  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 $180$ and $10^{10}$. 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 ($Re_\tau<10^3$) and high Reynolds numbers
($Re_\tau>10^6$). 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.
