Velocity and turbulent shear stress measurements were taken within the first 20 nozzle widths of incompressible turbulent air jets issuing from square apertures. Since the jet decays rapidly to axisymmetric configurations, the results are compared with classical closed form solutions and with finite difference solutions for round jets. Because of the nonsimilarity of velocity profiles the classical solutions are unsatisfactory, but finite difference solutions with a suitable mixing length model gives good predictions of axis velocity decay, velocity profiles and spreading of jets with both uniform and nonuniform starting profiles. The method presented here is useful to designers of free jet sensing and control devices since it enables fast and accurate prediction of square jet center-plane characteristics starting from known velocity profiles at the nozzle exit.

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