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• The hydrodynamics under a new energy competition force model in an oscillatory flow is studied. • The drag and excitation coefficients are inverse proportion to the KC number. • The added mass coefficients are independent of the KC number. • The empirical models for the drag and excitation coefficients in an oscillatory flow are developed. • A recommended value and modified curve for added mass coefficient in an oscillatory flow are established. The objective of this study is to reveal the characteristics of hydrodynamic force and coefficients undergoing vortex-induced vibration (VIV) of a flexible pipe in an oscillatory flow. The flexible pipe is forced to oscillate with different amplitudes and periods in the still water to simulate the equivalent oscillatory flow. Different from the traditional model, the hydrodynamic forces for the flexible pipe under the new energy competition force model in both in-line (IL) and cross-flow (CF) directions are divided into drag, excitation and added mass force. The identification methods of the hydrodynamic coefficients for the energy competition force model are then established. The detailed features of the hydrodynamic coefficients under the new force model in an oscillatory are illustrated. The distribution comparison of the two forces, i.e., drag force plays a role in energy dissipation and excitation force acts as energy input, explains the differences of the VIV response in magnitudes under different oscillatory flows. By summarizing the hydrodynamic coefficients, the drag and excitation coefficients are found to be inverse proportion to the Keulegan-Carpenter (KC) number. The independence of the added mass coefficients and KC number is revealed. On the basis of the observation, the empirical models for hydrodynamic coefficients are preliminarily proposed. The effects of the oscillatory flow on the hydrodynamics are established to be determined by the KC numbers under the same reduced velocity. The present investigations will provide a better guideline for developing flow-induced vibration prediction methods in future work. [Display omitted] [ABSTRACT FROM AUTHOR]

Hydrodynamic forces on a semi-submerged cylinder are experimentally investigated under the combination of steady and oscillatory flow. The hydrodynamic force in the in-line (IL) and cross-flow (CF) directions under combined flow with and without overtopping are respectively observed in detail. Then, the feasibility and validity of the force models in both IL and CF directions are further verified for the non-overtopping and overtopping regions. The modified Morison equation is then proposed by changing the flow velocity squared term to the higher-order index term. Through the least square method (LSM), the best fitting index and hydrodynamic coefficients, including the drag coefficients, added mass coefficients, and lift coefficients, are identified and summarized. The independency of the fitting index and hydrodynamic coefficients with Keulegan-Carpenter (KC) numbers has been witnessed. The empirical models for predicting hydrodynamic coefficients under the non-overtopping and overtopping flow states in combined flow are developed based on the experimental observation. The overtopping occurrence is proposed to be determined by the total Froude number. With these contributions, the present work provides a promising method to calculate the hydrodynamic force on a semi-submerged cylinder in unsteady flow. • The drag force is no longer proportional to the square of the relative velocity when overtopping occurs in combined flow. • The Morison equation is modified to be the applicant for semi-submerged cylinders when overtopping occurs in combined flow. • The independence of the coefficients with KC number was witnessed in combined flow. • The total Froude number is proposed to determine the hydrodynamic coefficients, higher-order index and overtopping occurrence. • The promising empirical models of the hydrodynamic coefficients in combined flow are developed. [ABSTRACT FROM AUTHOR]