Data Acquisition System

The HWA output is a continuous analog voltage signal; it has been sampled as a time series consisting of discrete values by an analog-to-digital converter (A/D board). The accuracy of the analog output signal is determined by the quality of the anemometer hardware. The accuracy of the reduced time series depends on the choice of the A/D board, the selection of sampling intervals, number of samples, N, and the extent of digitization. The values for sampling rate, SR and N depend primarily on the specific experiment, the required data analysis (time-averaged or spectral analysis), the available computer memory and the acceptable level of uncertainty. The level of digitization is specified as m-bit, indicating a resolution of the ratio of the full-scale reading and .The full scale reading is in the range of 0-10 V. Smaller voltages can be measured by using a gain of upto 1000. The final accuracy is thus a product of instrument specifications and data acquisition set-up related to the actual flow.

The full scale reading is in the range of 0-10 V. Smaller voltages can be measured by using a gain of upto 1000. The final accuracy is thus a product of instrument specifications and data acquisition set-up related to the actual flow. Time-averaged analysis, such as the determination of the time-averaged velocity and of velocity fluctuations requires uncorrelated samples. It can be achieved when the time elapsed between individual samples is at least two times larger than the integral time scale of the velocity fluctuations. On the other hand, spectral analysis requires the sampling rate to be at least twice the highest frequency in the flow oscillations. In the present experiments, a long signal, typically of 20 seconds duration with a sampling frequency of 1000 Hz was recorded from the hot-wire anemometer. A band pass filter (0:1 Hz- 1 kHz) and a gain setting (10) were additionally used. The A/D card was configured in the differential mode to avoid unwanted noise in the measured signal. The signal is amplified prior to digitization. The gain, ranges and resolution are selected on the basis of the characteristics (amplitude and spectral) of the input signal. The instantaneous voltage signals have been recorded by using a DAQ card (Keithley Instruments, KPCI-3108) of 16 bit resolution. The advent of graphical programming concept introduces the possibility of creating a new type instrumentation, not in hardware but in software. This new approach is called Virtual Instrumentation, (VI).In the present investigation, LabVIEW software has been used for programming the Keithley A/D card. LabVIEW (Laboratory Virtual Instrument Engineering Workbench) delivers a powerful graphical development environment for signal acquisition, measurements, analysis, and data presentation. It gives the flexibility of a programming language without the complexity of traditional development tools. Both data acquisition as well as cylinder actuation have been conducted in the LabVIEW environment.