Numerical study of the primary instability in a separated boundary layer transition under elevated free-stream turbulence

Numerical studies of laminar-to-turbulent transition in a separation bubble subjected to two free-stream turbulence levels (FST) have been performed using Large-Eddy Simulation (LES). Separation of the laminar boundary layer occurs at a curvature change over a plate with a semi-circular leading edge at Re = 3450 based on the plate thickness and the uniform inlet velocity. A numerical trip is used to produce the targeted free-stream turbulence levels and the decay of free-stream turbulence is well predicted. A dynamic sub-grid-scale model is employed in the current study and a good agreement has been obtained between the LES results and the experimental data. Detailed analysis of the LES data has been carried out to investigate the primary instability mechanism. The flow visualisations and spectral analysis of the separated shear layer reveal that the 2D Kelvin-Helmholtz instability mode, well known to occur at low FST levels, is bypassed at higher levels leading to earlier breakdown to turbulence

Large eddy simulation of separated boundary layer transition under free-stream turbulence

Physics of laminar-to-turbulent transition in a separated-reattached flow subjected to two free-stream turbulence levels have been explored using Large-Eddy Simulation (LES). Separation of the laminar boundary layer occurs at a curvature change over a flat plate with a semi-circular leading edge. A numerical trip has been used to generate the targeted free-stream turbulence levels. A dynamic Sub-grid-scale (SGS) model has been employed and excellent agreement has been achieved between the LES results and the experimental data. Detailed investigation of the LES data has been carried out to explore the primary instability mechanism at low (< 0.2%) and high free-stream turbulence (5.6%). The flow visualisations and spectral analysis of the separated shear layer reveal that the two-dimensional Kelvin-Helmholtz instability mode, well known to occur at low free-stream turbulence levels, is bypassed at a higher level leading to earlier breakdown to turbulence. The whole transition process leading to breakdown to turbulence has been revealed clearly by the flow visualisations and the differences between the low and high free-stream turbulence cases are clearly evident. Coherent structures are also visualised using iso-surfaces of the Q-criterion and for the high free-stream turbulence case the spanwise oriented two-dimensional rolls, which are clearly apparent in the low free-stream turbulence case, are not visible anymore. Detailed quantitative comparisons between the present LES results against experimental data and the previous LES results at low free-stream turbulence using a staggered grid have been done and a good agreement has been obtained, indicating that the current LES using a co-located grid with pressure smoothing can predict transitional flows accurately. Comprehensive spectral analysis of the separated shear layer at two free-stream turbulence levels has been performed. Under very low free-stream turbulence condition, a distinct regular vortex shedding and trace of the low-frequency flapping phenomena were detected. Under the higher free-stream turbulence however, a mild high-frequency activity was observed. No low frequency oscillations could be detected

A review of evaporative cooling system concepts for engine thermal management in motor vehicles

Evaporative cooling system concepts proposed over the past century for engine thermal management in automotive applications are examined and critically reviewed. The purpose of the review is to establish evident system shortcomings and to identify remaining research questions that need to be addressed to enable this important technology to be adopted by vehicle manufacturers. Initially, the benefits of evaporative cooling systems are restated in terms of improved engine efficiency, reduced CO2 emissions, and improved fuel economy. An historical coverage follows of the proposed concepts dating back to 1918. Possible evaporative cooling concepts are then classified into four distinct classes and critically reviewed. This culminates in an assessment of the available evidence to establish the reasons why no system has yet made it to serial production. Then, by systematic examination of the critical areas in evaporative cooling systems for application to automotive engine cooling, remaining research challenges are identified

Conjugate heat transfer predictions for subcooled boiling flow in a horizontal channel using a volume-of-fluid framework.

The accuracy of CFD-based heat transfer predictions have been examined of relevance to liquid cooling of IC engines at high engine loads where some nucleate boiling occurs. Predictions based on: i) the Reynolds Averaged Navier-Stokes (RANS) solution, and ii) Large Eddy Simulation (LES), have been generated. The purpose of these simulations is to establish the role of turbulence modelling on the accuracy and efficiency of heat transfer predictions for engine-like thermal conditions where published experimental data is available. A multi-phase mixture modelling approach, with a Volume-of-Fluid interface-capturing method, has been employed. To predict heat transfer in the boiling regime, the empirical boiling correlation of Rohsenow is used for both RANS and LES. The rate of vapour-mass generation at the wall surface is determined from the heat flux associated with the evaporation phase change. Predictions via CFD are compared with published experimental data showing that LES gives only slightly more accurate temperature predictions compared to RANS but at substantially higher computational cost.N/

On Transition Process in Separated-Reattached Flows

Laminar-to-turbulent transition in shear layer of separated flows is of practical importance. A thorough understanding of transition process is crucial for its prediction and control; to delay the turbulent phase where laminar flow characteristics are desirable or to accelerate it where high mixing and heat transfer rates of turbulent flow are of interest. This paper presents a review of transition process and unsteady behavior of shear layer in separated flows. Despite decades of intensive research in this area the transition process is still not fully understood. However, significant progress has been made with the simulation tools such as Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS). This paper will discuss several important issues such as instability, vortex shedding, coherent structures and so on in separated shear layer transition, trying to provide the current status of understanding and an appraisal of possible future developments

Large-eddy simulation of transitional flows using a co-located grid

A large-eddy simulation (LES) of a transitional separated flow over a plate with a semi-circular leading at low (<0.2%) and high (5.6%) free-stream turbulence (FST) has been performed, using a co-located grid with the Rhie–Chow pressure smoothing. A numerical trip is used to produce a high FST level and a dynamic subgrid-scale model is also employed in the current study. The entire transition process leading to breakdown to turbulence has been shown clearly by the flow visualisations using instantaneous spanwise vorticities, and the differences between the low- and high-FST cases are clearly visible. Coherent structures are also visualised using isosurfaces of the Q-criterion, and for the high-FST case, the spanwise-oriented quasi-two-dimensional rolls, which are clearly present in the low-FST case, are not visible anymore. Detailed quantitative comparisons between the present LES results and experimental data and the previous LES results at low FST using a staggered grid have been done and a good agreement has been obtained, indicating that the current LES using a co-located grid with pressure smoothing can also predict transitional flows accurately