Jet Momentum Coefficient Research Articles

Active Circulation Control on the Blunt Trailing Edge Wind Turbine Airfoil

Active circulation control on a thick blunt trailing edge wind turbine airfoil is proposed and numerically investigated by solving Reynolds-averaged Navier–Stokes equations along with Spalart–Allmaras one-equation turbulence model. The code is validated by comparing results with experiment based on the grid independency study. The circulation control greatly enhances the aerodynamic characteristics of the flatback airfoil by increasing the lift and simultaneously reducing the drag while using external energy. The drag can be reduced even to be negative due to combined contributions from the well-controlled wake separation and the jet reaction force. The two typical control regimes, namely, separation control and super-circulation control, are observed and analyzed. The enhancement in lift for both regimes can be further improved by increasing jet momentum coefficient or decreasing slot height, but at cost of higher external energy. Investigation shows that a great amount of net gain of energy can be obtained by implementing circulation control on the blunt trailing edge airfoil, and the circulation control with a relatively large slot height has a better control efficiency. It is demonstrated that the present proposed circulation control method is effective, efficient, and thus promising for the flow control in the blunt trailing edge wind turbine application.

Parametric analyses on dynamic stall control of rotor airfoil via synthetic jet

The effects of synthetic jet control on unsteady dynamic stall over rotor airfoil are investigated numerically. A moving-embedded grid method and an Unsteady Reynolds Averaged Navier-Stokes (URANS) solver coupled with k-ω Shear Stress Transport (SST) turbulence model are established for predicting the complex flowfields of oscillatory airfoil under jet control. Additionally, a velocity boundary condition modeled by sinusoidal function has been developed to fulfill the perturbation effect of periodic jet. The validity of present CFD method is evaluated by comparisons of the calculated results of baseline dynamic stall case for rotor airfoil and jet control case for VR-7B airfoil with experimental data. Then, parametric analyses are conducted emphatically for an OA212 rotor airfoil to investigate the effects of jet control parameters (jet location, dimensionless frequency, momentum coefficient, jet angle, jet type and dual-jet) on dynamic stall characteristics of rotor airfoil. It is demonstrated by the calculated results that efficiency of jet control could be improved with specific momentum coefficient and jet angle when the jet is located near separation point of rotor airfoil. Furthermore, the dual-jet could improve control efficiency more obviously on dynamic stall of rotor airfoil with respect to the unique jet, and the influence laws of dual-jet’s angles and momentum coefficients on control effects are similar to those of the unique jet. Finally, unsteady aerodynamic characteristics of rotor via synthetic jet which is located on the upper surface of rotor blade in forward flight are calculated, and asa result, the aerodynamic characteristics of rotor are improved compared with the baseline. The results indicate that synthetic jet has the capability in improving aerodynamic characteristics of rotor.

Experimental Validation of a Pulse-Modulated Flow-Control Actuator for Turbomachinery

An experimental investigation was undertaken to determine the effectiveness of a new design for a pulse-width-modulated actuator for flow control in turbomachinery. This new actuator was operated at angular velocities typical for a low-pressure compressor and was used to control the flow on an isolated airfoil. Jet-actuation parameters included the jet momentum coefficient, dimensionless actuation frequency, and operating pressure. Hot-wire anemometry was used to determine the frequency response of the actuator with a pseudoblade representative of a compressor blade, as well as determine the achievable jet momentum coefficients. Flow-control experiments in a low-speed wind tunnel with an instrumented NACA 0015 airfoil were performed to evaluate the ability of the actuator to control flow separation. The experimental data showed that the variation of the jet momentum coefficient with operating pressure and actuation frequency was marginal; therefore, the pulse modulator can operate effectively over a wide range of compressor wheel speeds. The flow-control data showed that flow separation was delayed for the entire range of jet momentum coefficients.

First principle analysis of Coandă Micro Air Vehicle aerodynamic forces for preliminary sizing

PurposeThe purpose of this paper is to reformulate the governing equations incorporating major variables and parameters for the design a Micro Air Vehicle (MAV), to meet the desired mission and design requirements.Design/methodology/approachMathematical models for various spherical and cylindrical Coandă MAV configurations were rederived from first principles, and the performance measures were defined. To verify the theoretical prediction to a certain extent, a computational fluid dynamic (CFD) simulation for a Coandă MAV generic models was performed.FindingsThe major variables and parameters of Coandă MAV have been formulated into practical guidelines, which relate the lift (or thrust) produced for certain input variables, particularly the Coandă MAV jet momentum coefficient. The influences of the geometrical parameters are elaborated.Research limitations/implicationsThe present analysis on Coandă jet-configured MAV is focused on the lift generation due to the Coandă jet effect through a meticulous analysis. The effects of viscosity, the Coandă jet thickness, the radius of curvature of the surface and the stability of Coandă jet are not considered and will be the subject of the following work.Practical implicationsThe results obtained can be used for sizing in the preliminary design of Coandă MAVs.Originality/valuePhysical and mathematical models were developed which can describe the physical phenomena of the flow field near the Coandă MAV surfaces influenced by Coandă jet sheets and for obtaining a relationship between relevant variables and parameters to the lift of practical interest.

Active Flow Control of a Low Reynolds Number S809 Wind Turbine Blade Model under Dynamic Pitching Maneuvers

A low Reynolds number wind turbine blade model based on the S809 airfoil was tested in a subsonic wind tunnel to study the structural vibration of the blade under dynamic pitching maneuvers. Piezoelectric-based synthetic jet actuators were embedded inside the blade and activated with a synthetic jet momentum coefficient, Cμ of 2.30 × 10-3. Structural vibration was quantified for a range of unsteady angles undergoing “pitch up and down” and “sinusoidal pitch” maneuvers at a Reynolds number of 5.28 × 104. The blade tip deflection amplitude and frequency were acquired utilizing a pair of strain gauges mounted at the root of the model. Using active flow control vibration reduction was more effective during the pitch up portion of the blade motion cycle compared to the pitch down portion. This effect is due to dynamic stall, where a leading edge vortex is shed during the pitch up motion and contributes to higher lift compared to static angles of attack and lower lift when the blade is pitched down. Dynamic stall was measured with phase-locked stereoscopic particle image velocimetry (SPIV), where global mean flow measurements reveal a shift in location and reduction in the size of a recirculating flow structure near the suction surface of the blade during the pitch up motion compared to the pitch down.

Performance analysis of a horn-type rudder implementing the Coanda effect

The Coanda effect is the phenomenon of a fluid jet to stay attached to a curved surface; when a jet stream is applied tangentially to a convex surface, lift force is generated by increase in the circulation. The Coanda effect has great potential to be applied practically applied to marine hydrodynamics where various lifting surfaces are being widely used to control the behavior of ships and offshore structures. In the present study, Numerical simulations and corresponding experiments were performed to ascertain the applicability of the Coanda effect to a horn-type rudder. It was found that the Coanda jet increases the lift coefficient of the rudder by as much as 52% at a jet momentum coefficient of 0.1 and rudder angle of 10°.

Performance and Energy Expenditure of Coflow Jet Airfoil with Variation of Mach Number

This paper conducts a numerical and experimental investigation of a coflow jet airfoil to quantify lift enhancement, drag reduction, and energy expenditure at a Mach number range from 0.03 to 0.4. The jet momentum coefficient is held constant at 0.08, and the angle of attack varies from 0 to 30 deg. The two-dimensional flow is simulated using a Reynolds–averaged Navier–Stokes solver with a fifth-order-weighted essentially non-oscillatory scheme for the inviscid flux and a fourth-order central differencing for the viscous terms. Turbulence is simulated with the one equation Spalart–Allmaras model. The predicted coflow jet pumping power has an excellent agreement with the experiment. At a constant Mach number, the power coefficient is decreased when the angle of attack is increased from 0 to 15 deg. When the Mach number is increased from 0.03 to 0.3, the suction effect behind the airfoil leading edge is further augmented due to the compressibility effect. This results in an increased maximum lift coefficient and reduced power coefficient at the higher Mach number because of the lower jet-injection pumping pressure required. At Mach 0.4, the lift coefficient is further improved. However as the angle of attack is increased, a shock wave interrupts the jet and triggers the boundary layer separation with increased drag and power coefficient. A corrected aerodynamic efficiency that includes the coflow-jet pumping power is introduced. Because of the high lift coefficient and low coflow-jet power required, the coflow-jet airfoil in this study achieves a comparable peak aerodynamic efficiency to the baseline airfoil, but the lift coefficient at peak efficiency is substantially increased by 120%. This study indicates that the coflow-jet airfoil is not only able to achieve very high maximum lift coefficient, but also able to improve cruise performance at low angle of attack when the flow is benign.

Dynamic Stall Control on the Wind Turbine Airfoil via a Co-Flow Jet

Dynamic stall control of a S809 airfoil is numerically investigated by implementing a co-flow jet (CFJ). The numerical methods of the solver are validated by comparing results with the baseline experiment as well as a NACA 6415-based CFJ experiment, showing good agreement in both static and dynamic characteristics. The CFJ airfoil with inactive jet is simulated to study the impact that the jet channel imposes upon the dynamic characteristics. It is shown that the presence of a long jet channel could cause a negative effect of decreasing lift and increasing drag, leading to fluctuating extreme loads in terms of drag and moment. The main focus of the present research is the investigation of the dynamic characteristics of the CFJ airfoil with three different jet momentum coefficients, which are compared with the baseline, giving encouraging results. Dynamic stall can be greatly suppressed, showing a very good control performance of significantly increased lift and reduced drag and moment. Analysis of the amplitude of variation in the aerodynamic coefficients indicates that the fluctuating extreme aerodynamic loads are significantly alleviated, which is conducive to structural reliability and improved life cycle. The energy consumption analysis shows that the CFJ concept is applicable and economical in controlling dynamic stall.

Microjet flow control in an ultra-compact serpentine inlet

Microjets are used to control the internal flow to improve the performance of an ultra-compact serpentine inlet. A highly offset serpentine inlet with length-to-diameter ratio of 2.5 is designed and static tests are conducted to analyze the internal flow characteristics in terms of pressure recovery, distortion and flow separation. Flow separation is encountered in the second S-turn, and two strong counter-rotating vortices are formed at the aerodynamic interface plane (AIP) face which occupy a quarter of the outlet area and result in severe pressure loss and distortion. A flow control model employing a row of microjets in the second turn is designed based on the internal flow characteristics and simplified CFD simulations. Flow control tests are conducted to verify the control effectiveness and understand the characteristics as a function of inlet throat Mach number, injection mass flow ratio, jet Mach number and momentum coefficient. At all test Mach numbers, microjet flow control (MFC) effectively improves the recovery and reduces the distortion intensity. Between inlet throat Mach number 0.2 and 0.5, the strong flow separation in the second S-turn is suppressed at an optimum jet flow ratio of less than 0.65%, resulting in a maximum improvement of 4% for pressure recovery coefficient and a maximum decrease of 75% for circumferential distortion intensity at cruise. However, in order to suppress the flow separation, the injection rate should retain in an effective range. When the injection rate is higher than this range, the flow is degraded and the distortion contour is changed from 90° circumferential distortion pattern to 180° circumferential distortion pattern. Detailed data analysis shows that this optimum flow ratio depends on inlet throat Mach number and the momentum coefficient affects the control effectiveness in a dual stepping manner.

Fluidic Control of a 155 Millimeter Spin-Stabilized Projectile Using Coanda Effect

The present paper aims at investigating numerically the control of a 155 mm spin-stabilized projectile using the Coanda effect. The flight Mach number ranges from to . A tangentially blowing jet exits over a convex surface at the base of the projectile and creates flow asymmetry, which generates significant aerodynamic loads. First, Reynolds-averaged Navier–Stokes simulations are performed without projectile rotation. The jet extends 18 deg azimuthally, and numerous continuous blowing conditions are assessed. The variation of the normal force coefficient with respect to the jet momentum coefficient is presented for different freestream Mach numbers. The efficiency loss of the Coanda effect with supersonic jets is specifically discussed. Additionally, comparisons between overexpanded, adapted, and underexpanded supersonic jets are presented. In the second part, unsteady Reynolds-averaged Navier–Stokes simulations, accounting for a 400 Hz projectile rotation, are performed. Both continuous and periodic blowing are assessed. A pulsed jet is used in phase with the rotation to always act in a 90 deg sector centered in the lift plane. The resulting normal force benefits are more important than in the nonspinning case, and spin-averaged values lead to a nonzero normal force increment. Then, four pulsed jets equally distributed along the circumference of the projectile are simulated to further improve the control efficiency. The jet separation process is also investigated to determine the influence of the jet on the base pressure distribution.

Numerical study of the S809 airfoil aerodynamic performance using a co-flow jet active control concept

A Co-Flow Jet (CFJ) active flow control concept is implemented on the S809 airfoil and numerically investigated by using an in-house code based on Reynolds-averaged Navier-Stokes equations and the Spalart-Allmaras turbulence model, aiming to systematically study the jet effect on the airfoil aerodynamic performance. The solver is validated by comparing the computed results with the baseline experiment measurement. The calculated aerodynamic force and moment coefficients agree fairly well with the experimental data, demonstrating the present solver can predict the attached as well as separated flows around the S809 airfoil with an acceptable precision. The CFJ jet-off geometry of S809 airfoil is simulated to study the jet channel effect on the baseline aerodynamic characteristic, showing that the jet channel could reduce the lift, increase the drag and cause an earlier abrupt stall at angle of attack (AoA) 16.24°. The CFJ jet-on geometry is elaborately studied at three jet momentum coefficient levels, showing that co-flow jet has a significantly positive effect in increasing lift, stall margin, and drag reduction. For the cases with jet momentum coefficients 0.12 and 0.18, the total drag even becomes negative. It is found that the drag from pressure and shear stress of CFJ airfoil is larger than that of baseline, and it is the negative mass-flow-produced drag that significantly reduces the total drag to be below zero. For cases in which AoA are below a critical value (e.g., 20.15° for the case with jet momentum coefficient 0.18), the power required to drive the jet flow decreases when AoA increases, demonstrating that the CFJ concept has an attractive advantage of minimum energy consuming. The present CFJ concept is proved to be a promising and effective active flow control method in the wind turbine application.

Numerical Simulation of the Effect of a Co-flow Jet on the Wind Turbine Airfoil Aerodynamic Characteristics

A co-flow jet active flow control method is applied to the wind turbine S809 airfoil, and numerically studied by solving Renolds-averaged Navier-Stokes equations. The co-flow jet airfoil is designed with an injection slot near the leading edge and a suction slot near the trailing edge on the airfoil suction surface. A high-speed jet is injected tangentially into the main flow, and in the meanwhile the same amount of mass flow is drawn into the suction slot. The solver is validated by comparing the computed results with the baseline experiment measurement. The flow around the co-flow jet airfoil is simulated at a jet momentum coefficient of 0.12, showing that the co-flow jet has a significantly positive effect in increasing lift, stall margin, and drag reduction. The present co-flow jet concept is proved to be an effective and promising active flow control method in the future wind turbine application.

Numerical Investigations of Synthetic Jet Control for TAU0015 Airfoil

The aerodynamic performance of TAU0015 airfoil was investigated with synthetic jet control method. The simplified mathematical model of the active flow control was established with unsteady velocity boundary condition at the specific location of airfoil surface. The aerodynamic performance was simulated with synthetic jet and the efficiency of jet momentum coefficient was conducted. The result shows that the flow control model could perform the minor jet flow characteristics and higher jet momentum coefficient result better control efficiency.

Design and Analysis of a Droop Nose for Coanda Flap Applications

The present study describes the fundamentals of droop nose design for improving the aerodynamics of airfoils with active high-lift using an internally blown Coanda-type flap. The main objectives are to increase the stall angle of attack and reduce the power required by the high-lift system. A two-dimensional sensitivity analysis explores the effects of varying airfoil camber and thickness in the first 20% of the chord. The resulting droop nose configuration improves the maximum lift coefficient by about 20% and increases the stall angle of attack by around 10–15 deg. A target lift coefficient of about 4.7 is reached with 28% less jet momentum coefficient, compared to the clean nose. As the modified leading-edge geometry presents different stall mechanisms, the aerodynamic response to variations of jet momentum is also different. In particular, for a jet momentum coefficient above 0.035, the stall angle of attack increases with jet momentum, in contrast with the behavior observed with the clean nose.

Closed-Loop Active Flow Control of a Three-Dimensional Turret Wake

An experimental study has been performed to investigate the wake of a three-dimensional turret with aspect ratio of 1.17 at a diameter-based Reynolds number of . Stereoscopic particle image velocimetry was used to evaluate the spatial structure of the wake, and dynamic surface-pressure measurements were used to study the time-dependent behavior. The uncontrolled wake was seen to be strongly three-dimensional with two main structures on opposing sides of the center plane. Cross correlations from the surface pressure indicated that opposite sides of the wake had nearly zero correlation in time. With the application of active flow control, both the spatial structure and time-dependent behavior of the wake were significantly altered. The downwash velocity along the center plane was seen to increase significantly, causing the wake to spread more rapidly near the support plate, and the spectra from the pressure sensors contained multiple peaks at different frequencies throughout the wake that were not seen in the baseline case. Closed-loop control achieved similar control authority as steady open-loop forcing, but with a 45% reduction in the mean jet-momentum coefficient.

Experimental study on synthetic jet array for aerodynamic drag reduction of a simplified car

This paper describes an experimental parametric study of synthetic jet array actuation to reduce the aerodynamic drag of a threedimensional simplified car. By using two configurations of an Ahmed body with 25° and 35° slant angles, we performed wind tunnel tests under different conditions of synthetic jet array. Several parameters, namely, jet location, jet direction, jet momentum coefficient, jet driving frequency, and number and position of activated jets within the actuator array, were considered. The total aerodynamic drag coefficients were compared, and the rear wake flows were studied by using the data obtained from rear surface pressure distribution and flow visualization tests. Results of the parametric study show that the aerodynamic drag exhibits different behavior depending on the location of the jet for each slanted model. Jet direction, jet momentum coefficient, and jet driving frequency affect only the amount of change in the aerodynamic drag. The distribution of the activated jets also affects jet efficiency.

Modification of a circular cylinder wake with synthetic jet: Vortex shedding modes and mechanism

The wake behind a circular cylinder is modified by a synthetic jet positioned at the front stagnation point. The flow field is measured with a time-resolved particle image velocimetry (PIV) system, and the proper orthogonal decomposition (POD) and λci methods are used to analyze the vortex dynamics. The synthetic jet vortex pair is induced near the exit orifice periodically and then moves upstream. The interaction between the synthetic jet and the oncoming flow gives rise to an envelope formed upstream of the circular cylinder, which acts as a virtual aerodynamic shape. It is found that the envelope can be categorized into the periodic closed envelope and the quasi-steady open envelope, leading to different shedding modes for the wake around the circular cylinder. In the present investigation, six kinds of vortex shedding modes under synthetic jet control have been classified as natural Karman vortex mode, bistable state mode I, symmetric mode, bistable state mode II, antisymmetric mode with shortened vortex formation length, vortex generation close to the rear stagnation point. The vortex dynamics analysis indicates that the wake vortex trajectory, vortex circulation, and convection velocity at the vortex core all exhibit regular variations for these typical shedding modes. The formation mechanisms for these shedding modes have been further revealed, which present some novel formation processes in comparison with the natural Karman vortex street. Moreover, the effects of the synthetic jet momentum coefficient and excitation frequency on the control are also compared, which suggests that the type of the front envelope is most important for the vortex shedding modes.

Experimental Evaluation of a Self-Contained Circulation-Control Wing

A low-speed experimental investigation detailing the design and evaluation of a self-contained circulation-control wing is presented. A moderate aspect-ratio wing (3.34) utilizing a low-Reynolds-number S8036 profile was tested at . The trailing-edge jet was produced using two internal impellers drawing air off the wing tips. The effect of trailing-edge bluntness was examined as was the jet’s quiescent characteristics. Achievable jet-momentum coefficients were low, limiting circulation control to the boundary-layer control regime. Force-balance testing indicated that the circulation-control system could achieve a moderate zero-lift angle-of-attack shift; however, the primary effect of the jet was flow control at high incidence, in which stall was delayed significantly. The blunt trailing edge (Coanda surface) caused a significant increase in the minimum drag coefficient, reducing both the range and endurance parameters.

Effect of Jet Nozzle Lip Momentum Loss on Circulation Control Airfoil Performance

*† Large-eddy simulations of flow around a circulation control airfoil (using a Coanda jet blowing over its trailing surface) are performed to investigate the influence of jet-nozzle-lip thickness on airfoil performance. The airfoil geometry is only slightly changed from our previous LES study [Nishino et al., Physics of Fluids, Vol. 22, 2010, 125105] to study three different nozzle-lip thickness cases; the geometry inside the nozzle is maintained the same. The results show that the jet profile across the nozzle exit is insensitive to the nozzle-lip thickness; however, the jet flow downstream of the nozzle exit decelerates more rapidly and thus the circulation around the airfoil decreases as the nozzle-lip thickness increases. It is subsequently shown that this effect is mostly cancelled out by adjusting the jet blowing rate in such a way that the difference of momentum loss arising from the nozzle lip is taken into account, demonstrating that the performance of a Coanda jet on a circulation control airfoil is determined not only by the jet momentum at the nozzle exit but also by the momentum loss behind the nozzle lip. These results suggest that it may be useful to define a new jet momentum coefficient that takes account of the momentum loss due to the nozzle lip, which can be roughly estimated once the velocity of the flow above the nozzle lip is known.

콴다 효과를 적용한 고정식 핀 안정기 설계를 위한 수치적 연구

Fins are widely used for roll stabilization of passenger ferries and high performance naval ships, among others. The Coanda effect is noticeable when a jet stream is applied tangentially to a curved wing surface since the jet can augment the lift by increasing the circulation. The Coanda effect has been found useful in various fields of aerodynamics and speculated to have practical applicability in marine hydrodynamics where various control surfaces are used to control motions of ships and the other offshore structures. In the present study, numerical computations have been performed to find proper jet momentum coefficients and trailing edge shapes suitable for the application of the Coanda effect to a stabilizer fin. The results show that the lift coefficient of the modified Coanda fin at the zero angle of attack identically coincides with that of the original fin at ${\alpha}\;