31

without any pattern the standard deviation values steadily increased as the mean velocity increased.

                    Table 3-2 Water tunnel velocity characteristics Mean velocity                    % Maximum        % Standard deviation
                  (m/s)        absolute deviation  (with respect to mean
                  0.16                1.02                 0.56

                  0.26                1.43                 0.54

                  0.38                4.30                 1.18

                  0.50                2.19                 0.86

                  0.61                1.36                 0.51

                  0.74                2.30                 1.22

                  0.82                2.21                 0.68

                  0.85                0.90                 0.49


                    Vibration Measurements of the Test Section The vibrations induced by the fan and pump, especially at high operating speeds, may vibrate the test section to a reasonable degree. Hence, the vibration influence of the fan and pump on the test section may become critical to the flow quality of both the air and water stream in the test sections. Therefore, vibration measurements at select operating conditions are performed. Experimental Set-up

    Three tri-axial accelerometers (Piezotronic Model 356A16) are used to measure the vibration levels. Two accelerometers are mounted close to the source of the vibration, i.e., at the fan and the pump as shown in Figure 3-9. A third accelerometer is placed approximately at the midpoint of the test section as shown in Figure 3-10. The accelerometers are mounted so as to measure accelerations in a coordinate system that is