ON THE VIRTUAL NONEXISTENCE OF MULTIPLE INSTABILITY REGIONS FOR SOME HEAT-EXCHANGER ARRAYS IN CROSS-FLOW

In fluidelastic analyses involving a time delay (or phase lag) between the motions of cylinders in an array and the resultant unsteady fluid forces on the cylinders, a succession of instability-stability region s is predicted theoretically at low values of the mass-damping paramet er m delta/rho D-2, below the ''ultimate'' fluidelastic instability be yond which the system is not restabilized. However, as experimenters h ave had difficulty in verifying the existence of these regions of inst ability, it is legitimate to ask (i) do these regions really exist, an d (ii) why are they so rarely observed In this paper, with the aid of the quasi-steady model of Price and Paidoussis and with expanded measu rements of lift and drag coefficients for a parallel triangular array with P/D = 1.375, it is shown that (a) the stability of the array stro ngly depends on geometric asymmetries; (b) whereas for a perfectly sym metric geometry the system may have several slab-ultimate instability regions, an asymmetry of as little as 0.02D may quench them and leave only the ultimate instability region intact. This suggests a possible explanation as to why the instability regions in question are so diffi cult to ''find'' experimentally. It also suggests that they may be of rather less practical importance for operating engineering systems tha n had heretofore been assumed, at feast for some array geometries.