Mean Flow Field Research Articles

Incident shock wave and supersonic turbulent boundarylayer interactions near an expansion corner

Direct numerical simulations of incident shock wave and supersonic turbulent boundary layer interactions near an expansion corner are performed at Mach number M∞ = 2.9 and Reynolds number Re∞ = 5581 to investigate the expansion effect on the characteristic features of this phenomenon. Four expansion angles, i.e. α = 00 (flat-plate), 20, 50 and 100 are considered. The nominal impingement point of the oblique shock wave with a flow deflection angle of 120 is fixed at the onset of the expansion corner, and flow conditions are kept the same for all cases. The numerical results are in good agreement with previous experimental and numerical data. Various flow phenomena, including the flow separation, the post-shock turbulent boundary layer and the flow unsteadiness in the interaction region, have been systematically studied. Analysis of the instantaneous and mean flow fields indicates that the main effect of the expansion corner is to significantly decrease the size and three-dimensionality of the separation bubble. A modified scaling analysis is proposed for the expansion effect on the interaction length scale, and a satisfactory result is obtained. Distributions of the mean velocity, the Reynolds shear stress and the turbulent kinetic energy show that the post-shock turbulent boundary layer in the downstream region experiences a faster recovery to the equilibrium state as the expansion angle is increased. The flow unsteadiness is studied using spectral analysis and dynamic mode decomposition, and dynamically relevant modes associated with flow structures originated from the incoming turbulent boundary layer are clearly identified. At large expansion angle (α=100), the unsteadiness of the separated shock is dominated by medium frequencies motions, and no low frequency unsteadiness is observed. The present study confirms that the driving mechanism of the low frequency unsteadiness is strongly related to the separated shock and the detached shear layer.

Nonintrusive reduced order modeling framework for quasigeostrophic turbulence.

In this study, we present a nonintrusive reduced order modeling (ROM) framework for large-scale quasistationary systems. The framework proposed herein exploits the time series prediction capability of long short-term memory (LSTM) recurrent neural network architecture such that (1) in the training phase, the LSTM model is trained on the modal coefficients extracted from the high-resolution data snapshots using proper orthogonal decomposition (POD) transform, and (2) in the testing phase, the trained model predicts the modal coefficients for the total time recursively based on the initial time history. Hence, no prior information about the underlying governing equations is required to generate the ROM. To illustrate the predictive performance of the proposed framework, the mean flow fields and time series response of the field values are reconstructed from the predicted modal coefficients by using an inverse POD transform. As a representative benchmark test case, we consider a two-dimensional quasigeostrophic ocean circulation model which, in general, displays an enormous range of fluctuating spatial and temporal scales. We first demonstrate that the conventional Galerkin projection-based reduced order modeling of such systems requires a high number of POD modes to obtain a stable flow physics. In addition, ROM-Galerkin projection (ROM-GP) does not seem to capture the intermittent bursts appearing in the dynamics of the first few most energetic modes. However, the proposed nonintrusive ROM framework based on LSTM (ROM-LSTM) yields a stable solution even for a small number of POD modes. We also observe that the ROM-LSTM model is able to capture quasiperiodic intermittent bursts accurately, and yields a stable and accurate mean flow dynamics using the time history of a few previous time states, denoted as the lookback time window in this paper. We show several features of ROM-LSTM framework such as significantly higher accuracy than ROM-GP, and faster performance using larger time step size. Throughout the paper, we demonstrate our findings in terms of time series evolution of the field values and mean flow patterns, which suggest that the proposed fully nonintrusive ROM framework is robust and capable of predicting chaotic nonlinear fluid flows in an extremely efficient way compared to the conventional projection-based ROM framework.

A Dual-Grid Hybrid RANS/LES Model for Under-Resolved Near-Wall Regions and its Application to Heated and Separating Flows

A hybrid RANS/LES model for high Reynolds number wall-bounded flows is presented, in which individual Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES) are computed in parallel on two fully overlapping grids. The instantaneous, fluctuating subgrid-scale stresses are blended with a statistical eddy viscosity model in regions where the LES grid is too coarse. In the present case, the hybrid model acts as a near-wall correction to the LES, while it retains the fluctuating nature of the flow field. The dual computation enables the LES to be run on isotropic grids with very low wall-normal and wall-parallel resolution, while the auxiliary RANS simulation is conducted on a wall-refined high-aspect ratio grid. Running distinct, progressively corrected simulations allows a clearer separation of the mean and instantaneous flow fields, compliant with the fundamentally dissimilar nature of RANS and LES. Even with the wall-nearest grid point lying far in the logarithmic layer, velocity and temperature predictions of a heated plane channel flow are corrected. For a periodic hill flow, the dual-grid system improves the boundary layer separation and velocity field prediction both for a constant-spaced and a wall-refined LES grid.

Scale-Adaptive Simulation of the Unsteady Turbulent Flow in a High-Speed Centrifugal Compressor with a Wedge-Type Vaned Diffuser

The present work investigates the details of unsteady flow behavior in a transonic centrifugal compressor with vaned diffuser. The analysis is based on solving three-dimensional, compressible, unsteady Navier-Stokes equations. The computational model is a high-compression ratio centrifugal compressor (4:1) consisting of an inlet duct, impeller, and diffuser vane. The hybrid scale-adaptive simulation (SAS) turbulent model is used to provide detailed flow information and characterize the transient flow structures within the compressor passages. A numerical sensitivity test is performed to validate the computational results in terms of pressure ratio and compressor efficiency. Instantaneous and mean flow field analyses are presented in the impeller and the vaned diffuser passages. Applying transient simulations, it is shown that the interaction between the pressure waves and the surface pressure of the diffuser blades leads to a pulsating behavior within the diffuser. Moreover, spectral analysis is evaluated to analyze the blade passing frequency (BPF) tonal noise as the main noise source of centrifugal compressors. In addition, the current SAS results are compared with those of the URANS-SST (shear stress transport) approach to show the ability of the SAS approach in the prediction of turbulent structures, where the SAS model leads to a much better resolution of the unsteady fluctuations. This study shows that the current SAS approach is promising in terms of the prediction of transient phenomena like LES, but offers a substantially reduced turn-around time.

Prediction of Turbulent Flow Over a Flat Plate With a Step Change From a Smooth to a Rough Surface Using a Near-Wall RANS Model

Abstract The present paper reports a numerical study of fully developed turbulent flow over a flat plate with a step change from a smooth to a rough surface. The Reynolds number based on momentum thickness for the smooth flow was Reθ=5950. The focus of the study was to investigate the capability of the Reynolds-averaged Navier–Stokes (RANS) equations to predict the internal boundary layer (IBL) created by the flow configuration. The numerical solution used a two-layer k−ε model to implement the effects of surface roughness on the turbulence and mean flow fields via the use of a hydrodynamic roughness length y0. The prediction for the mean velocity field revealed a development zone immediately downstream of the step in which the mean velocity profile included a lower region affected by the surface roughness below and an upper region with the characteristics of the smooth-wall boundary layer above. In this zone, both the turbulence kinetic energy and Reynolds shear stress profiles were characterized by a significant reduction in magnitude in the outer region of the flow that is unaffected by the rough surface. The turbulence kinetic energy profile was used to estimate the thickness of the IBL, and the resulting growth rate closely matched the experimental results. As such, the IBL is a promising test case for assessing the ability of RANS models to predict the discrete roughness configurations often encountered in industrial and environmental applications.

Acoustothermal heating in surface acoustic wave driven microchannel flow

Surface acoustic wave (SAW) is a well-proven tweezer serving various applications such as particle manipulation, cell trapping and separations, fluid mixing, and biosensing. SAWs can cause significant heat generation due to acoustothermal heating as established experimentally. Theoretical understanding of acoustothermal heating is limited, primarily due to the difficulties presented by multiple time scales inherent in this phenomenon. We present a theoretical model based on a multiple scale perturbation approach to solve the fluid flow and heat transfer equations for SAW-driven acoustothermal heating of a Newtonian fluid in a microchannel. The first order fields are oscillatory with the same frequency as that of the SAW, whereas the second order components are time-averaged to account for the mean flow and temperature fields. We find that the temperature rise depends solely on the acoustic energy density and its conversion into internal energy via pressure work on the fluid and hydrodynamic transportation of heat. For a fixed aspect ratio, an increase in system size essentially increases the conversion of acoustic energy into internal energy, leading to an increase in temperature rise. On the other hand, an increase in SAW frequency for a given system size causes the acoustic energy density to increase and thereby increases the temperature rise. Temperature rise is found to increase linearly with SAW power, in agreement with experimental results reported in the literature. The quantitative model for the temperature field presented in this work will find applications in designing biosensors, microreactors, and in other SAW driven controllable digital microfluidic heating applications.

Stability Analysis of the Swirling Majdalani–Fist Mean Flowfield in Solid Rocket Motors

Numerous studies have been performed to show the effects of spinning and swirling flows on increased flight stability characteristics, higher propellant consumption, increased burn rate, greater chamber pressures, and many more. The focus of this work is to understand and quantify the stability characteristics of the “swirling Majdalani–Fist” mean flowfield for solid rocket motors. The study is motivated by the need to quantify the hydrodynamic instability of the flow in cylindrically shaped solid rockets. The aforementioned mean flow profile has been adapted to the current biglobal stability framework. The biglobal stability framework developed here is not limited to a stream function formulation; instead it uses the complete Navier–Stokes equations to the extent of resulting in a linearized eigenproblem, which can be readily solved using a pseudo-spectral method. The governing equations are derived for a two-dimensional spatial wave with sinusoidal temporal dependence. Here a fully two-dimensional biglobal study is carried out using realistic boundary conditions that are consistent with solid rocket chambers. The swirling Majdalani–Fist profile results are compared with results found using the same framework with the well-known Taylor–Culick profile. Finally, future applications and areas of investigation such as headwall injection, multiphasic flow, and compressibility effects are then reviewed.

Large eddy simulation of turbulent heat transfer in a non-isothermal channel: Effects of temperature-dependent viscosity and thermal conductivity

In this work, we perform large eddy simulations (LES) to study the influence of variable viscosity and thermal conductivity on forced convection in a non-isothermal channel flow. To prevent thermal dilatational effect, the gas density is assumed to be constant. The temperature ratio T2/T1 is varied from 1.01 to 2.2, where T2 and T1 are the temperatures of hot and cold walls, respectively. The mean turbulent Reynolds number is kept the same at 395. The results indicated that the mean flow fields are significantly affected by the temperature-dependent fluid properties. Despite the modified velocity and temperature profiles, it is interesting to note that the molecular momentum and heat transport across the channel remain unchanged. Meanwhile, pronounced differences are exhibited for various turbulence statistics such as root-mean-square velocity and temperature fluctuations, Reynolds shear stress, and correlation between streamwise velocity and temperature. Compared with the isothermal flows, it is also found that the presence of the temperature gradient tends to diminish heat transfer. With increasing the temperature ratio, the Nusselt numbers for both sides are reduced. Moreover, the hot side has a higher Nusselt number than the one at the cold side.

A Fully Consistent Hybrid Les/Rans Conditional Transported Pdf Method for Non-premixed Reacting Flows

ABSTRACT A novel hybrid modeling approach that combines a finite-volume large eddy simulation (LES) with a Reynold-Averaged Navier-Stokes (RANS) conditional probability density function (PDF) method for turbulent non-premixed reacting flows is presented. The hybrid method incorporates the turbulent flow modeling of the LES with the chemistry–turbulence interaction modeling of the Lagrangian RANS-PDF. Specifically, the conditional composition RANS-PDF method is applied as the turbulence-chemistry closure for the LES. For a statistically steady-state case, the evolution of stochastic particles in a RANS framework reduces the computational effort, compared to the classical LES filtered density function (FDF), by performing the expensive chemical ODE integration at a lower spatial resolution. This approach is justified by the slow spatial variations of the reacting variables conditioned on the mixture fraction. Here, a consistent two-way coupling between the LES flow-field and the conditional composition RANS-PDF is formulated. The accuracy and the consistency of the method are established studying the Sandia flame series with detailed chemistry. Good predictions of the mean flow-fields and mean and conditional reactive scalars have been obtained, compared with the experimental data, with an overall decrease of the computational effort up to two orders of magnitude.

Large eddy simulation of film cooling flow from round and trenched holes

The thermal and flow fields of round and trenched holes in the flat-plate model are investigated using large eddy simulation (LES) after validated against the experimental results. The focus is on understanding the influence of the trenched hole on downstream vortex structures at blowing ratios M = 0.5 and 1.0, which may benefit its effective application in cooling design. At M = 1.0, the transverse trench increases turbulent fluctuations and augments the complexity of vortex structures. Dynamic mode decomposition analysis is employed to extract the primary vortex structures. The dominant vortices of the trenched hole include K-H vortices and hairpin-like vortices near the centerline. This series of hairpin-like vortices present a larger spatial size than the hairpin vortex downstream of the round hole. Besides, they correspond to the downstream counter-rotating vortex pair in the mean flow field, which is detrimental to the local film cooling effectiveness. At lower blowing ratio M = 0.5, the previous spatially large hairpin-like vortices are replaced by a smaller one which alternatively appears on both sides of the centerline. In the mean flow field, each branch of the CVP is caught between two anti-CVPs. The suppression of adjacent vortices guarantees the attachment of coolant to the surface.

Investigation of asymmetric flow past a slender body at high angles of attack

This paper presents an investigation of flow asymmetry around a slender body at high angles of attack. The paper investigated the numerical aspect of simulating such flows. The impact of three simulation parameters, including grid resolution, discretization scheme, and turbulent flow modeling, was assessed. It was shown that insufficient grid density resulted in highly dissipated solution. At high angles, where flow asymmetry is expected to develop around the body, the dissipation from poor grid resolution prevented the flow asymmetry. At higher grid resolution, the solution demonstrated a switch between two bistable states. Two spatial discretization schemes, namely central and bounded, were tested in the course of this study. The results illustrated the necessity to use non-dissipative unbiased discretization schemes. Large eddy simulation was performed using two sub-grid-scale models in addition to a run without a model. The sub-grid-scale models generated similar results except for switching of asymmetry direction and the axial location of separation foci. The study shows that grid resolution and solution scheme have a profound effect on the validity of the simulation of flow around slender bodies at high angles of attack. The study also showed that stringent grid requirements marginalized the effect of the sub-grid-scale model. Computations were then carried out at seven angles of attack $$\alpha = 30^{\circ }$$ , $$40^\circ $$ , $$50^\circ $$ , $$52.5^\circ $$ , $$55^\circ $$ , $$57.5^\circ $$ , and $$60^\circ $$ . Analysis was performed on mean and unsteady flow fields. The total normal force increased with increasing angle of attack. On the other hand, the total side force started to increase rapidly for angles of attack $$\alpha > 50^{\circ }$$ and reached a maximum at $$\alpha =57.5^{\circ }$$ before decreasing at $$\alpha =60^{\circ }$$ . A bistable mode was observed for $$\alpha > 50^{\circ }$$ in which the orientation of resultant forces switches with angle of attack. Comparison of computed dominant frequencies with experiment showed an acceptable agreement. Several dominant modes were identified: very low-frequency mode, low-frequency mode, intermediate-frequency mode, and high-frequency mode. The modes were shown to develop with increasing angle of attack. Surface flow pathlines revealed the existence of separation foci at $$\alpha =57.5^{\circ }$$ and $$60^{\circ }$$ , and a high-frequency tonal mode was observed to accompany the formation of separation foci.

Experimental Investigation on Flow Past Two and Three Side-by-Side Inclined Cylinders

A series of experiments were conducted to investigate the effect of the inclination angle of the cylinders on the wake flow characteristics for flow past two and three side-by-side inclined cylinders using the particle image velocimetry (PIV). Depending on the inclination angles, purely deflected gap flow, no-deflection gap flow, and flip-flop gap flow patterns are identified for both two and three cylinder cases. In both two and three cylinder cases, the flows through the gaps are found to be in purely deflected flow pattern at small inclination angles and flip-flop pattern at large inclination angles. For the three-cylinder case with flip-flop gap flow pattern, gap flows are predominantly in the outward deflection pattern (toward the two side cylinders) and are occasionally deflected inward (toward the middle cylinder). The gap flow deflection angles for all the tested inclination angles of the cylinders are quantified through statistical analysis, in addition to identifying the flow patterns. The deflection angle is found to decrease with increasing inclination angle for both two- and three-cylinder cases, and the outward deflection angle for the three cylinder cases is greater than the deflection angle of the two-cylinder case. The probability density distributions of the deflection angles approximately follow normal distribution. In the two-cylinder case, the mean flow field is asymmetrical about the x-axis when the possibility of the flow deflection toward one side of the gap is greater than that toward the other side.

Effects of nozzle-exit boundary-layer profile on the initial shear-layer instability, flow field and noise of subsonic jets

The influence of the nozzle-exit boundary-layer profile on high-subsonic jets is investigated by performing compressible large-eddy simulations (LES) for three isothermal jets at a Mach number of 0.9 and a diameter-based Reynolds number of $5\times 10^{4}$, and by conducting linear stability analyses from the mean-flow fields. At the exit section of a pipe nozzle, the jets exhibit boundary layers of momentum thickness of approximately 2.8 % of the nozzle radius and a peak value of turbulence intensity of 6 %. The boundary-layer shape factors, however, vary and are equal to 2.29, 1.96 and 1.71. The LES flow and sound fields differ significantly between the first jet with a laminar mean exit velocity profile and the two others with transitional profiles. They are close to each other in these two cases, suggesting that similar results would also be obtained for a jet with a turbulent profile. For the two jets with non-laminar profiles, the instability waves in the near-nozzle region emerge at higher frequencies, the mixing layers spread more slowly and contain weaker low-frequency velocity fluctuations and the noise levels in the acoustic field are lower by 2–3 dB compared to the laminar case. These trends can be explained by the linear stability analyses. For the laminar boundary-layer profile, the initial shear-layer instability waves are most strongly amplified at a momentum-thickness-based Strouhal number $St_{\unicode[STIX]{x1D703}}=0.018$, which is very similar to the value obtained downstream in the mixing-layer velocity profiles. For the transitional profiles, on the contrary, they predominantly grow at higher Strouhal numbers, around $St_{\unicode[STIX]{x1D703}}=0.026$ and 0.032, respectively. As a consequence, the instability waves rapidly vanish during the boundary-layer/shear-layer transition in the latter cases, but continue to grow over a large distance from the nozzle in the former case, leading to persistent large-scale coherent structures in the mixing layers for the jet with a laminar exit velocity profile.

Numerical analysis of the influence of early fuel injection on charge motion in a direct injection spark ignition engine using scale-resolving simulations

This work summarizes the numerical analysis of the effect of early fuel injection on the charge motion in a direct injection spark ignition engine concerning cyclic fluctuations of the flow field. The combination of the scale-resolving turbulence model “Scale Adaptive Simulation” and post-processing routines for vortex trajectory visualization allows for a detailed insight into the temporal resolved and cycle-dependent behavior of the charge motion. In the first part, a simplified engine set-up is presented and used as a validation case to ensure correct behavior of the turbulence model and post-processing routines. In the second part, the computational fluid dynamics model of the real engine is introduced. The application of the proposed vortex tracking algorithm is shown, and a short discussion about the transient behavior of the charge motion in this engine set-up is given. The third part describes the analysis of the influence of the fuel injection on the charge motion at different engine speeds from 1000 to 3000 r/min and variations of the intake pressure from 1 to 2 bar. Finally, the impact on different flow field properties at possible ignition timings is discussed. Changes in mean flow field quantities as well as in aerodynamic fluctuations are found as a consequence of fuel injection.

Effect of the numerical viscosity on reproduction of mean and turbulent flow fields in the case of a 1:1:2 single block model

Large-eddy simulations were performed for the velocity fields around a 1:1:2 single block model to clarify the effect of the numerical viscosity in different advection schemes. Six types of advection schemes with different numerical viscosities were employed: second-order central, first-order upwind, and blending schemes with ratios of 95:5, 90:10, 80:20, and 60:40. The central scheme alone or the blending schemes predicted values of the mean and turbulent kinetic energy that were comparable with those of the experiments, whereas the upwind scheme significantly underestimated the experimental values. In addition to the comparison with the experimental data, the turbulent flow fields among the schemes were compared by deriving the probability and power spectral densities. Blending of the upwind scheme indeed reduced the turbulence energy contribution at high frequency. However, such a reduction in energy became influential to the reproduction of the turbulent flows only when damping of the peak spectral energy occurred. The reduction of the statistical values became ∼10% when blending the upwind scheme by 20%. In contrast, a strong or weak velocity, evaluated by the percentile velocities, was more sensitive to the selection of the advection scheme than the mean velocities.

An investigation on mechanisms of equilibrium-stage scour and deposition process around a submerged L-head groyne

ABSTRACT A numerical model based on large eddy simulation code supported with a laboratory experiment was used to explore the mechanism of scour and deposition process around a submerged L-head groyne at equilibrium stage of flow. The numerical model used the equilibrium bathymetry, which was obtained from an experiment conducted in the laboratory. The components of Horse Shoe vortex system (HV) including the presence of aperiodic oscillatory motions between back flow and zero flow modes near the groyne faces junction are explored. Formations of elliptical-shaped vortex tubes along detached shear layer (DSL) and their merging phenomenon at different time scales of flow are also investigated. Increase in vorticity magnitude towards bottom as compared to upper planes for instantaneous and mean flow field is identified. Near bed region, concentration of DSL profile towards submerged mound situated in the wake region emerges as dominating factor over HV system for the production of larger vorticity magnitude. Transportation of DSL eddies, their interaction with wake vortices and flow within reattachment zone are also focussed. To emphasize, this innovative study was conducted first time to fulfil the gap of exploring mechanism of scour and deposition process around such a complex geometry under submerged condition over scoured bed.

An Experimental Investigation of Rotor–Box Aerodynamic Interaction1

Abstract Multirotor unmanned aerial vehicles (UAVs) are a promising means of package delivery. Such applications generally involve carrying bulky payloads under the vehicle. Understanding the aerodynamic interaction effects of payloads on the vehicle is the key to design such systems, in the low Reynolds number regime of small UAVs. High-speed particle image velocimetry (PIV), force, and torque measurements have been used with a rotor and a cubic box to investigate the rotor–box interactions and configurations typical of multirotor UAVs. The observed rotor and vehicle performance trends are explained by the mean flow field captured through PIV. Conditions similar to ground-effect operation are developed for the rotor at a high level of rotor-box overlap. A slight improvement in the vehicle performance is observed at conditions where the box is just out of the rotor wake. Some basic instantaneous flow phenomena due to rotor–box interaction have been identified. The interactions have been classified into three distinct modes based on observations at a range of box positions relative to the rotor. An empirical tip vortex trajectory model for isolated rotors is found to be instrumental in predicting the interaction mode at a given box position.

Numerical simulation of flow with large eddy simulation at Re = 3900

PurposeOver the past few decades, the flow around circular cylinders has been one of the highly researched topics in the field of offshore engineering and fluid-structure interaction (FSI). In the current study, numerical simulations for flow around a fixed circular cylinder are performed at Reynolds number (Re) = 3900 with the LES method using the ICEM-CFD and ANSYS Fluent tool for meshing and analysis, respectively. Previously, similar studies have been conducted at the same Reynolds number, but there have been discrepancies in the results, particularly in calculating the recirculation length and angle of separation. In addition, the purpose of this study is to address the impact of time interval averaging to obtain the fully converged solution.Design/methodology/approachThis study presents the LES method, using the ICEM-CFD and ANSYS fluent tool for meshing and analysis.FindingsIn the current study, turbulence statistics are sampled for 25, 50, 75 and 100 vortex-shedding cycles with the CFL value O (1). The recirculation length, angle of separation, hydrodynamic coefficients and the wake behind the cylinder are investigated up to ten diameters. The drag coefficient and Strouhal number are observed to be less sensitive, whereas the recirculation length appeared to be highly dependent on the average time statistics and the non-dimensional time step. Similarly, the mean streamwise and cross-flow velocity are observed to be sensitive to the average time statistics and non-dimensional time step in the wake region near the cylinder.Originality/valueIn the current investigation, turbulence statistics are sampled for 25, 50, 75 and 100 vortex-shedding cycles with the CFL value O (1), using large eddy simulation method at Re = 3900 around a circular cylinder. The impact of time interval averaging to obtain the fully converged mean flow field is addressed. No such consideration is yet published in the literature.

Role of fuel properties and piston shape in influencing soot oxidation in heavy-duty low swirl diesel engine combustion

The presented study evaluated effects on soot emissions of important diesel fuel properties (e.g. volatility T95, hydrogen to-carbon ratio H/C and oxygen content O/C) in a heavy-duty, low-swirl, direct injection diesel engine operated with cooled exhaust gas recirculation to control the emissions of nitrogen oxides. Variation of injection pressure was conducted in three representative operation points in the single-cylinder engine including conventional and wave piston bowl types. Additionally, detailed optical combustion studies were performed in a high pressure/high temperature combustion chamber with an engine-like flame-wall interaction geometry.The fuel properties mainly influenced parameters relating to events near the nozzle, such as the ignition delay, flame lift-off distance and spray air entrainment. Oxygenation of the fuel reduced soot formation (a known effect of spray-jet dilution), but the fuels’ H/C and T95 also influenced soot emissions. Viscosity, heating value and density, treated as secondary fuel parameters, affected the injected fuel’s kinetic energy, and hence mean flow field and local turbulent mixing late in the combustion cycle.The results suggested that fuel effects on soot oxidation during late phases of combustion are mainly governed by parameters related to the fuel injection profile under the specific cylinder gas conditions. With an oxygenated fuel, flow structures (local turbulent side vortices) which are typical for the investigated engine type were found to form in a similar way as with a reference fuel. Hence, improved internal flow using a wave piston in combination with a low-sooting oxygenated fuel gave additional reductions in net soot emissions.

The effect of an unsteady flow incident on an array of circular cylinders

In this paper we investigate the effect of an inhomogeneous and unsteady velocity field incident on an array of rigid circular cylinders arranged within a circular perimeter (diameter $D_{G}$) of varying solid fraction $\unicode[STIX]{x1D719}$, where the unsteady flow is generated by placing a cylinder (diameter $D_{G}$) upwind of the array. Unsteady two-dimensional viscous simulations at a moderate Reynolds number ($Re=2100$) and also, as a means of extrapolating to a flow with a very high Reynolds number, inviscid rapid distortion theory (RDT) calculations were carried out. These novel RDT calculations required the circulation around each cylinder to be zero which was enforced using an iterative method. The two main differences which were highlighted was that the RDT calculations indicated that the tangential velocity component is amplified, both, at the front and sides of the array. For the unsteady viscous simulations this result did not occur as the two-dimensional vortices (of similar size to the array) are deflected away from the boundary and do not penetrate into the boundary layer. Secondly, the amplification is greater for the RDT calculations as for the unsteady finite Reynolds number calculations. For the two highest solid fraction arrays, the mean flow field has two recirculation regions in the near wake of the array, with closed streamlines that penetrate into the array which will have important implications for scalar transport. The increased bleed through the array at the lower solid fraction results in this recirculation region being displaced further downstream. The effect of inviscid blocking and viscous drag on the upstream streamwise velocity and strain field is investigated as it directly influences the ability of the large coherent structures to penetrate into the array and the subsequent forces exerted on the cylinders in the array. The average total force on the array was found to increase monotonically with increasing solid fraction. For high solid fraction $\unicode[STIX]{x1D719}$, although the fluctuating forces on the individual cylinders is lower than for low $\unicode[STIX]{x1D719}$, these forces are more correlated due to the proximity of the cylinders. The result is that for mid to high solid fraction arrays the fluctuating force on the array is insensitive to $\unicode[STIX]{x1D719}$. For low $\unicode[STIX]{x1D719}$, where the interaction of the cylinders is weak, the force statistics on the individual cylinders can be accurately estimated from the local slip velocity that occurs if the cylinders were removed.