In this thesis, the aerodynamic responses of three circular cylinders in an equilateral-triangular arrangement were investigated by wind tunnel tests. The responses of three circular cyllinders for varius incidence angle α and spacing ratio S/D show rather severe aerodynamic instabilities due to flow interference than those of two circular cylinders, especially the areodynamic instabilities of downstream cylinder are remarkable. When the downstream cylinder is shielded by the shear layers of upstream cylinder, "wake galloping" occured at downstream cylinder. The incidence angle of shielding α varies with S/D, and $α_s$ is always lies below α=30˚ for all S/D because of presence of the other cylinder. At α about 60˚, "gap-flow induced oscillation" which maintains a uniform amplitude in spite of increasing wind velocity occurs at the downstream cylinder. If the amplitude of this oscillation exceeds a limit point, an oscollation with large amplitude occurs at downstream cylinder by interfernce of shear layers of upstream cylinders. The steady distribution on respective cylinder is varied as α is increased, especially this variation is remarkable in downstream cylinder. The pressure distribution of downstream cylinder must be varied as following processes for all S/D tested.
step 1 : Cpmax is located at θ = 60˚ ~ 70˚, only one peak exists.
step 2 : Cpmax is located at θ = 60˚ ~ 70˚, Cp at θ=0˚increases, two peak exists.
step 3 : the values of two peaks are reversed
step 4 : Cpmax is located at θ=0˚, Cp at θ=60˚~70˚decreases, two peak exists.
step 5 : Cpmax is located at θ=0˚, only one peak exists. pressure distribution for the axis θ=0˚, is not symmetric.
step 6 : Cpmax is located at θ=0˚, pressure distribution for the axis θ=0˚is symmetric.
Therefore it can be known that shielding of shear layers occurs at steps 3,4,5, thus in these process, wake galloping occurs at downstream cylicder. Also at step 6, the oscillation due to gap flow occurs at downstream cylicder because the pressure distribution is symmetric for the axis of θ=0˚ in this process.
