An experimental study has been made in a nearly two-dimensional 90˚ curved duct to investigate the effects of interaction between streamline curvature and mean strain on turbulence. The initial shear at the entrance to the curved duct was varied by an upstream shear generator to produce five different shear conditions ; a uniform flow (UF), a positive weak shear (PW), a positive strong shear (PS), a negative weak shear (NW) and a negative strong shear (NS).
The variations of surface pressure and the mean velocity profiles along the downstream direction under different initial shears are carefully measured. With the mean field data of the case UF, variations of the momentum thickness, the shape factor and the skin friction over the convex (inner) surface and the concave (outer) surface are scrutinized quantitatively in depth and they are compared with other previous data.
The responses of turbulent Reynolds stresses and triple velocity products to the curvature and the mean strain are also investigated. The evolution of turbulence under the curvature with the different shear conditions is described in terms of the turbulent kinetic energy and the various length scales vs the angular distance θ or a curvature parameter $S_c$ which is defined by $S_c$=(U/R)/(dU/dy-U/R). The results show that the turbulent kinetic energy and the integral length scale are augmented when $S_c<0.054$ whereas they are suppressed when $S_c>0.054$. It is also observed that the micro-length scales of Taylor and Kolmogoroff are relatively insensitive to the curvature.
The curvature effect on the eddy viscosity $v_t$ revealed that such effects can be quantified through a curvature parameter $R_f$ or $S_c$ and a non-equilibrium value of p/ε. When the streamline curvature persists for a sufficiently long period, the eddy viscosity is significantly affected by the curvature strain history, which may be parameterized by $\tau_c$. It is therefore suggested that proper curvature modifications, particularly the strain history effect, must be introduced into current eddy viscosity models for their applications to turbulent flows subjected to curvature straining field for a non-negligible period of time.
