Passive Flow Control Techniques Research Articles

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.

Flow Instabilities and Control Mechanisms in Cylindrical Cavities With Top Bounding Walls

Abstract Cavity flows studied over the past few decades have led to an increased understanding of the flow physics and instability modes in a range of configurations. While a large number of these studies focus on two-dimensional and three-dimensional rectangular/cuboidal cavities, significant variations are seen with three-dimensional cylindrical cavities with a top bounding wall. The present work details the flow physics of such cavities with a compressible (air) flow past the cavity at two pressure drops of 3850 Pa and 2000 Pa. Results from detached eddy simulations (DES) reveal the presence of the wake mode and shear layer instabilities, respectively, with modified dynamics and oscillatory modes owing to the top wall. In the interest of preventing mode switching with changes in operating conditions, which could lead to large-scale flow disruptions, a passive flow control technique is tested. The modified cavity (with a downstream ramp scaled by the size of shed structures) is seen to maintain the same mode throughout the range of operation offering valuable insights into design modifications for such cavities in practical settings.

Reduction of rotorcraft BVI using synthetic jets; computational studies using LES

In rotorcraft, the blade-vortex interaction (BVI) has been recognized as one of the main sources of noise and vibrations, affecting its aerodynamics performance and stability. Active and passive flow control techniques, for the reduction of BVI, have been investigated. The passive flow control techniques have proved to be inefficient due to the complex aerodynamics of rotorcraft. Thus, active flow control techniques has become of particular interest. In the present research we propose a novel approach, for the reduction of rotorcraft’s noise and vibrations due to BVI. The main idea of this approach is to inject air at the leading-edge of the blade to alter the characteristics (strength and core size) of the incoming vortex and thus, minimize the noise and vibrations associated with BVI. Computational studies are carried out using the large-eddy simulation (LES) with the dynamic Smagorisky sub-grid scale model. The computations are carried out for a flow of Reynolds number R e = 1.3 × 10 6 , based on the chord of NACA0012 airfoil and free-stream velocity. The studies show that injecting air at the leading edge of the blade, the influence of blade-vortex interaction on the aerodynamic coefficients is significantly reduced. Higher reduction of BVI effect, on the aerodynamic coefficients, was achieved with the increase of the speed of the synthetic jet. The reduction of the BVI, also, leads to the reduction of the vibrations.

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

This paper proposes a new strategy for extending centrifugal compressor stall margin through shroud passive flow control placed at the impeller radial bend. Relative to the usual leading-edge location, the radial bend presents fewer geometrical constraints for flow control devices and allows them to have potentially more impact on low-momentum flow structures that lead to stall. Two centrifugal compressors with fishtail pipe diffusers are chosen for study: a high-speed transonic design exhibiting diffuser stall and a low-subsonic design with exducer stall. Four passive flow control techniques placed on the impeller shroud at the radial bend are evaluated for each compressor, in terms of stall margin improvement and efficiency penalty at design mass flow, using unsteady RANS CFD simulations. These are circumferential groove, slots, impeller recirculation pipe and diffuser-impeller recirculation pipe. The results indicate that passive flow control at the impeller shroud radial bend can be significantly more effective than at the impeller leading edge. For each compressor, the flow mechanisms associated with stall initiation and its delay by the most effective studied flow control technique as well as the associated efficiency penalty are elucidated. The findings indicate that techniques having flow injection with high spanwise penetration and low relative streamwise momentum are most effective for delaying diffuser stall, while the opposite applies to exducer stall. The design point efficiency penalty is due to mixing/shear losses from the jet-in-cross flow phenomenon in the impeller and shear losses from flow redistribution in the diffuser.

Passive flow control for the E423 airfoil utilizing an equilateral triangular trip

A passive flow control technique of utilizing an equilateral triangular trip close to the leading edge was developed and tested for a micro-scale Eppler E423 airfoil at a Reynolds number based on the chord of 40 000. The analysis was carried out via high-order wall-resolved large eddy simulation using the computational solver HpMusic. Angles of attack of 5° and 20° were tested. It was shown that at an angle of attack of 5°, the trip height of two times the local boundary layer thickness outperformed existing passive flow control techniques from the literature by almost a factor of five in terms of the lift-to-drag ratio. To understand the underlying physics which allowed the trip to provide this very significant performance benefit, metrics such as pressure coefficient profiles, oil flows, iso-surfaces of Q-criteria, and leading-edge flow behavior were examined. It was found that this trip configuration simultaneously removed the flow separation regions on both the suction and pressure sides of the wing.

Passive elimination of flow-induced forces and vibrations of a circular cylinder using a slit

Here, passive flow control technique of incorporating a slit through a circular structure is utilized to dampen the flow-induced vibration (FIV). The study is segregated into two main sections, one where the effect of slit width ratio (S/D) and Reynolds number (Re) is investigated on the fixed stationary cylinder, and the other where the effect of slit size is explored on the behavior of the elastically mounted cylinder with varying reduced velocities (Ur) at Re = 100. The slit width ratio is varied from 0% (no slit) to 50% and Re from 100 to 300 for a stationary cylinder. For elastically mounted cylinder, the mass (m* = 5) and damping (ζ = 0.01) ratios and Re (100) are fixed, whereas Ur is varied from 0 to 10. The results are expressed as coefficient of lift and drag, Strouhal number, amplitude and frequency of vibration, force-displacement plots, and vorticity contours. They indicate that the presence of a slit reduces the lift and drag coefficients and suppresses the FIV. At Re = 100, the presence of 10% and 20% slit decreases the lift coefficient by 13.0% and 71.8%, respectively, as compared to the solid cylinder. Considering the maximum amplitude of vibration, a 10% slit resulted in a 5.7% amplitude drop, whilst a 20% slit resulted in a significant drop of 34.4% as compared to the solid cylinder. Further increasing the slit size from 20% to 30% completely vanished the vortex shedding and led to steady flow with zero lift and no vibration.

Turbulent boundary layer over porous media with wall-normal permeability

Porous walls are a widely used passive flow control technique, which shows potential in reducing skin friction and mitigating flow-introduced noise. In the present study, porous media with wall-normal permeability is applied to a flat plate to investigate its interaction with the turbulent boundary layer at the Reynolds number based on friction velocity of Reτ=225. Time-resolved planar and tomographic particle image velocimetry were employed to identify the impact on mean statistics and coherent structures. An overall skin friction reduction of 22% is achieved. The porous wall induces counter-rotating streamwise vortex pairs at the spanwise sides of each pore, leading to momentum transport and the generation of alternative low- and high-speed regions close to the wall. Slip velocity is obtained, associated with the reduction in turbulent fluctuations and Reynolds shear stress. The streamwise velocity streaks and the hairpin vortices are significantly distorted compared with the smooth wall condition due to the downwash and upwash motion, featuring a notable reduction in the number and scale of the coherent structures, in which the skin friction reduction mechanism is related to. The proper orthogonal decomposition analysis returns the most energetic unsteady modes. Although the wall-coherent mode type remains to dominate the production of turbulent fluctuations, the scale and energy content of wall-incoherent modes increase, confirming the modification of the distribution and scale of near-wall turbulent structures.

Experimental Investigation into the Drag Performance of Chevron-Shaped Protrusions in Wall-Bounded Turbulence

AbstractChevron-shaped protrusions have been proposed in the literature for turbulent skin friction reduction. However, there is no consensus on the performance of this passive flow control technique; both an increase and a decrease in drag have been observed in previous studies. There is also no experimental evidence to support the working mechanism behind the drag reduction effect that has been postulated in the literature. In this study, direct force measurements were used to replicate experiments from the literature and, in addition, were used to test new array configurations to characterise the effect of individual design parameters on drag performance. A total of 23 different protrusion configurations were investigated in a turbulent boundary layer flow. In addition to the integral force measurements, particle image velocimetry was used to measure wall-parallel velocity fields in order to extract the statistical sizing and energy of the near-wall cycle turbulence. All configurations increased the drag between 2% and 10% for a friction Reynolds number of 1700. The drag reduction reported in the literature could not be replicated; however, these findings agreed with an experimental and numerical study that reported drag increase. The trend observed in the low-speed streak spacing from the PIV experiments was consistent with that observed in the balance data. Nevertheless, no evidence was found to support the working mechanism proposed in the literature. These results cast doubt on the proposed drag reduction potential of chevron-shaped protrusions. In the authors’ view, the results of this study strengthen previous conclusions regarding their minor increase in drag. Future studies to further approach a consensus are proposed.

Enhanced energy extraction in small-scale wind turbines through slot-based passive blowing

The increasing demand for sustainable energy has spurred research and development of wind turbines. In this study, a passive flow-control technique using a non-uniform curved slot is proposed. The slot is engraved on the mid-chord region of the National Renewable Energy Laboratory (NREL) Phase VI wind turbine blade, between the quarter-span and tip radial stations. The impact of slot-induced flow control is investigated using three-dimensional transient computational fluid dynamics (CFD) modeling across a range of wind speeds, including pre-stall and deep-stall conditions. Aerodynamic analyses are performed to ascertain the influence of slot-flow on the complex three-dimensional flow dynamics around the blade. The slotted blade demonstrates significant improvements in low-speed shaft torque and power compared to the baseline blade. Additionally, the slotted blade experiences increment in the inherent thrust and flapwise bending moments. Detailed wake flow field and surface flow visualization indicate that the slot promotes a more streamlined attached flow across the blade span, delaying flow transition and suppressing turbulence on the blade surface and downstream wake. The slot-flow also reduces radial flow and tip-vorticity anomalies along the blade. Overall, flow control enhancement achieved by the slotted blade significantly boosts the annual energy production of the wind turbine.

Numerical investigation of aerodynamic characteristics of naca 23112 using passive flow control technique – gurney flaps

Numerical investigation of aerodynamic characteristics of naca 23112 using passive flow control technique – gurney flaps

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.

Base Bleed Flow Control Tool for Circular Cylinders with three Side-By-Side Arrangements in Shallow Water

In this study, Particulate Image Velocimetry (PIV) was utilized to determine the flow characteristics of three side-by-side circular cylinders with a base bleed. The height of the water was set as hW=20 mm which has characteristics of shallow water and flow images were captured from an elevation of hL/hW= 0.5 (mid-plane of water height). The freestream velocity of water and circular cylinder diameters were chosen as U∞=0.125 m/sec and D=40 mm, respectively which are the corresponding Reynolds number of ReD=5000. Three different gap/diameter ratios between the cylinders were tested as G/D=1.0, 1.25, and 1.5 throughout the experiments. As a passive flow control technique, base bleed slots located through the cylinders were used with the dimensionless width ratio of B/D= 0.05 to 0.25 with 0.05 increment. As a result of the study, the base bleed was found out an effective way to control unsteady wake. Besides, whereas the increasing height of VSP results in further decrement turbulence statistics, Increasing the width of base bleed leads to a reduction of maximum Reynolds shear stress concentration. Nevertheless, jet deflection flow cannot be avoided for the G/D=1.25 while, flow deflection is avoided with all base bleed case the gap ratio increases to G/D=1.5. The wake size of each cylinder is almost the same for G/D=1.5.

Flow instability control of an ultra-highly loaded transonic compressor rotor using self-excited casing bleed

Flow instability is a common issue encountered by high-speed compressors when they operate outside of their optimal range, especially in highly loaded compressors. This study investigates the potential of an unsteady passive flow control technique, self-excited bleed (SEB), which involves casing modification, to improve the base flow and stability characteristics of an ultra-highly loaded low reaction transonic compressor rotor. Through transient computational fluid dynamics simulations, we demonstrate that SEB can extend the rotor's operating range by up to 14.07%. The physical mechanism underlying this stability enhancement is the suppression of the shock-induced breakdown of the tip leakage vortex (TLV) near the blade leading edge and the attenuation of the double leakage flow by SEB. The unsteady excitation of the bleed effect dominates the tip flow and eliminates the spontaneous closed-loop feedback process based on the dynamic interaction between the TLV breakdown, the tip secondary vortex, and the blade loading. Time-resolved tip-region flow patterns elucidate the self-organization and reconstruction of this feedback mechanism. Frequency spectral analysis further reveals that the self-induced oscillation frequency of the tip leakage flow formed during the feedback process disappears, and the bleed excitation frequency replaces it as the main frequency of the tip flow field. However, increasing the bleed flow rate causes the boundary layer on the suction surface to migrate radially outward, resulting in increased flow blockage at the rear of the tip passage. These two influences of SEB are quantified by a blockage factor, and determining the optimal bleed flow rate requires a trade-off between beneficial and detrimental impacts.

Winglets effect on the aerodynamics of aircraft wing; computational studies

Aircraft drag reduction has been of interest to aerospace industry for long time. Therefore, in the past decade different flow control techniques have been used such as active and passive flow control techniques. Winglet design has been explored in the past both experimentally and computationally. Experimental studies pose significant challenges due to the wing-span and costly equipment and facilities. Therefore, this study concerns the computational studies of winglet design using the large-eddy simulation approach. The results show that the winglet design reduces the flow separation and drag force, while the pressure and lift force increases.

Combined passive and active flow control for fixed-wing micro air vehicles

This study presents the design, implementation, and assessment of a combined passive and active flow control technique with the aim of increasing the aerodynamic performance of fixed-wing Micro Air Vehicles (MAVs). Power consumption restrictions in MAVs support the choice of passive flow control solutions such as the use of a modified (tubercled) wing leading edge. This strategy successfully allows to delay and mitigate aerodynamic stall but detrimental effects are found at pre-stall operating conditions. In order to retrieve the lift-generation capabilities of the baseline wing at pre-stall, a subsidiary active flow control method making use of air blowing was designed and installed in the modified wing. Guidance to the selection of optimum settings was provided by experimental and computational analyses. The resulting hybrid flow control system demonstrated its effectiveness, thus producing generalized lift enhancements irrespectively of the attitude of the wing.

Geometric Optimization of Blunt Bodies with Aerodisk and Opposing Jet for Wave Drag and Heat Reduction

The use of passive, active, or hybrid flow control techniques is often investigated to reduce the acoustic signature, wave drag, and aerodynamic heating associated with the supersonic flow regime. This research explores passive and hybrid flow control techniques to achieve an optimal reduction in wave drag and aerodynamic heating on a blunt body using an aerodisk. While passive techniques use one or two aerospikes, hybrid techniques employ opposing jets and aerospikes. Numerical analysis is performed using Reynolds-Averaged Navier–Stokes (RANS) equations to analyze the bodies’ flow field. The statistical technique, Design of Experiments (DOE), is combined with Response Surface Method (RSM) to find the optimal configuration for four cases by generating design space. Two cases were considered for the optimization: single aerospike with and without opposing jet and double aerospike with and without opposing jet. Variables used for the design of the aerodisks were spike length and diameter, while the response variables were wave drag and normalized heat flux. The current study has established an optimum relationship between spike length and aerospike diameter located in front of the main blunt body for both single and double aerospikes. The study’s results suggest that a double aerodisk configuration is more beneficial for reducing drag and heat flux at supersonic speed than a single aerodisk. By incorporating an opposing jet at a pressure ratio of 0.8 from the frontal aerodisk to the spiked blunt body, it can reduce drag and heat flux by 86% and 95%, respectively. Finally, numerical verification is performed for statistically optimized designs.

Experimental investigation on cavitation and induced noise of two-dimensional hydrofoils with leading-edge protuberances

The applicability of leading-edge protuberances as a passive flow control approach inspired by humpback whale flippers has attracted significant research attention in aquatic and aeronautic systems because of their influence on critical hydrodynamic and aerodynamic aspects. An experimental investigation is conducted in a cavitation tunnel under various flow conditions to determine the effectiveness of leading-edge protuberances in controlling the detrimental effects of cavitation and suppressing flow-induced noise. The experiments are carried out on four National Advisory Committee for Aeronautics airfoil 0012 hydrofoils at 7° attack angles and free stream velocities up to 10 m/s. One of the four hydrofoils is considered the baseline, while the other models have wavy leading-edge modifications with different sinusoidal protuberances. These geometry modifications are defined by the amplitudes (A) (2% and 4% of the mean chord length) and wavelengths (λ) (12.5% and 25% of the mean chord length) of the sinusoidal protuberances. Investigations of flow over hydrofoils from top and side views at various Reynolds numbers exhibit that cavitation first appears in the modified hydrofoils' troughs and is restricted to just behind the protuberance troughs for the entire cavitating flow range. These results contrast the baseline geometry, where cavitation inception occurs at the flat leading edge, and the sheet cavity expands spanwise with extensive cloud shedding. Image processing under certain conditions reveals that the protuberances reduce cavitation by 25%–60%. The analysis of the sound pressure level demonstrates that the leading-edge protuberances effectively decrease flow-induced noise at higher flow velocities when cavitation is the dominant noise source. Finally, the direct comparison of cavitating flow characteristics, quantitative cavitation measurements, and noise production analysis between the baseline and modified hydrofoils, and their comparison among the modified geometries, provides a significant reference for future modeling of potential applications employing this passive flow control technique.

Aerodynamic Effects of Knitted Wire Meshes—CFD Simulations of the Flow Field and Influence on the Flow Separation of a Backward-Facing Ramp

Passive flow control techniques are needed to reduce flow separation and enhance aerodynamic performance. In this work, the effect of a knitted wire mesh on the flow separation of a backward-facing ramp was numerically investigated for a Reynolds number of 3000. A grid independence study and a RANS turbulence model sensitivity analysis were conducted. The CFD simulations exhibited counter-rotating streamwise vortices emerging from the knitted wire mesh, and the reattachment length was significantly reduced. A variation of the knitted wire rows revealed a maximum reduction of the reattachment length of 25.7% for the case of four rows. A comparison with a different knitted wire mesh geometry yielded a decreased reattachment length reduction.

Optimization of passive flow control above the ski jump ramp of an aircraft carrier by CFD and experimental investigation

Aircraft carriers are a very useful operational tool for military operations. They are designed to provide service to aircraft at sea. For that reason studying the aerodynamic flow on their decks is essential. During take-off and landing maneuvers the aircraft can be affected by the turbulent flow generated by the non-aerodynamic surfaces that compose the geometry of the ship. Specifically, this study uses Computational Fluid Dynamics (CFD) applied to an aircraft carrier. The goal is to analyze and modify the flow detachment generated by the ski jump ramp on the flight deck that can affect the aircraft performances during take-off maneuvers. Passive flow control techniques such as holes in the ramp to allow airflow entering over the deck or aerodynamic devices added in the ramp corner have been tested. Different parameters of the devices have been studied and partial and even complete elimination of the flow detachment has been achieved. Finally, the numerical model has been compared with experimental wind-tunnel tests. They have demonstrated a full elimination of the flow detachment and up to 80% of turbulence intensity reduction above the ski jump ramp of the aircraft carrier.