text: For all but the simplest of velocity profiles , numerical or asymptotic methods are required to calculate solutions.  Some typical flow profiles are discussed below.  In general, the spectrum of the equation is discrete and infinite for a bounded flow, while for unbounded flows (such as boundary-layer flow), the spectrum contains both continuous and discrete parts.
  For plane Poiseuille flow, it has been shown that the flow is unstable (i.e. one or more eigenvalues  has a positive imaginary part) for some  when  and the neutrally stable mode at  having , .  To see the stability properties of the system, it is customary to plot a dispersion curve, that is, a plot of the growth rate  as a function of the wavenumber .
  The first figure shows the spectrum of the Orr–Sommerfeld equation at the critical values listed above. This is a plot of the eigenvalues (in the form ) in the complex plane.  The rightmost eigenvalue is the most unstable one.  At the critical values of Reynolds number and wavenumber, the rightmost eigenvalue is exactly zero.  For higher (lower) values of Reynolds number, the rightmost eigenvalue shifts into the positive (negative) half of the complex plane.  Then, a fuller picture of the stability properties is given by a plot exhibiting the functional dependence of this eigenvalue; this is shown in the second figure.
  On the other hand, the spectrum of eigenvalues for Couette flow indicates stability, at all Reynolds numbers.  However, in experiments, Couette flow is found to be unstable to small, but finite, perturbations for which the linear theory, and the Orr–Sommerfeld equation do not apply.  It has been argued that the non-normality of the eigenvalue problem associated with Couette (and indeed, Poiseuille) flow might explain that observed instability.  That is, the eigenfunctions of the Orr–Sommerfeld operator are complete but non-orthogonal.  Then, the energy of the disturbance contains contributions from all eigenfunctions of the Orr–Sommerfeld equation.  Even if the energy associated with each eigenvalue considered separately is decaying exponentially in time (as predicted by the Orr–Sommerfeld analysis for the Couette flow), the cross terms arising from the non-orthogonality of the eigenvalues can increase transiently.  Thus, the total energy increases transiently (before tending asymptotically to zero).  The argument is that if the magnitude of this transient growth is sufficiently large, it destabilizes the laminar flow, however this argument has not been universally accepted.
  A nonlinear theory explaining transition, has also been proposed. Although that theory does include linear transient growth, the focus is on 3D nonlinear processes that are strongly suspected to underlie transition to turbulence in shear flows. The theory has led to the construction of so-called complete 3D steady states, traveling waves and time-periodic solutions of the Navier-Stokes equations that capture many of the key features of transition and coherent structures observed in the near wall region of turbulent shear flows. Even though "solution" usually implies the existence of an analytical result, it is common practice in fluid mechanics to refer to numerical results as "solutions" - regardless of whether the approximated solutions satisfy the Navier-Stokes equations in a mathematically satisfactory way or not. It is postulated that transition to turbulence involves the dynamic state of the fluid evolving from one solution to the next. The theory is thus predicated upon the actual existence of such solutions (many of which have yet to be observed in a physical experimental setup). This relaxation on the requirement of exact solutions allows a great deal of flexibility, since exact solutions are extremely difficult to obtain (contrary to numerical solutions), at the expense of rigor and (possibly) correctness. Thus, even though not as rigorous as previous approaches to transition, it has gained immense popularity.
  An extension of the Orr–Sommerfeld equation to the flow in porous media has been recently suggested.