Passive Flow Control Strategies Research Articles

Effects of Perforated Plates on Shock Structure Alteration for NACA0012 Cascade Configurations

To alleviate the shock boundary layer interaction adverse effects, various active or passive flow control strategies have been investigated in the literature. This research sheds light on the behavior of perforated plates as passive flow control techniques applied to NACA0012 airfoils in cascade configurations. Two identical perforated plates with shallow cavities underneath are accommodated on the upper and lower surfaces of each airfoil in the cascade arrangement. Six different cascade arrangements, including a baseline configuration with no control applied, are additively manufactured, with different perforated plate orifice sizes in the range of 0.5–1.2 mm. A high-speed wind tunnel with Schlieren optical diagnosis and wall static pressure taps is used to investigate the changes in the shock waves pattern triggered by the perforated plates. Steady 3D density-based numerical simulations in Ansys FLUENT are conducted for further analysis and validation. In the cascade configuration, the perforated plates alter the shock structure, and the strong normal shock wave is replaced by a weaker X-type shock structure. Eventually, a 1% penalty in overall total pressure loss is induced by the perforated plates because of the negative loss balance between the reduced shock losses and the enhanced viscous losses. Further studies on perforated plate geometrical features are needed to improve this outcome in a cascade arrangement.

Numerical investigation of drag reduction effects on a track bicycle fork using wings with a wavy leading edge

AbstractReynolds‐averaged Navier–Stokes (RANS) and large‐eddy simulations (LES) of the flow around wings with a wavy leading edge (WLE) are conducted in order to assess the capabilities of a passive flow control strategy for drag reduction. The intended application is indoor track cycling with controlled flow conditions. A section of a single fork rod is investigated in order to make the numerical simulations feasible. The present study reveals that net drag reduction is possible by a nonsinusoidal modification of the leading edge of the wing. However, the drag reduction effect remains limited to a few percent. While RANS and LES yield the same drag coefficient for a reference case, RANS underestimates the drag reduction effect for a longer wing and the WLE cases, but exhibits otherwise a qualitatively similar trend as the LES. With the aid of RANS, an optimal geometry is obtained defined by the wavelength‐to‐chord length ratio of and the amplitude‐to‐chord length ratio of . Corresponding LES results give an indication of the origin of drag reduction by a hampered vortex shedding. The generation of smaller and more streamwise oriented vortical flow structures at the trailing edge and behind the WLE wing is correlated with significantly reduced lift fluctuations and drag reduction.

Passive flow control of a Francis turbine operating in sand-laden rivers for mitigating sediment erosion

In a typical Francis turbine operating in sand-laden rivers, owing to its complicated geometry and variable operating conditions, vortex structures appear and cause severe erosion damage to turbine components. Here, we present a bioinspired method to mitigate severe sediment erosion on Francis turbines. The proposed method includes a passive flow control strategy using biomimetic convex domes for the inter-blade vortex, a major contributor to severe sediment erosion on the turbine runner. The effects of biomimetic convex domes on sediment erosion are investigated through numerical simulations and experiments. The results indicate that biomimetic convex domes significantly reduce the impact velocity and accretion rate of the particles, eventually reducing sediment erosion by at least 50 %. The mechanism underlying the effect of convex domes on sediment erosion is their inhibition of the development of the inter-blade vortex. The convex domes induce small-scale vortices from the blade boundary layer. When located in the nascent region of the inter-blade vortex, the small-scale vortex effectively inhibits its formation. Moreover, convex domes placed in severe erosion areas can accelerate the dissipation process of the inter-blade vortex.

Exploring a bio-inspired passive flow control strategy for wind turbines: Experimental characterization of the aerodynamics of a wind turbine blade section possibly equipped with leading-edge tubercles

Focusing on a bio-inspired passive flow control strategy with a potential application to wind turbines, this study consists of an experimental characterization of the aerodynamics of a representative wind turbine blade section to be possibly equipped with leading-edge tubercles. A total of five airfoils are tested in a low-speed wind tunnel, their aerodynamic characteristics being explored over a wide range of flow conditions by the means of load measurements and wall-flow visualizations. Results reveal that the leading-edge tubercles alter differently the airfoil's aerodynamics, depending on their specific design (amplitude and wavelength) as well as on the flow conditions (speed and incidence). The magnitude of these effects is seen to vary with the tubercles' amplitude and/or wavelength, ultimately leading to an optimal trade-off between pre- and post-stall regimes being offered by an intermediate amplitude-to-wavelength ratio. Second, the aerodynamic benefits/penalties induced by the leading-edge tubercles are explored through wall-flow visualizations; The tubercled airfoils exhibit pairs of counter-rotating vortices, whose characteristics are driven by the tubercles' design, the airfoil shape, and/or the flow conditions. Overall, these vortex pairs are more prominent for large-amplitude and/or small-wavelength tubercles, each design altering the near-wall flow and, thus, the aerodynamic performances in a specific fashion.

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.

Multi-input multi-output phononic subsurfaces for passive boundary layer transition delay

This paper documents progress towards developing a passive boundary layer transition delay strategy using a phononic subsurface (PSub). High-order implicit large-eddy simulation (ILES) is used to explore control requirements for boundary layer transition delay in the context of guiding passive flow control strategies. Positive phasing of surface displacement relative to forcing from a Tollmien-Schlichting (T-S) wave is shown to be effective at attenuating instability for the present flow conditions. Although difficult to achieve with a single input system, positive phasing can be realized with a multi-input, multi-output (MIMO) system. To this end, a MIMO PSub was developed and a reduced-order model was coupled with the flow solver. Positive phasing, and therefore a small degree of passive transition delay, was demonstrated on a flat plate boundary layer. Further optimization of subsurface properties and placement of multiple devices are likely to improve controller efficacy.

Bio-inspired variable-stiffness flaps for hybrid flow control, tuned via reinforcement learning

A bio-inspired, passively deployable flap attached to an airfoil by a torsional spring of fixed stiffness can provide significant lift improvements at post-stall angles of attack. In this work, we describe a hybrid active–passive variant to this purely passive flow control paradigm, where the stiffness of the hinge is actively varied in time to yield passive fluid–structure interaction of greater aerodynamic benefit than the fixed-stiffness case. This hybrid active–passive flow control strategy could potentially be implemented using variable-stiffness actuators with less expense compared with actively prescribing the flap motion. The hinge stiffness is varied via a reinforcement-learning-trained closed-loop feedback controller. A physics-based penalty and a long–short-term training strategy for enabling fast training of the hybrid controller are introduced. The hybrid controller is shown to provide lift improvements as high as 136 % and 85 % with respect to the flapless airfoil and the best fixed-stiffness case, respectively. These lift improvements are achieved due to large-amplitude flap oscillations as the stiffness varies over four orders of magnitude, whose interplay with the flow is analysed in detail.

Structured porous surface for drag reduction and wake attenuation of cylinder flow

In the current study, a new passive flow control strategy characterized by 3D printing structured porous surface was introduced and experimentally investigated to manipulate the dynamic wake evolution and flow characteristics around a circular cylinder at a Reynolds number of 2.02×104. With the 3D printing structured porous surface, parts of the incoming flow could go through the cylinder and communicate with the wake flow spontaneously. As a result, the vortex shedding was suppressed and the drag was reduced. From POD analysis results, we observed shifted modes and decreased vorticity distribution, indicating that the periodic vortex shedding process was weakened. This was also verified by the analysis of dynamic wake evolution and by the power spectrum analysis. In addition, experimental results showed that the time-averaged turbulent kinetic energy (TKE) and the Reynolds shear stress (RSS) distributions were restrained in the wake of the porous cylinder. Moreover, the mean estimated drag coefficients witnessed a significant decrease up to 49.47%.

Edge Race-Tracking during Film-Sealed Compression Resin Transfer Molding

Edge race-tracking is a frequently reported issue during resin transfer molding. It is caused by highly permeable channels and areas between the preform edge and cavity, which can significantly change the preform impregnation pattern. To date, information is scarce on the effect of edge race-tracking in compression resin transfer molding (CRTM). To close this gap, laboratory equipment was developed to study the CRTM preform impregnation via flow visualization experiments. The preform was thereby encapsulated in thin thermoplastic films sealing its impregnation. Film-sealed compression resin transfer molding (FS-CRTM) experiments of preforms with a small geometrical aspect ratio showed fast filling of the injection gap and a subsequent through-thickness preform impregnation. Creating an edge race-tracking channel, an additional lateral in-plane flow from the channel towards the preform center was observed, initiating soon after the injection started and caused by the spatial connection between the injection gap and the race-tracking channel. To diminish edge race-tracking, a passive flow control strategy was implemented via a split design of the upper tool to spatially isolate the injection gap from the channel and to pre-compact the preform edge. A delayed and reduced lateral race-tracking flow was observed, showing that the passive flow control strategy increases the process robustness of FS-CRTM regarding edge race-tracking effects.

Experimental study and passive control of the bistable dynamics of the three-dimensional air-wake flow of a finite-width double backward-facing step

Inspired by the study of Rao et al. (2019), the aerodynamics of a simplified frigate is investigated experimentally. The natural wake flow dynamics is shown to be bi-stable at zero yaw angles, at both the rear of the ship superstructure and the stern. The bi-stable dynamics is strongly weakened at the stern when the ship is lifted off the ground in the free-stream. Perimetric cavities of sufficient depth at the rear of the superstructure are able to suppress the bi-stable dynamics and to symmetrize the air-wake flow at both the superstructure and the stern. The base pressure increases with the cavity depth while the drag force slightly decreases. Our study experimentally confirms that perimetric rear cavities offer efficient devices for passive flow control strategies of the aerodynamic air-wake flow of bodies of complex geometries subject to bi-stable dynamics.

Horizontal axis wind turbines passive flow control methods: a review

In improving wind turbine torque and power output, the aerodynamic characteristics of wind turbines blades play an important role. Unleashing easy but efficient flow enhancement techniques over aerofoil sections used in horizontal axis wind turbines (HAWT) has become essential in the increasing demand for this source of renewable energy. These techniques or methods are primarily categorized into two kinds: active and passive. Active methods of flow control need energy expensewhereas passive flow control methods are needless of auxiliarypower. This paper investigates various passive flow control strategies that have great potential to boost the aerodynamics of blades of HAWT. The mechanisms and working principles, along with the findings from various experimental studies and CFD results for passive flow control systems are included in this article. Also, some of these studies are supported by demonstrating the lift and coefficient of drag (CD) variation with the angle of attack (AoA). The review suggests simple, cost-effective ways of improving lift and controlling the aerofoil stalling behaviors to obtain higher power efficiency for HAWT.

Application of Leading-Edge Tubercles to Enhance Propeller Performance

Aircraft propellers are known to experience flow separation at various flight conditions leading to efficiency variations from approximately 40–85%. By controlling flow separation, higher propeller efficiency and improved performance might be possible. In this paper a bioinspired passive flow control strategy that improves propeller performance is used by introducing protuberances, or tubercles, in the leading-edge region of a propeller blade. Experiments also revealed apparent improvements to propeller acoustic emissions. Propeller performance and noise signatures, evaluated against a reference propeller, are presented from experiments involving different propeller configurations modified with various combinations of tubercle geometry. Specific tubercle configurations exhibit improvements in efficiency up to 6% as well as redistributed directional and high-frequency broadband noise characteristics.

Optimal performance of adaptive flap on flow separation control

The stall caused by flow separation of airfoil seriously compromises the efficiency and safety of wind turbines. Inspired by bird feathers, a passive flow control strategy was investigated by configuring adaptive flaps on the suction surface of the airfoil to delay flow separation. The Computational Fluid Dynamics (CFD) software ANSYS Fluent 17.0 using SST k–ω turbulence model was performed to study the performance of the adaptive flap with different length and location. The performance was evaluated by the comparison of total lift coefficient between the flap airfoil and clean airfoil. In addition, the performance of the double flaps configuration was investigated by Fluid-Solid Interaction (FSI) method. The results show that when the separation point is at the upstream of flap hinge point, the total lift coefficient increases with the flap rising, and when the flap reaches the optimal flap angle, the lift coefficient begins to decrease. When the flap operates in optimal angle, the closer the flap is to the trailing edge of the airfoil, the more the lift enhancement for moderate flow separation, and the closer the flap is to the leading edge of the airfoil, the more the lift enhancement for serious flow separation. For the flaps with the same location, the longer flap can block more backflow resulting in a better performance, but when the flap is long enough to block the fluid, the shorter flap has better performance. Considering the variations of the flap aerodynamic moment with the flap angle, an external hinge moment was recommended to improve the swinging flap performance.

Improvement of the aerodynamic performance of vertical axis wind turbines with leading-edge serrations and helical blades using CFD and Taguchi method

The flow performance of vertical-axis wind turbines (VAWTs) is significantly deteriorated by dynamic stall at low tip-speed-ratios (TSR). Dynamic stall occurs periodically during the rotation of turbine blades and generates flow separations formed at leading-edges, which results in tremendous fluctuations in torque and essential reduction in power outputs. The target of this study is to apply a combination of CFD and Taguchi method to optimize the aerodynamic performance of a typical three-bladed VAWT at a low TSR of 2. Leading-edge wavy serrations and helical blade structures are implemented as the design parameters and the performance of these designs are compared with a corresponding baseline straight blade design of a traditional H-type VAWT. A comprehensive simulation study is carried out to obtain detailed flow characteristics. The Taguchi method is applied to find out the optimal combination of the selected factors such as amplitude and wave length of the serrations and twist angle of helical blade. A modified additive model is used to include the interaction effects among the selected factors. The optimal combination is found to appear in the case with an amplitude of 2.5%cb (cb is the blade chord length) and a wavelength of cb/6 for leading-edge serration as well as a blade twist angle of 60°. The simulation results show that the flow separations and torque fluctuations are significantly suppressed due to the passive flow control strategies implemented in the optimized VAWT model. The power output of the optimized wind turbine is found to increase by 18.3% in comparison to the baseline model. In addition, the force fluctuations are observed to be significantly reduced by employing the helical design, which is beneficial for improving the lifespan of turbines. The results derived from this study indicate a better VAWT design for the rural and built environments with relatively insufficient wind energy.

Turbulence Enhancement by Fractal Square Grids: Effects of Multiple Fractal Scales

Multi-scale fractal grids can be considered to mimic the fractal characteristic of objects of complex appearance in nature, such as branching pulmonary network and corals in biology, river network, trees, and cumulus clouds in geophysics, and the large-scale structure of the universe in astronomy. Understanding the role that multiple length scales have in momentum and energy transport is essential for effective utilization of fractal grids in a wide variety of engineering applications. Fractal square grids, consisted of the basic square pattern, have been used for enhancing fluid mixing as a passive flow control strategy. While previous studies have solidified the dominant effect of the largest scale, effects of the smaller scales and the interaction of the range of scales on the generated turbulent flow remain unclear. This research is to determine the relationship between the fractal scales (varying with the fractal iteration N), the turbulence statistics of the flow and the pressure drop across the fractal square grids using well-controlled water-tunnel experiments. Instantaneous and ensemble-averaged velocity fields are obtained by a planar Particle Image Velocimetry (PIV) method for a set of fractal square grids (N = 1, 2 and 4) at Reynolds number of 3400. The static pressure drop across the fractal square grid is measured by a differential pressure transducer. Flow fields indicate that the multiple jets, wakes and the shear layers produced by the multiple scales of bars are the fundamental flow physics that promote momentum transport in the fractal grid generated turbulence. The wake interaction length scale model is modified to incorporate the effects of smaller scales and thereof interaction, by the effective mesh size M e f f and an empirical coefficient β . Effectiveness of a fractal square grid is assessed using the gained turbulence intensity and Reynolds shear stress level at the cost of pressure loss, which varies with the distance downstream. In light of the promising capability of the fractal grids to enhance momentum and energy transport, this work can potentially benefit a wide variety of applications where energy efficient mixing or convective heat transfer is a key process.

Flow around a semicircular cylinder with passive flow control mechanisms

Wind tunnel experiments were performed to study the effect of passive flow control strategies on the wake and drag of a semicircular cylinder of infinite aspect ratio. High-resolution planar particle image velocimetry was used to obtain flow statistics around the semicircular cylinder at Reynolds number \(Re\approx 3.2\times 10^4\) based on the cylinder diameter. The control mechanisms under consideration include rigid flaps of various lengths placed at the edges of the structure and a small slot along the symmetry plane of the cylinder. Mean velocity fields reveal the distinctive effects of each passive mechanism on the flow, such as velocity recovery, size of the recirculation bubble and location of the reattachment point. The distributions of turbulence kinetic energy and kinematic shear stress show the modulation of each passive control mechanism on the wake, including the onset and location of the maximum turbulence levels. Instantaneous and mean fields of swirling strength further highlight the role of the passive mechanisms in the vortex dynamics. Drag coefficient for the various cases was estimated indirectly from the flow measurements using a momentum balance. This approach shows that long flaps and slot were able to reduce drag with respect to the base case. The rigid flaps with length coincident with the diameter of the cylinder offered the best performance with drag reduction of \(\sim\)25\(\%\).

Data-driven approach to design of passive flow control strategies

An approach to designing passive devices for control of unsteady flows is presented. The method requires only snapshots of the flow to be controlled as inputs. A temporal correlation based on proper orthogonal decomposition of both fluctuating velocity and nonlinear forcing serves to identify the spatial locations in which the forcing drives the different unsteady flow features. The installation of a passive device in these spatial locations inhibits the fluctuating motion. The potential of the methodology is demonstrated via the suppression of vortex shedding in flow past a square cylinder, paving the way to the control of more complex flows using passive devices. Connections in agreement with previous studies targeting the same flow using different passive flow control strategies are provided.

Three-dimensional instability analysis of boundary layers perturbed by streamwise vortices

A parametric study is presented for the incompressible, zero-pressure-gradient flat-plate boundary layer perturbed by streamwise vortices. The vortices are placed near the leading edge and model the vortices induced by miniature vortex generators (MVGs), which consist in a spanwise-periodic array of small winglet pairs. The introduction of MVGs has been experimentally proved to be a successful passive flow control strategy for delaying laminar-turbulent transition caused by Tollmien–Schlichting (TS) waves. The counter-rotating vortex pairs induce non-modal, transient growth that leads to a streaky boundary layer flow. The initial intensity of the vortices and their wall-normal distances to the plate wall are varied with the aim of finding the most effective location for streak generation and the effect on the instability characteristics of the perturbed flow. The study includes the solution of the three-dimensional, stationary, streaky boundary layer flows by using the boundary region equations, and the three-dimensional instability analysis of the resulting basic flows by using the plane-marching parabolized stability equations. Depending on the initial circulation and positioning of the vortices, planar TS waves are stabilized by the presence of the streaks, resulting in a reduction in the region of instability and shrink of the neutral stability curve. For a fixed maximum streak amplitude below the threshold for secondary instability (SI), the most effective wall-normal distance for the formation of the streaks is found to also offer the most stabilization of TS waves. By setting a maximum streak amplitude above the threshold for SI, sinuous shear layer modes become unstable, as well as another instability mode that is amplified in a narrow region near the vortex inlet position.

Transition to Turbulence Delay Using a Passive Flow Control Strategy

Since the mid–50s, until the first proof-of-concept experiment of transition delay using circular roughness elements in 2006, there was a strong consensus within the research community that roughness elements in general promoted the transition to turbulence process. From a series of wind tunnel experiments miniature vortex generators have emerged as being a coveted passive device for transition delay and hence skin-friction drag reduction. These devices are miniature with respect to classical vortex generators typically used for separation control and fall under the appealing category passive flow control device, since it uses the existing energy in the flow and hence no energy has to be added to the control system in order to accomplish the control. The underlying physical mechanism is attributed to an additional term in the perturbation energy equation, when the boundary layer is modulated in the direction orthogonal to the base flow by the control devices, which counteracts the wall-normal production term and, hence, stabilizes the flow. The present paper briefly reviews the current state-of-art of passive flow control using physical devices for transition delay with the implication of reducing skin-friction drag. This passive control strategy has potential to lead to an unforeseen positive impact on the broad spectrum of industrial applications where reducing drag is a daily challenge.

Passive boundary layer control of oblique disturbances by finite-amplitude streaks

AbstractRecent experimental results on the attenuation of two-dimensional Tollmien–Schlichting wave (TSW) disturbances by means of passive miniature vortex generators (MVGs) have shed new light on the possibility of delaying transition to turbulence and hence accomplishing skin-friction drag reduction. A recurrent concern has been whether this passive flow control strategy would work for other types of disturbances than plane TSWs in an experimental configuration where the incoming disturbance is allowed to fully interact with the MVG array. In the present experimental investigation we show that not only TSW disturbances are attenuated, but also three-dimensional single oblique wave (SOW) and pair of oblique waves (POW) disturbances are quenched in the presence of MVGs, and that transition delay can be obtained successfully. For the SOW disturbance an unusual interaction between the wave and the MVGs occurs, leading to a split of the wave with one part travelling with a ‘mirrored’ phase angle with respect to the spanwise direction on one side of the MVG centreline. This gives rise to $\Lambda $-vortices on the centreline, which force a low-speed streak on the centreline, strong enough to overcome the high-speed streak generated by the MVGs themselves. Both these streaky boundary layers seem to act stabilizing on unsteady perturbations. The challenge in a passive control method making use of a non-modal type of disturbances to attenuate modal disturbances lies in generating stable streamwise streaks which do not themselves break down to turbulence.