The issue of accuracy and convergence for numerical calculations in situations in which the exact solution of the Navier-Stokes equations (under the imposed initial and boundary conditions) is unstable is discussed. If one relies on roundoff error or truncation error (or the randomness of a stochastic scheme) in order to perturb the computed solution away from the unstable one, then the computed result cannot be accurate. Instead, one must explicitly provide perturbations in order to compute accurately flows of physical interest. Computational results for flow past an impulsively started cylinder are presented.
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