The influence of edge undulation on vortex formation for low-aspect-ratio propulsors

Kaiser, Frieder; Kriegseis, Jochen; Rival, David E.

doi:10.1017/jfm.2019.908

Cited by 5 publications

(6 citation statements)

References 46 publications

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“…The BDIM and BDIM-σ solution using d/D = 4/512 are indistinguishable and show very little change in the force profile as the Reynolds number is increased from Re = 100 to 1250 to 125000, especially during the acceleration phase (t * < 2), in agreement with the experimental findings [29,33]. In contrast, the Direct-forcing results only match BDIM and BDIM-σ for Re = 100, with large force errors at higher Re.…”

Section: Resultssupporting

confidence: 79%

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Immersed boundary simulations of flows driven by moving thin membranes

Lauber

Weymouth

Limbert

2022

Journal of Computational Physics

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Section: Resultssupporting

confidence: 79%

“…In addition to validating the initial impulse force with potential flow theory, we validate the force peak (F peak ) using published experimental data [29,33]. The results are presented in Table 2.…”

Section: Resultsmentioning

confidence: 94%

Immersed boundary simulations of flows driven by moving thin membranes

Lauber

Weymouth

Limbert

2022

Journal of Computational Physics

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“…As such, the memory effect in the present flow relative to a sudden change of boundary conditions is significantly smaller when compared to previous studies on other model geometries such as by Kaiser et al. (2020) and Marzanek & Rival (2019). Owing to the open separation on the elongated body of the prolate spheroid, the material volume of fluid that is directly affected by the acceleration quickly convects into the wake.…”

Section: Resultscontrasting

confidence: 50%

“…As shown in figure 6(k), the separation line moves outwards again at t * = 1.68, and vorticity and velocity fields already look very similar as in the steady state shown in figure 6(b). As such, the memory effect in the present flow relative to a sudden change of boundary conditions is significantly smaller when compared to previous studies on other model geometries such as by Kaiser et al (2020) and . Owing to the open separation on the elongated body of the prolate spheroid, the material volume of fluid that is directly affected by the acceleration quickly convects into the wake.…”

Section: Streamwise Vorticity and Velocity Distributionsmentioning

confidence: 51%

“…However, leading-edge vortex (LEV) formation, followed by the later shedding of the LEV, has led to an unsteady flow condition up to 14 chord lengths after acceleration (Mancini et al 2015). While the exact formation time, vorticity strength and vortex stability for the LEV are dependent on plate kinematics, other studies confirmed the existence of memory effects on accelerating plates (Onoue & Breuer 2016;Kaiser, Kriegseis & Rival 2020). As discussed earlier, a different type of memory effect was observed on a non-slender delta wing.…”

Section: Examples Of Unsteady 3-d Flow Separationmentioning

confidence: 78%

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Dynamic separation on an accelerating prolate spheroid

Guo,

Kaiser,

Rival

2023

J. Fluid Mech.

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Time-varying flow separation on an accelerating prolate spheroid has been studied at various angles of incidence. Instantaneous pressure and scanning stereoscopic particle image velocimetry were used to shed light on the evolution of cross-flow structures for the Reynolds number ( $Re$ ) range of $1.0\times 10^6\leq Re \leq 1.5\times 10^6$ . The movement of separation lines is examined for various model accelerations to investigate on the interplay between acceleration and flow separation. The results demonstrate that for axial accelerations, the streamwise pressure distribution in the rear part of the prolate spheroid switches from an adverse to a favourable pressure gradient. At the same time, the circumferential adverse pressure gradient present during steady motion vanishes during said accelerations. In contrast, both streamwise and circumferential adverse pressure gradients strengthen when the model is axially decelerated. These dynamic pressure distributions influence the location of the separation line, which in turn moves closer to the model meridian during accelerations while moving outwards during decelerations. The streamwise vorticity distribution and the streamwise circulation both show how the separation-line position impacts the vortex formation. A high-vorticity region near the model surface is established during acceleration. In contrast, a decelerating model leads to transport of high-vorticity fluid into the outer area of the cross-flow separation. We further assess the memory effects following the near-impulsive velocity changes. The cross-flow retains the memory of moving separation lines shortly after the acceleration. However, the separation recovers quickly to a steady state.

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Two models and the generation mechanisms of the drag on an accelerating starting disk

Xiang

Qin

et al. 2022

Physics of Fluids

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As a canonical problem, the impulsive starting of a circular disc contains the fundamental mechanisms of the force generation of the drag-based propulsions. In this paper, a circular disc is uniformly accelerated to a constant target velocity along a straight path, the instantaneous drag on and the flow fields around the disc are measured. A series of experiments were conducted by varying the two dimensionless numbers, i.e., the Reynolds number (Re) ranging from 40000 to 80000, and the acceleration number (A*)(double normalized uniform-acceleration distance) ranging from 0.5 to 2. Based on the quasi-steady and the impulse-based ideas, two analytical models are proposed for predicting and accounting for the drag force on the disc. Moreover, the two models distinguish the generation of the drag force into three phases. In the acceleration phase, the growth rate and initial peak of the drag on the disc strongly depend on A*, which make the drag-force histories exhibit a good scaling law for a given A*, and the whole drag is generally contributed by the increased growth-rate of the vortex ring circulation. In the transition phase, the drag decreases owing to the decrease of the circulation growth-rate of the vortex ring. In the vortex pinch-off phase, the circulation of the vortex ring nearly no longer grows and the size growth-rate of vortex ring gradually plays a dominant role in the drag generation. The present results suggest two implications for understanding the force generation in bio-propulsions.

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The influence of edge undulation on vortex formation for low-aspect-ratio propulsors

Cited by 5 publications

References 46 publications

Immersed boundary simulations of flows driven by moving thin membranes

Immersed boundary simulations of flows driven by moving thin membranes

Dynamic separation on an accelerating prolate spheroid

Two models and the generation mechanisms of the drag on an accelerating starting disk

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