A way to model the effects of streamwise vortices in a turbulent flow with one homogeneous direction is presented. The Reynolds averaged Navier-Stokes equations are solved with a differential Reynolds stress turbulence model. Assuming that the vortices can be approximated with the Lamb-Oseen model, wall-normal Reynolds stress distributions are calculated, corresponding to the spanwise variances of the estimated velocity distribution downstream of the vortex generators. The Reynolds stress contributions that are due to the vortex generators are added to the Reynolds stresses from the turbulence model so as to mimic the increased mixing due to the vortex generators. Volume forces are applied also in the mean momentum equations to account for the drag of the vortex generators. The model is tested and compared with experimental data from a plane asymmetric diffuser flow which is separating without vortex generators. The results indicate that the model is able to mimic the major features of vortex generator flow control and that the flow case in question is susceptible to separation control. The model results show that the pressure recovery of the diffuser could be increased by almost 10% by applying vortex generators and that, if keeping the shape of the vortex generators fixed, their optimal position is close to the diffuser inlet. Computations also indicated that the time to re-establish the separation zone when the control suddenly is turned off is substantially longer than the time it takes to remove the separation after the control is turned on again. Some work on adapting a differential Reynolds stress turbulence model was necessary in order to make it capable of realistic predictions of the asymmetric diffuser flow in which the vortex generator model is tested. However, the main focus of the article is on the modelling of vortex generator effects.

No abstract

The flow in a plane asymmetric diffuser with an opening angle of 8.5° has been studied experimentally using time-resolving stereoscopic particle image velocimetry. The inlet condition is fully developed turbulent channel flow at a Reynolds number based on the inlet channel height and bulk velocity of Re = 38000. All mean velocity and Reynolds stress components have been measured. A separated region is found on the inclined wall with a mean separation point at 7.4 and a mean reattachment point at 30.5 inlet channel heights downstream the diffuser inlet (the inclined wall ends 24.8 channel heights downstream the inlet). Instantaneous flow reversal never occurs upstream of five inlet channel heights but may occur far downstream the point of reattachment. A strong shear layer in which high rates of turbulence production are found is located in a region outside the separation. The static wall pressure through the diffuser is presented and used in an analysis of the balance between pressure forces and momentum change. It is demonstrated that production of turbulence causes a major part of the losses of mean flow kinetic energy. The character of the large turbulence structures is investigated by means of time-resolved sequences of velocity fields and spatial auto-correlation functions. Pronounced inclined structures are observed in the spanwise velocity and it is suggested that these are due to the legs of hairpin-like vortices.

Analyses and simulations of a lab‐scale device for determining floc strength are presented together with simulations of two different hermetic disc‐stack centrifuge inlets. Two methods for determining the turbulent shear stresses on flocs in hermetic centrifuge inlets are described. A model for the average shear rate in a hermetic centrifuge inlet is proposed. A correlation for estimating the floc size of the centrifuge feed after passing the high‐shear zone of the inlet is derived.

No abstract