Methods of Measuring Lock-In Strength and their Application to the Case of Flow over a Cavity Locking into a Single Side Branch Resonator
Lock-in is a non-linear interaction between a flow induced noise source and a resonator when their respective frequencies are near each other. Lock-in has been reported under many different labels and for many different applications. There is a need however for a consistent community wide method to measure the strength of lock-in so that data from different tests and different source/resonator combinations can be compared. This paper discusses three methods for measuring lock-in strength. The first, Resonant Response Method, (RRM) subtracts (in the decibel scale) the linear modal response of the resonator to broadband (BB) flow noise from the resonant response when lock-in occurs. The second, Quality Factor Method (QFM) tracks the change in quality factor of the resonant response. The third defines the strength in terms of the difference between peak response and the local BB levels. The RRM is applied to a fundamental test in water of a weak source from grazing flow over a cavity locking into acoustic resonant modes of a single side branch resonator. The major velocity effects are captured in the resonant response to BB flow and not in lock-in strength. However, Strouhal stage number and modal damping is shown to have a significant impact on strength. For two modes, strength versus flow rate using the RRM is compared to strengths obtained using the QFM; on a decibel scale the results are shown to be within experimental uncertainty. However, the QFM is noticeably more difficult to apply. The author recommends the use of the Resonant Response Method as the most tractable measure of lock-in strength.
- Research Organization:
- Lockheed Martin Corporation, Schenectady, NY 12301 (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC12-00SN39357
- OSTI ID:
- 821511
- Report Number(s):
- LM-03K030; TRN: US200411%%206
- Resource Relation:
- Other Information: PBD: 16 Apr 2003
- Country of Publication:
- United States
- Language:
- English
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