Hotwire measurements

Velocity was measured using a two channel hot wire anemometer along with an -wire probe. The was formed in the vertical plane, with the cylinder placed in the horizontal position. The probe was mounted on a traversing mechanism that facilitates all three orthogonal movements, to a positional accuracy in the most significant direction, namely the vertical at The commercially available anemometer and probes were employed in the present work. The two wires of the probe were calibrated in the wind tunnel itself. Small changes in room temperature were compensated through the use of a correction formula that assumes a temperature-independent heat transfer coefficient over this range. The probe was recalibrated for larger changes in room temperature. Both wires were operated at and their calibration curves were seen to be almost identical. The assumption of equal sensitivity coefficients of the two wires was occasionally employed during data reduction. The calibration curves were smoothed using a fifth order polynomial. A pitot-static tube connected to a 19:99 mm of (Furness Controls) digital manometer was used for calibration. Both DC and RMS values of voltages were recorded using true voltmeters supplied by the manufacturer. Integration time of typically 100 s was used to obtain all time-averaged quantities. For the range of velocities considered in the present work (namely, 1-5 m/s), incompressible flow conditions have been assumed to prevail for the sake of data analysis.

Before the start of measurements, the stability of the CTA bridge as well as the signal conditioner setup was checked. Static and dynamic balancing of the circuits had to be ensured. The static bridge balancing requires the use of a proper overheat setting while dynamic balancing is checked by square wave test.

Local time-averaged velocity and velocity fluctuations were measured using the -wire probe. The -wire probe was used for measuring two components of velocity along the x and y directions. The continuous output voltage from the anemometer was acquired independently for each wire via an analog to digital (A/D) converter. The filter settings were determined by examining the complete power spectrum of the velocity components. Voltage signals from the CTA were low-pass filtered at 1-3 kHz and highpass filtered at 0.1 Hz using the 56N20 amplifier/filter unit. Further, the 56N20 signal conditioner was used for amplifying the input signals with gain factors of 10. The anemometer output voltage was collected by a PC (HCL) through a data acquisition card (Keithley) with Lab VIEW software. In the low velocity regime, measurements with the pitot-static tube as well as the hotwire anemometer are prone to errors. These can arise from higher order physical phenomena as well as probe interference effects. The errors can be controlled by using a pitot-static tube of small diameter (3 mm in the present study); in addition the hotwire probe in the present work operated at a lower temperature (of around ). It was felt that the lower wire temperature minimized free convection and radiation errors, without excessive loss of sensitivity. The accuracy of measurement was validated by examining the published Strouhal number-Reynolds number data for a circular cylinder at low Reynolds numbers. The power spectra of the velocity fluctuations were determined using the FFT algorithm. The sampling frequency used was 1000 Hz, the signal length for RMS measurements being 20 seconds.