Aerodynamic characteristics of unsymmetrical aerofoil at various turbulence intensities

S. ARUNVINTHAN, S. NADARAJA PILLAI

Turbulence & Flow Control Lab, School of Mechanical Engineering, SASTRA Deemed University, Tamil Nadu 613401, India

KEYWORDS

Abstract A series of wind tunnel tests were performed to investigate the effect of turbulent inflows on the aerodynamic characteristics of the unsymmetrical airfoil at various turbulence intensities and Reynolds number. To assess the aerodynamic characteristics, surface pressure measurements were made over the unsymmetrical airfoil surface by using a simultaneous pressure scanner MPS4264 of Scanivalve make.Self-generated passive grids made of parallel arrays of round bars were placed at four different locations to generate various Turbulence Intensities(TI)in the wind tunnel.The location of the passive grid has been normalized in terms of considering the distance between the entry of the test section and the leading edge of the model.Based on the wind tunnel results,by comparing the baseline without grid low turbulence case TI=0.51%with other turbulence generated cases like TI=4.68%,4.73%,6.04%and 8.46%at different Reynolds number,it is found that the coefficient of lift increases with the increase in the turbulence intensity.Results also reveal that the flow featuring turbulence can effectively delay the stall characteristics of an airfoil by attaching the flow over the airfoil for an extended region. Additionally, attempts were made to understand the influence of turbulence on the aerodynamic hysteresis.

1. Introduction

Atmospheric turbulence is unpredictable and is often subjected to change;therefore,the assessment of aerodynamic characteristics of airfoils under turbulence in the wind tunnel is of primary importance in the field of aerodynamics and wind engineering.Recently,researchers started focussing their attention on small wind turbines,rooftop wind turbines,Unmanned Aerial Vehicles (UAV), Micro-Aerial Vehicles (MAV) and Nano-Aerial Vehicles (NAV). As airfoils are omnipresent in aircraft,UAV,MAV and wind turbine blades,predicting their aerodynamic performance under turbulent wind characteristics is deemed necessary to withstand and mitigate the negative effects. As the flow around the aircraft and the wind turbines are unstable, understanding the impact of the turbulence on the aerodynamic characteristics of the airfoil becomes necessary for the efficient design,operation and working of aircraft and wind turbines at optimum performance. Researchers suggested that the turbulence has the tendency to effectively alter the flow characteristics over the airfoil. The design regulation of wind turbines1states that turbulence has a significant impact on the aerodynamic performance of the blade and hence it is necessary to evaluate the effect of turbulence intensity over the airfoil section. Even though the turbulent fluctuations contribute to less than 1% of the mean speed, the aerodynamic measurements are greatly dependent upon them.

In 1931,Stack2initiated the research on the effect of turbulence over airfoils by conducting various wind tunnel experiments with and without turbulence grids and reported that the turbulence can effectively increase the maximum coefficient of lift and positively delay the stall behaviour of an airfoil.Devinant et al.3confirmed that the aerodynamic characteristics of the airfoil are strongly influenced by the turbulence intensity both qualitatively and quantitatively. Huang and Lin4reported that other than dynamically increasing the lift coefficient, the turbulence intensity also influences the flow separation characteristics.Following that,Sicot et al.5claimed that the increase in coefficient of lift and the stall delay behaviour is possibly due to the movement of separation point towards the vicinity of the trailing edge with the increase in the turbulence intensity and with the angle of attack. Seggidhi and Soltani6proved that increasing the Turbulence Intensities(TI) delays the stall until higher angles of attack (α). Similar results were reported by researchers like Karbasian et al.,7Al-Abadi et al.,8Swalwell et al.,9Maldonado10, and Kamada et al.11proved experimentally that the turbulence intensity prevents the flow separation over the airfoil surface and hence delays the stall. Jancauskas12reported that when the turbulence intensity is increased from 0.6% to 16%, the stall behaviour of the airfoil is further delayed. Wang et al.13analysed the influence of the turbulence intensity on the flow separation and concluded that the delay in flow separation is induced by the enhanced flow mixing characteristics which in turn results in the delayed stall.

In order to understand the lift and drag characteristics with its associated flow field, researchers started exploring the possible methods of producing desired turbulence in the wind tunnel. Researchers adopted several methods to generate the desired turbulence in the wind tunnel; some of them are listed in Table 1. It includes passive screens,14-20active grids,15,21geometries,22,23etc. Researchers have made significant progress in generating free stream turbulence over the years in the wind tunnel.Over the past fifty years,grids are considered as one of the traditional methods of producing desired turbulence due to its intrinsic simplicity.Grids being one of the most generalized ways of generating desired turbulence several researchers19,20have utilized this method of turbulence generation in their studies. Under certain circumstances, the effect of turbulence intensity is detrimental. For example, even a small packet of turbulence in the air can overstress the airframe or even cause aerodynamic performance deterioration like flow separation over the airfoils which in turn can result in loss of lift and in wind turbines result in the reduction of energy extraction.Based on this fact,Zbrozek24suggested that the turbulence must be considered while designing an airframe of an aircraft to provide sufficient strength to withstand sudden gust and turbulence without compromising the operational life of an aircraft. Similarly, several studies like Refs.3,5,11,18,25insisted on the importance of the effects of turbulence intensity over the wind turbines. Likewise, based on the quantitative data measured out of the wind tunnel,researchers also suggested that the aerodynamic forces and moments are found to be multivalued at the increasing and decreasing angles of attack. Yang et al.26confirmed this phenomenon and stated that ‘‘The coefficient of lift, drag and moment of the airfoil are found to be multiple-valued rather than single-valued functions of the angle of attack in the hysteresis loop”. Further, several studies like Refs.25-28reported aerodynamic hysteresis phenomenon as a common phenomenon prevalent for airfoils subjected to chord Reynolds number less than 500000. Other challenges associated with the turbulence were also studied by Luhur et al.29

Even though the investigations on the aerodynamics of the airfoils are carried out from the long back being driven by the needs of the applications, the knowledge obtained so far islimited due to the recent developments like small wind turbines, rooftop wind turbines, UAV, MAV and NAV. Since the aforementioned vehicles and systems operate around low Reynolds number in the order of 104-106,the effect of the turbulence intensity on the aerodynamic performance of the airfoil is significant. The presence of turbulence can effectively delay the flow separation induced by the transition and can cause significant aerodynamic performance changes at low Reynolds number. The ability of the turbulence to influence the aerodynamic characteristics over the airfoil demands a detailed experimental study to gain further insight.

Table 1 Methodologies utilized in generating turbulence in wind tunnel.

In this research, the aerodynamic characteristics of unsymmetrical airfoils at low Reynolds number are experimentally investigated in the wind tunnel. Surface pressure measurements have been made using Scanivalve MPS4264 simultaneous pressure scanners at a sampling rate of 700 Hz at different turbulence intensities. The influence of the turbulent inflow on the aerodynamic characteristics of airfoils like lift,drag and surface pressure measurements are studied in detail for better understanding. Attempts were made to gain further insight into the effect of turbulence on the aerodynamic characteristics of the airfoil at various Reynolds number. It is believed that the results will help aerodynamic designers and wind engineers in optimizing their aerodynamic performance under practical situations and it can also act as a guideline for them in better understanding the influence of TI over the aerodynamic characteristics of the unsymmetrical airfoil.

2. Computational methodology

The aerodynamic characteristics of a two-dimensional airfoil section were investigated by measuring the surface pressure measurement for different Reynolds number at various turbulence intensities and their results are described in detail in this section. In this study, the surface pressure measurements were obtained at five different Reynolds number: Re=1.09×105,1.54×105,2.0×105,2.44×105and 3.01×105corresponding to the average wind velocity of Vavg=17, 24, 31, 38, 47 m/s,respectively. The synthesis of experimental apparatus utilized in this study is discussed as follows:

(1) Wind tunnel. The experimental investigation has been carried out in SASTRA Deemed University low-speed subsonic wind tunnel facility. The rectangular test section of the wind tunnel is 300 mm×300 mm in the cross-section with the length of 1500 mm. The tunnel is operated by a fan powered by a 10HP motor and can reach the maximum wind velocity of around 60 m/s. The coordinate system of the wind tunnel was defined for the measurements, in which the X-direction signifies the freestream condition,Y-direction represents the vertical and Z-direction shows the lateral directions,respectively. The schematic representation of the experimental apparatus involved in this paper is shown in Fig.1. The flow field measurements were carried out over the models at various angles of attack ranging from α=-25° to +25°.

(2) Test airfoil. In this study, a rectangular wing model

made of wood with an unsymmetrical test airfoil model profile has been used. The test airfoil has a maximum camber of 2% with its maximum thickness lying at 15% of the chord location. The chord length of the model is 100 mm and the span of 300 mm as shown in Fig.2 is mounted horizontally in the test section with the aluminium holder protruding through the wind tunnel test section. The aluminium rod also acts as a pitch position holder using a mechanical holder arrangement.The airfoil model is equipped with a total number of 21 pressure taps uniformly distributed over the upper and the lower surface to obtain complete information of the flow behaviour. The distribution of pressure taps over the airfoil surface is displayed in Fig.3. Both the upper and the lower surface houses 10 pressure ports while one facing the freestream fluid can act as stagnation pressure port.The diameter of each port is approximately 1 mm.

(3) Turbulence grids.The turbulence in the flow is generated by a self-developed passive turbulence grid in our laboratory. A wide range of grid types and sizes have been examined, and the one which is used in this study is made up of parallel arrays of round bars as shown in Fig.4. With a view to generate multiple turbulence intensities, the self-developed turbulence grids were placed at four different locations. The turbulence intensities generated at each section along with its measurement position were tabulated in Table 2. The distance between the entry of the test section and the leading edge of the airfoil i.e. z is 750 mm. Based on the distance between the entry of the test section and the model location L, all the four locations have been normalized and represented as z/L. The choice of the normalized distance z/L is bi-fold: the uniformity of the turbulence intensity and the value of turbulence intensity. Once the turbulence generating grids were placed at corresponding locations, the turbulence intensities at each point has been measured using simultaneous pressure scanner. It was found that the turbulence intensities had a very good stability at each measurement locations and the results with their normalized locations are tabulated in Table 2.

(4) Simultaneous pressure scanner. The instantaneous pressures acting over the airfoil were simultaneously measured using an MPS4264 miniature pressure scanner of Scanivalve make. The pressure taps from the airfoils were pneumatically connected to the pressure scanner through the stainless steel pipes and flexible tubes. The sampling frequency of 700 Hz corresponded to the measurement with a total number of data samples being set at 10000.

The instantaneous pressure acting over the pressure ports was denoted as piand the distance between the measurement tap as Si. The pressure acting over the airfoil can be resolved to obtain the lift and the drag force i.e. by integrating the surface pressure acting over the airfoil for the entire blade surface,the lift force L and the drag force D can be obtained.From the overall lift force L and the drag force D, the lift coefficient CLand the drag coefficient CDare calculated based on the following equations:5,17,32-35

Fig.1 Experimental setup - wind tunnel.

Fig.2 NACA 2415: unsymmetrical airfoil with pressure taps.

Fig.3 Location of pressure taps in airfoil model.

The relation between the lift force and the drag force of the airfoil section with the measurement pressure taps can be seen from Fig.5.Similarly,α denotes the angle of attack i.e.the relative angle between the chord line and the freestream velocity.The angle of attack of the airfoil model was controlled manually with the help of a circular protractor mounted over the test section walls with cylindrical aluminium holder arrangement.

Based on the framework of the previous researchers, it is suggested that the uncertainties are either due to the dispersion of data or by the measuring instrument.It is further mentioned that‘‘uncertainty errors had no significant effect and could be easily overcome by a large number of samples”18. Likewise,repeated measurements showed that the average values of the measurements did not encounter any serious uncertainty effect.

3. Results and discussion

In order to get more information on the influence of TI and Reynolds number on the aerodynamic characteristics of the airfoil surface, the averaged surface pressure distribution acting over the airfoil was discussed in detail in this section.Pressure coefficient, lift and drag curves for unsymmetrical airfoil at different turbulence intensities and Reynolds number were analysed.

3.1. Effect of turbulence intensity on lift characteristics of unsymmetrical airfoil

(1) Re=1.09×105

Fig.6 illustrates the coefficient of lift against angle of attack (α) at various turbulence intensities for the case of Re=1.09×105. As shown in this figure, circle, triangle, square, diamond and x legends indicate the lift coefficient CL, corresponding to z/L=0.533, TI=8.46%, z/L=0.400,TI=6.04%, z/L=0.266, TI=4.73% and z/L=0.133,TI=4.68% respectively for both the increasing and decreasing direction of angles of attack. From the figure, it is evident that the characteristic regimes of surface flow shift appreciably at different freestream turbulence intensities.The coefficient of lift of z/L=0.533, TI=8.46% at the angle of attack of 0°＜α ＜25° in the increasing direction of angles of attack shows greater value than that of z/L=0.400, TI=6.04%and the low turbulence TI=0.51% i.e. without grid case.Similarly, at α=15°, for z/L=0.266, TI=4.73% and z/L=0.133, TI=4.68%, it shows relatively larger CLthan the baseline without grid case of extremely low turbulence TI=0.51%. The lift of the airfoil at TI=0.51% performs apparently poorer than its counterpart at TI=4.68% and 4.73%and even poorer than the TI=6.04%and 8.46%cases.From the figure, it can also be inferred that the mean coefficient of lift varies with the turbulence of the inflow which in turn clearly confirms that the aerodynamic loads are strongly correlated with the surface flow characteristics at Re=1.09×105. In the baseline low turbulence case without grid having TI=0.51%, the airfoil stalls at α=10° in the increasing direction of the angle of attack whereas the remaining other cases with turbulence generation grid continues to produce lift till α=15°.Therefore,it can be further added that the turbulence of the inflow influences the stall characteristics.The effect of turbulence intensity on the stall delay characteristics is positive and can be clearly seen from Fig.6. The stall angle of attack at Re=1.09×105increases from 10° at TI=0.51% to 15° at TI=8.46%. Results reveal that high freestream turbulence intensity can effectively delay the stall,when TI ≥6.04%, z/L ≥0.400. However, when TI ≤6.04%and z/L ≤0.400, the effect of turbulent inflow on the stall delay is relatively less.

Fig.4 A schematic representation of turbulence grid location and its measurement position in wind tunnel.

Table 2 Grid positions and resultant turbulence intensities.

Fig.5 A typical representation of relation between lift force and drag force over airfoil surface along with tap distance Si.

Fig.6 Lift coefficient vs angle of attack at Re=1.09×105.

Fig.7 represents the pressure coefficient Cpdistribution with various turbulence intensities for the angle of attack α=10° at Re=1.09×105. From Fig.7, it is evident that the pressure distribution for the baseline low turbulence case without grid TI=0.51% shows a small negative pressure on the leading-edge part following that the pressure increases with the increase in the chord-wise position. The increase in the pressure along the chord-wise position signifies that the flow is attached over the entire surface. On the other hand, in all the other cases TI ＞0.51%, the pressure distribution represents a strong negative pressure in the vicinity of the leading edge. Further, it can be seen that the negative pressure becomes strong with the increase in turbulence intensity.Additionally, it can also be noted that the constant pressure region in the vicinity of the trailing edge which corresponds to the flow separation becomes narrow with the increase of the TI.Thus, the turbulence intensity has a significant influence on the maximum coefficient of lift and the stall characteristics.Therefore,it can be claimed that the increase in the coefficient of lift associated with the increase in the turbulence of the inflow can be attributed to this strong pressure difference over the airfoil section.

(2) Re=1.54×105

The coefficient of lift for various turbulence intensities operating at Re=1.54×105is compared in Fig.8.From this figure, CLfor TI ＞0.51% represents linear increment as a function of an angle of attack in the increasing direction of angles of attack from 0°＜α ＜15°.However,the linear increment in the case of the baseline low turbulence case without grid is limited. It can be illustrated from the figure that the increase in the turbulence of the inflow keeps the flow attached to the airfoil thereby resulting in delayed stall characteristics.The influence of the freestream turbulence level on the lift coefficient and pressure distribution is shown in Figs. 8 and 9 respectively. The turbulence intensities are 0.51%, 4.68%,4.73%, 6.04% and 8.46% respectively for the angle of attack α=10° at Re=1.54×105.

From Fig.9, the pressure distribution for TI=0.51%-4.73% shows small negative pressures on the suction surface while TI ＞4.73% shows strong negative pressure on the leading edge indicating that the pressure distribution is greatly influenced by the turbulence of the inflow. In other words,the negative pressure starts dominating(the pressure difference between the upper and the lower surface of the airfoil increases) with the increase in the turbulence intensity. This is because the turbulence of the inflow supplies enough momentum to overcome the deceleration created by the adverse pressure gradient over the upper surface of the airfoil which in turn provides the additional lift by increasing the area over which the flow remains attached.Therefore,it can be considered that the turbulence of the inflow helps the flow to remain attached over the airfoil surface and this phenomenon holds in good agreement with the results of coefficient of lift of the airfoil operating at Re=1.54×105. This delay in flow separation over the airfoil surface with the increase in the turbulence intensity tends to alter the stall characteristics. Similarly, the constant pressure region which corresponds to the flow separation becomes narrow with the increase of TI as in the previous case.

Fig.7 Pressure coefficient vs x/c at Re=1.09×105, α=10o.

Fig.8 Lift coefficient vs angle of attack at Re=1.54×105.

Fig.9 Pressure coefficient vs x/c at Re=1.54×105, α=15o.

Fig.8 confirms that the coefficient of lift increases with the increase in the turbulence intensity both in the pre-stall as well as the post-stall region when z/L=0.266 and TI ＞4.73%.Even though a small negative suction pressure is seen from Fig.9 for TI ＜4.73%, it is believed that the amount of momentum infused by the turbulent flow is less likely to overcome the deceleration produced by the adverse pressure gradient formed over the top surface of an airfoil. From the above results,it can be inferred that the turbulence intensity has a significant effect on CLand stall characteristics on both the increasing and decreasing direction of angles of attack. However, the values of CLare expected to be the same for both the increasing and decreasing direction of angles of attack at the same Reynolds number whereas some small changes in the values of the coefficient of lift were observed. Further, it is also observed that the change in the characteristic lift curves between the increasing and decreasing direction of angles of attack gradually reduces with the increase in the turbulence of the inflow.The phenomenon which causes the change in values of the coefficient of lift with its associated effect needs to be discussed.

(3) Re=2.0×105

Fig.10 shows the variation of the coefficient of lift for various turbulence intensities in the case of Re=2.0×105. As can be seen from this figure, in the turbulent flow cases, it is considered that the flow becomes attached to the airfoil surface by the turbulent transition.Other than that,it can also be seen from the characteristic curves of the lift coefficient that the stall behaviour in the increasing and decreasing direction of angles of attack seems different. Therefore, it is believed that the characteristic curves of the lift coefficient CLwith α form a Hysteresis Loop (HL). For a given airfoil,at the same angle of attack, aerodynamic hysteresis can give different values of CL. In this section, it can be noticed that CL=0.09 at α=15° in the decreasing branch of the hysteresis loop whereas the values are found to become CL=0.13 for the same 15°at the increasing branch of the hysteresis loop for the baseline low turbulence case of TI=0.51%. Henceforth, in this section, the effect of turbulent inflow on the aerodynamic hysteresis has been described in detail.

Fig.10 Lift coefficient vs angle of attack at Re=2.0×105.

In order to better understand the effect of turbulent inflow on the aerodynamic hysteresis, the characteristic coefficient of lift curve is plotted individually for each turbulence intensity.It can be observed from Fig.11 that the hysteresis loop is clearly seen for all the TI along with the baseline without grid case. Further, it can be inferred from the figure that the hysteresis loops start disappearing with the increase in the turbulence of the inflow. In high turbulent cases like z/L=0.533 and TI=8.46%,the extent of the hysteresis loop is comparatively lesser than its counterparts like TI=6.04%,4.68%and 4.73% and the baseline low turbulence case of TI=0.51%.Additionally, the hysteresis loop was found to be in the counterclockwise direction for the variation of lift coefficient with various angles of attack at Re=2.0×105. Moreover, it has been found that the counterclockwise direction of loop formation persists for all the models including baseline as well as turbulence inflows generated using grids.

The surface pressure distribution over the airfoil at TI=0.51% and z/L=0.533; TI=8.46% at 25° angles of attack for both the increasing and decreasing direction of angles of attack is shown in Fig.12.It can be clearly seen that the flow pattern over the airfoil surface in the increasing and decreasing branch of the hysteresis loop is different. As can be seen from Fig.12(b),α=+25°lies in the increasing branch of the hysteresis loop and α=-25° lies in the decreasing branch of the hysteresis loop.Since the aerodynamic hysteresis is closely related to the separation and transitions, the corresponding changes in the pressure distribution are studied in detail. It can be seen from the surface pressure distribution graphs that the negative suction pressure or the favourable pressure gradient is significantly small relative to the increasing branch of the hysteresis loop. This surface pressure changes over the decreasing branch indicate that the airfoil has undergone a comparatively greater flow separation over the airfoil surface in the decreasing branch than in the increasing branch of the hysteresis loop.

Fig.11 Lift coefficient vs angle of attack.

Fig.12 Pressure coefficient vs x/c.

Fig.13 Lift coefficient vs angle of attack at Re=2.44×105.

Fig.14 Pressure coefficient vs x/c at Re=2.44×105, α=15o.

Fig.15 Lift coefficient vs angle of attack at Re=3.01×105.

Fig.16 Drag coefficient vs angle of attack at Re=1.09×105.

At α=-25°which lies in the decreasing branch of the hysteresis loop,since the flow separates near the leading edge and the airfoil experiences flow separation, the corresponding surface pressure distribution is limited between the values of 1.1 and -1.4. However, at α=+25° which lies in the increasing branch of the hysteresis loop,the favourable pressure gradient extends around 1.1 to -2.6. Since the same airfoil at the same angle of attack experiences different flow patterns in the increasing and decreasing direction, the formation of the hysteresis loop with α can be further confirmed.Yang et al.26confirmed similar flow behaviour. From Fig.12, it can be clearly seen that the favourable pressure gradient varies with the turbulence intensity. At TI=0.51%, the peak extends roughly between 0.8 and -0.6 which is apparently lower than the high turbulence case z/L=0.533 and TI=8.46% where the favourable pressure gradient extends between 1.1 and -2.6 which is the highest pressure difference induced among all the baseline and turbulence induced cases. Therefore, it becomes noticeable that with the increase in the turbulence intensity the negative suction pressure value increases. Consequently, it can also be substantiated that the increase in the turbulence of the inflow positively influences the favourable pressure gradient which in turn helps reducing the aerodynamic hysteresis.

(4) Re=2.44×105

Fig.13 illustrates the variation of the lift coefficient for various turbulence intensities in the case of Re=2.44×105.It is evident from the figure that the increase in the turbulent inflow keeps the flow attached to the airfoil surface.Additionally,the broken lines of Fig.13 represent the standard deviation of CLwhile the solid lines represent the actual CLfor various turbulence intensities at Re=2.44×105. It is evident from the figure that, in the pre-stall regime, the trend line of CLand its standard deviation seems stable whereas in the post-stall region the standard deviation of CLbecomes very large. It is also observed that the significant variation in standard deviation can be seen beyond the separation point where the flow is separated from the airfoil surface at Re=2.44×105. This standard deviation plot indicates that the surface pressure measured over the airfoil surface is widespread after the post-stall region and is most likely to be caused by the unstable flow induced in the vicinity of the separation point.Therefore,the coefficient of lift over the post-stall region shows a significantly larger deviation compared to the pre-stall region.Additionally, it can also be inferred that the variation of the mean with the standard deviation significantly increases with the increase in the turbulence of the inflow. This difference in the standard deviation of the plot between the baseline and the different turbulence intensities is believed to be caused by the turbulent nature of the flow influenced by the grid.Results further confirm that the variation of the coefficient of lift increases with the increase in the turbulence intensity as described in the previous sections. Thus, the changes in the pressure distribution are displayed in Fig.14 for the poststall angle i.e. α=15°.

Fig.14 indicates that, in the post-stall region, for the baseline low turbulence case i.e. without grid and TI ＜4.73%, a constant pressure distribution shows near the trailing edge.This implies that the flow is separated from the surface. Further, it can be explained that as the turbulence inflow is less for TI ＜4.73% cases, the favourable pressure gradient induced is not enough resulting in the separation of the flow.However,for TI ＞4.73%,the negative suction pressure keeps on increasing with the increase in the chordwise direction,showing that the flow remains attached to the surface.It is also found that with the increase in the turbulence intensity a strong negative pressure region near the leading edge is observed. The increased negative suction pressure, so-called the favourable pressure gradient region near the leading edge,contributes to the rise of the coefficient of lift. Therefore, it is determined that flow over the airfoil surface in the post-stall region is subjected to partial separation when the turbulence of the inflow is less than 4.73%.In other words,the separated flow region across the airfoil surface becomes narrower with the increase of the turbulence intensity.

(5) Re=3.01×105

Fig.17 Drag coefficient vs angle of attack at Re=1.54×105.

Fig.18 Drag coefficient vs angle of attack at Re=2.0×105.

The variation of the CLover the airfoil section at various angles of attack for Re=3.01×105is presented in Fig.15.The CLincreases with the increase in the turbulence of the inflow and the similar result has been discussed in detail in the aforementioned sections. As can be seen from the figure that the standard deviation of CLin the pre-stall region seems stable with respect to the post-stall region. As discussed in the previous section,the reason behind the significant variation in the standard deviation at post-stall angles is attributed to the unstable flow induced by the separation and transition of the flow. Similarly, in the decreasing direction of angles of attack also after the post-stall region, the surface flow becomes very unstable, thereby creating more disorderness in the flow than the other regions. An extensive study on the statistical property of the pressure data to identify the characteristics of the turbulent flows is deemed necessary for further understanding.

3.2. Effect of turbulence intensity on drag characteristics of unsymmetrical airfoil

(1) Re=1.09×105

The effect of turbulent inflow on the drag characteristics of the unsymmetrical airfoil is shown in Fig.16.In this figure,the mean drag coefficients corresponding to the turbulence intensities of TI=0.51%, 4.68%, 4.73%, 6.04% and 8.46% were displayed respectively. It is noteworthy that the mean drag coefficient plotted here describes only the pressure drag which is obtained from the airfoil surface pressure measurement.Further,it can be inferred from the figure that the drag coefficient increases with the increase in the turbulence intensity and the angle of attack.Also,the increase rate of the coefficient of drag with respect to turbulence intensity is noticeably larger when TI ＞4.73%than that when TI ＜4.73%.From the conventional point of view, it can be considered that the amount of momentum transferred from the turbulent inflow to delay the separation might lead to lesser drag when TI ＜4.73%compared to TI ＞4.73%. The reason behind the increase in the drag coefficient can be further explained based on the surface pressure distribution over the airfoil section at Re=1.09×105.

Fig.19 Drag coefficient vs angles of attack.

(2) Re=1.54×105

As discussed in the previous section, the mean drag coefficient increases with the increase in the turbulence intensity when TI ＞4.73%. The drag coefficient for various turbulent inflows at Re=1.54×105is discussed in Fig.17. From the figure,it can be represented that the measured drag coefficient values for different turbulent inflow rise monotonically till the point of separation. Following that, a rapid rise in the drag coefficient can be seen both in the increasing and decreasing direction of angles of attack. In this instance, for an extreme low turbulence case i.e. without grid, the drag increases monotonically till α=10°. However, CDat z/L=0.533 and TI=8.46% increases more rapidly than that at z/L=0.133 and TI=4.68% from α=15° since it shows a delayed separation relative to the baseline extreme low turbulence case i.e. without grid. This observation can be apparently linked to the occurrence of the stall and its associated characteristics with the increase in the turbulence intensity.With the increase in turbulence intensity, CDvaries relatively low in the pre-stall regime i.e. between α=0° and 15° but displays a discernable increase beyond the separation point or after the stall. Since the increase in the turbulence intensity of the inflow tends to enhance the entrainment of the free stream fluid, a low-pressure region formed after the separation point gives rise to the pressure drag on the airfoil. At smaller turbulence intensities, the energy imparted from the entrained free stream fluid to delay the separation was just enough leading to a relatively lesser drag when TI ＜4.73%.However, at higher turbulence intensities, the magnitude of the turbulence itself is more pronounced, which in turn further increases the pressure drag, thereby resulting in the increased drag coefficient.

(3) Re=2.0×105

Fig.18 shows the variation of the drag coefficient for various turbulence intensities in the case of Re=2.0×105. The characteristic curves of drag coefficient (CD) in the increasing and decreasing direction of angles of attack show a slight variation. As discussed earlier, the difference in the characteristic drag curves can be possibly explained by the aerodynamic hysteresis phenomenon.

Fig.19 displays the characteristic drag curves and its hysteresis loop for all the TI cases along with the baseline low turbulence without grid case. As can be seen from the figure, the aerodynamic hysteresis loop formed between the increasing and decreasing direction of angles of attack for the characteristic drag curve flows in the clockwise direction while it follows the counterclockwise direction at Re=2.0×105for the characteristic lift curve. In baseline low turbulence without grid case at TI=0.51%, the hysteresis loop forms between α=15° and 25°, whereas at z/L=0.533 and TI=8.46%i.e.in high turbulence intensity case the loop is limited between α=22°and 25°.From this,it can be inferred that the hysteresis loop decreases with the increase in the turbulence of the inflow.

The surface pressure distribution over the baseline low turbulence intensity TI=0.51% and the modified high turbulence intensity TI=8.46% at 25° angle of attack both in the increasing and decreasing direction of angle of attack.The surface pressure changes displayed in the figure clearly indicate the changes in the flow pattern over the airfoil surface. From the figure, it can be understood that the increase in the turbulence of the inflow tends to increase the favourable pressure gradient which in turn keeps the flow attached over the airfoil surface.Similarly,it can also be observed that the difference in the magnitude of the surface pressure over the increasing and decreasing direction of angle of attack reduces with the increase in the turbulence of the inflow which in turn might explain the reason behind the reduction in the aerodynamic hysteresis with the increase in the influence of the turbulent inflow. Similar results are reported in Ref.26stating that any increase in the Reynolds number or free stream turbulence can result in earlier transition and reattachment of the flow resulting in reduced aerodynamic hysteresis.

(4) Re=2.44×105

Fig.20 Drag coefficient vs angle of attack at Re=2.44×105.

Fig.20 shows the fluctuation of mean drag coefficient for various turbulence intensities operating at Re=2.44×105.It is evident from the figure that the increase in the turbulence of the inflow keeps the flow attached to the airfoil surface thereby generating a favourable pressure gradient in the vicinity of the leading edge and creating a pressure drag.In addition to the mean drag coefficient,the figure also shows the standard deviation of the drag coefficient for various turbulence intensities at Re=2.44×105.

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In the pre-stall regime i.e. before the separation point, the standard deviation of ΔCD/Δα varies relatively small with respect to the post-stall region.The large variation in the standard deviation of ΔCD/Δα in the post-stall region is most likely to be caused by the separated flow. The large variation of the CDvalues in the post-stall region can be further confirmed with the surface pressure distributions acting over the airfoil at α=15° shown in Fig.14. The noticeable change in ΔCD/Δα corresponding to the post-stall region is expected to be caused by the separation point associated with the stall as they are directly related to the increase in the drag.Therefore,the drag coefficient over the post-stall region shows a significantly larger deviation relative to the pre-stall region. Other than that,the difference in the magnitude of the standard deviation variation in the baseline low turbulence without grid case and the turbulent inflow cases generated by the grid is noticeable. It is believed that the effect of the turbulence inflow generated using grids played a vital role in this i.e.the turbulence nature of the inflow is one of the contributing factors for the increase in the magnitude of the standard deviation difference obtained between the baseline low turbulence and grid induced high turbulence methods. This further confirms that the drag coefficient CDincreases with the increase in the turbulence of the inflow. In addition to that, it can also be noted that the drag coefficient rises with the increase in the angle of attack as well.

(5) Re=3.01×105

The variation of the coefficient of drag(CD)over the airfoil section at various angles of attack for Re=3.01×105is presented in Fig.21. CDincreases with the increase in the turbulence of the inflow and the similar result has been described in the previous section above.As can be seen from the figure that the standard deviation of CDin the pre-stall regime exhibits a monotonic rise till the point of separation. However, when ΔCL/Δα changes from increasing to decreasing, the turbulence of the inflow along with the separation is more likely to induce disorderness in the flow,causing a larger difference in the standard deviation. It can be seen from the previous surface pressure distributions that with the increase in the Reynolds number the favourable pressure gradient increases to a new extent which in turn helps increasing CL. More quantitative relationship between the Re and the TI, and the TI and the CD& CLcould be established. It has been identified that with the increase in the angle of attack, CDstays higher though changing little in magnitude remains independent of turbulence intensity and Reynolds number.

Fig.21 Drag coefficient vs angle of attack at Re=3.01×105.

4. Conclusions

Wind tunnel tests are performed to examine the influence of the turbulence intensity over the aerodynamic characteristics of an airfoil at various Reynolds number. Various turbulence intensities have been generated using grids designed based on the barrier technique to understand the influence of the turbulent flow field on the aerodynamic characteristics of an airfoil.Surface pressure data were obtained for an unsymmetrical airfoil using Scanivalve simultaneous pressure scanner to identify the aerodynamic forces over angles of attack ranging from-25° to 25° in increment of 5°. Additionally, attempts were made to elucidate the aerodynamic hysteresis phenomenon of airfoils. Comparison of the aerodynamic forces along with the pressure distributions were also discussed in detail. Based on the detailed experimental investigation, the following conclusions were made:

(1) The lift characteristics of the airfoil at low turbulence intensity perform apparently poorer than its counterpart at higher turbulence intensity.

(2) In the case of extreme low turbulence i.e. without grid,the flow tends to separate in the vicinity of the leading edge itself.On the other hand,in the case of flow featuring turbulence, the flow remains attached for an extended region.

(3) It can be inferred that the turbulence intensity has a significant effect on the stall characteristics.

(4) The aerodynamic hysteresis loop which is predominant in the extremely low turbulence case i.e. without grid was found to be gradually disappearing with the increase in the turbulence of the inflow.In other words,it can be reported that the turbulence of the inflow influences the transient behaviour of the increasing and decreasing angles of attack, eventually resulting in the nearly same unsteady pattern.

The coefficient of lift and its standard deviation increase with the increase in the turbulence of the inflow as a function of an angle of attack. Moreover, the standard deviation of CLin the pre-stall region seems to be stable relative to the poststall region. This is most likely to be caused by the unstable flow induced in the vicinity of the separation point.

Acknowledgements

This research work was supported by the Science Engineering Research Board (SERB), Department of Science & Technology (DST) of India (No. ECR/2017/001199). The authors thank SERB/DST for their financial assistance in carrying out this research work through Early Career Research Award.