Passive flow control of jets

In this study, passive and active flow control methods were used together to manipulate the flow around a circular cylinder. The experiments were conducted in a wind tunnel for the Reynolds number range of 4000 and 10000 based on the diameter of the circular cylinder (D). A splitter plate was used as passive flow control device and its length was chosen to be about 3.75D. Plasma actuators were placed on the circular cylinder at a position of ±90° as an active flow control device. Combining the active and passive flow control methods, a greater reduction of the drag coefficient was achieved compared to that of the cases when using these methods separately. For Reynolds numbers of 5000 and 10000, the hybrid method gives a reduction in drag of 48% and 45%, respectively. The velocity measurements were carried out by using the hot-wire anemometry and velocity profiles were obtained in the wake region. The flow was visualized by using a smoke wire method. The results revealed that the wake region of the circular cylinder with plasma actuator and splitter plate has a narrower width than the plain cylinder and with splitter plate. Also, it can be seen from spectral analysis that the vortex shedding frequency was suppressed significantly by usage of the hybrid flow control method was used.

Aeroacoustic studies of porous cylinder coatings, which reduce the vortex shedding noise of cylinders in uniform flow, have received increased attention. To further understand how porous coatings suppress the vortex shedding tone, a numerical and experimental investigation of porous coated cylinders at a Reynolds number of 1.27×105 is presented herein. Two parameters were varied: (1) the ratio of the porous thickness to the bare cylinder diameter, and (2) the airflow resistivity of the porous media. Numerical simulations were performed using the large-eddy simulation approach with the Smagorinsky sub-grid length-scale model, and the Ffowcs-Williams and Hawkings acoustic analogy was used to predict the acoustic far-field pressure field. A numerical investigation of the internal flow field within the porous layer is presented, which provides a better understanding of the influence of the aforementioned parameters on the development of boundary layers within the porous media and the passive flow and noise control capability. Experiments were performed in an aeroacoustic open-jet wind tunnel to validate the acoustic and external flow field results, which typically showed very good agreement with the numerical results. For porous coatings with low airflow resistivity, the thickness of the porous coating, relative to the inner diameter, has a strong influence on passive flow and noise control. Vortex shedding originates from the inner diameter. Conversely, porous coatings with a higher airflow resistivity are less influenced by their thickness, and vortex shedding originates from the outer diameter (porous surface). The complex coupling of these parameters leads to the conclusion that the optimal selection of the thickness and airflow resistivity is typically dependent on the Reynolds number.

Passive flow control at impeller radial bend for stall delay in centrifugal compressors with fishtail pipe diffusers

With a view to finding the best approach to maximize the wind energy harvesting from wind turbines, this research has focused towards assessing the effects of different innovative modifications to wind turbines at different wind speed conditions and comparing them in terms of aerodynamic performance, economy, and environmental impact. This study illustrates strategies like, active and passive flow control, incorporation of power augmentation devices (PADs) and biomimetic approaches, have huge potential to increase the output power of both the horizontal‐axis wind turbine (HAWT) and vertical‐axis wind turbine (VAWT). Literature survey reveals that boundary layer suction method from active flow control, diffusers as PADs and inspiration from flapping flight of insects from biomimetic approach can enhance the power coefficients of the turbine by 18.08%, 96%, and 35%, respectively. Furthermore, capacity factor (CF) and economic analysis showed that turbine modifications providing high‐CFs and low cost of energy (COE) will have priorities over the others, while selecting suitable turbines in a particular region. Moreover, it is evident that combining the best modifications for each strategy (active control, passive control, PADs, and biomimetic approaches) can lead to the maximum improvement in the overall performance of the turbines in terms of energy and economy.

In this paper, we performed the numerical and experimental study of unsteady cavitation surge around a semi-circular leading-edge flat plate using a passive flow control method. We mounted a miniature spanwise wedge-type vortex generator on the suction side of the model close to its leading edge. To mitigate the destructive impact of this type of cavitation on the hydrofoil performance, we analyzed the effects of the passive control on the dynamics of cavitation surge. First, we investigated experimentally the unsteady cavitating flow around the semi-circular leading-edge flat plate without passive control using high-speed visualization, acoustic measurements and particle image velocimetry method. Next, we simulated numerically the dynamics of unsteady flow under the cavitation surge conditions with an open source code and validated the numerical results using the experimental data. We used a proper interaction between turbulence and cavitation model to capture a highly unsteady behavior of cavitation surge. Finally, we considered the effects of the passive control device on the mechanism of the cavitation surge instability. Our results revealed that using the passive control method, it is possible to stabilize the attached cavity on the suction side of the flat plate, to hinder the development of the spanwise instability of the attached cavity and to mitigate large-scale cavity structures. Furthermore, high-pressure pulsations in the wake region induced by unsteady cavitation surge were considerably reduced.

Rotor blades are essential for helicopter flight as they provide lift similar to an airplane’s wings, allowing the aircraft to take off, hover, and maneuver. However, they also generate aerodynamic drag, which can reduce maximum speed, stability, maneuverability, and fuel efficiency. The enhancement on the blade is usually done at the trailing edge area; however, this time it was done at the leading edge location. This study explored the use of passive flow control to enhance rotor blade performance by minimizing surface turbulence. Through Computational Fluid Dynamics (CFD) analysis, key parameters such as velocity and lift-to-drag ratios coefficient are examined at a Reynolds number (Re) of 3×10⁶. The research utilized the SST k-ω turbulence model and an unstructured mesh to simulate airflow around a NACA 0012 airfoil at a 15° angle of attack. Passive flow control was applied using slots of 5%, 10%, and 15% of the chord length, placed in turbulent flow regions and extending downstream. The findings indicate that positioning passive flow control further downstream increases the lift-to-drag ratio with 5% slot sizes delaying turbulence, improving velocity reattachment to the blade surface and improving the lift-to-drag ratio by approximately 77% compared with non slot blade. This highlights the effectiveness of passive flow control in improving the aerodynamic efficiency of rotor blades.

Large Eddy Simulation of Base Drag Reduction with Jet Boat-tail Passive Flow Control

Tonal noise suppression of a cylinder placed in uniform flow has been achieved, to some extent, by coating it with a structured porous material as a form of passive flow and noise control. A previously studied structured porous-coated cylinder is investigated in an anechoic wind tunnel to determine the relationship between the far-field vortex shedding noise and the pressure recorded on the outer porous surface. To date, no experimental studies have been conducted on the surface pressure of any type of porous-coated cylinder. Acoustic measurements are obtained using an equispaced microphone arc array and simultaneously unsteady surface pressure fluctuations are obtained around the cylinder mid-span circumference using remote-sensing techniques. By obtaining simultaneous time-dependent signals, more light is shed on the underlying noise-reduction mechanism of the structured porous-coated cylinder. In this paper, strong relationships between surface pressures and acoustic signals are revealed at the vortex shedding frequency. A spatio-temporal relationship between surface pressure and vortex shedding phenomena is also presented that helps explain the role of the structured porous media in passive flow and noise control.

This study explores the use of biomimetic designs in passive flow control to reduce wake length at low speed, aiming to enhance aerodynamic efficiency. Inspired by nature's optimization of fluid dynamics in birds and marine life, this research investigates the applicability of these biological principles to improve aerospace engineering designs, particularly for low Reynolds number flows relevant to Unmanned Aerial Vehicles (UAVs). The experimental setup involved a smoke visualization wind tunnel to examine flow patterns around several wing models. These models incorporated bio-inspired elements, mimicking shark skin riblets and humpback whale tubercles, strategically positioned to delay flow separation and minimize wake length. The wing models, designed using 3D modeling software and printed via a 3D printer, were tested at various angles of attack in a smoke visualization wind tunnel by measuring their wake length via image processing. Experiments were carried out in two stages; types of protrusion were investigated on the first stage and the position of a selected protrusions examined in the second stage. The findings indicate significant differences in wake length reduction across the models, with certain biomimetic adaptations, especially optimized triangular protrusions inspired by shark skin, showing notable improvements in aerodynamic performance at higher angles of attack. Among different models in the first stage, Model B was the most effective model in aerodynamic performance with a drag reduction effect of 18% compared to the reference model NACA 0018. In the second stage experiments, Model B-15 showed the most effective aerodynamic result by reducing the wake length by 30% in the range of α=0˚-20˚, while the reduction for Model B-20 was 26%. The results offer valuable insights into the design of efficient, low-speed flight vehicles, suggesting that biomimetics could lead to innovative designs with higher performance and energy efficiency. This research emphasizes the potential of integrating biomimetic principles into passive flow control strategies.

Passive flow control on Ahmed body by rear linking tunnels

In this paper, we explore the improvement of the aerodynamic characteristics of wind turbine blades under stall conditions using passive flow control with slots. The National Renewable Energy Laboratory (NREL) Phase II rotor, for which detailed simulations and experimental data are available, served as a baseline for assessing the flow control system effects. The position and configuration of the slot used as a flow control system were determined using CFD analysis. The 3D-RANS equations are solved with ANSYS FLUENT using the k-ω SST turbulence closure model. The pressure coefficient for different wind speeds for the baseline configuration is compared to the available experimental data. The comparison shows that CFD results were better for the attached flow. The current work consists of a 3-D CFD modeling of a rotating blade equipped with different flow control systems: single-slot (S-S) and two-slots (T-S). The computation provides a better understanding of the influence of these flow control devices on the performance of wind turbine blades, the control of boundary layer separation, and the rotation effect. These control systems increase the power output by over 60% at high wind speeds with large separated boundary layer regions. For the configuration with the control system, the slot has shown its ability to delay the boundary layer separation. However, the improved aerodynamic performance has been proven for medium and high angles of attack where the flow is generally in the stall condition. The addition of the second slot changed the flow behavior, and an improvement was observed compared to the single slot configuration. The results are helpful for the design and development of a new generation of wind turbine blades.

Typically, the fuselage of a modern military aircraft is designed in such a way that the propulsion system is integrated into it. The main reasons are reduction of installation space and minimisation of radar signature. Those requirements can be achieved by using highly bent engine intakes, which are occluding a direct line of sight to the compressor system. Depending on their design, secondary flows and flow separation can be expected due to the strong curvature of the intake system. In this study, a serpentine intake in front of the Larzac 04 test engine is investigated experimentally and its performance compared with and without flow stabilising measures. In detail, a configuration with vortex generators was compared experimentally with a configuration of active flow control by injected air. In order to analyse and compare the efficiency of both systems, the dimensionless total pressure coefficient, the distortion coefficient (DC60) and detailed surface pressure distributions as well as the aerodynamic interface plane are evaluated. In addition, different throttle lines were recorded for surge line evaluation of the low pressure compressor of the Larzac engine and compared for each flow-stabilising measure investigated. It was found that the application of injecting air showed a larger improvement in surge margin and reduction in distortion coefficients compared to the passive flow control.

Purpose The purpose of this paper is to study the transient cavitation process in torque converters with a particular focus on cavitation suppression with a passive flow control technique. Design/methodology/approach The transient fluid field in a torque converter was simulated by RANS-based computational fluid dynamics (CFD) in a full three-dimensional (3D) model. A homogeneous Rayleigh–Plesset cavitation model was used to simulate the transient cavitation process and the results were validated with test data. Various secondary flow passages (SFP) were applied to the stator blade. The cavitation behavior and hydrodynamic performance were simulated and compared to investigate the effect of SFP geometries on cavitation suppression. Findings Presented results show that cavitation in the torque converter is highly unstable at stall operating condition because of the combination of a high incidence angle and high flow velocity. The addition of an SFP to the stator blade produces a disruption of the re-entrant jet and reduces the overall degree of cavitation, consequently inhibiting the unstable cavitation and reducing performance degradation. Originality/value This paper provides unique insights into the complicated transient cavitation flow patterns found in torque converters and introduces effective passive flow control techniques useful to researchers and engineers in the areas of fluid dynamics and turbomachinery.

This study focused on reducing the drag force of a road vehicle using passive flow control. The aerodynamic performance of the model vehicle was improved by applying originally designed vortex generators. The drawing data of the model vehicle and vortex generators were designed in the Solid Works® program. The drag force measurements and flow visualizations were performed in the Fluent® program with the Reynolds numbers in the range of 2.8×105-6.6×105. Accordingly, the CD value of the base model vehicle decreased by 6.22%, 4.59%, 3.38%, and 3.04%, respectively, using the originally designed vortex generator. This aerodynamic improvement rate can decrease the fuel consumption of vehicles by up to 3.3% at high vehicle speeds. To verify the highest numerical drag reduction, the model vehicle and vortex generators were produced in a 3-D printer. The vortex generators were mounted on the vehicle models with the lowest CD value as in CFD analysis. The wind tunnel tests were conducted under the same test conditions for two vehicle models. It was determined that the experimental results supported numerical drag reduction.

An investigation was conducted in the model preparation area of the Langley 16-Foot Transonic Tunnel to study a passive cavity concept for improving the off-design performance of fixed-geometry exhaust nozzles. Passive cavity ventilation (through a porous surface) was applied to divergent flap surfaces and tested at static conditions in a sub-scale, nonaxisymmetric, convergent- divergent nozzle. As part of a comprehensive investigation, force, moment and pressure measurements were taken and focusing schlieren flow visualization was obtained for a baseline configuration and 27 passive cavity configurations. All tests were conducted with no external flow and high-pressure air was used to simulate jet-exhaust flow at nozzle pressure ratios from 1.25 to approximately 9.50. Results indicate that baseline nozzle performance was dominated by unstable shock-induced boundary-layer separation at off-design conditions, which came about through the natural tendency of overexpanded exhaust flow to satisfy conservation requirements by detaching from the nozzle divergent flaps. Passive cavity ventilation added the ability to control off-design separation in the nozzle by either alleviating separation or encouraging stable separation of the exhaust flow. Separation alleviation offers potential for installed nozzle performance benefits by reducing drag at forward flight speeds, even though it may reduce off-design static thrust efficiency as much as 3.2 percent. Encouraging stable separation of the exhaust flow offers significant performance improvements at static, low NPR and low Mach number flight conditions by improving off-design static thrust efficiency as much as 2.8 percent. By designing a fixed-geometry nozzle with fully porous divergent flaps, where both cavity location and percent open porosity of the flaps could be varied, passive flow control would make it possible to improve off-design nozzle performance across a wide operating range. In addition, the ability to encourage separation on one flap while alleviating it on the other makes it possible to generate thrust vectoring in the nozzle through passive flow control.