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pressure measurements
Electronic Pressure Scanner (ESP) - is generally used for acquiring mean surface pressure data. An in-situ method is adopted for calibrating the scanner using a Druck calibrator of model DPI-610 was utilized. The analog signal from the ESP scanner is acquired using 8 channel signal conditioner module from National Instruments of model SCXI-1520. The acquired analog signal is digitized using 16-bit Analog to Digital card (ADC) NI 6036 consisting of 16 channels with maximum sampling rate of 200 kS/s. The card has 16 bit resolution with output range of ± 10V. Depending on the experimental work, the analog signal is generally digitized at the sampling rate of 1000 S/s with 1000 Samples per port location leading to sampling time interval of 1s.
Fast piezo-resistive pressure sensors (Kulite Semiconductor, Inc.) - The fluctuations in the wall pressure and the associated unsteadiness in the intermittent region of separation are generally studied using piezo-resistive transducers of model Kulite XCQ-093 M-screen transducer (for wind tunnel measurements) or XT-140M (for sub-scale rocket nozzle studies). The Kulite transducers are calibrated statically using the Druck calibrator model no DPI-603. An in-situ method is followed for calibration, wherein 10 points calibration is done to get better accuracy. The pressure range for the calibration are chosen based on the expected pressure level in the experimental study.
As per the manufacturer’s specification, the Kulite transducers have a natural frequency of approximately 250 kHz. However, due to the presence of the M-screen, the maximum frequency response is limited to 50 kHz. These transducers have 0.071 cm of pressure sensitive area and is protected by an outer casing whose diameter is 0.26cm. These transducers are not flush mounted, but are placed inside a cavity that connects the transducer to the flow through a small of orifice 1mm length and 0.5mm in diameter. The sensitivity of the transducers is typically 3-4 mV/psi. An ADC card is used for the unsteady pressure measurements was NI-4495 DC series which is truly simultaneous type with 24 bit resolution. The data acquisition cards delivers up to 114 dB of dynamic range and simultaneous sampling on all 16 channels at rates up to 204.8 kS/s. The ADC card is equipped with an anti-alias filter with pass band value up to 0.4535×fs (fs, Sampling frequency). The sensor is powered with a DC power supply and the analog signal from the sensor is amplified with a gain of 10 and passed through the ADC card. Generally 200 records containing 4096 data points of data were acquired for each channel at a sampling frequency of 50 kHz making a total of 819200 data points per channel per run. To extract the frequency component from the unsteady signal, a narrow band Fast Fourier Transform (FFT) with frequency resolution of 12.2 Hz is usually carried out on each record containing 4096 points which will beveraged for all the 200 records.
