wall-normal jet array (peak magnitude: 1.07 m/s) similar to that generated in a micro-blowing technique, while the super-dense array plasma actuator created a wavy wall-parallel jet (magnitude: 0.94 m/s) due to the discrete spanwise electrostatic forces. Under a comparable electrical power consumption level, the super-dense array plasma actuator array significantly outperformed the grid-type configuration, reducing the total airfoil friction drag by approximately 22% at a free-stream velocity of 20 m/s. The magnitude of drag reduction was proportional to the dimensionless jet velocity ratio (r), and a threshold r = 0.014 existed under which little impact on airfoil drag could be discerned.

Wall shear stress measurement of turbulent bubbly flows using laser Doppler displacement sensor

A novel wall shear stress meter is developed to promote studies of frictional drag reduction by injecting air bubbles into a turbulent boundary layer. Such studies require a tool having a time resolution sufficiently high to capture the unsteady effect of turbulent characteristics due to various bubble passage patterns. The present study adopts the principle of the heterodyne interferometer to capture of time variation of wall shear stress in turbulent flow up to 500 Hz in sampling frequency. We demonstrate the performance of the tool by applying it to bubbly flows beneath a flat-bottom model ship running at 3 m/s in a towing water reservoir, where bubbles experience wavy passage around a few Hz in frequency. The data obtained by the proposed shear stress meter agree well with Dean's formula for single-phase turbulent channel flow. The scattering of the data across three trials is sufficiently small, at approximately 3 %. This high reproducibility is a great advantage over conventional direct measurements, which have low reproducibility and a large scatter of the data of approximately 15 %. The obtained results represent reasonable drag reductions by injecting bubbles with different void fractions. We also evaluate histogram of the wall shear stress fluctuation to characterize the effect of drag reduction by unsteady passage of bubbles.

Effect of wall-seeping gas film under different working media on stability of conical hypersonic boundary layer

Wall-seeping gas film (WSGF) is a promising method of controlling hypersonic boundary layer transition and reducing friction drag and heat transfer. Experiments are conducted in a Mach 6 hypersonic quiet wind tunnel by using nano-tracer planar laser scattering (NPLS) and high-frequency fluctuating pressure measuring technique. This work investigates the effects of wall-seeping helium, air, and carbon dioxide gas films under identical volume flow rate condition on conical boundary layer thickness, disturbance wave structure, wavelength, frequency, amplitude, and nonlinear interaction. The experimental results reveal that the WSGF significantly thickens the hypersonic boundary layer, with the thickest position appearing at the downstream boundary of the seeping zone. The boundary layer thickness is thinnest for helium gas film but thickest for carbon dioxide gas film. Generally, air gas film and carbon dioxide gas film induce the regular, rope-like, and interlaced second-mode waves to appear in advance in the boundary layer. However, under a higher volume flow rate for carbon dioxide gas film, the disturbance wave structure resembles interface fluctuations, with a characteristic wavelength of approximately 18 mm and a peak frequency as low as about 35 kHz, but no the rope-like interlaced characteristic. At this time, the influence of shear layer instability becomes significant. The disturbance waves do not exhibit second-mode wave characteristics for wall-seeping helium gas film, whose shape is irregular and undergoes deformation with time and space. Additionally, the power spectral density of wall fluctuating pressure exhibits insignificant variation with volume flow rate and flow direction, which is similar to the characteristic of power spectral density in the laminar boundary layer and has no peak frequency. The wavelength of second-mode waves is about 2－3 times the boundary layer thickness for air gas film, and increases to more than 3 times for carbon dioxide gas film. The application of carbon dioxide gas film results in smaller peak frequency and bandwidth of disturbance wave, larger characteristic wavelength and amplitude, longer propagation distance, and stronger nonlinear interaction than the application of air gas film. In the future, attention should be paid to understanding disturbance wave characteristics in the boundary layer for the helium gas film and shear layer instability under larger volume flow rates.

Lubricant-infused anodic aluminum oxide surface (AAO-LIS) for durable slipperiness under harsh conditions

The wettability of surfaces plays a crucial role in determining their applications, and slippery surfaces have been utilized in various fields. Especially, lubricant-infused surface (LIS) has been undergone extensive research over the past decade. However, LIS tends to lose its infused lubricant and consequently lose its slippery properties when exposed to harsh conditions. Therefore, there is a strong need to enhance the durability of the infused lubricant. Herein, two strategies are adopted to improve the durability of slippery properties against centrifugal forces and high-speed shear flows. One strategy is using re-entrant shaped nanocavities, and the other is grafting PDMS brush onto the surface. Anodic aluminum oxide (AAO) is employed to fabricate the re-entrant shaped nanocavities, and the durability of the infused lubricant is improved when the cavities have a smaller diameter. Furthermore, the re-entrant shape of nanocavity significantly improves the durability compared to nanocavity with a constant diameter. The grafting of PDMS brush is essential for enhancing the durability, and an increase in the length of the PDMS brush also improves the durability. The fabricated AAO-based LIS (AAO-LIS) maintains the slippery properties for 50 min under a high-speed water flow of 12 m/s. In addition, it maintains its superior slipperiness for more than a few days when exposed to actual seawater. Furthermore, the frictional drag reduction performance of the fabricated AAO-LIS is assessed. The proposed AAO-LIS maintains a certain drag reduction performance durably even after losing its superior slipperiness, demonstrating its strong potential for practical applications across various industrial fields.

Stability analysis of a streaky boundary layer generated by miniature vortex generators

Delaying laminar-turbulent transition is an attractive method to reduce friction drag on streamlined bodies. In the case of natural transition, in which turbulence is triggered by the growth of two-dimensional Tollmien–Schlichting (TS) instabilities, this growth of TS instabilities can be attenuated by introducing steady streamwise streaks into the boundary layer. These streamwise streaks are generated by streamwise vortices, which rearrange the flow via the lift-up mechanism. Such streamwise vortices can be induced by so-called miniature vortex generators (MVGs). Although recently considerable attention has been devoted to the investigation of MVGs, and multiple studies investigated MVGs with different parameters, the previously investigated MVG configurations are likely suboptimal. This study aims at (partially) filling this knowledge gap by complementing previous studies by conducting a parametric study of rectangular MVGs and assessing them with local linear stability analysis. Three parameters are varied simultaneously: the height, the spanwise distance of the MVG pairs, and the distance between MVGs in each pair. First, the modelling approach is presented, which consist of an efficient base flow computation, and BiGlobal stability analysis. The base flow calculation is validated by comparing our results with the experiments of Shattarzadeh and Fransson (2015). Then, the streak amplitude, the growth factors of the TS-waves and the secondary shear-layer instabilities are analysed in a parametric study. It is shown that with the variation of the spanwise distances, the maximum amplitude and the streamwise extent of the streaks in the boundary layer can be controlled, reminiscent of the behaviour of the optimal vortices. The findings and the test of the methodology provide valuable insight, which may be useful for the optimisation of MVGs.

Vertical diffusion of bubbles injected beneath a flat-bottomed ship for frictional drag reduction

There has been concern about the short effective distance of frictional drag reduction achieved by air lubrication, but recent model ship experiments have shown over 30 m longer than our expectation. Also, the reduction ratio can be improved through the artificial generation of void waves appearing beneath the hull. These topics relate to the spatiotemporal bubble distribution and an understanding of them requires clarification of the distribution. In this study, optical probes were used to measure the distributions of bubble states beneath a 20-m long flat-bottomed ship towed with 6–8 m/s. The measurements explained the long effective distance of drag reduction and the process of void wave generation and development. The drag reduction ratio was about 7–25% and mainly depended on the void fraction within 10 mm of the hull. The vertical distribution of the void fraction remained unchanged over long downstream distances. The development of void waves was affected by the streamwise velocity difference between bubble clusters near the hull. The velocity was determined by the number density of the bubbles, decreasing to the half of ship speed until reaching a certain number density, 100 per meter in our experiments, and then becoming constant at higher density.