The purpose of this work was to numerically study the area perturbation and phase lag in a staggered tube array, which comprised of an oscillating tube with a prescribed motion in the streamwise direction. CFD simulations were conducted for a parallel triangle array over a range of reduced flow velocities, varying displacement ratios, and pitch to diameter ratios. The velocity field was post-processed and the flow perturbations were extracted using an image processing technique. The obtained perturbation and the associated phase lag profiles were used to formulate and propose simplified ideal profiles for the area perturbation amplitude and phase lag. The proposed ideal perturbation amplitude and phase lag profiles were then utilized in the model of Hassan and Weaver (2016) to predict the stability threshold of Streamwise Fluidelastic Instability.