Insights into the effect of spacer filaments on the unsteady flow behavior in narrow spacer-filled channels were obtained using a computational fluid dynamics model. The flow was computed for different filament configurations for channel Reynolds numbers up to 2000. For the filament diameter, channel height, and spacer mesh length examined here, the transition to unsteady flow for channels obstructed with cylindrical spacer filaments occurs somewhere between Re-ch of 200 and 800, depending on the spacer configuration. When the flow is unsteady, vortex shedding occurs behind the spacer filaments and the shed vortices scour the channel walls as they move toward the next downstream filament, thereby increasing the wall shear stress. For each spacer configuration examined, a characteristic vortex formation pattern was found.