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arXiv 2009.10600math.APmath-phmath.MP
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用时间周期边界数据研究粘性液体中刚体的自推进

On the Self-Propulsion of a Rigid Body in a Viscous Liquid by Time-Periodic Boundary Data

Giovanni P. Galdi

摘要:

考虑一个刚体$\mathscr B$,它被约束在无限粘性液体中作平移运动。驱动机制是给定的时间周期速度场分布,$\bfv_*$,在界面处的体-液,大小为$δ$(在适当的函数类中)。主要目的是找到$\bfv_*$上的条件,确保$\mathscr B$执行非零净运动,即$\mathscr B$可以在有限时间内覆盖任何给定的距离。解决该问题的方法取决于在一段时间内$\bfv_*$的平均值是(情况(b))还是不是(情况(a))相同的零。在情况(a)中,我们以一种相对简单的方式解决了这个问题,通过表明,对于小的$δ$,它减少到一个合适的和充分研究的时间相关的斯托克斯(线性)问题的研究。然而,在情况(b)中,问题要复杂得多,因为我们证明了它{ \em不能}用于线性问题的研究。因此,在情况(b)中,自推进是一个真正的非线性问题,我们通过矛盾论证方法直接在非线性系统上解决。这样,我们也能够在情况(b)中给出自推进的充分条件(对于小的$δ$)。最后,我们证明,通过反例,这样的条件,一般来说,也是必要的。

英文摘要:

Consider a rigid body, $\mathscr B$, constrained to move by translational motion in an unbounded viscous liquid. The driving mechanism is a given distribution of time-periodic velocity field, $\bfv_*$, at the interface body-liquid, of magnitude $δ$ (in appropriate function class). The main objective is to find conditions on $\bfv_*$ ensuring that $\mathscr B$ performs a non-zero net motion, namely, $\mathscr B$ can cover any given distance in a finite time. The approach to the problem depends on whether the averaged value of $\bfv_*$ over a period of time is (case (b)) or is not (case (a)) identically zero. In case (a) we solve the problem in a relatively straightforward way, by showing that, for small $δ$, it reduces to the study of a suitable amd well-investigated time-dependent Stokes (linear) problem. In case (b), however, the question is much more complicated, because we show that it {\em cannot} be brought to the study of a linear problem. Therefore, in case (b), self-propulsion is a genuinely nonlinear issue that we solve directly on the nonlinear system by a contradiction argument approach. In this way, we are able to give, also in case (b), sufficient conditions for self-propulsion (for small $δ$). Finally, we demonstrate, by means of counterexamples, that such conditions are, in general, also necessary.

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