Strong Turbulence Interactions Research Articles

Experimental investigation of the flow past passive vortex generators on an airfoil experiencing three-dimensional separation

The use of passive vortex generators (VGs) as a simple and effective way to delay or suppress separation on an airfoil optimized for wind turbine blades is examined experimentally. The profile experiences three-dimensional separation of the Stall Cell type. Pressure, Flow Visualization and Stereo Particle Image Velocimetry experiments are discussed for the case of triangular counter rotating VGs with common flow up. For a Reynolds number of 0.87×106, Stall Cell formation is delayed for 5°, and lift increases up to α=15°. In total, maximum lift increases by 44%, while drag increases by 0.002 at pre-stall angles of attack. At α=16° the flow bifurcates between separated and attached flow conditions. At α=10° the flow is examined in detail and an investigation on the turbulence characteristics is carried out by correlating Reynolds stresses production to time averaged flow gradients. Strong turbulent interaction is observed between the two vortices and the underlying flow up to 37.2 VG heights downstream of the VGs, while further downstream (up to 47.2 VG heights) diffusion governs the flow. A wandering motion of the VG vortices leads to increased v`v`¯ normal stress values between the two vortices.

Turbulent advances of a breaking bore: Preliminary physical experiments

In an estuary, a tidal bore may be generated at the leading edge of the flood tidal wave during the early flood tide under spring tide conditions into a narrow funnelled channel. For Froude numbers greater than 1.4–1.6, the leading edge of the bore is characterised by a breaking roller. The roller is characterised by a sudden increase in water depth, a highly turbulent flow with large-scale vortical structures, some kinetic energy dissipation, a two-phase air–water flow region and strong turbulence interactions. New experiments were conducted in a large canal with a focus on breaking bore roller propagation. The upstream propagation of the roller toe was highly turbulent. The toe perimeter shape fluctuated rapidly with transverse distance and time. The celerity of the roller toe changed rapidly with time and space, although in a quasi-two-dimensional manner on average. The instantaneous longitudinal free-surface profile of the roller showed significant temporal and spatial fluctuations. New air–water flow measurements highlighted some distinctive air bubble entrainment at the toe of the roller. Bubbles with larger chord times were detected at higher vertical elevations in a more intermittent manner. Overall the study demonstrated that the propagation of breaking bore is a very turbulent, three-dimensional process.