5,736 research outputs found
A survey of machine learning wall models for large eddy simulation
This survey investigates wall modeling in large eddy simulations (LES) using
data-driven machine learning (ML) techniques. To this end, we implement three
ML wall models in an open-source code and compare their performances with the
equilibrium wall model in LES of half-channel flow at eleven friction Reynolds
numbers between and . The three models have ''seen'' flows at
only a few Reynolds numbers. We test if these ML wall models can extrapolate to
unseen Reynolds numbers. Among the three models, two are supervised ML models,
and one is a reinforcement learning ML model. The two supervised ML models are
trained against direct numerical simulation (DNS) data, whereas the
reinforcement learning ML model is trained in the context of a wall-modeled LES
with no access to high-fidelity data. The two supervised ML models capture the
law of the wall at both seen and unseen Reynolds numbers--although one model
requires re-training and predicts a smaller von K\'arm\'an constant. The
reinforcement learning model captures the law of the wall reasonably well but
has errors at both low () and high Reynolds numbers
(). In addition to documenting the results, we try to
''understand'' why the ML models behave the way they behave. Analysis shows
that the errors of the supervised ML model is a result of the network design
and the errors in the reinforcement learning model arise due to the present
choice of the ''states'' and the mismatch between the neutral line and the line
separating the action map. In all, we see promises in data-driven machine
learning models
Statistics of turbulence in the energy-containing range of Taylor-Couette compared to canonical wall-bounded flows
Considering structure functions of the streamwise velocity component in a
framework akin to the extended self-similarity hypothesis (ESS), de Silva
\textit{et al.} (\textit{J. Fluid Mech.}, vol. 823,2017, pp. 498-510) observed
that remarkably the \textit{large-scale} (energy-containing range) statistics
in canonical wall bounded flows exhibit universal behaviour. In the present
study, we extend this universality, which was seen to encompass also flows at
moderate Reynolds number, to Taylor-Couette flow. In doing so, we find that
also the transversal structure function of the spanwise velocity component
exhibits the same universal behaviour across all flow types considered. We
further demonstrate that these observations are consistent with predictions
developed based on an attached-eddy hypothesis. These considerations also yield
a possible explanation for the efficacy of the ESS framework by showing that it
relaxes the self-similarity assumption for the attached eddy contributions. By
taking the effect of streamwise alignment into account, the attached eddy model
predicts different behaviour for structure functions in the streamwise and in
the spanwise directions and that this effect cancels in the ESS-framework ---
both consistent with the data. Moreover, it is demonstrated here that also the
additive constants, which were previously believed to be flow dependent, are
indeed universal at least in turbulent boundary layers and pipe flow where
high-Reynolds number data are currently available.Comment: accepted in J. Fluid Mec
Log-law recovery through reinforcement-learning wall model for large-eddy simulation
This paper focuses on the use of reinforcement learning (RL) as a
machine-learning (ML) modeling tool for near-wall turbulence. RL has
demonstrated its effectiveness in solving high-dimensional problems, especially
in domains such as games. Despite its potential, RL is still not widely used
for turbulence modeling and is primarily used for flow control and optimization
purposes. A new RL wall model (WM) called VYBA23 is developed in this work,
which uses agents dispersed in the flow near the wall. The model is trained on
a single Reynolds number () and does not rely on high-fidelity
data, as the back-propagation process is based on a reward rather than output
error. The states of the RLWM, which are the representation of the environment
by the agents, are normalized to remove dependence on the Reynolds number. The
model is tested and compared to another RLWM (BK22) and to an equilibrium wall
model, in a half-channel flow at eleven different Reynolds numbers (). The effects of varying agents' parameters such as actions
range, time-step, and spacing are also studied. The results are promising,
showing little effect on the average flow field but some effect on wall-shear
stress fluctuations and velocity fluctuations. This work offers positive
prospects for developing RLWMs that can recover physical laws, and for
extending this type of ML models to more complex flows in the future.Comment: arXiv admin note: text overlap with arXiv:2211.0361
Extension of the law of the wall exploiting the concepts of strong and weak universality of velocity fluctuations in a channel
This paper explores the universality of the instantaneous velocity profiles
in a channel, which, despite the equilibrium nature of a channel flow, are
subjected to strong non-equilibrium effects. In the analysis, we employ a
one-dimensional scalar variant of the proper orthogonal decomposition (POD) and
exploit the concepts of strong and weak universalities. Strong universality
requires that all POD modes are universal with respect to the Reynolds number,
while weak universality only requires that the first few POD modes are
universal. As POD analysis concerns information at more than one location,
these universalities are more general than various similarities and
universality in the literature concerning single-point flow statistics, e.g.,
outer layer similarity or universality of the log law. We examine flows at
180, 540, 1000, and 5200. Strong universality is observed in the
outer layer, and weak universality is found in both the inner layer and the
outer part of the logarithmic layer. The presence of weak universality suggests
the existence of an extension to the law of the wall (LoW). Here, we propose
such an extension based on the results from one-dimensional POD analysis. The
usefulness of the LoW extension is assessed by comparing flow reconstructions
according to the extended LoW and the equilibrium LoW. We show that the
extended LoW provides strikingly accurate flow reconstruction in the wall layer
across a wide range of Reynolds numbers, capturing fine-scale motions that are
entirely missed by the equilibrium LoW.Comment: 20 pages, 16 figure
Water saturation effects on P-wave anisotropy in synthetic sandstone with aligned fractures
The seismic properties of rocks are known to be sensitive to partial liquid or gas saturation, and to aligned fractures. P-wave anisotropy is widely used for fracture characterization and is known to be sensitive to the saturating fluid. However, studies combining the effect of multiphase saturation and aligned fractures are limited even though such conditions are common in the subsurface. An understanding of the effects of partial liquid or gas saturation on P-wave anisotropy could help improve seismic characterization of fractured, gas bearing reservoirs. Using octagonal-shaped synthetic sandstone samples, one containing aligned penny-shaped fractures and the other without fractures, we examined the influence of water saturation on P-wave anisotropy in fractured rocks. In the fractured rock, the saturation related stiffening effect at higher water saturation values is larger in the direction across the fractures than along the fractures. Consequently, the anisotropy parameter ‘?’ decreases as a result of this fluid stiffening effect. These effects are frequency dependent as a result of wave-induced fluid flow mechanisms. Our observations can be explained by combining a frequency-dependent fractured rock model and a frequency-dependent partial saturation model
Incorporating basic calibrations in existing machine-learned turbulence modeling
This work aims to incorporate basic calibrations of Reynolds-averaged
Navier-Stokes (RANS) models as part of machine learning (ML) frameworks. The ML
frameworks considered are tensor-basis neural network (TBNN), physics-informed
machine learning (PIML), and field inversion & machine learning (FIML) in J.
Fluid Mech., 2016, 807, 155-166, Phys. Rev. Fluids, 2017, 2(3), 034603 and J.
Comp. Phys., 2016, 305, 758-774, and the baseline RANS models are the
one-equation Spalart-Allmaras model, the two-equation - model, and
the seven-equation Reynolds stress transport models. ML frameworks are trained
against plane channel flow and shear-layer flow data. We compare the ML
frameworks and study whether the machine-learned augmentations are detrimental
outside the training set. The findings are summarized as follows. The
augmentations due to TBNN are detrimental. PIML leads to augmentations that are
beneficial inside the training dataset but detrimental outside it. These
results are not affected by the baseline RANS model. FIML's augmentations to
the two eddy viscosity models, where an inner-layer treatment already exists,
are largely neutral. Its augmentation to the seven-equation model, where an
inner-layer treatment does not exist, improves the mean flow prediction in a
channel. Furthermore, these FIML augmentations are mostly non-detrimental
outside the training dataset. In addition to reporting these results, the paper
offers physical explanations of the results. Last, we note that the conclusions
drawn here are confined to the ML frameworks and the flows considered in this
study. More detailed comparative studies and validation & verification studies
are needed to account for developments in recent years
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