Supersonic Corner Flows in Rectangular Channels

Rectangular channel geometries are widely encountered in supersonic flows, such as in wind tunnels and in aircraft inlets. Shock-boundary-layer interactions in these flows are known to exhibit significant three-dimensionality, due to the presence of sidewalls and associated corner boundary layers. The main effect is on the local separation of these corner regions, which then affects the wider flow field. Successful prediction of the overall flow therefore relies on the corner separation to be determined accurately. This, in turn, requires knowledge of the flow momentum distribution within the corner boundary layers. However, numerical methods struggle to reliably compute these flows and there is not much experimental data on supersonic corner boundary layers for comparison. This thesis addresses the outstanding gap in knowledge by performing validation-quality experiments on the corner regions of a Mach 2.5 channel flow, with a unit Reynolds number of approximately 40 million per metre. The experiments are conducted in the rectangular test section of a supersonic wind tunnel at the University of Cambridge. An analysis of the wind tunnel experiments, alongside computational data provided by the US Air Force Research Laboratory, reveals that the corner boundary layers are strongly influenced by the geometry of the two-dimensional nozzles used to produce the supersonic flow. The dominant effect is related to bulk vertical velocities within the sidewall boundary layers, induced by vertical pressure gradients in the nozzle. For some very particular geometries, a second influence may be associated with a region of separated flow immediately ahead of the nozzle, which generates vortices within the sidewall boundary layer. Through these mechanisms, the nozzle geometry is seen to strongly influence both the thickness and the structure of the corner boundary layers. High-quality experimental data in the corner regions are used to validate relevant numerical methods. Simple linear eddy-viscosity type turbulence models are found to compute these flows particularly poorly, with a 7% discrepancy in streamwise velocity. This is largely due to the fact that they do not capture known, stress-induced, corner vortices. However, the quadratic constitutive relation improves prediction of the corner boundary-layer structure, reducing experimental-computational differences by as much as half. This improvement is associated with vorticity generation in these corner regions, albeit with slightly different properties to the physical vortices. This production of vorticity depends only on the presence of a quadratic term in the eddy-viscosity model and not on which particular quadratic term is used. A more general form of the quadratic constitutive relation with one additional term is proposed, which appears to exhibit substantial improvements in the prediction of turbulent stress anisotropies. The nozzle geometry effects are exploited to produce two otherwise-identical experimental setups with distinctly different momentum distributions in the corner boundary layers. A full-span wedge introduces an oblique shock with flow deflection angle, 8 degrees, which impinges on the floor boundary layer. The two setups exhibit quite dissimilar separation behaviour, not only in the corner regions but also on the tunnel's centre span, with a difference in central separation length of as much as 35%. The observed behaviour is consistent with expectations based on local flow momentum affecting corner separation size, and on the displacement effect of this corner separation influencing the wider flow.This material is based upon work supported by the US Air Force Office of Scientific Research under award FA9550–16–1–0430

Experimental investigation of transonic external fan cowl separation

When a civil aircraft engine is shut down during the cruise phase of flight and thus begins to windmill, a supersonic region forms on the external surface of the fan cowl. The terminating normal shock can separate the turbulent boundary layer developing on this external surface. A series of experiments are performed in a quasitwo-dimensional wind tunnel rig to investigate the influence of various parameters on this flow problem. As the engine mass-flow rate is reduced, an increase in normal shock strength results in the onset of flow separation which thickens the boundary layer developing on the external fan cowl surface by a factor of three. A reduction in incoming Mach number from the nominal value of 0.65 to 0.60 weakens the shock wave and thus delays flow separation. If the incoming boundary layer is laminar rather than turbulent, the normal shock Mach number is observed to increased by 10%. Despite the stronger shock, no significant flow separation can be detected even for the lowest engine mass-flow rates studied and the external nacelle surface boundary layer is measured to be thinner than for the turbulent case.This project has received funding from the Clean Sky 2 Joint Undertaking (JU) under grant agreement No 101007598. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union

Experimental investigation of external fan cowl separation for compact nacelles in windmilling scenarios

The slim fan cowl profiles used for ultra-high bypass ratio aircraft engines are designed considering off-design operating conditions, such as engine windmilling during take-off climb out or during cruise. The current paper describes wind tunnel experiments studying how incoming Mach number and engine mass-flow rate influence the aerodynamics governing external fan cowl flow separation in both these windmilling scenarios. A transonic region may form on the forebody surface if the engine becomes inoperative during take-off climb out, with peak Mach number up to 1.2. The subsequent adverse pressure gradient can separate the local boundary layer, resulting in flow separation which originates near the highlight and a more uniform fan cowl pressure distribution. Meanwhile, engine shut down during cruise results in a large supersonic region on the external fan cowl surface which terminates in a normal shock wave. When the Mach number of this shock exceeds about 1.35, a closed separation bubble develops, which causes up to a four-fold increase in the boundary-layer thickness downstream of the shock wave.European Union funding: 10100759

Characteristics of shock-induced boundary-layer separation on nacelles under windmilling diversion conditions

The boundary layer on the external cowl of an aeroengine nacelle under windmilling diversion conditions is subjected to a notable adverse pressure gradient due to the interaction with a near-normal shock wave. Within the context of computational fluid dynamics (CFD) methods, the correct representation of the characteristics of the boundary layer is a major challenge in capturing the onset of the separation. This is important for the aerodynamic design of the nacelle, as it may assist in the characterization of candidate designs. This work uses experimental data obtained from a quasi-2D rig configuration to provide an assessment of the CFD methods typically used within an industrial context. A range of operating conditions are investigated to assess the sensitivity of the boundary layer to changes in inlet Mach number and mass flow through a notional windmilling engine. Fully turbulent and transitional boundary-layer computations are used to determine the characteristics of the boundary layer and the interaction with the shock on the nacelle cowl. The correlation between the onset of shock-induced boundary-layer separation and the preshock Mach number is assessed, and it was found that the CFD is able to discern the onset of boundary-layer separation

Design of a new test rig to investigate transonic external fan cowl separation

Ultra high-bypass ratio engines, which show considerable promise in reducing the environmental impact of commercial aviation, generally adopt slim fan cowl profiles. These geometries can be more sensitive to separation on the external surfaces in engine windmilling conditions during take-off climb out or during cruise. This paper describes the development of a two-dimensional wind tunnel rig which can accurately replicate the separation mechanisms experienced by real aero-engine nacelles. This design process highlights the importance of considering factors such as Reynolds-number effects, tunnel-wall effects, the two-dimensional nature of the rig, and the tunnel boundary layers.This project has received funding from the Clean Sky 2 Joint Undertaking (JU) under grant agreement No 101007598. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union

Characteristics of shock-induced boundary layer separation on nacelles under windmilling diversion conditions

The boundary layer on the external cowl of an aero-engine nacelle under windmilling diversion conditions is subjected to a notable adverse pressure gradient due to the interaction with a near-normal shock wave. Within the context of Computational Fluid Dynamics (CFD) methods, the correct representation of the characteristics of the boundary layer is a major challenge to capture the onset of the separation. This is important for the aerodynamic design of the nacelle as it may assist in the characterization of candidate designs. This work uses experimental data obtained from a quasi-2D rig configuration to provide an assessment of the CFD methods typically used within an industrial context. A range of operating conditions is investigated to assess the sensitivity of the boundary layer to changes in inlet Mach number and mass flow through a notional windmilling engine. Fully turbulent and transitional boundary layer computations are used to determine the characteristics of the boundary layer and the interaction with the shock on the nacelle cowl. The correlation between the onset of shock induced boundary layer separation and pre-shock Mach number is assessed and the boundary layer integral characteristics ahead of the shock and the post-shock recovery evaluated and quantified. Overall, it was found that the CFD is able to discern the onset of boundary layer separation for a nacelle under windmilling conditions

Design of a quasi-2D rig configuration to assess nacelle aerodynamics under windmilling conditions

Aero-engine nacelles are typically designed to fulfil both design and off-design aircraft manoeuvres. Under-off design conditions one of the objective is to avoid large flow separation either on the external cowl or within the intake that can influence aircraft and engine operability. One particular scenario is represented by a low engine mass flow regime associated with one inoperative engine, also known as a windmilling condition. Under windmilling, the boundary layer on the external cowl of the nacelle can separate either due to the interaction with shockwaves or due to notable adverse pressure gradient towards the trailing edge. Both mechanisms are computationally difficult to model and there is a need for more validation of computational fluid dynamics (CFD) methods. The aim of this work is to develop a rig configuration which will provide CFD validation data for the aerodynamics of a nacelle under representative windmilling conditions. Two flight regimes are considered, namely windmilling diversion and end-of-runway. CFD simulations of a 3D nacelle are used to determine primary aerodynamic mechanisms associated with boundary layer separation. Two rig configurations are developed and both 2D and 3D CFD analyses are used to achieve the design objectives. Overall, this work presents the design philosophy and methods that were pursued to develop a quasi-2D rig configuration representative of the aerodynamics of 3D-annular aero-engine nacelles under windmilling conditions.European Union funding: 10100759

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

Experimental Investigation of Transonic External Fan Cowl Separation

When a civil aircraft engine is shut down during the cruise phase of flight and thus begins to windmill, a supersonic region forms on the external surface of the fan cowl. The terminating normal shock can separate the turbulent boundary layer developing on this external surface. A series of experiments are performed in a quasi- two-dimensional wind tunnel rig to investigate the influence of various parameters on this flow problem. As the engine mass-flow rate is reduced, an increase in normal shock strength results in the onset of flow separation which thickens the boundary layer developing on the external fan cowl surface by a factor of three. A reduction in incoming Mach number from the nominal value of 0.65 to 0.60 weakens the shock wave and thus delays flow separation. If the incoming boundary layer is laminar rather than turbulent, the normal shock Mach number is observed to increased by 10%. Despite the stronger shock, no significant flow separation can be detected even for the lowest engine mass-flow rates studied and the external nacelle surface boundary layer is measured to be thinner than for the turbulent case.This project has received funding from the Clean Sky 2 Joint Undertaking (JU) under grant agreement No 101007598. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union

Mach 3.5 Compression Corner Control Using Microvortex Generators

An experimental investigation has been performed to examine the effect of vortex generators (VGs) on a compression corner flow separation. Experiments are conducted at Mach 3.5 along a 23° compression corner with turbulent inflow boundary-layer and Reynolds number [Formula: see text] based on the 6.2-mm boundary-layer thickness. Micro-ramp, standard ramped-vane, and inverted ramped-vane VGs all cause the separation line to ripple and become more three-dimensional, but none eliminate it altogether. Vane-type VGs produce a stronger control effect than micro-ramps. Inverted vanes tend to generate large areas of near-wall low-momentum flow that locally increase separation length, making standard vane configurations more effective at reducing separation size. Velocimetry measurements show that the VG-induced vortices remain coherent and capable of exchanging momentum within the boundary-layer, even downstream of the interaction. Enhanced flow three-dimensionality causes an intensification of areas of increased and decreased momentum downstream of reattachment, resulting in significant flow distortion. Increased near-wall turbulent fluctuations are observed upstream of the interaction in areas where separation length is reduced. These findings are used to propose a mechanism of VG control, highlighting the role of VGs in enhancing mixing in the separated shear layer, leading to earlier reattachment and an overall reduction in separation length. This work was financially supported by the Winston Churchill Foundation of the United States. The blow-down wind tunnel used in this study is part of the United Kingdom National Wind Tunnel Facility and their support is gratefully acknowledged. The contributions of the technicians Dave Martin, Tony Luckett, and Ciaran Costello are gratefully acknowledged. The support of Tim Missing and Luke Dickinson in the design and operation of experiments is also recognized