The effects of curvature on sheared turbulence.

Abstract

The present experiment is an attempt to study the effect of curvature on sheared turbulence in relative isolation from wall and entrainment effects. The flow is an extension of previous studies on nearly homogeneous, parallel, shear flows. Uniformly sheared turbulence is allowed to reach a state of transverse statistical homogeneity in a straight rectangular duct, it is then passed into a curved duct, also of rectangular cross-section. The near homogeneity of the turbulence and the near uniformity of the shear are preserved. The present experiments span a range of $S = U\sb{c}/AR\sb{c}$, from $-$.49 to.65, ($U\sb{c}$ is the centerline velocity, $A = {dU\over dn}$ is the mean shear rate and $R\sb{c}$ is the radius of curvature at the centerline of the duct), using two curved tunnels with $R\sb{c}$ = 2m and 5m respectively as well as shear generators producing several values of A. Measurements indicate that the growth of the turbulent stresses and length scales is enhanced for $S 0.$ For $S > .05,$ the stresses decay. In cases where sufficiently large total strain was achieved, the stresses seem to grow or decay exponentially and to develop in a self-preserving manner. The magnitude of the dimensionless shear stress, $K\sb{uv} = {\overline{uv}\over q\sp2},$ decreases monotonically with increasing $S$. The effect of curvature on shear stress is more pronounced in the low wave number range.