# C. B. AllenUniversity of Bristol | UB · Department of Aerospace Engineering

C. B. Allen

PhD, Professor

## About

251

Publications

36,055

Reads

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3,614

Citations

Citations since 2016

## Publications

Publications (251)

View Video Presentation: https://doi.org/10.2514/6.2022-3439.vid Newton's method for optimisation presents an attractive quadratic convergence rate, however practical application to engineering optimisation problems is hampered by the computational cost and complexity associated with obtaining the required second-order information. In this paper, p...

A spacetime formulation is presented to solve unsteady aerodynamic problems involving large deformation or topological change such as store separation, slat and flap deployment or spoiler deflection. This technique avoids complex CFD meshing methods, such as Chimera, by the use of a finite-volume approach both in space and time, and permits a local...

Efficient optimization of an aeroelastic wing is presented through multidisciplinary analysis using low-dimensional modal design variables. Much work in wing optimization has concentrated on high-fidelity surface control, therefore utilising often hundreds of design variables. However, while fine surface control can be useful, problems can arise su...

View Video Presentation: https://doi.org/10.2514/6.2022-0028.vid The trend for increased processor core numbers in both consumer and server CPUs, as well as the increasing use of many-core GPU accelerators for computation, has lead to a new paradigm of massive parallelism. GPUs in particular offer significant advantages in terms of memory bandwidth...

View Video Presentation: https://doi.org/10.2514/6.2022-0013.vid A universal methodology is presented for generating orthogonal geometric modes for three-dimensional aerodynamic optimisation which is applicable to a variety of topologies without restriction. The method presented here builds upon previous work in two dimensions by the authors which...

View Video Presentation: https://doi.org/10.2514/6.2022-1864.vid In this paper, three different approaches to subdivision multi-resolution systems are presented in the context of Catmull-Clark subdivision. One of these involved a modification to the Catmull-Clark method to construct an interpolating subdivision scheme. Subsequently an algorithm to...

View Video Presentation: https://doi.org/10.2514/6.2022-1861.vid A two and three dimensional volume of solid parameterisation method is explored that is based on the method of cellular automata. It is capable of using geometric refinement and representing complex topology, with the ability to handle merging or splitting of objects and internal void...

This paper presents a high-fidelity aerodynamic optimisation framework designed to decrease the cost of optimisation of large wind turbine blades. The framework is presented in the context of the IEA 15MW reference turbine, but is applicable to all large turbine geometries. Optimisation is performed using a surrogate model, built through latin hype...

View Video Presentation: https://doi.org/10.2514/6.2021-2563.vid Typical algorithms used in optimisation problems can be classified into gradient-based or agent-based optimisers. Gradient-based optimisers boast very fast convergence due to their ability to extract topological information from the local gradient of the objective function, but can of...

Previous work by the authors investigated code structure and optimisation for a finite-volume test-bench code on highly parallel computing architectures such as GPUs. Preliminary results show an overall speedup of between 18x and 24x on two high performance consumer GPUs compared to 16 threads of a ninth generation Intel CPU. Further results are pr...

Acoupled, high-fidelity simulation framework on GPUs is presented, and is used to perform an aero-structural simulation of large wind turbines, where a turbine is fully-occluded in the wake of another. The GPU-enabled flow solver, zCFD, is coupled with a modal structural model using a multi-variate volume interpolation with radial basis functions (...

Local shape control methods, such as B-spline surfaces, are well-conditioned such that they allow high-fidelity design optimization; however, this comes at the cost of degraded optimization convergence rate as control fidelity is refined due to the resulting exponential increase in the size of the design space. Moreover, optimizations in higher-fid...

Detail is presented on the implementation of numerical derivatives with focus given to the discrete adjoint equations.
Two approaches are considered: a hybrid matrix-based scheme where the convective Jacobian is constructed explicitly; and a matrix-free method using reverse-mode automatic differentiation.
The hybrid matrix-based scheme exploits a c...

Consideration of the aerodynamic shape optimization problem definition is presented. An issue with drag minimization is that shocked-free solutions often result which have isolated performance improvements. As such, in this paper, optimizing the range parameter is considered enriched with the operating point as a design variable. A constraint on di...

Efficient multidisciplinary optimization is presented for aero-structural wing optimization using efficient low-dimensional modal design variables. Orthogonal aerofoil design variables are derived in the geometric space via singular value decomposition. Orthogo-nality of design variables leads to a well-conditioned design space and encourages posit...

The work presented here involves development and detailed investigations of niching methods for multimodal optimization of constrained functions. There is a lack of investigations in the literature on constrained multimodal optimization, hence a number of constrained niching algorithms have been developed here that leverage existing differential ev...

A topologically flexible parameterisation method developed for aerodynamic optimisation is tested ona variety of 2 dimensional structural topology problems. The background to the restricted snakes vol-ume of solid (RSVS) is explained. A topology optimisation framework is develloped using FreeFem++ as a linear elastic solver and differential evoluti...

This paper presents a topologically flexible parameterisation method suitable for the optimisation of 3 dimensional aerodynamics using traditional boundary fitted meshes. This parameterisation extends the restricted-snake volume of solid (RSVS) parameterisation previously developed by the authors.This is achieved by extending restricted snakes, a p...

Without addressing shape smoothness, gradient-based optimisation methods naturally amplify high-frequency shape components which can lead to poor convergence of the optimisation problem and a convergence rate exhibiting dependency on the fidelity of shape-control.
Recent work by the authors demonstrated that this problem arises due to the discrete...

The most common aerofoil optimization problem considered is lift-constrained drag minimization at a fixed design point; however, shock-free solutions can result, which can lead to poor off-design performance. As such, this paper presents a study of the construction of the aerofoil optimization problem and its effect on the performance over a range...

Without addressing shape smoothness, gradient-based optimisation methods naturally amplify high-frequency shape components which can lead to poor convergence of the optimisation problem and a convergence rate exhibiting dependency on the fidelity of shape-control. In this paper it is demonstrated that this problem arises due to the discrete shape p...

This paper uses previous developments in multi-fidelity shape parameterisation and aerodynamic topology optimisation to develop a framework which brings significant performance enhancements to topology optimisation cases. The combined framework comprises: a multilevel subdivision shape parameterisation used in combination with an adjoint solver and...

Aerodynamic shape optimization of a transonic wing using mathematically-extracted modal design variables is presented. A novel approach is used for deriving design variables using a singular value decomposition of a set of training aerofoils to obtain an efficient, reduced set of orthogonal ‘modes’ that represent typical aerodynamic design paramete...

An investigation into an aerodynamic optimization benchmark case that exhibits mul-timodality is presented using a global optimization approach. The recently suggested case 6 of the AIAA Aerodynamic Design Optimization Discussion Group involves the drag minimization of a rectangular NACA0012 wing subject to lift and root bending moment, as well as...

This work explores how smoothed particle hydrodynamics (SPH) may be applied for fluids optimisation problems. To achieve this, a newly developed volume of solid geometric parameterisation is applied that implicitly allows large geometric changes as well as topological changes. The meshless nature of SPH has long been an advantage, but when combined...

A generic constraint handling framework for use with any swarm-based optimization algorithm is presented. For swarm optimizers to solve constrained optimization problems effectively modifications have to be made to the optimizers to handle the constraints, however, these constraint handling frameworks are often not universally applicable to all swa...

Subdivision curves are defined as the limit of a recursive application of a subdivision rule to an initial set of control points. This intrinsically provides a hierarchical set of control polygons that can be used to provide surface control at varying levels of fidelity. This work presents a shape parameterization method based on this principle and...

A spacetime framework is presented to solve unsteady aerodynamics problems as an alternative to conventional approaches for complex problems involving large deformation or topological change such as store separation, slat and flap deployment or spoiler deflection. It avoids complex CFD meshing methods, such as Chimera, by the use of a finite-volume...

This paper presents improvements to the Restricted-Snake Volume of Solid (RSVS) parameterisa-tion method, published previously by the authors, for combined shape and topology optimisation for aerodynamics. Unlike established parameterisation method the RSVS supports topology optimisa-tion, allowing the number of bodies to be explored with a single...

This paper presents an investigation into the influence of shape parameterization and dimensionality on the optimization of a benchmark case described by the American Institute of Aeronautics and Astronautics Aerodynamic Design Optimization Discussion Group. This problem concerns the drag minimization of a National Advisory Committee for Aeronautic...

An investigation into a multimodal aerodynamic optimization benchmark case using an approximate global optimization approach is presented. A new benchmark case of the AIAA Aerodynamic Design Optimization Discussion Group that involves the drag minimization of a rectangular NACA0012 wing subject to lift and root bending moment, as well as other geom...

Radial basis function (RBF) interpolation is popular for mesh deformation due to robustness and generality, but the cost scales with the number of surface points sourcing the deformation as . Hence, there have been numerous works investigating efficient methods using reduced datasets. However, although reduced-data methods are efficient, they requi...

A new geometry and topology parameterisation method is presented which is based on creating a parameterisation grid of cells and reconstructing surfaces from the fraction of the cell volume defined to be solid, with the volume fractions acting as design variables. This method is able to include topological changes alongside fine-level geometric con...

A comprehensive review of aerofoil shape parameterization methods that can be used for aerodynamic shape optimization is presented. Seven parameterization methods are considered for a range of design variables: class-shape transformations; B-splines; Hicks-Henne bump functions; a radial basis function domain element approach; Bèzier surfaces; a sin...

The design of vortex generators on a tiltrotor-aircraft infinite wing is presented using an adaptive surrogate modeling approach. Particular design issues in tiltrotors produce wings that are thick and highly loaded, and so separation and early-onset buffet can be problematic, and vortex generators are commonly used to alleviate these issues. In th...

Aerodynamic shape optimization of aerofoils using efficient orthogonal design variables is considered using a global search algorithm. A novel approach is presented for deriving shape design variables, using a proper orthogonal decomposition of a set of training aerofoils to obtain an optimally efficient set of aerofoil deformation modes that repre...

This paper presents an investigation into the influence of shape parameterisation and dimensionality on the optimisation of a benchmark case described by the AIAA Aerodynamic Design Optimisation Discussion Group. This problem specifies the drag minimisation of a NACA0012 under inviscid flow conditions at M = 0.85 and α = 0 subject to the constraint...

A comprehensive review of aerofoil shape parameterisation methods that can be used for aerodynamic shape optimisation is presented. Seven parameterisation methods are considered for a range of design variables: class function/shape function transformations (CST); B-Splines; Hicks-Henne bump functions; a domain element approach using Radial Basis fu...

Tiltrotor wings are thick and highly loaded airfoils, and are thus particularly susceptible to separation and early onset buffet and vortex generators are commonly used to alleviate these issues. Hence. the design of counter-rotating vortex generators on tiltrotor wings to control separation is considered. A representative tiltrotor wing airfoil (a...

A multi-physics computational method is presented to model the effect of internally and externally-carried fuel on aeroelastic behaviour of a pitch–plunge aerofoil model through the transonic regime. The model comprises three strongly coupled solvers: a compressible finite-volume Euler code for the external flow, a two-degree of freedom spring mode...

Unsteady Computational Fluid Dynamics in AeronauticsTucker P. G.Springer, 2014. 413pp. Illustrated. £117. ISBN 978-94-007-7048-5. - Volume 119 Issue 1217 - C. B. Allen

Spatially varying signals are typically sampled by collecting uniformly spaced samples irrespective of the signal content. For signals with inhomogeneous information content, this leads to unnecessarily dense sampling in regions of low interest or insufficient sample density at important features, or both. A new adaptive sampling technique is prese...

Aerodynamic shape optimizations of aerofoils and wings using mathematically-derived design variables are presented. A novel approach is used for deriving design variables using a proper orthogonal decomposition of a set of training aerofoils to obtain an efficient, reduced set of deformation 'modes' that represent typical design parameters such as...

A novel mesh based volume geometry parameterisation method is presented that allows for optimisation of shape and topology simultaneously: a topology inclusive parameteri-sation. This uses a volume of solid (VOS) technique to describe the geometry by reconstructing surfaces from the volume fraction that solid in each parameterisation mesh cell. The...

Design of counter-rotating rectangular vortex generators on a representative tiltrotor wing section is considered by surrogate-based optimization. Due to structural requirements , tiltrotor wings are usually thick and highly loaded and are therefore susceptible to early onset buffet and separation. Boundary layer control is therefore essential and...

Within an aerodynamic shape optimization framework, an efficient shape parameterization and deformation scheme is critical to allow flexible deformation of the surface with the maximum possible design space coverage. Numerous approaches have been developed for the geometric representation of airfoils. A fundamental approach is considered here from...

Theoretical and Computational AerodynamicsSengupta T. K.John Wiley and Sons, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK. 2014. 505pp. Illustrated. £63.50. ISBN 978-1-118-78759-5. - Volume 119 Issue 1212 - C. B. Allen

Wind farms continuously grow in size given the importance of green energy. To optimize power production individual turbine design must be considered as well as the spatial localization of the turbines within the farm. While numerical predictions allow the assessment of the wind flow inside the wind farm, verification is limited. Moreover, such an a...

The optimization of wind farms with respect to spatial layout is addressed experimentally. Wake effects within wind turbine farms are well known to be deleterious in terms of power generation and structural loading, which is corroborated in this study. Computational models are the predominant tools in the prediction of turbine-induced flow fields....

The gravitational search algorithm (GSA) is a recent addition to the family of global optimization algorithms based on phenomena found in nature, specifically the gravitational attractive force between two bodies of mass. However, like almost all global search algorithms of this type, GSA has no direct method of handling a constrained optimization...

Agent-based optimization algorithms are an effective means of solving global optimization problems with design spaces containing multiple local minima, however, modifications have to be made to such algorithms to be able to solve constrained optimization problems. The gravitational search algorithm (GSA) is an efficient and effective agent-based me...

A comparison is presented of gradient-based and agent-based optimization schemes, ap-plied to constrained aerodynamic shape optimization in two dimensions. The optimizers developed are a feasible sequential quadratic programming (FSQP) gradient-based algo-rithm and a modified gravitational search algorithm (GSA), extended to include a new separatio...

Agent-based global search algorithms employ a set of search agents to traverse a given design space in pursuit of an optimum solution, and are normally accepted as the most reliable methods for finding a global optimum in a complex design space. However, when non-linear constraints are present in the optimization problem, modifications have to be m...