Appendix I 
Page A 1° 127 
4. ROOM CONDITION 
4.1 Justification of Air Velocity Sampling Frequency 
4.1.1 Minimum Sampling Frequency 
To measure the turbulence intensity in an airflow field, air velocity sampling frequency must be 
sufficiently high so that the original air velocity fluctuation patterns can be reconstructed. The 
minimum sampling frequency (f s ) should be twice as much as the fluctuation frequency of the 
measured variable (f v ), air velocity in this case, i.e., 
fs > 2 f v (4.1) 
The fluctuation frequency of air velocity is affected by at least the following factors: air velocity, 
boundary conditions of the airflow field, and the jet momentum. In this study, there are two 
subzones: jet zone or Zone 2 (within 0.30m (12”) of the diffuser) and occupied zone or Zone 1 
(the rest of the room airspace). 
4.1.2 Air Velocity of Jet Zone and Occupied Zone 
For the jet zone, the air discharge from the perforated diffuser is a free jet. Thus, only the 
discharge velocity and the jet momentum are considered to be key factors affecting the 
turbulence intensity. 
For the given perforated diffuser, the diameter of each discharge hole is 2.38e-3m (3/32”). There 
are 10,890 holes with a total discharge opening of 4.85 m" (0.52 ft 2 ). The dimension of the 
diffuser discharging area is 0.38m 2 (4.13 ft ) (0.69m (27”) long and 0.56m (22”) wide)). The air 
supply rate is 10 air changes per hour (ACH) which is 6.04e-2 m 3 /s (128 cfm). Because the effect 
of the vena contracta, the effective discharge opening is smaller than the actual area of the hole. 
This effect can be accounted for by applying a discharge coefficient Cd. Considering the flow as 
a flow in a virtual cylinder before being discharged from the hole, the area ratio A 2 to Ai is 12.7 
percent, the same as the opening ratio of the diffuser, where A 2 is the opening area of hole, and 
Ai is the cross-section area of the virtual cylinder. Thus the ratio of D 2 to Di is 0.36 and the Cd is 
0.6 (McQuiston and Parker, 1977). Thus the mean discharge air velocity from each hole is 
Ui = 128/(CdZA 2 ) 
= 128/(0.6*0.522) = 409 fpm 
(4.2) 
