  The observed baryon fraction and velocity--temperature relation in clusters
of galaxies are compared with hydrodynamic simulations in two cosmological
models : standard (Omega = 1) and a low-density flat (Omega=0.45 and
\lambda=0.55) CDM models, normalized to the COBE background fluctuations. The
observed properties of clusters include the velocity dispersion versus
temperature relation, the gas mass versus total mass relation, and the gas mass
fraction versus velocity dispersion relation. We find that, while both
cosmological models reproduce well the shape of these observed functions, only
low-density CDM can reproduce the observed amplitudes. The cluster gas mass
fraction reflects approximately the baryon fraction in the models, with a
slight anti-bias. Therefore, due to the low baryon density given by
nucleosynthesis, Omega = 1 models produce too few baryons in clusters compared
with observations. Scaling our results as a function of Omega, we find that a
low-density CDM model, with Omega approximately 0.3 - 0.4, best reproduces the
observed mean baryon fraction in clusters. The observed beta parameter of
clusters, beta = sigma^2/(k T/mu m_p) = 0.94 \pm 0.08 discriminates less well
between the models; it is consistent with that produced by low-density CDM
(1.10 \pm 0.22), while it is slightly larger than expected but still consistent
with Omega = 1 (0.70 \pm 0.14).
