The influence of nozzle aspect ratio on plane jets

Abstract

This paper reports a systematic investigation of the effect of nozzle aspect ratio (AR) on plane jets. The aspect ratio AR (≡ w/ h, where h and w are the nozzle height and width) was varied from 15 to 72. The present velocity measurements were performed using single hot-wire anemometry, over a downstream distance of up to 85 h and at a nozzle-height-based Reynolds number of Re h = 1.80 × 10 4. Results obtained reveal that both the extent and character of statistical two-dimensionality of a plane jet depend significantly on AR. Most aspects of the near field flow exhibit an asymptotic-like dependence on AR, but do not become independent of AR within the range of AR investigated. A region of statistically two-dimensional (2-D) mean velocity field is achieved only for AR ⩾ 20, and its axial extent increases with AR. However, the centerline turbulence intensity in the far field displays an asymptotic-like convergence only for AR ⩾ 30. In the self-similar region, both the mean decay and spreading rates of the jet increase as AR increases and do not reach an asymptotic value, even at AR = 72. The aspect ratio of the local jet ( w/local velocity half-width) at the end of the 2-D region becomes asymptotically independent of nozzle aspect ratio, for approximately AR ⩾ 30. That is, the plane jet ceases to be statistically 2-D at a fixed value of local jet aspect ratio for nozzle aspect ratios greater than 30. The skewness and flatness factors also depend on AR. These results imply that independence of AR, even in the near field, will require very much larger aspect ratios than have been investigated previously.