Slot Blowing Research Articles

Wind energy generation by forced vortex shedding

Oscillating wind energy conversion systems that rely on vortex-induced vibrations (VIV) suffer from low energetic efficiency, but active flow control can produce significant improvements. This research investigates wind energy generation resulting from alternating slot blowing (ASB) on a circular cylinder, experimentally and theoretically. On the basis of measured peak lift coefficients, a low momentum coefficient (<0.04) excitation regime and a high momentum coefficient (>0.04) forcing regime were identified. In the excitation regime, a clear amplitude peak was observed close to the nominal natural vortex shedding frequency. In the forcing regime, separation or circulation control time-scales resulted in mild peaks at approximately half of the natural shedding frequency. In all regimes, the natural vortex shedding amplitudes were exceeded; in particular, by up to a factor of three in the forcing regime. Using a mathematical simulation model, it was shown that an ASB system produces 3.8 times higher net peak power coeffcients − comparable to small wind turbines − with a 7.3 times greater bandwidth than a conventional VIV energy harvester. Economic benefits can potentially be realized by significantly upscaling the system and adding a second degree-of-freedom. Future research will focus on improved slot design, supercritical flow excitation and forcing, and non-linear mathematical modelling of the dynamical system.

Research on Drag Reduction and Heat Prevention of Hypersonic Vehicle Combined Model with Reverse Jet and Slot Blowing

A novel strategy combining reverse jet with slotted blowing is proposed in this paper, and two-dimensional (RANS) is used to numerically investigate the flow characteristics of a reverse jet. The numerical results show the jet from the aircraft head ejects backward, pushing the shock away and reducing aerodynamic heating and shock wave resistance. The jet from the middle slot ejects backward and the effective solution reduces the viscous resistance of the fuselage. This new strategy not only further reduces the total resistance but also provides good thermal protection for the aircraft. Compared with the aircraft model, the stagnation point temperature and resistance are reduced by 25.0% and 28.0%, respectively, with the addition of multiple jets.

Numerical investigations of the slot blowing technique on the hypersonic vehicle for drag reduction

In this paper, the slot blowing flow-control technique is numerically studied for the hypersonic vehicle, aiming for skin friction drag reduction. Firstly, the influence patterns of the slot blowing parameters on friction drag reduction are analyzed based on a flat-plate boundary layer flow. It is concluded that there is an optimal blowing Mach number to optimize the drag reduction effect with other parameters unchanged, while as the increase of blowing pressure ratio, the skin friction drag reduction effect first increases and then remains almost invariant. On this basis, a slot blowing methodology is designed for a typical lifting-body hypersonic vehicle with a 10 mm slot installed on the middle of the windward side. Under the typical freestream condition of Mach 15 and 60 km altitude, numerical results indicate that the slot blowing with the blowing maximum Mach number 2 and blowing pressure ratio 1 could achieve a remarkable control effect of increasing the lift-drag ratio by about 10.55%. After that, the parametric study of slot blowing for the lifting-body vehicle reveals that when the slot height is increased from 10 mm to 20 mm, the lift-to-drag ratio rises slightly after paying the price of doubling the blowing mass flow rate, which indicates that increasing the slot height does not obtain a more efficient result. As for the effect of the slot position along the streamwise, moving the slot position forward could receive a higher total drag reduction rate with less blowing mass flow rate while also leads to a slight decrease of lift. Thus, the practical application of the slot blowing technique needs to consider the variations of the drag and lift comprehensively.

High-Torque Oscillating Wind Energy Generator

This research investigates a novel concept in small-scale wind-energy generation. Instead of rotating turbine blades, the present concept produces reciprocating motion that is achieved by alternately activating boundary layer control (slot blowing) on opposite sides of a circular cylinder. This is commonly called the Coandă-effect, and here we deploy it periodically on a spring-stabilized vertical cylinder, to produce reciprocating motion. This Co andă-based Re ciprocating (CoRe) system converts wind energy to mechanical work. Compared to wind turbines and windmills, the CoRe system generates higher torque and is therefore well suited to directly driving positive displacement pumps for drip-irrigation or reverse osmosis water desalination. This new approach necessitates the investment of energy in order to extract energy from the wind and therefore a major challenge is to maximize the output/input ratio in order to render the concept viable. Previous preliminary wind tunnel tests conducted on the CoRe system produced gross and net power coefficients of 10% and 3% respectively. However, the blowing slot design has a decisive impact on output power. On the basis of data presented in the literature and our computational analysis, the CoRe system produces significantly greater torque than all types of wind turbines with net power coefficients over 10%.

Postponing the Onset and Alleviating the Load of Transonic Buffet by Using Steady and Periodic Tangential Slot Blowing

Transonic buffet not only influences the structural integrity, handling quality and ride comfort, but also limits the flight envelope of transporters and airliners. To delay buffet onset and alleviate the buffet load, the effects of both steady and periodic tangential slot blowing are investigated. The results show that steady tangential blowing on the airfoil upper surface can postpone the buffet onset margin and evidently increase the lift coefficient at incidence angles near and above the buffet onset case of the clean airfoil. Under buffeting conditions of the clean airfoil, unsteady aerodynamic loads can be greatly suppressed by both steady and periodic blowing. The control effort is depicted as reduced wedge effect and weakened dynamic effect. The buffet mechanism includes (a) the feedback loop between the Kutta wave and the separation bubble under the shock foot, and (b) the interaction between the shear layer shed by the shockwave and Kutta waves. Under blowing conditions, the upstream creeping Kutta waves are prevented, and the intensity of the shear layer shed by the shockwave into separated flows is evidently reduced. Parametric studies show that the control effect is reduced as the blowing slot moves downstream, and steady blowing at 41% x/c is the most favorable control case.

Effect of Leading-Edge Slot Blowing on a Vertical Axis Wind Turbine

or above the blade stall angle only. The results show significant potential for flow control by jet blowing using all methods and approaches. Single-sided blowing resulted in a 59% increase net energy yield, while double-sided blowing produced improvements in excess of 150%, even when secondary system losses were factored into the estimates.

Kelvin-Helmholtz Instability Due to Slot Blowing in Laminar Boundary Layers

B LOWING and suction are widely used to control wall bounded flows. In the case of turbulent boundary layers, the effects of both actuation methods have been thoroughly studied [1–3]. The same applies to the use of suction in laminar boundary layers. In contrast, systematic investigations on the effects of local blowing in initially laminar boundary layers are scarcely found in the literature. Information is especially lacking for studies covering a significant range of blowing rates [4]. In addition, one must bear in mind that laminar boundary layers are very prone to separation via application of concentrated blowing. Further increases of the blowing amplitude have been reported to produce unsteady motion near the dividing streamline. It has been suggested that such event is a consequence of Kelvin–Helmholtz instability [4], but convincing experimental evidence has not been provided. Given the role of unexcited and excited separated shear layers in the context of flow control [5], a survey with the specific aim of characterizing the aforementioned unsteadiness as a function of blowing intensity seemsmost valuable. The application of low-Reynolds-number boundary-layer control by excitation is envisaged for micro aerial vehicles, which bring new challenges relating to flight aerodynamics. Both for laminar and turbulent boundary layers in twodimensional configurations, the effects of suction/blowing have generally been assessed by a measure of the local suction/blowing rate [1–4] as follows:

Forebody Slot Blowing on Vortex Breakdown and Load Over a Delta Wing

the observed behavior, no concrete evidence was given. In this paper, we address this issue and further evaluate the effects of symmetric and differential blowing on the aerodynamic loads. Flow visualization and force measurement were carried out in a water tunnel at a Reynolds number of 8:5 10 4 . Our study shows that the differing effect of differential blowing can be traced to a combination of forebody slot blowing itself and the interaction of the vortices from both sides resulting in a side-slip-like effect. Moreover, symmetrical forebody slot blowing, apart from producing a significant delay in the formation of vortex breakdown, increases the lift by more than 5%. Differential blowing can be used to manipulate the vortex breakdown position and change the roll moment of the wing, which suggests that the method can be a potential means for roll control.

Control of Vortex Breakdown over a Delta Wing Using Forebody Spanwise Slot Blowing

In this paper we study the effectiveness of forebody slot blowing to control vortex breakdown over a generic delta wing-body configuration. The motivation is to exploit the benefits of both slot blowing and canards to control vortex breakdown over a delta wing. Parameters investigated include slot length, slot width, single and double-sided blowing, and the Reynolds number. Time-averaged flow images show that Reynolds number has little effect on vortex breakdown location, at least for the range of conditions studied here, and a single-sided blowing has favorable effects on the blowing side and unfavorable effects on the nonblowing side. It is postulated that when fluid is discharged from the slot at the forebody in the spanwise direction, it produces a vortex sheet that interacts with the freestream and is rolled up to form a trailing vortex further downstream. The rolled-up vortex sheet produces a downwash effect on the blowing side and a sideslip effect on the opposite side. The downwash modifies the flowfield around the leading edge of the wing, causing a delay in vortex breakdown. To compensate for the unfavorable effects of blowing on the opposite side, a much higher blowing momentum (more than two times the single-sided slot case) is needed to delay vortex breakdown on both sides of the wing when double-sided blowing is employed. Our results also show that for a given Reynolds number and angle of attack, increasing the blowing momentum leads to a substantial delay in the vortex breakdown position. In addition, for the same blowing momentum coefficient, varying the slot width has little effect on the breakdown location, whereas increasing the slot length has favorable effect, particularly at lower angles of attack.

Computational investigation of slot blowing for fuselage forebody flow control

This paper presents a computational investigation of a tangential slot blowing concept for generating lateral control forces on an aircraft fuselage forebody. The effects of varying both the jet width and jet exit velocity for a fixed location slot are analyzed. This work is aimed at aiding researchers in designing future experimental and computational models of tangential slot blowing. The primary influence on the resulting side force of the forebody is seen to be the jet mass flow rate. This influence is sensitive to different combinations of slot widths and jet velocities over the range of variables considered. Both an actuator plane and an overset grid technique are used to model the tangential slot. The overset method successfully resolves the details of the actual slot geometry, extending the generality of the numerical method. The actuator plane concept predicts side forces similar to those produced by resolving the actual slot geometry.

Effects of leading-edge lateral blowing on delta wing aerodynamics

An experimental and computational study was carried out to investigate the effects of lateral blowing on a delta wing at low to moderate angles of attack. Motivation for the present study is to use lateral blowing to increase lift and to provide roll control for a high-speed configuration during takeoff and landing approaches. For this purpose, lateral blowing was applied through blowing slots along the leading edge of a delta wing symmetrically to increase lift and asymmetrically to induce rolling moments. Computed solutions of the Navier-Stokes equations were obtained for the same wing geometry. Computations successfully captured the qualitative trends observed in the experiments. The physics of the blowing scheme, the underlying mechanisms of the control reversal phenomenon, and the higher efficiency of partial slot blowing compared with full slot blowing were determined by both examining the experimental and computational results.

An experimental investigation into yaw control at high alpha on a chined forebody using slot blowing

AbstractResults are presented from low-speed windtunnel tests on an isolated generic chined forebody with pneumatic methods of yaw control at high alpha. Measurements of forces and moments were made on the forebody at angles of attack in the range 0° ≤ ɑ ≤ 60°. Slot blowing through the chine edge at various angles and upper surface tangential blowing have been compared, and optimum configurations have been identified. The effects of forebody cross-sectional shape have also been investigated and have been found to be highly critical in determining blowing effectiveness. Limited flowfield visualisation is also presented in order to gain some understanding of the flow field characteristics.

Experimental investigation of tangential slot blowing on a generic chined forebody

An experimental investigation of tangential slot blowing was performed on a generic chined forebody. Lowspeed measurements of lateral forces and moments were made on the forebody at high angles of attack (0 deg < a < 45 deg). Tangential slot blowing was investigated as a means of forebody flow control to generate side force and yawing moment levels capable of overcoming flow asymmetries. The effects of jet mass flow ratio and jet longitudinal position were studied. Limited flowfield visualization was also conducted in order to gain a better understanding of the flowfield characteristics. Tangential slot blowing was found to overcome the natural flow asymmetry of the forebody throughout the angle-of-attack range studied.

Forebody flow control on a full-scale F/A-18 aircraft

A full-scale F/A-18 was tested in the 80- by 120-Foot Wind Tunnel at NASA Ames Research Center to measure the effectiveness of pneumatic forebody vortex control devices. By altering the forebody vortex flow, yaw control can be maintained to angles of attack greater than 50 deg. Two forebody vortex control devices were tested: a discrete circular jet and a tangential slot. The tests were conducted for angles of attack between 25 and 50 deg, and angles of sideslip from 0 to +/- 15 deg. The Reynolds number based on wing mean aerodynamic chord ranged from 4.5 x 10 exp 6 to 12.0 x 10 exp 6. The time-averaged side forces and yawing moments, along with both time-averaged and time-dependent pressures on the forebody of the aircraft are presented here for various configurations. Of particular interest was the results that the tangential slot blowing had a greater effect on the yawing moment than the discrete circular jet. Additionally, it was found that blowing very close to the radome apex was not as effective as blowing slightly farther aft on the radome, and that a 16-inch slot was more effective than either an 8- or 48-inch long slot.

Control of low-speed turbulent separated flow using jet vortex generators

A parametric study has been performed with jet vortex generators to determine their effectiveness in controlling flow separation associated with low-speed turbulent flow over a two-dimensional rearward-facing ramp. Results indicate that flow-separation control can be accomplished, with the level of control achieved being a function of jet speed, jet orientation (with respect to the free-stream direction), and jet location (distance from the separation region in the free-stream direction). Compared to slot blowing, jet vortex generators can provide an equivalent level of flow control over a larger spanwise region (for constant jet flow area and speed).

Oblique slot blowing into a supersonic laminar boundary layer

The response of a supersonic laminar boundary layer is considered when fluid is blown obliquely into it from a slot in the boundary. The velocity and length scales associated with the slot blowing are chosen in such a way that the triple-deck theory of Stewartson (9) may be exploited. It is shown that, depending upon the blowing angle, there can be either a compressive or expansive interaction between the boundary layer and free stream upstream from the slot.

Computational investigation of a pneumatic forebody flow control concept

The effectiveness of a tangential slot blowing concept for generating lateral control forces on an aircraft forebody is analyzed using computational fluid dynamics. The flow about a fighter forebody is computed using a multiple-zone, thin-layer Navier-Stokes code. Tangential slot blowing is modeled by the use of an actuator plane. The effects of slot location and slot length on the efficiency of the system are analyzed. Results of the study indicate that placement of the slot near the nose of the aircraft greatly enhances the efficiency of the system, while the length and circumferential location of the slot are of secondary importance. Efficiency is defined by the amount of side force or yawing moment obtained per unit blowing coefficient. The effect of sideslip on the system is also analyzed. The system is able to generate incremental changes in forces and moments in flows with sideslip angles up to 10 deg comparable to those obtained at zero sideslip. These results are used to determine a baseline configuration for an experimental study of the tangential slot blowing concept.

Aerodynamic Aspects of Boundary Layer Control for High Lift at Low Speeds

Summary:The usefulness of boundary-layer control (B.L.C.) at the knee of a trailing-edge flap, over the wing nose close to the leading-edge or at the knee of a leading-edge flap is first noted. Various methods of providing B.L.C. are outlined, comprising slot blowing, slot suction, area suction, inclined air-jets, and specially-designed aerofoil shapes. The aerodynamic aspects of slot blowing over trailing-edge flaps and the wing nose are then examined in detail and both slot suction and area suction are also considered. The associated practical design features required for good performance are discussed and some flight-handling implications are mentioned.