Passive Flow Control Method Research Articles

Passive Jet Approach to Control the Flow over a Circular Cylinder

AbstractA numerical investigation was conducted on a passive jet flow control method for alleviating unsteady wake oscillation behind a circular cylinder. The study focused on the effects of two cr...

Zamana Bağlı Kavite Akışı Aeroakustiğinin Büyük Girdap Simülasyonu ile Sayısal Olarak İncelenmesi

Flow along a cavity is of special interest for researchers due to the occurrence of free shear layer flow and related high levels of sound and pressure forces. In this study, turbulent flow along an open cavity at a low inlet Mach number (Ma = 0.034) is modelled by Large Eddy Simulations (LES). The velocity profiles at various stations inside the open cavity are compared to available experimental data. It is found that LES results agree with experimental data and detects the transient pressure change in the flow field satisfactorily. Transient pressure data in the flow field is evaluated in acoustic analogy. The noise generated by the cavity is compared with the established Rossiter modes and is found to be reasonable. To create an effect on the sound pressure levels (SPL), a small obstacle with quadrilateral cross section is immersed in the shear layer at three different locations. This causes that the SPL peaks are reduced compared to the case without any obstacle. Thus, cavity-induced noise form specific frequencies are redistributed to high frequency broadband noise as a result of this passive flow control method.

Experimental and numerical study of the cavitation surge passive control around a semi-circular leading-edge flat plate

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.

Large eddy simulation of passive jet flow control on the wake of flow around a circular cylinder

This study presents a passive jet flow control method to suppress the wake of a circular cylinder based on a computational fluid dynamics (CFD) numerical simulation at a high Reynolds number of 5.0 × 104. The investigation mainly focuses on the control effectiveness of the pattern of jet holes on the hollow pipe. First, the three-dimensional (3D) flow past a circular cylinder at a Reynolds number of 3900 is addressed based on the large-eddy simulation (LES) method to validate the feasibility of a numerical simulation. Then, the control effects of the passive jet flow on the aerodynamic forces and the wake flow of the circular cylinder are studied at different directions, angles, and heights of the jet holes. An optimal parameter scheme for suppressing the wake of a circular cylinder is then determined according to a comparison of the control effectiveness of the aerodynamic forces. The results indicate that the jet flow from the backward holes of the circular cylinder effectively separates the shear layers rolled up on both sides of the circular cylinder, which forces the vortex formation region downstream. This control measure can dramatically reduce the aerodynamic forces and suppress the wake of the circular cylinder.

VLES of drag reduction for high Reynolds number flow past a square cylinder based on OpenFOAM

The flow past a square cylinder attracts extensive interests in the past decades. It also has been used for the drag reduction research of flow past a bluff body. The present study performs Very-Large Eddy Simulation (VLES) of passive drag reduction for a square cylinder based on the open-source CFD (Computational Fluid Dynamics) toolbox, OpenFOAM. The newly developed VLES method is implemented in OpenFOAM and also assessed in the complex flow control simulations. For the drag reduction, a passive flow control method is used, i.e. cutting the corners at the front edge of the square cylinder. Firstly, a control method from previous studies (referred as to CM-1, i.e. control method 1) is adopted and numerical investigations are performed. Based on that, a new control method, i.e. cutting the corners with an extra cut (referred as to CM-2, i.e. control method 2) is proposed and the performance of the new control method for drag reduction is examined. The results show that, firstly, the reliability and accuracy of the VLES turbulence method is verified by the simulations, and also it is confirmed that the VLES method can give satisfactory predictions using a relatively coarse computational grid. Then, it is found that both the control methods of CM-1 and CM-2 can give significant drag reduction, up to 48% and 61% drag reductions compared with the original square cylinder. Also, the new proposed control method generally works better than the previous control method. The results can benefit to the high-fidelity numerical simulation and also the drag reduction studies for general bluff body flows which is widely encountered in ocean engineering applications.

Passive control of a front inclined hole on flow structures around a surface-mounted short cylinder

To control the wake flow of a short cylinder, a passive flow control method, a front inclined hole in the cylinder was drilled from the front surface side to the free end surface. A cylinder of an aspect ratio H/D = 1 with a height H = 70 mm and a diameter D = 70 mm is used. A circulation water tunnel is used to perform PIV measurements at Reynolds number of 8,570 for comparing the flow structures between the no hole and hole wakes. In order to consider the influence of the hole position, the different cylinder models of the hole height at the front from the wall are tested. Comparing with the flow structures around standard cylinder, it is found that the separation zone near the free end surface decreases and the rear separation ron increases because of the blown flow from the inclined hole. The vorticity and Reynolds shear stresses are also suppressed in the rear recirculation zone. The instantaneous large-scale eddies in the rear separation zone are divided into several small-scale eddies as increasing the angle of the inclined hole.

Structured Porous Material Design for Passive Flow and Noise Control of Cylinders in Uniform Flow.

Cylindrical bodies in uniform flows can be coated with a porous medium as a passive flow and noise control method in an effort to reduce the acoustic effects of vortex shedding. To date, the employed open-cell porous materials typically possess a randomized internal structure. This paper presents the design and validation of a novel 3-D printed structured porous coated cylinder that has significant flexibility, in that the porosity and pores per inch of the porous coating can be modified independently and relatively easily. The performance of the structured porous coating design is compared against porous polyurethane and metal foam with the same coating dimensions and similar pores per inch and porosity via an experimental acoustic investigation, revealing strong similarity in the passive noise control performance of each material type. A numerical comparison illustrates the similarities of the wake structure of the 3-D printed porous coated cylinder to an equivalent Darcy–Forchheimer model simulation that represents a randomized internal porous structure. The performance similarities of these different porous material types indicate that a structured porous geometry can be used to understand the internal flow behavior of the porous medium responsible for reducing the cylinder vortex shedding tone that is otherwise extremely difficult or impossible with typical randomized porous structures. Moreover, significant potential exists for the porous structure to be further optimized or smartly tailored by architectural design for different control purposes, coating geometries and dimensions, and working conditions.

Flow separation control of NACA-2412 airfoil with bio-inspired nose

PurposeThe purpose of this paper is to achieve an optimum flow separation control over the airfoil using passive flow control method by introducing bio-inspired nose near the leading edge of the NACA 2412 airfoil.Design/methodology/approachTwo distinguished methods have been implemented on the leading edge of the airfoil: forward facing step, which induces multiple accelerations at low angle of attack, and cavity/backward facing step, which creates recirculating region (axial vortices) at high angle of attack.FindingsThe porpoise airfoil (optimum bio-inspired nose airfoil) delays the flow separation and improves the aerodynamic efficiency by increasing the lift and decreasing the parasitic drag. The maximum increase in aerodynamic efficiency is 22.4 per cent, with an average increase of 8.6 per cent at all angles of attack.Research limitations/implicationsThe computational analysis has been done for NACA 2412 airfoil at low subsonic speed.Practical implicationsThis design improves the aerodynamic performance and increases structural strength of the aircraft wing compared to other conventional high-lift devices and flow-control devices.Originality/valueDifferent bio-inspired nose designs which are inspired by the cetacean species have been analysed for NACA 2412 airfoil, and optimum nose design (porpoise airfoil) has been found.

Reynolds-averaged Navier–Stokes modelling in transonic S-ducts with passive flow control

S-duct diffusers are used in aircraft with embedded engines to route ambient air to the fan face. Sizing and stealth considerations drive a need for high curvature ducts, but the curvature causes complex secondary flows that lead to total pressure distortion and swirl velocities at the engine face. These must be controlled for stable engine operation. In this paper, tubercles, a novel bio-inspired passive flow control method, are analysed numerically in a duct with transonic flow. The results are compared to experimental data obtained as part of a campaign at the Royal Military College, Canada to investigate the effects of S-duct geometry and novel passive flow control devices on the performance of transonic S-ducts. The performance of Reynolds-averaged Navier–Stokes turbulence models in the S-ducts is assessed – Menter's shear stress transport model predicts excessive losses due to the overactivity of its stress limiter. The realisable k–ɛ model gives a significant improvement in the prediction of static pressure distributions, but losses and distortion characteristics are predicted poorly due to the model's inability to resolve the effects of unsteadiness in separated regions. Large tubercle geometries are found to trigger earlier separation in the centre of the duct by concentrating low momentum fluid in valleys, but they also act as boundary layer fences away from the duct centre. Smaller geometries are found to generate vortices that re-energise the boundary layer, delaying flow separation. Methods are recommended for future computational analyses of S-ducts and new designs of tubercles.

Self-Suction-and-Jet Control in Flow Regime and Unsteady Force for a Single Box Girder

Self-suction-and-jet flow control is a passive flow control method involving self-organized suction at the leading edge and a wake jet at the trailing edge of a structure. In this study, it was used in a 1:125 physical model of the Great Belt East Bridge to decrease dynamic wind loads and modify the flow structure around the main girder. Experiments were performed in a wind tunnel at a wind speed of 12 m/s, yielding a Reynolds number of 2.8 × 104 for the height of the main girder model. The surface pressure (and therefore, the aerodynamic force) fluctuated less in the controlled models than in the uncontrolled model. The velocimetry data indicated that self-suction-and-jet flow control effectively changed the vortex shedding process in the wake of the model. The interactions between the self-organized jet flows and shear flows caused the trailing edge vortex shedding to transform from asymmetric to symmetric, eliminating the vortex shedding completely. Linear stability analysis of the mean wake flow structures indicated that self-suction-and-jet flow control pushed the unstable flow region farther downstream.

Drag reduction of a bus model by passive flow canal

In this study, the drag force of 1/33 scale bus model was improved by passive flow control method. The effect of passive air canal application to drag coefficient was experimentally and numerically investigated on a bus model. Experiments were carried out in the range of 3.8x105 - 7.9x105 Reynolds numbers. The similarity conditions were provided in experimental studies with the exception of the moving road. To provide the geometric similarity condition, the model bus was produced in 3D printer by scanning 3 dimensions. Reynolds number independence was used for dynamic similarity condition. The rate of blockage is 6.81% for kinematic similarity, which is lower than the rate of blockage accepted in the literature. Respectively 4.21%, 7.48% and 12.19% aerodynamic improvement achieved with 1, 3, and 5 passive air canals whose diameter 6 mm. Flow analysis was performed in Fluent® program of the model 3 bus to view the flow structure around the bus. The numerical results support to wind tunnel results. In this study the effect of obtained aerodynamic improvements by applied passive flow control method can decrease fuel consumption about 2-6% at high vehicle speeds.

축류 팬 허브의 홀 형상 최적화를 통한 모터 열 해석 및 유동 특성에 대한 수치해석적 연구

The motor used to drive an axial flow fan generates heat due to internal resistance, which negatively affects performance and shortens the fan’s life span. In this study, cooling of the fan motor using the passive flow control (PFC) method was employed via the hole shape optimization of the axial fan hub. Four design parameters were selected and design points were created using an optimal space-filling design method. The Kriging model was used to construct the response surface. The optimization process using the multi-objective genetic algorithm (MOGA) was performed until the error between the candidate value and the CFD value was less than 0.1%. By comparing the candidate with the initial shape, the optimized shape generated air flow from the back to the front of the axial fan hub. The maximum temperature of the motor was reduced by 17.47 K under the same rotational speed. Conversely, the temperature was further reduced by 6.21 K when compared with the initial shape and operated at the same total pressure.

On the Near-Wake of a Ground-Effect Diffuser with Passive Flow Control

A ground-effect diffuser is an upwardly-inclined section of an automobile’s underbody which increases aerodynamic performance by generating downforce. To understand the diffuser flow physics (force behaviour, surface and offsurface flow features), we established the near-wake (within one vehicle width of the base) velocity profiles and flow structures of an automotive ground-effect diffuser using a bluff body with a 17 degree slanted section forming the plane diffuser ramp surface (baseline geometry), and endplates extending along both sides of the ramp. Wind tunnel experiments were conducted at a Reynolds number of 1.8 million based on the bluff body length, and laser Doppler velocimetry was used to measure two-dimensional velocity components on three planes of the diffuser near-wake. We also measured the velocity field in the near-wake of diffusers with modified geometry (with an inverted wing or a convex bump) as passive flow control devices. The near-wake velocity profiles indicated that the passive flow control methods increased the diffuser flow velocity and that the longitudinal vortices along the diffuser determined the shape of the flow structures in the near-wake of the diffuser bluff body.

Large-eddy simulation of shock-wave/boundary-layer interaction control using a backward facing step

A new passive flow control method, putting a backward facing step ahead of the shock interaction position, is numerically investigated, aiming to control the shock-wave/boundary-layer interaction in a rocket-based combined-cycle scramjet engine. The height of the step is designed to be smaller than the boundary layer thickness. Implicit large-eddy simulations of an oblique shock wave impinging on a supersonic turbulent boundary layer are performed to examine the efficiency of this flow control method. Results show that with the flow control the length of shock-induced separation bubble is increased, but its height is reduced. The upstream extending of the reflected shock is suppressed, which indicates that the flow control method is able to relieve or avoid the adverse effects caused by the inner going of the reflected shock into the air-intake of the engine. The mechanism and the influence of the applied control method on instantaneous, mean and statistical flow-fields are discussed.

Application of endwall contouring in a high-subsonic tandem cascade with endwall boundary layer suction

A proper combination of active and passive flow control methods would be a promising way to further enhance the compressor performance. This numerical investigation presents a new attempt of application of endwall contouring in a high subsonic tandem cascade with the endwall boundary layer suction implemented in the forward blade. The study aims to further improve the corner flow in the rear blade at design and lower incidences. The planar endwall is redesigned using the optimization method. Nonuniform rational B-spline surface is employed to parameterize the endwall surface. A multi-points optimization strategy is selected to minimize the aerodynamic loss generated in the cascade. In the optimization, the suction strategy remains unchanged. To clarify the impacts of endwall contouring on the cascade performance, flow details in both mainflow passage and suction flow path at design and off-design incidences are analyzed.As a result, the total loss decreases by 8.4% at the design incidence via the endwall contouring. Furthermore, a more prominent loss reduction can be achieved at lower incidences. However, the control effects of endwall contouring is weakened towards higher positive incidences. Worse still, even a deterioration in the cascade performance can be detected at I=5° due to the presence of a small-scale corner stall in the forward blade. The results indicate that, as the gap flow is not strong enough to counteract the endwall secondary flow, the endwall contouring could be employed to further enhance the flow control ability of the previous proposed compound flow control method, while its negative impact on the cascade performance at higher positive incidences still remains to be addressed.

Effect of Hole-uniformity of Shrouds on Vortex Shedding Suppression behind a Circular Cylinder

A passive flow control method over a circular cylinder using perforated shrouds was computationally investigated to manipulate the unsteady wake behind the circular cylinder for vortex shedding suppression. The perforated shrouds were of 33% porosity with uniform and non-uniform holes. The computation was performed at low Reynolds number (based on the circular cylinder diameter), Re = 300. The objective of the study is to examine the effect of hole-uniformity of the perforated cylinders on the vortex shedding behind the circular cylinder. It was found that both perforated shrouds (uniform and non-uniform holes) significantly suppressed the vortex shedding behind the cylinder, although with an increased drag due to normalization according to cylinder diameter. The perforated shrouded cylinder with non-uniform holes has longer vortex formation length, suppressing vortex better than that of the uniform holes distribution. The intensity of vortices behind the shrouded cylinder wake with non-uniform holes was also considerably smaller than that of the uniform holes.

Passive flow control of the corner separation in an annular compressor cascade with a micro-blade

Corner separation contributes greatly to the loss and the passage blockage in a compressor stage. In order to mitigate the corner separation and improve the aerodynamic performance of compressors, a novel passive flow control method, an off-surface micro-blade installed upstream of the separation onset location, was proposed. A numerical investigation has was performed in an annular compressor cascade to assess the control effectiveness of the micro-blade. The results show that the location of the micro-blade affects the control effect significantly. The application of the well-designed micro-blade enhances the diffusion capacity considerably under the inflow incidence from −2° to +10°, accompanied by a slight loss reduction at some particular incidences. Detailed analysis of the predicted flow field was carried out to understand the underlying mechanism. It indicates that a “jet” forms upstream of the separation onset location with the application of the micro-blade. The formation of the jet reduces the thickness of boundary layer on the suction surface and builds a “jet barrier” near the endwall to hinder the accumulation of the low momentum fluid. The influence of the incidence was also investigated. It is concluded that the incidence increase has both positive and negative influences on the control effect of the micro-blade. As a result, the performance of the micro-blade is sensitive to the variation of inlet incidence.

Effect of a horizontal hole on flow structures around a wall-mounted low-aspect-ratio cylinder

To control the vortex structures and separation region of a low-aspect-ratio cylinder wake, a passive flow control method that is to drill a horizontal hole from the front side surface to the rear side surface was proposed for a short circular cylinder having an aspect ratio H/D = 1 with height H and diameter D of 70 mm. The PIV measurements were performed at Reynolds number of 8570 in a circulation water tunnel in order to compare the flow characteristics between the controlling and no-controlling wakes. Furthermore, to study the position effect of the horizontal hole, three kinds of the hole positions having different height h from wall were tested. It was found that the jet flow from a horizontal hole leads to suppress the vortex formation and to reduce the rear separation region. The vorticity, Reynolds shear stresses and TKE were evidently suppressed by the jet flow of the horizontal hole in the rear recirculation zone, resulting in the reduction of the drag acted on the cylinder. Meanwhile, the instantaneous large-scale vortical structures of the rear recirculation zone were broken down into several small-scale vortices by the jet. As increasing the height of hole, the downwash flow is evidently suppressed and the upwash flow is increased. Spanwise vortex shedding is altered from symmetric to asymmetric arrangement.

CFD Validation and Flow Control of RAE-M2129 S-Duct Diffuser Using CREATETM-AV Kestrel Simulation Tools

The flow physics modeling and validation of the Royal Aircraft Establishment (RAE) subsonic intake Model 2129 (M2129) are presented. This intake has an 18 inches long S duct with a 5.4 inches offset, an external and an internal lip, forward and rear extended ducts, and a center-positioned bullet before the outlet. Steady-state and unsteady experimental data are available for this duct. The measurements include engine face conditions (pressure recovery, static pressure to free-stream total pressure ratio, and distortion coefficient at the worst 60 ∘ sector or DC60), as well as wall static pressure data along the duct. The intake has been modeled with HPCMP CREATE TM -AV Kestrel simulation tools. The validation results are presented including the effects of turbulence models on predictions. In general, very good agreement (difference errors are less than 6%) was found between predictions and measurements. Secondary flow at the first bend and a region of flow separation are predicted at the starboard wall with an averaged DC60 coefficient of 0.2945 at the engine face. Next, a passive and an active flow control method are computationally investigated. The passive one uses vane-type vortex generators and the active one has synthetic jet actuators. The results show that considered passive and active flow control methods reduce the distortion coefficient at the engine face and the worst 60 ∘ sector to 0.1361 and 0.0881, respectively. The flow control performance trends agree with those obtained in experiments as well. These results give confidence to apply the Kestrel simulation tools for the intake design studies of new and unconventional vehicles and hence to reduce the uncertainties during their flight testing.

Experimental Investigation of Flow Structures around a Torpedo-like Geometry Placed in a Boundary Layer Flow

Defense applications for both under oceans and seas, particularly underwater vehicles have been considered in this research. With this aim, flow characteristics around a torpedo-like geometry under the effect of the boundary layer flow over a smooth flat plate have been experimentally examined by using PIV technique. All of the experiments have been done for Re=20000 and Re=40000 based on the length (L) of the geometry as a characteristic length. As a result, time-averaged streamwise velocity components , velocity vectors , streamline topologies and Reynolds stress correlations in the wake region of the torpedo-like geometry have been acquired in the range of 0 ≤ G/D ≤ 1.5. Here, G is the space between the bottom point of the geometry and flat plate surface; D stands for the diameter of the geometry. It is found that at the smallest value of G/D=0.25, jet-like flow occurs between the plate and the model which causes a powerful scouring. As the gap ratio is increased to G/D=0.5 and G/D=1.0, the jet-like flow diminishes slightly and then the flow structure in the wake region becomes similar to the uniform incoming flow condition for G/D=1.50. Due to the effect of the jet-like flow and boundary layer flow, time-averaged flow patterns present asymmetrical distributions which are clearly shown a bigger size focus close to the plate in streamline topology. Reynolds stress patterns form more powerful viscous forces in the boundary layer flow due to the occurrence of eddy vortices and viscosity effect. It is observed from the aforementioned flow patterns that interaction between the flow structure, the model and boundary layer flow yields very complex structure. In order to decrease the energetic flow in this condition, passive or active flow control method can be integrated on the torpedo-like geometry.