Transitional region of a round synthetic jet

Spreading and decay rates of several synthetic jets are experimentally measured up to 70–100 diameters ($d$) away from the orifice. It is found that, consistent with previous studies, after $10d–15d$ from the nozzle a synthetic jet displays a self-similar velocity profile together with enhanced spreading and decay behaviors than a continuous jet. However, this enhancement does not persist throughout the downstream region, as the spreading and decay rates around $30d–50d$ away from the orifice decrease to asymptotic values that are comparable to continuous jets. This intermediate region with enhanced spreading and decay rates is dubbed the transitional region, while the region beyond $30d–50d$, where the spreading and decay rates approach the continuous jet values, is referred to as the actual far field of a synthetic jet. Physically, the enhanced spreading and decay rates of the transitional region are caused by an enhanced mode of mixing related to the forced instability of the pulsed large vortices, which eventually break down into smaller eddies in the fully turbulent far field.

Figure 1

(a) Axial variation of the jet half width $b_{1/2}$ for six different synthetic jets listed in Table 1. The slope of a typical continuous jet (CJ) is also plotted for comparison. (b) Axial variation of the jet spreading rate $S_b$.

Figure 2

(a) Axial variations of the inverted scaled centerline velocity for the same cases shown in Fig. 1. (b) Axial variations of the nondimensional decay rate.

Figure 3

The beginning of the far field $x_f$ vs the stroke ratio $L/d$ for 50 different synthetic jets reported in Ref. [32].

Figure 4

(a) Mean vorticity contour for SJ33. $\omega$ is the vorticity, so $\omega d/fL$ is the nondimensional vorticity. (b) A sample phase-locked vorticity contour averaged over 50 different cycles of SJ33. (c) The instantaneous vorticity contour of a sample cycle corresponding to (b). (d) The locations of large vortices for the 50 different cycles of (b). The vortex locations are determined by the $Q$ criterion [47].

Figure 5

The left schematic shows the mean velocity field and the corresponding vortical structures in different regions of a synthetic jet. The right figure presents the power spectrum results in different regions of a sample synthetic jet (SJ17).