LARGE EDDY SIMULATION OF TURBULENT RIBBED-PIPE FLOW

초록

Mounting ribs on a pipe wall is a popular way of enhancing heat transfer in turbulent pipe flow [1]. In the present study, we consider periodically-mounted ribs in the axial direction (Fig. 1) as vortex generators to disturb the flow. Flow topology can be altered depending on the distance between the two neighboring ribs called “pitch”. When the pitch is short, fluid is trapped inside the grooves, resembling a cavity flow. No significant interaction is seen between the cavity flow and the core flow. When the pitch is long enough, however, the ribs disturb the flow by shedding vortices. The vortices are advected downstream and entrained into the grooves, leading to the high turbulence intensity in the vicinity of the pipe wall. In the current study, we aim at identifying the turbulent near-wall structures induced by the shed vortices in order to find a clue to heat-transfer enhancement. The present investigation is based on our LES (Large Eddy Simulation) data base obtained for Re=24,000, Pr=0.71, p/e(ratio of pitch to rib height)=2, 4, 6, 8, 10, 18, e/D(blockage ratio of rib height to pipe diameter)=0.0625. Here, the bulk velocity and the pipe diameter are used as the velocity and length scales, respectively. The ribs were implemented in the cylindrical coordinate system by means of an immersed boundary method. The dynamic subgrid-scale models [2, 3] were used for LES. Our analysis on the instantaneous flow fields (Fig. 1) reveals that the vortices shed from the ribs are highly correlated with high turbulent kinetic energy in the vicinity of the pipe wall. The regions of high turbulent kinetic energy are, in turn, correlated with those of high heat-transfer rate. We focus on the near-wall distributions of the higher-order turbulence statistics. The cross-correlations and joint probability density functions of velocity and temperature fluctuations are presented to identify the correlations among the fluctuations. The present result of octant analysis (performed in