Assessment methods for unsteady flow distortion in aero-engine intakes

Abstract

Peak events of unsteady total pressure and swirl distortion generated within S-duct intakes can affect the engine stability, even when within acceptable mean distortion levels. Even though the distortion descriptors have been evaluated in S-duct intakes, the associated flow field pattern has not been reported in detail. This is of importance since engine tolerance to distortion is usually tested with representative patterns from intake tests replicated with steady distortion generators. Despite its importance in intake/engine compatibility assessments, the spectral characteristics of the distortion descriptors and the relationship between peak unsteady swirl and both radial and circumferential total pressure distortion has not been assessed previously. The peak distortion data is typically lowpass filtered at a frequency associated with the minimum response time of the engine. However the engine design is not always known a priori in intakes investigations and a standard approach to reporting peak distortion data is needed. In addition, expensive and time-consuming tests are usually required to capture representative extreme distortion levels. This work presents a range of analyses based on Delayed Detached-Eddy Simulation and Stereo Particle Image Velocimetry data to assess these aspects of the unsteady flow distortion. The distorted pattern associated with different swirl distortion metrics is identified based on a conditional averaging technique, which indicates that the most intense swirl events are associated with a single rotating structure.. The main frequencies of the flow distortion descriptors in a representative S-duct intake are found to lie within the range in which the engine stability may be compromised. The peak total pressure and swirl distortion events are found to be not synchronous, which highlights the need to assess both types of distortion. Peak swirl and total-pressure distortion data is reported as a function of its associated time scale in a more general way that can be used in the assessment of intake unsteady flow distortion. Extreme Value Theory has been applied to predict peak distortion values beyond those measured in the available dataset, and whose measurement would otherwise require testing times two orders of magnitude longer than those typically considered.

Unsteady swirl distortion characteristics for S-ducts using Lattice Boltzmann and time-resolved, stereo PIV methods

“…This is a consequence of the LBM's under prediction on the swirl unsteadiness (see Figure 4). As previously described by Gil-Prieto et al [35] and Tanguy et al [36], these peak events may exceed the tolerance level of a propulsion system causing operability issues. As such, their characterisation for a coupled engine-intake system is of key importance to the specification of the system's operability range during operation, even if their probability of appearance is relatively low and stochastic.…”

CFD Validation and Flow Control of RAE-M2129 S-Duct Diffuser Using CREATETM-AV Kestrel Simulation Tools

“…A second intake diffuser was modeled at the U.S. Air Force Academy (USAFA) using M2129 intake data and Equations (1) and (2). These two geometries (USAFA and NASA) do not match everywhere as shown in Figure 3.…”

“…In a subsonic intake diffuser, the flow distortion is caused by the ingestion of fuselage boundary layer or aircraft vortices into intake, flow separation at the cowl lips during maneuvering conditions, or formation of secondary and separated flow regions at the intake bends with small radius of curvature. These airflow distortions will lead to total pressure loss and non-uniformity at the engine face which reduces the compressor surge margin and may eventually cause the compressor to stall, engine instability, and the performance deviation from design conditions [2]. Careful consideration should therefore be given to the design of the shape of the cowl lip and diffuser of these intakes.…”

Impact of the Intake Vortex on the Stability of the Turbine Jet Engine Intake System

“…its maneuverability. In the area of work of the intake systems, the following problems can be distinguished: the boundary layer issues for various types of inlet systems [1], [2] the influence of the jet engine inlet operating conditions on the fan [3], [4] or the inlet vortex problem [5]. In the case of the analysis of the problem of the phenomenon of the inlet vortex itself, as in [5], the mechanism of generating a ground vortex has been analyzed at different variants of the gust (side wind and head wind).…”

“…The above is due to the finite response time of the compressor or fan. These devices need time to adapt to unstable inlet conditions [15].…”