A water tunnel study was conducted to study the effect of asymmetric stiffness on a parallel triangular array of tubes with a pitch ratio of 1.375. The tubes were cantilevered from rectangular support rods so that the stiffness, and hence natural frequencies, were different in directions parallel and transverse to the flow. This arrangement was designed to simulate the difference in in-plane and out-of-plane natural frequencies of curved tubes. A test was conducted with symmetric stiffness for datum purposes and then eight tests were run with differences between streamwise and transverse frequencies ranging from 6.3 to 57 percent. It was found that the critical reduced velocity based on the lower frequency was increased only slightly over the symmetric case. This effect is essentially independent of the difference in frequency and the direction of the lower frequency relative to the flow.

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