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

Bypass transition in separated-reattached flows under elevated free-stream turbulence

Laminar-to-turbulent transition is of great practical interest as it occurs in many engineering flows and often plays a critical role in aerodynamics and heat transfer performance of those flow devices. There could be many routes through transition, depending on flow configuration, geometry and the way in which transition is initiated by a wide range of possible background disturbances such as free-stream turbulence, pressure gradient, acoustic noise, wall roughness and obstructions, periodic unsteady disturbance and so on. This paper presents a brief overview of transition in general and focuses more on the transition process in the free shear layer of separated-reattached flows, demonstrating that above certain free-stream turbulence intensity a so called bypass transition could occurN/

On bypass transition in separation bubbles: a review

Transition from laminar flow to turbulent flow is of great practical interest as it occurs in many engineering flows and often plays a critical role in aerodynamics and heat transfer performance of those flow devices. There could be many routes through transition, depending on flow configuration, geometry and the way in which transition is initiated by a wide range of possible background disturbances such as free-stream turbulence, pressure gradient, acoustic noise, wall roughness and obstructions, periodic unsteady disturbance and so on. This paper presents a brief overview of wall bounded flow transition in general and focuses more on the transition process in the free shear layer of separation bubbles, demonstrating that at elevated free-stream turbulent intensity the so called bypass transition could occur in geometrically induced separation bubbles where the separation point is fixed.N/

High fidelity numerical simulations of gas turbine flows.

Traditionally the so called Reynolds-Averaged Navier-Stokes (RANS) and Unsteady RANS (URANS) have been the main numerical tools for computing gas turbine flows due to their computational efficiency and reasonable accuracy. However, the limitations of RANS and URANS to resolve appropriate details and capture some essential flow features associated with turbulence are also well known, in some cases such as transition they could fail to predict the flow behaviors completely. Therefore, the desire for greater accuracy has led to the development and application of high fidelity numerical simulation tools for gas turbine flows. Two conventional such tools are Direct Numerical Simulation (DNS) which captures directly all details of turbulent flow in space and time, and Large Eddy Simulation (LES) which computes large scale motions of turbulent flow directly in space and time while the small scale motions are modelled. DNS is computationally very expensive and even with the available most powerful supercomputers today or in the foreseeable future it is still prohibitive to apply DNS for gas turbine flows. LES is the most promising simulation tool which has already reasonably widely used for gas turbine flows. This paper will very briefly review first the applications of LES in turbomachinery flows and then focus on two gas turbine combustor related flow cases, demonstrating the superiority of LES in those cases where the RANS performs poorlyN/

Numerical study of separated boundary layer transition under pressure gradient

Large-eddy simulation (LES) is conducted to study the transition process of a separated boundary layer on a flat plate with an elliptical leading edge. A streamwise pressure distribution is imposed and the free stream turbulence intensity is 3% to mimic the suction surface of a low-pressure turbine (LPT) blade. A dynamic sub-grid scale model is employed in the study and the current LES results compare well with available experimental data and previous LES results. The transition process has been analysed with a particular focus on primary instabilities at work. Streaky structures further upstream of the separation, known as the Klebanoff Streaks, have been observed. Typical two-dimensional Kelvin-Helmholtz (K-H) rolls are distorted in the separated region. When Klebanoff streaks passing over a full-span K-H roll, portion of the two-dimensional roll merges with the Klebanoff streaks and develop into chaotic three-dimensional structures, whereas the remaining undisrupted two-dimensional K-H rolls develop into Λ-vortex indicating that despite the disturbances before separation, the K-H instability may still be the main instability at work

Meshing strategy for PEM fuel cells CFD modelling – a systematic approach

Typical PEM fuel cell models usually involve more than one million mesh elements making the computation very intense. This necessitates an effective way to mesh the computational domain with a minimum number of mesh points while, at the same time, maintaining good accuracy. The meshing strategy in each flow direction is investigated systematically in the current study and it has been found that mesh resolution in different directions has a different degree of influence on the accuracy of solutions. The proposed meshing strategy is capable of greatly reducing the number of mesh elements, hence computation time, while preserving the characteristics of important flow-field variable

Impacts of the gap size between two bluff bodies on the flow field within the gap

When two bluff bodies is in close tandem, i.e, the distance between blocks D is less than half of the height of the body H, the flow field is very similar to that in the gap between tractor and trailer in a truck, and hence understanding such a flow field would help to reduce the gap drag of a truck. This paper presents a numerical study of the flow field in the gap between two identical cubes in tandem arrangement, in particular, focusing on the impact of gap size on the flow field within the gap and around the two cubes. Simulations have been carried out for four different gap sizes. The numerical model has been validated first against a test case before further studies are carried out to study the impact of gap width on the flow field. The predicted mean velocity profiles compare well against the experimental data for the validation test case. Detailed analysis has been carried out to reveal the change of the flow fields when the gap size changes, leading to a better understanding of the drag coefficient variations for the four cases studied.N/

Numerical analysis of flow in the gap of a simplified tractor-trailer model with cross vortex trap device

Heavy trucks are aerodynamically inefficient due to their un-streamlined body shapes, leading to more than of 60% engine power being required to overcome the aerodynamics drag at 60 m/hr. There are many aerodynamics drag reduction devices developed and this paper presents a study on a drag reduction device called Cross Vortex Trap Device (CVTD) deployed in the gap between the tractor and the trailer of a simplified tractor-trailer model. Numerical simulations have been carried out at Reynolds number 0.51×106 based on inlet flow velocity and height of the trailer using the Reynolds-Averaged Navier-Stokes (RANS) approach. Three different configurations of CVTD have been studied, ranging from single to three slabs, equally spaced on the front face of the trailer. Flow field around three different configurations of trap device have been analysed and presented. The results show that a maximum of 12.25% drag reduction can be achieved when a triple vortex trap device is used. Detailed flow field analysis along with pressure contours are presented to elucidate the drag reduction mechanisms of CVTD and why the triple vortex trap configuration produces the maximum drag reduction among the three configurations tested