Linear stability analysis of finite length journal bearings in laminar and turbulent regimes
The ever increasing demand for injecting natural gas underground at high pressures was the incentive for the design of multistage compressors. One common fault in such compressors had been excessive lateral vibrations that were identified as sub-synchronous whirl.1 Ideally a rotor is designed just to undergo the spin mode of motion. However, rotors undergo a different preceding type of motion known as rotor whirling that can be caused by a variety of mechanisms such as imbalance and misalignment with whirl frequencies of equal (synchronous) and higher (supersynchronous) than the spin speed of the shaft, respectively. In turbomachinery, an instability is usually referred to rotor whirling at frequencies other than the rotating spin frequency of the shaft. (850.7Kb)
Gadala, Mohamed S.
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Dynamic coefficients of a finite length journal bearing are numerically calculated under laminar and turbulent regimes based on Ng–Pan–Elrod and Constantinescu models. Linear stability charts of a flexible rotor supported on laminar and turbulent journal bearings are found by calculating the threshold speed of instability associated to the start of instable oil whirl phenomenon. Local journal trajectories of the rotor-bearing system were found at different operating conditions solely based on the calculated dynamic coefficients in laminar and turbulent flow. Results show no difference between laminar and turbulent models at low loading while significant change of the size of the stable region was observed by increasing the Reynolds number in turbulent models. Stable margins based on the laminar flow at relatively low Sommerfeld numbers S≤0.05 were shown to fall inside the unstable region and hence rendering the laminar stability curves obsolete at high Reynolds numbers. Ng-Pan turbulent model was found to be generally more conservative and hence is recommended for rotor-bearing design.