Technical Guide to the TAD-2 Wind Tunnel
8. Data Acquisition and Processing System
8.1 Basic system
The system of measuring of aerodynamic forces is realized on modern personal computer, multifunctional extension board PCLabCard-711 with 8-channal multiplexor and 12-bit analog-to-digital converter, 12-bit digital-to-analog converter, 16-channel digital input, and 16-channel digital output.
For pressure distribution researches the wind tunnel is equipped by multichannel (256 sensors) unmanned system of acquisition and handling of information providing automated data processing in the rate of experiment. Every measuring channel includes the pressure sensors of MPXV5004G type (Freescale Semiconductor, USA) and 16 bit analog-digital converter of 16 channel interface board PCI-6221 (National Instruments, USA) for introduction of analog information of the unmanned subsystem. Static pressure is measured in the drain points of residential district lot model by a measuring channel. The drain points join in with the pressure sensors by tubes with length to 12 meters. At necessity of pulsating component researches the length of tubes is done minimum, and sensor block is placed directly in model.
Software of measuring of aerodynamic forces is written in C++ language. It includes programs for conducting tests of different types and common files with calibration coefficients and other necessary data used by the programs.
At pressure distribution researches the interface board operates out in the environment of the integrated package of the graphic programming LabVIEW.
8.1.3 Real Time Data Processing
As a rule at measuring of aerodynamic forces 256 samples of output signals are acquired in each point. Using measured signals physical values (loads, dynamic pressure, etc.) are calculated. Aerodynamic loadings are obtained by subtracting tare loadings from loadings in airstream. Using this, the aerodynamic coefficients are calculated. For a sample of 256 values of each aerodynamic coefficient digital denoising is performed, outliers are removed, and mean value and standard deviation are calculated. Support loadings are calculated and subtracted from coefficient mean values and angle-of-attack is corrected for upwash interference. Final values of aerodynamic coefficients together with angle-of-attack and velocity values are written into the output protocol file.
At pressure distribution measuring is conducting at every drain point of model as nonexpendable measurings of initial sensor signals (by 1000 measurings) which are undergone to polynomial transformation of approximating functions and statistical treatment.
8.1.4 Hardware Calibration
Calibration of tensiometry balance is performed for the entire measuring path. To the input end of each path a known reference signal (e.g. loading, pressure, etc.) is applied, as described in section 5. The resultant output signals are then processed and written in the calibration protocol file. Calculated values of coefficients of calibration function are written into the coefficient file to be read by an appropriate program and used to calculate physical values using measured voltage ones.
Calibration of all pressure sensors is executed for the whole measuring channel. ÌÊÂ-250 is used as standard micromanometer with absolute error limit of pressure measuring of 0,02 ìì of water column (0,2 Pascal). Calibration is conducted by standard pressure supply on sensors and standard manometer which readings are registered and entered in computer by which the initial signals of pressure sensors are measured also. Measurings at each the pressure value are executed as nonexpendable measurings of initial signals (more than 500 measurings) which are statistically operated on. Conducting of the nonexpendable measurings allows considerably to decrease (more than in 20 times) influencing of quantizing noises of analog-digital converter and measuring channel noises on the measuring results.
8.2 Basic System Extension
Measuring system of aerodynamic forces can be extended up to 40 channels using modern PC and multifunctional extension board ICPDAS PCI-1802, which has 32 channels of analog input, 12-bit analog-to-digital converter, 32-channel digital input, and 32-channel digital output.
Some measures for multiplying the amount of the simultaneously investigated drain ports from 256 to 512 are developed.
Software includes board drivers and data processing programs, written in C++ and LabVIEW language. Algorithms and functions of software are similar to those in basic system and additionally include the possibility for real-time graphic output of obtained data onto the monitor.