An Inquiry of the Friction Factor “Jump” Phenomenon in Hole-Pattern Seals

Experiments measuring the leakage rate through hole-pattern seals operating at high pressures of up to 8400 kPa reveal the presence of an abrupt variation in the friction factor (up to a factor of three) with change in the pressure difference across the seal. The measured pressure fluctuations suggest that this observation may, perhaps, be understood as a change in fluid instability modes across the cavities of the seal. A more detailed investigation via large eddy simulations (LES) suggest that the switching from shear layer instability mode to a wake instability mode, a phenomenon well known in fundamental studies of cavity flow (without a top bounding wall), is responsible for the observed variation.