Suppression Of Flow Separation Research Articles

Effects of lateral wind gusts on vertical axis wind turbines

This paper investigates the effects of lateral wind gusts on the aerodynamic performance of vertical axis wind turbines. A synthetic approach coupling computational fluid dynamics (CFD) simulations and the double multiple streamtube method is utilized to calculate vertical axis wind turbine performance. For simulating the wind gust response, the resolved gust approach (RGA) models the gust transport throughout the wind field. The effects of wind gusts on the effective wind velocity, angle of attack, torque and power performance are evaluated. The presence of gusts slightly changes the effective wind velocity and angle of attack. Lateral gusts increase the angles of attack in the upwind half period causing premature flow separations in the flowfield, while suppress flow separation by decreasing the angles of attack in the downwind period. The mean torque and power of the three-bladed turbines are enhanced by the lateral sharp-edge gust of 2 m/s by 5% in comparison to the steady wind. Reducing the number of blades to one and doubling the gust velocity make the performance further enhances by 14.7% and 34%, respectively. Finally, the effects of several gust shapes are investigated and discussed.

Direct numerical simulation of turbulent channel flow with spanwise alternatively distributed strips control

The effect of spanwise alternatively distributed strips (SADS) control on turbulent flow in a plane channel has been studied by direct numerical simulations to investigate the characteristics of large-scale streamwise vortices (LSSVs) induced by small-scale active wall actuation, and their potential in suppressing flow separation. SADS control is realized by alternatively arranging out-of-phase control (OPC) and in-phase control (IPC) wall actuations on the lower channel wall surface, in the spanwise direction. It is found that the coherent structures are suppressed or enhanced alternatively by OPC or IPC, respectively, leading to the formation of a vertical shear layer, which is responsible for the LSSVs’ presence. Large-scale low-speed region can also be observed above the OPC strips, which resemble large-scale low-speed streaks. LSSVs are found to be in a statistically-converged steady state and their cores are located between two neighboring OPC and IPC strips. Their motions contribute significantly to the momentum transport in the wall-normal and spanwise directions, demonstrating their potential ability to suppress flow separation.

A qualitative study of vortex trapping capability for lift enhancement on unconventional wing

Lift enhancement by using passive vortex trapping technique offers great advantage in small aircraft design as it can improve aerodynamics performance and reduce weight of the wing. To achieve this aim, a qualitative study on the flow structures across wing models with cavities has been performed using smoke wire visualisation technique. An experiment has been conducted at low Reynolds number of 26,000 with angle of attack (α) = 0°, 5°, 10° and 15° to investigate the vortex trapping capability of semi-circular leading edge (SCLE) flat-plate wing model and elliptical leading edge (ELE) flat-plate wing model with cavities, respectively. Results from the qualitative study indicated unique characteristics in the flow structures between the tested wing models. The SCLE wing models were able to trap stable rotating vortices for α ≤ 10° whereas the ability of ELE wing models to suppress flow separation allowed stable clockwise vortices to be trapped inside the cavities even at α > 10°. The trapped vortices found to have the potential to increase lift on the unconventional wing models.

Effects of Wall-Normal and Angular Momentum Injections in Airfoil Separation Control

The objective of this computational study is to quantify the influence of wall-normal and angular momentum injections in suppressing laminar flow separation over a canonical airfoil. Open-loop control of fully separated, incompressible flow over a NACA 0012 airfoil at and is examined with large-eddy simulations. This study independently introduces wall-normal momentum and angular momentum into the separated flow using swirling jets through model boundary conditions. The responses of the flowfield and the surface vorticity fluxes to various combinations of actuation inputs are examined in detail. It is observed that the addition of angular momentum input to wall-normal momentum injection enhances the suppression of flow separation. Lift enhancement and suppression of separation with the wall-normal and angular momentum inputs are characterized by modifying the standard definition of the coefficient of momentum. The effect of angular momentum is incorporated into the modified coefficient of momentum by introducing a characteristic swirling jet velocity based on the nondimensional swirl number. With this single modified coefficient of momentum, each controlled flow can be categorized into separated, transitional, and attached flows.

Roll aerodynamic characteristics study of an unmanned aerial vehicle based on circulation control technology

To explore the control efficiency of circulation flow control technology, a circulation control actuator with an independent gas source has been designed and applied in roll attitude control of a small unmanned aerial vehicle. The circulation control devices are arranged at the two ends of the wing on an unmanned aerial vehicle scale model, the changes in aerodynamic force and aerodynamic moment caused by turning on the actuator are measured in a wind tunnel, and the flow field characteristics are analysed using particle image velocimetry technology. The flight control effect of the roll attitude is verified via a flight test. Experimental and flight test results show that the control of roll attitude can be achieved by turning on the circulation control actuator on one side, and the maximum efficiency that the circulation control generates is equivalent to 8° aileron deflection with production of a favorable yaw moment to achieve a coordinated turn. The circulation control actuator can increase lift and reduce drag when opened on both sides simultaneously. The maximum lift-to-drag ratio of the UAV increased from 5 to 9, and this approach can also suppress flow separation and delay stall at high angles of attack. The aileron or trailing edge flaps can be replaced with circulation control actuators, and the circulation control technology can also be applied to aerodynamic performance improvement and flight control in other types of aircraft.

Large-Scale Streamwise Vortices in Turbulent Channel Flow Induced by Active Wall Actuations

Direct numerical simulations of turbulent flow in a plane channel using spanwise alternatively distributed strips (SADS) are performed to investigate the characteristics of large-scale streamwise vortices (LSSVs) induced by small-scale active wall actuations, and their role in suppressing flow separation. SADS control is obtained by alternatively applying out-of-phase control (OPC) and in-phase control (IPC) to the wall-normal velocity component of the lower channel wall, in the spanwise direction. Besides the non-controlled channel flow simulated as a reference, four controlled cases with 1, 2, 3 and 4 pairs of OPC/IPC strips are studied at M = 0.2 and R e = 6,000, based on the bulk velocity and the channel half height. The case with 2 pairs of strips, whose width is Δz + = 264 based on the friction velocity of the non-controlled case, is the most effective in terms of generating large-scale motions. It is also found that the OPC (resp. IPC) strips suppress (resp. enhance) the coherent structures and that leads to the creation of a vertical shear layer, which is responsible for the LSSVs presence. They are in a statistically steady state and their cores are located between two neighbouring OPC and IPC strips. These motions contribute significantly to the momentum transport in the wall-normal and spanwise directions showing potential for flow separation suppression.

Investigation on aerodynamic performance of horizontal axis wind turbine by setting micro-cylinder in front of the blade leading edge

For NREL Phase VI horizontal axis wind turbine, a flow control method to suppress the flow separation by setting micro-cylinder in front of the blade leading edge is proposed, and the corresponding numerical simulation analysis for the aerodynamic performance of wind turbine is conducted. Firstly, the results predicted by simulation are confirmed experimentally. Under the same operating condition, the simulation and experimental results of low-speed shaft torque are compared, along with results from other studies. It can be found that the simulation results can accurately reflect the basic physical characteristics of flow field for NREL Phase VI wind turbine. Secondly, the influence of different diameters and positions of micro-cylinders on aerodynamic performance of wind turbine is discussed. Numerical results suggest that under different stall conditions, setting appropriate micro-cylinders in front of the blade leading edge can effectively suppress flow separation on wind turbine blades without increasing the load of wind turbine. Moreover, under different wind speeds, micro-cylinders with different diameters and positions have various impacts on aerodynamic performance of wind turbine. Through numerical calculation, the blade torque can maximally have an increase of 27.3% by setting a micro-cylinder with proper diameter and position in front of the blade leading edge.

Suppression of flow separation of power-law fluids flow around a confined circular cylinder by superimposed thermal buoyancy

This paper presents a comprehensive computational work on hydrodynamic and thermal phenomena of flow separation around a confined circular cylinder by superimposed thermal buoyancy. For that purpose, let us consider a confined flow of Non-Newtonian power-law fluid around a heated circular cylinder in a two-dimensional vertical channel. The effects of thermal buoyancy and power-Law index on the flow separation and the average Nusselt number are studied for the conditions: (10 ≤ Re ≤ 40), (0.4≤ n ≤ 1.2), (0 ≤ Ri ≤ 0.8), Pr = 50 and blockage ratio β = 0.2. The Reynolds numbers of flow are chosen in low range such that the flow remains steady and separated without imposition of the thermal buoyancy. In the steady flow regime the results show that the augmentation of the power-law index in the absence of thermal buoyancy causes a separation to grow. The thermal buoyancy delays the separation in different power-law indexes gradually and at some critical value of the buoyancy parameter it completely disappears resulting a stuck flow around a cylinder. Moreover, the recirculation length and skin friction are calculated to support the above finding. The decrease in the power-Law index increases the heat transfer rate. The Nusselt numbers are computed to predict the heat transfer rates of power-law fluids under the superimposed thermal buoyancy condition.DOI: http://dx.doi.org/10.5755/j01.mech.23.2.14342

Visualization of vortical flows around a rapidly pitching wing and propeller

This study was conducted to visually investigate flows related to fixed-wing vertical-takeoff-and-landing micro air vehicles, using the smoke-wire technique. In particular, the study examines transition between forward flight and near-hover. The experimental model consists of a rigid Zimmerman wing and a propulsion system with contra-rotating propellers arranged in a tractor configuration. The model was pitched about the wing’s aerodynamic center at approximately constant rates using a five-axis robotic arm. Constant-rate pitching angles spanned 20° to 70°. No-pitching and four pitching-rates were used, along with three propulsive settings. Several observations were made during no-pitching tests. Turbulent wakes behind blades and laminar flow between them produces pulsations in the boundary layer. These pulsations alter the boundary layer from a laminar to turbulent state and back. An increase in lift and drag in the presence of a slipstream is a result of competing effects of the propulsive slipstream: (a) suppression of flow separation and increased velocity over the wing and (b) decrease of the effective angle of attack. Higher nose-up pitching-rates generally lead to greater trailing-edge vortex-shedding frequency. Nose-up pitching without a slipstream can lead to the development of a traditional dynamic-stall leading-edge vortex, delaying stall and increasing wing lift. During nose-up pitching, a slipstream can drive periodically shed leading-edge vortices into a larger vortical-structure that circulates over the upper-surface of a wing in a fashion similar to that of a traditional dynamic-stall leading-edge vortex. At lower nose-up pitching-rates, leading-edge vortices form at lower angles of attacks. As a slipstream strengthens, a few things occur: separation wakes diminish, separation occurs at a higher angle of attacks, and downward flow-deflection increases. Similar effects are observed for nose-up pitching, while nose-down pitching produces the opposite effects.

A pulsed generator for synchronous discharges of high-energy plasma synthetic jet actuators

Plasma synthetic jet (PSJ) has a wide application prospect in flow separation suppression and boundary layer control. Comparing with traditional control methods, synchronous discharges of PSJ with high repetition frequency and high energy could achieve a better flow control effect because it can meet multi-angle, wide-range and long-time flow control requirements. In this paper, a microsecond-pulse power supply for synchronous discharges of three plasma synthetic jet actuators (PSJA) is developed. Parameters of output pulses are as follows: the maximum voltage pulse is 10 kV, the pulse repetition rate is 1000 Hz and the full width is 15 μs. The generator can withstand a discharge current with an amplitude of 320 A and a discharge energy of 150 mJ. The electrical characteristics of the synchronous discharges of PSJAs including the voltage-current waveforms, discharge images and Schlieren images are investigated. The experimental result shows that the microsecond-pulse generator could sustain three discharge channels in PSJAs when the output voltage is 10 kV. These discharge channels have preferable performance in synchronicity when the repetition frequency is 1000 Hz. Furthermore, the Schlieren images show that the power supply can sustain three PSJAs with a maximum jet velocity of up to 150 m/s.

Instability of cylinder wake under open-loop active control

Instability of a wake controlled by a streamwise Lorentz force is investigated through a Floquet stability analysis. The streamwise Lorentz force, which is a two-dimens- ional control input created by an electromagnetic actuator located on the cylinder surface, adjusts the base flow to affect the three-dimensional wake instability and achieve wake stabilization and transition delay. The instability mode at a Reynolds number Re = 300 can be transformed from B to A with N = 1.0, where N is an interaction number repre- senting the strength of the Lorentz force relative to the inertial force in the fluid. The wake flow is Floquet stable when N increases to 1.3. The spanwise perturbation wavelengths are 3.926D and 0.822D in the modes A and B, respectively, where D is the cylinder diameter. In addition, the oscillating amplitudes of drag and lift are reduced with the increase in the interaction number. Particle tracing is used to explore the essential physical mechanism for mode transformation. The path lines show that suppression of flow separation hinders the fluid deformation and rotation, leading to the decrease in elliptic and hyperbolic instability regions, which is the material cause of mode transformation. All of the results indicate that wake stabilization and transition delay can be achieved under open-loop active control via the streamwise Lorentz force.

Characteristics of the alula in relation to wing and body size in the Laridae and Sternidae

ABSTRACTThe alula is a small structure present on the leading edge of bird wings and is known to enhance lift by creating a small vortex at its tip. Alula size vary among birds, but how this variation is associated with the function of the alula remains unclear. In this study, we investigated the relationship between the size and shape of the alula and the features of the wing in the Laridae and Sternidae. Laridae birds have generally longer wings and greater loadings than Sternidae birds. The two families differed in the relationships between body size or wing length and the size or shape of the alula. In the Laridae, the aspect ratio of the alula was smaller in the species that have relatively longer wings, but the pattern was opposite in the Sternidae. The aspect ratio of the alula was greater in the species that are relatively heavier in the Sternidae but not in the Laridae. Combined, these results suggest that the species with high loading potential and long wings exhibit long alula. We hypothesize that heavier species may benefit from having longer alula if they perform flights with higher attack angles than lighter species, as longer alula would better suppress flow separation at higher attack angles. Our results suggest that the size and shape of the alula can be explained in one allometric landscape defined by wing length and loading in these two closely related families of birds with similar wing shapes.

Arrangement effects of inclined teardrop-shaped dimples on film cooling performance of dimpled cutback surface at airfoil trailing edge

Cooling at trailing edge of gas turbine airfoil is one of the most difficult problems because of its thin shape: high thermal load from both surfaces, hard-to-cool geometry of narrow passages, and at the same time demand for structural strength are the reasons. In this study, heat transfer coefficient and film cooling effectiveness on pressure-side cutback surface were measured by a transient infrared thermography technique with consideration of three-dimensional heat conduction. The cutback surface was roughened by teardrop-shaped dimples. Three different rotation angles (30, 45, and 60deg) of the dimpled surface with in-line and staggered arrangements were examined for blowing ratio of 0.5, 1.0, 1.5, and 2.0. Within the present arrangements, the film cooling effectiveness remained almost constant, although the heat transfer coefficient became a maximum at 30deg in-line arrangement. The overall film cooling performance evaluated by net heat flux reduction was the highest for the 30deg in-line arrangement and it was 14–24% higher than the 30deg staggered arrangement. Three-component PTV results explained this effect by the suppression of flow separation in the 30deg in-line arrangement and it attained the favorable inflow condition on the downstream dimples.

Computational Fluid Dynamics-Based Pump Redesign to Improve Efficiency and Decrease Unsteady Radial Forces

In this study, a double volute centrifugal pump with relative low efficiency and high vibration is redesigned to improve the efficiency and reduce the unsteady radial forces with the aid of unsteady computational fluid dynamics (CFD) analysis. The concept of entropy generation rate is applied to evaluate the magnitude and distribution of the loss generation in pumps and it is proved to be a useful technique for loss identification and subsequent redesign process. The local Euler head distribution (LEHD) can represent the energy growth from the blade leading edge (LE) to its trailing edge (TE) on constant span stream surface in a viscous flow field, and the LEHD is proposed to evaluate the flow field on constant span stream surfaces from hub to shroud. To investigate the unsteady internal flow of the centrifugal pump, the unsteady Reynolds-Averaged Navier–Stokes equations (URANS) are solved with realizable k–ε turbulence model using the CFD code FLUENT. The impeller is redesigned with the same outlet diameter as the baseline pump. A two-step-form LEHD is recommended to suppress flow separation and secondary flow encountered in the baseline impeller in order to improve the efficiency. The splitter blades are added to improve the hydraulic performance and to reduce unsteady radial forces. The original double volute is substituted by a newly designed single volute one. The hydraulic efficiency of the centrifugal pump based on redesigned impeller with splitter blades and newly designed single volute is about 89.2%, a 3.2% higher than the baseline pump. The pressure fluctuation in the volute is significantly reduced, and the mean and maximum values of unsteady radial force are only 30% and 26.5% of the values for the baseline pump.

Effect of particle momentum transfer on an oblique-shock-wave/laminar-boundary-layer interaction

Numerical simulations of solid particles seeded into a supersonic flow containing an oblique shock wave reflection were performed. The momentum transfer mechanism between solid and gas phases in the shock-wave/boundary-layer interaction was studied by varying the particle size and mass loading. It was discovered that solid particles were capable of significant modulation of the flow field, including suppression of flow separation. The particle size controlled the rate of momentum transfer while the particle mass loading controlled the magnitude of momentum transfer. The seeding of micro- and nano-sized particles upstream of a supersonic/hypersonic air-breathing propulsion system is proposed as a flow control concept.

Plasma actuator scaling down to improve its energy conversion efficiency for active flow control in modern turbojet engines compressors

The present work was performed to investigate the employment of micro dielectric barrier discharge plasma actuators for mitigating separation, thereby decreasing wake losses and increasing efficiency, on a highly loaded compressor cascade. To this aim, the experimental characterization of the control device was initially done. A dedicated activity was devoted to microelectronic technology adoption for micro plasma actuator fabrication, together with batch production of electrodes with photolithographic techniques. The actuation effect on quiescent flow was evaluated by measuring the induced wall-jet with particle image velocimetry. The actuator power consumption was estimated by recording the applied voltages and resulting currents.Experimental results were then used to calibrate a multi-physics numerical model, for the prediction of the body forces induced by plasma actuator. Different algebraic models were compared. Numerical modelling was applied to predict the capability of micro plasma actuation to suppress flow separation into a highly-loaded subsonic compressor stator cascade. At first, simulations of the compressor cascade without active flow control were carried out and the results were compared with the literature experimental data. A good agreement was found between the experimental and the numerical results. Active flow control by the micro plasma actuator was then tested under different sinusoidal voltage amplitudes. It was found that the compressor pressure losses were reduced by increasing the applied voltage; actuation brought to a reduction in the pressure loss coefficient up to 14% and to an increase in static pressure up to 3%. When the actuator was on, the isosurface of the Q-criteria showed the reduction of secondary flow structures and the shape factor at the trailing edge of the midspan section was always lower than 2.2, confirming a reattachment of the flow. Furthermore, a conventional macro actuator found in the literature was also modelled and its actuation effect was compared to the one of the micro plasma actuator.In conclusion, the analysis of the actuation cost underlined that the adoption of micro actuation allowed reaching a higher gain when operating at lower voltage and same frequency.

Influence of Aiding Buoyancy on the Suppression of Flow Separation for Power-Law Fluids Around a Circular Object

Numerical computations are performed to analyze the influence of aiding thermal buoyancy on the phenomenon of suppression of flow separation in power-law fluids around a circular object. The idea has been borrowed from some recent similar works in Newtonian fluids. Owing to the contradictory behavior of shear-thinning and shear-thickening fluids in regard to the separation mechanism, we intend to understand the role of superimposed thermal buoyancy on the suppression phenomena in non-Newtonian power-law fluids, for which a range of power-law indices (0.4 to 1.8) is considered. The Reynolds numbers are kept intentionally low, within 10 to 40, such that the isothermal flow remains steady and separated without imposition of thermal buoyancy. The buoyancy causes a delay in the separation, thereby affecting the suppression phenomena. We determine the critical heating parameter (Richardson number) for the complete suppression of the flow separation and from there we construct a bifurcation diagram to show the typical flow regime evolved due to the complex interplay between the aiding thermal buoyancy and fluid rheology. The Richardson number in the simulation lies in the range 0 to 0.35, keeping the Prandtl number fixed at 50. The heat transfer rates from the object are also obtained and important inferences are drawn in regard to the inhibition/augmentation of heat transfer due to fluid rheology.

波型マイクロプラズマアクチュエータによる剥離制御

波型マイクロプラズマアクチュエータによる剥離制御

NUMERICAL STUDY OF CONTROL OF FLOW SEPARATION OVER A RAMP WITH NANOSECOND PLASMA ACTUATOR

The nanosecond plasma discharge actuator driven by high voltage pulse with typical rise and decay time of several to tens of nanoseconds is emerging as a promising active flow control means in recent years and is being studied intensively. The characterization study reveals that the discharge induced shock wave propagates through ambient air and introduces highly transient perturbation to the flow. On the other hand, the residual heat remaining in the discharge volume may trigger the instability of external flow. In this study, this type of actuator is used to suppress flow separation over a ramp model. Numerical simulation is carried out to investigate the interaction of the discharge induced disturbance with the external flow. It is found that the flow separation region over the ramp can be reduced significantly. Our work may provide some insights into the understanding of the control mechanism of nanosecond pulse actuator.

Optimization of Printed Circuit Heat Exchanger Using Exergy Analysis

A printed circuit heat exchanger (PCHE) with zigzag flow channels in a double-faced configuration was optimized to enhance its thermal–hydraulic performance. Using exergy analysis, the objective function was defined as the net exergy gain of the system considering the exergy gain by heat transfer and exergy loss due to friction in the channels. A Reynolds-averaged Navier–Stokes (RANS) analysis and surrogate modeling techniques were used for the optimization. Three geometric variables were selected as the design variables. The objective function was calculated at each design point through RANS analysis in order to construct a response surface surrogate model. Through the optimization, both the thermal and hydraulic performances of the PCHE were improved with respect to the reference geometry by suppressing flow separation in the channels.