A NUMERICAL STUDY OF FLUTTER IN A TRANSONIC FAN

The bending mode flutter of a modern rr transonic fan has been studied using a quasi-three-dimensional viscous unsteady CFD code. The type o f flutter in this research is that of a highly loaded blade with a tip relative Mach number just above unity, commonly refer red to as trans onic stall flutter. This type of flutter is often encountered bz moder n wide chord fans without a part span shr-oud. The CFD simulation uses an upwinding scheme with Roe's third-order flux differencing, and Joh nson and King's turbulence model with the later modification due to Jo hnson and Coakley. A dynamic transition point model is developed using the e(n) method and Schubauer and Klebanoff's experimental data. The calculations of the flow in this fan reveal that the source of the flu tter of IHI transonic fan is an oscillation of the passage shock, rath er than a stall. As the blade loading increases, the passage shock mov es forward. Just before the passage shock unstarts, the stability of t he passage shock decreases, and a small blade vibration causes the sho ck to oscillate with a large amplitude between unstarted and started p ositions. The dominant component of the blade excitation force is due to the foot of the oscillating passage shock on the blade pressure sur face.