MEASUREMENT OF THE DIFFUSION-COEFFICIENT IN A HEATED PLANE AIRSTREAM

In order to measure the diffusion coefficient of a heated plane airstream, a single laser beam is passed through the jet, perpendicularly to the flow direction. The thermic turbulence in the airstream causes random fluctuations of the refractive index. Consequently, the beam direction undergoes, in the flow, random perturbations. After having traversed the jet, the beam produces a luminous trace on a photoelectric cell placed outside the jet. An experimental setup for measuring the probabilities of the beam impact positions on the cell is described. From the Markovian process model, applied along the whole random path of the beam, it has been possible to compute these probabilities by solving the Einstein-Fokker-Kolmogorov equation. The diffusion coefficient can be determined by adjusting the numerical solution to agree with the experimental results. In addition, the calculation procedure gives the order of magnitude of an integral scale, characterizing the dimension of the turbulent structures in which the propagation of light can be considered rectilinear. A good agreement between the results and the published data obtained by means of the cold-wire anemometer technique proves the validity of the method.