Numerical Study of Passive and Active Flow Separation Control ...

at x = 0.217C shows a wave packet with a higher peak centered at f sep . In Case 1, the
second separation bubble starts at x = 0.21C, and it is not surprise that its spectrum at x =
0.217C has an isolated peak at f sep .
(a) x = 0.163C
Fig. 24. Spectra of pressure fluctuations of Case 1
28
(b) x = 0.217C
4.3 Case 2 – Flow Separation Control with Active Vortex Generators
In controlled case 2, a pair of oscillating vortex generators was used for active flow
separation control. The vortex generator layout can be found in Section 3. The location of
the vortex generators is the same as in controlled case 2. The motion of two vortex
generators was synchronized and controlled by a sinusoidal duty cycle with a frequency
of fe = 15 U ∞ / C that equals the natural frequency of the separation region in the
baseline case. It has been reported that the range of effective frequency of excitation
includes bands of naturally amplified frequencies centered at f sep and the normalized
+
frequency of excitation should be = f / f = O(
1)
[24, 39]. Given the length of
F e sep
natural separation bubble X = 0.225C
in the uncontrolled baseline case, there is an
B
alternative way to normalize fe as fX e B / U∞ = 3.4 , which is consistent with the general
range of 0.3 ≤ fX e B / U∞≤ 4 obtained by experiments [14].