Viscous Drag Reduction Research Articles

Pressure drag reduction via imposition of spanwise wall oscillations on a rough wall

The present study tests the efficacy of the well-known viscous drag reduction strategy of imposing spanwise wall oscillations to reduce pressure drag contributions in transitional and fully rough turbulent wall flow. This is achieved by conducting a series of direct numerical simulations of a turbulent flow over two-dimensional (spanwise-aligned) semi-cylindrical rods, placed periodically along the streamwise direction with varying streamwise spacing. Surface oscillations, imposed at fixed viscous-scaled actuation parameters optimum for smooth wall drag reduction, are found to yield substantial drag reduction ( $\gtrsim$ 25 %) for all the rough wall cases, maintained at matched roughness Reynolds numbers. While the total drag reduction is due to a drop in both viscous and pressure drag in the case of transitionally rough flow (i.e. with large inter-rod spacing), it is associated solely with pressure drag reduction for the fully rough cases (i.e. with small inter-rod spacing), with the latter being reported for the first time. The study finds that pressure drag reduction in all cases is caused by the attenuation of the vortex shedding activity in the roughness wake, in response to wall oscillation frequencies that are of the same order as the vortex shedding frequencies. Contrary to speculations in the literature, this study confirms that the mechanism behind pressure drag reduction, achieved via imposition of spanwise oscillations, is independent of the viscous drag reduction. This mechanism is responsible for weakening of the Reynolds stresses and increase in base pressure in the roughness wake, explaining the pressure drag reduction observed by past studies, across varying roughness heights and geometries.

Design of Hybrid-Laminar-Flow-Control Wing and Suction System for Transonic Midrange Aircraft

Hybrid laminar flow control (HLFC) has shown significant promise in the viscous drag reduction of aircraft. However, the use of HLFC for commercial applications requires further simplification. The current study proposes tools for the conceptual design of transonic HLFC wing and suction system. In the first part of the study, airfoil sections for the wing are optimized for minimum total drag using a multi-objective genetic algorithm approach at six spanwise locations. The induced drag of the wing is estimated using a vortex lattice method solver. In the second part of the study, suction system design is performed using ASPeCT, an in-house solver for HLFC system design. A simplified inner structure for the suction system is proposed, which can be integrated easily within the wing structure. A total drag penalty approach is proposed to establish a tradeoff between matching the target suction distribution and the complexity of the suction system. Finally, the additional weight and off-design performance of the suction system are analyzed for a ±0.1 change in the design lift coefficient. A maximum fuel reduction of 7% can be expected with the HLFC system taking into account the additional weight added and power off-take from the engine.

Evolution of Air Plastron Thickness and Slip Length over Superhydrophobic Surfaces in Taylor Couette Flows

Drag reduction (DR) using superhydrophobic surfaces (SHSs) has received intensive interest due to the emergence of SH coating technology. The air layer (plastron “δ”) trapped between the SHS and the water controls the flow slip over the SHSs. We demonstrate slippage over three fabricated SHSs in laminar and low turbulent Taylor–Couette flows. We experimentally investigate how the slip length increases with a higher Reynolds number (Re) over the tested SHSs; simultaneously, the air plastron thickness investigates using a viscous model. The mean skin friction coefficient (Cf) can be fitted to a modified semi-empirical logarithmic law expressed in the Prandtl–von Kármán coordinate. An effective slip length is estimated in the 35–41 µm range with an achieved 7–11% DR for the tested surfaces. Statistical analysis is used to develop a regression model from the experimental data. The model shows an R2 of 0.87 and good agreement with the experimental data. This shows the relation between the dimensionless slip length (b+), the dimensionless plastron thickness (δ+), and the Reynolds number, which is directly proportional. The regression model shows that b+ and Reynolds numbers have a higher impact on the δ+ than the surface wettability, which attribute to the small difference in the wetting degree between the three tested surfaces. The practical importance of the work lies in its ability to provide a deep understanding of the reduction in viscous drag in numerous industrial applications. Furthermore, this research serves as a groundwork for future studies on hydrophobic applications in internal flows.

Riblet Performance Beneath Transitional and Turbulent Boundary Layers at Low Reynolds Numbers

Several high-resolution scale-resolving simulations are carried out to examine the effect of riblets on the mean and turbulent statistics of a zero-pressure-gradient boundary layer. The Reynolds number is chosen such that the riblets are exposed to both the transitional and turbulent regimes of the boundary layer. This is in contrast to the turbulent channel flow or fully turbulent boundary layers studied in the literature. The boundary layer is subjected to freestream turbulence and roughness tripping. The transition process and the extent of the turbulent regime on the riblets are altered by tripping the boundary layer with an isolated hemispherical roughness element. The influence of the riblets on the transition onset and the viscous drag reduction is demonstrated through time-averaged, phase-averaged, and instantaneous flow quantities. The effect of the V-shaped riblets with both sharp and curved tips (and valleys) is also explored. With riblets, the viscous drag in the turbulent regime is reduced by 4–6% when compared to a smooth surface, and the efficacy of sharp V-shaped riblets is shown to be marginally higher than for the curved riblets. In the transitional flow regime, the coherent structures over riblets are predominantly spanwise oriented. In particular, a separated shear layer forms over the riblet leading edge as the flow encounters an abrupt surface transition from the smooth surface onto the riblets. A leading-edge ramp is shown to effectively minimize the additional spurt in the turbulent kinetic energy and the associated losses incurred due to this abrupt surface change.

Predicting the drop size passing through a superhydrophobic orifice

Superhydrophobic surfaces can be utilized in various applications, such as enhanced heat transfer, anti-icing, self-cleaning, and viscous drag reduction. In this work, we investigated the water droplet size separation using superhydrophobic surfaces, which is relatively new and unexplored research field, but yet promising for pharmaceutical and medical applications. We developed a theoretical model for predicting the diameter of a droplet passing through a smaller superhydrophobic orifice by considering the balance of forces, geometrical characteristics, and the surface wettability. For verification of the model, experimental water droplet size separation was conducted using a thin superhydrophobic copper foil with a laser-cut orifice with a diameter ranging from 1.2 to 2.1 mm. A comparison of the experimental and analytical results has shown that the error of the model is less than 20% within the model's validity range with the upper limit at the capillary length of the fluid. By moving away from this limit toward smaller droplet diameters, the accuracy of the model improves and reaches an error of less than 7% at the smallest used orifice diameter of 1.2 mm.

Experimental Study on Performance of Transonic Compressor Cascade with Microgroove Polyurethane Coatings

Due to the harsh operating environment of aero-engines, a surface structure that provides excellent aerodynamic performance is urgently required to save energy and reduce emissions. In this study, microgroove polyurethane coatings fabricated by chemical synthesis are investigated in terms of their effect on aerodynamic performance, which is a new attempt to investigate the impact on aerodynamic performance of compressor cascade at transonic speeds. This method reduces manufacturing and maintenance cost significantly compared with traditional laser machining. Wake measurements are conducted in the high-speed linear compressor cascade wind tunnel to evaluate the performance of cascade attached with different microgroove polyurethane coatings. Compared with the Blank case, the microgroove polyurethane coatings have the characteristic of reducing flow loss, with a maximum reducing rate of 5.87% in the area-averaged total pressure loss coefficient. The mechanism of flow loss control is discussed through analyzing the correlation between the total pressure distribution and turbulence intensity distribution. The results indicate that a large quantity of energy loss in the flow field due to turbulence dissipation and the reduction in viscous drag by microgroove polyurethane coatings relates to its effect on turbulence control. This paper demonstrates a great perspective on designing micro-nano surface structure for aero-engine applications.

Quasi-Analytical Solution of Optimum and Maximum Depth of Transverse V-Groove for Drag Reduction at Different Reynolds Numbers

Reducing the skin-friction drag of a vehicle is an important way to reduce carbon emissions. Previous studies have investigated the drag reduction mechanisms of transverse grooves. However, it is more practical to investigate which groove geometry is optimal at different inflow conditions for engineering. The purpose of this paper is to establish the physical model describing the relationship between the dimensionless depth (H+=Huτ/υ) of the transverse groove, the dimensionless inflow velocity (U∞+=U∞/uτ), and the drag reduction rate (η) to quasi-analytically solve the optimal and maximum transverse groove depth according to the Reynolds numbers. Firstly, we use the LES with the dynamic subgrid model to investigate the drag reduction characteristics of transverse V-grooves with different depths (h = 0.05~0.9 mm) at different Reynolds numbers (1.09×104~5.44×105) and find that H+ and U∞+ affect the magnitude of slip velocity (Us+), thus driving the variation of the viscous drag reduction rate (ην) and the increased rate of pressure drag (ηp). Moreover, the relationship between Us+, ην, and ηp is established based on the slip theory and the law of pressure distribution. Finally, the quasi-analytical solutions for the optimal and maximum depths are solved by adjusting Us+ to balance the cost (ηp) and benefit (ην). This solution is in good agreement with the present numerical simulations and previous experimental results.

Vol4 num3-16 Bioinspired hybrid materials for reduction of viscous drag in ships

Nature has evolved structural materials along millions of years of incremental changes to fulfil functional requirements of very different situations. Let´s take as an example the sharks, one of the best adapted species for swimming across the oceans, and their smooth skin that is composed of millions of small scales (denticles), with complex geometries formed by sharp spines and micro-grooves aligned with flow direction, varying in size and shape at different locations of the sharkskin. The purpose of this microstructure is to reduce the viscous drag produced by the water when it adheres to the hull forming the fluid boundary layer, which can stick out the added mass to the weight of the fish and increase dramatically its peak speed. Inspired by this natural solution, we have started the design of a bioinspired new material, to be incorporated to the hull wet surface, reducing the viscous drag and increasing the velocity and the efficiency (less fuel consumption) of the ship. In this way, it will be easier to control the green effect gas emissions while sailing. We present the development of this new bioinspired, hybrid material with scales obtained by additive manufacturing using polymeric materials. To control the orientation of the denticles respect to flow direction, the scales have been mounted on a substrate of hydrogel, who interact with the fluid field of pressures to adapt the angle of the moving scales as the velocity increases. No mechanical devices are needed, as the hydrogel act as a nonlinear spring accommodating the movements of the scales. CFD code helps in the development of the most efficient shape and coating design. A water channel has been developed to measure the actual forces acting on the surface of coated panels.

Numerical Analysis of Drag Reduction Characteristics of Biomimetic Puffer Skin: Effect of Spinal Height and Tilt Angle.

Based on the migratory phenomenon of the puffer and the cone-shaped structures on its skin, the effects of spinal height and tilt angle on the drag reduction characteristics is presented by numerical simulation in this paper. The results show that the trend of total drag reduction efficiency changes from slow growth to a remarkable decline, while the viscous drag reduction efficiency changes from an obvious increase to steady growth. The total and viscous drag reduction efficiencies are 19.5% and 31.8%, respectively. In addition, with the increase in tilt angle, the total drag reduction efficiency decreases gradually; the viscous drag reduction efficiency first increases and then decreases, finally tending to be stable; and the total and viscous drag reduction efficiency reaches 20.7% and 26.7%, respectively. The flow field results indicate that the pressure drag mainly originates at the front row of the spines and that the total pressure drag can be effectively controlled by reducing the former pressure drag. With the increase in low-speed fluid and the reduction in the near-wall fluid velocity gradient, the viscous drag can be weakened. Nevertheless, the drag reduction effect is achieved only when the decrement of viscous drag is greater than the increment of pressure drag. This work can serve as a theoretical basis for optimizing the structure and distribution parameters of spines on bionic non-smooth surfaces.

Airfoil profile surface drag reduction characteristics based on the structure of the mantis shrimp abdominal segment

To reduce the drag of fluid on a surface, the parameterized structure of the mantis shrimp abdominal segment was analyzed to design the airfoil profile, and a calculation model was established based on the biological characteristics of the mantis shrimp. By using a numerical simulation method, the change in the viscous sublayer and drag reduction of the biological surface under different flow velocities and with different numbers of airfoil profiles was analyzed, and the mechanism of the drag reduction of the biological airfoil profile was revealed. The results show that the biological surface with airfoil profiles exhibited a better drag reduction effect. With an increase in airfoil profile numbers, the drag reduction rate first decreased and then increased. When the flow velocity was 10 m/s and the number of airfoil profiles was 5, the drag reduction rate exhibited the largest value of 15.33%. This result showed that the biological airfoil profile could affect the structure of the wall turbulent boundary layer and diminish the velocity gradient of the boundary layer, which finally changed the interaction pattern between the fluid and wall. In addition, the vortex cushioning effect was created to change the friction between the fluid and the wall into a rolling friction with a lower friction coefficient, thus reducing the shear stress on the wall, which achieved drag reduction.

Actively Reduced Airfoil Drag by Transversal Surface Waves

The flow over a DRA2303 wing section at a Reynolds number of Re = 400,000 is actively controlled by spanwise traveling transversal surface waves. The actuated low-Mach number flow is investigated by a high-resolution large-eddy simulation. Approximately 74% of the solid surface on both sides of the wing section is deflected by a sinusoidal space- and time-dependent function in the wall-normal direction. The turbulence intensitites and wall-normal vorticity fluctuations are significantly reduced and a shift from one-dimensional turbulence to two-dimensional turbulence is observed. Besides a viscous drag reduction by 8.6% with a strong decrease of skin-friction in the favorable pressure gradient region and an overall drag decrease by 7.5%, a slight increase in lift is achieved for an external flow over a realistic geometry.

Numerical research on the turbulent drag reduction mechanism of a transverse groove structure on an airfoil blade

The optimization of the airfoil has a significant impact on the reduction of energy consumption for a rotary machine. In this paper, ANSYS Fluent is used to study the influence of different groove structure sizes on the turbulence drag of NACA0012 airfoil blades, and the drag reduction mechanism is analyzed. The results show that the groove structure can significantly reduce the drag during the working speed of the fan. The optimal groove size is s = 0.1 mm and the drag is reduced by 9.65%. The secondary vortices reduce the normal velocity gradient at the top of the groove structure, resulting in a reduction in viscous drag. However, as the groove size increases, the drag reduction effect decreases, and even the drag increases. The overall shear stress of the airfoil surface with the transverse groove structure is smaller than the original airfoil, and the velocity gradient of the airfoil surface is reduced. The two sides work together to reduce the turbulence drag of the airfoil. Besides, the spacing between the grooves increases the shear stress in some areas, but reduces the mutual interference of the vortices, so there is an optimum value for the groove spacing.

Active and Passive Turbulent Boundary-Layer Drag Reduction

A 50 year journey through flow control strategies that seek to achieve viscous drag reduction in turbulent boundary layers is presented. These are shown to focus on different mechanisms underlying coherent motions that have been observed in the early period of turbulent boundary-layer research. The approaches have generally focused either on modifying the large-scale outer motions or the small-scale motions near the wall that are particularly associated with low-speed wall streaks. The degree of success of these approaches is highlighted.

Material selection and manufacturing of riblets for drag reduction: An updated review

Riblets are a well-researched and understood passive method for achieving viscous drag reduction. Since the 1970s, researchers have found that, with riblets, viscous drag reduction in the order of 8% is achievable in turbulent air and fluid flows. Most of the relevant literature provides insight into the drag-reductive mechanisms of riblets and the effect of riblet morphological design in varying flow conditions. A few recent studies have begun to investigate the influence of material properties on the drag-reductive ability of riblet surfaces with promising results. We here provide an updated review of material selection and riblet manufacture and show current trends. A brief summary is provided of the theories of riblet drag-reductive ability, riblet surface design, the role of material selection for drag reduction and current manufacturing techniques.

Drag Reduction by Boundary-Layer Combustion: Effects of Flow Disturbances from Rectangular-to-Elliptical-Shape-Transition Inlets

A key issue that can affect the implementation of boundary-layer combustion for viscous drag reduction in scramjet combustors is the potential of nonuniform flow entering the combustor from the inlet. Experiments were conducted in the T4 Stalker tube to study the effects of these flow nonuniformities on boundary-layer combustion in a circular constant-area combustor. A rectangular-to-elliptical shape-transition inlet, typical of the type proposed for self-starting scramjets, was designed, manufactured, and attached upstream of the combustor in the current experiments. Numerical simulations of the flow through the rectangular-to-elliptical shape-transition inlet showed that the inlet generated a nonuniform outflow that had vortices, shock waves, and expansion waves. When tested at design-point inflow conditions, the total skin friction drag measured in the combustor was reduced by 61% when hydrogen burned in the boundary layer. These levels of reduction are similar to those measured in experiments conducted without a realistic scramjet inlet upstream of the combustor, thus demonstrating that the drag reduction brought about by boundary-layer combustion is not affected by the flow disturbances generated from the REST inlet. Tests conducted at off-design conditions further demonstrated that the drag reduction brought about by boundary-layer combustion was not affected by changes in the inflow conditions.

An Experimental Investigation on the Fabrication of Micro/Meso Surface Features by Metallic Roll-to-Plate Imprinting Process

Metallic components with large-area functional surface micro/mesostructures have been increasingly utilized in various industrial fields, such as friction/wear reduction, viscous drag reduction, and energy efficiency enhancement. Roll-to-plate (R2P) imprinting process is an efficient and economical method in fabricating micro/mesofeatures on the large-area surface of the metal parts. However, process design methods based on scale law cannot be directly used due to size effects. Its formability is greatly influenced by tool feature size and material grain size. In this study, a lab-scale R2P imprinting system was developed to fabricate the microsructures on the surface of metallic materials. The specimens of pure aluminum and pure copper with various size grains were prepared. Rigid die with geometric dimensions was fabricated and series of experiments were conducted. The microfeature height of the imprinted workpiece was measured to evaluate the effects of tool feature dimensions (width, spacing, and fillet) and metal grain sizes. It is found that the groove width and fillet had more significant effect on the microfeature formation among the die cavity geometric parameters. Wider groove could enhance the microforming ability and large fillet could improve the flowing ability. From the viewpoint of polycrystalline material, grain structures significantly affected the microfeature formation. When the grain size was smaller than the groove width, the material flowed more easily into the die cavity with increasing of the grain size because of the decrease of grain boundary strengthening effect.

Experimental investigation of viscous drag reduction of superhydrophobic nano-coating in laminar and turbulent flows

In this research, effects of superhydrophobic nano-coating on frictional drag force have been investigated. The result of this study could be considered to be used as a method in applications concerned with fuel consumption reduction, less CO2 emission and environmental problems as well as speed increase; while, its significance can also be of great use in marine applications. A rotating disc apparatus was used as the experimental set-up to compare the frictional drag force on an aluminum disc with TiO2 superhydrophobic nano-coating and a smooth coatless aluminum disc. The superhydrophobic nano-coating was prepared using sol–gel method and was shown to be able to produce a contact angle of about 160°. Experiments were performed in the Reynolds numbers, ranging from 105 to 2×106. Results indicated the drag reduction values of up to 30% and 15% in laminar and turbulent flows, respectively.

Simulations of laminar flow past a superhydrophobic sphere with drag reduction and separation delay

Superhydrophobic surfaces have potential for reducing hydrodynamic drag by combining a structured surface and hydrophobicity to retain a lubricating air layer (plastron) at the surface. In the present contribution, numerical simulations of laminar flow past a superhydrophobic sphere are conducted using a two-phase flow representation. The results show drag reductions in Stokes flow of up to 19% for an air-water system, in agreement with previous analytic work, and demonstrate an increased effect as the Reynolds number is increased to 100. Drag reductions of up to 50% are achieved due to reduction in viscous drag and suppression of separation by the plastron, resulting in a narrower wake. To explore a less idealised model of the plastron, baffles have also been introduced to simulate the support of a plastron by roughness elements. The baffles lead to the attached vortex regime no longer being suppressed, but separation is delayed and drag reductions are evident in comparison to a solid sphere. Increasing the area solid fraction results in a diminished drag reduction due to the plastron, however drag reductions of up to 15% can still be achieved with solid fractions of 10%.

Optimal Application of Riblets on Compressor Blades and Their Contamination Behavior

During the last decades, riblets have shown a potential for viscous drag reduction in turbulent boundary layers. Several investigations and measurements of skin-friction in the boundary layer over flat plates and on turbomachinery-type blades with ideal riblet geometry have been reported in the literature. The question of where riblets must be applied on the surface of a compressor blade is still not sufficiently answered. In a first step, the profile loss reduction by ideal triangular riblets with a trapezoidal groove and a constant geometry along the surface on the suction and pressure sides of a compressor blade is investigated. The results show a higher potential on the profile loss reduction by riblets on the suction side. In a second step, the effect of laser-structured ribs on the laminar separation bubble and the influence of these structures on the laminar boundary layer near the leading edge are investigated. After clarifying the best choices where riblets should be applied on the blade surface, a strategy for locally adapted riblets is presented. The suction side of a compressor blade is laser-structured with segmented riblets with a constant geometry in each segment. The measured profile loss reduction shows the increasing effect on the profile loss reduction of this locally adapted structure compared to a constant riblet-geometry along the surface. Furthermore, the particle deposition on a riblet-structured compressor blade is investigated and compared to the particle deposition on a smooth surface. Results show a primary particle deposition on the riblet tips followed by an agglomeration. The particle deposition on the smooth surface is stochastic.

Numerical Investigation of a Blade Riblet Surface for Drag Reduction Applications with Large Eddy Simulation Method

Fully developed turbulent channel flow with a blade riblet surface has been simulated numerically at Reynolds number by Large Eddy Simulations (LES). The blade riblet is shown to provide a total viscous drag reduction approximately 9% with the riblet spacing and the cross section . For the sake of investigating the interaction of the turbulent flow with riblets, the mean velocity profiles, velocity fluctuations, and instantaneous flow visualization have been analyzed. It has been found that the riblet of certain size reduces drag by damping the dynamics and weakening the cross motions in the near-wall boundary layer, revealing beneficial turbulence controlling.