The Research On Vortex-induced Vibration Characteristics Of Plate And Hydrofoil Based On Bi-directional Fluid-structure Interaction

As what we acknowledged, flow-induced noise of propeller is considered as one of the major predisposing factor that reduce the acoustic stealth capability of submarine. As the propeller section of the basic shape,VIV of hydrofoil is to understand the basis of propeller singing.In this thesis, structure and flow field has been solved synchronously with method of FEM and CFD individually base on fundamental theory from computational fluid dynamics and structural dynamics. Moreover, a two-way coupling between structure and fluid has been established with interpolative transferring displacement and stress within the same time step, to simplify blade as two-dimensional plate, two-dimensional airfoil and three-dimensional trapezoidal airfoil, to investigate VIV characteristics separately.The investigation started from two-dimensional plate model, to analyze how Young's modulus impact on the pattern of VIV base on bi-directional flow-solid coupling algorithm. Result of calculation has good coincide with literature values. Hereafter more research has been focused on engineering application, analysis on VIV of two-dimensional cantilever plate in different stream velocity, to capture the structure frequency locking phenomenon in the second-order natural frequency. Furthermore, unidirectional algorithm has been applied to set condition for result comparison. It turned out to shows that periodic generation of stern vortex is likely to induce massive vibration response from plate, to generate frequency locking phenomenon when vortex excitation frequency is coincide with the natural frequency of plate. In Reynolds locked section, the output result of total displacement response from unidirectional algorithm is always larger than those result from two-dimensional method. Meanwhile, difference between two methods turned out to be neglectable in other unlocked section. The calculated vortex shedding frequency from two methods is closed to each other, which indicates that feedback from structure to field has only minor effects on vortex shedding frequency.The thesis has made deep-going efforts on investigating self-sustained oscillation forming condition and influence of two-dimensional plate. When vortex shedding frequency is close to certain order natural frequency of structure, surface fluctuation pressure distribution is according to modal, the trailing edge has achieve certain thickness and wake strength has reached certain level, strong coupling effect will generated between structure and flow. Meanwhile, structure amplitude will reach a maximum phase, accompany with vortex shedding frequency locking phenomenon. However, though VIV occurs in other natural frequencies, solid-flow coupling is found to be relatively weak, vibration level of these frequencies is far lower than locking occurs.Major cause of self-sustained oscillation is flow velocity and material of structure.Base on the shape of propeller section, this thesis has chosen a two-dimensional object which is coincide with engineering for researching, to apply a two-dimensional solid-flow coupling algorithm on object, to obtain the results of how flow velocity and trailing edge effects on zero attack angle two dimensional cantilever airfoil vortex-induced vibration characteristics. The result shows that vortex shedding frequency of airfoil in zero angle of attack has consanguineous relationship with trailing edge, and it is irrelevant with airfoil chord. When altering vortex shedding frequency under such Reynolds numbers is close to certain natural frequency of structure, VIV is likely to be excited or even occurs frequency locking phenomenon. Thickness of trailing edge has significantly effects on VIV, the higher value thickness reaches, the higher possibility of high-intensity trailing vortex shedding goes, to excite more intensive structural response.In the end, the blade has been simplified as an end clamped trapezoidal wing. Research of unsteady surface force and wake spread characteristics reveals that series of discrete vortex shedding frequency exist on spanwise of trapezoidal wing, and the maximum amplitude of frequency happens accompany with maximum value of pulse pressure frequency appears. Investments on vortex induced vibration under diverse stream velocity has also been finished with unidirectional algorithm, from which indicates that maximum displacement response of structure generates nearby natural frequency and discrete vortex shedding frequency. Furthermore, when one of the vortex shedding frequency approaches one of the natural frequency, the magnitude of trapezoidal wing will reaches a peak value under that specific stream velocity.The location of VIV is tending to move toward big end with the increase of stream velocity till vortex shedding frequency and structure vibration frequency is unlocked. When stream velocity increase continuously, vortex shedding frequency is going to increase accordingly and lock up with higher order natural frequency, to induce resonance.