Comparison to Two-Dimensional Experiments

f =3.3 Hz yields a reduced frequency ofk=0.0196. The higher pitching frequency f =9.9 Hz yields a reduced frequency ofk=0.0588. At this frequency, the maximum suction peak and consequently stall is postponed to higher angles of attack for all sections. Due to the stronger motion induced downwash, thec_p-distributions are less symmetric around the maximum angle of attack for f =9.9 Hz than for f =3.3 Hz. The reattachment is postponed to higher phase angles for the higher frequency, similar to the results of McCroskey [16] and Carr [14].

At section S2, one notices two discernible peaks even though the data is phase averaged. More distinct, repeatable vortices seem to travel across the blade tip at the higher frequency.

Figure 5.25:Phase-averagedc_p-distributions for f =3.3Hz vs f =9.9Hz

5.6 Comparison to Two-Dimensional Experiments

In the following, the test case LC-DS3 is compared to 2D experimental data of the airfoils EDI-M112 and EDI-M109 [24]. Thus, the differences between the advanced planform and the midsection of a 2D rectangular wing can be studied. A straight-forward comparison between two test cases is not possible. Due to the twist distribution of the blade tip (Fig. 3.1) and the

78 5 DYNAMICSTALL AT THEDOUBLE-SWEPTBLADETIP

availability of certain measurement points only, different test cases are compared at the span-wise sections S2, S4 and S6, shown in Fig 5.26. The phase-averaged data is presented. For the convenience of the reader the standard deviation is only indicated for the sensors from the lead-ing edge tox/c=0.16. At section S2, the 3D test case LC-DS4 (α=11^◦±5^◦) is used instead of LC-DS3. Thereby, the airfoil, the oscillation amplitude and the geometric angle of attack match approximately at each section. The Mach numberMa=0.5 and the pitching frequency f =6.6 Hz are equal for all cases. Due to higher chord length the Reynolds number isRe_2D=3.0·10⁶ and the reduced frequency isk_2D=0.07 for the 2D-cases.

The strongest discrepancy between the 2D-case and the blade tip is detected at the kink. The suction at the leading edge is decreased for the blade tip over the whole cycle. Again, this indi-cates that the spanwise velocities which have opposite signs at the forward and backward swept part lead to a less accelerated flow at the leading edge. At the trailing edge the flow is less decelerated. Consequently the suction peak is lower and stall is postponed to higher angles of attack for the blade tip. It is remarkable, that the phase-averagedcp-data of the 2D-case shows strong oscillations after the first drop in thecp-distribution. Nevertheless, the standard deviation remains high, so that no stronger similarity from cycle to cycle can be deduced.

At section S6, the suction peak is higher, sharper and located closer to the leading edge then for the 2D case. The deflection of the inflow due to the sweep seems to lead to a stronger accelerated flow at the leading edge and consequently to the increased suction peak. Flow separation occurs around the same phase angles but is more pronounced for the 3D-case. Similar to the differ-ences at section S2 at high angles of attack, this can be caused by the lower Reynolds number and the lower reduced frequency, as shown in the preceding sections. One can state, that the agreement between the 2D- and 3D-cp-distributions at the section S2 is well for the first half of the cycle. The differences in thecp-distributions are smaller at the trailing edge. The suction level is similar, even as flow separates aroundψ≈225^◦. Atx/c=0.86, ac_p-distribution of a pressure transducer located further inboard at section S1 is depicted. From the lower suction level aroundψ ≈270^◦, one can deduce that the twist reduction towards the root is sufficient to avoid separation to be triggered by the corner stall at the wind tunnel wall. As shown in Gardner et al. [77], the gap between wind tunnel model and wind tunnel wall also reduces the tendency towards flow separation.

5.6 Comparison to Two-Dimensional Experiments 79

Figure 5.26:Comparison of the blade tip to 2D airfoil experiments

80 5 DYNAMICSTALL AT THEDOUBLE-SWEPTBLADETIP

5.7 Inuence of the Surface Roughness - Pressure Sensitive Paint

In the second measurement campaign the pressure distribution on the upper surface of the model was investigated by means of unsteady pressure sensitive paint (iPSP). As shown by the static polars in Fig. 4.2, the surface roughness and the thickening of the airfoil seems to change the flow significantly. Before investigating the influence of the oscillation amplitude by means of iPSP, the pressure distributions of LC-DS2 with and without coating are shown. In Figs. 5.27 and 5.28, the phase-averaged data is shown for the sections S2 and S4-S8. The iPSP-measurements have a sampling frequency of 64 points per period.

Figure 5.27:Ma=0.5,α =8^◦±5^◦with and without iPSP

One can deduce, that the coating enhances trailing edge separation at the inboard section S2 where the airfoil thickness is 12% (EDI-M112). The suction level at the trailing edge is signifi-cantly increased around the maximum angle of attack. However, the suction peak at the leading edge does not collapse, but is slightly decreased. According to Schlichting [58], the increased surface roughness shifts the laminar-turbulent transition in upstream direction. This can be seen in thecp-distribution of section S4 atx/c=0.15. In case of the coated surface the transition