Flow Control Efficiency Research Articles

Flow Separation Control of Nacelle in Crosswind by Microsecond Pulsed Surface Dielectric Barrier Discharge Plasma Actuator

Flow separation under crosswind conditions seriously jeopardizes the quality of the nacelle’s flow field. In this paper, microsecond pulsed surface dielectric barrier discharge (μSDBD) is used to suppress the flow separation and reduce the crosswind distortion of the nacelle. The flow structure induced by the μSDBD is first explored by a high-speed schlieren system. The pressure waves composed of a cylindrical wave surrounding the electrodes and a flat wave at the top of the cylindrical one can be perceived, which indicates the fast gas heating produced by the μSDBD. A set of wind tunnel tests are then conducted to verify the ability of μSDBD to suppress the nacelle flow separation and study the influence laws of pulse frequency, coverage area, and the actuator layout on the flow control effects. Results show that plasma actuation can not only improve the total pressure at the exit of the nacelle but also suppress the flow distortion caused by the crosswind. The best flow control effect can be achieved at the pulse frequency of 500 Hz, with the value of sectional distortion coefficient reduced by 57.76% compared with the baseline condition. The flow control effect with the plasma actuator covering 120° of the nacelle perimeter is better than that of 60° and 180° coverage, showing the highest flow control efficiency in the 120° condition. The μSDBD can improve mixing between the boundary layer and the main flow, enhancing the ability of the boundary layer to resist the adverse pressure gradient, which is beneficial to flow separation control.

Sliding discharge plasma actuation for forebody vortex control on a slender body at high angles of attack

Two innovative kinds of sliding discharge plasma actuators based on different formation hypotheses of forebody asymmetric vortices are designed using a slender body model. Particle image velocimetry and surface pressure measurements are synchronously used to compare the control effect of antiflow and along-flow sliding discharge actuators for a forebody asymmetric vortex at high angles of attack. The experimental results show that the two kinds of sliding discharge plasma actuators can effectively change the random lateral force caused by the asymmetric vortices, and a better control effect is manifested using the along-flow sliding discharge plasma actuator, which contributes to the approximately linear proportional control of the lateral force. This finding suggests that the convective instability of the forebody vortex system in the leeward region of the slender body has a significant influence on the random lateral forces and moments. Through improving the spatial stability of forebody vortices, the flow control efficiency can be effectively improved. An optimal pulse frequency exists in each of the three actuation modes, but it may vary due to the different geometry configuration of the plasma actuator. In this regard, the research findings will potentially provide technical guidance for improving the efficiency of plasma actuators and understanding the formation mechanism of asymmetric vortices.

Shock wave/turbulence boundary layer interaction control with the secondary recirculation jet in a supersonic flow

The separation induced by the shock wave/boundary layer interaction is not beneficial for the engine operation, and it should be mitigated by the active or passive control approaches. In the current study, the flow control induced by the secondary recirculation jet has been evaluated numerically, and this approach combines the boundary layer bleed and the boundary layer suction together. At the same time, the influences of the height and length of the recirculation channel, as well as the dimensions and positions of the bleed and suction slots, on the flow control efficiency have been analyzed in detail. The obtained results show that the potions of the bleed and suction slots have a great impact on the flow control efficiency, and the separation length would decrease when the suction slot is placed on the windward side of the separation zone, as well as the peak pressure along the bottom wall. The secondary shock wave system induced by the secondary recirculation jet is beneficial for the mixing augmentation in the supersonic flow. However, the dimensions of the bleed and suction slots only have a slight impact on the flow control efficiency, and these factors can be omitted in the design process of the shock wave/boundary layer interaction model.

Investigation on large responses of the streamwise velocity with the effect of spanwise Lorentz force

The flow of a weakly conductive fluid (i.e. seawater) can be controlled by Lorentz forces generated by the suitably chosen magnetic and electric fields, which has significant effects for applications in the drag reduction and oscillatory suppression. However, the control efficiency is very low due to the application of large amplitude for Lorentz force. Therefore, the large response, induced by a small Lorentz force, is the key to enhance the flow control efficiency. In this paper, the amplification mechanism of the velocity response with the effect of Lorentz force in weakly conductive fluids is investigated, where the Lorentz force is applied on the lower wall in the channel. The analytic solutions of the velocity responses in linear stage are derived with linear stability theory, when the amplitude of Lorentz force is far less than 1. From the discussions of the analytic solutions, the mechanism is revealed on large responses which is induced by the small Lorentz force in the flow field. The results show that the flow along the spanwise direction is induced by Lorentz force, which leads to the momentum exchange of fluids in the wall-normal direction. Therefore, the large responses of velocity are generated due to the high-speed fluids transferring to the wall. Moreover, the responses depend on the parameters of Lorentz force, i.e. wave number Kz , effective penetration Δ, including proportional to amplitude A and square of Reynolds number Re2. With the increase of Kz , the response decreases monotonously. However, with the increase of Δ, the response increases first (Δ<0.4), reaches the maximum at Δ=0.4, and then decreases (Δ>0.4). Finally, the maximum response is obtained and the corresponding amplification is 480, when the two parameters are optimized, i.e. Kz=2, Δ=0.4.

Drag reduction and flow structures of wing tip sails in ground effect

The hydrodynamics and flow structures of a base wing slotted with tip sails in proximity to the ground were studied experimentally in order to investigate the flow control efficiency of wing tip sails in ground effect. The experiment was conducted in a towing tank at a Reynolds number 1.5×105. The lift and drag forces were measured by a transducer, the velocity fields of the wing tip vortices were measured using a time-resolved particle image velocimetry system (TR-PIV). The tip-sails and ground clearance were both effective in reducing the total drag, the lift coefficients of the tip-sails wings were increased as compared with that of a base wing. The lift-drag ratios of the tip-sails wings were improved obviously in a range of angles of attack from 2° to stalling angle. The tip-sails played more important role in unwinding the concentrated wing tip vortices at higher angle of attack, the intensity of the tip vortices were much weaker than that of the base wing. The development of the wing tip vortices was suppressed as well due to the inhibition of the ground, the downwash speed was reduced and the induced drag was decreased.

Active control of flow over a rounded ramp by means of single and double adjacent rectangular synthetic jet actuators

Active control of flow over a rounded ramp with Re=13,700 was studied numerically by using rectangular synthetic jet actuators. Wall-resolved large eddy simulation was utilized to accurately predict the separation and reattachment of the flow. The solution of uncontrolled case as well as controlled by a single synthetic jet in quiescent conditions were validated against available numerical or experimental data. To investigate active flow control, five actuated cases were considered: a single synthetic jet (SS), double synthetic jets with opposite-phase actuation (DS opposite-phase), double synthetic jets with in-phase actuation (DS in-phase), a wide synthetic jet (WSS) with the same momentum coefficient of the DS in-phase but with twice the opening width, and a high momentum single synthetic jet (HSS) similar to SS but with twice the momentum coefficient of DS in-phase. The aim was to compare the flow control efficiency of these cases and identify the most effective configuration. The DS opposite-phase case was identified in previous studies as being able to keep the expelled fluid inside the near-wall region. We found that among the first three cases, the DS in-phase was the most effective in reducing the length of the separated region. In order to illuminate the flow control mechanisms of each case, phase-averaged streamlines of the flow were generated. Also, a passive scalar variable was introduced into the flow to allow for tracking the expelled fluid from the synthetic jets. This passive scalar was used to evaluate and compare the flow control mechanisms in the cases of WSS and HSS. We found that HSS was the most efficient mechanism among the five configurations studied.

Technique for Enhanced Flow Control Efficiency Through Thermal Actuation

Thermal active flow control has been developed to enhance the efficiency of active flow control systems. The actuation concept is derived from gas-dynamics principles, and it is based on thermal control of the air supply. It is shown that higher air supply temperatures result in reduced mass flow rate with no degradation in active flow control performance. A computational method has been used to systematically investigate the hot air supply approach for isolated blowing actuators and fluidic oscillators. Subsequently, the thermal control concept has been computationally evaluated for airplane applications. These include enhanced control authority of a vertical tail and an airplane high-lift system, confirming the trends observed from the gas-dynamics analysis with regard to reduced actuation input as a function of supply temperature. Further confirmation of the thermal actuation concept was experimentally obtained for a bench-top actuator and a vertical-tail model in a wind-tunnel setting. The paper introduces potential approaches for system integration associated with heated supply, while highlighting the benefit of using available high-temperature sources for active flow control.

Numerical Investigation of Three-dimensional Separation Control on a High-speed Compressor Stator Vane with Tailored Synthetic Jet

Abstract A numerical study on the performance of synthetic jet for flow separation control on a high-speed compressor stator vane is performed. Four control schemes including full-span and part-span configurations are investigated at both design and off-design conditions. Results indicate that both full-span and part-span schemes could effectively delay flow separation and reduce total pressure loss for the compressor stator vane, the adaptability of the flow control under off-design conditions is also validated. Within the investigated incidence range, the full-span configuration is able to gain the most significant performance improvement, by which a maximum loss reduction of 23.8 % is obtained at i=2 deg. The part-span configuration could reorganize the vortex structures more efficiently and cut off the interaction between the ring-like vortex and the passage vortex, thus improving its performance in the corner region. In terms of flow control efficiency, the part-span configurations turn out to be more superior, where the highest control efficiency of 614.0 % is achieved at i=0 deg with the total height of the actuator being 40 %H. The flow control efficiency for all the schemes is higher than 100 % within the whole operating range, demonstrating a promising prospect for the application of synthetic jet in axial compressors.

Numerical investigation of boundary layer suction on certain highly loaded aspirated compressor at low speeds

Boundary layer suction is considered to be an available approach to restraining or even eliminating flow separation and to improve the aerodynamic performance of the compressor. In this paper, a highly loaded axial-flow aspirated compressor based on a low-reaction design concept is investigated in detail to find an appropriate flow control strategy for boundary layer suction to achieve significant performance benefit. The flow control strategy consists mainly of the arrangement of suction hole and the aspirated flow rate. The geometrical models including aspiration cannulas, stator cavity and aspiration channel are novelly applied in this research to approach a real engineering application. Complete compressor maps are predicted by a three-dimensional computational fluid dynamics simulation. The distribution of the typical aerodynamic parameters and the partial flow structures are analysed at design and off-design conditions. Three-dimensional separation near the stall point is effectively suppressed by the aspiration on both hub and shroud, and better performance is achieved by a reasonable increase of aspirated flow rate; a peak efficiency of 0.91 and a total pressure ratio of 1.055 are attained at a total aspirated flow rate of 0.024 kg/s per passage. However, sudden turning of compressor maps at low inlet flow rate occurs without the aspiration on shroud, and a noticeable deterioration in performance occurs with the decreasing of the inlet flow rate. The main reason of performance deterioration is that the flow control efficiency of aspiration is not enough for effectively suppressing three-dimensional separation near the casing corner. The difference in the aspirated flow source is owed to the distinct flow feature at various locations caused by the aspiration efficiency of suction holes. A partial auto-readjustment feature of the suction flow rate with increasing flow separation has been found. The boundary layer suction with an appropriate flow control strategy is necessary for a higher efficiency, highly loaded aspiration compressor.

Dynamic characterization of piezoelectric micro-blowers for separation flow control

A micro-blower device, made by Murata™ Manufacturing Co. and originally designed for cooling electronic microprocessors, is used in the present study as an actuator for separation flow control. The dynamical performance of the device, working in pulsed mode, is experimentally investigated to deduce its abilities and limitations regarding active flow control. Both the device design and its dynamical characterization are addressed in the paper, paying particular attention to its ability to control flow. Both hot-wire anemometry and particle image velocimetry have been employed to investigate the dynamical evolution of the pulsed jet and allow the identification of the jet organized flow structures. The phase-averaging technique is applied to distinguish the large scale structure dynamics induced during the ejection phase. An array of active devices is also used to evaluate the flow control efficiency by forcing frequency strategy in order to reduce the separation area of a back facing step flow. Results show good performance in open loop flow control and applicability of such actuator for further closed loop flow control research.

Experimental Study on Ground Effect of a Wing with Tip Sails

The experimental results of Ground Effect of a wing mounted with tip sails are introduced in the paper. The objective of the study is to simulate and evaluate the flow control efficiency of primary feathers to the wing of a pelican skimming over water surface. Compared with a NACA4412 prototype wing, the experimental results show that, for the same ground clearance, the lift coefficient of the tip-sails wing increases significantly and the stalling angle of attack decreases, the drag coefficient keeps nearly unchanged at small angle of attack (AOA) and decreases obviously at higher AOA. The tip sails increase the slope of the lift coefficient curves, the maximum lifts rise by 11.03% and 8.26% compared to those of the prototype wing for ground clearance h*=0.15 and h*=0.5, respectively. The lift-to-drag ratio is improved as well at higher AOA and lift coefficient range. The vorticity contours of tip vortices of two types of wings show that, the vortices cores move outboard due to the ground effect. The concentrated vorticity distribution of the prototype wing is scattered by the tip sails, and spread over a larger area. For the tip-sails wing, the vortices at h*=0.15 are scattered much more than the vortices at h*=0.5 and the intensity of the vortex is much lower. The dissipation of the tip vortices is accelerated due to the inhibition by the ground clearances and the spreading by the tip sails to the vortices.

Tapered-Bean Steam Chokes Revisited

This paper (SPE 144615) was accepted for presentation at the SPE Western North American Regional Meeting, Anchorage, 7–11 May 2011, and revised for publication. Original manuscript received for review 5 July 2011. Paper peer approved 31 August 2011. Summary Controlling and monitoring flow rates at continuousand cyclicsteam-injection wells are important elements of reservoir-heat management. For nearly 30 years, critical flow chokes have proven to be the most reliable and cost-effective means of controlling steam injection into heavy-oil reservoirs. Flow-control efficiency has been further improved with tapered-bore bean inserts to achieve critical flow, with only 10 to 15% pressure loss across the choke. For the past 10 years, the standard steam-choke assembly has consisted of a 1-in.-outer-diameter (OD) and 6-in.-long bean with a 6° tapered bore inserted inside a 2-in.-OD cage nipple or housing. Larger-diameter cage nipples and bean inserts have been required for steam-injection rates exceeding 500 B/D. More recently, a costcutting practice has been employed using shorter tapered beans inserted in standard choke assemblies. This paper presents the results of field tests conducted to evaluate the effectiveness of shorter tapered-bean length for controlling steam-injection rates. Transition from subcritical to critical flow and overall pressure loss for different tapered-bean lengths are presented. A modified Thornhill-Craver flow-rate equation is provided for criticaland subcritical-flow regions. Calculated and measured rates are compared, and their relative uncertainties are assessed.