One-equation turbulence models applied to practical scramjet inlet

Reynolds-averaged Navier–Stokes equations are used to simulate a practical scramjet inlet geometry using the shock-unsteadiness modified Spalart–Allmaras (SA) turbulence model. The geometry consists of fore-body ramps, expansion corners, and inlet ducts. The focus is to study the impingement of the cowl shock on the opposite wall boundary-layer. The resulting separation bubble can lead to blockage and inlet unstarts. The shock-unsteadiness correction is employed and is found to improve the computational fluid dynamics (CFD) prediction of flow separation in shock/boundary-layer interactions. The shock-unsteadiness parameter is calibrated against available experimental data of canonical flows, and the predicted flow-field is analyzed in detail. A large separation bubble size normalized to the upstream boundary-layer thickness of 4.6 is observed in the interaction region. Across the reattachment region in the interaction region, a peak value of wall pressure is observed. The inlet performance parameters are also calculated. The total pressure losses of 62% are observed across different shock waves, with an additional loss of 15% due to viscous boundary-layer effects.

Evaluation of Reattaching Shear-Layer in Compressible Turbulent Flows: A Large Eddy Simulation Approach

The boundary-layer separation and subsequent reattachment due to the free shear-layer and Shockwave interaction have a significant impact on the aerothermal design of supersonic aerospace systems. This problem is prevalent in high-speed flights and can significantly affect the skin friction, aerodynamic loads, and heat transfer. In recent years, considerable progress has been achieved in the prediction of turbulent compressible flows using high-fidelity models. However, the prediction of reattaching free shear-layer and shockwave interactions still needs to be modified for accurate predictivity. The objective of this study is to investigate the ability of a new computational fluid dynamics model to predict these critical flow phenomena accurately. The new high-fidelity model is based on a collocated central scheme, which has the advantage of being a Riemann free solver, and therefore easy to implement on unstructured grids. It is developed to capture any discontinuities at shocks while it is able to capture broadband spatial and temporal variations in turbulent flows with minimal dissipation and dispersion. Large Eddy Simulation is performed on a compression corner at a Mach number of 2.92 and a high Reynolds number. The geometry of the model is specifically designed to isolate the reattachment process of a high-speed separated flow. To examine the accuracy of the predicted results, results of velocity profiles in the free shear-layer, boundary layer development, turbulent fluctuations, and pressure are compared to an experimental effort by Princeton. Excellent agreement is observed, and it is recommended that the model can be used to investigate the physics of the shock unsteadiness due to interaction with a free shear-layer.

Toward Future Installations: Mutual Interactions of Short Intakes With Modern High Bypass Fans

In this paper, we investigate the coupled interaction between a new short intake design with a modern fan in a high-bypass ratio civil engine, specifically under the off-design condition of high incidence. The interaction is expected to be much more significant than that on a conventional intake. The performance of both the intake-alone and rotor-alone configurations are examined under isolation. Subsequently, a comprehensive understanding on the two-way interaction between intake and fan is presented. This includes the effect of fan on intake angles of attack (AoA) tolerance (FoI) and the effect of circumferential and radial flow distortion induced by the intake on the fan performance (IoF). In the FoI scenario, the rotor effectively redistributes the mass flow at the fan-face. The AoA tolerance of the short-intake design has increased by ≈4 deg when compared with the intake-alone configuration. Dynamic nature of distortion due to shock unsteadiness has been quantified. ST plots and power spectral density (PSD) of pressure fluctuations show the existence of a spectral gap between the shock unsteadiness and blade passing, with almost an order of magnitude difference in the corresponding frequencies. In the IoF scenario, both the “large” (O(360 deg)) and “small” scale distortion (O(10–60 deg)) induced by the intake results in a non-uniform inflow to the rotor. Sector analysis reveals a substantial variation in the local operating condition of the fan as opposed to its steady characteristic. Streamline curvature, upwash, and wake thickening are identified to be the three key factors affecting the fan performance. These underlying mechanisms are discussed in detail to provide further insights into the physical understanding of the fan-intake interaction. In addition to the shock-induced separation on the intake lip, the current study shows that shorter intakes are much more prone to the upwash effect at higher AoA. Insufficient flow straightening along the engine axis is reconfirmed to be one of the limiting factors for the short-intake design.

Numerical Investigation of Unsteady Shock Wave Motion in a Transonic Centrifugal Compressor

In order to meet the requirements of automobile engines and marine-use diesel engines, turbochargers must be developed with high boost pressure and appreciably high levels of efficiency. The high pressure rise typically achieved in transonic compressors lead to a stage characterized by high inlet relative Mach numbers. Losses generated in transonic compressors are to a large extent due to the formation of shockwaves at the inducer with interactions between the shock, tip leakage vortex and boundary layer. Significant efficiency reduction occurs at the tip region of the impeller due to the complex interaction of the tip clearance flow and shocks, resulting in significant overall performance degradation. A study has been conducted on the unsteady motion of shockwaves in a transonic centrifugal compressor with vaned diffuser using time-resolved three-dimensional Reynolds average Navier-Stokes simulation. Focus is placed on the impact of the shock motion and post shock unsteadiness on stage performance and impeller-diffuser interaction. The key findings were that the interaction of the shockwave with the tip leakage flow and the boundary layer were the most influential in loss generation with a consequence of increased aerodynamic loss. For the unsteady blade row interaction, the influence of upstream flow unsteadiness on diffuser vanes had significant effect on the flow incidence angle. Periodic jet and wake structure from the impeller and the progressive pressure waves which interacts with the vanes at the interface strongly determines the intensity and position of the vane shock. This has implications on performance in terms of stall inception and static pressure rise across the diffuser.