ASYMPTOTIC VELOCITY DEFECT PROFILE IN AN INCIPIENT SEPARATING AXISYMMETRICAL FLOW

Presence of a new asymptotic mean velocity profile (velocity defect law) was found concomitant with appreciable incipient separation (instantaneous detachment greater than or equal to 20% of the time) and extremely high-turbulence intensities in the final stages of an 8-deg total divergence angle conical diffuser fed with a fully developed turbulent pipe flow. Pulsed-wire anemometry was used for the measurement of incipient flow separation, mean velocities and skin-friction distributions. The conical diffuser now shows some attributes of quasiequilibrium in the final one-third length. In the second-half of the diffuser the mean velocity profiles indicate a distinct point of inflection, the wall regions have less sensitivity to the initial conditions, and the flow exhibits some insensitivity toward the local boundary conditions. A comparison with the other asymptotic velocity defect profiles in Perry-Schofield coordinates revealed the distinctly different shape of the present asymptotic profile found in the conical diffuser. The validity of this asymptotic mean velocity profile was confirmed at two Reynolds numbers. Application of rapid distortion theory indicated coexistence of the regions of slowly and rapidly changing turbulence in the conical diffuser. Results of the present study also support the theory that turbulence is not universal and depends on the initial and boundary conditions.