Hohyun Kim

Development of flow separation control system to reduce the vibration of wind turbine blades

The size of wind turbine blade has been continuously increased. Large-scale wind turbine blades induce loud noise, vibration; and maintenance difficulty is also increased. It causes the eventual increases of the cost of energy. The vibration of wind turbine blade is caused by several reasons such as a blade rotation, tower shadow, wind shear, and flow separation of a wind turbine blade. This wind speed variation changes in local angle of attack of the blades and create the vibration. The variation of local angle of attack influences the lift coefficient and causes the large change of the lift. In this study, we focus on the lift coefficient control using a flow control device to reduce the vibration. DU35-A15 airfoil was employed as baseline model. A plasma actuator was installed to generate the upwind jet in order to control the lift coefficient. Wind tunnel experiment was performed to demonstrate of the performance of the plasma actuator. The results show the plasma actuator can induce the flow separation compared with the baseline model. In addition, the actuator can delay the flow separation depending on the input AC frequency with the same actuator configuration.

The Value of Cash Holdings, Ownership Disparity, and Corporate Governance: Evidence from Korean Business Groups

This study examines the effect of business group, ownership disparity, and corporate governance on the value of excess cash using the business group (chaebols) data in Korea. Through the Fama and French (1998), we find that Korean chaebol firms have significantly higher value of excess cash compared to the non-chaebol firms, but the ownership disparity between cash flow right and control right of the controlling shareholders decreases the value of excess cash in chaebol firms. However, corporate governance enhances the value of excess cash in the whole samples of chaebol- and non-affiliated firms, and it also alleviates the negative effect of the ownership disparity in the value of excess cash of chaebol-affiliated firms. More specifically, we find that independency of the board of directors and audit committee enhance the value of excess cash in business group, and mitigates the negative effect of ownership disparity by controlling shareholders on the effect of excess cash on the frim value. Finally, we find that the impact of the use of excess cash on operating performance is significantly positive, and this positive effect is enhanced by corporate governance.This study examines the impacts of business group affiliation, ownership disparity, and corporate governance on cash holdings and the value of excess cash of firms, using business group data from Korea, where such organizations are called “chaebols.” We find that Korean chaebol-affiliated firms have lower (higher) cash holdings (value of excess cash) compared to non-chaebol-affiliated firms; however, ownership disparity increases (decreases) cash holdings (value of excess cash) in chaebol firms. Furthermore, corporate governance enhances the value of excess cash and alleviates the negative effect of the ownership disparity in cash value for chaebol firms.