Delay Flow Separation Research Articles

Transitional Flow Dynamics Behind a Micro-Ramp

Micro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.

New combinational active control strategy for improving aerodynamic characteristics of airfoil and rotor

In order to improve the aerodynamic characteristics of rotor, a new active flow control strategy by combining a synthetic jet actuator and a variable droop leading-edge or a trailing-edge flap has been proposed. Their control effects are numerically investigated by computational fluid dynamics (CFD) method. The validated results indicate that variable droop leading-edge and synthetic jet can suppress the formation of dynamic stall vortex and delay flow separation over rotor airfoil. Compared with the baseline state, Cdmax and Cmmax are significantly reduced. Furthermore, parametric analyses on dynamic stall control of airfoil by the combinational method are conducted, and it indicates that the aerodynamic characteristics of the oscillating rotor airfoil can be significantly improved when the non-dimensional frequency ( k*) of variable droop leading-edge is about 1.0. At last, simulations are conducted for the flow control of rotor by the combinational method. The numerical results indicate that large droop angle of variable droop leading-edge can better reduce the torque coefficient of rotor and the trailing-edge flap has the capability of increasing the thrust of rotor. Also, the synthetic jet could further improve the aerodynamic characteristics of rotor.

Experimental and Numerical Analysis of the Effect of Vortex Generator Installation Angle on Flow Separation Control

In order to explore the effect of the installation angle of vortex generator (VG) on boundary-layer flow control, the vortex characteristics of plate VG and their effect on the aerodynamic characteristics of an airfoil was studied numerically and using wind tunnel experiments. The effects of five VG installation angles (β) of 10°, 15°, 20°, 25°, and 30° on the characteristics of vortices were studied. The results show that the strength of vortices on the leeward side of VG increases with an increased installation angle until, eventually, the vortex core breaks down. During the downstream development of the VG leading-edge separation vortices, these vortices deviate in the radial direction. The larger the installation angle, the larger this deviation distance in the radial direction becomes. The effects of installation angle on the aerodynamic performance of airfoils were studied in a wind tunnel using the same five VG installation angles. The results show that VG can delay flow separation on the airfoil suction surface, thereby increasing lift and reducing drag. The stall angle of the airfoil with VG was increased by 10°. When the installation angle of the VG was 20°, the maximum lift coefficient of airfoil increased by 48.77%. For an airfoil angle of attack (AoA) of 18°, the drag of the airfoil decreased by 88%, and the lift-drag ratio increased by 1146.04%. Considering the best overall distribution of lift-drag ratio, the positive effect of the VG was found to be when β = 20° and the worst VG effectiveness was observed at β = 30°.

Aerodynamic analysis of basic and extended lead-trail formation using numerical technique

Abstract This paper uses a numerical post-stall predictive tool based on ‘decambering’ approach to study the aerodynamic characteristics of a lead-trail formation in pre and post-stall flow conditions. A basic lead-trail formation consisting of 2 wings and an extended formation consisting of 5 wings are studied with a view to the possibility of fuel savings, increase in range of operation, delayed flow separation and efficient positioning of the wings with respect to each other. Whether increasing the number of wings in a configuration is more useful is also looked into. The optimum operational angles of attack for maximum advantage in terms of fuel efficiency of all wings is studied including post-stall angles of attack. Numerical results for C L , C D i , section C l distribution and their dependence on vertical offsets and angle of attack are reported.

Research on the Effects of Nonsmooth Surfaces on Backward-facing Step Flow

To achieve the purpose of backward-facing step flow control, a passive control approach, which consists in introducing nonsmooth structures in solid walls, is applied to the upstream of the backward-facing step. Based on STAR CCM+ software, the standard k-ε turbulence model was established to simulate flow characteristics of the right angle step, fillet step and nonsmooth fillet step. The introduction of the nonsmooth surface leads to a significant reduction in recirculation region length (17.5%) and a decrease in downstream wall pressure coefficient. According to the analysis of separation point position and turbulent kinetic energy, the delay of separation point and the enhancement of momentum exchange are the main reasons for the success of flow control. Compared with the right angle step, the delay of flow separation of the fillet step leads to a reduction in step expansion ratio (ER), and the existence of the nonsmooth structure enhances the turbulent kinetic energy at the fluid separation points and the momentum mixing at the downstream of the step, thus reducing the reattachment length and the downstream wall pressure coefficient.

The function of the alula on engineered wings: a detailed experimental investigation of a bioinspired leading-edge device

Birds fly in dynamic flight conditions while maintaining aerodynamic efficiency. This agility is in part due to specialized feather systems that function as flow control devices during adverse conditions such as high-angle of attack maneuvers. In this paper, we present an engineered three-dimensional leading-edge device inspired by one of these specialized groups of feathers known as the alula. Wind tunnel results show that, similar to the biological alula, the leading-edge alula-inspired device (LEAD) increases the wing’s ability to maintain higher pressure gradients by modifying the near-wall flow. It also generates tip vortices that modify the turbulence on the upper-surface of the wing, delaying flow separation. The effect of the LEAD location and morphology on lift production and wake velocity profile are investigated using force and hot-wire anemometer measurements, respectively. Results show lift improvements up to 32% and 37% under post and deep stall conditions, respectively. Despite the observed drag penalties of up to 39%, the aerodynamic efficiency, defined as the lift-to-drag ratio, is maintained and sometimes improved with the addition of the LEAD to a wing. This is to be expected as the LEAD is a post-stall device, suitable for high angles of attack maneuvers, where maintaining lift production is more critical than drag reduction. The LEAD also accelerates the flow over the wing and reduces the wake velocity deficit, indicating attenuated flow separation. This work presents a detailed experimental investigation of an engineered three dimensional leading-edge device that combines the functionality of traditional two dimensional slats and vortex generators. Such a device can be used to not only extend the flight envelope of unmanned aerial vehicles (UAVs), but to also study the role and function of the biological alula.

Entropy analysis of pitching airfoil for offshore wind turbines in the dynamic stall condition

Dynamic stall is a phenomenon that frequently occurs inside the offshore wind turbine blades. The unsteadiness within the flow leads to an alternative increase in the angle of attack to a higher value than the static stall condition and as a consequence, a dynamic stall occurs. In this paper, the airfoil is simulated in the oscillating case with different oscillatory frequencies in order to observe the dynamic stall. Then entropy generation rate is considered as a criterion in the analysis of this mechanism and also a tool for accurate designing. The simulations have been done in a 2D unsteady state flow field. The hysteresis charts show that S822 and SD7062 airfoils, which have a delay in flow separation, have almost lower drag coefficient compared with S809 airfoil. In this study, using entropy generation rate contours, the relation between different phenomenon and irreversibility or losses have been determined. The drag coefficient, separation, dynamic stall, and effective work have been calculated with considering entropy generation criteria for the airfoils in the practical condition. By focusing on the entropy generation in a transient stall condition, could be a strong tool to design accurate and offshore wind turbines with lower losses in near future.

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.

Leading-edge tubercles delay flow separation for a tapered swept-back wing at very low Reynolds number

Flow separation characteristics for two tapered swept-back wings, one with straight leading-edge (LE) and the other with tubercled LE, were investigated in a water tunnel using time-resolved particle image velocimetry (TR-PIV) technique. The two wings were based on the SD7032 aerofoil profile, with Reynolds number Re = 1.4 × 104, close to the working condition for common underwater gliders. The LE tubercles were designed such that the amplitude decreased linearly from the wing root to wing tip, while retaining constant wavelength. Results indicate that the baseline wing shows significantly separated flow in the outboard region at pitch angle of 10° and 20°, and the flow remains attached in the inboard region due to relatively larger local Reynolds number. Implementation of LE tubercles can mitigate flow separation downstream of both troughs and peaks. At higher pitch angles, the separated flows cover most of the baseline wing surface, whereas flow remains attached downstream most of tubercle peaks. Streamwise aligned counter-rotating vortex pairs (CVPs) formed over the tubercles are significantly tilted and asymmetrical due to the sweep and amplitude difference between the two sides of tubercle. Consequently, weaker vortices in CVPs close to the wing root are rapidly dissipated, allowing the CVPs to evolve into a series of co-rotating vortices (CVs), which exerted significant impact on flow separation characteristics downstream of the tubercles.

Performance Characteristics of Airfoils with Leading-Edge Tubercles and an Internal Slot

The combined effect of an internal slot in an airfoil and transverse leading-edge tubercles on its performance is studied. The internal slot allows flow ventilation from the pressure face to the suction face of the airfoil, effectively delaying flow separation and stall. Previous studies have demonstrated good stall characteristics of airfoils with leading-edge tubercles. It is shown here that the slot results in further improvement of the post-stall performance. The performance characteristics are investigated both numerically and experimentally, with angles of attack in the range of 0°–30° at a chord-Reynolds number of lift and drag coefficients of slotted airfoils with leading-edge tubercles predicted from the computational fluid dynamics study correlate well with the corresponding coefficients determined via experiments. The experimental results show that the lift and drag coefficients of airfoils with leading-edge tubercles are significantly improved for angles of attack exceeding 10° with the inclusion of the internal slot. The slotted airfoil with small leading-edge tubercles outperformed others for angles of attack in the range of 10°–20° with the most improved maximum lift coefficient and maximum lift-to-drag ratio, whereas the slotted airfoil with larger tubercles achieved the best performance for angles of attack in the range of 20°–30°, including more favorable stall characteristics.

Improved performance of a slotted blade using a novel slot design

Efficient capture of wind energy requires optimal dynamic performance of turbine blades, desirably including delaying stall to higher angles of attack (AoAs). Stall is associated with flow separation close to the leading edge on the suction side of a blade so that delaying flow separation mitigates this condition. Merits of passive slots in wings for increasing their lift are well known. Typically, however, the increase in lift is accompanied by a corresponding increase in drag so that the overall aerodynamic efficiency is at most marginally improved. Here, we discuss application of a custom optimized-design internal slot on a NACA 634-021 airfoil blade to allow ventilation of flow through the slot from the pressure side to the suction side of the blade, in support of delaying flow separation, and stall. Delaying flow separation, results in increase in lift and decrease in drag at significantly high angles of attack. Slot width and inclined angle were varied to determine the optimum configuration. Dynamic performance of the blade was investigated both numerically and experimentally, with angles of attack in the range 0o≤AoA≤30o at a Reynolds number of 105. Lift and drag coefficients of the slotted airfoils predicted from the computational fluid dynamics (CFD) study correlate well with the corresponding coefficients determined via experiments. Experimental results show that a 58% increase in maximum lift coefficient and an 14% increase in maximum lift-to-drag ratio could be achieved with the optimum design case. Flow details obtained from the CFD study provide better insight into the underlying control mechanism of the internal slot. The findings could have significant implications for aeronautics and for improving capture of renewable energy from wind.

Flow over hydrofoils with varying hydrophobicity

The effect of the shape of a hydrofoil on its force characteristics is well documented; however, the role of the surface on the ensuing flow is not yet understood. Here, we report the effect of hydrophobicity on the flow dynamics of NACA0015 hydrofoil based on more than 100 particle image velocimetry based experiments. Acrylic, Cellophane tape, Teflon coated, and superhydrophobic painted surfaces are studied for Reynolds number range of 6500–30,800 at five angles of attack (0–20°) in this work. The static contact angle varies from 70° to 152° for these surfaces. The superhydrophobicity helps to reduce the range of Reynolds number where the flow separation occurs. We observe up to 40% reduction in drag at 15° angle of attack for Re = 30,800, and superior hydrodynamic performance for the superhydrophobic painted surface as compared to the bare Acrylic surface. The superhydrophobic painted surface is also found to delay flow separation and stall angle (by up to 5° based on the gliding ratio), which is attributed to a reduction in the separation bubble size (by up to 56%). The Teflon coated surface behaves similar to the superhydrophobic painted surface until an angle of attack of 5°. The vortex strength is reduced for the superhydrophobic painted surface at 15° angle of attack compared to the Acrylic surface. The results of this study show that the hydrodynamic efficiency can be increased and also sustained over a relatively large range of angle of attack by employing superhydrophobic surfaces.

Microfiber Coating for Drag Reduction by Flocking Technology

The biomimicry of using a hair-like structure is introduced as a drag reduction coating. The hair-like structure consists of an array of microfiber that is introduced as a passive drag reduction device. An effective flow control for a transition delay or a flow attachment is expected via an interaction or counteraction of flexible fibers, compared to the existing passive methods that use a solid or rigid surface device. The effect of the microfiber coating on drag reduction over a bluff-body was experimentally investigated using a circular cylinder in a wind tunnel at Reynolds number of 6.1 × 104. A drag reduction of 32% was obtained when the microfiber coating with a length of 0.012D was located at 40° from the stagnation point. Smoke flow visualization showed that flow separation delay was induced by the microfiber coating when the drag reduction occurred.

Non-equilibrium effects on flow past a circular cylinder in the slip and early transition regime

This paper presents a comprehensive investigation into flow past a circular cylinder where compressibility and rarefaction effects play an important role. The study focuses on steady subsonic flow in the Reynolds-number range 0.1–45. Rarefaction, or non-equilibrium, effects in the slip and early transition regime are accounted for using the method of moments and results are compared to data from kinetic theory obtained from the direct simulation Monte Carlo method. Solutions obtained for incompressible continuum flow serve as a baseline to examine non-equilibrium effects on the flow features. For creeping flow, where the Reynolds number is less than unity, the drag coefficient predicted by the moment equations is in good agreement with kinetic theory for Knudsen numbers less than one. When flow separation occurs, we show that the effects of rarefaction and velocity slip delay flow separation and will reduce the size of the vortices downstream of the cylinder. When the Knudsen number is above 0.028, the vortex length shows an initial increase with the Reynolds number, as observed in the standard no-slip continuum regime. However, once the Reynolds number exceeds a critical value, the size of the downstream vortices decreases with increasing Reynolds number until they disappear. An existence criterion, which identifies the limits for the presence of the vortices, is proposed. The flow physics around the cylinder is further analysed in terms of velocity slip, pressure and skin friction coefficients, which highlights that viscous, rarefaction and compressibility effects all play a complex role. We also show that the local Knudsen number, which indicates the state of the gas around the cylinder, can differ significantly from its free-stream value and it is essential that computational studies of subsonic gas flows in the slip and early transition regime are able to account for these strong non-equilibrium effects.

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.

Computational Modelling of Three Different Sub-Boundary Layer Vortex Generators on a Flat Plate

Flow separation is the source of several problems in a wind turbine including load fluctuations, lift losses, and vibrations. Vortex generators (VGs) are passive flow control devices used to delay flow separation, but their implementation may produce overload drag at the blade section where they are placed. In the current work, a computational model of different geometries of vortex generators placed on a flat plate has been carried out throughout fully meshed computational simulations using Reynolds Averaged Navier-Stokes (RANS) equations performed at a Reynolds number of R e θ = 2600 based on local boundary layer (BL) momentum thickness θ = 2.4 mm. A flow characterization of the wake behind the vortex generator has been done with the aim of evaluating the performance of three vortex generator geometries, namely Rectangular VG, Triangular VG, and Symmetrical VG NACA0012. The location of the primary vortex has been evaluated by the vertical and lateral trajectories and it has been found that for all analyzed VG geometries the primary vortex is developed below the boundary layer thickness δ = 20 mm for a similar vorticity level ( w x m a x ). Two innovative parameters have been developed in the present work for evaluating the vortex size and the vortex strength: Half-Life Surface S 05 and Mean Positive Circulation Γ 05 + . As a result, an assessment of the VG performance has been carried out by all analyzed parameters and the symmetrical vortex generator NACA0012 has provided good efficiency in energy transfer compared with the Rectangular VG.

Control of flow around a low Reynolds number airfoil using longitudinal strips

Longitudinal strips are suggested as a new device that can significantly increase the aerodynamic performance of a low Reynolds number airfoil at post-stall angles of attack. Its mechanism relies on the generation of corner vortices that delay flow separation on the airfoil suction surface.

Flow structure modifications by leading-edge tubercles on a 3D wing

Leading-edge tubercles on a humpback whale flipper are known to enhance its hydrodynamic performance at post-stall angles of attack (Miklosovic et al 2004 Phys. Fluids 16 39–42). We investigate vortical structures above a three-dimensional wing with tubercles using surface-oil-flow visualization and particle image velocimetry measurement. Two wing models with and without tubercles, previously studied by Miklosovic et al (2004 Phys. Fluids 16 39–42), are considered at the Reynolds number of 180 000 based on the free-stream velocity and mean chord length. At this Reynolds number, tubercles delay the stall angle by 7° and increase the maximum lift coefficient by about 22%. At a low angle of attack, flow separation first occurs near the tip region for both wing models. While flow separation rapidly progresses inboard (toward the wing root) for the model without tubercles with increasing angle of attack, tubercles produce two types of vortical motions and block the inboard progression of flow separation, resulting in delayed stall from α = 8° to 15°. One of these two vortical structures is pairs of counter-rotating streamwise vortices evolving from hemi-spherical separation bubbles near the leading-edge troughs at pre-, near-, and post-stall angles of attack, and the other is asymmetric pairs of streamwise vortices evolving from separated flow regions after the mid-chord region at near-stall angle of attack. At a post-stall angle of attack (α = 16°), strong clockwise and counter-clockwise streamwise vortices are generated from foci at the root and tip near the trailing edge, respectively, and delay flow separation in the mid-span, resulting in a higher lift coefficient than that without tubercles.

Separation delay through contoured transverse grooves on a 2D boat-tailed bluff body: Effects on drag reduction and wake flow features

Abstract The effectiveness of properly contoured transverse grooves in delaying the flow separation occurring on a two-dimensional boat-tailed bluff body is assessed through numerical simulations. The body has a cross-section with a 3:1 elliptical forebody and a rectangular main part followed by a circular-arc boat tail. Three-dimensional Variational Multiscale Large Eddy Simulations are carried out at R e = D u ∞ ∕ ν = 9 . 6 × 1 0 4 , using a mixed finite-volume/finite-element method. The introduction of one contoured groove on each of the boat-tail lateral surfaces produces a significant delay of flow separation and a consequent increase of the base pressure, with a global drag reduction of the order of 9 . 7 % . The wake dynamical structure remains qualitatively similar to the one typical of blunt-based two-dimensional bodies, with quantitative variations that are consistent with the reduction in wake width caused by boat tailing and by the grooves. The introduction of the grooves leads also to a regularization of the vortex shedding downstream of the body, which is more correlated in the spanwise direction. Finally, a few supplementary simulations show that the effect of the grooves is also robust to the variation of the geometrical parameters defining their location and shape.

Reduction of Pressure Gradient and Turbulence Using Vortex Generators in Prosthetic Heart Valves.

Blood damage and platelet activation are inherent problems with present day bi-leaflet mechanical heart valve designs. Passive flow control through different arrangements of vortex generators (VG) as means of improving pressure gradients and reducing turbulence are investigated. Rectangular VG arrays were mounted on the downstream surfaces of a 23mm 3D printed mechanical valve. The effect of VGs on the resulting flow structures were assessed under pulsatile physiological flow conditions where high resolution particle image velocimetry measurement was performed. The co-rotating VGs showed lower Reynolds shear stresses and improved pressure gradients (PG) compared with the counter-rotating ones and the no-VG control one (that showed higher turbulence). RSS was found 38.13 ± 0.89, 12.95 ± 0.32, 15.75 ± 0.71, 24.54 ± 0.84 and 16.33 ± 0.58Pa for the control, co-rotating VGs, 8 counter-rotating VGs, 4 far-spaced VGs and 4 closely-spaced VGs, respectively. PG of 10.45 ± 0.94mmHg was obtained with co-rotating VGs and the difference was significant compared with the other configurations (control 14.88 ± 0.4mmHg; 8 counter-rotating VGs 13.76 ± 0.51mmHg; 4 far-spaced VGs 13.84 ± 0.09mmHg; and 4 closely-spaced VGs 15.37 ± 0.16mmHg). Co-rotating VGs for this application induce a more delayed flow separation and a more homogenized and streamlined transition of flow compared with the counter-rotating VGs. Passive flow control techniques deployed on BHMVs is potentially beneficial as significant control of flow at small length scales without inducing large-scale design modifications of the valve.