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Design Handbook on Animal Research Facilities Using Static Microisolators 
Since this discharge velocity from the free jet has a very small momentum, the air velocity will 
attenuate quickly. Using the free jet theory, the jet velocity (U x ) at a distance (jc) from the 
discharge opening is given by (Wilson, Ed: Hellickson and Walker, 1983) 
U x = 7.6 UiA/x 
(4.3) 
Where A is the opening area of the jet: 0.0000479 ft 2 . 
The first measurement point is two inches from the diffuser. The vena contracta from the 
discharging surface is less than 0.5 £>2 = 3/64 ” and therefore is negligible compared with the jc. 
At 5.0e-2m (2”) distance from the opening, the air jet velocity will be reduced to 
U I x = 2 " = 7.6 *409 • 0.0000479 1/2 + 0.1667 = 129 fpm (4.4) 
Thus, the air velocity will not exceed 129 fpm at any measurement point in the jet zone (Zone 2) 
unless the airflow field is disturbed. Compared with the conventional air diffusers, the air 
velocity of this perforated diffuser is very low, and hence, low turbulence intensities. 
At an outside measurement point in the jet zone (x = 12”), the jet velocity will be: 
U I x =i 2 " = 7.6 * 409 * 0.0000479 1/2 + 1 = 22 fpm (4.5) 
Thus, on average, the air velocity will not exceed 22 fpm at any measurement point in the 
occupied zone (Zone 1), except in vicinity of the exhaust outlet, unless the airflow field is 
disturbed. 
4.1.3 Air Velocity Fluctuation in the Jet Zone 
Air velocities in a jet zone of a room air space have a strong random feature. The air velocities in 
the jet zone contains more high fluctuation frequencies and more turbulence compared to those 
in occupied zones (Zhang, 1991). Using a hot wire anemometer and at a sampling frequency of 
250 Hz, figures 4.1.01 and 4.1.02 (Zhang, 1991) illustrate examples of air velocity fluctuations in 
jet zones with a discharge velocity (U<j) of 350 fpm and 150 fpm, respectively. 
Within each primary velocity fluctuation cycle (time period between two peak to peak values), 
there are high frequency fluctuations containing negligible energy that can be considered to be 
secondary velocity fluctuation. By the definition of the turbulence intensity, which is the ratio of 
the standard deviation of velocity fluctuations to the mean velocity, the secondary velocity 
fluctuation has only negligible influence on the turbulence intensity, i.e., the secondary high 
frequency fluctuations contribute very little to the overall variation of the velocity fluctuation. 
This can be further explained in the following example. 
