Wake-induced transition in the low-Reynolds-number flow over a multi-element airfoil

The flow past a cylinder in proximity to a plane wall is investigated numerically for small gap ratios. Three vortex dynamic processes associated with different hairpin vortex generation mechanisms are identified for the first time, and the wake-induced turbulent transition is analysed. The vortex shedding is suppressed at $G/D = 0.1$ , while the spanwise vortex is generated via a Kelvin–Helmholtz instability and evolves into hairpin vortices. For $G/D= 0.3$ , the upper and lower rollers alternatively shedding from the cylinder, interact with the secondary vortex. The split secondary vortex merges with the upper roller and results in a new vortex downstream, which develops into hairpin vortices. When $G/D = 0.9$ , the secondary vortex interacts with the lower roller and then evolves into hairpin vortices. A tertiary vortex induced by the secondary vortex is observed, rotating in the opposite direction to the secondary vortex the wake-induced transitions share the same route. The velocity fluctuations deviate from the optimal growth theory in the pre-transitional region. In the transitional region low-frequency disturbances penetrate the sheltering edge to generate streaks where the disturbance energy declines. In the turbulent region the logarithmic layer is formed, indicating that the turbulent equilibrium is established.

Figure 13.Shape factor of the wall boundary layer for different gap ratios.…”

Section: Resultsmentioning

confidence: 99%

Three-dimensional vortex dynamics and transitional flow induced by a circular cylinder placed near a plane wall with small gap ratios

Wang

Qiu

et al. 2022

“…This approach has been adopted by Wang et al. (2018) and Wang & Wang (2021 b ). The evolution of (shown in figure 13) illustrates that the curves of for all three aspect ratios progressively cross the line (the laminar stage) along the streamwise direction after the reattachment point.…”

Section: Resultsmentioning

confidence: 99%

“…2015; Zhang & Zhou 2024) are significant mode analysis tools, and were widely used in the previous studies (He, Wang & Pan 2013; Wang et al. 2018; Wang & Wang 2021 a , b ; Li et al. 2022 a ).…”

Section: Numerical Methodologymentioning

confidence: 99%

“…However, for the upstream reverse boundary layer, the boundary layer edge is defined as the separatrix between positive ω Z and negative ω Z , where ω Z represents the timeand spanwsie-averaged spanwise vorticity. This approach has been adopted by Wang et al (2018) and Wang & Wang (2021b). The evolution of H (shown in figure 13) illustrates that the curves of H for all three aspect ratios progressively cross the line H = 2.59 (the laminar stage) along the streamwise direction after the reattachment point.…”

Section: Transitional Characteristicsmentioning

confidence: 99%

See 1 more Smart Citation

Vortex dynamics and boundary layer transition in flow around a rectangular cylinder with different aspect ratios at medium Reynolds number

Li,

Wang,

Qiu

et al. 2024

The numerical investigation focuses on the flow patterns around a rectangular cylinder with three aspect ratios ( $L/D=5$ , $10$ , $15$ ) at a Reynolds number of $1000$ . The study delves into the dynamics of vortices, their associated frequencies, the evolution of the boundary layer and the decay of the wake. Kelvin–Helmholtz (KH) vortices originate from the leading edge (LE) shear layer and transform into hairpin vortices. Specifically, at $L/D=5$ , three KH vortices merge into a single LE vortex. However, at $L/D=10$ and $15$ , two KH vortices combine to form a LE vortex, with the rapid formation of hairpin vortex packets. A fractional harmonic arises due to feedback from the split LE shear layer moving upstream, triggering interaction with the reverse flow. Trailing edge (TE) vortices shed, creating a Kármán-like street in the wake. The intensity of wake oscillation at $L/D=5$ surpasses that in the other two cases. Boundary layer transition occurs after the saturation of disturbance energy for $L/D=10$ and $15$ , but not for $L/D=5$ . The low-frequency disturbances are selected to generate streaks inside the boundary layer. The TE vortex shedding induces the formation of a favourable pressure gradient, accelerating the flow and fostering boundary layer relaminarization. The self-similarity of the velocity defect is observed in all three wakes, accompanied by the decay of disturbance energy. Importantly, the decrease in the shedding frequency of LE (TE) vortices significantly contributes to the overall decay of disturbance energy. This comprehensive exploration provides insights into complex flow phenomena and their underlying dynamics.

“…The boundary layer thickness δ is defined as the wall-normal distance between the airfoil surface and the boundary layer edge . The location of is at the separatrix between the boundary layer flow and the inviscid free stream (Wang & Wang 2021). However, the standard boundary layer assumption (the velocity at the boundary layer edge equals to 0.99 U ∞ ) remains invalid due to the curved airfoil surface.…”

Section: Unsteady Aerodynamics and Fluid–structure Interactionmentioning

confidence: 99%

Aerodynamics and fluid–structure interaction of an airfoil with actively controlled flexible leeward surface

Guo

et al. 2023

Self Cite

Piezoelectric macro-fibre composite (MFC) actuators are employed onto the flexible leeward surface of an airfoil for active control. Time-resolved aerodynamic forces, membrane deformations and flow fields are synchronously measured at low Reynolds number (Re = 6 × 104). Mean aerodynamics show that the actively controlled airfoil can achieve lift-enhancement and drag-reduction simultaneously in the angle of attack range of 10° ≤ α ≤ 14°, where the rigid airfoil encounters stall. The maximum increments of lift and lift-to-drag ratio are 27.1 % and 126 % at the reduced actuation frequency of ${f^ + } = 3.52$ . The unsteady coupling features are further analysed at α = 12°, where the maximum lift-enhancement occurs. It is newly discovered that the membrane vibrations and flow fields are locked into half of the actuation frequency when ${f^ + } > 3$ . The shift of the dominant vibration mode from bending to inclining is the reason for the novel ‘half-frequency lock-in’ phenomenon. To the fluid–structure interaction, there are three characteristic frequencies for the actively controlled airfoil: $S{t_1} = 0.5{f^ + }$ , $S{t_2} = {f^ + }$ , and $S{t_3} = 1.5{f^ + }$ . Here, St1 and its harmonics (St2, St3) are coupled with the natural frequencies of the leading-edge shear layer, resulting in the generation of multi-scale flow structures and suppression of flow separation. The lift presents comparable dominant frequencies between St1 and St3, which means the instantaneous lift is determined by the flow structures of St1 and St3. The local membrane bulge and dent affect the instantaneous swirl strength of flow structures near the maximum vibration amplitude location, which is the main reason for the variation of instantaneous lift.