Publications
20 results found
Hamzehloo A, Lusher DJ, Sandham ND, 2023, Direct numerical simulations and spectral proper orthogonal decomposition analysis of shocklet-containing turbulent channel counter-flows, International Journal of Heat and Fluid Flow, Vol: 104, ISSN: 0142-727X
Counter-flow configurations in a confined channel flow provide an efficient framework to study high intensity turbulent mixing processes. In a previous study (Physical Review Fluids, 6(9), p.094603), a wall-bounded counter-flow turbulent channel configuration was presented as an effective framework for addressing certain challenges related to the study of compressibility effects on turbulence as an alternative to free shear layer and Poiseuille/Couette type flows. Here, the previous direct numerical simulations are extended to a higher Mach number (M = 0.7) to quantify direct and indirect effects of compressibility. It is found that the configuration is able to produce large numbers of embedded shocklets, leading to significant asymmetry in probability density functions of dilatation. Reducing the Prandtl number from 0.7 to 0.2 increases the compressibility effect further by reducing the bulk heating in the channel. A peak turbulent Mach number close to unity is obtained, for which the contribution of the dilatational dissipation to the total dissipation is nevertheless found to be limited to . Indirect effects of compressibility are much larger, with changes of up to 40% in Favre normal stresses, despite the mean flow and shear stress being almost unaffected by compressibility in this configuration. Given the inflectional nature of the turbulent mean flow it is also interesting to identify large structures. Spectral Proper Orthogonal Decomposition (SPOD) reveals a full spectrum with a slow decay of energy with mode number. Mode shapes are three-dimensional with the low frequencies displaying elongated streaks in the velocity field at the channel centre plane.
Hamzehloo A, Lusher DJ, Laizet S, et al., 2022, Direct numerical simulations of shocklet-containing turbulent channel counter-flows, 12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12), Publisher: International Symposium series on Turbulence and Shear Flow Phenomena, Pages: 1-6
Counter-flow or counter-current configurations can maintain high turbulence intensities and exhibit a significant level of mixing. We have previously introduced a wall-bounded counter-flow turbulent channel configuration (Physical Review Fluids, 6(9), p.094603.) as an efficient framework to study compressibility effects on turbulence. Here, we extend our previous direct numerical simulation study to a relatively higher Mach number (M = 0.7) to investigate strong compressibility effects (also by reducing the Prandtl number from Pr = 0.7 to 0.2), and the formation and evolution of unsteadyshocklet structures. It is found that the configuration is able to produce highly turbulent flows with embedded shocklets and significant asymmetry in probability density functions of dilatation. A peak turbulent Mach number close to unity is obtained, for which the contribution of the dilatational dissipation to total dissipation is nevertheless found to be limited to 6%.
Hamzehloo A, Bahlali ML, Salinas P, et al., 2022, Modelling saline intrusion using dynamic mesh optimization with parallel processing, ADVANCES IN WATER RESOURCES, Vol: 164, ISSN: 0309-1708
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Hamzehloo A, Lusher D, Laizet S, et al., 2021, Direct numerical simulation of compressible turbulence in acounter-flow channel configuration, Physical Review Fluids, Vol: 6, Pages: 1-21, ISSN: 2469-990X
Counter-flow configurations, whereby two streams of fluid are brought together from oppositedirections, are highly efficient mixers due to the high turbulence intensities that can be maintained.In this paper, a simplified version of the problem is introduced that is amenable to direct numericalsimulation. The resulting turbulent flow problem is confined between two walls, with one non-zeromean velocity component varying in the space direction normal to the wall, corresponding to asimple shear flow. Compared to conventional channel flows, the mean flow is inflectional and themaximum turbulence intensity relative to the maximum mean velocity is nearly an order of magnitude higher. The numerical requirements and turbulence properties of this configuration are firstdetermined. The Reynolds shear stress is required to vary linearly by the imposed forcing, witha peak at the channel centreline. A similar behaviour is observed for the streamwise Reynoldsstress, the budget of which shows an approximately uniform distribution of dissipation, with largecontributions from production, pressure-strain and turbulent diffusion. A viscous sublayer is obtained near the walls and with increasing Reynolds number small-scale streaks in the streamwisemomentum are observed, superimposed on the large-scale structures that buffet this region. Whenthe peak local mean Mach number reaches 0.55, turbulent Mach numbers of 0.6 are obtained,indicating that this flow configuration can be useful to study compressibility effects on turbulence.
Özbay AG, Hamzehloo A, Laizet S, et al., 2021, Poisson CNN: Convolutional neural networks for the solution of the Poisson equation on a Cartesian mesh, Data-Centric Engineering, Vol: 2, Pages: 1-31, ISSN: 2632-6736
<jats:title>Abstract</jats:title> <jats:p>The Poisson equation is commonly encountered in engineering, for instance, in computational fluid dynamics (CFD) where it is needed to compute corrections to the pressure field to ensure the incompressibility of the velocity field. In the present work, we propose a novel fully convolutional neural network (CNN) architecture to infer the solution of the Poisson equation on a 2D Cartesian grid with different resolutions given the right-hand side term, arbitrary boundary conditions, and grid parameters. It provides unprecedented versatility for a CNN approach dealing with partial differential equations. The boundary conditions are handled using a novel approach by decomposing the original Poisson problem into a homogeneous Poisson problem plus four inhomogeneous Laplace subproblems. The model is trained using a novel loss function approximating the continuous <jats:inline-formula> <jats:alternatives> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2632673621000071_inline1.png" /> <jats:tex-math>$ {L}^p $</jats:tex-math> </jats:alternatives> </jats:inline-formula> norm between the prediction and the target. Even when predicting on grids denser than previously encountered, our model demonstrates encouraging capacity to reproduce the correct solution profile. The proposed model, which outperforms well-known neural network models, can be included in a CFD solver to help with solving the Poisson equation. Analytical test cases indicate that our CNN architecture is capable of predicting the correct solution of a Poisson problem with mean percentage errors below 10%, an improvement by comparison to the first step of conventional iterative methods. Predictions from our model, used as the initial guess to iterative algorithms like Multigrid, can reduce the root mean square error af
Hamzehloo A, Bartholomew P, Laizet S, 2021, Direct numerical simulations of incompressible Rayleigh–Taylor instabilities at low and medium Atwood numbers, Physics of Fluids, Vol: 33, Pages: 1-23, ISSN: 1070-6631
Direct numerical simulations of two-dimensional (2D) and three-dimensional (3D), single-mode and multi-mode, incompressible immiscible Rayleigh–Taylor (RT) instabilities are performed using a phase-field approach and high-order finite-difference schemes. Various combinations of Atwood number, Reynolds number, surface tension, and initial perturbation amplitude are investigated. It is found that at high Reynolds numbers, the surface tension, if significant, could prevent the formation of Kelvin–Helmholtz type instabilities within the bubble region. A relationship is proposed for the vertical distance of the bubble and spike vs the Atwood number. The spike and bubble reaccelerate after reaching a temporary plateau due to the reduction of the friction drag as a result of the formation of the spike vortices and also the formation of a momentum jet traveling upward within the bubble region. The interface for a 3D single-mode instability grows exponentially; however, a higher Reynolds number and/or a lower Atwood number could result in a noticeably larger surface area after the initial growth. It is also shown that a 3D multi-mode RT instability initially displays an exponential interface growth rate similar to single-mode RT instabilities. Due to the collapse and merging of individual single-mode instabilities, the interface area for a multi-mode RT instability is strongly dependent to the mesh resolution after the exponential growth rate. However, the ratio of kinetic energy over released potential energy exhibits an almost steady state after the initial exponential growth, with values around 0.4, independently of the mesh resolution.
Hamzehloo A, Lusher D, Laizet S, et al., 2021, On the performance of WENO/TENO schemes to resolve turbulence in DNS/LES of high-speed compressible flows, International Journal for Numerical Methods in Fluids, Vol: 93, Pages: 176-196, ISSN: 0271-2091
High‐speed compressible turbulent flows typically contain discontinuities and have been widely modelled using Weighted Essentially Non‐Oscillatory (WENO) schemes due to their high‐order accuracy and sharp shock capturing capability. However, such schemes may damp the small scales of turbulence, and result in inaccurate solutions in the context of turbulence‐resolving simulations. In this connection, the recently‐developed Targeted Essentially Non‐Oscillatory (TENO) schemes, including adaptive variants, may offer significant improvements. The present study aims to quantify the potential of these new schemes for a fully‐turbulent supersonic flow. Specifically, DNS of a compressible turbulent channel flow with M = 1: 5 and Re τ = 222 is conducted using OpenSBLI, a high‐order finite difference CFD framework. This flow configuration is chosen to decouple the effect of flow discontinuities and turbulence and focus on the capability of the aforementioned high‐order schemes to resolve turbulent structures. The effect of the spatial resolution in different directions and coarse grid implicit LES are also evaluated against theWALE LES model. The TENO schemes are found to exhibit significant performance improvements over the WENO schemes in terms of the accuracy of the statistics and the resolution of the three‐dimensional vortical structures. The 6th order adaptive TENO scheme is found to produce comparable results to those obtained with non‐dissipative 4th and 6th order central schemes and reference data obtained with spectral methods. Although the most computationally expensive scheme, it is shown that this adaptive scheme can produce satisfactory results if used as an implicit LES model.
Bontitsopoulos S, Hamzehloo A, Aleiferis P, et al., 2020, Large Eddy Simulations of In-Nozzle Cavitation Phenomena for Cold Fuel Injection, ASME 2020 Power Conference, Publisher: The American Society of Mechanical Engineers
Bontitsopoulos S, Hamzehloo A, Aleiferis P, et al., 2020, Numerical Simulations of the Effect of Cold Fuel Temperature on In-Nozzle Flow and Cavitation Using a Model Injector Geometry, SAE Technical Papers, ISSN: 0148-7191
Large Eddy Simulations (LES) were performed using a 3D model of a step nozzle injector. The focus has been on modelling injections with pentane, chosen as a representative single component of the high-volatility components in gasoline. The influence of fuel temperature was investigated with comparisons primarily made between 20 deg C and -10 deg C. The test cases provided a description of the in-nozzle cavitating flow and the macroscopic near-nozzle spray jet structure across different cavitation regimes in order to shed light on engine cold-start effects, a phenomenon prevalent in a number of combustion applications, albeit not extensively studied. The results showed that the size and intensity of the cavitation features tend to become suppressed as the temperature of the fuel decreases. The 20 deg C cases (supercavitating regime) depicted a sporadic shedding of vapour nuclei from a continuous cavitation region that extended to the nozzle outlet surface. Collapse-induced wave dynamics in that region caused a transient entrainment of air from the discharge chamber towards the nozzle inlet. The extent of air entrainment appeared noticeably reduced at the coldest temperature of -10 deg C (incipient cavitation regime) due to the shorter length of the cavitation region, which impeded the backflow of air. Temporally averaged data showed that the near-nozzle jet appearance was also affected by the fuel temperature. The -10 deg C case produced a relatively symmetric jet, in contrast to the supercavitating cases that demonstrated an increased opening angle and a concave surface on the side of the step nozzle edge due to the intense cavitation and parallel air entrainment.
Price C, Hamzehloo A, Aleiferis P, et al., 2020, Numerical Modelling of Droplet Breakup for Flash-Boiling Fuel Spray Predictions, International Journal of Multiphase Flow, Vol: 125, ISSN: 0301-9322
Flash-boiling of fuel sprays can occur under injection of superheated fuel into ambient pressure that is lower than the saturation pressure of the fuel and can dramatically alter spray formation due to complex two-phase flow effects and rapid droplet evaporation phenomena. Such phenomena exist in-cylinder at low-load in-city driving conditions where strict engine emission regulations apply, hence the need for faithful flash-boiling fuel spray models by engine designers. To enhance the current modelling capability of superheated fuel sprays, with focus on near-nozzle plume expansion, a flash-boiling breakup modelling approach was developed to introduce the thermal breakup mechanism of droplets caused by nucleation and bubble growth. This model was particularly aimed at sprays where levels of superheat introduced noticeable radial expansion of the plumes upon discharge from the nozzle orifice. The model was able to simulate droplet shattering by introducing Lagrangian child parcels at breakup sites with additional radial velocity components instigated by rapid bubble growth and surface instabilities. Combination of the flash-boiling droplet breakup model with a flash-boiling effective nozzle model that was used as boundary condition for the spray plumes offered a more complete modelling approach, where both in-nozzle phase change effects and near-nozzle flashing through droplet shattering were incorporated into the Eulerian-Lagrangian two-phase computational framework. Sensitivity studies were carried out to investigate important parameters which are inherently difficult to measure experimentally and offered valuable insight into modelling superheated sprays. The model was able to capture important flash-boiling spray characteristics and quantitative validation was achieved through comparison to experimental data in the form of penetration lengths and droplet sizes with a good level of agreement.
Hamzehloo A, Aleiferis P, 2019, LES and RANS Modelling of Under-Expanded Jets with Application to Gaseous Fuel Direct Injection for Advanced Propulsion Systems, International Journal of Heat and Fluid Flow, Vol: 76, Pages: 309-334, ISSN: 0142-727X
A density-based solver with the classical fourth-order accurate Runge-Kutta temporal discretization scheme wasdeveloped and applied to study under-expanded jets issued through millimeter-size nozzles for applications in highpressuredirect-injection (DI) gaseous-fuelled propulsion systems. Both large eddy simulation (LES) and ReynoldsaveragedNavier-Stokes (RANS) turbulence modelling techniques were used to evaluate the performance of the newcode. The computational results were compared both quantitatively and qualitatively against available data from theliterature. After initial evaluation of the code, the computational framework was used in conjunction with RANSmodelling (k-ω SST) to investigate the effect of nozzle exit geometry on the characteristics of gaseous jets issued frommillimeter-size nozzles. Cylindrical nozzles with various length to diameter ratios, namely 5, 10 and 20, in addition toa diverging conical nozzle, were studied. This study is believed to be the first to provide a direct comparison betweenRANS and LES within the context of nozzle exit profiling for advanced high-pressure injection systems with theformation of under-expanded jets. It was found that reducing the length of the straight section of the nozzle by 50%resulted in a slightly higher level of under-expansion (~2.6% higher pressure at the nozzle exit) and ~1% higher massflow rate. It was also found that a nozzle with 50% shorter length resulted in ~6% longer jet penetration length. At aconstant nozzle pressure ratio (NPR), a lower nozzle length to diameter ratio resulted in a noticeably higher jetpenetration. It was found that with a diverging conical nozzle, a fairly higher penetration length could be achieved if anunder-expanded jet formed downstream of the nozzle exit compared to a jet issued from a straight nozzle with the sameNPR. This was attributed to the radial restriction of the flow and consequently formation of a relatively smallerreflected shock angle. With the conical
Price C, Hamzehloo A, Aleiferis P, et al., 2018, Numerical modelling of fuel spray formation and collapse from multi-hole injectors under flash-boiling conditions, Fuel, Vol: 221, Pages: 518-541, ISSN: 0016-2361
Flash-boiling of fuel sprays can occur when the fuel enters a metastable superheated state, which is common in direct-injection spark-ignition engines operating at low in-cylinder pressures and/or hot fuel temperatures. The effect of flash-boiling on the resultant spray formation can be both detrimental and advantageous to engine operation, hence numerical modelling capability is essential in future engine optimisation and design. A recently-developed new model by the current authors that can be applied as zero-dimensional boundary condition for multi-hole flash-boiling fuel spray predictions was investigated over a wide range of injection systems, focusing on the model’s ability to quantify in-nozzle phase change effects and automatically predict important global spray characteristics such as spray collapse, droplet recirculation and plume merging within a Lagrangian particle tracking framework. Mesh-type sensitivity was highlighted using a uniform Cartesian and a non-uniform polyhedral mesh. The model was also normalised through a dimensionless parameter for a wide range of single component fuels. The model was validated both qualitatively and, where possible, quantitatively against experimental data. The model’s ability to deal with a wide range of injection configurations and operating conditions was confirmed and a number of limitations are highlighted and discussed with respect to future work.
Price C, Hamzehloo A, Aleiferis P, et al., 2016, An Approach to Modelling Flash-Boiling Fuel Sprays for Direct-Injection Spark-Ignition Engines, Atomization and Sprays, Vol: 26, Pages: 1197-1239, ISSN: 1936-2684
Hamzehloo A, Aleiferis PG, 2016, Numerical Modelling of Transient Under-Expanded Jets under Different Ambient Thermodynamic Conditions with Adaptive Mesh Refinement, International Journal of Heat and Fluid Flow, Vol: 61, Pages: 711-729, ISSN: 0142-727X
Hamzehloo A, Aleiferis PG, 2016, Gas Dynamics and Flow Characteristics of Under-Expanded Hydrogen and Methane Jets under Various Nozzle Pressure Ratios and Ambient Pressures, International Journal of Hydrogen Energy, Vol: 41, Pages: 6544-6566, ISSN: 1879-3487
The current study used large eddy simulations to investigate the sonic and mixing characteristics of turbulent under-expanded hydrogen and methane jets with various nozzle pressure ratios issued into various ambient pressures including elevated conditions relevant to applications in direct injection gaseous-fuelled internal combustion engines. Due to the relatively low density of most gaseous fuels such as hydrogen and methane, DI requires high injection pressures to achieve suitable mass flow rates for fast in-cylinder fuel delivery and rapid fuel-air mixing. Such pressures typically form an under-expanded fuel jet past the nozzle exit. Test cases of hydrogen injection with nozzle pressure ratio (NPR) of 10 issued into quiescent air with pressure P∞ ≈ 1, 5 and 10 bar were simulated. Direct comparison between hydrogen and methane jets with NPR = 8.5 and P∞ ≈ 1 was also made. The effect of ambient pressure on features of transient development of the near-nozzle shock structure and tip vortices (vortex ring) was investigated. It was observed that at constant NPR, higher ambient pressure resulted in slightly faster formation of the Mach reflection and shorter Mach disk settlement time. Different mechanisms were observed between hydrogen and methane with regards to transient formation of their initial tip vortex rings. It was found that the initial transient tip vortices of hydrogen jets may also contribute to the flow instabilities at the boundary of the intercepting shock and, unlike for methane, promote fuel-air mixing before the Mach reflection. It was also shown that the near-nozzle shock structure was only affected by NPR regardless of the ambient pressure. Furthermore, no flow recirculation zone was found just downstream of the Mach disk, a finding comparable to all previous experimental investigations. Also, it was observed that a locally richer mixture was created for jets with higher NPR or with higher ambient pressure at constant NPR.
Price C, Hamzehloo A, Aleiferis PG, et al., 2015, Aspects of Numerical Modelling of Flash-Boiling Fuel Sprays, SAE Technical Paper Series, ISSN: 0148-7191
Hamzehloo A, Aleiferis PG, 2014, Large Eddy Simulation of Highly Turbulent Under-Expanded Hydrogen and Methane Jets for Gaseous-Fuelled Internal Combustion Engines, International Journal of Hydrogen Energy, Vol: 39, Pages: 21275-21296, ISSN: 0360-3199
Burning hydrogen in conventional internal combustion (IC) engines is associated with zero carbon-based tailpipe exhaust emissions. In order to obtain high volumetric efficiency and eliminate abnormal combustion modes such as preignition and backfire, in-cylinder direct injection (DI) of hydrogen is considered preferable for a future generation of hydrogen IC engines. However, hydrogen's low density requires high injection pressures for fast hydrogen penetration and sufficient in-cylinder mixing. Such pressures lead to chocked flow conditions during the injection process which result in the formation of turbulent under-expanded hydrogen jets. In this context, fundamental understanding of the under-expansion process and turbulent mixing just after the nozzle exit is necessary for the successful design of an efficient hydrogen injection system and associated injection strategies. The current study used large eddy simulation (LES) to investigate the characteristics of hydrogen under-expanded jets with different nozzle pressure ratios (NPR), namely 8.5, 10, 30 and 70. A test case of methane injection with NPR = 8.5 was also simulated for direct comparison with the hydrogen jetting under the same NPR. The near-nozzle shock structure, the geometry of the Mach disk and reflected shock angle, as well as the turbulent shear layer were all captured in very good agreement with data available in the literature. Direct comparison between hydrogen and methane fuelling showed that the ratio of the specific heats had a noticeable effect on the near-nozzle shock structure and dimensions of the Mach disk. It was observed that with methane, mixing did not occur before the Mach disk, whereas with hydrogen high levels of momentum exchange and mixing appeared at the boundary of the intercepting shock. This was believed to be the effect of the high turbulence fluctuations at the nozzle exit of the hydrogen jet which triggered Gortler vortices. Generally, the primary mixing was observed to
Hamzehloo A, Aleiferis PG, 2014, Numerical Modelling of Mixture Formation and Combustion in DISI Hydrogen Engines with Various Injection Strategies, SAE Technical Paper Series, Vol: 2014, ISSN: 0148-7191
Hamzehloo A, Aleiferis PG, 2014, Large Eddy Simulation of Near-Nozzle Shock Structure and Mixing Characteristics of Hydrogen Jets for Direct-Injection Spark-Ignition Engines, 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2014)
Hamzehloo A, Aleiferis PG, 2013, Computational Study of Hydrogen Direct Injection for Internal Combustion Engines, SAE Technical Paper Series, Vol: 2013, ISSN: 0148-7191
Hydrogen has been largely proposed as a possible fuel forinternal combustion engines. The main advantage of burninghydrogen is the absence of carbon-based tailpipe emissions.Hydrogen’s wide flammability also offers the advantage ofvery lean combustion and higher engine efficiency thanconventional carbon-based fuels. In order to avoid abnormalcombustion modes like pre-ignition and backfiring, as well asair displacement from hydrogen’s large injected volume percycle, direct injection of hydrogen after intake valve closure isthe preferred mixture preparation method for hydrogenengines. The current work focused on computational studies ofhydrogen injection and mixture formation for direct-injectionspark-ignition engines. Hydrogen conditions at the injector’snozzle exit are typically sonic. Initially the characteristics ofunder-expanded sonic hydrogen jets were investigated in aquiescent environment using both Reynolds-Averaged NavierStokes(RANS) and Large-Eddy Simulation (LES) techniques.Various injection conditions were studied, including areference case from the literature. Different nozzle geometrieswere investigated, including a straight nozzle with fixed crosssection and a stepped nozzle design. LES captured details ofthe expansion shocks better than RANS and demonstratedseveral aspects of hydrogen’s injection and mixing. Incylindersimulations were also performed with a side 6-holeinjector using 70 and 100 bar injection pressure. Injectiontiming was set to just after inlet valve closure with duration of6 μs and 8 μs, leading to global air-to-fuel equivalence ratios typically in the region of 0.2–0.4. The engine intake airpressure was set to 1.5 bar absolute to mimic boostedoperation. It was observed that hydrogen jet wall impingementwas always prominent. Comparison with non-fuelled engineconditions demonstrated the degree of momentum exchangebetween in-cylinder hydrogen injection and air motion. LEShighlighted details of hydroge
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