Numerical investigation on the aerodynamic efficiency of bio-inspired corrugated and cambered airfoils in ground effect

Abstract

This research numerically investigates the flapping motion effect on the flow around two subsonic airfoils near a ground wall. Thus far, the aerodynamic efficiency of the dragonfly-inspired flapping airfoil has not been challenged by an asymmetric cambered airfoil considering the ground effect phenomenon, especially in the MAV flight range. The analysis is carried out on the basis of an unsteady Reynolds-averaged Navier-stokes (URANS) simulation, whereby the Transition SST turbulence model simulates the flow characteristics. Dragonfly-inspired and NACA4412 airfoils are selected in this research to assess the geometry effect on aerodynamic efficiency. Moreover, the impacts of Reynolds number (Re), Strouhal number (St), and average ground clearance of the flapping airfoil are investigated. The results indicate a direct relationship between the airfoil's aerodynamic performance ([Formula: see text]/[Formula: see text]) and the ground effect. The [Formula: see text]/[Formula: see text] increases by reducing the airfoil and ground distance, especially at [Formula: see text]. At [Formula: see text], by increasing the St from 0.2 to 0.6, the values of [Formula: see text]/[Formula: see text] decrease from 10.34 to 2.1 and 3.22 to 1.8 for NACA4412 and dragonfly airfoils, respectively. As a result, the [Formula: see text]/[Formula: see text] of the NACA4412 airfoil is better than that of the dragonfly airfoil, especially at low oscillation frequency. The efficiency difference between the two airfoils at St=0.6 is approximately 14%, indicating that the [Formula: see text]/[Formula: see text] difference decreases substantially with increasing frequency. For [Formula: see text], the results show the dragonfly airfoil to have better [Formula: see text]/[Formula: see text] in all frequencies than the NACA4412 airfoil.

Figure 1

(a) Three cross-sections of a dragonfly forewing (Aeshna Cyanea)^, (b) Final cross section profile based on profile 2.

Figure 2

Computational domain and applied boundary conditions around airfoil.

Figure 3

(a) Unstructured grids around airfoils (b) Close-up view of grid near NACA 4412 airfoil (c) Close-up view of grid near dragonfly airfoil.

Figure 4

NACA 4412 airfoil drag coefficient variations for different grids with close-up views.

Figure 5

Dragonfly-inspired airfoil drag coefficient variations for different grids with close-up views.

Figure 6

Drag coefficient variations versus time for different grids with close-up views.

Figure 7

Comparison of aerodynamic coefficients with Refs.^,.

Figure 8

Flow over a flapping airfoil in ground effect.

Figure 9

The $C_d$ of dragonfly airfoil in different $h_0$ over one oscillation period (OGE means out of ground effect).

Figure 10

The $C_l$ of dragonfly airfoil in different $h_0$ over one oscillation period.

Figure 11

The $C_d$ of NACA 4412 airfoil in different $h_0$ over one oscillation period.

Figure 12

The $C_l$ of NACA 4412 airfoil in different $h_0$ over one oscillation period.

Figure 13

Comparison of $C_d$, $C_l$ and $\overline{C_l}/\overline{C_d}$ between dragonfly and NACA 4412 airfoils in different distant from the ground at $St=0.4$ and $Re=5\times {10}^4$.

Figure 14

The $C_d$ and $C_l$ of dragonfly airfoil in different St at $Re=5\times {10}^4$.

Figure 15

The $C_d$ and $C_l$ of NACA 4412 airfoil in different St at $Re=5\times {10}^4$.

Figure 16

Comparison of $C_d$, $C_l$, and $\overline{C_l}/\overline{C_d}$ between dragonfly and NACA 4412 airfoils in different St at $Re=5\times {10}^4$.

Figure 17

Contours of momentary velocity coefficients across a cycle with $Re=5\times {10}^4$ and St=0.6 for both dragonfly and NACA4412 airfoils.

Figure 18

The mean aerodynamic performance variations in different St.

Figure 19

Contours of momentary pressure coefficients across a cycle with streamlines at $Re=5\times {10}^3$ and $St=0.6$ for (a) NACA4412 and (b) dragonfly airfoils.

Figure 20

Contours of momentary turbulent kinematic energy (T.K.E) across a cycle with $Re=5\times {10}^4$ and $St=0.6$ for both dragonfly and NACA4412 airfoils.