Unsteady motions in the turbulent separation bubble of a two-dimensional wing

Abstract

The unsteadiness of a turbulent separation bubble (TSB) formed close to the trailing edge of a two-dimensional wing was investigated using time-resolved particle image velocimetry. The angle of attack was set to 9.7° and the chord-based Reynolds number was 720 000. The TSB consisted of two shear layers and formed a triangular shape in the streamwise–wall-normal plane. The vertices of this triangle consisted of an intermittent detachment point, a fixed corner close to the airfoil trailing edge and an intermittent endpoint in the wake region. The velocity field had three energetic regions each with different Strouhal numbers (Stl): (a) an upstream turbulent boundary layer (TBL) with Stl = 0.1 to 4, (b) a TSB with Stl = 0.03 to 0.08 and (c) two shear layers with Stl = 0.4 to 0.8. The low-frequency motions in the TSB consisted of large zones of positive and negative streamwise velocity fluctuation that were several times wider than the large-scale structures of the upstream TBL. These zones forced an undulation of the separation line and were attributed to Görtler structures. They were also correlated with the velocity fluctuations between the two shear layers. The breathing motion of the TSB occurred at Stl = 0.05. This breathing correlated with the location of the TSB endpoint and the flapping of the upper shear layer. The detachment point of the TSB featured broad fluctuations and did not demonstrate a strong correlation with the breathing motion.

Unsteady wake interference of unequal-height tandem cylinders mounted in a turbulent boundary layer

“…Figure 4 shows profiles of the normalized streamwise mean velocity (), root mean square of the streamwise velocity fluctuations () and the streamwise integral length scale () of the approach unperturbed TBL. Following Wang & Ghaemi (2022), the integral length scale was determined by integrating the autocorrelations of the streamwise velocity fluctuations () from the self-correlation point to the downstream location where , where is the exponential constant. For submerged cylinders, the section of the TBL that directly interacts with the cylinders has a stronger influence on the flow separation and wake dynamics around the cylinders (Lim, Castro & Hoxey 2007; Fang & Tachie 2019; Essel et al.…”

Comparative Analysis of the Hydrodynamic Performance of Arc and Linear Flapping Hydrofoils

“…These occurred because, compared with linear flapping, the lateral displacement velocity of arc flapping occurs in the direction of the X axis, as shown in Figure 2, thereby changing the velocity value of the equivalent incoming flow, as shown in Equation ( 8), and then causing a change in the attack angle, as shown in Equation ( 9) and Figure 4. It is known from the literature that a change in the attack angle will affect the strength of the leading-edge vortex of the hydrofoil [23,24]. Therefore, the different flapping modes had significant effects on the generation and evolution of the leading-edge vortices.…”

“…A. Andersen et al found, using the soap film technique, that the wake diagrams for pure heave and pure pitch motions are qualitatively similar [22]. Abbaspour and Song conducted research using three flapping modes-pure pitch, pure heave, and coupled heave-pitch-by means of experiments and numerical calculations, and concluded that an anti-Kármán vortex street always appears in the wake when employing coupled heave-pitch motion [23][24][25]. On this basis, Du et al further subdivided the flapping mode employing coupled heave-pitch motion, and compared the propulsion performance of the four resulting flapping modes, concluding that the greatest average thrust occurs when the maximum pitch angle is at the initial position and the pitch direction is the same as the heave direction, and no resistance is generated [26].…”