Effective Separation Control Research Articles

Effect of peanut-skin fortification on oil separation and consumer acceptance of sesame butter

Background: Sesame butter offers a rich source of health benefits, but its commercial viability is hampered by oil separation during storage. Aims: This study aimed to develop a sesame butter formulation with improved oil retention capacity using peanut skins (PS) as a natural thickening agent, while also evaluating consumer acceptance of the resulting product. Material and Methods: Sesame butter was prepared from roasted sesame seeds. The formulation was then reformulated with the incorporation of PS at varying concentrations (0.0%, 0.5%, 1.0%, 2.0%, 5.0% and 10.0%). Oil separation in these formulations was monitored throughout a storage period of 63 days, with measurements recorded at regular intervals (1, 3, 5, 7, 14, 21, 28, 35, 42, 49, 56 and 63 days). To assess consumer acceptability, 50 untrained panelists evaluated the sensory attributes (flavor, texture, and overall acceptance) of the sesame butter formulations using a 9-point hedonic scale. Results: Consumer acceptance of the sesame butter was primarily influenced by the formulation's sensory characteristics, particularly flavor and color. Texture did not significantly impact overall acceptance (p = 0.975). Notably, the degree of oil separation significantly decreased (p = 0.00) with increasing PS concentration. A marked reduction in oil separation occurred after 14 days of storage. Formulations containing 10.0% and 5.0% PS exhibited the most effective oil retention, while the 1.0% PS formulation received the highest consumer ratings for flavor, texture, and overall acceptability. Conclusion: This study demonstrates the potential of peanut skins as a viable food additive to enhance oil retention capacity and consumer acceptance of sesame butter. Notably, the optimal PS concentration for improved functionality and consumer preference appears to be at a lower level (1.0%). This finding suggests the potential for effective oil separation control without compromising consumer perception of the product.

Direct numerical simulation of shock-wave/boundary layer interaction controlled with convergent–divergent riblets

In this paper, a section of convergent–divergent (C–D) riblets is applied upstream of a compression ramp in a supersonic turbulent boundary layer in a Mach 2.9 flow with a Reynolds number of Reθ=2240. Direct numerical simulations are undertaken to examine the impact of C–D riblets on the shock wave/boundary layer interaction and the feasibility of using them to mitigate flow separation. Over the riblet section, a large-scale secondary roll mode is produced by C–D riblets with the downwelling motion occurring around the diverging region and upwelling motion near the converging region. This consequently leads to a spanwise heterogeneity in mean quantities and turbulent structures over the riblet section and also in the interaction zone. Compared with the baseline case, the area of the separation zone for the riblet case experiences a dramatic local reduction of 92% in the diverging region, owing to the downwelling motion that injects the high-momentum fluid toward the wall and the near-wall spanwise velocity that transports the low-momentum fluid away. The enhanced upwelling motion around the converging region induced by C–D riblets, on the one hand, contributes to the decrease of the near-wall momentum and subsequently the increase of the local separation area. On the other hand, the upwelling motion effectively reduces the incoming Mach number upstream of the compression corner. This appears to reduce the strength of the separation shock, leading to a more gradual compression of the incoming flow that helps ease the enlargement of the separation area nearby. Overall, the area of the mean flow separation is reduced by 56%, indicating an effective flow separation control by the C–D riblets.

Double-Ducted Fan as an Effective Lip Separation Control Concept for Vertical-Takeoff-and-Landing Vehicles

This paper describes a novel inlet flow conditioning concept that significantly improves ducted fans’ performance and controllability. High angle-of-attack operation of ducted fans is very common in vertical-takeoff-and-landing systems. The new concept named double-ducted fan (DDF) uses a secondary shroud to control inlet lip-separation-related momentum deficit at the inlet of the fan rotor during edgewise flight. The DDF is helpful in a wide edgewise flight velocity range, and its corrective aerodynamic effect becomes more pronounced with increasing flight velocity. A conventional baseline duct operating at 90 deg angle of attack is compared to other novel double-ducted fan designs via three-dimensional, viscous, and turbulent flow computations. A custom actuator-disk model replaces the zonal computation of the complex and three-dimensional rotor flowfield in the rotating frame of reference. Both hover and edgewise flight conditions are considered around the ducted fan unit and inside the ducts. Significant improvements from several novel ducted fan designs are in the areas of vertical thrust enhancement, nose-up pitching moment reduction, and recovery of fan through-flow mass flow rate in a wide horizontal flight range. A variable DDF design, a partial DDF design, and a fan-in-wing implementation of the DDF are introduced.

Investigation of leading-edge protuberances for the performance improvement of thick wind turbine airfoil1

An effective flow separation control method is urgently needed to improve the aerodynamic performance of the thick wind turbine airfoil. The purpose of the present study is to explore the effect of leading-edge protuberances (LEPs) on the aerodynamic performances for thick wind turbine airfoil during stall region. A direct force measurement technique employing three-component force balance was used to analyze the aerodynamic characteristics of airfoils with LEPs at a Reynolds number of Re = 2 × 105 through the wind tunnel experiment. Particle Image Velocimetry measurement was also performed to understand the effects of LEPs on flow separation and to correlate these effects with the aerodynamic performances of airfoil. The mean velocity and vorticity results indicate that the LEPs can effectively suppress flow separation for the thick airfoil in the stall region, moreover the airfoil with smaller amplitudes and wavelengths tends to perform significantly better in flow separation control. The LEPs method promotes the wavy airfoil with a better flow separation characteristics, which can effectively improve the aerodynamic performance and reduce the intensity of aerodynamic fluctuations for the thick airfoil. In addition, the stall process of the wavy airfoil is much gentler without sharp deterioration. Therefore, the LEPs method can help maintain high aerodynamic performance and prevent structural problems for the inboard region of wind turbine blades.

A calculation method for modeling the flow characteristics of the wind turbine airfoil with leading-edge protuberances

An effective flow separation control method is urgently needed to improve the aerodynamic performance of the wind turbine blades. The leading-edge protuberances (LEPs) are the method that can inhibit flow separation and improve the aerodynamic performance of wind turbine airfoil. In order to achieve the optimal design of the LEPs on the wind turbine airfoil, it is necessary to develop a theoretical model of the effect of the LEPs on the flow characteristics. This paper proposes a novel modeling calculation method as achieved by substituting the momentum source into the N–S equation, and establishes the relationship between the geometrical parameters of the LEPs and the flow characteristics. The results show that the aerodynamic and flow characteristics of the modeled and solid airfoil with LEPs maintain good consistency under different stall conditions. Moreover, the high-strength streamwise vortices induced by LEPs increase the area of low pressure in the airfoil suction side, which effectively suppresses the flow separation and improves the aerodynamic performance of the airfoil. The modeling method provides the basis for the optimal design of the wind turbine airfoil with LEPs. The process of model construction explains the flow control mechanism of the LEPs from a new perspective.

Aerodynamics of vertical-axis wind turbine with boundary layer suction – Effects of suction momentum

Vertical axis wind turbine (VAWT) has become a research hotspot because of its structural advantages and omni-direction capability. While a further performance improvement is needed for its lower power coefficient and complex flow on blades for VAWT. Boundary layer suction (BLS) as an effective separation control method for airfoil can be a potential way to enhance the performance of VAWT. The current study first investigates boundary layer suction momentum on NACA0021 airfoil by numerical simulation. Suction momentums from 0.001 to 0.4 are investigated systematically using steady simulation with SST κ-ω turbulence model. Three zones are concluded by the corrected lift-drag ratio with suction momentum. The corrected lift-drag ratio can be increased to more than 8–14 times higher compared with the airfoil without suction within the 2°–26° angles and suction momentum within 0.02–0.2. The percentage increase in power coefficient for BLS applied on VAWT can reach to about 34.25% at the optimal tip speed ratio (TSR) for boundary layer suction VAWT at TSR = 2.33 for Cμ = 0.0075. The instantaneous power coefficient during one blade revolution and the behavior of vortex and flow structures around the suction blade are also studied to explore the physical mechanism of improving aerodynamic performance.

O uticaju skaliranja dimenzija i varijacije masenog protoka na frekvenciju aktuatora sa oscilirajućim mlazom

This paper presents an experimental and numerical study of the sweeping jet oscillation frequency which is a crucial parameter for effective separation control. Measurements of the time-resolved static pressure are made in the feedback channels using high-frequency pressure transducers. Experiments are performed for eight different inlet mass flow rates (from 1g/s to 8 g/s). The effects of geometry scaling are examined for the halfand double-size of the baseline actuator. The results reveal a relationship between the oscillating frequency and the inlet mass flow rate and demonstrate a linear trend of the Strouhal number versus the inlet Mach number for all the geometries. The experimental results are compared to results obtained from numerical simulations using Ansys Fluent v17.2 software.A high correspondence is found between experimental results and numerical data from computationally inexpensive 2D-URANS simulations. This result opens possibilities of using these simple numerical models to perform extensive studies of the geometric parameters of the sweeping jet actuator in the future.

Active Control of Laminar Separation: Simulations, Wind Tunnel, and Free-Flight Experiments

When a laminar boundary layer is subjected to an adverse pressure gradient, laminar separation bubbles can occur. At low Reynolds numbers, the bubble size can be substantial, and the aerodynamic performance can be reduced considerably. At higher Reynolds numbers, the bubble bursting can determine the stall characteristics. For either setting, an active control that suppresses or delays laminar separation is desirable. A combined numerical and experimental approach was taken for investigating active flow control and its interplay with separation and transition for laminar separation bubbles for chord-based Reynolds numbers of Re ≈ 64,200–320,000. Experiments were carried out both in the wind tunnel and in free flight using an instrumented 1:5 scale model of the Aeromot 200S, which has a modified NACA 643-618 airfoil. The same airfoil was also used in the simulations and wind tunnel experiments. For a wide angle of attack range below stall, the flow separates laminar from the suction surface. Separation control via a dielectric barrier discharge plasma actuator and unsteady blowing through holes were investigated. For a properly chosen actuation amplitude and frequency, the Kelvin–Helmholtz instability results in strong disturbance amplification and a “roll-up” of the separated shear layer. As a result, an efficient and effective laminar separation control is realized.

Passive Boundary Layer Flow Control Using Porous Lamination

The flow over a porous laminated flat plate is investigated from a flow control perspective through experiments and computations. A square array of circular cylinders is used to model the porous lamination. We determine the velocities at the fluid–porous interface by solving the two-dimensional Navier–Stokes and the continuity equations using a staggered flow solver and using LDV in experiments. The control parameters for the porous region are porosity, $$\phi $$ and Reynolds number, Re, based on the diameter of the circular cylinders used to model the porous lamination. Computations are conducted for $$0.4< \phi < 0.9$$ and $$25< Re < 1000$$ , and the experiments are conducted for $$\phi = 0.65$$ and 0.8 at $$Re \approx 391,\ 497$$ and 803. The permeability of the porous lamination is observed to induce a slip velocity at the interface, effectively making it a slip wall. The slip velocity is seen to be increasing functions of $$\phi $$ and Re. For higher porosities at higher Re, the slip velocity shows non-uniform and unsteady behavior and a breakdown Reynolds number is defined based on this characteristic. A map demarcating the two regimes of flow is drawn from the computational and experimental data. We observe that the boundary layer over the porous lamination is thinner than the Blasius boundary layer and the shear stress is higher at locations over the porous lamination. We note that the porous lamination helps maintain a favorable pressure gradient at the interface which delays separation. The suitable range of porosities for effective passive separation control is deduced from the results.

Reduced-order coherent structures in active flow separation control using a bio-inspired flapping actuator

An experimental study was conducted to investigate large-scale coherent structures induced by a bio-inspired flapping actuator in a turbulent backward-facing step flow. The flow field velocity dataset in the horizontal-vertical plane was obtained by planar particle image velocimetry. The flapping actuator was implemented over the step edge and it was driven to vertically oscillate, generating periodic small perturbations into the separated shear layer. As a result of active flow control, time-averaged reattachment length was reduced by 31% and Reynolds stresses as well as in-plane turbulent kinetic energy in the separated shear layer were considerably increased. In proper orthogonal decomposition analysis, the first two modes are found to be phase-correlated in the phase portrait of the coefficients. Therefore, reduced-order coherent structures are reconstructed by the first two modes, showing the phase evolution of the vortex shedding process. The extracted reduced-order coherent structures provide a better understanding of the underlying flow physics and are beneficial to effective flow separation control.

Scaling of separated shear layers: an investigation of mass entrainment

We report an experimental investigation of the separating/reattaching flow over a descending ramp with a $25^{\circ }$ expansion angle. Emphasis is given to mass entrainment through the boundaries of the separated shear layer emanating from the upper edge of the ramp. For this purpose, the turbulent/non-turbulent interface and the separation line inferred from image-based analysis are used respectively to mark the upper and lower bounds of the separated shear layer. The main objective of this study is to identify the physical parameters that scale the development of the separated shear layer, by giving a specific emphasis to the investigation of mass entrainment. Our results emphasise the multiscale nature of mass entrainment through the separated shear layer. The recirculation length $L_{R}$, step height $h$ and free-stream velocity $U_{\infty }$ are the dominant scales that organise the separated flow (and related large-scale quantities as pressure distribution or shear layer growth rate) and set mean mass fluxes. However, local viscous mechanisms seem to be responsible for most of local mass entrainment. Furthermore, it is shown that large-scale mass entrainment is driven by incoming boundary layer properties, since $L_{R}$ scales with $Re_{\unicode[STIX]{x1D703}}$, and in particular by its turbulent state. Surprisingly, the relationships evidenced in this study suggest that these dependencies are established over a large distance upstream of separation and that they might also extend to small scales, at which viscous entrainment is dominant. If confirmed by additional studies, our findings would open new perspectives for designing effective separation control systems.

Improvement of effectiveness of EMHD flow separation control

The present numerical study shows that the effectiveness of flow separation control using Electro-Magneto-Hydro-Dynamic (EMHD) method can be improved by choosing the dimensions of the actuator based on the Reynolds number and the shape of the model studied. The width of the electrode/magnet for most effective flow separation control should be approximately 2.5 times the boundary layer thickness, which in turn is dependent on the Reynolds number. The length of the EMHD actuator for most effective flow separation control can be the shortest based on manufacturing constraints as well as on the possibility of shifting of separation point. The application of Lorentz force inside the boundary layer near the separation point provides the most effective flow separation control. The studies are done for flow over a cylinder in low Reynolds number regime (Reynolds number = 200) and for flow over an elliptical airfoil in high Reynolds number regime (Reynolds number = 1.6 × 105).

Nonlinear Adaptive Approach to Microjet-Based Flow Separation Control

Boundary-layer separation, a critical phenomenon in the operation of aerodynamic surfaces, limits the performance of compressor and turbine blades, fixed and rotary wings, as well as bluff bodies moving through a fluid. Flow separation leads to increased drag, decreased lift, and unpredictable vibrations due to unsteadiness. On these systems, effective control of separation could provide greater maneuverability and performance, and reduced vibration. Separated flow is a macroscale phenomenon governed by complex flow interactions, but it can be controlled by microscale actuation. Recently, the emergence of closed-loop methods has enhanced robustness. Modern processors enable the use of sophisticated adaptive control methods that achieve separation control with adaptive models. This paper considers control of flow separation over a NACA-0025 airfoil using microjet actuators. Experimental results are presented for a novel approach to nonlinear model predictive control, referred to as adaptive sampling-based model predictive control, which applies the minimal resource allocation network algorithm for nonlinear system identification and the sampling-based model predictive optimization algorithm to achieve effective nonlinear control. Through pressure data and flow characterization from wind-tunnel experiments, effective and robust separation control is demonstrated. The method’s computational efficiency is sufficient for successful real-time experimental implementation.

Effects of vortex generators on a blunt trailing-edge airfoil for wind turbines

Vortex generators (VGs) are commonly-used effective flow separation control devices, and are proved to have potential to improve the aerodynamic performance of large wind turbines. In this paper, the flow physics of VGs and how their size affects the aerodynamic performance of a blunt trailing-edge airfoil DU97-W-300 have been investigated using CFD simulations. Based on wind turbine dedicated airfoil with and without VGs respectively, three-dimensional numerical models were established and further validated through the comparisons between the numerical results and the experimental data. The effects of VGs' size were analyzed from several perspectives, such as trailing-edge height, length, short and long spacing between an adjacent pair of VGs. The results indicate that drag penalty is more sensitive to the increase of VG height than lift; an increment of VG length leads to negative effects on both lift and drag; increases of the spacing between an adjacent pair of VGs have positive impact on suppression of separated flow. Additionally, the flow field characteristics were further revealed by the analysis of streamlines and vortices in the wake region.

Discrete Adjoint based Sensitivity Analysis for Optimal Active Flow Control of High‐Lift Configurations

AbstractBlowing and suction type of active flow control techniques can be used to delay the flow separation on the flap and to enhance the aerodynamic performance of high‐lift configurations. Effective separation control and maximum enhancement in the mean lift coefficient are achieved by finding the optimal actuation parameters. The optimal set of actuation parameters can be obtained by combining the gradient based algorithms with discrete adjoints. In the present work, an unsteady discrete adjoint incompressible RANS solver is developed for the optimal active separation control. The adjoint solver is applied to the test case of active flow control on the flap of a 2D high‐lift configuration. Sensitivity gradients are presented to demonstrate the accuracy of unsteady adjoint RANS solver. (© 2013 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Influence of air-jet vortex generator diameter on separation region

Control of shock wave and boundary layer interaction continues to attract a lot of attention. In recent decades several methods of interaction control have been investigated. The research has mostly concerned solid (vane type) vortex generators and transpiration methods of suction and blowing. This investigation concerns interaction control using air-jets to generate streamwise vortices. The effectiveness of air-jet vortex generators in controlling separation has been proved in a previous research. The present paper focuses on the influence of the vortex generator diameter on the separation region. It presents the results of experimental investigations and provides new guidelines for the design of air-jet vortex generators to obtain more effective separation control.

A model for Nanosecond Pulsed Dielectric Barrier Discharge (NSDBD) actuator and its investigation on the mechanisms of separation control over an airfoil

In order to simulate the flow control problem by using Nanosecond Pulsed Dielectric Barrier Discharge (NSDBD), a one-zone inhomogeneous phenomenological model is constructed based on the experimental and theoretical results. The model is coupled with the unsteady Navier-Stokes equations, which can well predict the compression-expansion wave structures and wave speed compared with experimental results and can be applied to the simulation of the flow control by using NSDBD. The model is adopted to investigate the separation control over NACA0015 airfoil using the NSDBD plasma actuator. The separation-control mechanisms are revealed that the spanwise vortices produced by the plasma actuation play the key role. Each plasma actuation can produce a spanwise vortex around the separation point near the leading edge. The spanwise vortices make the separated free-shear layer unstable and shed away, move downstream along the upper wall, control the flow near the wall, and bring outer flow with high kinetic energy into the near wall region to realize the effective separation control over the upper surface of the airfoil.

The Application of Flow Control to an Aft-Loaded Low Pressure Turbine Cascade With Unsteady Wakes

The synchronous application of flow control in the presence of unsteady wakes was studied on a highly loaded low pressure turbine blade. At low Reynolds numbers, the blade exhibits a nonreattaching separation bubble under steady flow conditions without upstream wakes. Unsteady wakes from an upstream vane row are simulated with a moving row of bars. The separation zone is modified substantially by the presence of unsteady wakes, producing a smaller separation zone and reducing the area-averaged wake total pressure loss by more than 50%. The wake disturbance accelerates transition in the separated shear layer but stops short of reattaching the flow. Rather, a new time-averaged equilibrium location is established for the separated shear layer. The focus of this study was the application of pulsed flow control using two spanwise rows of discrete vortex generator jets. The jets were located at 59% Cx, approximately the peak cp location, and at 72% Cx. The most effective separation control was achieved at the upstream location. The wake total pressure loss decreased 60% from the wake-only level and the cp distribution fully recovered its high Reynolds number shape. The jet disturbance dominates the dynamics of the separated shear layer, with the wake disturbance assuming a secondary role only. When the pulsed jet actuation was initiated at the downstream location, synchronizing the jet to actuate between wake events was key to producing the most effective separation control. Evidence suggests that flow control using vortex generator jets (VGJs) will be effective in the highly unsteady low pressure turbine environment of an operating gas turbine, provided the VGJ location and amplitude are adapted for the specific blade profile.

Separation Control on a Very High Lift Low Pressure Turbine Airfoil Using Pulsed Vortex Generator Jets

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. Jet pulsing frequency, duty cycle, and blowing ratio were all varied. Computational results from a large eddy simulation of one case showed reattachment in agreement with the experiment. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was possible even at the lowest Reynolds number. Pulsed VGJs were more effective than steady jets. At sufficiently high pulsing frequencies, separation control was possible even with low jet velocities and low duty cycles. At lower frequencies, higher jet velocity was required, particularly at low Reynolds numbers. Effective separation control resulted in an increase in lift and a reduction in total pressure losses. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can reseparate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times.

Numerical Simulation of Low-Pressure Turbine Blade Separation Control

Low-pressure turbines are a common and important element of many modern jet engines. Any performance improvement will lead to net savings in aircraft operating costs. At low operating Reynolds numbers or for reduced stage solidities laminar separation from the constituent blades can noticeably deteriorate overall engine performance. Under such conditions active control of laminar separation may eliminate or reduce associated losses resulting in increased engine performance. A high-order-accurate finite volume Navier-Stokes code for investigating separation control for the PackB blade geometry at a Reynolds number based on axial chord of 25,000 was employed. For this Reynolds number laminar separation was observed in the experiments. First, a grid resolution study for the uncontrolled flow was carried out, which shows grid convergence for our simulations. Then separation control by pulsed vortex generator jets was investigated. It is shown how this control results in an earlier transitioning of the flow and the introduction of spanwise coherent structures. The increased wall-normal mixing associated with these two mechanisms results in an effective separation control. An even more efficient separation control can be accomplished by harmonic blowing through a slot. The astounding effectiveness of the latter control scheme is attributed to an amplification of the disturbance input through a hydrodynamic instability mechanism and to the suppression of three-dimensional structures, which weaken the spanwise structures. Finally, separation control by streamwise vortices that were generated by volume forces was investigated. As the flow does not amplify such structures the energy input required for accomplishing an effective control was found to be large compared with the other flow control techniques. Because the structures are steady they can, however, likely be generated with passive devices such as vortex generators.