Total Drag Reduction Research Articles

The Response of Turbulent Channel Flow to Standing Wave-like Wall Motion

The effect of wall deformations on wall turbulence is a topic of fundamental importance for the advancement of flow control strategies aimed at reducing the frictional drag. Dynamic wall deformations in the form of wall-normal of in-plane oscillations represents an area that is yet to be fully explored, and that can open a new realm of drag control strategies, as well as provide fresh insights into the structure of wall turbulence. In this study, we present several results from direct numerical simulations aimed at understanding the response of wall turbulence to standing wave-like wall motion, arranged in a checkerboard pattern. More specifically, here we target a low Reynolds number turbulent channel flow, featuring standing wave-like dynamic wall deformations on both bottom and top walls, with parameterizations in terms of the frequency of oscillations, roughness height, and spacing. We quantify the effect that this type of wall motion can have on the skin friction drag, various turbulent statistics, and turbulent flow structures. In addition, taking advantage of the periodicity and the symmetry of the flow, an improved phase-averaging procedure is introduced, which enhances the smoothness of the data. The results show that this type of dynamic wall deflection can have a significant effect on the turbulent flow in proximity to the wall, and that the variation of the spatial wavenumbers of the wall deflections can make a big difference. A slight total drag reduction, in the order of 2%, was observed for some combinations of wavenumbers and frequencies, especially for the highest streamwise wavenumber case.

Numerical study of hydrodynamic drag effects of streamwise riblet structures on SUBOFF bare hull model

This study performs a full-scale numerical simulation on meter-scale underwater vehicles (UWVs) with micrometer-scale V-type streamwise riblet structures, focusing on hydrodynamic drag using the SUBOFF model. The periodic simulation along the azimuthal direction enables the full-scale analysis. Two main scenarios are discussed: (1) effect of riblet structure size on drag at a constant maneuvering speed and (2) effect of maneuvering speed on drag for representative riblet cases. In scenario (1), the average wall shear stress at the central region of the body exhibited significant reduction of approximately 50%. However, owing to larger effective area at the central surface of the body fixed with riblets, the friction drag may either decrease or increase depending on the riblet size. Consequently, R100 case showed maximum total drag reduction of ∼3%, whereas R500 case showed maximum increase in drag of ∼3%, where R100 and R500 indicate riblet-applied SUBOFF models with riblet heights of 100 and 500 μm, respectively. In scenario (2), friction drag in riblet-applied region exhibited smaller variations in R100 case, showing universal drag reduction effect at all speeds. In contrast, R500 case showed significant variations, with drag increments ranging from minimum of 1% to maximum of 12% at all speeds.

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.

Research on the drag reduction of high-speed train based on bottom two-multistage wing deflector

To address the challenge of reducing aerodynamic drag while further enhancing the speed of high-speed trains, this research employs the concept of flow control for the bottom parts and draws inspiration from the front wings of Formula 1 (F1) race cars. Three kinds of two-multistage wing deflectors are designed and systematically analyzed by unsteady Reynolds-averaged Navier–Stokes (URANS) turbulent model. The most suitable design is determined by the single bogie model with a simplified train body. Using the improved delayed detached eddy simulation method, the aerodynamic drag of 1:8 three-car train models with or without two-multistage wing deflector is studied at different operational speeds. The results present the total drag reduction is higher at higher speeds. The reductions of 4.26%, 3.92%, 3.63%, and 3.49% are obtained at the operating speeds of 400, 350, 300, and 200 km/h, respectively. The two-multistage wing deflector desirably improves the flow structure at the bottom of the train, which leads to the reduction of aerodynamic drag and a corresponding reduction in the positive pressure zones within the bogie area. Furthermore, the deflector restricts shedding vortices, effectively narrowing the interference range of airflow under the train, which will provide a potential drag reduction method for the next generation high-speed train.

Numerical Analysis of the Drag Reduction Performance of a Double M-Ship Boat With Stepped Planning-Air Coupling

Abstract In this paper, analyze the influence of the stepped planning structure on the drag performance by observing waveform diagrams at the stern of the double M-ship and water–air and pressure distribution diagrams at the bottom of the ship. This study uses the combined stepped planning-air drag reduction technology to improve the sailing characteristics of the double M-ship. Research findings: The stepped planning contributes to a reduction in bottom pressure, enhances water–air distribution, and augments the amplitude of hull movement. Within the design speed range, the maximum drag reduction rate achieved by the stepped planning is 7.574%. However, this enhancement comes at the expense of increased viscous pressure resistance, which becomes the predominant resistance when sailing at full speed; Injecting air at the stepped planning can effectively reduce the viscous pressure resistance increased by the stepped planning. The combined drag reduction technology of stepped planning and air successfully realizes the total drag reduction at the double-M ship's high speed. The total resistance experienced when air is injected at the stepped planning is reduced by up to 20.981% compared to the original hull.

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.

A Novel Squid-Inspired AUV Design: Revolutionizing Coral Reef Preservation Through CFD and Experimental Insights into Drag and Maneuverability

This study focuses on conceptualizing and optimizing an advanced bionic robotic solution to surmount the limitations of current observational vehicles in coral reef conservation. Human divers are traditionally subjected to physiological challenges, while current robotic alternatives grapple with operational impediments, including anthropogenic marine disturbances. Utilizing advanced robotics tailored for marine ecosystems with intricate topologies and fragile corals, this study aims to enhance conservation precision in the face of climatic shifts and marine pollution from anthropogenic activities.This paper develops a squid-inspired Autonomous Underwater Vehicle (AUV), whose behavior is then examined and improved utilizing both experiments and Computational Fluid Dynamics.(CFD). The governing equations considered here are the incompressible Reynolds-Averaged Navier-Stokes (RANS) with an iterative solver for both the pressure-Poisson and momentum equations. Apart from CFD analysis, a prototype was assembled and experimentally tested to validate the CFD outcomes and assess the lift from its bionic rudders. The maneuverability is optimized through careful designs and parameter studies of several features.A head length is determined to balance viscous and pressure drag while achieving the least total drag in the AUV design. Additionally, a disk is introduced at the AUV’s front, which reduces fluid contact, thereby decreasing the viscous drag. While this modification slightly increased pressure drag, it yielded an overall reduction in total drag. Experimental observations confirmed a propulsion speed and underscored the necessity for maximizing rudder extension.The developed AUV’s squid-like design, optimized via CFD, ensures drag reduction by 5.34% and energy efficiency. By mimicking squid dynamics, disturbances to marine biota are minimized. The incorporation of bionic fins, empirically fine-tuned, enhances the AUV’s agility. These attributes establish it as an exceptional prototype, setting a precedent for future coral reef monitoring systems and augmenting conservation strategies.

Drag reduction in turbulent channel flow by gas perfusion through porous non-hydrophobic and hydrophobic interfaces: Part I

Skin-friction drag in liquid flows can be reduced by several methods including bubble injection into the near-wall region, air layer creation, air cavities, and applied super-hydrophobic coatings. The first three of these methods each has major drawbacks. In the fourth method, the depletion of the trapped air pockets on the super-hydrophobic surfaces (SHSs) in turbulent flows may cause a significant drag increase. To improve FDR in turbulent flows, a novel method is investigated that perfuses air through a porous medium with and without a hydrophobic treatment. A set of experiments has been conducted in a recirculating water tunnel at downstream-distance-based Reynolds numbers to 8.2million. The test model was a flat porous plate and the total wall shear stress was measured by a load cell apparatus. Air was perfused through such interfaces at mass flow rates to 50 L/min. It is noted that downstream persistence is not an issue with this technique as long as perfusion occurs near uniformly along the surface. The results showed that the method with maximum airflow exhibited a total drag reduction of around 10–25% with an untreated surface and about 20–30% with a treated surface.

Drag reduction using bionic groove surface for underwater vehicles.

Introduction: The reduction of drag is a crucial concern within the shipping industry as it directly influences energy consumption. This study addresses this issue by proposing a novel approach inspired by the unique ridge structure found on killer whale skin. The objective is to develop a non-smooth surface drag reduction method that can effectively decrease drag and improve energy efficiency for ships. Methods: The study introduces a technique involving the creation of transverse bionic groove surfaces modeled after the killer whale skin's ridge structure. These grooves are aligned perpendicular to the flow direction and are intended to modify the behavior of turbulent boundary layer flows that form around the ship's hull. Numerical simulations are employed using the Shear Stress Transport k-ω model to analyze the effects of the proposed groove surface across a wide range of flow conditions. The research investigates the impact of various parameters, such as the width-to-depth ratio (λ/A), groove depth, and inlet velocity, on the drag reduction performance of the bionic groove surface. Results: The study reveals several key findings. Optimal shape parameters for the bionic groove surface are determined, enabling the most effective drag reduction. The numerical simulations demonstrate that the proposed groove surface yields notable drag reduction benefits within the velocity range of 2∼12m/s. Specifically, the friction drag reduction ratio is measured at 26.91%, and the total drag reduction ratio at 9.63%. These reductions signify a substantial decrease in the forces opposing the ship's movement through water, leading to enhanced energy efficiency. Discussion: Comparative analysis is conducted between the performance of the bionic groove surface and that of a smooth surface. This investigation involves the examination of velocity gradient, streamwise mean velocity, and turbulent intensity. The results indicate that the bionic groove structure effectively mitigates viscous stress and Reynolds stress, which in turn reduces friction drag. This reduction in drag is attributed to the alteration in flow behavior induced by the non-smooth surface. Conclusion: The study proposes a novel approach for drag reduction in the shipping industry by emulating the ridge structure of killer whale skin. The transverse bionic groove surface, aligned perpendicular to flow direction, demonstrates promising drag reduction outcomes across diverse flow conditions. Through systematic numerical simulations and analysis of key parameters, the research provides insights into the drag reduction mechanism and identifies optimal design parameters for the groove surface. The potential for significant energy savings and improved fuel efficiency in maritime transportation underscores the practical significance of this research.

Alula Effect on Aerodynamic Performance of Small UAVs Wing Configuration

The application range of small UAVs for civil purposes such as aerial imaging, agricultural pest control and, corps management as well as drone delivery has been increasing throughout recent years. Promising results are observed from recent studies which support the effectiveness of the bio-inspired approach in improving the aerodynamic efficiency of small UAVs. However, very limited studies have investigated the effect of Alula on the aerodynamic performance of wing geometries. This research investigates the effects of employing Alula on the aerodynamic efficiency of the wing to overcome the challenge of small UAVs efficiency degradation. Numerical and experimental investigations are conducted on 6 different Alula configurations. The findings of the investigations shows a reduction in drag for wings and an increase in aerodynamic efficiency by 9% for wings with Alula compared to the conventional wing. It is observed that using Alula configuration helps in delaying flow separation by adding momentum into the flow which resulted in total drag reduction without having a significant effect on the lift. Consequently, the aerodynamic efficiency is increased, and this can be utilized in drone industry to increase the endurance of the flight.

Bistability in the wake of a circular cylinder with passive control using two leeward rods

This paper presents an experimental investigation on the flow around a circular cylinder controlled by a pair of rods placed close to the separated shear layers from the leeward face of the cylinder. Past research of Cicolin et al. (2021) has shown that one control rod, placed at specific positions, induces a late flow separation on the main cylinder’s surface in a typical occurrence of the Coandă effect. As a result the control rod reduced the mean drag acting on the cylinder but also produced a net mean lift force. We now present an extension of the aforementioned study to consideration of a pair of symmetrically arranged control rods, aimed at eliminating the mean lift force whilst maintaining a drag reduction. The experiments were carried out at a Reynolds number Re=20,000 based on the diameter of the main cylinder D, with the diameter of the control rods being ten times smaller than that of the cylinder. The centre-to-centre distances between each control rod and the cylinder was 0.7D, and the angular position of the rods varied from θ=120∘ to 125∘, where θ is measured from the front stagnation point. Time-resolved PIV velocity fields and hydrodynamic forces were measured for the different setups. Results show that the mean flow is asymmetric in spite of the symmetry of the geometric model and position of the rods. The pair of control rods induces a bistable flow, induced by the imbalance of two colliding jets in the near wake. The dynamics were found to be random, with the average switching time between stable states depending on the position of the rods. The mean drag force was reduced by up to 15%, and the mean lift force was reduced by 80% compared to the cases with a single control rod. It was also observed that the control rod can induce two different types of flow separation from the main cylinder, one in which the flow separates from the main cylinder only once and one in which a small separation bubble forms in proximity to the control rod before reattaching and then permanently separating. These distinct features play an important role in the dynamics of the bistability and hint at a possible extension of the total drag reduction to 30% if the bistability is suppressed.

Numerical research on drag-reduction characteristics of a body of revolution based on periodic forcing

Effective drag-reduction technology is important for enhancing navigation speed, extending endurance, and enabling efficient energy usage of underwater vehicles. In this study, we investigated the effectiveness of a body of revolution to influence the turbulent boundary-layer properties by performing numerical simulations and varying a periodic sinusoidal force in a series of annular slots. The selected control parameters included the normalized periodic force amplitude and periodic force frequency; the effects of these parameters on drag reduction have not been demonstrated in previous studies. The surface pressure and velocity analyses revealed that frictional drag accounted for 71.7% of the total drag reduction, and it was the dominant contributor to the drag-reduction effect. Unsteady vortices were generated in the boundary layer, and a retarded fluid region was created on the surface. A clockwise vortex was generated downstream of the annular slots during blowing, reducing the pressure drag on the surface of the revolution body. The clockwise vortex weakened when suction was applied. The results of this research might provide ideas for solving the drag-reduction problem in the design of revolving-body-type underwater vehicles.

Experimental Investigation and Intelligent Optimization of Airfoil Zero-Lift Drag Reduction with Plasma Actuators

Micro aerial vehicles flying at low speeds are becoming increasingly popular in military and daily life. Nevertheless, the short cruise time related to the poor aerodynamic efficiency of the wing at low Reynolds numbers is still a key issue. To deal with this, a spanwise plasma actuator array is used to reduce the zero-lift drag of a low-Reynolds-number airfoil, and experimental optimization of the electrical parameters is performed with intelligent algorithms. Results show that for efficient drag reduction, an unsteady unidirectional jet working mode should be preferred by the plasma actuator. In this mode, the drag reduction maps are mostly flat, and the drag reduction magnitude is insensitive to the variation of input voltage amplitude. There exists a threshold particle-observed Strouhal number (0.2) below which the drag reduction effectiveness drops sharply. As a comparison, the map of the power saving ratio shows a steep gradient, and its maximum always resides on the lower bound of duty cycle. With increasing freestream velocity, the mean drag reduction decreases monotonically. A genetic algorithm shows superior performance over surrogate-based optimization by reaching a maximum drag reduction of 40% and a peak power saving ratio of 0.7. Particle image velocimetry results reveal that there exists a laminar separation bubble on the airfoil. With plasma actuation, the transition location is shifted upstream, and the separation region is eliminated significantly. At low speeds, this pressure drag reduction exceeds the friction drag increase, resulting in a net drag decrease. However, transition-induced drag variation can only explain part of the total drag reduction, and the rest is inferred to be turbulent friction drag reduction.

Minimizing airfoil drag at low angles of attack with DBD-based turbulent drag reduction methods

Dielectric Barrier Discharge (DBD) based turbulent drag reduction methods are used to reduce the total drag on a NACA 0012 airfoil at low angels of attack. The interaction of DBD with turbulent boundary layer was investigated, based on which the drag reduction experiments were conducted. The results show that unidirectional steady discharge is more effective than oscillating discharge in terms of drag reduction, while steady impinging discharge fails to finish the mission (i.e. drag increase). In the best scenario, a maximum relative drag reduction as high as 64 % is achieved at the freestream velocity of 5 m/s, and a drag reduction of 13.7 % keeps existing at the freestream velocity of 20 m/s. For unidirectional discharge, the jet velocity ratio and the dimensionless actuator spacing are the two key parameters affecting the effectiveness. The drag reduction magnitude varies inversely with the dimensionless spacing, and a threshold value of the dimensionless actuator spacing of 540 (approximately five times of the low-speed streak spacing) exists, above which the drag increases. When the jet velocity ratio smaller than 0.05, marginal drag variation is observed. In contrast, when the jet velocity ratio larger than 0.05, the experimental data bifurcates, one into the drag increase zone and the other into the drag reduction zone, depending on the value of dimensionless actuator spacing. In both zones, the drag variation magnitude increases with the jet velocity ratio. The total drag reduction can be divided into the reduction in pressure drag and turbulent friction drag, as well as the increase in friction drag brought by transition promotion. The reduction in turbulent friction drag plays an important role in the total drag reduction.

Flexible polymeric tail for micro robot drag reduction bioinspired by the nature microorganisms

In nature, most microorganisms have flexible micro/nanostructure tails, which help them create propulsion, reduce drag, or search for food. Previous studies investigated these flexible structures mostly from the propulsion creation perspective. However, the drag reduction and the underlying physical mechanisms of such tails are less known. This scientific gap is more significant when multi-polymeric/hierarchical structures are used. To fill the gap, we use the dissipative particle dynamics (DPD) method as a powerful fluid–polymer interaction technique to study the flexible tails' influences on drag reduction. Note that the flow regime for these microorganisms is in the range of laminar low Reynolds number; hence, the effects of both pressure and viscous drag forces are crucial. On the other hand, in the DPD method, only the total drag force is obtained. Therefore, this paper first proposes a way to determine the contribution of viscous and pressure drags and then investigates their effects on the body of the micro-robot separately. As a bioinspired-templated micro-robot simulation, the flow over a circular cylinder with an attached flexible tail is investigated. The problem is carried out for the Reynolds numbers from 10 to 25 for different polymer lengths (single/multi) and hierarchical structure tails. Our results show that long polymer tails strongly affect pressure drag, such that the longer polymeric tails (single/multi), the more drag reduction, particularly the pressure drag. Moreover, the hierarchical structures (containing short and long tails) caused the total drag reduction mainly by decreasing the viscous drag rather than the pressure one.

Drag reduction of a slanted-base cylinder using sweeping jets

In this work, a pair of sweeping jet actuators is installed underneath the endplate of a slanted-base cylinder at ReD = 200 000. The sweeping jets form a 30° inclined angle with the endplate and are placed at different streamwise locations, and their strength is varied with a momentum coefficient, Cμ, ranging from 3.8 × 10−3 to 6.0 × 10−2. For all the cases examined in this paper, it is found that while a higher Cμ produces a higher drag reduction, the flow control energy efficiency decreases rapidly as Cμ increases. A net energy saving is achieved when Cμ is less than 0.01, and the highest energy efficiency obtained in the present study is 2.8% when the actuator pair is placed at the most upstream location tested. The drag reduction is attributed to the reaction force and an increase in the surface pressure force acting on the endplate produced by the jet pair. The contribution from the former constitutes an increasing proportion of the total drag reduction as Cμ increases leading to lower energy efficiency in flow control. Depending on the relative positions between the trajectory of the sweeping jet and afterbody vortex, sweeping jets are not only capable of altering the surface pressure distributions via directly imposing a footprint of high pressure on the surface, but also affecting the roll-up of the afterbody vortex and/or reducing its strength via injecting turbulence into the afterbody vortex.

RANS-based optimization of a T-shaped hydrofoil considering junction design

Hydrodynamic lifting surfaces usually include junctions. High-fidelity simulations are necessary to capture critical physics near these regions, such as separation, junction vortices, and cavitation. We present RANS-based hydrodynamic optimizations of a T-shaped hydrofoil, including changes in the junction geometry. The optimized hydrofoils avoid separation and delay cavitation compared to the baseline. The full optimization design with planform, cross-section, and junction geometry variables yields a total drag reduction of 6.4%. The optimized results show that the relative locations of the maximum foil thickness and the maximum strut thickness significantly impact the junction cavitation. Including the translation between the strut and the foil and more strut geometric variables as design variables will provide further improvement. The comparison between optimized designs demonstrates that optimizing planform and detailed junction geometry provides further improvement in addition to designing the cross-sectional geometry. The hydrostructural analyses show that the optimized T-foils have lower stress at the junction than the baseline because of the resultant junction fairing. However, these hydrodynamic-only optimized T-foils have higher deformation and maximum stress, which could result in accelerated fatigue, highlighting the need for hydrostructural responses in design optimization. The results demonstrate that the developed methodology is useful for designing next-generation complex hydrodynamic lifting surfaces.

Drag Reduction by Riblets on a Commercial UAV

Riblets are micro-grooves capable of decreasing skin-friction drag, but recent work suggests that additional benefits are possible for other components of aerodynamic drag. The effect of riblets on a fixed-wing, low-speed Unmanned Aerial Vehicle (UAV) on the total aerodynamic drag are assessed here for the first time by means of RANS simulations. Since the microscopic scale of riblets precludes their direct representation in the geometric model of the UAV, we model riblets via a homogenised boundary condition applied on the smooth wall. The boundary condition consists in a suitably tuned partial slip, which assumes riblets to be locally aligned with the flow velocity, and to possess optimal size. Several configurations of riblets coverage are considered to extract the potential for drag reduction of different parts of the aircraft surface. Installing riblets with optimal size over the complete surface of the UAV leads to a reduction of 3% for the drag coefficient of the aircraft. In addition to friction reduction, analysis shows a significant additional form of drag reduction localised on the wing. By installing riblets only on the upper surface of the wing, total drag reduction remains at 1.7%, with a surface coverage that is only 29%, thus yielding a significant improvement in the cost–benefit ratio.

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.

Pressure Gradient Effects on the Performance of Riblets Installed on Aircraft Surfaces

A method for estimating the drag reduction performance of riblets installed on the surfaces of an aircraft was extended to take the effect of the pressure gradient over the aircraft into account. This method simply and roughly estimates the effectiveness of riblets applied to arbitrary surfaces of any aircraft by fusing wind tunnel test data and computational fluid dynamics analysis. The drag reduction rate is calculated from the relationships between friction drag reduction , wall-normalized riblet spacing , and Clauser pressure gradient parameter . To obtain this relationship, a wind tunnel test with straight riblets in an adverse pressure gradient was carried out. The resulting shift in the value in the intercept of the log-law region of the mean velocity profile for riblet cases indicates that the drag reduction effect decreases as the Clauser parameter increases. Based on these results, an approximate surface for the relationship between , , and was proposed, and the drag reduction performance of riblets installed on the surfaces of a conceptual aircraft was estimated. The total drag reduction rates estimated by the extended method considering pressure gradient effects and the original method case were both about 2%, which indicates that the effects of the pressure gradient on the aircraft on overall riblet performance are small.