Passive Flow Control Method Research Articles

Drag Reduction of Truck and Trailer Combination with Different Passive Flow Control Methods

In this study, drag force and surface pressure measurements were conducted on a 1/32 scaled truck-trailer combination model. The experimental tests were carried out between the ranges of 312×103- 844×103 Reynolds Numbers in a suction type wind tunnel. The aerodynamic drag coefficient (CD) and distribution of pressure coefficient (CP) were experimentally determined on the truck and trailer combination. The regions where has big pressure coefficients were determined on the truck-trailer by using flow visualizations. The aerodynamic structure of truck-trailer combination models was improved by passive flow control methods on 4 different models. By using newly designed spoiler on the model 1, drag coefficient was reduced 10.01 %. On the model 2, adding trailer rear extension with a spoiler, the reduction was obtained as 11.35 %. For the model 3 which is obtained adding side skirt to model 2, the improvement reached 18.85 %. The model 4 was composed of model 2 and a bellow between the truck and trailer. The drag force improvement was obtained as 22.80 % for model 4.

Novel passive flow control method using leading-edge prism-shaped cylinder: Performance enhancement of vertical-axis wind turbines

Due to periodic dynamic stall at low tip speed ratios (TSRs), vertical-axis wind turbines (VAWTs) experience notable performance challenges during rotation, which leads to fluctuations in torque and a decrease in energy capture. This research aims to boost the aerodynamic performance of Darrieus VAWTs by employing a leading-edge (LE) prism cylinder (PC) to enhance energy extraction. This novel small-scale device functions as a passive method for controlling flow separation, aiming to energize the boundary layer and adjust the pressure distribution on the blades. Its effectiveness depends on factors such as size, shape, and placement, necessitating careful optimization. A three-dimensional (3D) computational fluid dynamics (CFD) analysis, combined with Taguchi optimization and analysis of variance, is conducted to determine the optimal design parameters for the LE PC tool. This 3D CFD method captures the full complexity of flow dynamics, including vortex structures and wake behavior, leading to more accurate wind turbine performance predictions than two-dimensional (2D) CFD models. The results highlight the crucial role of PC size (Factor A), which contributes nearly 85% to the total contribution factor, while the angle of PC influence is minimal. The optimized rotor demonstrates a 36% increase in maximum average power coefficient (CP) compared to an uncontrolled rotor at TSR = 1.5. However, the effectiveness of this control method diminishes at higher TSRs because the blades encounter angles of attack below the critical stall angle throughout the rotation cycle, naturally preventing flow separation and making the flow separation control method unnecessary. The PC installed on the optimized blade delays flow separation to 55% of the blade chord length, compared to 40% for the base blade. Consequently, the rotor operates efficiently, ensuring consistent, and reliable power generation without flow separation issues.

Optimization Design of Bionic Leading-edge Protuberances on Horizontal Axis Wind Turbine Blades

Abstract The complex operating environments of horizontal axis wind turbines has made the phenomenon of blade flow separation more obvious. As a passive flow control method, the bionic humpback whale leading-edge protuberances have been shown to effectively enhance the performance of the wind turbine. This study employs a multi-objective genetic algorithm and Computational Fluid Dynamics method to analyse the impact of leading-edge protuberances on the aerodynamic performance and flow characteristics of the blades. The results indicate a significant improvement in blade performance after optimization in the wind turbine’s output power. The streamwise vortices generated by the bionic protuberances not only reduce the suction surface pressure, but also suppress the spanwise flow over the blade surface. The length of the flow separation Region is reduced from 0.410 to 0.368 radius. In the process of wind speed change, the tangential forces on the bionic blade exhibit a wave-like variation. The bionic protuberance alters the pressure coefficient of the position. Compared with the original blade, the pressure coefficient of the trough sections on both sides of the bionic blade protuberance is improved. In this study, the bionic protuberance applied to the leading-edge of horizontal-axis wind turbine blades is proposed for the first time, and holds practical value for guiding practical applications.

The groove effect on wake characteristics of rotating cylinders

In the present study, active and passive flow control methods have been implemented to investigate their effects on the wake flow structures of a circular cylinder. Grooves having circular, rectangular, and triangular cross sections have been applied to the cylinders exposed to the rotation rates, α, from 0 to 0.79. The experiments have been conducted by particle image velocimetry at a Reynolds number of Re = 5 × 103. The contour graphics of time-averaged results have been presented. Moreover, the variations in velocity profiles have also been depicted. The experimental results revealed significant variations for flow patterns, wake structures, and turbulence parameters due to the effects of both groove geometries and rotational motion. In the stationary cases, for turbulence intensity, the circular grooved cylinder exhibited a 15% increase, while the triangular grooved cylinder showed a slightly higher increase of around 20% compared to that of the bare cylinder (BC). Conversely, in non-stationary cases, the rectangular grooved cylinder displayed the most prominent reduction in turbulence intensity, decreasing by approximately 10% compared to that of the BC. The groove type has considerably affected the flow structures of the wake regions, especially for the lower rotation rates.

COMPARISON OF MECHANICAL DESIGN AND FLOW ANALYSIS OF QUADRUPED ROBOTS

In recent years, there has been a trend of developing innovative technologies inspired by living creatures in nature. Quadruped robots, in particular, have emerged as walking mobile systems with articulated leg structures that can skilfully perform dynamic movements that wheeled systems are limited to. These robots offer advantages in technical criteria such as manoeuvrability, cross-capability, controllability, terrain adaptability and stability. It is important to note that this evaluation is based on objective technical criteria rather than subjective opinions. This study compares the advantages and disadvantages of quadruped robots to wheeled systems. It highlights that quadruped robots outperform wheeled systems in manoeuvrability, obstacle overcoming, and speed, particularly in rough terrains. The study also suggests that designers of quadruped robots should consider aerodynamic factors, which are often overlooked. Flow analysis using the finite element method is crucial in robot design to enhance aerodynamic performance. This paper aims to comprehensively analyse the flow structure around quadruped robots using Computational Fluid Dynamics and investigate passive flow control methods to reduce the drag coefficient (Cd). The study examines four different robots, and the resulting Cd average percentage calculations are presented. The aerodynamic efficiency of Robot 4 compared to Robot 2 was found to be 95%. Similarly, the aerodynamic efficiency of Robot 3 was determined to be 28% compared to Robot 2. Additionally, it was determined that the aerodynamic efficiency of Robot 2 was 76% compared to Robot 1. These results provide an important comparison to understand the energy efficiency, differences in aerodynamic performance and relative effectiveness of quadruped robots.

Enhanced heat transfer for subway linear motor through underbody flow regulation

The low-speed flow, restricted tunnel environment, and degraded motors have resulted in a serious challenge in the underbody equipment of subway trains, where overheating of multiple longitudinally aligned linear induction motors is one of the most urgent challenges to be resolved. In the face of this problem, the flow field distribution of a 4-car formation subway train is investigated. Two deflector installation designs are proposed to regulate the underbody flow using the passive flow control method to enhance the heat dissipation of motors. The influence of deflector installation is analyzed through an experimentally verified conjugated heat transfer numerical method. The results showed that the installation of deflectors can significantly enhance the longitudinal underbody flow. On the one hand, the deflector can effectively enhance the momentum exchange between underbody flow and the surrounding undisturbed flow field. On the other hand, the deflectors can break the large lateral vortices, such that the flow resistance along the longitudinal direction is reduced. Besides, the deflectors can also reduce the size of the wake behind the motor. As a result, the increased incoming flow rate ahead of motors effectively alleviated the overheating issue. The enhancement of heat transfer between different deflector installation designs was compared. It was found that the vertical edge of deflectors should be installed as close to the lateral edge of the subway car as possible. The maximum temperature of motors has been reduced by 9 % according to the installation design proposed in this study. The outcomes of this study can also provide theoretical guidance as well as engineering solutions for the cooling design of bottom heat sources and other similar overheating devices in thousands of railway trains with exposed underbody compartment. The convection enhancement around the heating devices can be realized by the surrounding flow field regulation method proposed in this study.

Numerical analysis of novel wavy wall based control of turbulent boundary layer separation

In this work, a comprehensive insight into the new passive flow control method, i.e. a two-dimensional streamwise waviness (recently proposed by Dróżdż et al. [1]) is provided using Large Eddy Simulations. This approach was demonstrated to be effective in postponing turbulent boundary layer separation at high Re, which is out of reach for other commonly known passive flow control strategies. Although the effectiveness of the wavy wall in delaying flow separation has been confirmed, the physical mechanisms standing behind the skin friction enhancement are not fully understood which opens a space for future work. Of particular interest is an insight into the fluid flow in the near-wall region just above the waviness and this paper is aimed at doing so. To separate the effect of the global adverse pressure gradient, the results were confronted with the ones obtained under zero pressure gradient conditions for the identical wavy wall geometry and the same Reτ=2500. Also additional simulations of the turbulent boundary layer building up on the flat wall were conducted to have an additional reference for the flow fields obtained above the wavy wall under zero and adverse pressure gradients respectively. A detailed inspection of the streamwise velocity and its fluctuation profiles indicated that the contribution of the global adverse pressure gradient to the flow statistics begins when the Rotta-Clauser pressure gradient parameter β≥3. The paper confirms an enhancement of the wall shear stress τw downstream the corrugation, which is the necessary condition to postpone the separation. Interestingly, the effect of increased τw was not observed for the case with the same waviness and without pressure gradient.

Method development for estimation of bleed momentum coefficient using surface pressure measurements and in situ hot wire calibration

Recently, as a passive flow control method, bleeding technique allowing flow of upstream towards suction side by interior passages, has been utilized on non-slender delta wing planforms, where effective geometries can enhance the flow field characteristics. In order to quantify the effect of the flow control, estimation of the momentum induced by the technique is of significant importance in addition to quantified aerodynamic favor. In the current work, passive bleeding momentum coefficient estimation method is developed and presented based on surface pressure measurements by utilizing Bernoulli equation and loss coefficient for a 45° swept non-slender delta wing. For the characterization of the presented technique in-situ hot wire measurements were performed allowing to extract across bleed slot pressure loss coefficient. Results represent that proposed approach can be used to compute the bleeding momentum where further advancements might lead to an important control parameter for potential active bleeding systems in different flow regimes.

Flow properties of an Ahmed Body with different passive flow control methods

A numerical simulation by utilizing the FloEFD software was carried out in order to investigate the flow topology formed on slant surface and wake region of an Ahmed Body with and without passive flow control techniques. The effects of those flow controllers on flow at the slant surface and wake region by influencing the flow topology as well as aerodynamic drag coefficient examined carefully. The numerical findings clearly revealed that the best performance in terms of providing the drag reduction obtained when sphere and hemispherical shape flow control techniques were applied at the rear part of slant surface of Ahmed Body. Sphere and hemispherical shape flow controllers positioned at the rear part of slant surface led to have drag reduction of 6% and 7%, respectively. Besides, the results of current study compared with the results obtained from published studies in the literature. It was clearly observed that they are consistent with each other even though they were found by different software.

Effects of Perforated Plates on Shock Structure Alteration for NACA0012 Airfoils

This research investigated a passive flow control technique to mitigate the adverse effects of shock wave–boundary layer interaction on a NACA 0012 airfoil. A perforated plate with a strategically positioned cavity beneath the shock wave anchoring spot was employed. Airfoils with perforated plates of varying orifice sizes (ranging from 0.5 to 1.2 mm) were constructed using various manufacturing techniques. Experimental analysis utilized an “Eiffel”-type open wind tunnel and a Z-type Schlieren system for flow visualization, along with static pressure measurements obtained from the bottom wall. Empirical observations were compared with steady 3D density-based numerical simulations conducted in Ansys FLUENT for comprehensive analysis and validation. The implementation of the perforated plate induced a significant alteration in shock structure, transforming it from a strong normal shock wave into a large lambda-type shock. The passive control case exhibited a 0.2% improvement in total pressure loss and attributed to the perforated plate’s capability to diminish the intensity of the shock wave anchored above. Significant fluctuations in shear stress were introduced by the perforated plate, with lower stress observed in the plate area due to flow detachment from cavity blowing. Balancing shock and viscous losses proved crucial for achieving a favorable outcome with this passive flow control method.

Aerodynamic Instabilities in High-Speed Air Intakes and Their Role in Propulsion System Integration

High-speed air intakes often exhibit intricate flow patterns, with a specific type of flow instability known as ‘buzz’, characterized by unsteady shock oscillations at the inlet. This paper presents a comprehensive review of prior research, focused on unraveling the mechanisms that trigger buzz and its implications for engine stability and performance. The literature survey delves into studies concerning complex-shaped diffusers and isolators, offering a thorough examination of flow aerodynamics in unstable environments. Furthermore, this paper provides an overview of contemporary techniques for mitigating flow instability through both active and passive flow control methods. These techniques encompass boundary layer bleeding, the application of vortex generators, and strategies involving mass injection and energy deposition. The study concludes by discussing future prospects in the domain of engine-intake aerodynamic compatibility. This work serves as a valuable resource for researchers and engineers striving to address and understand the complexities of high-speed air induction systems.

Experimental and Numerical Investigation of the Control of the Flow Structure on Surface Modified Airfoils

In this study, experimental and numerical flow analysis was performed on three different blade profiles with a chord length of 165 mm using passive flow control method. The first of the airfoil is the standard NACA 0018 profile. The second airfoil type has a NACA 0018 profile with a gap in the suction surface. The last airfoil is the NACA 0018 profile which is 66% of the trailing edge cut from the chord length. All airfoil profiles were analyzed at the Reynolds number, Re=2x104, and angles of attack α=0o, 5o, 10o, 12o and 14o in both experiment and numerical studies. The experiments were carried out using the Particle Image Velocimetry (PIV) method in a closed-loop open water channel, and the time-averaged velocity vectors, streamlines, and vorticity contours of the flow field were obtained. Subsequently, numerical analyses were performed using the ANSYS Fluent package program, one of the Computational Fluid Dynamics (CFD) programs used frequently in the literature. The streamlines and pressure contours of the airfoil profiles have been compared visually at the same Re and different angles of attack. In addition, according to the angle of attack of the airfoil profiles, lift coefficient CL, drag coefficient CD, and the ratio of lift coefficient to drag coefficient CL/CD graphs were presented. It has been shown that the gap on the airfoil at high attack angles caused changes in lift (up to 0.7) and drag (up to 0.15). These features can allow these models to be used for different purposes in the aerodynamics field.

Proof of concept study on a self-driven pulsed jet on a compressor stator blade by numerical simulation

This study presents a new concept of self-driven pulsed jet flow control on a compressor stator blade. This passive unsteady flow control method has the advantage that neither external flow nor electrical source is needed. This study’s preliminary proof-of-concept study on a low-speed compressor stator blade is performed using numerical simulation. When the pulsed jet frequency is 100 Hz, the optimum control performance and control efficiency are reached, and the total pressure loss coefficient is reduced by 8.9%. As the valve’s rotational speed increases, the pulsed jet’s momentum coefficient decreases gradually. The analysis of the unsteady characteristics of the self-driven pulsed jet shows that the jet velocity is close to a periodic square wave signal, and the typical reduced jet velocity ranges from approximately 0.15 to 0.8. Moreover, the time-averaged driving torque on the valve depends on the rotational speed and is relevant to the self-starting and self-driven characteristics of this passive flow control method. Under different bearing resistance torque, the self-driven valve behaves differently in three cases that can self-start and be self-driven, cannot self-start but can be self-driven, and cannot self-start nor be self-driven.

Influence of the chordwise distribution of tip winglets on the stability of a high-load compressor stage

Compressor rotor blade tip winglet technology has been confirmed to be a passive flow control method that is effective in increasing the operating stability of compressors. As revealed by extensive research, the structural parameters of tip winglets are capable of affecting their stability expansion effect, whereas their mechanism and influence law of the compressor stage remains unclear. In this study, the effect exerted by the chordwise distribution of tip winglets on the stability of a high-load compressor stage was investigated. As indicated by the result of this study, the tip winglet exhibiting a proper widest position and an adequate chordwise length can effectively increase the stall margin. To be specific, the widest position was located at 25% chord, and the stall margin was increased by up to 25.39%. A winglet exhibiting a chordwise length of 50% achieved the stability expansion ability equivalent to winglet with a full chordwise length, which can be increased by 22.58%. The tip leakage flow turned out to be weakened as the axial momentum was increased, and the circumferential momentum of the flow that arose from the tip winglet was reduced. The result of this study also suggested that tip winglets can play a certain role in combing the inflow of the stator, such that the blockage of the stator channel can be reduced.

Parameter Study of a Loss Reducing Passive Flow Control Method in a Square-to-square Sudden Expansion

The energy consumption of mechanical ventilation in buildings needs to be reduced. An efficient way to achieve this goal is to reduce the hydraulic resistance of the ventilation duct system elements, for example, that of sudden expansions. Ventilation ducts and pipe fittings are frequently of rectangular cross-section. The present paper investigates a passive flow control method in order to reduce the loss coefficient of a square-to-square sudden expansion, where the loss-reducing appendages are short guide vanes, termed as miniflaps, placed at the step edge of the sudden expansion. The turbulent flow is examined numerically using the generalized k-ω model of the Ansys Fluent software for different area ratios of the sudden expansion, miniflap lengths, and miniflap angle setups. The Reynolds number is kept constant at 1.08·105. Based on the results of the numerical simulations, the loss coefficient of the sudden expansion can be reduced by ~20–25% for an optimum miniflap angle between 9° and 12°. Increasing the length of the miniflaps leads to a greater reduction of the loss coefficient up to a miniflap length of 0.3 dh1, where dh1 is the upstream hydraulic diameter of the duct.

Performance analysis of a horizontal axis current turbine blade section with inserted tube

Generating more usable power annually from the river and tidal currents is essential to improving cost-effectiveness. Among various alternative options, the performance improvement of the blade foil has been considered in this work. When the fluid over the blade surface loses kinetic energy, flow separation occurs. The lift forces are reduced by flow separation, which finally results in less power production by the horizontal axis current turbine. To extract more power, it is necessary to overcome this flow separation. This paper presents a passive flow control method using tubes at regular intervals on the blade section to improve its performance considering its application on a horizontal axis current turbine. The tube inlet and outlet positions are determined by analyzing the force coefficients, glide ratio, and stall angle for a specific angle of attack. Finally, the performance characteristics are compared between the baseline and the modified hydrofoil. The maximum lift coefficient of the hydrofoil is increased by 15.7%. Also, the maximum glide ratios are considerably increased beyond the stall of the baseline profile. From the numerical results, it can be concluded that tubes inserted at regular intervals on the hydrofoil significantly increase its performance at a higher angle of attack.

Aerodynamic performance improvement of Savonius wind turbine through a passive flow control method using grooved surfaces on a deflector

A Savonius rotor is a Vertical-Axis Wind Turbine (VAWT) that converts wind force into torque on a rotating shaft. The poor efficiency of this rotor is caused by the unfavorable negative torques generated as the returning blade rotates against the incoming flow. The current study aims to improve the aerodynamic performance of a Savonius wind rotor by installing a cylinder deflector with different grooved surfaces as a unique deflector system to offset the negative impact at the wake zone downstream of the deflector. The grooved surface is essentially one way to alter the surface roughness of a cylinder, which has shown to be quite successful to control vortex shedding and wake formation behind the cylinder. In contrast to previous research, a grooved surface of a cylinder deflector was used instead of traditional deflectors to increase the aerodynamic performance by disrupting the wake zone downstream of the deflector. With this motivation, a systematic parametric study via Computational Fluid Dynamics (CFD) method was implemented for different combinations of the flow and geometric characteristics including Tip Speed Ratio (TSR) value, groove shapes (rectangular, U, and V), groove number, and size. Besides, the flow over a smooth cylinder was analyzed and compared with that of grooved cases to determine the ideal groove configurations for enhancing the total output power. The results demonstrated that the optimal deflector with the U-grooved surface augmented the aerodynamic performance the most for all TSRs. The optimal deflector has 12 U-grooved surfaces with a diameter size of 0.025 of rotor diameter. The average power coefficient enhanced dramatically up to 24.2% and 15.8% through the deflector with a U-grooved surface at TSR = 0.9 in comparison to without deflector case and smooth cylinder deflector case, respectively.

Passive control of boundary layer flow separation on a wind turbine airfoil using vortex generators and slot

Passive Flow Control (PFC) technique has been established as an effective approach in the mitigation/elimination of flow separation phenomenon to enhance the aerodynamic performance of the wind turbine blades. The paper sheds light on the working mechanism of slot and Vortex Generator (VG) in the transitional boundary layer which has received far less attention from researchers. Also, the idea of the combination of two PFC techniques including the slot and the VG treatments on the wind turbine blade has been tested. The lift to drag coefficient (CL/CD) improves up to 112% at incidence angle of 16.3° using VG in comparison to uncontrolled case; however, this coefficient deteriorates down to 12.3% on average for pre-stall angles. The results revealed that the slot exit jet at the aft portion of chord length (70% C) improves the lift to drag coefficient (CL/CD) by 91% at the post-stall condition, a reduction of aerodynamic characteristics over the airfoil profile is detected for the lower inflow angles. The combined PFC method shows better performance than the slotted profile in terms of lift coefficient in most operating angles, but still has lower values than the controlled case with VG.

Thermal performance and structural behaviour of steel hyperbolic cooling tower with holes

Wind load is the control load for the structural design of steel cooling towers, reducing wind load is beneficial to reduce the construction cost of a steel cooling tower. In this study, based on a passive flow control method, a design concept for setting holes in a cooling tower was proposed to reduce the wind load. Taking a 220 m high hyperbolic natural draft dry steel cooling tower as the research object, five elevations and two orientations of holes were established in models. Thermal performance analysis, wind field characteristic analysis, and structural behaviour analysis were used to evaluate the design concept of setting holes. The results showed that in the case where the relative elevation of holes is lower than 0.667, holes will significantly reduce the thermal performance of the cooling tower under wind conditions. In addition, holes in the middle area of the tower will reduce the wind suction in the crosswind direction to a certain extent. Compared with the cooling tower without holes, the steel consumption of the cooling tower with holes can be reduced by 951 t. The research results verified the feasibility of the design concept of setting holes in a cooling tower.

Self-adaptive turbulence eddy simulation of flow control for drag reduction around a square cylinder with an upstream rod

The use of an upstream circular rod to control the flow around a downstream square cylinder is found to efficiently reduce the drag of the square cylinder. The complex flow characteristics and mechanism of the rod-square system in tandem arrangement as a passive flow control method for various centre-to- centre gap ratios (L/D) are investigated by numerical simulation. Turbulence modelling of the rod-square cylinder system is challenging. To select a suitable turbulence modelling, the recently developed Self-Adaptive Turbulence Eddy Simulations (SATES), as well as IDDES (Improved Delayed Detached Eddy Simulation) and SBES (Stress Blended Eddy Simulation) are first assessed for single circular and single square cylinder flow test cases. The SATES method performs best among the three models for bluff body flow simulation and is thus used in the drag flow control study. For the rod-square flow control system, the Reynolds number is 34000 based on freestream velocity U0 and the edge length of the square cylinder D. The diameter of the control rod is kept constant at d/D = 0.18. The flow characteristics and mechanism are explored in detail, varying the rod and square cylinder distance of L/D = 1.5, 2.0, 2.5and 3.0. According to the absence or presence of vortex shedding from the upstream rod cylinder, two flow patterns (pattern I and pattern II) are observed. Large drag reduction is observed for all the simulation cases the maximum drag reduction is approximately 63% with L/D = 2.0, and the minimum drag reduction approximately 53% with L/D = 3.0. The secondary lock-frequency, which presents as pattern II, is caused by the excitation of upstream vortices. It is found that the side recirculation is associated with the wake recirculation length of the square cylinder, which is similar to the elongated cylinder in the flow pattern I. The flow mechanism of suppression of the vortices from the rod can be attributed to the effective diffusion of vorticity in the separating shear layer caused by the presence of the front wall of the square cylinder.