Quantitative smoke visualization: An optical field measurement technique for measuring scalar concentration using Mie scattering from smoke particles

We present an optical field measurement technique for measuring scalar concentration field in wind tunnels using smoke particles, demonstrating its application through measurements of particle concentration and particulate dispersion in the turbulent wake of a simplified vehicle model (Ahmed body). Experiments involved the release of smoke particles (Ethylene-Glycol) from a source placed behind Ahmed body in the wind tunnel (at model length based Reynolds no., Re-l=1.9x10(5)). The smoke particles were illuminated using a laser sheet and recorded using a digital camera. An optical technique similar to PLIF (Planar Laser-Induced Florescence) but adapted for the Mie-scattering of light by smoke particles is developed and implemented. A method for compensating the laser beam attenuation in the smoke medium is also discussed and implemented on the test flow. Calibration of the image intensities with absolute particle concentration has been done, and the Beer-Lambert law is shown to apply. Laser attenuation along the beam path is corrected using an extinction model based on the Beer-Lambert law. An iterative approach for the laser attenuation correction is applied. Using these, quantitative measurements of smoke concentrations are obtained. These experimental results are compared with earlier cold wire thermometry measurements. The results obtained by the current technique are in good agreement with the earlier results. It is observed that not accounting for the attenuation correction can lead to significant errors in the measured concentration field. The presented technique can be used to obtain instantaneous and time-averaged pollutant concentration fields that are resolved in space and time.