The Measurement of Sphere Drag in Rarefield Gas Using a Magnetic Wind Tunnel Balance

摘要

Drag measurements on spheres in high speed transition regime flow are presented. The spheres are suspended electromagnetically in the low density flow field from a jet expanding freely from a small sonic nozzle into a vacuum. This arrangement provides sting-free measurements under hypersonic conditions. The current in the control coil of the electromagnetic balance is proportional to the applied force and provides a sensitive determination of the small forces encountered. 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 give the best agreement with the experimental results. (Author)