It is well established in the natural transition of an attached boundary layer that the transition process starts with a two-dimensional primary instability (Tollmien-Schlichting wave, denoted as TS wave), followed by usually a three-dimensional secondary instability (fundamental mode or subharmonic mode) leading to the breakdown to turbulence. However, the transition process of a separation bubble (laminar flow or laminar boundary layer at separation and transition occurs downstream of the separation, leading to turbulence at reattachment) is less well understood, especially on the nature of secondary instability. The focus of this paper is on trying to advance our understanding of secondary instability of a transitional separation bubble on a flat plate with a blunt leading edge (separation is induced geometrically at the leading edge) under a very low free-stream turbulence level (<0.1%). Large-Eddy Simulation (LES) is employed in the current study with a dynamic sub-grid-scale model. The numerical flow visualisation together with the spectral analysis has indicated that a three dimensional secondary instability, the elliptical instability, which occurs for fundamental frequency is the main mechanism at work whereas the subharmonic mode in the form of vortex-pairing is hardly active. There is no evidence for the existence of hyperbolic instability in the braid region either. (C) 2018 Elsevier Ltd. All rights reserved.

An experimental work was conducted to obtain an insight on the variation of fundamental thermo-physiochemical properties for a range of mixtures of red Diesel (referred to here as D100), and Biodiesel (referred to here as B100) and the performance of such blends in a Compression Ignition Engine (CIE). The results for some specific properties such as density and viscosity; water content, pour and cloud points for D100, B100 and blended fuels (referred to here as B10-B90, the numbers indicate the percentage of Biodiesel in the blend), strongly indicate that the resultant mixture is an ideal solution and display a linear increase with increasing the B100 percentage in the blend. The cetane number showed a linear variation between B10-B80 but high value for B90 and B100. Sulphur content decreases continuously with increasing the Biodiesel percentages in the blend while the acid content increases with increasing B100 ratio. All these results carries no controversial issues with previous related studies. The FTIR data obtained using gas analyser has shown that D100, B100 and fuels blends include a range of sub-hydrocarbons with Alkanes (=C-H) compounds as the main hydrocarbon. All Biodiesel blends has shown significant existence of Aldehydes (-C=O) and Ketones (-C=Ost). Nitro-compounds (-NO2) exist in almost all fuels while Alcohols, ethers, acids and esters (-C-O-) are mainly associated with B100 and its blends. Allowing more time for B100 and its blends, the FTIR data strongly indicate that chemical composition changes and water contents increases in B100 and its blends leading to instability issues. Using D100, B100 and 3 blends, mainly B25, B50 and B75 in a Diesel (CIE), the results showed that D100 releases higher rate of energy as expected compared to B100 and its blends. However the results showed that the blends burn more efficiently in the CIE used with the blend B75 producing the best engine efficiency and reasonably low fuel consumption. The emission data showed that the B100 and its blends produce less unburned hydrocarbon, CO, CO2 and NOx emission compared to D100. The better thermal and emission performance of the blends is most likely due to their balanced chemical composition as revealed by the FTIR spectra. The current work also indicates that blended fuels with higher ratio of B100 are recommended to use in CIEs to ensure efficient combustion.

Large-scale organized motions (commonly referred to coherent structures) and flow topology of a transitional separated-reattached flow have been visualised and investigated using flow visualisation techniques. Two geometrical shapes including two-dimensional flat plate with rectangular leading edge and three-dimensional square cylinder are chosen to shed a light on the flow topology and present coherent structures of the flow over these shapes. For both geometries and in the early stage of the transition, two-dimensional Kelvin-Hehnholtz rolls are formed downstream of the leading edge. They are observed to be twisting around the square cylinder while they stay flat in the case of the two-dimensional flat plate. For both geometrical shapes, the two-dimensional Kelvin-Helmholtz rolls move downstream of the leading edge and they are subjected to distortion to fonn three-dimensional hairpin structures. The flow topology in the flat plate is different from that in the square cylinder. For the flat plate, there is a merging process by a pairing of the Kelvin-Helmholtz rolls to form a large structure that breaks down directly into many hairpin structures. For the squire cylinder case, the Kelvin-Helmholtz roll evolves topologically to form a hairpin structure. in the squire cylinder case, the reattachment length is much shorter and a forming of the three-dimensional structures is closer to the leading edge than that in the flat plate case.

The current study sheds a light on two fundamental aspects of a transitional separated-reattached flow induced over a two-dimensional blunt flat plate and three-dimensional square cylinder employing large eddy simulation conducted with Open FOAM CFD code. These aspects are different vortices shedding frequency modes and large scale structures and their development. The current paper is the first study to investigate a transitional separated-reattached flow in a three-dimensional square cylinder and compare between transition aspects of this case and that in a two-dimensional flat plate. It is not clear whether all transitional separated-reattached flows have low frequency shear layer flapping and selective high shedding frequency. This issue is addressed. The current LES results show that the characteristic shedding frequency value for the square cylinder is different from that in the flat plate. Coherent structures and their development are visualized at different stages of transition for both geometers. In the square cylinder, Kelvin-Helmholtz rolls are twisting around this geometry and evolve topologically to form hairpin structures. In the flat plate, Kelvin-Helmholtz rolls stay flat and hairpin structures formed by a braking down process

Combustion process of conventional liquid fuels and BioFuels depend on many factors including thermo - physicochemical properties associated with such fuels, their chemical structure and the combustion infrastructure used. This manuscript summarises the computational results of a steady cfd simulation for reactive flows performed to validate advanced reaction mechanisms for both conventional and BioFuels. The computational results have shown good agreement with the available experimental data with the differences thoroughly discussed and explained. An important observations and findings reported in this work was that when comprehensive reaction models were used, the injected fuels burned at a slower rate compared to the situation when reduced models were employed. While such comprehensive models predicted better flame structure and far better biproducts compared to the existing experimental results, it has also led to over-predicting the temperature field. The computational results have also shown that BioDiesel produces a marginally higher rate of CO2 compared to Diesel. Such results are thought to be due to the Oxygenated nature of the fuel and how such feature influences the development of a comprehensive reaction mechanism for such fuels.

Numerical studies based on steady Computational Fluid Dynamics (CFD) for reactive flows were performed with the objective of validating advanced reaction mechanisms used to study spray combustion for both conventional and Biofuels.The SST-4 equation model was used to model turbulence, while more than one (comprehensive) reaction mechanisms were used to model the combustion of methanol, diesel and biodiesel using CHEMKIN-CFD and Fluent CFD code.Some of the reaction mechanisms used in modelling the current reactive flow simulation was already tested while others were developed during the course of this work.The computational results have shown good agreement with the available experimental data of Widmann and Presser (Combustion and Flame, 129, 47-86, 2002) with the developed reaction mechanism slightly over predicting the temperature range.The CFD results have also shown that most of the harmful emission of the combustion of liquid fuels is less for Biodiesel compared to conventional diesel with the exception of CO 2 .This is in line with the finding of many experimental data.

The transitional separated-reattached flow on a flat plate with a blunt leading edge under 2% free-stream turbulence (FST) is numerically simulated using the Large-eddy simulation (LES) approach. The Reynolds number based on the free-stream velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed and the LES results compare well with the available experimental data.It is well known that FST enhances shear-layer entrainment rates, reduces the mean reattachment distance, and causes early transition to turbulence leading to an early breakdown of the separated boundary layer. Many experimental studies have shown that different vortex shedding frequencies exist, specially the so called low-frequency flapping when there is a separation bubble but its mechanism is still not completely understood. The previous study by us without free-stream turbulence (NFST) did not show the existence of such a low-frequency flapping of the shear layer and it is not clear what the effects of FST will have on these shedding modes. Detailed analysis of the LES data has been presented in the present paper and the low-frequency flapping has not been detected in the current study. (C) 2009 Elsevier Inc. All rights reserved.

Large-eddy simulation (LES) of transitional separating-reattaching flow on three different geometries including a square surface mounted obstacle (referred to hereafter as the obstacle), a forward facing step (FFS) and square leading edge plate aligned horizontally to a flow field, has been performed using a dynamic sub-grid scale model. The Reynolds number based on the uniform inlet velocity and the plate thickness, obstacle/step height varies in the range of 4.5 - 6.5 x 10(3). The mean LES results for three geometries compare reasonably well with the available experimental and DNS data.As the obstacle and FFS are characterised by an additional separated region upstream the separation line compared to the square leading edge plate, this is thought to have led to some differences observed both on the flow topology and turbulence spectrum downstream the leading edge for the three geometries. The spectra obtained using standard Fourier transform for positions downstream the leading edge plate has clearly captured the characteristic shedding frequency but not in the case of the obstacle and FFS. However, the spectra content at locations within the upstream separated region for the obstacle and FFS indicates that the upstream bubble is unstable via the Kelvin-Helmholtz (K-H) mechanism which might have an influence on both the spectra content, instability mechanism and the flow topology of the downstream separated region.

The structure of a buoyant plume above a point heat source in a ventilated enclosure has been investigated using large-eddy simulation (LES). The aim of the work is to assess the performance and the accuracy of LES for modelling buoyancy-driven displacement ventilation of an enclosure and investigate the role of coherent structures in the plume entrainment mechanism which is important in these flow types because, for example, entrainment determines the ventilation flow rate. The Smagorinslcy subgrid-scale model is used for the unresolved small-scale turbulence. The Rayleigh number Ra is chosen in the range where spatial transition from laminar to turbulent flow takes place (Ra=1.5 x 10(9)). The stratification height and temperature of the stratified layer deduced from the mean field of the LES data is in good agreement with the theory of Linden, Lane-Serff and Smeed (1990). The plume entrainment coefficient is in good agreement with experimental values determined by Morton, Taylor and Turner (1956), Rouse, Yih and Humphreys (1952), and Baines and Turner (1969). Instantaneously, the plume develops through expansion and contraction phases, where the expansion phase is associated with the existence of coherent large-scale structures leading to an outward stretching of the plume and the contraction occurs as a result of partial breakdown and/or loss of coherence of these structures. As a result, the instantaneous entrained mass (and thus the entrainment coefficient) at different heights below the mean interface height were found to fluctuate about a mean value. Visualization of the computed flow showed that the stretching mechanism of the large-scale structures, which governs the expansion-contraction behaviour of the plume, occurs in such a way that the coherent structure dominating the flow below the interface height takes a spiral shape.

Large-eddy simulation (LES) of transitional separating-reattaching flow on a two-dimensional square surface mounted obstacle and a forward facing step has been performed using a dynamic sub-grid scale model. The Reynolds number based on the uniform inlet velocity and the obstacle/step height is 4.5 × 103. The mean LES results for both the obstacle and step flow compare reasonably well with the available experimental and DNS data. The flow structures upstream of the surface-mounted obstacle (referred to hereafter as obstacle) and the forward-facing step (referred to hereafter as FFS) consist of unstable two-dimensional structures and coherent rib-shaped structures. These structures with the aid of 3D streamline visualisation strongly indicate that the upstream separation bubble is a closed one rather than an open one in the sense that there is little evidence to suggest that there is fluid injection from the upstream separation region into the downstream separated region for the two geometries. The spectra and time history for the velocities and pressure fields at locations immediately upstream of the obstacle and FFS (including the recirculation region) were analysed using both the Fourier and wavelet transforms and revealed the unsteady nature of the recirculation region upstream of the obstacle and FFS. The transition process has been elucidated using both 2D and 3D flow visualisation of the flow. In both geometries (obstacle and FFS), the separated boundary layer downstream of the leading edge shows 2D nature and roll-up shortly downstream of the separation line leading to 2D K-H rolls to be shed from the leading edge. Coherent structures such as the λ-shaped and rib-like vortices commonly associated with a flat plate boundary layer and also found in the separated-reattached flow of a blunt leading edge plate aligned horizontally to a flow are not common in the separated-reattached flow over the obstacle and FFS.

Large Eddy Simulation (LES) has been employed to simulate a separated boundary layer transition with 2% free stream turbulence level and without free stream turbulence on a flat plate with a blunt leading edge. The entire transition process leading to breakdown to turbulence has been shown by flow visualization and large-scale coherent structures have been identified at different stages of the transition process. However, the Kelvin-Helmholtz rolls, which are clearly visible under no free stream turbulence (NFST), are not as apparent under 2% free steam turbulence (FST). The Lambda-shaped vortical structures which can be clearly seen in the NFST case can hardly be identified in the FST case. Generally speaking, the effects of free-stream turbulence have led to an early breakdown of the boundary layer and hence increased the randomisation in the vortical structures, degraded the spanwise coherence of those large-scale structures.

It is well known that large-scale organised motions, usually called coherent structures, exist in many transitional and turbulent flows. The topology and range of scales of coherent structures change widely in different flows. However, it is not well established what kind of large-scale coherent structures exists in separated/reattached transitional flows. Large Eddy Simulation (LES) with a dynamic subgrid-scale mo del is employed to investigate a separated boundary layer transition under 2% free stream turbulence level and without free stream turbulence on a flat plate with a blunt leading edge. Flow visualization is employed to show the entire transition process leading to breakdown to turbulence and large-scale coherent structures have been identified at various stages of the transition process. However, there are some noticeable difference between the flow case with and without free stream turbulence. The Kelvin-Helmholtz rolls, which are clearly visible under no free stream turbulence (NFST), are not so clearly visible under 2% free steam turbulence (FST) case. The Lambda-shaped vortical structures which can be clearly seen in the NFST case can hardly be identified in the FST case. Generally speaking, the effects of free-stream turbulence have led to an early breakdown of the separated boundary layer and hence increased the randomisation in the vortical structures, degraded the spanwise coherence of those large-scale structures.

It is well known that large-scale organised motions, usually called coherent structures, exist in many transitional and turbulent flows. The topology and range of scales of coherent structures change widely in different flows. However, it is not well established what kind of large-scale coherent structures exists in separated/reattached transitional flows. Large Eddy Simulation (LES) with a dynamic subgrid-scale model is employed to investigate a separated boundary layer transition under 2% free stream turbulence level and without free stream turbulence on a flat plate with a blunt leading edge. Flow visualization is employed to show the entire transition process leading to breakdown to turbulence and large-scale coherent structures have been identified at various stages of the transition process. However, there are some noticeable difference between the flow case with and without free stream turbulence. The Kelvin-Helmholtz rolls, which are clearly visible under no free stream turbulence (NFST), are not so clearly visible under 2% free steam turbulence (FST) case. The Lambda-shaped vortical structures which can be clearly seen in the NFST case can hardly be identified in the FST case. Generally speaking, the effects of free-stream turbulence have led to an early breakdown of the separated boundary layer and hence increased the randomisation in the vortical structures, degraded the spanwise coherence of those large-scale structures.

Rising buoyant plumes from a point heat source in a naturally ventilated enclosure have been investigated using large-eddy simulation (LES). The aim of the work is to assess the performance and the accuracy of LES for modelling buoyancy-driven displacement ventilation of an enclosure and to shed more light on the transitional behaviour of the plume and the coherent structures involved. The Smagorinsky sub-grid scale model is used for the unresolved small-scale turbulence. The Rayleigh number, Ra is chosen to be in the range where spatial transition from laminar to turbulent flow takes place (Ra = 1.5 × 109). The plume properties (source strength and rate of spread) as well as the ventilation properties (stratification height and temperature of stratified layer) estimated using the theory of Linden et al. are found to agree reasonably well with the LES results. The variation of the plume width with height indicates a linear variation of the entrainment coefficient rather than a constant value used by Linden et al. for a fully turbulent thermal plume. Flow visualisation revealed the nature of the large-scale coherent structures involved in the transition to turbulence in the plume. The most excited modes observed in the velocity, pressure and temperature fields spectra correspond to Strouhal number in the range 0.3 ≤ St ≤ 0.55 which is in agreement with those observed by Zhou et al. for a turbulent forced plume. Excited modes less than thisvalue (St = 0.2) were observed and may be due to low-frequency motions felt throughout the flow.

Large-eddy simulations (LES) of transitional separating-reattaching flow over a square surface mounted obstacle (SSMO) and a forward-facing step (FFS) have been performed. The Reynolds number based on the uniform inlet velocity and the obstacle height is 4.5 x 10(3). A dynamic subgrid-scale model is employed in this work. The mean LES results compare favourably with the available experimental and DNS data.This paper addresses the characteristic shedding modes associated with the separated-reattached flows on the SSMO and the FFS and sheds light on the use of the wavelet transform (WT) in extracting the content of a time history of a (velocity and/or pressure) signal compared to the traditional Fourier transform (FT). The turbulence spectra for the geometries revealed amplified frequency modes both upstream and downstream of the separation edge with those associated with the SSMO showing more clearly compared to the FFS. A frequency peak was detected at a location upstream of the separation line and immediately above the SSMO. The value of this frequency suggests that the upstream separated region is unstable via the Kelvin-Helmholtz instability and the peak can not be attributed to the flapping of the separated shear layer which is a phenomenon commonly associated with this class of flows. The WT captured events that are characterised by narrow periods (scales) and which happened over shorter times. Such events are smoothed out by the Fourier transform indicating the superiority of the WT over the FT.

Large‐eddy simulation (LES) of transitional separating–reattaching flow on a square surface mounted obstacle has been performed. The Reynolds number based on the uniform inlet velocity and the obstacle height is 4.5 × 103. A dynamic subgrid‐scale model is employed in this work. The mean LES results compare favourably with the available experimental and direct numerical simulation (DNS) data. Extensive analysis of the time series signals of the velocity and pressure fields at different locations including positions close to solid surfaces, at the centre and edge of the separated–reattached boundary layer using the windowed Fourier transform (WFT) and the wavelet transform was performed. The spectra analysis revealed the nature of the amplified frequencies at all the important locations of the flow field. Excited modes that could be due to the movement (shedding) of large‐scale structures and pairing of such types of structures are identified. A clear frequency peak was captured just upstream of the separation line. The value of the frequency peak and the low percentage of the back flow velocity compared to the freestream velocity in the current case strongly support the idea that this amplified frequency is most likely due to the Kelvin–Helmholtz (K–H) instability mechanism of the shear layer forming in the boundary of the small upstream separated region rather than being attributed to the flapping of the shear layer. Copyright © 2006 John Wiley & Sons, Ltd.

It has been well established that large-scale structures, usually called coherent structures, exist in many transitional and turbulent flows. The topology and range of scales of those large-scale structures vary from flow to flow such as counter-rotating vortices in wake flows, streaks and hairpin vortices in turbulent boundary layer. There has been relatively little study of large-scale structures in separated and reattached transitional flows.Large-eddy simulation (LES) is employed in the current study to investigate a separated boundary layer transition under 2% free-stream turbulence on a flat plate with a blunt leading edge. The Reynolds number based on the inlet free stream velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed to compute the subgrid-scale stresses more accurately in the current transitional flow case. Flow visualization has shown that the Kelvin-Helmholtz rolls, which have been so clearly visible under no free-stream turbulence (NFST) are not as apparent in the present study. The Lambda-shaped vortical structures which can be clearly seen in the NFST case can hardly be identified in the free-stream turbulence (FST) case. Generally speaking, the effects of free-stream turbulence have led to an early breakdown of the boundary layer, and hence increased the randomization in the vortical structures, degraded the spanwise coherence of those large-scale structures. Copyright (c) 2005 John Wiley & Sons, Ltd.

Large-eddy simulation (LES) of transitional separatingreattaching flow on a flat plate with a blunt leading edge has been performed. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6.5103. A dynamic subgrid-scale model is employed in the transitional flow case. The LES results compare reasonably well with the available experimental data. The entire transition process has been visualized by using the LES data, and large-scale vortical structures have been observed at different stages of transition. It is known that different vortex shedding frequencies exist, especially the so-called low-frequency flapping when there is a separation bubble. It is not clear whether all transitional and turbulent separatingreattaching flows have different vortex shedding frequencies. It is also not clear what the working mechanisms are behind the so-called low-frequency flapping as reported widely. These issues are addressed.

It is well known that large-scale organized motions, usually called coherent structures, exist in many transitional and turbulent flows (if not all). The topology and range of scales of those large-scale structures change widely from flow to flow such as counter-rotating vortices in wake flows, streaks and hairpin vortices in turbulent boundary layers. However, it is not well established what kind of large-scale structures exists in separated/reattached transitional flows.

Laminar separated flows are known to become unstable at relatively low Reynolds numbers. As a result, both the mean and instantaneous flow patterns are highly influenced by instabilities leading to transition to turbulence. Large-Eddy Simulation (LES) is employed to investigate the primary and secondary instabilities of a separated boundary layer transition on a flat plate with a blunt leading edge. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed to compute the subgrid-scale stresses more accurately in the transitional flow case. Statistics of the LES are found to be in acceptable agreement with the available experimental data. Based on the characteristic frequency from the velocity and pressure spectra, the LES results confirm that transition starts with the primary 2D instability originating from the free shear in the bubble as the free shear layer is inviscidly unstable via the Kelvin–Helmholtz mechanism. The flow visualisation together with the spectral analysis for the velocity components and pressure give strong indication of the dominance of the helical-pairing instability which could be mainly responsible for the breakdown to turbulence.