Flow over a Magnetically Suspended Cylinder in an Axial Free Stream

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II. Experimental Set Up A. Magnetic Model Suspension, Force Balance and Wind Tunnel The experiment was conducted using the Magnetic Suspension and Balance System at the Institute of Space Technology and Aeronautics, Japan Aerospace Exploration Agency (formerly the National Aerospace Laboratory). The system was developed by the second author and is described in Ref. 4. Figure 1 shows schematically the coil arrangement used in the system, and specifications of the individual coils are listed in Table 1. The position and attitude of the model were monitored by two CCD cameras oriented perpendicular to each other. Two colors and filters were used to separate reflected lights in two directions. The position was controlled at a feedback frequency of 248Hz. The magnet current was calibrated against known force applied through pulleys and weights for each model. The calibration was repeatable within 0.02%. The models were made of aluminum alloy and housed a rod-shaped magnet along their centerline. The balance system was installed in the subsonic wind tunnel with a 60cmx60cm test section. The available flow speed ranges from 10 to 35 m/s. The free stream turbulence level was less than 0.05%. The measured flow angle at the center is 0.05 degree in downwash and 0.15 degree about z-axis shown in Fig. 1. Further detail of the MSBS and the wind tunnel can be found in Ref. 4. model coil # turn number size purposes 0,9 50 620 x 620 drag 1,3,5,7 97 + 97 200 x 200 lift, pitch moment side force , yawing moment, rolling moment 130V, 120A in continuous mode … 3 units 130V, 60A in continuous mode … 4 units 5 DOF for models with a main magnet only 6 DOF for special models with pair magnets 2,4,6,8 100 200 x 200 coil drive units control Table 1 Specifications of the 60cm MSBS (Ref. 4) position sensing camera Fig. 1 MSBS coil arrangement (not to scale, only one camera is shown) B. Test Models Figures 2 illustrates cylindrical models supported magnetically in the test section. In addition to the model edges, the sensing cameras use a vertical stripe in the middle to detect the model position and attitude. Instantaneous model position was monitored and held within ±0.2mm. In order to provide a wide range of the fineness ratio, models with different diameters were used. One had 45mm diameter and shared the same center section enclosing the magnet. By changing the end caps of different lengths, nine fineness ratios between 4.13 and 8.13 were achieved. Second set of model had a diameter of 85mm and the third 110mm. These larger diameter models provided the fineness ratio down to 1.272. An additional model of 25mm diameter and fineness ratio of 6 was also used. The Reynolds number based on the diameter ranged from 60,000 to 100,000. - 2 – American Institute of Aeronautics and Astronautics
Fig. 2(a) A magnetically supported 45mm diameter cylinder model Fig. 2(b) A magnetically supported 85mm diameter cylinder model III. Results and Discussion A. Wake behind magnetically supported cylinder The mean velocity distribution behind the L/D=5.02 model is presented in both vertical (z) and horizontal (y) directions in Fig. 3. The distance is measured from the trailing edge of the model. An earlier test employed the PIV system on loan and the results indicated excellent results in the wake while limiting the illumination away from the position sensing cameras. Results from another PIV measurement in water will be also discussed oater. However, these velocity measurements relied on a more conventional total head probe as well as a hot-wire probe. Naturally, the results are limited to regions where these instrumentations are applicable. Figure 3 nonetheless demonstrates excellent flow axisymmetry measured with the total head probe. The results clearly benefited from the absence of the model support in the present system. The symmetry of the flow and the repeatability of the measurements are further illustrated in Fig. 4 by vertical and horizontal probe traverses. - 3 – American Institute of Aeronautics and Astronautics
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Flow over a Magnetically Suspended Cylinder in an Axial Free Stream

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