Effect of local forcing on backward-facing step flow with laminar separation

Effects of single-and double-frequency forcings on backward-facing step flow with laminar separation have been investigated by employing hot-wire measurement and multi-smoke-wire flow visualization. In the case of single-frequency forcing, the level of the saturated subharmonic is much lower than that of the saturated fundamental when the flow is forced at either the natural instability frequency or at the maximum amplification frequency. However, when the flow is forced at the frequency of maximum amplification rate, vortices or organized structures are not observed. In the case of double-frequency forcing, the saturated subharmonic is amplified to a level which is the same as or higher than those of the saturated fundamental and the corresponding subharmonic of the single-frequency forcing over a wide range of initial phase difference. From the phase measurement, it is observed that the initial phase difference plays a dominant role on the stability of vortex pairing process.

Keywords:

• Double-frequency forcing
• Fundamental
• Subharmonic
• Phase difference
• Vortex pairing

Acknowledgement

This work has been supported by the Micro Thermal System Research Center under the auspices of the Korea Science and Engineering Foundation, and the second author acknowledges the support from Ministry of Science and Technology, Republic of Korea, through the National Research Laboratory Program.