Experimental Comparisons of Derivative Free Optimization Algorithms

In this paper, the performances of the quasi-Newton BFGS algorithm, the NEWUOA derivative free optimizer, the Covariance Matrix Adaptation Evolution Strategy (CMA-ES), the Differential Evolution (DE) algorithm and Particle Swarm Optimizers (PSO) are compared experimentally on benchmark functions reflecting important challenges encountered in real-world optimization problems. Dependence of the performances in the conditioning of the problem and rotational invariance of the algorithms are in particular investigated.Comment: 8th International Symposium on Experimental Algorithms, Dortmund : Germany (2009

Adaptive unstructured remeshing using gradient-only optimisation algorithms for shape optimisation

In this 'work-in-progress' paper, we show how gradient-only optimization algorithms can effectively be used for optimization problems characterized by noise and discontinuities; an example of such a function being the unstructured remeshing shape optimization problem. For this problem, gradient information is known to be (reasonably) reliable, as opposed to function value information, which may be deceptive. In performing the shape optimization, we use the quadratically convergent unstructured remeshing strategy we have previously proposed; it is based on a truss structure analogy, for which computationally efficient analytical gradients are available. While the unstructured remeshing strategy per se allows for increased flexibility (e.g. large shape changes per iteration), this comes at the cost of the discontinuities mentioned, due to remeshing and changes in finite element connectivity. We therefore reflect on optimization strategies for the discontinuous unstructured remeshing shape optimization problem. To do this, we present two simple algorithms based on the well-known Broyden- Fletcher-Goldfarb-Shanno (BFGS) updating scheme, in which we modify the line searches used in the updating formulae. We then present two relatively simple example problems, being a discontinuous one-dimensional function, and the shape optimisation of a Michell-like structure. © 2006 Civil-Comp Press

Identifiability of Tyre Force Contact Prediction from Deformation Measurements

The possibility of accurately inferring the external forces applied to a vehicle can directly contribute to better safety systems and thus lowers the chance of injury or loss of life. These external forces are applied to a vehicle through the tyres and are challenging to measure directly. Still, it is possible to measure acceleration, deformation, or strain on the inner surface of a tyre. These measurements are theorized to be strongly linked to the forces produced by the tyre. However, it is still unknown whether or not one can always identify external forces from internal measurements in this way. Research has mainly focused on obtaining estimates of tyre forces rather than establishing to what extent these tyre forces are identifiable. This paper investigates this by conducting a virtual experiment that simulates known external forces applied to the tyre and computes the strains and displacements inside the tyre. A virtual inverse simulation then recovers the external forces from either the deformation or strain computed on the inside of the tyre. The identifiability of the forces recovered by the virtual inverse simulation is investigated by adding artificial measurement noise and initial guess perturbations to quantify the variance in the identified forces

Analysis of parallel spatial partitioning algorithms for GPU based DEM

International audienceThe capability of solving a geotechnical discrete element method (DEM) applications is determined by the complexity of the simulation and its computational requirements. Collision detection algorithms are fundamental to resolve the mechanical collisions between millions of particles efficiently. These algorithms are a bottleneck for many DEM applications resulting in excessive memory usage or poor computational performance. In particular, for GPU based DEM, there are many factors for a user to consider when deciding on an algorithm. This study discusses a set of diverse classes of geotechnical problems and the impact of algorithm choice. Four factors were considered: i) the world domain size, number of particles and particle density, ii) polydispersity in size, iii) the time evolution and iv) the particle shape. This study shows that for spherical particles, the choice of broad-phase collision detection algorithm has the most impact on computational performance. The computational cost for convex polyhedral particles is dominated by the selection of the particles’ bounding volumes and their intersection tests over the selection of the broad-phase collision detection algorithm. On average for convex polyhedral particles, the broad-phase occupies at most 1.3% of the total runtime, while the narrow-phase collision detection and collision response require more than 87% of the runtime. A combination of bounding spheres and axis-aligned bounding boxes for use as bounding volumes of particles showed the best performance reducing the computational cost by 20%. This study serves as a guide for further research in the field of GPU based DEM collision detection and the application in geotechnics. © 2020 Elsevier Lt

Long-term follow-up of patients with atherosclerotic renal artery disease

Atherosclerotic renal artery stenosis (ARAS) is a predictor of increased morbidity and mortality. However, whether ARAS itself accelerates the arteriosclerotic process or whether ARAS is solely the consequence of atherosclerosis is unclear. We imaged renal arteries of 1561 hypertensive patients undergoing coronary angiography and followed this cohort for 9 years (range, 2.4-15.1 years; median, 31.2 months, interquartile range, 13.4/52.9 months). All patients received aspirin, renin-angiotensin system blockade, statins, and beta blockade as indicated. One hundred seventy-one patients had ARAS >50% diameter stenosis and 126 patients an arteriosclerotic plaque (ARAP) without significant stenosis. Blood pressures were not different in ARAS, ARAP, and non-ARAS patients. After adjustment for cardiovascular risk factors by propensity scores and matched pair analysis, ARAS patients had a lower ejection fraction and more coronary artery disease (CAD) than non-ARAS patients. The same was true for brain natriuretic peptide values, troponin I, and highly sensitive C-reative protein. Over 9 years, more ARAS patients died of any cause (34% vs 23%; P < .05). The prevalence of CAD in ARAP patients was higher than in non-ARAS patients and lower than in ARAS patients. The mortality of the ARAP patients at 9 years was 37%, not different from the ARAS patients. Atherosclerotic renal artery disease appears to be a marker for the severity of atherosclerosis rather than a causative factor for atherosclerosis progression

Long-term follow-up of patients with atherosclerotic renal artery disease

Atherosclerotic renal artery stenosis (ARAS) is a predictor of increased morbidity and mortality. However, whether ARAS itself accelerates the arteriosclerotic process or whether ARAS is solely the consequence of atherosclerosis is unclear. We imaged renal arteries of 1561 hypertensive patients undergoing coronary angiography and followed this cohort for 9 years (range, 2.4-15.1 years; median, 31.2 months, interquartile range, 13.4/52.9 months). All patients received aspirin, renin-angiotensin system blockade, statins, and beta blockade as indicated. One hundred seventy-one patients had ARAS >50% diameter stenosis and 126 patients an arteriosclerotic plaque (ARAP) without significant stenosis. Blood pressures were not different in ARAS, ARAP, and non-ARAS patients. After adjustment for cardiovascular risk factors by propensity scores and matched pair analysis, ARAS patients had a lower ejection fraction and more coronary artery disease (CAD) than non-ARAS patients. The same was true for brain natriuretic peptide values, troponin I, and highly sensitive C-reative protein. Over 9 years, more ARAS patients died of any cause (34% vs 23%; P < .05). The prevalence of CAD in ARAP patients was higher than in non-ARAS patients and lower than in ARAS patients. The mortality of the ARAP patients at 9 years was 37%, not different from the ARAS patients. Atherosclerotic renal artery disease appears to be a marker for the severity of atherosclerosis rather than a causative factor for atherosclerosis progression

The neutron imaging system fielded at the National Ignition Facility

We have fielded a neutron imaging system at the National Ignition Facility to collect images of fusion neutrons produced in the implosion of inertial confinement fusion experiments and scattered neutrons from (n, n′) reactions of the source neutrons in the surrounding dense material. A description of the neutron imaging system is presented, including the pinhole array aperture, the line-of-sight collimation, the scintillator-based detection system and the alignment systems and methods. Discussion of the alignment and resolution of the system is presented. We also discuss future improvements to the system hardware

The neutron imaging system fielded at the National Ignition Facility

We have fielded a neutron imaging system at the National Ignition Facility to collect images of fusion neutrons produced in the implosion of inertial confinement fusion experiments and scattered neutrons from (n, n′) reactions of the source neutrons in the surrounding dense material. A description of the neutron imaging system is presented, including the pinhole array aperture, the line-of-sight collimation, the scintillator-based detection system and the alignment systems and methods. Discussion of the alignment and resolution of the system is presented. We also discuss future improvements to the system hardware