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).