Near field characteristics of swirling flow past a sudden expansion

作者：Mak H; Balabani S^*

来源：Chemical Engineering Science, 2007, 62(23): 6726-6746.

DOI：10.1016/j.ces.2007.07.009

• sudden expansion
• swirl
• PIV
• POD
• coherent structures

The effect of swirl on the flow characteristics of an axisymmetric sudden expansion chamber with an expansion ratio of 2.5 was examined experimentally. Particle image velocimetry (PIV) was employed to capture the instantaneous flow field. The experiments were carried out for three swirl numbers of 0, 0.17 and 0.65 and a Reynolds number of 10,000. The measured time-mean and fluctuating velocities downstream of the expansion showed that the introduction of weak swirl does not affect the flow significantly whereas stronger swirl (S = 0.65) results in the formation of a central recirculation region, a typical manifestation of vortex breakdown, which alters the character of the flow. Proper orthogonal decomposition (POD) was employed to further characterise the flow by decomposing it into eigenmodes and identifying dominant structures and their spatial distribution. Both velocity-based and vorticity-based POD were implemented. The analysis indicated that the first two eigenmodes are the most energetic ones and are associated with the instability of the jet, whereas higher modes are associated with rolling vortices along the shear layer and smaller scale motion. Phase portraits of the time-dependent amplitude coefficients also indicated the existence of low-dimensional dynamics in the swirling flow for swirl numbers well below the critical ones for vortex breakdown. This low dimensionality at low swirl numbers might be due to a precessing motion of the flow.

• 出版日期2007-12