CERES :: Browsing by Author "Shapiro, Evgeniy"

Browsing by Author "Shapiro, Evgeniy"

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Open Access
Advanced computational modelling and simulation of transition to turbulence in separated suddenly-expanded channel flows
(Cranfield University, 2010-09) Vamvakoulas, Christos; Drikakis, Dimitris; Shapiro, Evgeniy
The main scope of this PhD thesis is the analysis of unsteady laminar and transitional suddenly expanded flows. For this reason Implicit Large Eddy Simulation (ILES) approach was used in combination with high order, high resolution numerical methods. The numerical methods examined are a 2nd order Monotonic Upwind Scheme for Scalar Conservation Laws (MUSCL) with Van Leer limiter, a high order (3rd) interpolation and a 5th order Weighted Essentially Non-Oscillatory scheme (WENO). First the numerical data for three low (steady state) Reynolds numbers and for two unsteady ( in the form of primary frequencies) were compared to the experimental data and were found in good agreement. A grid convergence study was undertaken for two Reynolds numbers demonstrating grid convergence and justifying the selection of the grid. The three numerical methods were evaluated for two Reynolds numbers showing good agreement for Reynolds number 412 and discrepancies at Reynolds number 900 between MUSCL and WENO with the MUSCL demonstrating a very dissipative behavior. The physical behavior of the flow in a wide range of Reynolds numbers were examined. For this range the flow behavior changed from steady to unsteady laminar and finally exhibiting localized transition to turbulence. The behavior of the main recirculation areas was described and the vortex shedding that occur there and how this change with the Reynolds number. The flow was observed to change from an unsteady quasi three dimensional flow at Reynolds number 412 to an increased transitional state with three dimensional vortical structures at Reynolds number 550. Kinetic energy spectra were calculated for the aforementioned range of Reynolds numbers. The primary frequencies are increasing with Reynolds number as expected. The slopes that were calculated for the inertial subrange revealed a trend. As the Reynolds number is increasing the slopes are decreasing approaching the value given by Kolmogorov -5/3.
Open Access
Artificial compressibility, characteristics-based schemes for variable density, incompressible, multi-species flows. Part I. Derivation of different formulations and constant density limit
(Elsevier, 2005-12-10) Shapiro, Evgeniy; Drikakis, Dimitris
The paper presents various formulations of characteristics-based schemes in the framework of the artiﬁcialcompressibility method for variable-density incompressible ﬂows. In contrast to constant-density incompressible ﬂows, where the characteristics-based variables reconstruction leads to a single formulation, in the case of variable density ﬂows three diﬀerent schemes can be obtained henceforth labeled as: transport, conservative and hybrid schemes. The conservative scheme results in pseudo-compressibility terms in the (multi-species) density reconstruction. It is shown that in the limit of constant density, the transport scheme becomes the (original) characteristics-based scheme for incompressible ﬂows, but the conservative and hybrid schemes lead to a new characteristics-based variant for constant density ﬂows. The characteristics-based schemes are combined with second and third-order interpolation for increasing the computational accuracy locally at the cell faces of the control volume. Numerical experiments for constant density ﬂows reveal that all the characteristics-based schemes result in the same ﬂow solution, but they exhibit diﬀerent convergence behavior. The multigrid implementation and numerical studies for variable density ﬂows are presented in Part II of this study.
Open Access
Artificial compressibility, characteristics-based schemes for variable-density, incompressible, multispecies flows: Part II. Multigrid implementation and numerical tests
(Elsevier, 2005-12-10) Shapiro, Evgeniy; Drikakis, Dimitris
The paper presents an investigation of the accuracy and efficiency of artificial compressibility, characteristics-based (CB) schemes for variable-density incompressible ﬂows. The CB schemes have been implemented in conjunction with a multigrid method for accelerating numerical convergence and a fourth-order, explicit Runge–Kutta method for the integration of the governing equations in time. The implementation of the CB schemes is obtained in conjunction with first-, second- and third-order interpolation formulas for calculating the variables at the cell faces of the computational volume. The accuracy and efficiency of the schemes are examined against analytical and experimental results for difusion broadening in two- and three-dimensional microﬂuidic channels, a problem that has motivated the development of the present methods. Moreover, unsteady, inviscid simulations have been performed for variable-density mixing layer. The computations revealed that accuracy and eﬃciency depend on the CB scheme design. The best multigrid convergence rates were exhibited by the conservative CB scheme, which is obtained by the fully conservative formulation of the variable-density, incompressible equations.
Open Access
Comparative investigation of large eddy simulation and RANS approaches for external automotive flows
(Cranfield University, 2011-07) Brondolo, Luca; Shapiro, Evgeniy
This thesis investigates the accuracy and scalability of RANS and LES approaches applied to external automotive aerodynamics. Due to the availability of considerable experimental and computational data available on the Ahmed body, this reference model was chosen for this study. The relative simple geometry of the Ahmed body model is able reproduce the common flow features of a hatch back style vehicle. The 25° slant angle configuration was used as it is a major challenge in terms of flow prediction. The RANS model used included the Standard K-ε, RNG K-ε, Realizable k-ε and K-ω SST. The LES simulations were run with the Smagorinsky-Lilly SGS model. Three grids with different level of refinement were generated. A viscous hybrid mesh approach was used for all the simulations. This type of mesh is commonly used by automotive manufactures and motorsport organizations. The commercial package Fluent 12 was used as a solver. The K-ω SST and LES models showed good agreement with the experimental data. LES in particular was the only model to predict flow re-attachment over the slant angle as seen on the experimental and computational data available in literature. The richness of the unsteady data available from the LES simulations and correct interpretation of flow topology balance in part the major computational requirements compared to the RANS models. Taking into account the hardware resources available to automotive manufactures, the LES is suitable to be part of the design process.
Open Access
Comparison of high-order methods on unstructured grids
(Cranfield University, 2013) Iqbal, Kashif H.; Shapiro, Evgeniy
A high-order Discontinuous Galerkin (DG) method is formulated and implemented on the Cranfield University’s 3D unstructured Finite Volume Method (FVM) code (UCNS3D), for both linear and non-linear hyperbolic conservation laws and for test-cases which exhibit both smooth and discontinuous solutions. As both DG and FVM are developed on the same solver platform, this enables the use of any procedures which are common to both the methods, thus, ensuring the closest possible compari-son. The initial part of the thesis details the basic concepts and derivation of the discon-tinuous Galerkin method in the 1D space for the advection equation, which is then extended to the 3D space for a hyperbolic system. Prior to comparing the FVM and DG methods, the DG method implementation is verified. The verification is a combination of a theoretical and numerical approach which endeavours to minimize any potential programming errors. Following the verification of the DG method, the FVM and DG methods are compared for numerous flows: the linear advection equation and Euler equations, sufficiently smooth testcases, and testcases which require a limiter to suppress Gibb’s oscillations.
Open Access
Computational Analysis of the Flow Around an Exposed Wheel
(Cranfield University, 2010-11) Takeuchi, S.; Shapiro, Evgeniy
This thesis presents an investigation of RANS and LES models for the isolated rotating exposed wheel with moving ground with the aim of analyzing the flow behavior and comparing the results in cost/quality context. This is motivated by general demand for the drag reduction in automobile industry. In addition the demand is even stronger from high-performance car competition where the tyre wake has a great impact on total performance such as lift force or aerodynamic stability aspect. Cont/d.
Open Access
Computational Modelling of Cavity Arrays with Heat Transfer using Implicit Large Eddy Simulations
(Cranfield University, 2010-01) Malick, Zeshan; Drikakis, Dimitris; Shapiro, Evgeniy
This PhD programme was sponsored by the United Kingdom Atomic Energy Authority (UKAEA). The aim of this study is to conduct advanced computational modelling of a cooling device used in the fusion process which recycles waste energy. The development of efficient, water cooled tiles, that can sustain heat loads of approximately 20 MW (in quasi-steady state conditions) is the motivation of the current work. The information presented here will contribute to thermal-mechanical analysis, to be conducted at the Joint European Torus (JET) in future years. The devices known as “Hypervapotrons” have been used successfully at JET to provide a ion dump that dissipates residual energy from the fusion process. A capability to model the flow structure and heat transfer, across a large number of geometric and material options is provided within. Differences in geometry, result in changes to the flow structure and heat transfer rates. The desire to optimise such designs relies upon the fundamental understanding of the flow field within the main section, where the geometry may be defined as a cavity array. The benchmark case of a lid driven cavity flow was used for the validation of the flow field solution. Solutions using high resolution methods in the formulation provided a good comparison with established experimental data. Therefore, validation of incompressible, Implicit Large Eddy Simulations (ILES) for a wall bounded, three dimensional, turbulent flow is provided within. The sensitivity of the high order reconstruction in conjunction with the characteristics based scheme (Drikakis & Rider, 2005), to resolve turbulent flow structure is provided here. The solution response to grid resolution and a regularised velocity profile at the upper lid surface is also detailed. The investigation provided insight and confidence in the turbulence modelling approach which is relatively recent. It was also demonstrated through the lid driven case (and later in the Hypervapotron cases) that high order reconstruction was a simulation prerequisite, based on grid resolutions used within. Additional validation was also provided against numerical and analytical solutions for the Conjugate Heat Transfer (CHT) and scalar temperature field. Where appropriate both unsteady and steady problems based on a composite, three layer medium are detailed to provide preliminary validation for the implementation of the temperature scalar and conjugate boundary conditions. Unfortunately, it was not feasible to solve the coupled problem with an explicit solver as used in this study. However, it is suggested that the initial stages of thermal boundary layer development may be observed leading to the locations of incipient boiling. Two different Reynolds numbers were considered for the Hypervapotron ”Standard” geometry, Re=12000 and Re=18000. The different flow structures show that the cavity aspect ratio of the Standard design promotes lower flow speeds at the cavity base, since two or three counter rotating vortices coexist inside the cavities depending on Reynolds number. A detailed analysis on the impact of the number of repeating units within the computational domain is also provided. Results are presented of ensemble averaged quantities based on the Reynolds decomposition. The temperature distribution present in the solid, fluid and its interface for the thermally developing case is achieved. In addition the total and decomposed heat fluxes are presented for the Hypervapotron (Standard design) which provides similar comparison with recent Reynolds Averaged Navier-Stokes (RANS) simulations.
Open Access
Effects of turbulence modelling on the analysis and optimisation of high-lift configurations
(Cranfield University, 2011-09) Guo, Chuanliang.; Shapiro, Evgeniy; Kipouros, Timoleon
Due to the significant effects on the performance and competitiveness of aircraft, high lift devices are of extreme importance in aircraft design. The flow physics of high lift devices is so complex, that traditional one pass and multi-pass design approaches can’t reach the most optimised concept and multi-objective design optimisation (MDO) methods are increasingly explored in relation to this design task. The accuracy of the optimisation, however, depends on the accuracy of the underlying Computational Fluid Dynamics (CFD) solver. The complexity of the flow around high-lift configuration, namely transition and separation effects leads to a substantial uncertainty associated with CFD results. Particularly, the uncertainty related to the turbulence modelling aspect of the CFD becomes important. Furthermore, employing full viscous flow solvers within MDO puts severe limitations on the density of computational meshes in order to achieve a computationally feasible solution, thereby adding to the uncertainty of the outcome. This thesis explores the effect of uncertainties in CFD modelling when detailed aerodynamic analysis is required in computational design of aircraft configurations. For the purposes of this work, we select the benchmark NLR7301 multi-element airfoil (main wing and flap). This flow around this airfoil features all challenges typical for the high-lift configurations, while at the same time there is a wealth of experimental and computational data available in the literature for this case. A benchmark shape bi-objective optimization problem is formed, by trying to reveal the trade-off between lift and drag coefficients at near stall conditions. Following a detailed validation and grid convergence study, three widely used turbulence models are applied within Reynolds-Averaged Navier-Stokes (RANS) approach. K- Realizable, K- SST and Spalart-Allmaras. The results show that different turbulent models behave differently in the optimisation environment, and yield substantially different optimised shapes, while maintaining the overall optimisation trends (e.g. tendency to maximise camber for the increased lift). The differences between the models however exhibit systemic trends irrespective of the criteria for the selection of the target configuration in the Pareto front. A-posteriori error analysis is also conducted for a wide range of configurations of interest resulting from the optimisation process. Whereas Spalart-Allmaras exhibits best accuracy for the datum airfoil, the overall arrangement of the results obtained with different models in the (Lift, Drag) plane is consistent for all optimisation scenarios leading to increased confidence in the MDO/RANS CFD coupling.
Open Access
An efficient multi-scale modelling approach for ssDNA motion in fluid flow
(Elsevier Science B.V., Amsterdam, 2008-12-01T00:00:00Z) Benke, Matyas; Shapiro, Evgeniy; Drikakis, Dimitris
The paper presents a multi-scale modelling approach for simulating macromolecules in fluid flows. Macromolecule transport at low number densities is frequently encountered in biomedical devices, such as separators, detection and analysis systems. Accurate modelling of this process is challenging due to the wide range of physical scales involved. The continuum approach is not valid for low solute concentrations, but the large timescales of the fluid flow make purely molecular simulations prohibitively expensive. A promising multi-scale modelling strategy is provided by the meta-modelling approach considered in this paper. Meta-models are based on the coupled solution of fluid flow equations and equations of motion for a simplified mechanical model of macromolecules. The approach enables simulation of individual macromolecules at macroscopic time scales. Meta-models often rely on particle-corrector algorithms, which impose length constraints on the mechanical model. Lack of robustness of the particle- corrector algorithm employed can lead to slow convergence and numerical instability. A new FAst Linear COrrector (FALCO) algorithm is introduced in this paper, which significantly improves computational efficiency in comparison with the widely used SHAKE algorithm. Validation of the new particle corrector against a simple analytic solution is performed and improved convergence is demonstrated for ssDNA motion in a lid-driven micro-cavity.
Open Access
Exploring continuum computational models for neutral gas flow in the JET neutraliser
(Cranfield University, 2010-05) Porton, Michael; Drikakis, Dimitris; Surrey, E.; Shapiro, Evgeniy
Effective understanding of gas flow is important to ensure efficient operation of gas neutralizer systems such as those used at the Joint European Torus (JET), which form part of invaluable heating systems for nuclear fusion experiments. Computational fluid dynamics modelling of the neutral gas flow in the JET neutraliser has been undertaken, motivated by the shortfall in neutralisation efficiency and apparent loss of gas target observed in the JET neutraliser system. This has presented a challenging modelling endeavour due to the interaction of beam, background plasma and rarefied neutral gas. Utilising the continuum flow approximation, the Navier-Stokes and Augmented Burnett equations have been implemented and applied in conjunction with secondorder slip boundary conditions to form a gas solver accurate within the continuum-transition regime. Simulations in the presence of the ionic beam and background neutraliser plasma encountered during tokamak heating operations have been achieved via the development of a coupled beamplasma- gas solver. The gas flow governing equations have been supplemented by a series of source/sink terms for mass/energy that describe the complex web of interactions between the neutraliser constituents. The developed solver has been validated against experimental data, both in the absence and presence of beam. The design of future gas neutraliser systems has also been considered, with variation of several model and geometry parameters in order to better understand the loss of neutralisation efficiency and how future systems might be optimised. The neutraliser design for the forthcoming International Thermonuclear Experimental Reactor (ITER) has also been evaluated.
Open Access
High resolution and high order methods for RANS modelling and aerodynamic optimization
(Cranfield University, 2008-10) Zachariadis, Zacharias Ioannis; Drikakis, Dimitris; Shapiro, Evgeniy
With the optimisation of fixed aerodynamic shapes reaching its limits, the active flow control concept increasingly attracts attention of both academia and industry. Adaptive wing technology, and shape morphing airfoils in particular, represents a promising way forward. The aerodynamic performance of the morphing profiles is an important issue affecting the overall aerodynamic performance of an adaptive wing. A new concept of active flow, the Active Camber concept has been investigated. The actuator is integrated into the aerofoil and aerofoil morphing is realized via camber deformation. In order to identify the most aerodynamically efficient designs, an optimisation study has been performed using high resolution methods in conjunction with a two equation eddy viscosity model. Several different types of previously proposed compressible filters, including monotone upstream-centered schemes for conservation laws (MUSCL) and weighted essential non-oscillatory (WENO) filters, are incorporated and investigated in the present research. The newly developed CFD solver is validated and the effect that high resolution methods have on turbulent flow simulations is highlighted. The outermost goal is the development of a robust high resolution CFD method that will efficiently and accurately simulate various phenomena, such as shock/boundary layer interaction, flow separation and turbulence and thus provide the numerical framework for analysis and aerodynamic aerofoil design. With respect to the latter a multi-objective integrated design system (MOBID) has been developed that incorporates the CFD solver and a state-of-the-art heuristic optimisation algorithm, along with an efficient parametrization technique and a fast and robust method of propagating geometric displacements. The methodologies in the MOBID system resulted in the identification of the design vectors that revealed aerodynamic performance gains over the datum aerofoil design. The Pareto front provided a clear picture of the achievable trade-offs between the competing objectives. Furthermore, the implementation of different numerical schemes led to significant differences in the optimised airfoil shape, thus highlighting the need for high-resolution methods in aerodynamic optimisation.
Open Access
High resolution methods for incompressible, compressible and variable density flows.
(2004-01-01T00:00:00Z) Drikakis, Dimitris; Hahn, Marco; Patel, Sanjay; Shapiro, Evgeniy
The uid dynamics community has dealt with a number of numerical challenges since the 1950's. These include the development of numerical methods for hyperbolic conservation laws with particular interest in capturing shock wave propagation and related phenomena, solution algorithms for the solution of the incompressible Navier-Stokes equations - a numerical challenge arises here due to the absence of the pressure term from the continuity equation - methods/ techniques for the acceleration of the numerical convergence, modelling of turbulence and grid generation techniques. Within each of those areas di erent numerical approaches have been pursued by various researchers aiming to achieve higher accuracy and ef ciency of the numerical solution. Continuous interest exists in relation to the development of accurate and e cient numerical methods for the computation of instabilities, transition and turbulence. It has been observed for more than a decade that high-resolution methods can be used in (under-resolved) turbulent ow computations without the need to resort to a turbulence model, but this approach has only recently gained some theoretical support and structural explanation for the observed results [3, 15]. Because of this there is a necessary overlap between the classical modelling of turbulence and its computation through high resolution methods [5]. These methods are currently used to simulate a broad variety of complex ows, e.g., ows that are dominated by vorticity leading to turbulence, ows featuring shock waves and turbulence, and the mixing of materials [23]. Such ows are extremely dif cult to practically obtain stably and accurately in under-resolved conditions (with respect to grid resolution) using classical linear (both second and higherorder accurate) schemes. Further, new applications at micro-scale, e.g. micro uidics, microreactors and lab-on-a-chip, have raised a number of challenges for computational science methods. In this paper we provide a brief overview of highresolution methods in connection with some of the above problems. An extensive description of these methods for incompressible and low-speed Flows can be found in [5].
Open Access
High-order methods for steady, unsteady and transitional flow over a cylinder
(Cranfield University, 2012-07) Starr, Maxwell; Shapiro, Evgeniy
In this thesis, the flow around a cylinder is chosen as a test case for higher-order numerical reconstruction techniques. No direct comparison of these higher-order methods has been carried out for this particular test case. Especially for low Mach number, incompressible flows with Implicit Large Eddy Simulation method. The cylinder test case is both a proven test case in literature, as well as a test case that can be scaled up in terms of flow speed with other parameters remaining unchanged. The scaling of flow speed around the cylinder allows ease of flow regime change. Thus the flow was modelled in this thesis from laminar flow to turbulent flow, going through a transitional regime in between. The simulations were set up such that numerical reconstruction methods could be directly compared to one another at the range of flow speeds, and subsequently in both two-dimensional and three-dimensional flows. The numerical reconstruction methods for the ILES cases ranged from first order reconstruction through to higher-order methods as high as ninth-order (in the weighted essentially non-oscillatory scheme). With the speed of computation for the twodimensional simulations, it was possible to test all of these schemes directly with one another. However, three-dimensional simulations require a significantly greater CPU run-time. Therefore, based on the results of the two-dimensional simulations, a group of the higher-order methods were chosen for continuing analysis in the three-dimensional simulations. In the laminar flow regime, all the numerical schemes agreed very well with literature data. As the flow speed increased, discrepancies started to appear in the results, to varying degrees based on the flow speed, the numerical scheme used, and the dimensionality of the flow. An analysis of the results showed that two-dimensional simulations were suitable up to Reynolds 300. From this flow speed onwards, three-dimensional simulations are deemed necessary. At lower Reynolds number flows the two-dimensional simulations provided good predictions of the flow. At the higher Reynolds numbers, the 3D simulations outperformed the 2D simulations. Specific numerical reconstruction schemes were found to perform better at certain aspects of the flow. For example, the coefficients around the cylinder or the velocities in the wake varied based solely on the numerical scheme used. Finally, during the course of the post-processing of the simulations, a spectral analysis was carried out. The flow field was analysed at specific points in the wake (ranging from near, medium and far wake). The spectral analysis proved suitable for examining the fluctuations in the wake of the cylinder, showing the redistribution of energy in the wake towards higher frequencies. In addition, the wake showed increased power densities for the fluctuations as the flow moved away from the cylinder, before then decreasing again as dissipation into the surrounding flow occurred.
Open Access
High-Resolution Computational Modelling of Multi-Material Flow
(Elsevier, 2005-06-14) Shapiro, Evgeniy; Drikakis, Dimitris
The paper concerns the development of high-resolution methods for variable density flows and the implementation in multi-species flow studies in microfluidics.The high resolution discretisation is obtained by numerically reconstructing the flow variables using information for the eigenstructure of the system of equations. Three variants of high-resolution methods are presented and their accuracy is assessed against analytic and experimental results for diffusion broadening in microfluidics. Results from numerical convergence studies are also presented to demonstrate the relative efficiency of the three reconstruction variants in conjunction with first, second and third-order of accuracy in spatial discretisation.
Open Access
Implicit Large Eddy Simulation of Environmental Urban Flows
(Cranfield University, 2010-07-18) Papachristou, Charikleia; Drikakis, Dimitris; Shapiro, Evgeniy
Many environmental flows are turbulent flows. Depending on the physical aspects of the wind and the urban topology, turbulence might result into unfavourable or even dangerous conditions for the pedestrians. Turbulence can also play a very important role in the transport of toxic pollutants from accidental or intentional releases. Thus, it is vital to understand its complex characteristics so that its features are accurately predicted when computational methods are used. Real urban environment involving separation and reattachment regions provides an excellent testcase for investigating such complex flows. This thesis is focused on analysing the physics involved in flows around building models pertinent to environmental flows in urban areas and to evaluate the applicability of Implicit Large-Eddy Simulation in simulating the specific type of flows. For this purpose, a number of high resolution schemes in the context of Implicit Large-Eddy Simulation (each representing di erent degrees of spatial discretisation accuracy) was assessed. The evaluation of the schemes involved direct validation against experimental data as well as comparisons with DNS and LES data regarding flows within roughness element arrays in staggered arrangements. Initially, the flow within an uniform height cubical matrix was simulated. Four numerical schemes were tested in three di erent grid resolutions. The results were found in very good agreement with the Laser Doppler Anemometry data and they even exhibit DNS-like characteristics in specific locations of comparisons. Thus, it was concluded that high order spatial discretisation schemes allow the accurate representation of reality even in relatively coarse computational meshes. The second case under investigation involved flows within a more realistic representation of urban topology. Results obtained within an array of sixteen elements with five di erent heights reveal that although the roughness of the area is increased, the wind’s velocity profile above the obstacles shares almost the same slope as in the case of the array of the four cubical element. It is believed that this thesis has expanded the range of applications in the context of Implicit Large Eddy Simulation using high resolution schemes and contributed in persuading the scientific community for its potentials.
Open Access
Implicit large eddy simulation of turbulent duct flows
(Cranfield University, 2010-04) Mylonas, Antonios Athanassios; Drikakis, Dimitris; Shapiro, Evgeniy
Ducts can be found in ventilation systems, cooling ducts and blade passages of turbines, centrifugal pumps and many other engineering installations. The properties of the flow in ducts can significantly aﬀect the performance and eﬃciency of these installation areas. The majority of the flows in ducts and engineering applications are turbulent. The work presented in this thesis focuses on the analysis of turbulent flows inside square sectioned ducts and ducts with bends. The accuracy of three diﬀerent high resolution high order schemes in the context of Implicit Large Eddy Simulation (ILES) is analysed. The influence of a low Mach limiting technique, Low Mach Number Treatment (LMNT) is also studied. The schemes employed are Monotonic Upwind Scheme for Scalar Conservation Laws (MUSCL) with a 2nd order Monotonized Central (MC) and 5th order limiter, and a 9th order Weighted Essential Non-Oscillatory (WENO) limiter. The first case studied is a duct of square cross section . In the absence of experimental data for the duct case, the data from a plain channel flow is used to shed light on the results. The flow analysis points out the generation of secondary motions created by the existence of surrounding walls. All schemes employed lead to a similarly developed turbulent flow that is used to provide the turbulent boundary profile for the following case. LMNT proves to significantly assist MUSCL 2nd and 5th, that use it, in providing a turbulent profile similar to that of WENO 9th that did not employ the technique but is inherently less dissipative. The second case under study is that of a square sectioned duct with a 90o bend. The simulation output is in good agreement both qualitatively and quantitatively with the experimental data available in the literature. The generation of secondary flows inside the bend is observed without flow separation. Although the turbulent flow entering the domain is almost the same for all cases, diﬀerences between the schemes are noticed especially after the middle of the bend. LMNT leads to an overprediction of turbulence after that area for both schemes employing it while WENO 9th without LMNT provides the most accurate results compared to those provided by the experiment. The results demonstrate applicability of ILES to strongly confined flows with secondary motions and shed light on cognitive properties of a wide range of state of the art schemes.
Open Access
Implicit Large-Eddy Simulations of Wall-Bounded Turbulent Flows.
(2007-01-01T00:00:00Z) Drikakis, Dimitris; Hahn, Marco; Malick, Zeshan; Shapiro, Evgeniy
The paper presents application of Implicit Large Eddy Simulation(ILES)to wall- bounded turbulent flows. A characteristics-based scheme in conjunction with total variation diminishing(TVD)Runge-Kutta time stepping and slope-limiting variants,for the com-pressible flow case, have been employed. Results are pre- sented for an incompressible lid-driven cavity flow,using an incompressible solver, and low-Mach number flows over a hill and around a delta wing, using a compressible solver. Good agreement with experimental data and numerical results using classical LES has been obtained. Future ILES developments are also discussed
Open Access
Investigation of high-resolution methods in large-eddy simulation of subsonic and supersonic wall turbulent flows
(Cranfield University, 2009-04-30) Kokkinakis, Ioannis William; Drikakis, Dimitris; Shapiro, Evgeniy
This thesis presents the motivation, objectives and reasoning behind the undertaken PhD to investigate the capability of compressible Implicit Large Eddy Simulation (ILES) in simulating wall-bounded inhomogeneous flows with particular interest in the near wall region and further presents the progress achieved to date. Investigation includes the assessment of current ILES methods to resolve inhomogeneous turbulence as well as compressible turbulent boundary layers and to improve on those models further. A channel flow is an excellent problem to use to investigate the properties of a SGS model near a wall. The presence of a solid boundary tends to alter the behaviour of the turbulent flow in a number of ways that need to be modeled by the SGS model in order to correctly represent the flow near the wall and most importantly the boundary layer. The presence of the wall inhibits the growth of the small scales, alters the exchange mechanisms between the resolved and unresolved scales and finally gives rise in the SGS near wall region to important Reynolds-stress producing events. A literature survey was carried out to identify other numerical investigations in simulating channel flow as well as data that could be used for validation purposes. The main parameters used to validate the level of resolution in simulating channel flow are identified and a number of tools are developed. The primary parameters extensively used to validate LES simulations of channel flow throughout the literature are mean flow velocity profiles, turbulent kinetic energy, dissipation and shear stress profiles, wall shear stress and friction velocities as well as energy spectra in the spanwise and streamwise homogeneous directions. Compressible viscous ILES of inhomogeneous anisotropic turbulence in an incompressible channel flow at wall normal grid resolutions of 68, 96 and 128 cells are carried out with grid clustering applied to the wall normal direction. Initial results conducted in the compressible regime show that in order to obtain satisfactory results, medium and fine grids are required whereas on coarser grids, some additional numerical method is required. Each reconstruction scheme introduces a numerical dissipation characteristic to itself that maybe regarded as a sort of turbulence model. Thus depending on the required dissipation, a suitable limiter can be chosen. The investigation then moves on to supersonic turbulent flow incorporating shockboundary layer interaction. Only the slope-limiters that prove to simulate the flow in the fully developed turbulent channel best are favoured and then also utilised in the subsequent compressible ramp simulations. The capabilities of modelling the shock boundary layer interaction, mean turbulent profiles and shockwave angle are investigated and compared against those obtained by DNS simulations. It is found that the grid at the inlet of the ramp plays a significant role, since it needs to be fine enough to maintain the turbulent in flow at an acceptable level before reaching the shock-boundary layer interaction zone. Further, very high-order numerical reconstructions were found to have difficulties in remaining stable in the high gradient regions of the flow when formulated in conservative form and therefore solutions were not possible to obtain. Nonetheless, lower order reconstruction methods run smoothly and the momentum profiles obtained, matched closely those obtained by DNS.
Open Access
Linear stability of ice growth under a gravity-driven water film
(American Institute of Physics, 2006-07) Shapiro, Evgeniy; Timoshin, Sergei
In this paper we consider linear stability of ice growth under a gravity-driven water film on a sloping wall. First, we derive an analytic solution of the stability problem in the long-wave limit, which shows that the presence of the ice layer generates an additional wave mode. Further, using a long-wave solution as an initial guess, we find the additional wave mode in the numerical solution of the complete Orr-Sommerfeld problem and investigate its behavior numerically for a wide range of problem parameters. We show that the ice mode can become unstable even at moderate Reynolds numbers, and that the ice layer alters the behavior of the mode corresponding to the waves on the liquid film surface. We also demonstrate that the presence of the ice layer stabilizes wave disturbances on the water surface and that, depending on the angle of the incline, the critical Reynolds number of the surface mode can be either increased or decreased.
Open Access
Mechanical behaviour of DNA molecules-Elasticity and migration.
(Elsevier Science B.V., Amsterdam., 2011-09-30T00:00:00Z) Benke, Matyas; Shapiro, Evgeniy; Drikakis, Dimitris
A novel multi-scale simulation method developed to describe mesoscale phenomena occurring in biofluidic devices is presented. The approach combines the macro-scale modelling of the carrier fluid and the micro-scale description of the transported macromolecules or compounds. Application of the approach is demonstrated through mesoscale simulations of DNA molecules. The investigated phenomena include elastic relaxation of dsDNA molecules and migration of ssDNA molecules in a microchannel flow. The results of the first study demonstrate that the elastic behaviour of the DNA molecules can be captured sucessfully. The second study proves that the migration of ssDNA in pressure-driven microchannel flows can be explained by the hydrodynamic interaction with the carrier liquid.