Modification of Flow Characteristics on the Blowing Surface by Inserting a Diaphragm

Abstract

Numerical calculations on a pipe flow with wall blowing have been performed using large-eddy simulation methodology to capture the intrinsic changes of axial small-scale vortices and radial fluctuating motions near the flow surface upon the insertion of a diaphragm. Special attentions were devoted to the dynamic role of turbulent coherent vortices over the diaphragm and the reaction of small vortices in the boundary layer. Numerical results, as well as streakline analysis, showed that a large-scale vortex over a diaphragm interfered with the existing vortices near the surface. The growth of the small-scale vortices was suppressed until a large-scale vortex shed in the downstream pseudoperiodically. In addition, proper orthogonal decomposition analysis revealed that inserting a diaphragm decreased the energy content in the primary flow mode and reallocated flow energy in lower modes 2–5. The increase of flow energy in lower modes 2–5 was found to be directly associated with the experimentally observed local increase in regression rate after a diaphragm. Finally, the estimation of the skin-friction coefficient in the flow with a diaphragm showed a very good qualitative agreement with the axial profile of the regression rate obtained both by numerical and experimental studies.