The spatial growth rate (ki = 2πf/U) Muscimol proportional to the characteristic frequency of the shear layer instability. At a fixed Reynolds number, the velocity along the shear layer can be assumed nearly constant. Thus, small spatial growth rate (or slope) implies low characteristic frequency. In Fig. 12(c), as the gap ratio reduces, the spatial growing rate of the outer shear layer of the wide wake decreases mildly at G∗ = 1.25 and 0.75 and significantly at G∗ = 0.25. This corresponds to slow variations of StwStw at large gap ratio and rapid change of StwStw at small gap ratio in Fig. 3. On the other hand, in Fig. 12(d), the slope of um∗(fn∗) component increases slightly as the gap ratio decreases. So do the values of StnStn increase as the gap ratio decreases in Fig. 3. For D/d = 2 in Fig. 12(c) and (d), as the gap ratio decreases, the reduced spatial growing rate of the shear layer of wide wake is subatomic particles more significantly than the increasing spatial growing rate of the narrow wake. This explains the rapid increase of the frequency ratio at small gap ratio shown in Fig. 4 for D/d = 2.