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Self-Excited Oscillations of Wiping Air Knives Part III: Mitigation of Opposing Jets Oscillation
Eric Salt, Samir Ziada

Last modified: 2018-04-15


During the hot dip galvanizing process, planar air jets are used on either side of a steel sheet to wipe away excess zinc material. This process is known to produce two distinct self-excited jet oscillations, each of which are summarized in Part I of this paper series. While Part II of this series discusses effective methods to suppress the oscillations produced by the jets impinging on the steel sheet, this paper is concerned with the mitigation of the jet oscillations near the edges of the sheet. In this region, the jets on either side of the sheet interact and exhibit robust oscillations characterized by periodic, out-of-phase lateral (or flapping) deflections of the two jet columns. The oscillations persist over wide ranges of jet velocity and offset distance between the jets. Even though this phenomenon has been reported for a variety of practical applications, the underlying mechanism which sustains the oscillations is still not fully understood. This excitation mechanism is further examined by means of mapping the unsteady velocity, vorticity and pressure fields. Periodic establishment and dissipation of circulation zones at the opposite sides of each jet and the resulting pressure oscillations in the entrainment regions are shown to be the main source which derives the jet oscillations. An effective way to eliminate the system oscillations is to disturb the flow in the circulation zones by locating small splitter plates in strategic locations far from the jet exit and the impingement region.


Jets; Opposing; PIV; Pressure; Self-excited; Oscillation

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