1,902 research outputs found
A machine learning framework for LES closure terms
In the present work, we explore the capability of artificial neural networks
(ANN) to predict the closure terms for large eddy simulations (LES) solely from
coarse-scale data. To this end, we derive a consistent framework for LES
closure models, with special emphasis laid upon the incorporation of implicit
discretization-based filters and numerical approximation errors. We investigate
implicit filter types, which are inspired by the solution representation of
discontinuous Galerkin and finite volume schemes and mimic the behaviour of the
discretization operator, and a global Fourier cutoff filter as a representative
of a typical explicit LES filter. Within the perfect LES framework, we compute
the exact closure terms for the different LES filter functions from direct
numerical simulation results of decaying homogeneous isotropic turbulence.
Multiple ANN with a multilayer perceptron (MLP) or a gated recurrent unit (GRU)
architecture are trained to predict the computed closure terms solely from
coarse-scale input data. For the given application, the GRU architecture
clearly outperforms the MLP networks in terms of accuracy, whilst reaching up
to 99.9% cross-correlation between the networks' predictions and the exact
closure terms for all considered filter functions. The GRU networks are also
shown to generalize well across different LES filters and resolutions. The
present study can thus be seen as a starting point for the investigation of
data-based modeling approaches for LES, which not only include the physical
closure terms, but account for the discretization effects in implicitly
filtered LES as well
Investigating Model-Data Inconsistency in Data-Informed Turbulence Closure Terms
In the present work, we investigate the stability of turbulence closure predictions from neural network models and highlight the role of model-data-inconsistency during inference. We quantify this inconsistency by applying the Mahalanobis distance and demonstrate that the instability of the model predictions in practical large eddy simulations (LES) correlates with a deviation of the input data between the training dataset and actual simulation data. Moreover, the method of 'stability training' is applied to increase the robustness of recurrent artificial neural networks (ANN) against small perturbations in the input, which are typically unavoidable in any practical scenario. We show that this method can increase the stability of simulations with ANN-based closure term predictions significantly. The models also achieve good accuracy on the blind testing set in comparison to the baseline model trained without stability training. The work presented here can thus be seen as a building block towards long-term stable data-driven models for dynamical systems and highlights methods to detect and counter model-data-inconsistencies
Structured Deep Kernel Networks for Data-Driven Closure Terms of Turbulent Flows
Standard kernel methods for machine learning usually struggle when dealing
with large datasets. We review a recently introduced Structured Deep Kernel
Network (SDKN) approach that is capable of dealing with high-dimensional and
huge datasets - and enjoys typical standard machine learning approximation
properties. We extend the SDKN to combine it with standard machine learning
modules and compare it with Neural Networks on the scientific challenge of
data-driven prediction of closure terms of turbulent flows. We show
experimentally that the SDKNs are capable of dealing with large datasets and
achieve near-perfect accuracy on the given application
Effect of Intraoperative Goal-directed Balanced Crystalloid versus Colloid Administration on Major Postoperative Morbidity
BACKGROUND:
Crystalloid solutions leave the circulation quickly, whereas colloids remain for hours, thus promoting hemodynamic stability. However, colloids are expensive and promote renal toxicity in critical care patients. This study tested the hypothesis that goal-directed colloid administration during elective abdominal surgery decreases 30-day major complications more than goal-directed crystalloid administration.
METHODS:
In this parallel-arm double-blinded multicenter randomized trial, adults having moderate- to high-risk open and laparoscopically assisted abdominal surgery with general anesthesia were randomly assigned to Doppler-guided intraoperative volume replacement with 6% hydroxyethyl starch 130/0.4 (n = 523) or lactated Ringer's solution (n = 534). The primary outcome was a composite of serious postoperative cardiac, pulmonary, infectious, gastrointestinal, renal, and coagulation complications that were assessed with a generalized estimating equation multivariate model. The primary safety outcome was a change in serum creatinine concentration up to 6 months postoperatively, compared to baseline concentrations.
RESULTS:
A total of 1,057 patients were included in the analysis. Patients assigned to crystalloid received a median [quartile 1, quartile 3] amount of 3.2 l [2.3, 4.4] of crystalloid, and patients assigned to colloid received 1.0 l [0.5, 1.5] of colloid and 1.8 l [1.2, 2.4] of crystalloid. The estimated intention-to-treat common effect relative risk for the primary composite was 0.90 for colloids versus crystalloids (95% CI: 0.65 to 1.23, P = 0.51), and 18% (91 of 523) of colloid patients and 20% (103 of 534) of crystalloid patients incurred at least one component of the primary outcome composite. There was no evidence of renal toxicity at any time.
CONCLUSIONS:
Doppler-guided intraoperative hydroxyethyl starch administration did not significantly reduce a composite of serious complications. However, there was also no indication of renal or other toxicity
Precise ultra fast single qubit control using optimal control pulses
Ultra fast and accurate quantum operations are required in many modern
scientific areas - for instance quantum information, quantum metrology and
magnetometry. However the accuracy is limited if the Rabi frequency is
comparable with the transition frequency due to the breakdown of the rotating
wave approximation (RWA). Here we report the experimental implementation of a
method based on optimal control theory, which does not suffer these
restrictions. We realised the most commonly used quantum gates - the Hadamard
(\pi/2 pulse) and NOT (\pi pulse) gates with fidelities
( and
), in an excellent agreement with the
theoretical predictions ( and
). Moreover, we demonstrate magnetic
resonance experiments both in the rotating and lab frames and we can
deliberately "switch" between these two frames. Since our technique is general,
it could find a wide application in magnetic resonance, quantum computing,
quantum optics and broadband magnetometry.Comment: New, updated version of the manuscript with supplementary informatio
An Efficient Sliding Mesh Interface Method for High-Order Discontinuous Galerkin Schemes
Sliding meshes are a powerful method to treat deformed domains in
computational fluid dynamics, where different parts of the domain are in
relative motion. In this paper, we present an efficient implementation of a
sliding mesh method into a discontinuous Galerkin compressible Navier-Stokes
solver and its application to a large eddy simulation of a 1-1/2 stage turbine.
The method is based on the mortar method and is high-order accurate. It can
handle three-dimensional sliding mesh interfaces with various interface shapes.
For plane interfaces, which are the most common case, conservativity and
free-stream preservation are ensured. We put an emphasis on efficient parallel
implementation. Our implementation generates little computational and storage
overhead. Inter-node communication via MPI in a dynamically changing mesh
topology is reduced to a bare minimum by ensuring a priori information about
communication partners and data sorting. We provide performance and scaling
results showing the capability of the implementation strategy. Apart from
analytical validation computations and convergence results, we present a
wall-resolved implicit LES of the 1-1/2 stage Aachen turbine test case as a
large scale practical application example
Crystalloids versus colloids for goal-directed fluid therapy in major surgery
INTRODUCTION: Perioperative hypovolemia arises frequently and contributes to intestinal hypoperfusion and subsequent postoperative complications. Goal-directed fluid therapy might reduce these complications. The aim of this study was to compare the effects of goal-directed administration of crystalloids and colloids on the distribution of systemic, hepatosplanchnic, and microcirculatory (small intestine) blood flow after major abdominal surgery in a clinically relevant pig model. METHODS: Twenty-seven pigs were anesthetized and mechanically ventilated and underwent open laparotomy. They were randomly assigned to one of three treatment groups: the restricted Ringer lactate (R-RL) group (n = 9) received 3 mL/kg per hour of RL, the goal-directed RL (GD-RL) group (n = 9) received 3 mL/kg per hour of RL and intermittent boluses of 250 mL of RL, and the goal-directed colloid (GD-C) group (n = 9) received 3 mL/kg per hour of RL and boluses of 250 mL of 6% hydroxyethyl starch (130/0.4). The latter two groups received a bolus infusion when mixed venous oxygen saturation was below 60% ('lockout' time of 30 minutes). Regional blood flow was measured in the superior mesenteric artery and the celiac trunk. In the small bowel, microcirculatory blood flow was measured using laser Doppler flowmetry. Intestinal tissue oxygen tension was measured with intramural Clark-type electrodes. RESULTS: After 4 hours of treatment, arterial blood pressure, cardiac output, mesenteric artery flow, and mixed oxygen saturation were significantly higher in the GD-C and GD-RL groups than in the R-RL group. Microcirculatory flow in the intestinal mucosa increased by 50% in the GD-C group but remained unchanged in the other two groups. Likewise, tissue oxygen tension in the intestine increased by 30% in the GD-C group but remained unchanged in the GD-RL group and decreased by 18% in the R-RL group. Mesenteric venous glucose concentrations were higher and lactate levels were lower in the GD-C group compared with the two crystalloid groups. CONCLUSIONS: Goal-directed colloid administration markedly increased microcirculatory blood flow in the small intestine and intestinal tissue oxygen tension after abdominal surgery. In contrast, goal-directed crystalloid and restricted crystalloid administrations had no such effects. Additionally, mesenteric venous glucose and lactate concentrations suggest that intestinal cellular substrate levels were higher in the colloid-treated than in the crystalloid-treated animals. These results support the notion that perioperative goal-directed therapy with colloids might be beneficial during major abdominal surgery
Towards Exascale CFD Simulations Using the Discontinuous Galerkin Solver FLEXI
Modern high-order discretizations bear considerable potential for the
exascale era due to their high fidelity and the high, local computational load
that allows for computational efficiency in massively parallel simulations. To
this end, the discontinuous Galerkin (DG) framework FLEXI was selected to
demonstrate exascale readiness within the Center of Excellence for Exascale CFD
(CEEC) by simulating shock buffet on a three-dimensional wing segment at
transsonic flight conditions. This paper summarizes the recent progress made to
enable the simulation of this challenging exascale problem. For this, it is
first demonstrated that FLEXI scales excellently to over 500 000 CPU cores on
HAWK at the HLRS. To tackle the considerable resolution requirements near the
wall, a novel wall model is proposed that takes compressibility effects into
account and yields decent results for the simulation of a NACA 64A-110 airfoil.
To address the shocks in the domain, a finite-volume-based shock capturing
method was implemented in FLEXI, which is validated here using the simulation
of a linear compressor cascade at supersonic flow conditions, where the method
is demonstrated to yield efficient, robust and accurate results. Lastly, we
present the TensorFlow-Fortran-Binding (TFFB) as an easy-to-use library to
deploy trained machine learning models in Fortran solvers such as FLEXI.Comment: 15 pages, 5 figure
Meperidine and skin surface warming additively reduce the shivering threshold: a volunteer study
INTRODUCTION: Mild therapeutic hypothermia has been shown to improve outcome for patients after cardiac arrest and may be beneficial for ischaemic stroke and myocardial ischaemia patients. However, in the awake patient, even a small decrease of core temperature provokes vigorous autonomic reactions-vasoconstriction and shivering-which both inhibit efficient core cooling. Meperidine and skin warming each linearly lower vasoconstriction and shivering thresholds. We tested whether a combination of skin warming and a medium dose of meperidine additively would reduce the shivering threshold to below 34 degrees C without producing significant sedation or respiratory depression. METHODS: Eight healthy volunteers participated on four study days: (1) control, (2) skin warming (with forced air and warming mattress), (3) meperidine (target plasma level: 0.9 mug/ml), and (4) skin warming plus meperidine (target plasma level: 0.9 mug/ml). Volunteers were cooled with 4 degrees C cold Ringer lactate infused over a central venous catheter (rate asymptotically equal to 2.4 degrees C/hour core temperature drop). Shivering threshold was identified by an increase of oxygen consumption (+20% of baseline). Sedation was assessed with the Observer's Assessment of Alertness/Sedation scale. RESULTS: Control shivering threshold was 35.5 degrees C +/- 0.2 degrees C. Skin warming reduced the shivering threshold to 34.9 degrees C +/- 0.5 degrees C (p = 0.01). Meperidine reduced the shivering threshold to 34.2 degrees C +/- 0.3 degrees C (p < 0.01). The combination of meperidine and skin warming reduced the shivering threshold to 33.8 degrees C +/- 0.2 degrees C (p < 0.01). There were no synergistic or antagonistic effects of meperidine and skin warming (p = 0.59). Only very mild sedation occurred on meperidine days. CONCLUSION: A combination of meperidine and skin surface warming reduced the shivering threshold to 33.8 degrees C +/- 0.2 degrees C via an additive interaction and produced only very mild sedation and no respiratory toxicity
Hemodynamic effects of intraoperative 30% versus 80% oxygen concentrations: an exploratory analysis
BackgroundSupplemental oxygen leads to an increase in peripheral vascular resistance which finally increases systemic blood pressure in healthy subjects and patients with coronary artery disease, heart failure, undergoing heart surgery, and with sepsis. However, it is unknown whether this effect can also be observed in anesthetized patients having surgery. Thus, we evaluated in this exploratory analysis of a randomized controlled trial the effect of 80% versus 30% oxygen on intraoperative blood pressure and heart rate.MethodsWe present data from a previous study including 258 patients, who were randomized to a perioperative inspiratory FiO2 of 0.8 (128 patients) versus 0.3 (130 patients) for major abdominal surgery. Continuous arterial blood pressure values were recorded every three seconds and were exported from the electronic anesthesia record system. We calculated time-weighted average (TWA) and Average Real Variability (ARV) of mean arterial blood pressure and of heart rate.ResultsThere was no significant difference in TWA of mean arterial pressure between the 80% (80 mmHg [76, 85]) and 30% (81 mmHg [77, 86]) oxygen group (effect estimate −0.16 mmHg, CI –1.83 to 1.51; p = 0.85). There was also no significant difference in TWA of heart rate between the 80 and 30% oxygen group (median TWA of heart rate in the 80% oxygen group: 65 beats.min−1 [58, 72], and in the 30% oxygen group: 64 beats.min−1 [58; 70]; effect estimate: 0.12 beats.min−1, CI –2.55 to 2.8, p = 0.94). Also for ARV values, no significant differences between groups could be detected.ConclusionIn contrast to previous results, we did not observe a significant increase in blood pressure or a significant decrease in heart rate in patients, who received 80% oxygen as compared to patients, who received 30% oxygen during surgery and for the first two postoperative hours. Thus, hemodynamic effects of supplemental oxygen might play a negligible role in anesthetized patients.Clinical Trail Registrationhttps://clinicaltrials.gov/ct2/show/NCT03366857?term=vienna&cond=oxygen&draw=2&rank=
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