Acoustic Characteristics of Orifice Holes Exposed to Grazing Flow

Abstract

Experimental and theoretical studies were conducted on the acoustic characteristics of orifice holes exposed to a grazing flow. The impedance tube method was applied and the grazing flow was given by an open jet to avoid standing wave effects in the radiation field. The radiation resistance was found to become negative at specific conditions determined by flow Mach number, frequency and hole diameter. The open area ratio of orifice holes was pointed out to have a stronger influence on the impedance than considered. The concept of ' constant volume flow through orifice hole' is not necessarily valid. A potential flow theory was presented on the radiation impedance of a tube opening exposed to a grazing flow. In the theory the flow region and the tube region were connected to satisfy the coincidence conditions of displacement and pressure at the opening. This theory was found to predict fairly well the various aspects of impedance obtained in the experiments.