Flow structure evolution for laminar vortex rings impinging onto a fixed solid wall

• A near wall large-scale spiral vortex ring is observed for the first time.
• Secondary vortex ring plays a vital role in the formation of near wall flow structure.
• Characteristics of radial wall jet are associated with near wall flow structure.

Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) techniques are used to investigate the synthetic jet actuated laminar vortex rings impinging onto a solid wall. Four cases with different stroke lengths (19 mm ⩽ L0 ⩽ 50 mm) are adopted to generate laminar vortex rings with Reynolds number (ReV0 ≈ 151) and orifice-to-wall distance (H = 40 mm) unchanged. It is found that the dimensionless stroke length normalized by the orifice-to-wall distance (L0/H) has a significant influence on the evolution of near wall flow structure of the impinging synthetic jet. Specifically, as the stroke length is relatively small (L0/H = 0.475 and 0.775), the impinging vortex ring could only induce a weak secondary vorticity near the wall, which cannot roll up into a coherent structure. It is illustrated with LIF visualization and Finite Time Lyapunov Exponents (FTLEs) that a large-scale spiral vortex ring is generated in the near wall region for these two cases, which is explored for the first time. However, as the stroke length becomes larger (L0/H = 0.95 and 1.25), the strong secondary vorticity rolls up and pairs with the impinging vortex ring to form a vortex dipole. As a result, the impinging vortex rings are prevented to merge with the vorticity clustering near the wall causing a rapid reduction of the near wall vortex strength. The analysis of the radial wall jet shows that the induced large-scale spiral vortex ring at the case of the smaller stroke length could slow down both the decay rates of radial mass flow rate and momentum flux, and this might be of benefit for the mass and heat transfer of wall surface when applying the impinging synthetic jet.