S-Ducts have wide application on air vehicles with embedded engines. The complex geometry is known to lead to separation downstream of curved profiles. This paper reports the influences on that flow of passive flow control geometries. In these experiments, stream-wise tubercles were applied in an effort to improve the internal performance of S-duct diffusers, parameters including pressure recovery, distortion and swirl. The test articles were tested with the high subsonic (Ma = 0.8) flow and were manufactured using 3D printing. Stream-wise static pressure and exit-plane total pressure were measured in a test rig using surface pressure taps and a 5-probe rotating rake, respectively; the baseline and variant S-ducts were simulated through computational fluid dynamics. The experiments showed that some subtle improvements to the S-Duct distortion could be achieved through careful selection of tubercle geometry. Generally, the recovered flow downstream of the inner radius of the second bend of the S-duct deteriorated, but overall pressure recovery improved. The simulations were useful in characterizing swirl, whereas experiments were not so equipped. Adjustments to the numerical approaches resulted in reasonable agreement with the experiments.

Across the open literature, there is no clear consensus on what the most suitable modelling fidelity is for rotating cavity flows. Although it is a widely held opinion that URANS approaches are unsuitable, many authors have succeeded in getting reasonable heat transfer results with them. There is also a lack of research into the validity of hybrid URANS/LES type approaches such as DES. This paper addresses these research challenges with a systematic investigation of a rotating cavity with axial throughflow at Grashof numbers of 3.03 × 109 and 3.03 × 1011. The disk near-wall layers remained laminar at both conditions, meaning that a turbulence model should not be active in these regions. The disk heat transfer was observed to affect the near-disk aerodynamics, which in turn affect the disk heat transfer: this feedback loop implies that conjugate heat transfer computations of rotating cavities may be worth investigating. On the shroud, additional eddy viscosity in URANS and DES was found to interfere with the formation of heat transfer enhancing streaks, whilst these were always captured by LES. DES exhibited a concerning behaviour at the higher Grashof number. Locally generated eddy viscosity from the shroud was injected into the bulk fluid by the radial inflow. This contaminated the entire cavity with non-physical modelled turbulence. As the radial inflow is a characteristic feature of rotating cavity flows, these results show that caution is necessary when applying hybrid URANS/LES approaches to this type of flow.

S-duct diffusers are used in aircraft with embedded engines to route ambient air to the fan face. Sizing and stealth considerations drive a need for high curvature ducts, but the curvature causes complex secondary flows that lead to total pressure distortion and swirl velocities at the engine face. These must be controlled for stable engine operation. In this paper, tubercles, a novel bio-inspired passive flow control method, are analysed numerically in a duct with transonic flow. The results are compared to experimental data obtained as part of a campaign at the Royal Military College, Canada to investigate the effects of S-duct geometry and novel passive flow control devices on the performance of transonic S-ducts. The performance of Reynolds-averaged Navier–Stokes turbulence models in the S-ducts is assessed – Menter's shear stress transport model predicts excessive losses due to the overactivity of its stress limiter. The realisable k–ɛ model gives a significant improvement in the prediction of static pressure distributions, but losses and distortion characteristics are predicted poorly due to the model's inability to resolve the effects of unsteadiness in separated regions. Large tubercle geometries are found to trigger earlier separation in the centre of the duct by concentrating low momentum fluid in valleys, but they also act as boundary layer fences away from the duct centre. Smaller geometries are found to generate vortices that re-energise the boundary layer, delaying flow separation. Methods are recommended for future computational analyses of S-ducts and new designs of tubercles.

Across the open literature, there is no clear consensus on what the most suitable modelling fidelity is for rotating cavity flows. Although it is a widely held opinion that URANS approaches are unsuitable, many authors have succeeded in getting reasonable heat transfer results with them. There is also a lack of research into the validity of hybrid URANS/LES type approaches such as DES. This paper addresses these research challenges with a systematic investigation of a rotating cavity with axial throughflow at Grashof numbers of 3.03 × 10 9 and 3.03 × 10 11 .The disk near-wall layers remained laminar at both conditions, meaning that a turbulence model should not be active in these regions. The disk heat transfer was observed to affect the near-disk aerodynamics, which in turn affect the disk heat transfer: this feedback loop implies that conjugate heat transfer computations of rotating cavities may be worth investigating. On the shroud, additional eddy viscosity in URANS and DES was found to interfere with the formation of heat transfer enhancing streaks, whilst these were always captured by LES. DES exhibited a concerning behaviour at the higher Grashof number. Locally generated eddy viscosity from the shroud was injected into the bulk fluid by the radial inflow. This contaminated the entire cavity with non-physical

The flow and heat transfer within rotating cavities is often discussed as a conjugate problem: the temperature distribution within the cavity disks drives the large-scale flow structure within the cavity, and the cavity aerodynamics influence the heat transfer to the disks. However, most simulations of rotating cavities only consider the fluid domain in isolation. This is particularly true for turbulence resolving approaches such as large eddy simulation (LES). The large timescale disparity between the fluid time steps used in LES and the characteristic solid time-scale complicates the use of LES with conjugate heat transfer (CHT). A further issue is that an under-resolved solid domain mesh can artificially amplify higher frequency temperature fluctuations in the fluid domain. This paper addresses these challenges with a new method for LES-CHT where the low-frequency temperature fluctuation caused by the large-scale flow structure is accounted for using a multi-scale frequency domain approach. We investigate two cases: axially heated disks made of a low conductivity material and disks made from a higher conductivity material with a temperature set by radial conduction from the shroud. The formation of small-scale flow structures on both the disk and shroud is dependent on the heating configuration of the cavity — this interaction indicates that high-fidelity thermal boundary conditions should be used when simulating rotating cavities. The formation of heating induced vortical flow structures on the disk is particularly interesting, as this is unexpected from the laminar Ekman layer modelling argument usually used to consider this region of the flow.

Heat transfer inside rotating cavities plays an important role in gas turbine engineering. Flows in both compressors and turbine internal flow cavities exhibit self-generated large-scale inertial wave structures, and buoyancy effects are often important. Across the open literature on the topic, there seems to be no clear consensus on what the most suitable modelling fidelity is — although it is a widely held opinion that URANS approaches are less suitable than LES, many authors have succeeded in getting reasonable heat transfer results with URANS. There is also little knowledge of the validity of hybrid URANS/LES type approaches (such as DES) when it comes to predicting the heat transfer in these flows, and furthermore, on the sensitivity of the flow model validity to local driving aerothermal mechanisms in different parts of the cavity. This paper presents the results of a systematic investigation of a rotating cavity with axial throughflow at a Grashof number of 3 × 109. It is found that, for the case investigated, the disk Ekman layers remain laminar. This causes the disk heat transfer to be relatively insensitive to the modelling fidelity used with URANS, DES, and LES giving similar results. The effect of the disk thermal boundary condition is also investigated — it is found to have a significant effect on the direction of the near-wall flow at high radii, despite the large-scale flow structure within the cavity remaining essentially unchanged. This feedback of the disk heat transfer to the near-disk aerodynamics implies that conjugate heat transfer computations of rotating cavities may be worth investigating. On the shroud, URANS fails to resolve the heat transfer enhancement from small-scale buoyancy driven streaks, whilst these are captured by LES. DES also captures these streaks, as the URANS layer within which they are located returns a very small eddy viscosity, and behaves in a similar manner to LES.

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The flow and heat transfer within rotating cavities is often discussed as a conjugate problem: the temperature distribution within the cavity disks drives the large-scale flow structure within the cavity, and the cavity aerodynamics influence the heat transfer to the disks. However, most simulations of rotating cavities only consider the fluid domain in isolation. This is particularly true for turbulence resolving approaches such as large eddy simulation (LES). The large timescale disparity between the fluid time steps used in LES and the characteristic solid time-scale complicates the use of LES with conjugate heat transfer (CHT). A further issue is that an under-resolved solid mesh artificially amplifies higher frequency temperature fluctuations from the fluid. This paper addresses these challenges with a new method for LES-CHT where the low-frequency temperature fluctuation caused by the large-scale flow structure is accounted for using a multi-scale frequency domain approach. We investigate two cases: axially heated disks made of a low conductivity material, and disks made from a higher conductivity material with a temperature set by radial conduction from the shroud. The formation of small-scale flow structures on both the disk and shroud is dependent on the heating configuration of the cavity - indicating that high-fidelity thermal boundary conditions should be used when simulating rotating cavities. The formation of heating induced vortical flow structures near the disk is particularly interesting, as this is unexpected from the laminar Ekman layer modelling argument usually used to consider this region.