Wake Rake Research Articles

Experimental investigation of mini Gurney flaps in combination with vortex generators for improved wind turbine blade performance

Abstract. This wind tunnel study investigates the aerodynamic effects of mini Gurney flaps (MGFs) and their combination with vortex generators (VGs) on the performance of airfoils and wind turbine rotor blades. VGs are installed on the suction side aiming at stall delay and increased maximum lift. MGFs are thin angle profiles that are attached at the trailing edge in order to increase lift at pre-stall operation. The implementation of both these passive flow control devices is accompanied by a certain drag penalty. The wind tunnel tests are conducted at the Hermann-Föttinger Institut of the Technische Universität Berlin based on two airfoils that are characteristic of different sections of large rotor blades. Lift and drag are determined using a force balance and a wake rake, respectively, for static angles of attack between −5 and 17∘ at a Reynolds number of 1.5 million. The impact of different MGF heights including 0.25 %, 0.5 % and 1.0 % and a VG height of 1.1 % of the chord length is tested and evaluated. Furthermore, the clean and the tripped baseline cases are considered. In the latter, leading-edge transition is forced with Zig Zag (ZZ) turbulator tape. The preferred configurations are the smallest MGF on the NACA63(3)618 and the medium-sized MGF combined with VGs on the DU97W300. Next, the experimental lift and drag polar data are imported into the software QBlade in order to design a generic rotor blade. The blade performance is simulated with and without the add-ons by means of two case studies. In the first case, the retrofit application on an existing blade mitigates the adverse effects of the ZZ tape. Stall is delayed and the aerodynamic efficiency is partly recovered leading to an improvement of the power curve. In the second case, the new design application allows for the design of a more slender blade while maintaining the rotor power. This alternative blade appears to be more resistant against the adverse effects of forced leading-edge transition.

Study of wake characteristics behind an oscillating NACA 0012 airfoil in deep dynamic stall regime

PurposeThis study aims to investigate the wake behind an oscillating airfoil at a various angle of incidence and Reynolds number in a deep dynamic stall condition.Design/methodology/approachNACA 0012 airfoil is allowed to undergo harmonic pitching motion about the quarter chord axis at Reynolds numbers of 0.5 * 105, 1.17 * 105, 1.7 * 105 and 2.12 * 105, and the reduced frequency of 0.1. The experiments are conducted at a set of mean and amplitude angle of attack that covered the angle of incidence from −5° to 25°. The wake rake is placed at a distance of one chord from the trailing edge of the airfoil.FindingsThe hysteresis of the flow during the upstroke and the downstroke motion are captured. The huge growth in the velocity defect and the wake thickness beyond the angle of attack of 15° explicate the appearance of the strong unsteady effects on the wake. The results also show that at the reduced frequency of 0.1, the wake structure is of drag producing type due to the momentum deficit.Originality/valueStreamwise velocity profile and the turbulent intensity profiles are presented to show the effects of Reynolds number and angle of incidence on the wake behind the oscillating airfoil at the reduced frequency of 0.1, and in the intermediate range of Reynolds number is the novelty of the study.

Experimental Aerodynamic Comparison of Active Camber Morphing and Trailing-Edge Flaps

Unlike hinged flaps (for example, ailerons, elevators, and rudders), camber morphing devices vary camber distribution in a smooth and continuous way, resulting in aerodynamic efficiency improvements due to the absence of sharp changes in airfoil camber. One such camber morphing concept, the Fish Bone Active Camber (FishBAC) device, has shown significant aerodynamic benefits when compared to a flap. In this work, a quasi-two-dimensional wind-tunnel test was performed to investigate the aerodynamic performance of a FishBAC device and compare it to a trailing-edge plain flap. A combination of quasi-steady force balance and wake rake pressure measurements was used to determine aerodynamic force coefficients. These measurements are complemented by a flow visualization study performed using particle image velocimetry, allowing for comparison of the near-field wakes in terms of size and vortical structure. Additionally, displacement measurements were performed on the FishBAC to track deformed shapes under aerodynamic loads. Results show that the FishBAC achieves greater than a 50% improvement in lift-to-drag ratio for moderate to high lift coefficients where the device is significantly deflected and an at least 16% higher lift-to-drag ratio at all lift coefficients, even when the device deflections are low.

Estimation and Analysis of Hybrid Laminar Flow Control on a Transonic Experiment

This paper investigates the method for predicting the laminar-to-turbulent transition in the context of hybrid laminar flow control. Transonic wind tunnel experiments are used to support our study. The relation between the discrete hole velocity in wind tunnel tests and the continuous surface velocity used in the simulation is established. With the calibrated relation, the method, consisting of a boundary-layer code and linear stability theory framework, has successfully predicted the transition location at various conditions, within 4% chord error compared with the experimental results. Additionally, the simulation profile drag by the integration of momentum loss method matches the results of the experimental wake rake, and the difference is within 1.85 counts. The simulation results show that the microsized wall suctioning reduces the amplitude of cross-flow velocity and lowers the inflection point in the boundary layer, hence suppressing the cross-flow vortices. Hence, the laminar-to-turbulent transition is pushed aft significantly. Meanwhile, the experimental and simulation results show that the wall blowing degrades the HLFC benefit, and it needs to be avoided in applications. This paper concludes that the method can be applied in the hybrid laminar flow control transition prediction with the given perforated panel properties.

An innovative study on low surface energy micro-nano coatings with multilevel structures for laminar flow design

Laminar flow design is one of the most effective ways to reduce the drag of a commercial aircraft by expanding the laminar flow region on the surface of the aircraft. As material science develops, the emergence of new materials such as low surface energy materials has offered new choices for laminar flow design of commercial aircraft. Different types of low surface energy micro-nano coatings are prepared to verify the effects on the boundary layer transition position and the drag of the airfoil through wind tunnel tests. The infrared thermal imaging technology is adopted for measuring the boundary layer transition, while the momentum integral approach is employed to measure the drag coefficient through a wake rake. Infrared thermal imaging results indicate that the coatings are capable of moving backward the boundary layer transition position at both a low velocity of Mach number 0.15 and a high velocity of Mach number 0.785. Results of the momentum integral approach demonstrate that the drag coefficients are reduced obviously within the cruising angle of attack range from 1° and 5° by introducing the low surface energy micro-nano coating technology.

Design Study of a Heavily Loaded Ice Class Propeller Using an Advance Panel Method

A design and optimization procedure developed and used for a propeller installed on a twin-semitunnel-hull ship navigating in very shallow and icy water under heavy load conditions is presented. The base propeller for this vessel was first determined using classic design routines under open-water condition with existing model test data. In the optimization process, a panel method code (PROPELLA) was used to vary the pitch values and distributions and take into account the inflow wake distribution, tunnel gap, and cavitation effects. The optimized propeller was able to improve a ship speed of 0.02 knots higher than the desired speed and 0.06 knots higher than the classic B-series propeller. The analysis of the effect of inflow wake, hull tunnel, cavitation, and blade rake angle on propulsive performance is the focus of this paper.

Surface pressure and aerodynamic loads determination of a transonic airfoil based on particle image velocimetry

The present investigation assesses a procedure to extract the aerodynamic loads and pressure distribution on an airfoil in the transonic flow regime from particle image velocimetry (PIV) measurements. The wind tunnel model is a two-dimensional NACA-0012 airfoil, and the PIV velocity data are used to evaluate pressure fields, whereas lift and drag coefficients are inferred from the evaluation of momentum contour and wake integrals. The PIV-based results are compared to those derived from conventional loads determination procedures involving surface pressure transducers and a wake rake. The method applied in this investigation is an extension to the compressible flow regime of that considered by van Oudheusden et al (2006 Non-intrusive load characterization of an airfoil using PIV Exp. Fluids 40 988–92) at low speed conditions. The application of a high-speed imaging system allows the acquisition in relatively short time of a sufficient ensemble size to compute converged velocity statistics, further translated in turbulent fluctuations included in the pressure and loads calculation, notwithstanding their verified negligible influence in the computation. Measurements are performed at varying spatial resolution to optimize the loads determination in the wake region and around the airfoil, further allowing us to assess the influence of spatial resolution in the proposed procedure. Specific interest is given to the comparisons between the PIV-based method and the conventional procedures for determining the pressure coefficient on the surface, the drag and lift coefficients at different angles of attack. Results are presented for the experiments at a free-stream Mach number M = 0.6, with the angle of attack ranging from 0° to 8°.

Two-Dimensional Low-Reynolds Number Wind Tunnel Results for Airfoil NACA 0018

The two-dimensional characteristics of airfoil NACA 0018 have been measured for Reynolds numbers between 0.15×106 and 1.0×106 to establish the lift, drag and moment curves that serve as input to performance calculations of vertical axis wind turbines. At the lower surface laminar separation occurs at low to medium angles of attack, which is of significant influence on the characteristics and the radiated noise. For the situation with a lower surface laminar separation bubble, span wise wake rake traverse measurements showed an irregular three-dimensional pattern. Noise reduction could be achieved with zigzag tape at the 70% to 80% lower surface chord station. Significant post-stall hysteresis loops occurred showing a high loss in lift.

Aerodynamic load characterisation of a low speed aerofoil using particle image velocimetry

AbstractParticle image velocimetry (PIV) measurements of the flow around a wing section are employed as a basis for non-intrusive aerodynamic mean loads characterisation, providing sectional lift, drag and pitching moment. The technique relies upon the application of control-volume approaches in combination with the deduction of the pressure from the PIV experimental data through application of the momentum equation. The treatment can also be applied when the flow is unsteady; in that case time-mean loads are obtained from velocity statistics, through the use of Reynolds-averaged formulation of the governing equations. The procedure was applied in the experimental investigation of a NACA 642A015 aerofoil, in which the PIV approach is validated against standard pressure-based methods (surface pressure distribution and wake rake). The chord Reynolds number considered in the investigation ranges between 1 – 7 × 105. In addition, the consistency and potential performance of the method was assessed by means of synthetic velocity field data obtained from a numerical flow simulation.

Non-intrusive load characterization of an airfoil using PIV

An assessment is made of the feasibility of using PIV velocity data for the non-intrusive aerodynamic force characterization (lift, drag and pitching moment) of an airfoil. The method relies upon the application of control-volume approaches in combination with the deduction of the pressure from the PIV experimental data, by making use of the momentum equation. First, the consistency of the method is verified by means of synthetic data obtained from CFD. Subsequently, the procedure was applied in an experimental investigation, in which the PIV approach is validated against standard pressure-based methods (surface pressure distribution and wake rake).

107 パワード・リフト機模型の後流測定

The aircraft employing the Upper Surface Blowing(USB) system as a poweredlift technology makes use of the energy of the engine exhoust to augument wing lift. The aircraft with the USB system achieves higher powered-lift by turning full downward the engine exhaust flow. To studying powered-lift characteristics, the wakes of the semi-span aircraft model with USB Powered high lift system were measured by a taversing wake rake with 5-pitot tube in NAL 2×2m Gust Wind Tunnel.In this report, the characteristics for the wake having different turning angle of exhaust flow are described.

The Profile Drag of A Hawk’s Wing, Measured by Wake Sampling in A Wind Tunnel

ABSTRACT The distribution of dynamic pressure behind a Harris’ hawk’s wing was sampled using a wake rake consisting of 15 pitot tubes and one static tube. The hawk was holding on to a perch, but at an air speed and gliding angle at which it was capable of gliding. The perch was instrumented, so that the lift developed by the wing was known and the lift coefficient could be calculated. The mean of 92 estimates of profile drag coefficient was 0.0207, with standard deviation 0.0079. Lift coefficients ranged from 0.51 to 1.08. Reynolds numbers were nearly all in the range 143000–194000. The estimates of profile drag coefficient were reconcilable with previous estimates of the wing profile drag of the same bird, obtained by the subtractive method, and also with values predicted by the ‘Airfoil-ii’ program for designing aerofoils, based on a digitized wing profile from the ulnar region of the wing. The thickness of the wake suggested that the boundary layer was mostly or fully turbulent in most observations and separated in some, possibly as an active means of creating drag for control purposes. It appears that the bird could momentarily either increase or decrease the profile drag of specific parts of the wing, by active changes of shape, and it appeared to use the carpo-metacarpal region especially for such control movements. Further investigation in a low-turbulence wind tunnel would help to resolve doubts about the possible influence of airstream turbulence on the behaviour of the boundary layer.

A213 パワード・リフト機模型のエンジン後流測定

Aircraft adopting USB(Upper Surface Blowing) concept as a powered lift system achieves powered lift augumentation by deflecting the engine exhaust flow over the wing and the extended USB flap by Coanda principles. To obtain higher powered lift, optimal engine exhaust nozzles design and engine allocation which enhance effectively exhaust flow turning performance are important factors.This report describes the exhaust flow characteristicsin the case of varying space between two engines installated on the semi-span USB powered lift model. Measurements of Exhaust flows behind two engines were made by a traversing wake rake with 5-holes pitot tube in NAL 2m×2m Gust Wind Tunnel.