A model problem of the form -i\epsilon y''+q(x)y=\lambda y, y(-1)=y(1)=0, is
associated with well-known in hydrodynamics Orr--Sommerfeld operator. Here
(\lambda) is the spectral parameter, (\epsilon) is the small parameter which is
proportional to the viscocity of the liquid and to the reciprocal of the
Reynolds number, and (q(x)) is the velocity of the stationary flow of the
liquid in the channel (|x|\leqslant 1). We study the behaviour of the spectrum
of the corresponding model operator as (\epsilon\to 0) with linear, quadratic
and monotonous analytic functions. We show that the sets of the accumulation
points of the spectra (the limit spectral graphs) of the model and the
corresponding Orr--Sommerfeld operators coincide as well as the main terms of
the counting eigenvalue functions along the curves of the graphs.