The research work described in this thesis includes two topics: 1). An experimental and computational study on the aerodynamic and acoustic characteristics of case fans for computer cooling applications; and 2). A comparative study on the aeromechanic performances and wake characteristics of innovative dual-rotor wind turbines (DRWTs) and conventional single-rotor wind turbine (SRWT). For the first topic, by using a commercially-available cooling fan as the baseline, a number of acoustically tailored modifications were implemented in order to reduce the noise level of the cooling fan, which includes optimizing the rotating blades and guide vanes according to axial fan design theory, adding an intake cone in the front of the hub to guide the airflow into the axial fan smoothly, and reducing the tip clearance to lower the noise generation due to tip vortex structures. In order to assess the effects of the modifications on the fan noise reduction, a comparison study was conducted to measure the sound pressure level (SPL) and the sound spectra of the newly designed axial fan in an anechoic chamber, in comparison to that of the baseline fan. The measurement results of our preliminary study revealed that, at the same flow rate, the SPL of the newly designed fan would be up to 5 dB lower than that of the baseline fan. The spectra results demonstrated that the sound power energies in both of peak frequency and broadband frequency for the baseline fan were higher than that of the newly designed fan regardless of flow rate. In addition, a digital particle image velocimetry (PIV) system and numerical simulation were also used to conduct detailed flow field measurements to reveal the inner and outer flow characteristics and unsteady vortex structures associated with the modifications.

For the second topic, a comprehensive study was conducted to investigate the aerodynamics and wake characteristics of innovative dual-rotor wind turbines (DRWTs) consisting of twin-rotor, co-, and counter-rotating configurations, in comparison to a conventional single-rotor wind turbine (SRWT). In addition to measuring the dynamic wind loads acting on the SRWT and DRWT models, a Cobra Probe Anemometry, a conventional 2D and a stereoscopic Particle Image Velocimetry (PIV) system were used to attain the detailed flow field measurements to quantify the flow characteristics in the turbine wakes and to quantitatively visualize the time evolution of the unsteady vortex structures in the wake flows. Furthermore, the power outputs of a duplicate model turbine operating in the wakes behind the DRWT and SRWT models at different downstream locations were also measured and compared quantitatively. The detailed flow field measurements were correlated with the dynamic wind loads and power output measurements to elucidate the underlying physics to explore/optimize design paradigms for higher total power generation and better durability of the wind turbines.