Numerical study of separated cross‐flow near a two‐dimensional rough wall with narrow apertures and suction

Abstract

The turbulent flow (Re = 1.5 × 10 5 ) near a rough wall with narrow apertures has been numerically analysed to study the effect of the aperture geometry and wall suction on the flow characteristics. The aperture entry geometry is characterized by roughness height and roughness width. The roughness height is varied from 0.3 to 1.2 mm and roughness width is varied from 2.6 to 4.0 mm. The wall suction is characterized by slot velocity which is varied from 0.25 to 5 m/s. The flow characteristics in terms of fluid streamlines, flow resistance, wall pressure, and wall shear have been presented for several cases. The results show that the flow through the apertures is dominated by a separation vortex that covers the aperture. As roughness height increased (or slope of the roughness), the vortex size increased. With increasing wall suction, the vortex size decreased and moved towards the aperture opening. The flow resistance characterized by pressure drop across the aperture is significantly high for very low wall suction and it is increased with increasing roughness slope. At higher wall suction the slot velocity and roughness geometry has less influence on flow resistance. Wall pressure and skin friction coefficients are dependent on the ratio of roughness height to width.Le débit turbulent (Re = 1,5 × 10 5 ) près d'une paroi rugueuse avec orificesétroits a fait l'objet d'une analyse numérique afin d'étudier l'effet de la géométrie de l'orifice et de la succion de la paroi sur les caractéristiques du débit. La géométrieà l'entrée de l'orifice est caractérisée par la hauteur de la rugosité et la largeur de la rugosité. La hauteur de la rugosité variait de 0,3à 1,2 mm et la largeur de la rugosité de 2,6à 4,0 mm. La succion de la paroi est caractérisée par la vélocité de la fente qui variait de 0,25à 5 m/s. Les caractéristiques du débit en termes de lignes de courant du fluide, de résistance au débit, de pression de paroi et de cisaillement de paroi ontété présentées pour plusieurs cas. Les résultats montrent que le débit par les orifices est dominé par un vortex de séparation qui recouvre l'orifice.À mesure que la hauteur de la rugosité augmente (ou pente de la rugosité), le vortex augmente. En augmentant la succion de la paroi, le vortex diminue et se déplace vers l'ouverture de l'orifice. La résistance au débit caractérisée par une chute de pression au travers de l'orifice est significativementélevée en présence d'une succion de paroi très faible et augmente en présence d'une augmentation de la pente de la rugosité. En présence d'une succion de paroiélevée, la vélocité de la fente et la géométrie de la rugosité ont un moindre impact sur la résistance au débit. Les coefficients de pression de la paroi et le frottement du revêtement sont fonction du ratio hauteurà largeur de la rugosité.

Experimental study of a time‐varying turbulent cross‐flow near a two‐dimensional rough wall with narrow apertures

“…The effect of flow conditions and contour height on vortex size are shown in Figures 6 and 7. This agrees with Mokamati et al; [16] however, the present study suggests a stagnation point closer to the slot entry than they found, which may be a consequence of differences in the channel geometry and the type of particles used in the studies. [11] Vortex size also increases with increasing roughness (i.e., increased contour height).…”

“…[11] Vortex size also increases with increasing roughness (i.e., increased contour height). This agrees with Mokamati et al; [16] however, the present study suggests a stagnation point closer to the slot entry than they found, which may be a consequence of differences in the channel geometry and the type of particles used in the studies. The average velocity between the smooth rotor and the screen surface, i.e., the upstream velocity, V u , was found to be proportional to rotor speed (i.e., V u : 40 % V t ) but independent of the wall roughness and slot velocity.…”

“…Mokamati et al [16] developed a CFD simulation of the flow through a screen slot and considered the effect of various screen contours. As contour height increased, turbulence intensity near the wall increased.…”

Some Fundamental Aspects of Pulp Screen Capacity

“…Mokamati et al [19] developed a Computational Fluid Dynamic (CFD) simulation of the ow through a screen slot and considered the effect of various screen contours. As contour height increased, turbulence intensity near the wall increased.…”

Screening of chemical pulp, revisiting technology options, and the state‐of‐the‐art equipment—A critical review

“…[ 19 ] Fibre separation can be further improved by selectively choosing an advantageous roughness geometry for apertures. [ 20 ] Different apertures have their gains and losses, and it depends on the objectives of a screen room which aperture type is the most beneficial. According to a previous study by Olson, [ 21 ] the best fractionation is usually achieved with slotted apertures.…”