From Performance and Energy Expenditure of Coflow Jet Airfoil with Variation of Mach Number page 1758
Figure 5 compares the measured lift coefficient of several discrete
CFJ (DCFJ) airfoils with the baseline airfoil at a constant jet mass
flow rate [16,17]. The DCFJ airfoils in Fig. 5 have different slot
blockages to generate discrete injection holes, and hence different jet
velocity, while keeping the same mass flow rate. For example, the
open slot (black solid circles) has zero blockage. The obstruction
factor (OF; i.e., blockage), indicated after “DCFJ” in the figure
legend, is the percentage of the slot area blocked. An OF of 3/4 means
that 75% of the injection slot area is blocked, and it results in many
small discrete holes for the CFJ injection. Figure 5 shows that the
open-slot CFJ airfoil increases the maximum lift coefficient by about
50%, whereas the discrete CFJ airfoil with OF of 2/3 increases the lift
by about 100%. When the mass flow is increased, the measured
maximum lift coefficient is further augmented, as shown in Fig. 6.
Figure 6 shows all the airfoils generate thrust (negative drag) in the
wind tunnel testing, with the maximum amount produced by a CFJ
airfoil using discrete jets with OF of 3/4. The minimum drag is
reduced by 4000% to an enormous thrust coefficient of about 0.8. By
comparing with the open-slot CFJ airfoil, the discrete CFJ airfoil
needs half of the mass flow rate to achieve the same lift augment and
drag reduction [17]. However, the power consumed by the DCFJ is
significantly higher than for the open-slot CFJ airfoil, because the
smaller holes create more blockage loss for the jets. Nonetheless, the
extraordinary high lift and high thrust generated by the DCFJ deserve
the extra energy cost [17]. In nature, the only system that generates
both lift and thrust at the same time is flapping bird wings. In the manmade fixed wing systems, CFJ airfoils appear to be the only system
that can achieve such effect.
Dano et al. [17] experimentally investigated the energy
expenditure at Mach number of 0.03. Their study indicates that the
CFJ airfoil gains drastic performance enhancement at high AoA for a
low energy expenditure. Additional numerical studies performed by
Lefebvre et al. [23,24] confirm the trends. However, no study has
been conducted to investigate CFJ airfoil performance enhancement
and energy expenditure with Mach number effect.