Drag reduction of three-dimensional bodies by base blowing with various gas densities

This study reveals that injecting a light fluid of density ${\rho }_b$ in the recirculating bubble of a bluff body at $\text{Re}\approx 6.4\times {10}^4$ has a greater drag reduction potential than blowing fluid of a density greater than or equal to that of the free stream $\rho$. It is found that the maximum drag reduction scales as ${({\rho }_b/\rho )}^{-\frac{1}{6}}$. This power law combines the ability of the recirculating bubble to diffuse the injected momentum and the effectiveness of the injection to increase the recirculating bubble length.

Figure 1

(a) Sketch of the experimental setup. (b) Base of the bluff body depicting the location of pressure taps (with black dots) and the blowing slit of surface $s$ placed at the bottom of the body base.

Figure 2

Base suction coefficient $C_B$ vs (a) the volumetric flow rate coefficient $C_q$, and (b), (c) the scaled coefficients $C_q{({\rho }_b/\rho )}^{\alpha }$ as power laws of the density ratio. Vertical dashed lines in (a) indicate the transition from the mass to the momentum regimes for each gas as described in Lorite-Díez et al. [9].

Figure 3

Averaged streamwise velocity $U_x^{*}$ contours at the $y^{*}=0$ plane along with averaged flow streamlines (gray lines). The black continuous line denotes the values $U_x^{*}=0$. (a) Case with no injection, cases with (b) air at $C_q^{\text{Opt}}=0.008$ and (c) helium at $C_q^{\text{Opt}}=0.021$. The blowing slit is indicated with the white band.

Figure 4

(a) Recirculation length $L_r^{*}$ and (b) drag coefficient ${\tilde{C}}_d$ vs base suction $C_B$ for the three gases and the case with no blowing.

Figure 5

Mean velocity profiles of the mixing layer that develops from the bottom edge of the base. Time-averaged velocity profiles $U_x^{*}$ at several streamwise locations for the nonblowing case and two blowing flow rates employing He and air which produce the same recirculation region length, $L_r^{*}=1.54$.