3.3 | The streamlines diagram and pressure contour analysis

Figure 6 shows the streamline diagrams and pressure contours of CA models at the investigated frequencies.
At the beginning of the downstroke (t/T = 0), vortices are seen on the upper surface of the airfoil trailing edge (Figure 6a). They gradually increase with the increase of frequency. At 75 Hz, the LEV appears on the leading edge of AP1 and AP2, but at AP3 an LEV is not visible. The TEV (trailing edge vortex) gradually increases and it is about shedding. The upper surface’s pressure of each CA model is negative pressure, which is less than the lower surface. The pressure difference of AP1 is the largest. By contrast, the AP3 is the least. So, the aerodynamic performance of AP1 is better than AP2 and AP3 with the increase of flapping frequency at this stage of the stroke cycle.
At the middle of the downstroke (t/T = 1/4), a large vortex appears on the upper surface near the leading edge, as shown in Figure 6b. This is the point where the maximum thrust is generated in a single flapping cycle (Yang et al., 2015). The TEV begins to shed with the increase of frequency, which indicates that the frequency has an important influence on the lift and thrust. In addition, the pressure difference is higher than the initial stage of the downstroke. At this time, the development of a LEV and a TEV are the main reasons for the change of the flapping wing aerodynamic performance (Gong et al., 2019). Duo to the corrugated groove more obvious at the leading edge of AP1 model, the LEV can be captured more persistently. But AP2 and AP3 cannot stably capture LEV by comparison AP1. So, the aerodynamic performance of AP2 and AP3 is lower than AP1. In other words, the parameterization of corrugated grooves may be beneficial for further research on the aerodynamic performance of corrugated wings.
At the middle of the upstroke (t/T = 3/4), there is a vortex on the lower surface near the leading edge of AP1, as shown in Figure 6c. But no vortex is found in AP2 and AP3. Because the corrugated structure at the leading edge of AP1 is more obvious, the unsteady effect of the airflow is higher. Moreover, the shedding of the TEV is obvious with the increase of frequency. At this time, the pressure difference between upper and lower surface is positive value. Therefore, the lift effect isn’t almost effective at this stage. In addition, the result also can see that the influence of corrugated structural parameters and flapping frequency on the aerodynamic performance is important.
In summary, the aerodynamic performance of the AP1 is the optimal among CA models. And, it has been confirmed that the position of the microstructure of the first groove affects the aerodynamic performance of the airfoil (CA models).