Passive flow control study in a convoluted intake using Stereo Particle Image Velocimetry

The ability of vortex generators (VG) to reduce the unsteady distortion at the exit plane of an S-duct (AIP) is investigated. The 3 components of the velocity at the AIP were measured using a Stereo Particle Velocimetry system with high spatial resolution. This enabled an assessment of the synchronous swirl distortion at the duct exit. A total of nine VG cases have been investigated with a systematic variation of key design variables. Overall the VGs change the duct secondary flows and separation and are able to substantially restructure the flow field at the AIP. The pressure distortion could be reduced up to 50% and a reduction in pressure loss of 30% was achieved for the mean flow field. The VGs have a substantial influence on the unsteadiness of the flow field with a reduction in peak swirl unsteadiness of 61% and an overall reduction of unsteady swirl distortion of 67%. They also suppress the primary unsteady flow switching mechanism of the datum configuration which is associated with the oscillation of bulk and twin swirl regimes. Consequently, extreme events which leads to high swirl intensity are suppressed which lower by 45% the maximum swirl intensity for the VG cases

Dynamic flow distortion investigation in an S-duct using DDES and SPIV data

The dynamic flow distortion generated within convoluted aero-engine intakes can affect the performance and operability of the engine. There is a need for a better understanding of the main flow mechanisms which promote flow distortion at the exit of S-shaped intakes. This paper presents a detailed analysis of the main coherent structures in an S-duct flow field based on a Delayed Detached Eddy Simulation (DDES). The DDES capability to capture the characteristics of the highly unsteady flow field is demonstrated against high resolution, synchronous Stereoscopic Particle Image Velocimetry (SPIV) measurements at the Aerodynamic Interface Plane (AIP). The flow field mechanisms responsible for the main AIP perturbations are identified. Clockwise and counter-clockwise stream-wise vortices are alternately generated around the separation region at a frequency of St=0.53, which promotes the swirl switching at the AIP. Spanwise vortices are also shed from the separation region at a frequency of St=1.06, and convect downstream along the separated centreline shear layer. This results in a vertical modulation of the main loss region and a fluctuation of the velocity gradient between the high and low velocity flow at the AIP

Pressure flowfield and inlet flow distortion metrics reconstruction from velocity data

Complex engine intakes are susceptible to unsteady flow distortions that may compromise the propulsion system operability. Hence, the need for high spatial and temporal resolution flow information is essential to aid the development of distortion tolerant, closely coupled propulsion systems. Stereoscopic PIV methods have been successfully applied to these flows offering synchronous velocity datasets of high spatial resolution across the Aerodynamic Interface Plane. However, total pressure distortion measurements are still typically provided by low bandwidth, intrusive total pressure rakes of low spatial resolution which results in limited characterisation of the total pressure distortion. This limitation can potentially be addressed by pressure field reconstruction from non-intrusive, high resolution velocity data. A range of reconstruction methods are assessed based on representative data from steady and unsteady computational simulations of an S-duct configuration. In addition to the reconstructed total pressure field, the impact on the key distortion metrics is assessed. The effect of Mach number is considered. Overall the reconstruction methods show that the distortion metrics can be determined with sufficient accuracy to indicate that there is a potential benefit from exploiting high resolution velocity measurements in evaluating total pressure distortion

Complex aero-engine intake ducts and dynamic distortion

For many embedded and partially-embedded engine systems, the complexity of the flow field associated with convoluted intakes presents an area of notable research challenges. The convolution of the intake duct introduces additional flow distortion and unsteadiness which must be understood and quantified when designing the turbo machinery components. The aim of the current work is to investigate the capabilities of modern computational methods for these types of complex flows, to study the unsteady characteristics of the flow field and to explore the use of proper orthogonal decomposition methods to understand the nature of the unsteady flow distortion. The unsteady flow field for a range of S-duct configurations has been simulated and assessed using a delayed detached eddy simulation method. The configurations encompass the effects of Mach number, Reynolds number and S-duct centre line offset distance. Analysis of the conventional distortion criteria highlights the main sensitivities to the S-duct configuration and quantifies the unsteady range of these parameters. These results illustrate the strongly dynamic nature of the flow field for both total pressure as well as swirl based distortion. Analysis of the unsteady flow field shows signature regions of unsteadiness which are postulated to be related to the classical secondary flows as well as to the stream wise flow separation. The more aggressive duct, with a larger centre line offset, shows some similar characteristics, but the unsteadiness is more broadband and the distinction between these two mechanisms is less clear. A proper orthogonal decomposition of the total pressure field at the duct exit identifies the underpinning flow modes which are associated with the overall total pressure unsteadiness distributions. For the more aggressive duct, the flow modes are notably different and highlight the reduced demarcation between the unsteady flow field mechanism

Pressure flowfield and inlet flow distortion metrics reconstruction from velocity data

Complex engine intakes are susceptible to unsteady flow distortions that may compromise the propulsion system operability. Hence, the need for high spatial and temporal resolution flow information is essential to aid the development of distortion tolerant, closely coupled propulsion systems. Stereoscopic PIV methods have been successfully applied to these flows offering synchronous velocity datasets of high spatial resolution across the Aerodynamic Interface Plane. However, total pressure distortion measurements are still typically provided by low bandwidth, intrusive total pressure rakes of low spatial resolution which results in limited characterisation of the total pressure distortion. This limitation can potentially be addressed by pressure field reconstruction from non-intrusive, high resolution velocity data. A range of reconstruction methods are assessed based on representative data from steady and unsteady computational simulations of an S-duct configuration. In addition to the reconstructed total pressure field, the impact on the key distortion metrics is assessed. The effect of Mach number is considered. Overall the reconstruction methods show that the distortion metrics can be determined with sufficient accuracy to indicate that there is a potential benefit from exploiting high resolution velocity measurements in evaluating total pressure distortion

Complex aero-engine intake ducts and dynamic distortion

For many embedded engine systems, the intake duct geometry introduces flow distortion and unsteadiness, which must be understood when designing the turbomachinery components. The aim of this work is to investigate the capabilities of modern computational methods for these types of complex flows, to study the unsteady characteristics of the flowfield, and to explore the use of proper orthogonal decomposition methods to understand the nature of the unsteady flow distortion. The unsteady flows for a range of S-duct configurations have been simulated using a delayed detached-eddy simulation method. Analysis of the conventional distortion criteria highlights the main sensitivities to the S-duct configuration and quantifies the unsteady range of these parameters. The unsteady flowfield shows signature regions of unsteadiness, which are postulated to be related to the classical secondary flows as well as to the streamwise flow separation. A proper orthogonal decomposition of the total pressure field at the duct exit identifies the underpinning flow modes, which are associated with the overall total pressure unsteadiness distributions. Overall, the unsteady distortion metrics are not found to be solely linked to a particular proper orthogonal decomposition mode, but are dependent on a wider range of modes

Assessment methods for unsteady flow distortion in aero-engine intakes

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 low-pass 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 characteristics of S-duct intake flow distortion

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 some embedded engine arrangements, the nature of the inlet distortion is influenced by the boundary layer characteristics at the inlet plane of the intake. This research presents the first quantitative assessment on the influence of inlet boundary layer thickness and asymmetry on the swirl distortion at the exit of an S-shaped intake. Measurements of high spatial and temporal resolution have been acquired at the outlet plane of the S-duct using time-resolved particle image velocimetry. When boundary layer profiles typical of embedded engines are introduced, the characteristic secondary flows at the outlet plane are intensified. Overall, the peak swirl intensity increases by 40% for a boundary layer which is 7 times thicker than the reference case. The unsteady modes of the S-duct remain, although the dominant fluctuations in the flow arise at a frequency 50% lower. When the inlet boundary layer profile becomes asymmetric about the intake centerline the peak swirl events at the hub are reduced by up to 40%. At the tip the peak swirl intensity increases by 29%. The results demonstrate that the effects of inlet boundary layer thickness and asymmetry must be carefully considered as part of engine compatibility tests for complex intakes

Passive flow control study in an S-duct intake using Stereo Particle Image Velocimetry

The ability of vortex generators to reduce the unsteady distortion at the exit plane of an S duct is investigated. The three components of the velocity at the aerodynamic interface plane were measured using a stereo particle velocimetry system with high spatial resolution. This enabled an assessment of the synchronous swirl distortion at the duct exit. A total of nine vortex generator cases have been investigated with a systematic variation of key design variables. Overall, the vortex generators change the duct secondary flows and separation and are able to substantially restructure the flowfield at the aerodynamic interface plane. The pressure distortion could be reduced up to 50%, and a reduction in pressure loss of 30% was achieved for the mean flowfield. The vortex generators had a substantial influence on the unsteadiness of the flowfield with a reduction in peak swirl unsteadiness of 61% and an overall reduction of unsteady swirl distortion of 67%. They also suppressed the primary unsteady flow switching mechanism of the datum configuration, which is associated with the oscillation of bulk and twin swirl regimes. Consequently, extreme events that lead to high swirl intensity are suppressed, which lower by 45% the maximum swirl intensity for the vortex generator cases