Coherent structure of a magnetically suspended sphere

Conventional wind tunnel testing has the problems that real phenomena are different from the test environments. One of the problems is the support interference. In conventional testing, the problem that mechanical suspensions may affect the test condition is always present. The solution to the problem is a Magnetic Suspension and Balance System (MSBS). An MSBS can suspend a model in a wind tunnel by using magnetic forces. So an environment where no mechanical suspension exists, can be realized in a test section. In present research, the advantages of MSBS were applied to the measurement of the coherent structures produced behind a sphere. First, in order to verify the advantage of the environment where no mechanical suspension exists, drag coefficients are compared with and without a dummy sting in the MSBS. When the dummy stings were placed closely behind a sphere, the drag of a sphere with dummy sting decreased compared with that without dummy sting. It was due to the reduction of the wake width when dummy sting was put in. After that, the coherent structures in the wake of a sphere were examined using hot wire measurements. The two dominant frequencies were found at Re=3.6×10⁴. One was the lower frequency that is called the very low mode in present paper. The very low mode was suspected to be an axisymmetric structure and to be related to the movement of the recirculation bubble. The other was the low mode, which was common to the low mode of conventional researches. The low mode was quantitatively indicated to be the alternating structure, which fluctuated with the amplitude of about 0.3D in the plane of x/D=3.0 and the wavelength in the freestream direction was about 3.6D.