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Sphere drag measurements are very popular in aerodynamic test with wind tunnels. Sphere drag coefficients decrease rapidly around a Reynolds number of 0.3 million. This is due to the fact that the boundary layer separation point moves rapidly downstream on a sphere around this Reynolds number because of the transition from a laminar boundary layer to a turbulent one. The Reynolds number where the drag coefficient decreases rapidly depends on the turbulence in the flow. Naturally, measured drag coefficients show different values depending on wind tunnel tests. Figure 1 shows some reported relations between measured drag coefficients and Reynolds numbers, quoted from Ref. 1. The Reynolds number at which the drag coefficient becomes 0.3 is commonly called the "critical Reynolds number". In these tests, a sphere is supported with wires or a sting. Either support affects the surface flow around model or in its wake. This effect on the flow from the support system is called "support interference". The interference must be quite large around the critical Reynolds number because the flow around the sphere changes drastically and rapidly as the Reynolds number increases. However this effect has not been adequately studied. A 'magnetic suspension and balance system' (MSBS) is a means of magnetically supporting a model in the flow and measuring the force on the model. MSBS is free of support interference, and is suitable for measuring sphere drag around the critical Reynolds number, where the wake of the sphere changes drastically. The NAL 60 cm MSBS is referred from Ref. 2.

Sphere Drag Measurements with the NAL 60cm MSBS