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Air flowing through a narrow slit sometimes generates noise. This phenomenon has been identified in some parts of automobile HVAC systems. The noise annoys drivers and passengers and causes deterioration in the automobile’s quality. Simply sealing the gap is a way to avoid such flow noise. In an HVAC system, however, the doors that control the air flow are openable, and the door slits, which are difficult to eliminate, emanate the noise in transient conditions. The noise is well known, but the mechanism of noise generation has not been well studied to date. Computational fluid dynamics (CFD) has been a useful tool for air flow studies, but commercial CFD programs usually use a turbulence model such as Reynolds-averaged Navier-Stokes (RANS); in addition, the flow behavior result depends on the model selected in the calculation.