Boundary-layer instability induces spiral vortices on rotating cones. As they grow along the cone, the vortices enhance mixing of high- and low-momentum fluid, and subsequently, cause the boundary-layer to transition into a turbulent state. This transition process is scientifically enticing as it is one of the classical problems in fluid mechanics. In practice, the transitions of rotating cone boundary-layer are relevant in several engineering applications, including rotating nose-cones of aero-engines. The instability-induced spiral vortices on rotating aero-engine-nose-cones are expected to influence the aerodynamics at the fan root. This will potentially affect the loss mechanism at junctions between fan blades and the hub (the central rotating body of the engine, including the nose-cone). Accurate assessment of these losses requires knowing the boundary-layer instability behaviour on rotating cones in aero-engine-like flow conditions. The past literature on this classical instability problem has only focused on low-speed (low-Reynolds number incompressible) axisymmetric inflow conditions. In reality, aero-engine-nose-cones often experience high-speed (high-Reynolds number compressible) inflow during a cruise. Moreover, several concepts of future-aircraft feature engines embedded in the airframe, or engines with ultra high bypass ratio with short nacelles. Owing to the associated inflow distortions, the nose-cones of these engines will experience non-axisymmetric inflow. However, limitations of the past experimental techniques pose hurdles in investigating the boundary-layer instability on rotating cones in non-axisymmetric as well as high-speed inflows. This dissertation explores the boundary-layer instability on rotating cones with the inflow conditions pertaining to a typical aero-engine, i.e. non-axisymmetric as well as high-speed inflow. First, an experimental method is developed to measure the coherent flow structures on rotating cones. This method uses infrared thermography (IRT) with proper orthogonal decomposition (POD) to detect the thermal footprints of the spiral vortices on rotating cones. The POD modes are selectively used to reconstruct different instability-induced flow features. For this selection, a new criterion is formulated to determine the physical admissibility of the POD modes for reconstructing the flow-feature of interest. This method overcomes the limitations of the past experimental methods and has allowed quantitative measurements of spiral vortex growth, angle and azimuthal number, for the first time in complex flow environment, i.e. axial as well as non-axial inflow and high-speed inflow. The asymmetry of the non-axial inflow has been found to delay the spiral vortex growth on the investigated case of a rotating slender cone (half-cone angle ψ=15º). Here, the spiral vortex growth appears at higher local Reynolds number Rel and local rotational speed ratio S compared to the axial inflow case at same operating conditions. It is postulated that the azimuthal asymmetry of the flow conditions (local Rel and S) disturbs the azimuthal coherence of the instability characteristics, i.e. angle and wavelength of the dominant mode. This inhibits the spiral vortex growth. However, at high rotational speed ratio S, when the instability characteristics approach the azimuthal coherence, the spiral vortices are found to be growing in the asymmetric flow field. Furthermore, the dissertation extends the axial flow investigations from the most addressed case of a rotating slender cone of ψ=15º to the broader cones of ψ=22.5º, 30º, 45º, and 50º. Here, the boundary-layer instability mechanism changes from the centrifugal instability for slender cones ψ ≤ 30º to the cross-flow instability for the broad cones ψ ≥ 30º. The exact half-cone angle where this change occurs still remains unclear. While the past literature majorly focused on rotating slender cones in axial inflow, theoretical studies expressed the lack of experimental data for the rotating broad cones in axial inflow. This dissertation has provided this experimental data on the instability-induced spiral vortices for the rotating broad cones of ψ=45º and 50º in axial inflow. The experimental method developed in this work has enabled studying the boundary-layer instability behaviour on rotating cones, for the first time in high-speed conditions, i.e. local Reynolds number Rel =0—3 × 106, rotational speed ratio S<1—1.5, and inflow Mach number M=0.5. These conditions are typically expected on the aero-engine-nose-cones during the transonic cruise of a large passenger aircraft (like A320, A350, etc.). These high-speed measurements revealed that the spiral vortices grow on the investigated rotating cones (ψ=15º, 30º and 40º) as expected from the low-speed studies. This confirms that the right circular type nose-cones of the transonic cruise aircraft will experience the spiral vortex growth in transitional boundary-layer. The dissertation also conceptually discusses the potential effects of the spiral vortices on the fan aerodynamics. The spiral vortices are expected to influence the aerodynamics within the blade passage, especially, near the hub. Flow at the hub and fan-blade junction corner often separates on the suction side of the blade. This reduces the total pressure rise and efficiency of the engine. Presence of the spiral vortices is expected to affect the local aerodynamics at the hub, including the hub-corner separation, however, quantifying this effect needs further investigation. Furthermore, the dissertation has also shown a typical asymmetric flow field around the nose-cones when the fan is subjected to an inflow distortion. The fan-driven redistribution of the distorted inflow reduces the flow-field asymmetry near the nose-cone wall in the symmetry plane. This is a favourable condition for the spiral vortex growth. Overall, this doctoral research has presented a new experimental approach to the classical problem of the boundary-layer instability on rotating cones. This has allowed furthering the fundamental knowledge about the instability-induced spiral vortex growth on rotating cones in following parameters: local Reynolds number Rel =0—3 × 106, rotational speed ratio S=0—250, inflow Mach number M=0—0.5, inflow incidence angle α=0º—10º and half-cone angle ψ=15º—50º. @en

Boundary-layer instability on a rotating cone induces coherent spiral vortices that are linked to the onset of laminar–turbulent transition. This type of transition is relevant to several aerospace systems with rotating components, e.g., aeroengine nose cones. Because a variety of options exist for the nose-cone shapes, it is important to know how their shape affects the boundary-layer transition phenomena. This study investigates the effect of varying cone angle on the boundary-layer instability on rotating cones facing axial inflow. It is found that increasing cone angle has a stabilizing effect on the boundary layer over rotating cones in axial inflow. The parameter space of Reynolds number Re l and local rotational speed ratio S is experimentally explored to find the spiral vortex growth on rotating cones of half angle ψ 22.5°, 45°, and 50°. The previously addressed cases of ψ 15° and 30° are also revisited. Increasing half-cone angle is found to have a stabilizing effect on the boundary layer on the rotating cones with ψ ≲ 45°; i.e., the spiral vortex growth is delayed to higher Re l and S. This effect diminishes when the half-cone angle increases from ψ 45° to 50°. The spiral vortex angle ϵ decreases with increasing rotational speed ratio S for all the investigated cones, irrespective of the half-cone angle. However, the instability on the broader cones is found to induce shorter azimuthal wavelengths. @en

The physical relation between the geometry and the flow topology of the wake of a micro ramp is investigated by means of a parametric study. Various micro ramp geometries are placed in a supersonic turbulent boundary layer at a free-stream Mach number of 2. The flow field is measured with schlieren and Particle Image Velocimetry (PIV). The effect of geometry on different aspects of the flow field is studied and from this a physical model of the flow phenomena is formulated. This model includes the concept of captured momentum, a simplified expression which allows relating the geometrical features to vortex circulation and from this a new scaling factor is proposed. The physical model is validated with the experimental data.@en

In the pursuit of reducing the fuel burn, future aircraft configurations will feature several types of improved propulsion systems, e.g. embedded engines with boundary layer ingestion, high-bypass ratio engines with short intakes, etc. Depending on the design and phase of flight, the engine fan will encounter inflow distortion of varying strength, and fan performance will be adversely affected. Therefore, investigation of the flow phenomena causing performance losses in fan and distortion interaction is important. This experimental study shows the effect of varying distortion index on four aspects of fan performance: distortion topology, upstream redistribution, performance curve, and flow unsteadiness. A low speed fan is tested under 60? circumferential distortion of varying strength, generated using distortion screens. The flow field in the upstream redistribution region is measured using PIV (planar and stereo). The fan performance is obtained using total pressure measurements. The noise spectra measured by a microphone are used to quantify the unsteadiness in the flow field. The distortion index (DC60) varies linearly with the grid porosity at constant wall thickness and aspect ratio of the grid cells. However, the distortion topology is significantly different as a stream-wise vortex pair appears in distorted flow at higher DC60. The vortices are stronger at higher DC60, but their order of magnitude is much lower than the circulation corresponding to fan itself. The spinner, distortion index and topology significantly affect the upstream redistribution mechanism. The vortex pair redistributes the flow which results in lower asymmetry in the symmetry plane. With increasing distortion, the performance is reduced and the unsteadiness is increased.@en

Centrifugal instability of the boundary layer is known to induce spiral vortices over a rotating slender cone that is facing an axial inflow. This paper shows how a deviation from the symmetry of such axial inflow affects the boundary layer instability over a rotating slender cone with half-angle. The spiral vortices are experimentally detected using their thermal footprint on the cone surface for both axial and non-axial inflow conditions. In axial inflow, the onset and growth of the spiral vortices are governed by the local rotational speed ratio and Reynolds number in agreement with the literature. During their growth, the spiral vortices significantly affect the mean velocity field as they entrain and bring high-momentum flow closer to the wall. It is found that the centrifugal instability induces these spiral vortices in non-axial inflow as well; however, the asymmetry of the non-axial inflow inhibits the initial growth of the spiral vortices, and they appear at higher local rotational speed ratio and Reynolds number, where the azimuthal variations in the instability characteristics (azimuthal number and vortex angle) are low.@en

This paper reports on the efficacy of the Görtler number in scaling the laminar-turbulent boundary-layer transition on rotating cones facing axial inflow. Depending on the half-cone angle and axial flow strength, the competing centrifugal and cross-flow instabilities dominate the transition. Traditionally, the flow is evaluated by using two parameters: the local meridional Reynolds number comparing the inertial versus viscous effects and the local rotational speed ratio accounting for the boundary-layer skew. We focus on the centrifugal effects, and evaluate the flow fields and reported transition points using Görtler number based on the azimuthal momentum thickness of the similarity solution and local cone radius. The results show that Görtler number alone dominates the late stages of transition (maximum amplification and turbulence onset phases) for a wide range of investigated and half-cone angle , although the early stage (critical phase) seems to be not determined by the Görtler number alone on the broader cones (and) where the primary cross-flow instability dominates the flow. Overall, this indicates that the centrifugal effects play an important role in the boundary-layer transition on rotating cones in axial inflow.@en

This work shows the behaviour of an unstable boundary-layer on rotating cones in high-speed flow conditions: high Reynolds number Rel > 106, low rotational speed ratio S < 1–1.5, and inflow Mach number M = 0.5. These conditions are most-commonly encountered on rotating aero-engine-nose-cones of transonic cruise aircraft. Although it has been addressed in several past studies, the boundary-layer instability on rotating cones remained to be explored in high-speed inflow regime. This work uses infrared-thermography with POD approach to detect instability-induced flow structures by measuring their thermal footprints on rotating cones in high-speed inflow. Observed surface temperature patterns show that the boundary-layer instability induces spiral modes on rotating cones, which closely resemble the thermal footprints of the spiral vortices observed in the past studies at low-speed flow conditions: Rel < 105, S > 1, and M ≈ 0. Three cones with half-cone angles y = 15◦, 30◦, and 40◦ are tested. For a given cone, the Reynolds number relating to the maximum amplification of the spiral vortices is found to follow an exponential relation with the rotational speed ratio S, extending from low- to high-speed regime. At a given rotational speed ratio S, the spiral vortex angle appears to be as expected from the low-speed studies, irrespective of the half-cone angle.@en

Microramp vortex generators are robust, reliable, and simple devices for passively controlling the boundary layer in several aerospace applications. Various past studies have investigated the effectiveness of microramps in controlling the flow separation induced by shock-wave/boundary-layer interactions. Building upon the past knowledge, this paper reports a systematic investigation of the relation between the microramp geometry and the downstream flow characteristics. A simplified flow model of the microramp wake is provided to explain and predict the influence of changing the geometry on the circulation of the primary vortex pair. The model also provides scaling relations for the evolution of the wake characteristics (that is, wake velocity, wake location, and added incompressible momentum), incorporating the effect of all geometry parameters. Extensive experimental data have been used to validate the model.@en

Abstract: Infrared thermography is applied to measure the spiral vortices in the boundary layer over a rotating cone under axial inflow. The data sets are analysed using proper orthogonal decomposition (POD). A criterion based on the signal-to-noise ratio is defined for the selection of relevant POD modes, such that a low-order reconstruction with reduced measurement noise is obtained without affecting the thermal footprint of the spiral vortices. The resulting reconstruction still includes the large-scale modulations in the local vortex strength, relating to low-frequency phenomena like amplification, changing vortex states, disturbances in outer flow, etc. The effect of coherent vortical structures is further separated from such phenomena by selective reconstruction of the POD modes based on the number of observed vortices (n) along the circumference. The counter-rotating nature of these vortices is confirmed by PIV measurements. The number of spiral vortices shows good agreement with previously reported methods in the literature. The spiral vortex angle is in good agreement with the previous methods at low rotation ratio (S) , but deviates towards the direction of the local wall shear for high values of S. Graphic abstract: [Figure not available: see fulltext.].@en

Boundary layer instabilities and the formation of coherent spiral vortices over a rotating cone and ellipsoid are studied experimentally. It is found that under a non-axial inflow, the breaking of symmetry in the flow field significantly disturbs the coherence and delays the formation of spiral vortices to higher local Reynolds numbers (Re_l) and higher rotation ratios (S). Consequently, it also delays boundary layer transition region. A conceptual reasoning for the observed phenomena is given based on the sensitivity of the local spiral vortex characteristics (number n and angle ϵ) to the asymmetry in the local rotation ratios.@en