Unsteady characteristics of S-duct intake flow distortion

Abstract

The unsteady distorted flow fields generated within convoluted intakes can have a detrimental effect on the stability of an aero-engine. The frequency signature in the distorted flow field is of key importance to the engine's response. In this work, time-resolved particle image velocimetry is used to obtain the three-component velocity field at the outlet plane of two S-duct intake configurations for a range of inlet Mach numbers. Proper orthogonal decomposition of the time-resolved velocity data allows the identification of the main frequencies and coherent structures in the flow. The most energetic unsteady structures comprise an in-plane vortex switching mode, associated with a lateral oscillation of the main loss region, and a vertical oscillation of the main loss region. The switching structure occurs at a frequency of St=0.42 and 0.32 for the high and low offset ducts, respectively. The vertical perturbation is associated with a more broadband spectrum between approximately St=0.6-1.0 and St=0.26-1.0 for the high and low offset configurations, respectively. The determined frequencies for these main unsteady flow structures are within the range, which is expected to be detrimental to the operating stability of an aero-engine. The results provide a novel, time-resolved dataset of synchronous velocity measurements of high spatial resolution that enables analysis of the unsteady flows at the exit of complex aero-engine intakes.

Influence of Upstream Total Pressure Profiles on S-Duct Intake Flow Distortion

“…For the reference case of in=0.07Din, the flow unsteadiness is distributed across a range of bands from St = 0.0 to 1.0. For St > 1.0 the contribution to the overall unsteadiness becomes negligible, in agreement with previous measurements [12]. The greatest contribution to the flow field unsteadiness is located within a spectral band range of St=0.4 to 0.6 ( Figure 9) which is close to the lower limit on the range of frequencies of interest (St = 0.7).…”

“…Therefore, the higher values of <> as reported in the current research (<>=14.2) are expected due to the larger value of duct offset (H/Din=2.44), compared to H/Din=1.49 in [15]. Moreover, the unsteady measurements are expected to depict peak swirl events which have a magnitude greater than time-averaged ones [12]. Further increases in the intensity of the spoiled region and the secondary flows are observed for in=0.51Din (Figure 3).…”

“…Uncertainty contributions have been considered from the effects of particle image diameter, particle image density, image quantization levels, displacement gradients, and background noise. The uncertainty for the in-plane and out-of-plane velocities are estimated as 4.9% and 7.0% of the average out of plane velocity at the AIP [12]. These uncertainty figures account for no particle tracking errors due to centrifugal forces caused by the flow to the particles.…”

“…As a consequence, the timeaveraged flow at the outlet plane contains a pair of counter-rotating vortices in addition to a region of total pressure loss on the lower half of the outlet plane of the duct. Zachos et al [3] and Gil-Prieto et al [4] [12] performed extensive studies using Particle Image Velocimetry (PIV) at the outlet plane of two representative S-duct intakes. It was found that although the time-average flow field corresponds to a twin-swirl structure, the flow oscillates between positive and negative bulk swirl topologies with an intermediate condition of two contra-rotating vortices.…”

“…The temporal measurement rate of the time resolved PIV measurements was established to ensure sufficient resolution of the key unsteady fluctuations inside the duct. Previous experimental and computational studies for the same intake geometry and operating conditions[5][12] revealed that the dominant flow fluctuations are found for Strouhal numbers (St=f Dout/Wref) of up to 1.0, where Wref is the average out of plane velocity at the AIP. Therefore, the key flowfield oscillations are expected to occur at frequencies below 410 Hz for an inlet Mach number of Min=0.27 and an outlet duct diameter of Dout=150 mm.…”