On Using an Analog Computer to Study Hydrodynamic Stability

The use of analog computers to study hydrodynamic stability is complicated by the fact that the governing differential equation has two rapidly varying solutions (one growing and one decaying), and two which are well behaved. Thus, errors in setting initial conditions and noise in an analog computer excite the rapidly growing solution, making the computer behave in an unstable way. This unstable behavior has its origin in the role played by fluid viscosity. Approximate techniques are discussed whereby these instabilites can be circumvented. The techniques are applied to the case of the Blasius boundary layer on a flat plate, and the results are compared with results available in the literature, obtained with a digital computer. The analog-and digital-computer results are in good agreement.