## 5. Comparison of Rapid Scanning and Static Hot Wire Measurements

### The aim of this section is to compare the results about the wandering features obtained from the experimental campaigns carried out in the present work. The vortex wandering is analyzed both through the parameters directly linked to the wandering as the wandering amplitude σ _{yz} and the anisotropy parameter e, and through the quantities inherent to the mean velocity profiles that reveal the wan- dering effects: the peak tangential velocity V _{θ1} , the vortex core radius r _{1} and the axial velocity deficit at the vortex core centre U _{D} .

### In order to compare these quantities were taken several tests series into account:

### the rapid scanning (denoted as RS in the following) data, whose values are re- ported in Sec. 3.5.1, Sec 3.5.2 and Sec 3.5.3, the static measurements carried out by traversing the 3HFP, whose values are reported in Sec. 4.4, and the static mea- surements carried out by Iungo & Skinner [24] by traversing the 5HP using the same set-up of the above-mentioned tests.

### The analysis of the results is organized as follows:

### • downstream distance variation;

### • effects of the variation of the angle of attack;

### • Reynolds dependency.

### 5.1. Downstream Distance Variation

### In the present section an analysis of the downstream variation of the wandering pa-

### rameters and of the main features of the wing-tip vortex is presented. The compared

### 0 1 2 3 4 5 6 0

### 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

### x/c σ

yz### /c

### 5HP RS 5HP RS Devenport 5HP Traverses 3HFP Traverse

### 0 1 2 3 4 5 6

### −0.2

### −0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

### x/c

### e

### 5HP Traverses 3HFP Traverse

### (a) (b)

### Figure 5.1. Downstream variation of the non-directional wandering amplitude (a) and of the anisotropy parameter (b) at α = 8 ^{◦} and U _{∞} = 10 m/s. Com- parison between 5HP rapid scanning data (analyzed through the PDF fitting and through the Devenport et al [12] method), 5HP traverses data (from Iungo & Skinner [24] measurements) and 3HFP traverses data.

### test series are: RS measurements performed at x/c = 2, 2.5, 3, 3.5, 3.88, 5, 5.5, 3HFP traverses at x/c = 1, 2, 3, 4, 5 and 5HP traverses at x/c = 0.1, 0.33, 0.66, 1, 1.5, 2, 3, 5, 6. All these tests series were carried out with a free-stream velocity of U _{∞} = 10 m/s and a wing incidence of α = 8 ^{◦} . After a preliminary comparison of the vortex wandering amplitude, the anisotropy parameter and the location of the mean vortex centre, the effects of wandering on the mean velocity profiles are taken into account by comparing the wandering affected data with the re-centred data.

### Firstly, it should be pointed out that the data carried out through the RS tech-

### nique were analyzed using the PDF fitting of the distribution of the vortex cen-

### tre locations, whereas the data carried out through the static measurements are

### processed using the method proposed by Devenport et al in [12]. In addition, in

### Fig. 5.1 (a) is also reported the non-directional wandering amplitude obtained from

### RS data processed with the Devenport et al [12] method. This quantity is signif-

### icant to compare the results yielded by applying different processing procedure to

### the same data set, even though the number of samples for each acquisition point

### in the RS tests is considerably lower than in the traverses. The Fig. 5.1 (a) shows

### a good agreement between the different data sets. In particular, the 3HFP data

### 5.1. Downstream Distance Variation

### confirms the linear rate of the wandering amplitude with increasing the streamwise distance already pointed out by RS data, see Sec. 3.5.1. The difference between the RS data, both evaluated using the PDF fitting and the Devenport et al [12]

### method, and the 3HFP data is very low for each analyzed location. Moreover, considering the non-directional wandering amplitude σ yz , the difference between the RS data processed using the PDF fitting and the data obtained from 3HFP is lower than the difference between the RS data processed using the PDF fitting and the same data processed using the Devenport et al [12] method. This consideration excludes to ascribe the small differences in the wandering amplitude evaluation to a bias error introduced by one of the two processing methods. Finally, it is significant to note that the values of the non-directional wandering amplitude as carried out by Iungo & Skinner [24] follow an analogous trend with respect to the other data sets.

### The values of the anisotropy parameter, as obtained from the cross-correlation coefficients vw measured by the 3HFP traverses is fairly constant in all the tested conditions and e = 0.5 ÷ 0.6, as reported in Fig. 5.1 (b). This finding is coherent with the values of e calculated by Iungo & Skinner [24] for a slightly wider range of downstream locations. Indeed, they show an abrupt increasing of the anisotropy parameter from x/c = 0.1 to x/c = 2 followed by a roughly constant behavior from x/c = 2 to x/c = 6. This suggests an early evolution of the direction of principal axes of wandering up to 2 chord-lengths downstream of the wing-tip, and a successive stabilization of this parameter. However, the data carried out with static measurements present an extremely different behavior with respect to the RS data (reported in Tab. 3.12) with varying the streamwise distance. It can be due to the different method used to evaluate e from the velocity signals of the 3HFP traverses and the RS. Moreover, the insufficient reliability of the estimate of the parameter e both using the fitting of the experimental PDF and the cross- correlation coefficient vw is extensively discussed in Sec. 3.5.1 and in Sec. 4.3.2, respectively.

### Considering the 3HFP data reported in Tab. 4.5, the direction of principal axes

### of wandering Θ was derived from the anisotropy parameter e and it changes from

### about 40 ^{◦} to about 50 ^{◦} . It is compared with the analogous parameter obtained by

### Iungo & Skinner [24] measurements. Despite the fact that Θ evaluated from 3HFP

### data is slightly higher than Θ evaluated from 5HP data, the excursion of the values between x/c = 1 and x/c = 6 is comparable for both methods and it highlights a fairly constant behavior of Θ with varying the streamwise distance. However, it is not advisable to make further considerations because of the amplification of the uncertainties in the calculation of e in the estimate of Θ.

### In Fig. 5.2 (a) and Fig. 5.2 (b) is reported the trajectory of the mean vortex centre in the cross-plane in terms of spanwise (Y c ) and normal coordinates (Z c ), respectively. The 3HFP measurements of Y _{c} and Z _{c} matches the RS measure-

### 0 1 2 3 4 5 6

### −0.3

### −0.25

### −0.2

### −0.15

### −0.1

### −0.05 0

### x/c Y

c### /c

### 5HP RS 5HP Traverses 3HFP Traverses

### 0 1 2 3 4 5 6

### −0.1

### −0.05 0 0.05 0.1 0.15

### x/c Z

c### /c

### 5HP RS 5HP Traverse 3HFP Traverse

### (a) (b)

### Figure 5.2. Downstream variation of the trajectory of the mean vortex centre at α = 8 ^{◦} and U _{∞} = 10 m/s: spanwise coordinate (a) and normal coor- dinate (b). Comparison between 5HP rapid scanning, 5HP traverses (from Iungo & Skinner [24] measurements) and 3HFP traverses.

### ments very well, confirming the observations reported in Sec. 3.5.1 as regard to the downstream evolution of the mean vortex centre position. In particular, the Y _{c} position is slightly overestimated by the 3HFP data, but the maximum difference for x/c = 3 is the 18% of the total excursion of the vortex centre in the spanwise direction from x/c = 1 to x/c = 5. The Y _{c} values at different downstream sections exhibited by Iungo & Skinner [24] measurements are always comprise between the two data sets carried out in the present work.

### According to the substantial agreement about the evaluation of Y _{c} with proceed-

### ing downstream, the different data sets show analogous results also in the estimate

### of the normal coordinate of the vortex centre. The RS data exhibit a fairly constant

### 5.1. Downstream Distance Variation

### trend of the Z _{c} with a slight decrement at x/c = 5.5, and the static measurements suggest a weak increment of Z _{c} by proceeding downstream up to 2 chord-lengths, followed by a slow decrease from x/c = 2 to x/c = 6. Moreover, the differences between the 3HFP and the RS data in the determination of Z _{c} are always lower than the differences in the determination of Y c .

### 0 1 2 3 4 5 6

### 0.2 0.25 0.3 0.35 0.4 0.45 0.5

### x/c V

θ 1### /U

∞### 5HP RS no−wandering 5HP RS wandering 5HP Traverse 3HFP Traverse

### 0 1 2 3 4 5 6

### 0.025 0.03 0.035 0.04 0.045 0.05 0.055 0.06 0.065

### x/c r

1### /c

### 5HP RS no−wandering 5HP RS wandering 5HP Traverses 3HFP Traverse

### (a) (b)

### Figure 5.3. Downstream variation of the peak tangential velocity (a) and the core radius (b) at α = 8 ^{◦} and U _{∞} = 10 m/s. Comparison between re- centred 5HP rapid scanning data, 5HP rapid scanning data affected from wandering, 5HP traverses (from Iungo & Skinner [24] measure- ments) and 3HFP traverses.

### In Fig. 5.3 (a) and in Fig. 5.3 (b) are reported the downstream evolutions of the peak tangential velocity and the vortex core radius, respectively. The first significant output is the good agreement between the RS data affected by wandering (reported in Tab. 3.13) and the 3HFP data (reported in Tab. 4.9) for both the analyzed quantities.

### Observing the downstream evolution of V _{θ1} plotted in Fig. 5.3 (a) it is evident

### the roughly constant decrease of this quantity with increasing the downstream

### distance. The rate of decreasing of the peak tangential velocity measured by the

### RS from x/c = 2 (V _{θ1} = 0.419) to x/c = 5.5 (V _{θ1} = 0.376) is roughly the same of

### the one measured by the 3HFP traverses from x/c = 1 (V _{θ1} = 0.437) to x/c = 4

### (V _{θ1} = 0.332). The same rate is detectable in the 5HP data, but the values reported

### by Iungo & Skinner [24] are always lower than the data carried out in the present

### work. This shift is probably due to a slightly different calibration of the 5HP.

### Concluding, the 3HFP data confirms the evaluation of the vortex wandering effects on the tangential velocity peaks, detectable from the difference between the RS data and the data carried out from static measurements.

### A similar conclusion regards the vortex core radius, which values at different streamwise locations, for the data affected from wandering, are determined by the 3HFP traverses measurements and by the RS uncorrected data. The results of these two techniques agree for all the analyzed streamwise locations in describing an increase of the vortex size with proceeding downstream. The only exception is for x/c = 5, where the RS yields a considerably larger value of r _{1} than the 3HFP.

### In particular, the RS measurements affected by wandering show an abrupt increase of the r 1 value between x/c = 3.88 and x/c = 5.5, and the 3HFP data follow the same trend for all the tested locations. Consequently, RS data affected by wandering at x/c = 2 ÷ 3.88 and 3HFP data indicate the same rate of increasing for r _{1} , whereas this rate is double if all the tested locations are taken into account for the RS uncorrected data. The data carried out with the static measurements using the 5HP indicate an increase rate similar to the one suggested by the 3HFP data, although the deviation between the 5HP data and the other data sets is fairly consistent. Nevertheless, the results of the measurements affected from wandering are very different with respect to the results of the RS measurements corrected for wandering effects, that only show a slight increase. The increase rate of the vortex radius corrected for wandering effects is 3 times lower than the rate of the vortex radius affected by wandering with proceeding downstream.

### In Fig. 5.4 is reported the streamwise variation of the axial velocity deficit in

### correspondence of the vortex centre. The three series of measurements affected

### by wandering agree to indicate a fairly constant trend of the module of the the

### axial velocity defect (U _{D} ' 0.1 U ∞ ) from x/c = 1 to x/c = 6. In particular, the

### 3HFP measurements do not register the abrupt decrease that the RS uncorrected

### data present between x/c = 3.5 and x/c = 3.88. The 5HP traverses confirm the

### constant trend of U _{D} with increasing the downstream distance, apart from two

### outliers that indicate very low values of the U _{D} module. Nevertheless, the RS re-

### centred data show an increase of the module of the axial velocity deficit that is very

### different from the trend obtained with the others measurements technique affected

### 5.2. Effects of the Variation of the Angle of Attack

### 0 1 2 3 4 5 6

### −0.4

### −0.35

### −0.3

### −0.25

### −0.2

### −0.15

### −0.1

### −0.05 0 0.05

### x/c U

D### /U

∞### 5HP RS no−wandering 5HP RS wandering 5HP Traverses 3HFP Traverses

### Figure 5.4. Downstream variation of the axial velocity deficit at the core centre at α = 8 ^{◦} and U _{∞} = 10 m/s. Comparison between re-centred 5HP rapid scanning data, 5HP rapid scanning data affected from wander- ing, 5HP traverses (from Iungo & Skinner [24] measurements) and 3HFP traverses.

### by wandering. For instance, at x/c = 5 the module of the velocity defect corrected for wandering is 5/3 the value registered from the 3HFP.

### 5.2. Effects of the Variation of the Angle of Attack

### In this section an analysis of the behavior of the vortex wandering with increasing the incidence of the wing is provided.

### In Fig. 5.5 (a) the estimate of the non-directional wandering amplitude σ _{yz} as carried out with different measurements techniques is presented for x/c = 3, x/c = 5 (RS and 3HFP traverses) and x/c = 6 (5HP traverses performed by Iungo

### & Skinner [24]). It is evident the good agreement between the estimate of the wandering amplitude with the RS measurements and with the 3HFP traverses. In spite of the fact that the 3HFP slightly overestimates the σ _{yz} at x/c = 3 and slightly underestimates the σ _{yz} at x/c = 5, the 3HFP data confirm the roughly invariance of the wandering amplitude with increasing the angle of attack for U _{∞} = 20 m/s.

### However, the results obtained by Iungo & Skinner [24] at x/c = 6 and U _{∞} = 20 m/s

### indicate a completely different behavior of the analyzed parameter. They found a

### decay of the wandering amplitude from σ yz = 0.119 for α = 4 ^{◦} to σ yz = 0.035

### 4 6 8 10 12 14 0

### 0.02 0.04 0.06 0.08 0.1 0.12

### α [°]

### σ

yz### /c

### 5HP RS x/c=3 5HP RS x/c=5 5HP Traverses x/c=6 3HFP Traverses x/c=3 3HFP Traverse x/c=5

### 4 6 8 10 12 14

### −0.2

### −0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

### α [°]

### e

### 5HP Traverse x/c=6 3HFP Traverse x/c=3 3HFP Traverse x/c=5

### (a) (b)

### Figure 5.5. Non-directional wandering amplitude (a) and anisotropy parameter of wandering (b) as a function of the angle of attack at U _{∞} = 20 m/s.

### Comparison between 5HP rapid scanning, 5HP traverses (from Iungo

### & Skinner [24] measurements at U _{∞} = 10 m/s and x/c = 6) and 3HFP traverses.

### for α = 12 ^{◦} . Consequently, the wandering response to wing incidence variations is probably regulated by a threshold value in the vortex strength: below this threshold the wandering is sensitive to the variations of the angle of attack, viz. the wandering amplitude reduces with increasing the angle of attack, whereas over this threshold the wandering amplitude becomes insensitive to variations of the angle of attack.

### In particular, investigating the effects of the angle of attack, the wing-tip vortex strength was below the threshold for the base flow condition tested by Iungo &

### Skinner [24], whereas it was above the threshold for the base flow condition tested in the present work.

### The values of the anisotropy parameter with varying the angle of attack are reported in Fig. 5.5 (b) for the 3HFP traverses at x/c = 3, 5 and for the 5HP traverses at x/c = 6. It is immediate to observe that the three data sets are not in agreement in the evaluation of e. The deviation between the data is high and only the 5HP data show a clear trend.

### In Fig. 5.6 (a) and in Fig. 5.6 (b) are shown the spanwise and the normal coordi- nates of the mean vortex centre, respectively, as a function of the angle of attack.

### The data of the three test series agree to individuate the substantial invariance of

### 5.2. Effects of the Variation of the Angle of Attack

### 4 6 8 10 12 14

### −0.3

### −0.25

### −0.2

### −0.15

### −0.1

### −0.05 0 0.05

### α [°]

### Y

c### /c

### 5HP RS x/c=3 5HP RS x/c=5 5HP Traverses x/c=6 3HFP Traverses x/c=3 3HFP Traverses x/c=5

### 4 6 8 10 12 14

### −0.3

### −0.25

### −0.2

### −0.15

### −0.1

### −0.05 0 0.05

### α [°]

### Z

c### /c

### 5HP RS x/c=3 5HP RS x/c=5 5HP Traverses x/c=6 3HFP Traverses x/c=3 3HFP Traverses x/c=5

### (a) (b)

### Figure 5.6. Trajectory of the mean vortex centre at U _{∞} = 20 m/s: spanwise coor- dinate (a) and normal coordinate (b) as a function of the angle of at- tack. Comparison between 5HP rapid scanning, 5HP traverses (from Iungo & Skinner [24] measurements at U _{∞} = 10 m/s and x/c = 6) and 3HFP traverses.

### the mean vortex centre in the spanwise direction with varying the angle of attack, although the 3HFP data slightly overestimates Y _{c} at x/c = 3 and slightly underes- timates it at x/c = 5 with respect to the vortex centre position individuated from the RS data, see Fig. 5.6 (a).

### The difference between the 3HFP data and the RS data reduces taking Z _{c} into account. As it is exhibited in Fig. 5.6 (b), the two data sets agree in the individua- tion of the rate of the downward motion of Z _{c} . Moreover, both the 3HFP and the RS data fix at 5/3 the ratio between the rate of downward motion of the vortex centre at x/c = 3 with respect to the same quantity at x/c = 5 with increasing α.

### The 5HP data are different to the results carried out in the present work, however, these data were obtained for a free-stream velocity (U _{∞} = 10m/s) lower than the one of the present work (U _{∞} = 20m/s) and for a streamwise location (x/c = 6) greater than the ones used for our tests (x/c = 3 and x/c = 5).

### In the following, the results carried out by Iungo & Skinner [24] are not reported,

### because the tested conditions chosen for the analysis of the angle of attack depen-

### dency of the vortex parameters are sensibly different, thus the comparison of the

### analyzed quantities is not reliable.

### 4 6 8 10 12 14 0.1

### 0.2 0.3 0.4 0.5 0.6 0.7 0.8

### α [°]

### V

θ 1### /U

∞### 5HP RS no−wandering 5HP RS wandering 3HFP Traverses

### 4 6 8 10 12 14

### 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

### α [°]

### V

θ 1### /U

∞### 5HP RS no−wandering 5HP RS wandering 3HFP Traverses

### (a) (b)

### Figure 5.7. Peak tangential velocity as a function of the angle of attack at U _{∞} = 20 m/s, x/c = 3 (a) and x/c = 5 (b). Comparison between re- centred 5HP rapid scanning data, 5HP rapid scanning data affected from wandering, 5HP traverses (from Iungo & Skinner [24] measure- ments at U _{∞} = 10 m/s and x/c = 6) and 3HFP traverses.

### In Fig. 5.7 (a) and (b) the peak tangential velocity variation is shown as a function of the angle of attack for U _{∞} = 20 m/s, x/c = 3 and x/c = 5, respectively. The increment of the vortex strength due to the increase of the angle of attack produces an enhance of V _{θ1} , clearly visible in all the data sets carried out both with the RS and with the static measurements. The roughly linear dependency of the V _{θ1} from the angle of attack is fairly the same both for x/c = 3 and x/c = 5. Moreover, the difference between the data affected and not affected by wandering is very low at x/c = 3, whereas it is appreciable at x/c = 5, where the wandering effects are significant. It should be pointed out the evident concordance of the results produced from the 3HFP data and from the RS uncorrected data.

### For the conditions tested in the present work at U _{∞} = 20 m/s, x/c = 3 and x/c = 5, see Fig. 5.8 (a) and (b), respectively, r _{1} as obtained from RS re-centred data increases by following a trend well represented by a square root law with increasing the angle of attack, see Sec. 3.5.2. The 3HFP results, showing a good agreement with the RS uncorrected data, suggest a roughly linear dependency of r _{1} from the angle of attack for both the analyzed streamwise locations.

### In Fig. 5.9 the axial velocity deficit at the core centre is reported as a function

### 5.3. Effects of the Variation of the Reynolds Number

### 4 6 8 10 12 14

### 0.02 0.022 0.024 0.026 0.028 0.03 0.032 0.034 0.036 0.038 0.04

### α [°]

### r

1### /c

### 5HP RS no−wandering 5HP RS wandering 3HFP Traverse

### 4 6 8 10 12 14

### 0.02 0.022 0.024 0.026 0.028 0.03 0.032 0.034 0.036 0.038 0.04

### α [°]

### r

1### /c

### 5HP RS no−wandering 5HP RS wandering 3HFP Traverse

### (a) (b)

### Figure 5.8. Radius of the vortex core as a function of the angle of attack at U _{∞} = 20 m/s, x/c = 3 (a) and x/c = 5 (b). Comparison between re- centred 5HP rapid scanning data, 5HP rapid scanning data affected from wandering, 5HP traverses (from Iungo & Skinner [24] measure- ments at U _{∞} = 10 m/s and x/c = 6) and 3HFP traverses.

### of the angle of attack at U _{∞} = 20 m/s, x/c = 3 and x/c = 5 as obtained from the RS measurements and from the 3HFP traverses. As pointed out in Sec. 4.4, U _{D} measured by the 3HFP displays the same behavior registered by the RS data affected from wandering. The agreement between these two data sets is very good for both the downstream locations. The wandering effects are more evident at x/c = 5, where, at high angles of attack, for the RS re-centred data the jet flow in the core preserves an abrupt decay of axial velocity defect in the vortex core.

### For data affected by wandering, the velocity deficit in this very small region was averaged, by the wandering effects, with the velocity excess in the region that surrounds the vortex centre and it disappears completely. Consequently, neither the RS uncorrected data neither the 3HFP data registered a deficit of the axial velocity in the jet flow at high angles of attack.

### 5.3. Effects of the Variation of the Reynolds Number

### In Fig. 5.10 the variation of the non-directional wandering amplitude is presented

### as a function of the free-stream velocity, for the tests performed at α = 8 ^{◦} and

### x/c = 5 with the RS and by traversing the 3HFP. Furthermore, in the same figure

### 4 6 8 10 12 14

### −0.2

### −0.15

### −0.1

### −0.05 0 0.05 0.1 0.15 0.2

### α [°]

### U

D### /U

∞### 5HP RS no−wandering x/c=3 5HP RS wandering x/c=3 3HFP Traverses x/c=3

### 4 6 8 10 12 14

### −0.2

### −0.15

### −0.1

### −0.05 0 0.05 0.1 0.15 0.2

### α [°]

### U

D### /U

∞### 5HP RS no−wandering x/c=5 5HP RS wandering x/c=5 3HFP Traverses x/c=5

### (a) (b)

### Figure 5.9. Axial velocity deficit at the core centre as a function of the angle of attack at U _{∞} = 20 m/s, x/c = 3 (a) and x/c = 5 (b). Comparison between re-centred 5HP rapid scanning data, 5HP rapid scanning data affected from wandering, 5HP traverses (from Iungo & Skinner [24]

### measurements at U _{∞} = 10 m/s and x/c = 6) and 3HFP traverses.

### the data carried out by Iungo & Skinner [24] at α = 8 ^{◦} and x/c = 6 are plotted.

### The data obtained in the present work (RS and 3HFP) show an excursion in the σ _{yz} values with increasing the Reynolds number much lower than the excursion shown by the 5HP traverses. However, the 3HFP data and the 5HP data assess the trend observed in the RS tests (Sec. 3.5.3): the reduction of the wandering amplitude with increasing U _{∞} is not linear and the wandering amplitude seems to reach an asymptotical value with proceeding towards the highest free-stream velocities. This statement is in agreement with the different response of the vortex wandering to a variation of the flow condition depending on the vortex initial intensity, as it was observed in Sec. 5.2 from Fig. 5.5 (a). Indeed, a free-stream velocity variation over U _{∞} = 20 m/s produces a negligible change in the wandering amplitude, whereas the wandering is sensitive to a free-stream velocity variation if it occurs below 20 m/s.

### Both the vortex parameters exhibited in Fig. 5.11 (a) and (b), the peak tangential

### velocity and the vortex radius, respectively, referred to data affected by wandering,

### seem to reach the value corrected for wandering with increasing the free-stream

### velocity. Indeed, the RS data corrected for wandering effects vary roughly linear

### 5.3. Effects of the Variation of the Reynolds Number

### 10 20 30

### 0 1 2 3

### x 10

^{−4}

### U

_{∞}

### [m/s]

### σ

yz### /c

### 5HP RS x/c=5 5HP Traverses x/c=6 3HFP Traverses x/c=5

### Figure 5.10. Non-directional wandering amplitude as a function of the free-stream velocity at α = 8 ^{◦} . Comparison between re-centred 5HP rapid scan- ning data (at x/c = 5), 5HP rapid scanning data affected from wan- dering (at x/c = 5), 5HP traverses (from Iungo & Skinner [24] mea- surements at x/c = 6) and 3HFP traverses (at x/c = 5).

### with increasing the U _{∞} . Conversely, the other data sets affected from wandering show an increase of the peak tangential velocity and a decrease of the vortex radius, changing from U _{∞} = 10 m/s to U _{∞} = 20 m/s, much higher than the one that occurs for an increment of the asymptotic velocity from U _{∞} = 20 m/s to U _{∞} = 30 m/s.

### Furthermore, the variation rate of V _{θ1} and r _{1} from U _{∞} = 20 m/s to U _{∞} = 30 m/s

### is very similar to the variation rate of the corrected RS data.

### 10 20 30 0.2

### 0.25 0.3 0.35 0.4 0.45 0.5

### U

_{∞}

### [m/s]

### V

θ 1### /U

∞### 5HP RS no−wandering x/c=5 5HP RS wandering x/c=5 5HP Traverses x/c=6 3HFP Traverses x/c=5

### 10 20 30

### 6 7 8 9 10 11 12 13 14

### U

_{∞}