Uncertainty

The seeding of flow with oil particles, calibration, laser light reflection, background illumination, image digitization, cross correlation calculation, velocity gradients and out of- plane particle motion affect the accuracy of PIV measurements. Tracer particles need to follow the main air flow without any lag. For the particle size utilized and the range of frequencies in the wake, an expected slip velocity error of relative to the instantaneous local velocity is expected. A second source of error in velocity measurement is due to the weight of the particle. In the present experiments, the effect of the weight of the seed particles was examined by conducting experiments at a fixed Reynolds number by varying the size of the cylinder and fixed in flow velocity. The streamline plot and the dimensionless size of the recirculation region were found to be identical in each case, and independent of the fluid velocity. The noise due to background light was minimized by using a band-pass filter (at the wavelength of the laser) before the camera sensor. The hotwire measurements are affected by the error due to calibration, free convection effect at small velocity, curve fitting error, electrical noise, digitization error and turbulence intensity in the incoming fluid stream. The pitot static tube, hotwire anemometry and PIV measurements of mean velocity compared exceedingly well with each other indicating less than error in velocity measurements. The drag coefficient calculation has been carried out at various stream-wise location and found to be within of each other. From repeated measurements at the zero degree cylinder orientation (Figure 3.35, with Reynolds number kept constant to within the uncertainty in drag coefficient has been determined to be within . The Strouhal number has been calculated at different and locations from both and -velocity measurements indicating the uncertainty on Strouhal number to be %. The influence of particle density on the measured flow field is shown in Figure 3.36; the influence is seen to be small.

Figure 3.35: Variation of drag coefficient with cylinder orientation. Experiments were conducted on three different days under nominally similar conditions.