Multi-objective optimization of a wing fence on an unmanned aerial vehicle using surrogate-derived gradients

In this paper, the multi-objective, multifidelity optimization of a wing fence on an unmanned aerial vehicle (UAV) near stall is presented. The UAV under consideration is characterized by a blended wing body (BWB), which increases its efficiency, and a tailless design, which leads to a swept wing to ensure longitudinal static stability. The consequence is a possible appearance of a nose-up moment, loss of lift initiating at the tips, and reduced controllability during landing, commonly referred to as tip stall. A possible solution to counter this phenomenon is wing fences: planes placed on top of the wing aligned with the flow and developed from the idea of stopping the transverse component of the boundary layer flow. These are optimized to obtain the design that would fence off the appearance of a pitch-up moment at high angles of attack, without a significant loss of lift and controllability. This brings forth a constrained multi-objective optimization problem. The evaluations are performed through unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations. However, since controllability cannot be directly assessed through computational fluid dynamics (CFD), surrogate-derived gradients are used. An efficient global optimization framework is developed employing surrogate modeling, namely regressive co-Kriging, updated using a multi-objective formulation of the expected improvement. The result is a wing fence design that extends the flight envelope of the aircraft, obtained with a feasible computational budget

Non-equilibrium wall-bounded turbulence and associated noise generation

Abstract : The present study investigates the response of turbulence in a non-equilibrium flows such as transient periodic channel flows and spatially developing boundary layers subjected to pressure gradients. Such a fundamental study is important to understand noise generation in complex wall-bounded turbulent flows. First, to understand the flow dynamics in transient accelerating flows, direct numerical simulations (DNS) of periodic channel flows responding to an impulse acceleration are carried out. The turbulent flow undergoes reverse transition toward a quasi-laminar state, followed by a retransition phase to the new equilibrium state. To reduced simulation cost, the minimal-span methodology is applied and evaluated for simulations of transient flows. Detailed comparisons with a full-span case show that the small-span test case captures the essential dynamics during the transition process despite small, quantitative differences attributed to a slower streak transient growth. A small span is used to characterize accelerating channels with riblets. Results indicate that riblets delay the transition to high Reynolds number state, as it reduces streak meandering. Next, to study non-equilibrium boundary layer flows in the presence of convex wall curvature, DNS simulations over an airfoil (suction side) and a flat plate are compared. Both cases are characterized by matching adverse pressure gradient (APG) along the streamwise direction. For the airfoil boundary layer, existing DNS data obtained by \cite{wu2019effects} of flow around a controlled-diffusion (CD) airfoil is used. For the flat-plate boundary layer, a DNS simulation is carried out, with prescribed pressure gradient distribution that matches that of the airfoil flows in the APG region. Comparison between the two cases shows how the wall curvature affects turbulence in an APG boundary layer, important in industrial applications such as fan flows. Overall, the comparison shows that the boundary layer developments are very similar. This indicates that a flat-plate boundary layer can serve as a low-cost surrogate of an airfoil boundary layer in numerical studies of important features of an airfoil flow. The difference between the two cases represents the effect of a mild convex wall curvature. Specifically, in the region of weak APG, the curvature effect dominates that of the pressure gradient and yields a lower friction coefficient. In high-APG regions (near the trailing edge of the airfoil) the effects of wall curvature and APG appear to interact. Lastly, various existing analytical models are evaluated on their predictions of wall pressure fluctuations, which are essential for noise prediction in non-equilibrium boundary layer turbulent flows that develop on fan blades. Limitations of the existing models are evaluated; new parameters that do not involve the ill-defined wall friction in a boundary layer under strong adverse pressure gradients are proposed. The primary role of the mean velocity logarithmic layer in affecting the overlap range of the wall pressure spectrum is also demonstrated. A new wall pressure spectrum model is proposed and tested in a wide range of boundary layer flows under different Reynolds numbers and zero, adverse and favorable pressure gradients. The test database includes existing experimental data and various DNS flat-plate simulations. The new wall pressure spectrum model is the first generalized model designed for boundary layer flows with a wide range of pressure gradients and Reynolds numbers.Ce mémoire étudie la réponse de la turbulence dans des écoulements hors équilibre, tels que les écoulements transitoires dans un canal périodique et les couches limites se développant spatiallement soumises à des gradients de pression. Une telle étude fondamentale est importante pour comprendre la génération du bruit dans des écoulements complexes turbulents. Premièrement, pour comprendre la dynamique d’écoulements transitoires soumis à une accélération, des simulations directes d’écoulements instationnaires dans un canal périodique soumis à une accélération impulsionnelle ont été réalisées. L’écoulement turbulent subit une transition inversée vers un état quasi-laminaire, suivi par une nouvelle phase de transition vers un nouvel équilibre. Pour réduire le coût de calcul, la méthode de l’envergure minimale du domaine de calcul est appliquée et validée pour de telles simulations instationnaires. Des comparaisons détaillées avec un cas d’envergure complète montrent que la simulation avec une envergure minimale capture l’essentiel de la dynamique de l’écoulement durant la phase de transition et ce malgré quelques petites différences attribuées à la croissance plus lente des tourbillons longitudinaux le long de la paroi (“streaks”). Une envergure réduite est ensuite appliquée à l’étude d’un écoulement accéléré dans un canal avec de micro-sillons ou “riblets”. Les résultats montrent que les riblets retardent la transition du fait qu’ils stabilisent la turbulence de proche paroi. Deuxièmement, pour étudier les couches limites hors équilibre sur une paroi convexe, des simulations directes sur l’extrados d’un profil aérodynamique et d’une plaque plane sont comparées. Les deux cas sont caractérisés par le même gradient de pression adverse dans la direction de l’écoulement. Pour la couche limite sur le profil, on utilise les données existantes de la simulation directe de Wu et al. (2019) autour du profil à diffusion controllée (CD). Pour la couche limite sur la plaque plane, une nouvelle simulation directe a été réalisée avec le même gradient de pression adverse que sur le profil. La comparaison des deux cas montre que la courbure de la paroi convexe peut modifier la turbulence dans une couche limite soumise à un gradient de pression adverse qui est important dans les applications industrielles comme les écoulements dans des ventilateurs. Cependant les modifications restent mineures et la comparaison montre que le développement des couches limite turbulentes dans les deux cas est semblable. Ceci implique que la couche limite sur une plaque plaque sur un domaine réduit peut servir de substitut à celle sur un profil aérodynamique qui requiert un domaine plus grand et des ressources de calcul plus importante. La différence observée entre les deux cas permet d’évaluer l’effet d’une paroi faiblement convexe. Spécifiquement, dans la région de faible gradient de pression adverse, les effets de courbure dominent ceux du gradient de pression et réduisent le coefficient de frottement pariétal. Dans les zones de fort gradient de pression adverse, près du bord de fuite, les effets de gradient de pression et de courbure interagissent. Finalement, la dernière étape a été d’évaluer les différents modèles analytiques de fluctuations de pression pariétale qui sont au centre des prédictions de bruit dans les couches limites turbulentes hors équilibre qui se développent sur les pales de ventilateurs. Les limites des modèles précédents sont évaluées et de nouveaux paramètres ne faisant pas intervenir le frottement pariétal mal défini dans une couche limite à fort gradient de pression adverse sont proposés. Le rôle primordial de la zone logarithmique dans la couche limite turbulente sur le gabarit spectral des spectres de pression pariétale est aussi mis en évidence. Le nou veau modèle de spectre de pression pariétale est ensuite testé sur plusieurs couches limites attachées avec des gradients de pression favorables, adverses, et des écoulements décollés à divers nombres de Reynolds basés sur l’épaisseur de quantité de mouvement. Les données proviennent de bases de données expérimentales et numériques existantes. Des simulations directes supplémentaires ont également été réalisées pour étendre les résultats numériques (notamment sur le profil CD) à des nombres de Reynolds plus élevés. Pour la première fois, un modèle est capable de reproduire les spectres de pression pariétale pour tous ces types d’écoulement

Confronting Grand Challenges in environmental fluid mechanics

Environmental fluid mechanics underlies a wealth of natural, industrial and, by extension, societal challenges. In the coming decades, as we strive towards a more sustainable planet, there are a wide range of grand challenge problems that need to be tackled, ranging from fundamental advances in understanding and modeling of stratified turbulence and consequent mixing, to applied studies of pollution transport in the ocean, atmosphere and urban environments. A workshop was organized in the Les Houches School of Physics in France in January 2019 with the objective of gathering leading figures in the field to produce a road map for the scientific community. Five subject areas were addressed: multiphase flow, stratified flow, ocean transport, atmospheric and urban transport, and weather and climate prediction. This article summarizes the discussions and outcomes of the meeting, with the intent of providing a resource for the community going forward

Direct multiscale coupling of a transport code to gyrokinetic turbulence codes

Direct coupling between a transport solver and local, nonlinear gyrokinetic calculations using the multiscale gyrokinetic code TRINITY [M. Barnes, Ph.D. thesis, arxiv:0901.2868] is described. The coupling of the microscopic and macroscopic physics is done within the framework of multiscale gyrokinetic theory, of which we present the assumptions and key results. An assumption of scale separation in space and time allows for the simulation of turbulence in small regions of the space-time grid, which are embedded in a coarse grid on which the transport equations are implicitly evolved. This leads to a reduction in computational expense of several orders of magnitude, making first-principles simulations of the full fusion device volume over the confinement time feasible on current computing resources. Numerical results from TRINITY simulations are presented and compared with experimental data from JET and ASDEX Upgrade plasmas.Comment: 12 pages, 13 figures, invited paper for 2009 APS-DPP meeting, submitted to Phys. Plasma

An Irregularly Portioned FDF Solver for Turbulent Flow Simulation

A new computational methodology is developed for large eddy simulation (LES) with the filtered density function (FDF) formulation of turbulent reacting flows. This methodology is termed the "irregularly portioned Lagrangian Monte Carlo finite difference" (IPLMCFD). It takes advantage of modern parallel platforms and mitigates the computational cost of LES/FDF significantly. The embedded algorithm addresses the load balancing issue by decomposing the computational domain into a series of irregularly shaped and sized subdomains. The resulting algorithm scales to thousands of processors with an excellent efficiency. Thus it is well suited for LES of reacting flows in large computational domains and under complex chemical kinetics. The efficiency of the IPLMCFD; and the realizability, consistency and the predictive capability of FDF are demonstrated by LES of several turbulent flames

In-situ Estimation of Time-averaging Uncertainties in Turbulent Flow Simulations

The statistics obtained from turbulent flow simulations are generally uncertain due to finite time averaging. The techniques available in the literature to accurately estimate these uncertainties typically only work in an offline mode, that is, they require access to all available samples of a time series at once. In addition to the impossibility of online monitoring of uncertainties during the course of simulations, such an offline approach can lead to input/output (I/O) deficiencies and large storage/memory requirements, which can be problematic for large-scale simulations of turbulent flows. Here, we designed, implemented and tested a framework for estimating time-averaging uncertainties in turbulence statistics in an in-situ (online/streaming/updating) manner. The proposed algorithm relies on a novel low-memory update formula for computing the sample-estimated autocorrelation functions (ACFs). Based on this, smooth modeled ACFs of turbulence quantities can be generated to accurately estimate the time-averaging uncertainties in the corresponding sample mean estimators. The resulting uncertainty estimates are highly robust, accurate, and quantitatively the same as those obtained by standard offline estimators. Moreover, the computational overhead added by the in-situ algorithm is found to be negligible. The framework is completely general and can be used with any flow solver and also integrated into the simulations over conformal and complex meshes created by adopting adaptive mesh refinement techniques. The results of the study are encouraging for the further development of the in-situ framework for other uncertainty quantification and data-driven analyses relevant not only to large-scale turbulent flow simulations, but also to the simulation of other dynamical systems leading to time-varying quantities with autocorrelated samples

Confronting Grand Challenges in Environmental Fluid Dynamics

Environmental fluid dynamics underlies a wealth of natural, industrial and, by extension, societal challenges. In the coming decades, as we strive towards a more sustainable planet, there are a wide range of grand challenge problems that need to be tackled, ranging from fundamental advances in understanding and modeling of stratified turbulence and consequent mixing, to applied studies of pollution transport in the ocean, atmosphere and urban environments. A workshop was organized in the Les Houches School of Physics in France in January 2019 with the objective of gathering leading figures in the field to produce a road map for the scientific community. Five subject areas were addressed: multiphase flow, stratified flow, ocean transport, atmospheric and urban transport, and weather and climate prediction. This article summarizes the discussions and outcomes of the meeting, with the intent of providing a resource for the community going forward