Low frequency duct flow noise mitigation is a great challenge in noise control of extensive flow duct networks of ventilation/air-conditioning systems. Conventional silencer designs with porous/fibrous materials are usually unbearably bulky because their size characteristic dimensions have to be comparable to dominant long noise wavelengths for achieving effective mitigation. Coupled with their high flow resistance, they are usually impractical for situations with strict flow speed and tight spatial requirements. To overcome such constraints, many silencer designs equipped with acoustic metamaterials have been proposed due to their high potential in providing remarkable low frequency absorption within compact dimensions. However, their effectiveness in the practical flow duct remains an open question because the effect of grazing flow on metamaterial surfaces is entirely unknown. The present paper reports a study of the effectiveness of acoustic metamaterial silencer designs exposed to grazing flows. A duct silencer catering various membrane-type metamaterial liner designs is fabricated. Coefficients of acoustic reflection, transmission, absorption, and transmission loss (TL) across the duct silencer at various mean duct flow velocities and liner designs are measured and compared. Generally, the silencer designs show promising mitigation with TL = 10 dB over frequencies 600 Hz - 1100 Hz. The mitigation mechanisms are discussed.