Predictions are presented for hole pattern seals for which the hole depth is varied axially, showing that various depth patterns can significantly improve damping performance in terms of both increasing the seal’s effective damping and reducing its cross-over frequency — the frequency at which effective damping changes from positive to negative. Test results are presented for the seal with the best variable hole depth (VHD) pattern showing an increase in peak damping by a factor of 1.6 over a constant hole depth (CHD) design, versus predictions of 2.7. The cross over frequency is reduced by approximately 40%. The VHD seal has a significant negative static stiffness that probably arises from a friction-factor jump phenomena, not flow path divergence. The VHD seal leaks less than a comparable CHD design.

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