Physics of Transient Stall on a Rotating Blade Due to Blockage

Transcription

1 Physics of Transient Stall on a Rotating Blade Due to Blockage Vrishank Raghav And Narayanan M Komerath School of Aerospace Engineering, Georgia Institute of Technology Atlanta, GA 30332, USA ABSTRACT Rotating blades encounter a blocked flow in many instances, for example the tower wakes of downwind wind turbine blades, the blades of the radiator fan of an automobile. One of the two schools of thought models the blocked flow as causing a transient stall on the blade and the other models it as a lift deficiency function. In this work the extreme case of a blocked flow is studied, where the blockage covers a considerable sector of the rotating disc. The blade undergoes a transient stall and consequently the radial flow over the blade becomes significant. Pulsed laser sheet imaging and particle image velocimetry (PIV) are used to verify the occurrence of separation, and to capture features of the separated flow. The results verify the occurrence of stall in the obstructed-inflow sector. In the stalled region, radial flow develops, but does not increase on moving outboard. This appears to be due to the presence of separation cells along the radius, which contribute to exchange of fluid between the near-blade surface and the freestream. Keywords: Transient Stall, Rotating Blade, Blockage, and Radial Flow. 1. INTRODUCTION Rotating blades experience varying dynamic loads due to blockage effects. Blockage is defined as a flow that is obstructed from freely interacting with the rotating blade. This dynamic loading is attributed to the blade transient stall as a result of its interaction with the blocked flow. The underlying source of the stall being the pitch increase on the blade in the low dynamic pressure region coupled with rapidly changing inflow velocity, causing the effective angle of attack to exceed the static stall limit rapidly and substantially. This triggers a sharp dynamic lift increase followed by a sudden loss of lift, and the stall persists until the blade is well under the static stall limit. As such this phenomenon has been studied extensively, two dimensional dynamic stall of 2-D airfoils has been well studied by McCroskey (1). Concepts for stall alleviation by Carr (2) and elimination by McAlister (3) have been developed for rotating blades. But much less is known about the complexities and intricacies of 3-D dynamic stall, due to the presence of strong radial acceleration. On the other hand, Leishman (4) postulates that the dynamic lift acting on a wind turbine blade when passing through the tower wake can be modeled by fitting a lift deficiency function to the airfoil section. He suggests using a Kussner Function, which models a vertical gust impingement onto the airfoil section. Coton (5) cited discrepancies in the above method when blade experiences very high angles of attack, even though 3-D effects were included. He also cited that the predictions overshoot the impulsive response of the blade at the exit from the tower shadow region. The two schools of thought both suggest that 3-D effects cannot be neglected to determine the effect of blockage on rotating blades. It is important that further studies include three dimensional effects. Raghav (6) cites that the nature of the separation line on the rotating blade must involve interaction between the stall vortex and the details of separated flowfield downstream. Thus, study of this region opens the way to understanding 3-D flow separation leading to Dynamic Stall, Raghav (7). The motivation of the present studies is two-fold. The first is to study the effect of blockage on a rotating blade. Second is to study the significance of radial flow during transient or dynamic stall. In this work the inflow onto a rotating blade is obstructed to study the characteristics of the flow over the blade under the imposed conditions. The obstruction here is an extreme case of the practically occurring cases of the wind turbines and radiator fans. The flow behind the stall line is of interest as the radial acceleration is strongest in that region. In practice a rotating blade interacting with a blocked flow occurs in the case of downwind Horizontal Axis Wind Turbines (HAWT), when the blade passes through the tower shadow. Such stall also occurs on automobile radiator fan blades, where the flow is blocked by the other parts. This phenomenon gives rise to dynamic loading on the blades which is hard to predict. In case of HAWT it leads to increased fatigue loading leading to structural failure and lower life expectancy. The dynamic loading on a radiator fan leads to increased noise generation by the blades of the fan. 2. PREVIOUS WORK The effect of blockage over a part of the azimuth of a rotating blade is of interest in many practical applications. It is known that flow separation leading to stall, occurs on a rotating blade interacting with a blocked flow. This flow separation is dynamic in nature due to which the reattachment is delayed in a hysteresis loop

2 which persist well into the next quadrant of the rotation. Butterfield (8) cites that dynamic stall has been observed on wind turbine blades due to the effect of the tower shadow. Efforts to capture this phenomenon have proceeded through 2-D experiments on pitching airfoils in wind and water tunnels McCroskey (1) (9). Numerical predictions proceeding from inviscid formulations to full Navier-Stokes simulations have also been performed. Research on 3-D flow separation leading to dynamic stall showed that the stall initiation was a localized event, thus making the timing of stall difficult to predict and control. Influence of Radial Acceleration The effect of radial acceleration on the lift and pitching moment evolution on a dynamically stalled blade is of great interest. Coton (10) cites the substantial pressure differences in the inboard pressure distribution on rotating wind turbine blades compared to 2-D models, preceding dynamic stall. He also cites the delay in forwards movement of the separation region (prior to liftoff of the dynamic lift vortex) due to spanwise flow. Corten (11) has used a stall flag method to capture the occurrence of stall on full scale wind turbine blades. His analysis shows that in dynamic stall region, there is a substantial radial pressure gradient and accompanying radial flow. Corten's (12) laser velocimeter results shows the formation of vorticity directed parallel to the rotor axis along the aft portions of the blade. Hence radial accelerations must have a strong influence on the lift and pitching moment evolution after stall occurs. Xu (13) shows that the influence of rotation (centrifugal effects in the boundary layer) on a wind turbine blade, when modeled using a full Navier-Stokes formulation, shows a substantial stall delay in the 3-D case compared to 2-D cases where there is no rotation effect. facility. Refer to Yang (14) for the complete details of the experiment. Description Value Units Blade Radius 0.61 Meters Blade Chord 0.13 Meters Aspect Ratio 4.8 Airfoil Section Blade NACA 0012 Un-Tapered, Untwisted Motor 11.2 KW Motor RPM RPM Table 1 Hover Facility Specifications 3. MEASUREMENT APPROACH The objective of this study is to study the flow field behind the point of separation on inboard region of a rotating blade under blockage effects. The approach was to use a single-bladed rotor in a hover facility and considerably alter its effective angle of attack over a part of the azimuth. The sudden change in angle of attack was achieved by using a flow obstructer plate in the inflow region of the rotor. The primary concern is to determine the effect of blockage on a rotating blade. The secondary concern is the behavior of radial flow downstream of the separation line on the blade. Experimental Facility The experiment was conducted in a rotorcraft hover facility in the School of Aerospace Engineering at Georgia Institute of Technology. A single bladed rotor of the specifications shown in Table 1 was used for the experiments. The Figure 1 shows a picture of the experimental setup and a schematic front view of the Figure 1: Inflow obstruction picture and sketch of inflow region 4. FLOW MEASUREMENTS From prior experiments in this facility, Liou (15), it is known that the blade used here operates with attached flow at 15 degrees pitch, at 500 RPM (30 m/s tip speed).

3 Qualitative Visualization Pulsed Laser Sheet Imaging (PLSI) studies were conducted at 70 RPM. The RPM was slow enough to capture successive video images, showing the same seeding during a single-event approach and passage of the blade through the seeding cloud, albeit at low Reynolds number. The PLSI showed that the flow across the disc (axial velocity component), was being stopped suddenly in contrast to being sharply accelerated as a lifting blade passes by, this indicated blade stall. In Figure 3, as the blade moves through the seeding cloud, very little axial flow movement can be seen ( a) and b) ), showing the absence of strong suction. Soon after the blade passes through, the seeding resumes its downward movement. (c). Viewed in detailed image sequences, this visualization made clear that the blade was essentially just a blunt body moving through the flowfield, and was not generating any significant lift. The inflow during the rest of the cycle was due to the lift generated when the blade was beyond the inflow-obstruction region. This finding is quantified by PIV. Quantitative Visualization For PIV, four image sets of fifty image pairs were taken for each blade azimuth of the measuring plane. The spanwise axial section was selected to capture the radial flow velocity (Figure 4). The measuring plane was illuminated along the span with a rectangular viewing region of 12.8 cm (spanwise) x 9.6 cm (axial). (a) (b) 5. EVIDENCE OF STALL The experiment was conducted under conditions where the aerodynamic load is small compared to the inertial loads. Also, no direct thrust measurement is made, and no pressure sensors are used. Thus, the occurrence of stall was verified by comparing the axial velocity induced by blade passage with the local level attributable to the tip vortex system and lift generation through the rest of the blade cycle. Previous measurements with a liftgenerating rotor have shown that a very large inflow velocity spike occurs during blade passage, followed by settling to the local level very quickly for the rest of the cycle. However, if the blade is stalled as it passes the measuring location, and generating lift through much of the rest of the cycle, we should expect to see only the smaller perturbation in the inflow velocity during blade passage, associated with passage of a blunt body. It is expected that the flow will be pushed away by thickness of the blade, followed by a briefly increased inflow. Velocity results from the 70 RPM tests, followed by cleaner results at 500 RPM, verified that this was occurring. Figure 4 shows that at mid-span, at 270 degrees azimuth (blade is at the opposite side of the rotor disc) the measured inflow velocity was about 2.5 m/s for the obstructed case and 3 m/s for the control case, as shown in Figure 4. This is in line with rough expectations from Momentum Theory. (c) Figure 3 Sequential Blade and Seeding Visualization and Schematic View 6. FLOW CHARACTERISTICS RADIAL FLOW Figures 5 through 8 show contours of the radial velocity component, measured at various locations along the spanwise viewing plane covering 12.8 cm of the blade centered on the 38 cm radial location (midspan). At the leading edge plane, (Figure 5), without the obstruction, at 90 degrees azimuth, when the blade is assumed to be lifting and at the measurement region, the radial velocity component is in the range of 0.5 m/s (red region).

4 Figure 4 Axial Velocity Profile at Mid-span However, when the blade is at 270 degrees (as far away as possible from the measuring region) the radial velocity is on the order of 1.4 m/s, as shown in Figure 6. This is the local level of radial velocity, attributed to the tip vortex. Figure 6 Radial Velocity Contour with blade at 270 degrees azimuth, no obstruction Figure 5 Radial Velocity Contour at Leading edge, No Obstruction Comparing Figures 7 and 8 for the same cases as above, but with the obstruction present, we see that the radial velocity remains near the local level. Thus in the case with obstruction, the radial velocity above the blade leading edge region is not very different from the local. This sets the context to view the change of radial velocity from leading edge to trailing edge over the blade, as it Moves through the measurement plane, and compare the cases with and without obstruction. Figure 7 Radial velocity contours at leading edge, obstructed case. The measurements at mid-span are shown. In Figures 9 and 10, the blue lines show the deceleration from leading edge to quarter chord at 5 cm from the surface of blade, while the orange line is 10 cm from the surface. Figure 10 shows the radial velocity as a function of chord at mid-span without the obstruction. There is less discontinuity after the quarter chord than in the case with obstruction (Figure 10).

5 Figure 8 Radial velocity contours with blade at 270 degrees azimuth, obstructed case The case with the obstruction, in Figure 10, shows a quick change in the magnitude of the radial velocity. The velocity fluctuation increases after the quarter chord. The reason why the radial inflow velocity is initially higher in the obstructed case is that while the axial flow has been obstructed, the radial flow has not been obstructed. So when the pressure drop occurs near the upper surface, the radial inflow is accelerated as opposed to the controlled case where axial component of the flow would be sufficient in pressure recovery. Figure 10 Radial Velocity Profile at Mid-Span with Obstruction In contrast, the obstructed case had the same deceleration at 5 cm and 10 cm away from the surface. The results show that the inboard-directed radial component of the inflow velocity decreases as the blade passes through the obstruction. This suggests that the centrifugal effect dominates, downstream of quarter chord. After the blade has passed through, the radial velocity returns to the local level. Figure 11 Instance of Recirculation at 0.25 x/c Figure 9 Radial Velocity Profile at Mid-Span without obstruction (Control Case) The range of the radial inflow velocity variation for both cases (controlled and obstructed) is the same up to the quarter chord, where flow is believed to be attached for both cases. However the rate of deceleration of radial inflow in the control case decreased over the observed distance from the blade surface. 7. AXIAL FLOW Figures 11 and 12 show two instances of the span-wise cells in the separated flow. This process does not appear to be periodic at the rotor frequency, and thus differs substantially in location and strength from one rotor cycle to another. Thus this feature does not show up in averaged measurements. However examination of several PIV frames confirms that the phenomenon persists.

6 Figure 12 Instance of Recirculation at 0.28 x/c 8. CONCLUSIONS A single-blade hover facility is used as a test-bed to study the temporal evolution of obstructed flow phenomena close to a rotating blade. The transient stall phenomenon is studied, given that the interest is in the region downstream of the separation line. 1. PLIS and PIV validate the experiment in that stall indeed occurs, based on the blockage of the inflow. 2. Downstream of the stall line, the radial velocity along the blade sharply develops an outward direction. However, the formation of stall cells along the radius serves to exchange fluid away from the blade boundary layer. 3. The phase-locked radial velocity profile shows higher deceleration of radial out-flow from leading edge to chord-wise location where flow recirculation begins. This suggests that while reattached flow s inboard velocity component started to dominate over the centrifugal effect from leading edge to separation point, the inflow directional preference is nullified by the separation/recirculation zone created. 4. The cells of radial flow separation, while in the comparable scale as the blade chord, is time varying. This is possibly due to the dynamic nature of the extent of the radial out-flow due to domination of the centrifugal effect. 3. McAlister, K. W. and Tung, C. Suppression of Dynamic Stall with a Leading-Edge Slat on a VR- 7 Airfoil.NASA, March TP Leishman, J.G. Challenges in Modelling the Unsteady Aerodynamics of Wind Turbines. Wiley - Wind Energy, Tongguang Wang, Frank N. Coton. A High Resolution Tower Shadow model for Downwind Wind Turbines. Journal of Wind Engineering And Industrial Aerodynamics - Elsevier, Raghav, V., Richards, P., Smith,M., Komerath,N. Three- Dimensional Features of the Stalled Flow Field of a Rotor Blade in Forward Flight. Seoul Oct Raghav, V., Richards, P., Smith,M., Komerath,N. An Exploration of the Physics of Dynamic Stall. AHS, Hansen, A.C., Butterfield, C.P Aerodynamics of Horizontal-Axis Wind turbines..annual Review of Fluid Mechanics, 1993, Vol McCroskey, W. J Some Current Research in Unsteady Fluid Dynamics - The 1976 Freeman Scholar. Journal of Fluids Engineering, 1977, Vol Coton, F. N., Wang, T., and Galbraith, R. A. McD. An Examination of Key Aerodynamic Modelling Issues Raised by the NREL Blind Comparison. Wind Energy, Corten, Gustave.P.,. Flow Separation on Wind Turbine Blades. University of Utrecht : Doctoral Dissertation, Bunnes, P., Fiddes, S.,. Laser Doppler Results Contributed to 'Dynamic Stall and Three Dimensional Effects. Glasgow, UK Joule II contractors meeting, Xu, Guanpeng. Computational Studies of Horizontal Axis Wind Turbines. PhD Thesis, Georgia Instiute of Technology, Jaesuk Yang, Balakrishnan Ganesh, Narayanan Komerath. Radial Flow Measurements Downstream of Forced Dynamic Separation on a Rotor Blade. San Francisco, California AIAA Fluid Dynamics Conference and Exhibit, Liou, S-G., Hyun, J-S.,Komerath, N.M. Flowfield of a Swept Blade Tip at High Pitch Angles AIAA, Bousman, W. A Quantitative Examination of Dynamic Stall from Flight Test Data. AHS Forum, Bousman, W. Evaluation Of Airfoil Dynamic Stall Characteristics For Maneuverability. European Rotorcraft Forum, REFERENCES 1. McCroskey, W.J., McAlister, K.W., Carr, L.W., and Pucci, S.L. An Experimental Study of Dynamic Stall on Advanced Airfoil Sections. s.l. : NASA-TM, Chadrashekara, M.S., Wilder, M.C., Carr, L.W. Compressible Dynamic Stall Control Using Dynamic Shape Adaptation. : AIAA,