Spectral Proper Orthogonal Decomposition Downstream of a Vortex Tube Separator Array

Vortex tube separator (VTS) arrays are frequently used to filter foreign particles, such as sand, from helicopter engine intakes. While studies have been conducted on singular separators, these devices are implemented in array configurations of dozens or hundreds of separators, to which little research is devoted. VTS arrays surround all separators with a common plenum known as the “scavenge chamber” that allows them to aerodynamically interact, potentially increasing the complexity of the flow. In the current study, experimental particle image velocimetry (PIV) measurements are conducted downstream of a VTS array model to identify array-specific phenomena that do not necessarily appear in single-VTS configurations. Mean flow data reveals the rapid development of a recirculation zone just 0.23DVTS downstream of the central outlet in the array (where �������� is the inlet tube diameter of each separator), which likely leads to axial flow gradients in the associated separator. This behavior was not seen in single-VTS configurations. Spectral proper orthogonal decomposition reveals high-energy tonal modes at very low Strouhal numbers of 9.8 × 10−3, 1.8 × 10−2, and 2.6 × 10−2 that depict back-and-forth pumping motions at the center of the duct. These motions appear to comprise the central recirculation zone, which appears as merely stagnant in the mean flow. Additional high-energy precession motions are observed at a Strouhal number of 1.5 × 10−1, which appear to represent the rotation of a spanwise vortex within the central recirculation zone. Studies of precession in similar cyclone separators have concluded that such behaviors can resonate with low-frequency oscillations and negatively affect filtration performance.