FLOW AROUND A CYLINDER - istiarto

– 2.13 –
transducer were used, a focused-type and a plane-type transducers. The focused-type
transducer was used for measurements at depth h ≤ 18 [cm] where the time-gate was
specified to Tg = 4 [�s]. The measuring volume has a dimension of (see Fig. 2.2b):
thickness �d � 3 [mm], diameter �� � 6 [mm]. The plane-type transducer was used for
measurements at depth h > 18 [cm] with the time-gate fixed at Tg = 6 [�s]. In these
measurements the measuring volume has a dimension of: thickness �d � 4.5 [mm],
diameter 9 � ��[mm] � 26 .
To obtain the time series of the instantaneous velocities of a target, the recorded
intensities and phases of the signals are grouped in which each group contains a number
of intensity-phase data-pairs obtained from several pulses (the number of pulse-pairs,
NPP). A value of NPP of 32 is used in the present work. From each group, one Doppler
frequency is obtained at each receiver. The instantaneous velocity can then be derived
from the Doppler frequency according to Eqs. 2.7 and 2.8. There exists, therefore, a
relation between the pulse frequency, PRF, the number of pulse-pairs, NPP, and the
frequency of the data acquisition, fV:
f V � PRF NPP (2.17)
If the acquisition is done within the duration of Tacq, the number of instantaneous
velocities, NV, for one measurement is:
N V � f V � T acq
(2.18)
The measurements in the present work were conducted with a data acquisition frequency
of fV = 20.8 to 28.4 [Hz] (PRF = 667 to 909 [Hz]). The duration of the acquisition was
Tacq = 150 [s] for measurements at the uniform approach flow and was Tacq = 60 [s] for
measurements around the cylinder. There are thus 3,140 instantaneous velocity data at
every gate (measurement point) for the measurements at the uniform approach flow,
while for the measurements around the cylinder there are 1,250 to 1,700 data.
2.2.3 Other measuring instruments
Besides the ADVP, some other measuring equipment were employed for different
purposes:
� Point-gauge limnimeter: to map the water surface and the channel bed.
� Periscope: to measure the scour depth. The equipment is inserted in the cylinder,
which is transparent. The scour bed can be easily viewed through the mirror provided
at the periscope.
� Electro-magnetic discharge meter: to detect the discharge passing through the circuit.
� Theodolite: to measure the slope of the channel bed.