Influence of tripping devices in hastening transition in a flat plate submitted to zero and favorable pressure gradients

Flow similarity rules permit the measurement of airplane aerodynamics in small-scale wind tunnels. These similarity rules are only valid if the physical phenomena in small-scale resembles the full-size application. The associated Reynolds number in small-scale wind tunnel testing is typically much lower compared to the full-size application due to practical limitations. Of primary importance is therefore the similarity of the turbulent boundary layer. Hence, tripping devices are commonly used in wind tunnel testing to hasten the laminar-turbulent transition of the boundary layer in order to mimic the aerodynamic effects present in the full-scale application. In this paper, we experimentally investigated the effectiveness of two types of tripping devices, e.g., zigzag strips and randomly distributed grits of varied sizes, in hastening the transition in a flat plate. Flows of zero and favorable pressure gradients were considered. Hot-wire anemometry was used to quantify the velocity profile in the streamwise direction. The acceleration coefficient for the sink flow was K = 5.3 x 10^(-7). All studies were performed at chord-based Reynolds number of 1 x 10^6. The research demonstrates that the absence of a tripping device led the boundary layer to remain laminar along the entire flat plate length for both zero and positive pressure gradients cases. All trips hastened the transition from laminar to turbulent flow. Zigzag strips introduced stronger disturbances inside the boundary layer compared to grits, and consequently, induced a faster transition. However, the zigzag strips overstimulated the inner region of the boundary layer, requiring a more significant recovery length compared to the grits. The grits with the height of 59% of the boundary layer thickness presented the most consistent results for both pressure gradients since this trip did not overstimulate the boundary layer and developed a turbulent flow faster than the smallest grit investigated, corresponding to 25% of the boundary layer thickness. Furthermore, a recommendation is made regarding the trip height to guarantee the development of a self-similar turbulent boundary layer valid for zero and favorable pressure gradients.