## 2 Overall Layout and Structural Design  
Because of the strengths in velocity control accuracy, the dynamic calibration method is adopted in the calibration facility design in this paper. The motion manner of the probe is selected to be rectilinear, in which a motor drives a synchronous pulley, the pulley drives a synchronous belt, and the belt pulls a carriage (in fact a flat plate) along a rectilinear guide track. The hot-wire probe strut is mounted on the carriage, and moves with the carriage. When the hot-wire probe fixed on the top of the strut moves at a constant speed, a \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Institution,** & & & & & & \\
**Country** & **Facility Type** & **Velocity Range/m/s** & **Temperature Range/\({}^{\circ}\)C** & **Temperature Control Accuracy/\({}^{\circ}\)C** & **Humidity Range/\%RH** & **Humidity Control Accuracy/\%RH** \\ \hline \hline \multirow{4}{*}{CETIAT, France} & \multirow{4}{*}{Wind tunnel} & \multirow{4}{*}{0.05–2} & \multirow{4}{*}{10–50} & Control accuracy: & & Control accuracy: \(\pm\)4; homogeneity: \(\pm\)0.5; \(\pm\)0.5; \(\pm\)0.2. & Time necessary to reach target temperature and humidity \(\leq\)2 h \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \hline \hline \end{tabular}  
* * Velocities exceed 1.0 m/s.  
\end{table} 
Table 2: Temperature and humidity control performances of world-wide calibration facilities in extremely low speed. uniform air flow toward the probe is achieved and the hot-wire anemometer is calibrated (see Figure 1). As observed in the experiments in Ref. [4], a carriage with a blunt frontal surface will exert a severe disturbance to the air upstream of the carriage, thus in the layout design of the calibration rig in this paper, the hot-wire probe is placed in an enclosed chamber (Figures 1 and 2), while all other blunt objects such as the hot-wire master computer and the acquisition computer are put on the moving carriage outside and below the enclosed chamber so that the perturbation effects of blunt bodies can be isolated from the chamber air. Besides, the enclosed chamber also facilitates the control of air temperature and humidity. To further minimize the disturbance to the upstream air, the hot-wire strut is designed to have a cross section shape of a thin airfoil with sharp leading and trailing edges, so that there are only a thin plate strut and a probe left inside the enclosed chamber (see Figure 2b). Since the vertical probe strut is fixed to the moving flat plate carriage, which is below the chamber bottom wall and is exposed to the ambient air, the strut has to penetrate the chamber bottom wall with its upper part confined in the chamber. Besides, the strut is designed to move in a horizontal direction with the carriage, hence a slot is made on the chamber bottom wall to allow the strut to move through. Two sealing blocks made of  
Figure 1: Structural schematic diagram of the calibration facility in conception. 1. Synchronous pulley; 2. Synchronous belt; 3. Master control and motion control computer; 4. Master control and motion control cabinet; 5. Moving flat platecarriage; 6. Mounting platform; 7. Guide track pedestal; 8. Guide track; 9. Propping pillars for the chamber; 10. Copper tubes for heating; 11. Water return pipe; 12. Water intake pipe; 13. Strut and probe; 14. Enclosed chamber. Figure 2: Exterior and interior of the enclosed chamber and its associated accessories and other parts nearby. 1. Enclosed chamber; 2. Temperature and humidity transducer; 3. Moving flat plate carriage; 4. Chamber propping pillars; 5. Guide track; 6. Guide track pedestal; 7. Synchronous pulley; 8. Synchronous belt; 9. Mounting platform; 10. Water return pipes; 11. Water intake pipes; 12. Sealing blocks; 13. Probe strut; 14. Hot-wire probe; 15. Entrance or exit of moist air (steam).