A low aspect-ratio of 1.67 flexible polyethylene terephthalate membrane located within the wake region of a square cylinder is studied to understand the effects of Reynolds number (Re_D=3200–12,000) upon its resultant flapping behaviour and strain energy distribution. For Reynolds number up to Re_D=4000, the membrane exhibits limited cantilever-like oscillations (i.e. Mode A), while a mixture of Mode A response and limited travelling waves is produced between Re_D=4000–6800 (i.e. Mode B). On the other hand, quasi-periodical flapping occurs between Re_D=6800–12,000 (i.e. Mode C). Results further demonstrate similarities to high aspect-ratio polyvinylidene difluoride membranes studied previously: firstly, flapping amplitude increases with Reynolds number and secondly, an optimal flapping frequency exists in Mode C whereby it “locks-in” to the wake vortex-shedding frequency. Flapping intermittencies are observed to occur in all modes and found to decrease with increasing Reynolds number. In addition, non-uniform strain energy distributions along the membrane length and the total harvestable energy levels are deduced to increase with the Reynolds number. Lastly, results also indicate that Modes A and B lead to linear energy growth rates, while Mode C produces growth rates that scales with Re_D^3.4.