Numerical Study Of Several Fluid-Induced Vibration Problems In Turbomachineries By Fluid-Structure Coupling Approach

Fluid induced Oscillation is one of the most important problems that threaten the turbomachinery's safety. A new fluid-structure coupling numerical method is developed in the present dissertation. 2D/3D Navier-Stokes equations and low Renolds number turbulence model are solved in the fluid zone, while the structure models are solved in the solid zone. The boundary conditions are transferred between the two zones after each time step. A high accuracy, high resolution numerical method (LU-SGS-GE scheme and 4th order MUSCL TVD scheme) is applied to calculate the unsteady flow field. Various numerical methods are used to solve the structure models, such as Fourth order Rounge-Kutta method and Lax method. This numerical approach is used to analyze three typical fluid-induced vibration problems in turbomachinery.Firstly, the airfoil's classic flutter (at small attack angle), stall flutter (near the static stall angle) and response (at large attack angle) are analyzed. It is found from the numerical results that "lock-in" will occur at certain freestream velocity range near the static stall angle, where the frequency of the vortex will be equal to the natural frequency and the flutter has the characteristic of self-induced oscillation. When the attack angle is far from the static stall angle, the vortex will have its own frequency, which differs from the natural frequency, and the flutter has the characteristic of forced oscillation. The characteristic of "lock-in" can be used to explain all kinds of nonlinear phenomenon of the stall flutter. At large attack angle the frequency of the vortex hasn't a apparent zone of "lock-in". The fluid-induced vibration at large attack angle belongs to the stable dynamic response problems. But a sudden skip of amplitude will occur at large inlet velocity. The oscillatory blowing technique to control the flutter is also studied. The influence of the blowing factors to the flutter and the lift coefficient is analyzed. Secondly, the self-induced vibration of seals are analyzed by fluid-structure coupling numerical method. The self-induced oscillation of the seals will occur at large rotational speed, however, this phenomenon can't be simulated with thecustom linear aerodynamic model. The seal's fluid-induced vibration is studied by the present fluid-structure coupling method. The "lock-in" of the frequency is obtained from the study and the amplitude of the natural frequency will increase with the rotational speed, which verifies that the self-induced oscillation will occur at large rotational speed. In addition, the influence of the pressure ration and the pre-swirling to the fluid-induced vibration is also studied.Finally, the fluid-structure interaction problems of a 3-d turbine blade are studied. During the numerical simulation of flutter at large negative attack angle, it is found that the frequency of the propagating stall will move to the natural frequency gradually, which is an important signal of the flutter. The figures, which indicate the influence of the attack angle and pressure ratio to the fluid, are obtained, from which we can learn not only the stability of the oscillation, but also the oscillation's intensity. The study have shown the advantage, efficiency and usage of the present fluid-structure coupling numerical method. The numerical results will be very useful in the design and theory of turbomachinery.