Paraphrased from Ewing, Bixby, and Knacke, [1] some challenges associated with wind tunnel testing include:

• Designing mounts that don't impact the flow
• Need to test above critical Reynolds numbers
• Wind tunnel wall and blocking effects
  • Subsonic testing:
    • Opening-shock measurements:
    • Coefficient of drag measurements:

Here at MIT, we have the 1" x 1" subsonic open wind tunnel and the Wright Brothers Wind Tunnel, which has a 7 X 10-foot elliptical cross-section.

Resources

[1] Ewing, Bixby, and Knacke, Recovery Systems Design Guide