Topology Rule-Based Methodology for Flow Separation Analysis in Turbomachinery

Abstract

Boundary-layer flow separation is a common flow feature in many engineering applications. The consequences of flow separation in turbomachinery can be disastrous in terms of performance, stability and noise. In this context, flow separation is particularly difficult to understand because of its three-dimensional and confined aspects. Analyzing the skin friction lines is one key point to understanding and controlling this phenomenon. In the case of separation, the flow at the wall agglutinates around a manifold while the fluid from the boundary layer is ejected toward the flow away from the wall. The analysis of a three-dimensional separation zone based on topology is well addressed for a simple geometry. This paper aims at providing simple rules and methods, with a clear vocabulary based on mathematical background, to conduct a similar analysis with complex turbomachinery geometry (to understand a surface with a high genus). Such an analysis relies on physical principles that help in understanding the mechanisms of flow separation on complex geometries. This paper includes numerous typical turbomachinery surfaces: the stator row, vaneless diffuser, vaned diffuser, axial rotor and shrouded and unshrouded centrifugal impeller. Thanks to surface homeomorphisms, the generic examples presented can easily be converted into realistic shapes. Furthermore, classical turbomachinery problems are also addressed, such as periodicity or rotor clearance. In the last section, the proposed methodology is conducted on a radial diffuser of an industrial compressor. The flow at the wall is extracted from LES computations. This study presents the different closed separation zones in a high-efficiency operating condition.

An Open–Closed Flow Separation in a Compressor Radial Diffuser

“…The three-dimensional streamlines have been observed to identify the different separation zones. Since this study has been already presented in [18], only the main steps are recalled hereafter. The geometry of the analysis domain has a genus (g) of two and the topology rule becomes :…”

“…The analysis domain proposed is not the complete system but only a portion of the radial diffuser. To respect the topology rule in this case, some critical points need to be added for the boundary condition (refer to Surface Partition and Splitting Critical Points or SCP in Table 3 or in [18]).…”

Beneficial Effects of Guide Fins on Corner Separation

“…Skin friction lines (streamlines infinitely close to the hub) are now used to further investigate the near hub flow. Their analysis helps characterizing the near hub flow orientation and the nature of the separated regions [26]. Locations where the friction field vanishes are symptomatic of separated regions.…”

Numerical investigation of a transonic centrifugal compressor at high rotational speed

“…Low Mach number zones are observed in the radial diffuser, as shown in Figure 13. To detect a separation zone, analysis of critical points is necessary (Duquesne et al, 2022). Figure 14 presents the average topological structure of the velocity field near the hub in two adjacent channels in the radial diffuser.…”