A parallel, block-based, adaptive mesh refinement, finite-volume scheme is developed and validated for the solution of the Favre-Averaged Navier-Stokes equations governing three-dimensional flow of a polytropic gas. The two-equation k-omega turbulence model is used to model the unresolved turbulent scales and their influence on the mean solution. The finite-volume spatial discretization is accomplished by using a finite-volume procedure on multiblock, body-fitted, hexahedral mesh. The inviscid flux functions make use of Roe's approximate Riemann solver. The viscous flux is evaluated using a diamond path reconstruction procedure on each cell boundary. Verification and validation of the solution method is accomplished through the application of the algorithm to a number of flow problems. The results from the application of the solution method to the flow problems are in good agreement with available experimental data. Therefore, the validity of the solution method for solving three-dimensional, turbulent flows is confirmed.

In hybrid RANS/LES, Reynolds-averaged Navier-Stokes (RANS) equations method is used to treat the near-wall region and large-eddy simulation (LES) is applied to the core turbulent region. Owing to the incompatibility of these two numerical modelling approaches, an artificial (i.e., non-physical) buffer layer forms along the interface where the model switches from RANS to LES. In this thesis, a novel dynamic forcing scheme incorporating backscatter is proposed in order to remove the artificial buffer layer. In contrast to previous forcing techniques, the proposed forcing is determined dynamically from the flow field itself, and does not require any extraction of turbulent fields from reference direct numerical simulation (DNS) or high-resolution LES databases. The proposed forcing model has been tested on three types of wall-bounded turbulent flows, namely, turbulent flow in a plane channel; turbulent flow in a spanwise rotating channel; and turbulent flow in a spanwise rotating rib-roughened channel. In order to validate the present hybrid approach, turbulence statistics obtained from hybrid RANS/LES simulations are thoroughly compared with the available DNS results and laboratory measurement data. Based on the study of a plane channel flow, transport equations for the resolved turbulent stresses and kinetic energy are introduced to investigate the effects of dynamic forcing on reduction of the thickness and impact of the artificial buffer layer. As long as the dynamic forcing is in use, the artificial buffer layer have been successfully removed, indicating that the proposed hybrid approach is insensitive to the choices of the forcing region or interface location. The predictive performance of the dynamic forcing scheme has been further evaluated by considering turbulent flows subjected to a special type of body force, i.e., the non-inertial and non-conservative Coriolis force. Due to the effects of system rotation, turbulence level is enhanced on the pressure side and suppressed on the suction side of the rotating channel. Furthermore, it is reported in this thesis that the classification of the roughness type now relies not only on the pitch ratio, but also on the rotation number in the context of rotating rib-roughened flows. / February 2016

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第14139号 / 工博第2973号 / 新制||工||1441(附属図書館) / 26445 / UT51-2008-N456 / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 細田 尚, 教授 後藤 仁志, 准教授 牛島 省 / 学位規則第4条第1項該当

Cette thèse s’inscrit dans le cadre de l’amélioration des prévisions aérothermiques qui suscite l’intérêt croissant des industriels aéronautiques. Elle consiste à évaluer l’apport des méthodes URANS avancées de type SAS dans la prévision des flux de chaleur turbulents et pariétaux pour des écoulements turbulents chauffés. Elle vise aussi à situer ces approches par rapports aux modèles URANS classiques de type DRSM et hybrides RANS/LES comme la ZDES. Une extension de l’approche SAS à un modèle DRSM a d’abord été proposé afin d’obtenir une meilleure restitution des tensions de Reynolds résolues et modélisées. Ce modale SAS-DRSM a été implanté dans le code elsA de l’ONERA. Nous avons ensuite évalué les approches SAS disponibles avec ce code sur la prévention d’écoulements aérothermiques rencontrés sur avion dans un compartiment de moteur. Ces études ont montré que les approches SAS améliorent la représentation des écoulements par rapport aux modèles URANS classiques. Elles aboutissent à des écoulements fortement tridimensionnels avec de nombreuses structures turbulentes. Ces structures induisent un mélange turbulent accru et donc une meilleure prévision du flux de chaleur pariétal. De plus, nos travaux ont situé les approches de type SAS comme des méthodes plus précises que les méthodes URANS classiques sans augmentation importante du coût de calcul. Les modèles SAS ne résolvent pas les plus petites structures caractéristiques du mouvement turbulent par rapport à la ZDES qui montre des prévisions supérieures. Le modèle SAS-RDSM offre néanmoins la meilleur alternative de type SAS. Enfin, l’étude du flux de chaleur turbulent semble retrouver le fait que l’hypothèse classique de nombre de Prandtl turbulent constat n’est pas valable dans toutes les zones de l’écoulement. / The improvement of aerothermal predictions is a major concern for aeronautic manufacturers. In line with this issue, SAS approaches are assessed on the prediction of wall and turbulent heat fluxes for heated-turbulent flows. This study also aims at evaluating these advanced URANS methods in regard to DRSM models and hybrid RANS/LES approaches as ZDES. Firstly, we proposed to combine the SAS approach and a DRSM model in order to better reproduce both resolved and modelled Reynolds stresses. This new model, called SAS-DRSM, was implemented in ONERA Navier-Strokes code elsA. Unsteady simulations of two heated turbulent flows encountered in an aircraft engine compartment were then performed to evaluate all the SAS models available in the code. These numerical studies demonstrated that SAS approaches improve prediction of the flows compared to classical URANS models. They lead to full 3D flows with many turbulent structures. These structures favour turbulent mixing and thus induce a better prediction of the wall heat fluxes. Moreover, the numerical simulations showed that SAS methods are more accurate than classical URANS models without increasing significantly calculation costs. SAS approaches are not able to resolve the smallest turbulent structures in relation to ZDES which provides better predictions. Finally, the investigation of the turbulent heat flux suggested that the constant turbulent Prendtl number assumption, that is characteristic of classical URANS models, may not be valid in some regions of the flow.

Dans le domaine de la modélisation des écoulements turbulents, les approche hybrides RANS/LES ont reçu récemment beaucoup d’attention car ils combinent le coût de calcul raisonnable du RANS et la précision de la LES.Parmi elles, le TPITM (Temporal Partially Integrated Transport Model) est une approche hybride RANS/LES temporelle qui surmonte les inconsistances du raccordement continu du RANS et de la LES grâce à un formalisme de filtrage temporel. Cependant, le modèle TPITM est relativement difficile à mettre en œuvre et, en particulier, nécessite l’utilisation d’une correction dynamique, contrairement à d’autres approches, notamment la DES (Detached Eddy Simulation).Cette thèse propose alors une approche hybride RANS/LES similaire à la DES, mais basée sur un filtrage temporel, déduite du modèle TPITM par équivalence, c’est-à-dire en imposant la même partition entre énergies résolue et modélisée. Ce modèle HTLES (Hybrid Temporal LES) combine les caractéristiques de la DES (facilité de mise en œuvre) et du TPITM (formalisme consistant, justification théorique des coefficients).Après calibration en turbulence homogène, l’approche est appliquée à des cas d’écoulements autour de cylindres carrés puis rectangulaires. La modélisation des tensions de sous-filtre est une adaptation au contexte hybride du modèle RANS k-wSST. / In the field of modelling of turbulent flows, hybrid RANS/LES approaches have recently received a considerable attention due to the combination of the computational cost of RANS and the accuracy of LES.Among them, TPITM (Temporal Partially Integrated Transport Model) is a hybrid RANS/Temporal LES approach that overcomes the inconsistency of the continuous bridging of RANS and LES by using a temporal filtering formalism. However, TPITIM is relatively difficult to implement and, in particular, requires a dynamic correction, contrary to other approaches, in particular DES (Detached Eddy Simulation).The present thesis then proposes a hybrid RANS/LES approach similar to DES, but based on temporal filtering, derived from TPITM using an equivalence criterion, i. e., imposing the same partition of among resolved and modeled energies. This HTLES approach (Hybrid Temporal LES) combines the characteristics of DES (ease of implementation) and of TPITM (consistent formalism, theoretical justification of the coefficients).

In order to increase results accuracy and to provide some time-dependency to CFD results, embedded RANS/LES simulations are getting more and more interesting. In order to run these simulations accurate LES boundary conditions are required, not to affect the downstream results with a poor quality synthetic turbulence generation. Considering the currently developped methodologies, it is not possible to generate a divergence free turbulent flow which satisfy a non isotropic state of turbulence. The author started from the Synthetic Eddy Method (SEM) defined by Jarrin (2009), and defined a new shape function with the ability to satisfy continuity. The new methodology, named Divergence Free SEM (DFSEM), is able to reproduce almost any kind of turbulence anisotropy by using a special shape function and adapting the eddies intensities in order to match the Reynolds stress tensor rather than using the Lund coefficients, as most of the precursor methodologies did. Results comparisons against SEM and some other very popular synthetic turbulence models in some academic cases, proved that a reduce influence on the downstream flow was achieved. In most of the cases the friction coefficient Cf , used as a performance parameter, benefit by reducing the downstream developping zone by almost 50% in most cases, when compared against SEM. Another issue that has been tackled regards the unphysical pressure fluctuations present because of the synthetic turbulence, due to non perfectly constant mass-flow rate imposed in stochastic methodologies. The new methodology also showed an increased flexibility as it has been tested in embedded DDES simulation, by using the blending function to activate/deactivate it, and again it showed improved performances when compared against standard SEM.

The commercial CFD software STAR-CCM+ is applied as a RANS solver for comparison with potential flow methods in the calculation of vertical plane radiation and diffraction problems.  A two-dimensional rectangular cylinder oscillating in an unbounded fluid is first considered, and the added mass result shown to agree well with the analytical potential flow solution.  Hydrodynamic coefficients are then determined for the cylinder oscillating in heave and sway about a calm free surface.  Predicted values are observed to coincide with available experimental and linear potential flow results for most amplitudes and frequencies of oscillation examined.   A three-dimensional radiation problem is then studied in which 1-DoF heave and pitch motions are prescribed to the ONR Tumblehome hullform in calm water at zero forward speed and Fn 0.3.  Combinations of amplitude and frequency of oscillation ranging from small to large are considered.  Results are compared with several potential flow codes which utilize varying degrees of linearization.  Differences in the force and moment results are attributed to particular code characteristics, and overall good agreement is demonstrated between RANS and potential flow codes which employ a nonlinear formulation.  The ONR Tumblehome is next held static in incident head waves of small and large steepness and zero forward speed or Fn 0.3.  Force and moment time histories of the periodic response are compared with the same set of potential flow codes used in the radiation problem.  Agreement between potential flow and RANS is reasonable in the small wave steepness case.  For large wave steepness, the nonlinear wave response is seen to be important and the RANS solution does not generally agree well with potential flow results. / Master of Science

Computational investigations were carried out to evaluate the effectiveness and usability of hybrid RANS/LES techniques for predicting the unsteady separated flow over wings with ice accretion. RANS and hybrid RANS/LES computations were performed using the viscous flow solver CHEM with the SST turbulence model. Two configurations were considered during the study: an extruded wing with a glaze-ice shape and a swept wing with a simulated glaze-ice accretion. Hybrid RANS/LES results, in general, predict a less active shear layer ``roll up' than seen in the experimental data. Qualitative improvements are seen in the hybrid RANS/LES results over corresponding RANS results. The extruded wing results show that the CHEM hybrid RANS/LES results are similar to the AVUS DES results. The use of preconditioning and a different turbulent model in CHEM showed a slight improvement in results.

Flow in the Moderator of a CANDU reactor can be very complex due to the interplay of convective and buoyant effects. Experiments have been performed to measure temperature and velocity fields for these kind of flows, although concerns still exist. As a result a Moderator test facility has been built in order to validate CFD models for future predictions and safety analysis. To properly validate this experiment an accurate set of inlet flow conditions must be established in order to ensure a fair comparison. A series of flow conditions indicative of the header assemblies which feed flow into the moderator test facility have been investigated through experimentation, empirical evaluation and numerical simulation. They include flow through curved tubes, turbulent free jets and flow through dividing manifolds. The goal of the present study is to establish the modelling approach to predict the flow distribution inside the manifold and velocity field out of the J-nozzles. A variety of RANS based turbulence models and computational meshes were employed in the numerical study. The turbulence model that was found to perform best was the realizable k- model. It was also found that the velocity field of the J-nozzles is constant between Reynolds numbers of 6800-9300. These Reynolds numbers are indicative of those expected out of the header assemblies. / Thesis / Master of Applied Science (MASc)

A set of second-moment closure turbulence models are implemented for the study of wake evolution in an oceanic environment. The effects of density stratification are considered, and the models are validated against laboratory experiments mimicking the stratified ocean environment, and against previous experimental study of wakes subjected to a density stratification. The turbulence models are found to reproduce a number of important behaviors which differentiate stratified wakes from those in a homogeneous environment, including the appropriate decay rates in turbulence quantities, buoyant suppression of turbulence length scales, and canonical stages in wake evolution. The existence of background turbulence is considered both through the introduction of production terms to the turbulence model equations and the replication of scale-resolved simulations of wakes embedded in turbulence. It is found that the freestream turbulence causes accelerated wake growth and faster decay of wake momentum. Wakes are then simulated at a variety of Re and Fr representative of full-scale vehicles operating in an ocean environment, to downstream distances several orders of magnitude greater than existing RANS studies. The models are used to make some general predictions concerning the dependence of late-wake behavior on these parameters, and specific insights into expected behavior are gained. The wake turbulence is classified using "fossil turbulence" and stratification strength criteria from the literature. In keeping with experimentally observed behavior, the stratification is predicted to increase wake persistence. It is also predicted that, regardless of initial Re or F r, the wake turbulence quickly becomes a mixture of overturning eddies and internal waves. It is found that the high Re wakes eventually become strongly affected by the stratification, and enter the strongly-stratified or LAST regime. Additional model improvements are proposed based on the predicted late wake behavior. / Doctor of Philosophy / A set of advanced turbulence models are implemented and used to study ship wakes in an oceanic environment. The flows in the ocean are subject to a density stratification due to changes in temperature and salinity; the associated effects are included in the turbulence models. The models are validated against laboratory experiments mimicking the stratified ocean environment, and against previous experimental study of wakes subjected to a density stratification. The turbulence models are found to reproduce a number of important behaviors expected under such conditions based on experimental study. Additional modifications are made to the models to include the effect of pre-existing freestream turbulence. Wakes are then simulated under conditions representative of full-scale vehicles operating in an ocean environment. The models are used to make some general predictions concerning late-wake behavior. Specific insights into expected behavior are gained. The wake turbulence is classified using ``fossil turbulence'' and stratification strength criteria from the literature. In keeping with experimentally observed behavior, the stratification is predicted to increase wake persistence. Additional model improvements are proposed based on the predicted late wake behavior.