Both potential and wake interactions are usually the primary mechanisms related to the unsteadiness in turbomachines. However, it is usually difficult to distinguish their effects on flow evolution. This paper develops a direct and visual method to investigate the effects of wake and potential interactions on features of the flow velocity in a two-stage centrifugal refrigeration compressor. The centrifugal compressor consists of six cascades of blades/vanes, i.e. inlet guide vanes in the first stage, impeller blades and vaned diffusers in both stages and return channel in the second stage. The detailed features of flow velocity, such as time-averaged flow velocity, instantaneous flow velocity and power density spectrum of the flow velocity, downstream each cascade of blades/vanes are studied in detail to investigate the potential and wake interactions on the flow evolution in the centrifugal compressor.

Accurately identifying sources of flow loss and hydroacoustics and clarifying the mechanism of their generation are crucial for directing the optimal design of efficient and quiet pump-jet propulsors (PJPs). In this paper, numerical simulations of steady and unsteady flow are performed for a PJP equipped with pre-swirl stationary vanes, based on which both sources of flow loss and hydroacoustics are investigated at multi-level granularity. Analyses of flow efficiency and entropy generation rate are performed to identify the sources of flow loss, and analyses of thrust fluctuation and wall pressure fluctuation are conduced to identify the sources of hydroacoustics. The results indicate that the pressure drag accounts for 76% of the total drag and is mainly contributed from the stator and the duct, but the flow efficiency of the rotor is much smaller than that of the stator and the sources of the flow loss are mainly located at three regions of the rotating blades: the leading edge, the tip, and the corner of the suction surface. The hydroacoustic sources are mainly located at the leading edge and the tip of the rotating blades due to stator–rotor and duct–rotor interactions, respectively, but the Taylor's frozen turbulence hypothesis is inappropriate to describe the wake evolution of the stationary vanes owing to the potential interaction caused by the blade rotation.

Radial inlet chambers are widely used in various multistage centrifugal compressors, although they induce extra flow loss and inlet distortions. In this paper, the detailed flow characteristics inside the radial inlet chamber of an industrial centrifugal compressor have been numerically investigated for flow control and performance improvement. First, the numerical results are validated against the experimental data, and flow conditions inside the inlet chambers with different structures are compared. They indicate that, in the non-guide vane scheme, sudden expansions, tangential flows and flow separations in the spiral and annular convergent channels are the major causes of flow loss and distortions, while using guide vanes could introduce additional flow impacts, separations and wakes. Based on the flow analysis, structure improvements have been carried out on the radial inlet chamber, and an average increase of 4.97% has been achieved in the inlet chamber efficiencies over different operating conditions. However, the results further reveal that the increases in the performance and overall flow uniformity just in the radial inlet chamber do not necessarily mean a performance improvement in the downstream components, and the distribution of the positive tangential velocity at the impeller inlet might be a more essential factor for the efficiency of the whole compressor.

Porous materials fixed on and downstream the cylinder can reach a much better effect in suppressing wall pressure fluctuations. In the present paper, numerical comparative studies have been conducted to investigate passive control of flow past a cylinder surface, in which three schemes with different porous treatments are applied to compare their pros and cons. The results show all of the three schemes of porous materials increase the time-averaged flow drag and reduce fluctuations of lift and drag forces. It can be concluded the velocity gradient reduction inside the boundary layer and the vortex shedding delay through porous coating, as well as reverse transition from turbulent vortex shedding into laminar through porous treatment downstream the cylinder, are main flow control mechanisms of porous materials. These mechanisms all reduce fluctuations of lift and drag fluctuations, but have a distinct effect on the features of wake evolution, such as the wake width and length as well as the fluctuating components of the flow velocity. In addition, the wake evolution is highly affected by the location of porous materials.