Drag studies with magnetically supported sphere

Drag measurements on spheres in high speed transition regime flow are presented. An electromagnetic suspension technique is employed to obtain sting free drag measurements while maintaining control over the sphere models in a wind tunnel. The gradient coil current is proportional to the applied force and gives a sensitive determination of the small forces encountered. A freely expanding jet from a small sonic nozzle into vacuum is used as the hypersonic low density flow field.
Data were taken using nitrogen and argon gases. A number of nozzle and sphere sizes were employed covering a Knudsen number range of 0.05 to 5. The results exhibit a smooth increase in the transition regime drag coefficient toward the free molecular limit for diffuse reflection and complete thermal accommodation.
Comparison is made with the available experimental results of other techniques. Improved repeatability and an extension of range of flow parameters is obtained with the present methods. The data are compared with current near-free molecular flow theories and the modified Krook solution of Willis is found to be in good agreement with the experimental results.