数学物理学报, 2023, 43(3): 691-701

二维可压缩Prandtl方程倒流点的存在性

邹永辉,, 徐鑫,*

中国海洋大学数学科学学院 山东青岛 266100

Existence of Back-Flow Point for the Two-Dimensional Compressible Prandtl Equation

Zou Yonghui,, Xu Xin,*

School of Mathematical Sciences, Ocean University of China, Shandong Qingdao 266100

通讯作者: *徐鑫, E-mail: xx@ouc.edu.cn

收稿日期: 2022-09-27   修回日期: 2022-11-28  

基金资助: 国家自然科学基金(12001506)

Received: 2022-09-27   Revised: 2022-11-28  

Fund supported: NSFC(12001506)

作者简介 About authors

邹永辉,E-mail:zouyonghuimath@163.com

摘要

该文研究了二维非稳态可压缩Prandtl边界层方程倒流点的存在性, 在Oleinik单调性假设下, 作者首先利用极值原理得到了第一个倒流点如果出现, 那么一定出现在边界$\left\{y=0\right\}$上. 其次, 当压力满足一致逆压梯度条件并且初始值满足一定增长条件时, 作者通过Lyapunov泛函方法得到倒流点的存在性. 最后给出倒流点存在的实例.

关键词: 可压缩Prandtl方程; 倒流点; 极值原理; 逆压梯度; Lyapunov泛函

Abstract

In this paper, we study the back-flow problem of the two-dimensional unsteady compressible Prandtl boundary layer equations. By using the maximum principle, we obtain that a first back-flow point should appear on the boundary $\left\{y=0\right\}$ if back-flow occurs under Oleinik's monotonicity assumption. Moreover, when the pressure gradient of the outer flow is adverse and the initial velocity satisfies certain growth condition, we obtain the existence of a back-flow point of the compressible Prandtl boundary layer by Lyapunov functional method. Finally, an example of the existence of the back-flow point is given.

Keywords: Compressible Prandtl equation; Back-flow point; The maximum principle; Adverse pressure gradient; Lyapunov functional

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本文引用格式

邹永辉, 徐鑫. 二维可压缩Prandtl方程倒流点的存在性[J]. 数学物理学报, 2023, 43(3): 691-701

Zou Yonghui, Xu Xin. Existence of Back-Flow Point for the Two-Dimensional Compressible Prandtl Equation[J]. Acta Mathematica Scientia, 2023, 43(3): 691-701

1 引言

本文在区域$\Omega_{T}=\left\{(t,x,y)| 0\leq t\leq T, 0\leq x\leq L, 0\leq y<\infty \right\}$中研究二维非稳态可压缩Prandtl方程

$\begin{matrix} \left\{\begin{array}{ll} \partial_{t}u+u\partial_{x}u+v\partial_{y}u-\frac{1}{\rho}\partial^{2}_{y}u=-\frac{\partial_{x}P(\rho)}{\rho},\\[2mm] \partial_{x}(\rho u)+\partial_{y}(\rho v)=-\partial_{t}\rho,\\ u|_{y=0}=v|_{y=0}=0, \quad \lim\limits_{y\to \infty}u=U(t,x),\\ u|_{t=0}=u_{0}(x,y),\quad u|_{x=0}=u_{1}(t,y), \end{array}\right. \end{matrix}$

这里$u(t,x,y)$$v(t,x,y)$分别表示边界层的切向速度和法向速度. $\rho(t,x),U(t,x),P(\rho)$分别表示可压Euler外流的密度, 切向速度和压力在边界$\left\{y=0\right\}$上的迹, 并且满足如下Bernoulli 法则

$\begin{matrix}\label{Bernoulli} \partial_{t}U+U\partial_{x}U+\frac{\partial_{x}P(\rho)}{\rho}=0. \end{matrix}$

流体的压力$P(\rho)$是关于$\rho$$0<\rho_{0}\leq\rho$上严格递增的函数,其中$\rho_{0}$ 是一个正常数, 逆压梯度条件$\partial_{x}P>0$ 意味着 $\partial_{x}\rho>0$.

Ludwig Prandtl于1904年在文献[21]中首次提出边界层的相关理论, Prandtl得到一个退化的抛物方程与椭圆方程耦合的方程组, 即著名的Prandtl方程. 至今为止, 许多学者建立了Prandtl边界层的数学理论. 在文献[19-20]中, Oleinik和Samokhin假设初始值$u_{0}$以及来流$u_{1}$满足如下条件

$\begin{matrix}\label{initial} u_{0}(x,y)>0,\quad u_{1}(t,y)>0, \quad \forall t\in[L], y\in[0,+\infty) \end{matrix}$

$\begin{matrix}\label{monotonic} \partial_{y}u_{0}(x,y)>0,\quad \partial_{y}u_{1}(t,y)>0, \quad \forall t\in[L], y\in[0,+\infty), \end{matrix}$

然后通过引入von Mises变换和Crocco变换分别得到了稳态和非稳态Prandtl方程的局部适定性. 最近, 在单调性假设条件下, 文献[1,17]通过引入一种新变量来克服导数损失的困难, 然后利用能量方法在加权Sobolev空间中建立二维非稳态Prandtl方程的局部适定性. 在文献[2]中, 作者利用Littlewood-Paley理论在各向异性Sobolev空间中得到了适定性, 有望为无粘极限问题提供一种新的可能途径. 当初始值是一些单调剪切流的小扰动时, 文献[35]用能量方法证明了在加权Sobolev空间中解的长时间适定性. 在单调性假设以及顺压条件下, 文献[33]利用粘性拆分的方法得到了弱解的全局存在性, 最近在[34]中又进一步利用双变量方法得到了弱解的唯一性和在常外流条件下弱解在区域内部的光滑性. 在无单调性假设时, 二维非稳态Prandtl方程在解析函数或者Gevrey函数框架下也有一部分学者进行了研究. 最早由Sammartino和Caflisch在文献[23-24]中利用Cauchy-Kowalevski理论, 在初始值满足一定的相容性条件以及关于$x,y$解析时得到了局部适定性. 最近, 文献[36]用能量的方法讨论了解析解的局部存在性, 更多关于在解析框架下的结果可以参见[11,22]及文中的参考文献. 关于解的爆破相关结果可以参见文献[6,12], 这说明在没有单调性假设时, 二维非稳态Prandtl方程在Sobolev空间中可能就不再适定. 目前为止, 关于三维边界层的数学理论还很少, 因为当三维Prandtl边界层中出现二次流时, 其非线性稳定性和不稳定性非常复杂, 这在物理上有比较多刻画[18]. 在文献[15]中, 作者通过构造三维非稳态Prandtl方程的一种特殊结构的解, 避免了有可能出现促使三维边界层不稳定的“二次流”, 进而利用能量方法得到了适定性, 更多关于三维边界层的研究可以参见[13-14,18]及参考文献. 最近关于MHD和热传导黏性流体边界层等更加复杂边界层的适定性也有一些讨论, 参见文献[16,30]及参考文献. 更多关于边界层的理论可以参见Schlichting的专著[25].

二维非稳态可压缩Prandtl方程(1.1)最早由Wang和Williams在文献[28]中提出. 关于方程(1.1)的适定性最近有一部分研究结果, 在文献[8]中, 作者利用Oleinik在文献[20]中提出的方法, 通过von Mises变换和Crocco变换分别研究了稳态和非稳态情形的局部适定性. 在文献[1,17]的基础上, 文献[7,29]利用能量方法在加权Sobolev空间中建立了局部适定性. 在单调性假设以及顺压条件下, 关于弱解的全局存在性可以参见文献[4].

边界层分离是物理上非常重要的一个现象, 对于稳态边界层而言, 倒流点就是分离点, 关于稳态边界层的分离问题已有一部分数学理论, 参见文献[3,5,27]. 对于非稳态边界层而言, 边界层分离是一个非常复杂的问题, 正常情况下倒流点就不再是分离点[26], 倒流点作为研究非稳态边界层分离的切入点意义重大, 关于二维非稳态Prandtl方程倒流问题最早由Wang和Zhu 在文献[31]提出, 他们利用极值原理和Lyapunov泛函方法分别证明了倒流点首次出现的位置和倒流点的存在性. 在逆压梯度条件的作用下, Oleinik单调性条件将可能会被破坏, 即有可能出现倒流点, 这时候二维非稳态Prandtl方程将可能不再适定. 最近他们在文献[32]中研究了二维不可压热传导黏性流体边界层的倒流问题, 在温度的影响下顺压也有可能产生倒流点. 关于其他边界层倒流问题的数学理论可以参见文献[9-10]. 到目前为止, 还没有关于二维可压缩非稳态Prandtl 方程倒流点的相关数学理论, 本文将在文献[31]基础上来研究可压缩边界层的倒流点存在性. 倒流点$(t_{*}, x_{*},0)$ 的定义如下

$\partial_{y}u(t_{*}, x_{*},0)=0, \quad\mbox{且}\ \partial_{y}u(t,x,0)>0, \quad\forall0<x<L, 0<t<t_{*}. $

因为(1.1)式是一个退化的抛物方程与椭圆方程耦合的方程组, 因此不能直接利用抛物型方程的极值原理, 所以需要在单调性假设(1.4)下, 利用Crocco变换

$\tau=t,\quad \xi=x,\quad \eta=\frac{u}{U},\quad\omega=\frac{\partial_{y}u}{U},$

将方程(1.1)变为在区域$\Omega^{*}_{T}=\left\{0\leq\tau\leq T, 0\leq\xi \leq L, 0\leq\eta<1\right\}$中的单个退化抛物方程

$\begin{matrix}\label{Crocco} \left\{\begin{array}{ll} \partial_{\tau}\omega+\eta U\partial_{\xi}\omega+A\partial_{\eta}\omega+B\omega= \frac{1}{\rho}\omega^{2}\partial_{\eta}^{2}\omega,\\[3mm] \omega\partial_{\eta}\omega|_{\eta=0}=\frac{\partial_{\xi}P}{U}, \quad \omega|_{\eta=1}=0,\\[3mm] \omega|_{\tau=0}=\omega_{0}=\frac{\partial_{y}u_{0}}{U},\quad \omega|_{\xi=0}=\omega_{1}=\frac{\partial_{y}u_{1}}{U}, \end{array}\right. \end{matrix}$

其中

$ A=(1-\eta)\frac{\partial_{t}U}{U}+(1-\eta^{2})\partial_{x}U,\quad B=\frac{\partial_{t}U}{U}+\eta \partial_{x}U -\frac{\partial_{t}\rho}{\rho}-\frac{\partial_{x}\rho}{\rho}\eta U. $

现在我们的主定理可以表述如下.

${\bf定理1.1}$ (1)假设$(\rho,U)$在边界上满足

$\begin{eqnarray*} (\rho, U)\in C^{1}([T]\times[L]),\quad \mbox{且}\ U(t,x)>0,\quad\forall t\in[T], x\in[L], \end{eqnarray*}$

若单调性条件(1.4)成立, 函数$(u, v)\in C^{2}(\Omega_{T})$是问题(1.1)的解, 则如果$\partial_{y} u(t,x,y)$ 的第一个零点在某一个时刻$t\in(0,T)$出现, 一定先出现在边界$\left\{y=0\right\}$上.

(2)此外, 若压力满足一致逆压梯度条件

$\begin{matrix}\label{comm} \partial_{x}P>0,\quad \forall t\in[T], x\in[L], \end{matrix}$

并且初始值$u_{0}(x,y)$满足

$\begin{matrix}\label{first} \int_{0}^{\infty}\int_{0}^{L}\frac{(L-x)^{\frac{3}{2}}\partial_{y}u_{0}}{\sqrt{(\partial_{y}u_{0})^{2}+ u_{0}^{2}}}{\rm d}x{\rm d}y\geq C^{*}, \end{matrix}$

这里$C^{*}$是依赖于$L, T, U, \rho $以及$\partial_{x}P$的正常数,则存在一个倒流点$(t_{*}, x_{*})\in(0,T)\times[L]$使得

$ \left\{\begin{array}{ll} \partial_{y}u(t_{*},x_{*},0)=0,\\ \partial_{y}u(t,x,y)>0, \quad\forall0<t<t_{*}, x\in[L],y\geq 0, \end{array}\right. $

并且有$\partial_{y}^{2}u(t_{*}, x_{*}, 0)\ne0.$

${\bf注1.1}$ 关于定理1.1有如下两点注释:

(1) 相对于经典的二维非稳态Prandtl方程而言, 可压流不再有$\partial_{x}u+\partial_{y}v=0$, 因此对(1.1)式进行Crocco变换时会带来一些新的项, 如(1.5)式中$B$的后两项. 这对第3节的证明会带来一定的困难, 例如会出现带有$\eta^{3}$的项$D_{4}$ (见第3节). 上述定理第一部分不需要满足一致逆压梯度条件, 这对文献[31]中经典Prandtl方程得到的结果有所提升.这是因为本文在证明$\omega$有上界的时候构造的函数$H(\tau, \xi, \eta)$与文献[31]构造的函数$f(\tau, \xi, \eta)=e^{-N\tau}\omega^{2}$不一样,具体可以参见引理2.1的证明.

(2) 此外, 定理第二部分说明即使对于可压缩边界层, 只要压力满足逆压梯度条件和初始值满足一定的增长条件, 就有可能发生倒流, 并且这里有$\partial_{y}^{2}u(t_{*}, x_{*}, 0)=\partial_{x}P(t_{*}, x_{*})\ne0$, 说明倒流点是非退化的. 因为我们考虑的是可压缩流, 所以最后我们给出的具体例子考虑了一致逆压梯度的大小对倒流点出现的影响. 这个实例表明逆压梯度越大时, 倒流点将会更早的出现, 这从物理上理解这一结果是合理的.

本文随后几节的安排如下: 在第2节中将利用极值原理证明如果倒流点出现, 那么一定先出现在边界. 然后在第3节将在(1.6)和(1.7)式的假设下, 通过构造Lyapunov泛函来证明倒流点的存在性. 最后在第4节将给出倒流点存在的实例.

2 倒流点首次出现的位置

本节将通过构造适当的函数, 然后利用抛物型方程的极值原理来证明倒流点如果出现, 那么一定先出现在边界.

${\bf定理2.1}$ 在定理1.1(1)的条件假设下, 假设$(u,v)\in C^{2}(\Omega_{T})$上问题(1.1)的局部经典解, 则如果$\partial_{y}u(t,x,y)=0$出现, 那么一定首次出现在边界$\left\{y=0\right\}$.

${\bf证}$ 利用反证法证明, 不妨设第一个倒流点是一个内点$(t_{0}, x_{0}, y_{0})(0<t_{0}<T, 0\leq x\leq L, 0<y_{0}<\infty$), 即有

$ \omega(\tau_{0}, \xi_{0}, \eta_{0})=\frac{\partial_{y}u(t_{0}, x_{0}, y_{0})}{U(t_{0}, x_{0})}=0, $

上式意味着在$(\tau_{0}, \xi_{0}, \eta_{0})$的某个远离边界$\left\{\eta=1\right\}$的邻域$\Omega_{0}$上有

$\begin{matrix}\label{com} \inf_{(\tau, \xi, \eta)\in\Omega_{0}}\omega(\tau, \xi, \eta)=0. \end{matrix}$

此外, 对于任意的$0\leq t< t_{0}$

$\begin{matrix} \omega(\tau, \xi, \eta)>0, \mbox{在}\left\{(\tau, \xi, \eta)|0\leq \tau<\tau_{0}, 0\leq\xi\leq L, 0\leq \eta <1\right\}, \end{matrix}$

此时Crocco变换是有意义的.下面利用极值原理得到与(2.1)矛盾, 从而证明定理.

设函数$\phi$是一个足够光滑的函数, 并且满足

$\begin{eqnarray*} \phi(\eta)=\left\{\begin{array}{ll} 1, &\eta=0,\\ 1-\eta, &\eta \in[\eta_{1},1], \end{array}\right. \end{eqnarray*}$

这里$\eta_{1}\in(0,1)$, 并且当$\eta\in[\eta_{1}]$时有$\phi(\eta)>0$. 为了能用极值原理, 在区域$\Omega_{\varepsilon}=\left\{(\tau, \xi, \eta) | 0\leq \tau<\tau_{0}, 0\leq\xi\leq L, 0\leq \eta <1\right\}$中定义函数$F(\tau, \xi, \eta)$

$\begin{eqnarray*} F(\tau, \xi, \eta)=\omega(\tau, \xi, \eta)-\varepsilon\phi(\eta)e^{-M\tau}, \end{eqnarray*}$

此时$F$$\Omega_{\varepsilon}$中满足的方程为

$\begin{matrix}\label{F} \left\{\begin{array}{ll} \partial_{\tau}F+\eta U\partial_{\xi}F+A\partial_{\eta}F+BF=\frac{1}{\rho} \omega^{2}\partial_{\eta}^{2}F+\varepsilon{\cal F},\\ F|_{\eta=0}=\omega(\tau, \xi, 0)-\varepsilon e^{-M\tau},\quad F|_{\eta=1}=0,\\ F|_{\tau=0}=\omega_{0}-\varepsilon\phi(\eta),\quad F|_{\xi=0}=\omega_{1}-\varepsilon\phi(\eta)e^{-M\tau}, \end{array}\right. \end{matrix}$

这里

${\cal F}=M\phi(\eta)e^{-M\tau}-A\partial_{\eta}\phi(\eta)e^{-M\tau}-B\phi(\eta)e^{-M\tau}+\frac{1}{\rho}\omega^{2}\partial_{\eta}^{2}\phi(\eta)e^{-M\tau}.$

下面来证明只要选取合适的$M$, 就能保证${\cal F}\geq0$$\Omega_{\varepsilon}$ 中成立.

(1)若$\eta\in[\eta_{1}]$, 显然$\phi(\eta)$ 是有下界的, 结合引理2.1可知$\omega$有界,因此只需要取$M$充分的大, 就有

${\cal F}(\tau,\xi,\eta)=\left(M\phi(\eta)-A\partial_{\eta}\phi(\eta)-B\phi(\eta) +\frac{1}{\rho}\omega^{2}\partial_{\eta}^{2}\phi(\eta)\right)e^{-M\tau}\geq0, $

(2)若$\eta\in(\eta_{2},1]$, 将$A$重写为$A=(1-\eta)\left((1+\eta)\partial_{x}U+\frac{\partial_{\tau}U}{U}\right)$,结合$\partial_{\eta}^{2}\phi(\eta)=0$, 因此有

$\begin{eqnarray*} {\cal F}(\tau, \xi, \eta)&=&\bigg[M+(1+\eta)\partial_{\xi}U+\frac{\partial_{\tau}U}{U}-B\bigg]\phi(\eta)e^{-M\tau}\\ &=&\left(M+\partial_{\xi}U+\frac{\partial_{\tau}\rho}{\rho}+\frac{\partial_{\xi}\rho}{\rho}\eta U\right)(1-\eta)e^{-M\tau},\end{eqnarray*}$

只要取$M$充分的大, 就能保证$ {\cal F}(\tau,\xi,\eta)\geq0.$

因此结合上面的讨论, 只要取合适的$M$就能使得${\cal F}(\tau,\xi,\eta)\geq0.$ 接下来固定$M$, 为了对问题(2.3)使用抛物型方程极值原理, 令$G=e^{-N\tau}F(\tau, \xi, \eta)$, 并且取$N+B\geq0$, 则$G$$\Omega_{\varepsilon}$中满足的方程为

$\begin{matrix}\label{G} \left\{\begin{array}{ll} \partial_{\tau}G+\eta U\partial_{\xi}G+A\partial_{\eta}G+(N+B)G=\frac{1}{\rho}\omega^{2}\partial_{\eta}^{2}G+\varepsilon{\cal F}e^{-N\tau},\\[2mm] G|_{\eta=0}=[\omega(\tau, \xi, 0)-\varepsilon e^{-M\tau}]e^{-N\tau},\quad G|_{\eta=1}=0,\\ G|_{\tau=0}=\omega_{0}-\varepsilon\phi(\eta),\quad G|_{\xi=0}=[\omega_{1}-\varepsilon\phi(\eta)e^{-M\tau}]e^{-N\tau}, \end{array}\right. \end{matrix}$

取充分小的$\varepsilon>0$, 使得

$\begin{eqnarray*} & &G|_{\tau=0}=\omega_{0}-\varepsilon\phi(\eta)>0,\\ & &G|_{\xi=0}=[\omega_{1}-\varepsilon\phi(\eta)e^{-M\tau}]e^{-N\tau}>0,\\ & &G|_{\eta=0}=[\omega(\tau, \xi, 0)-\varepsilon e^{-M\tau}]e^{-N\tau}>0, \end{eqnarray*}$

此外, $G|_{\eta=0}=\omega(\tau, \xi, 0)e^{-N\tau}>0$是显然的. 由极值原理可知$G$ 的最小值只能在边界$\left\{\tau=0\right\}, \left\{\xi=0\right\}, \left\{\eta=0\right\}$$\left\{\eta=1\right\}$取到, 因此对于任意$(\tau, \xi, \eta)\in\Omega_{\varepsilon}$都有

$G(\tau, \xi, \eta)=e^{-N\tau} F(\tau, \xi, \eta)=\left(\omega(\tau, \xi, \eta)-\varepsilon\phi(\eta)e^{-M\tau}\right)e^{-N\tau}\geq0, $

进而有$\omega(\tau, \xi, \eta)\geq\varepsilon\phi(\eta)e^{-M\tau}, $上式意味着

$\begin{matrix} \inf_{(\tau, \xi, \eta)\in\Omega_{0}}\omega(\tau, \xi, \eta)\geq\inf_{(\tau, \xi, \eta)\in\Omega_{0}}\varepsilon\phi(\eta)e^{-M\tau}>0, \end{matrix}$

这与(2.1)式产生了矛盾, 因此若有倒流点出现, 则一定先出现在边界$\left\{y=0\right\}$.

接下来, 我们通过下面这个引理证明$\omega$有上界.

${\bf引理2.1}$ 设问题(1.5)在$0\leq\tau<t_{0}$上有一个解$\omega(\tau,\xi,\eta)\in C^{1}(\Omega^{*}_{t_{0}})$, 在定理2.1的条件假设下, 存在一个与$U, \rho, \partial_{\xi}U, \partial_{\xi}\rho, \partial_{\tau}U, \partial_{\tau}\rho$有关的常数$N$以及$R$满足

$\begin{matrix}\label{R} R\geq\max\left\{\lambda^{-1}, \|\frac{\omega_{0}}{1-\eta}\|_{L^{\infty}([L]\times[T]\times[0,1 ))}\right\}, \end{matrix} $

其中$\lambda$是一个正常数, 使得在区域$\Omega^{*}_{t_{0}}$中满足

$\begin{matrix}\label{wbound} \omega(\tau,\xi, \eta)\leq R(1-\eta)e^{N\tau}. \end{matrix}$

${\bf证}$ 在定理2.1证明时我们假设内点$(t_{0}, \xi_{0}, \eta_{0})$是第一个倒流点, 那么也就意味着在边界$[t_{0}]\times [L]$上有$\partial_{y}u(t,x,0)>0$, 其等价于存在一个正常数$\lambda$使得在区域$\Omega^{*}_{t_{0}}$

$\begin{matrix}\label{ww} \omega(\tau, \xi, 0)\in[\lambda, \lambda^{-1}]. \end{matrix}$

为证明(2.7), 我们定义函数$H(\tau, \xi, \eta)$

$ H(\tau, \xi, \eta)=\omega e^{-N\tau}-R(1-\eta), $

$H(\tau, \xi, \eta)$在区域$\Omega^{*}_{t_{0}}$ 满足的方程为

$\begin{matrix} \left\{ \begin{array}{ll} &\partial_{\tau}H+\eta U\partial_{\xi}H+A\partial_{\eta}H+(B+N)\omega=\frac{1}{\rho}\omega^{2}\partial_{\eta}^{2}H-{\cal H},\\ &H|_{\eta=0}=\omega(\tau, \xi, 0)e^{-N\tau}-R, \quad H|_{\eta=1}=0,\\ &H|_{\tau=0}=\omega_{0}-R(1-\eta), \quad H|_{\xi=0}=\omega_{1}e^{-N\tau}-R(1-\eta), \end{array} \right. \end{matrix}$

其中

${\cal H}=R(1-\eta)(N-\partial_{\xi}U-\frac{\partial_{\tau}\rho}{\rho}-\frac{\partial_{\xi}\rho}{\rho}\eta U). $

$N$充分的大使得

$N\geq\max\left\{\max_{\Omega_{T}^{*}}|B|,\max_{\Omega_{T}^{*}}\left|\partial_{\xi}U+\frac{\partial_{\tau}\rho}{\rho}+\frac{\partial_{\xi}\rho}{\rho}\eta U\right|\right\}, $

此外结合(2.6)和(2.8)式易知

$\begin{eqnarray*} &&H|_{\tau=0}=\omega_{0}-R(1-\eta)\leq0, \\ &&H|_{\xi=0}=\omega_{1}e^{-N\tau}-R(1-\eta)\leq0,\\ & &H|_{\eta=0}=\omega(\tau, \xi, 0)e^{-N\tau}-R\leq\omega(\tau, \xi, 0)-R\leq0. \end{eqnarray*}$

因此由极值原理可知, $H(\tau, \xi, \eta)$在区域$\Omega^{*}_{t_{0}}$的最小值只能在边界取到, 即有

$\omega(\tau, \xi, \eta)\leq R(1-\eta)e^{N\tau}, $

这就证明了$\omega(\tau, \xi, \eta)$有上界.

3 倒流点的存在性

本节将利用反证法以及Lyapunov泛函方法来证明倒流点的存在性. 由定理2.1可知, 如果$\partial_{y}u(t,x,y)=0$出现, 则一定不可能先出现在内部. 不妨假设$\partial_{y}u>0$在区域$\Omega_{T}$上是恒成立的, 则此时, Crocco变换在区域$\Omega_{T}$上是有意义的, 并且进一步可知$\omega=\frac{\partial_{y}u}{U}>0$.

$W(\tau, \xi, \eta)=(\omega^{2}+\eta^{2})^{-\frac{1}{2}}$, 则$W$$\Omega^{*}_{T}=\left\{0\leq\tau\leq T, 0\leq\xi \leq L, 0\leq\eta<1\right\}$ 上满足的方程为

$\begin{matrix}\label{W} \left\{\begin{array}{ll} \partial_{\tau}W+\eta U\partial_{\xi}W+A\partial_{\eta}W= BW-\frac{\omega^{3}}{\rho(\omega^{2}+\eta^{2})^{\frac{3}{2}}}\partial_{\eta}^{2}\omega+\left(\eta\frac{\partial_{\xi}P}{\rho U}+\frac{\partial_{\tau}\rho}{\rho}\eta^{2}+\frac{\partial_{\xi}\rho}{\rho}\eta^{3} U\right)W^{3},\\[3mm] \left(\partial_{\eta}W+\frac{\partial_{\xi}P}{U}W^{3}\right)|_{\eta=0}=0,\quad W|_{\eta=1}=1,\\[2mm] W|_{\tau=0}=(\omega_{0}^{2}+\eta^{2})^{-\frac{1}{2}},\quad W|_{\xi=0}=(\omega_{1}^{2}+\eta^{2})^{-\frac{1}{2}}. \end{array}\right. \end{matrix}$

接下来, 我们通过证明下面这个引理来证明主定理的第二部分.

${\bf引理3.1}$ 在定理1.1$(2)$的条件假设下, 存在一点$(\tau_{*},\xi_{*})\in(0,T)\times[L]$使得

$ W(\tau_{*},\xi_{*},0)=+\infty. $

${\bf证}$ 与文献[31]类似定义Lyapunov泛函为

${\cal G}(\tau)=\int_{\Omega}W(\tau, \xi, \eta)\varphi(\xi){\rm d}\xi {\rm d}\eta, $

其中$\Omega=[L]_{\xi}\times[0,1)_{\eta}, \varphi(\xi)=(L-\xi)^{\frac{3}{2}}$, 结合(3.1)式有

$\begin{matrix}\label{GG} \frac{\rm d}{{\rm d}\tau}{\cal G}&=&-\int_{\Omega}\eta U \partial_{\xi}W\varphi {\rm d}\xi {\rm d}\eta-\int_{\Omega}(A\partial_{\eta}W-BW)\varphi {\rm d}\xi {\rm d}\eta+\int_{\Omega}\eta\frac{\partial_{\xi}P}{\rho U}W^{3}\varphi {\rm d}\xi {\rm d}\eta\\ &&+\int_{\Omega}\eta^{2}\left(\frac{\partial_{\tau}\rho}{\rho}+\frac{\partial_{\xi}\rho}{\rho}\eta U\right)W^{3}\varphi {\rm d}\xi {\rm d}\eta-\int_{\Omega}\frac{\omega^{3}}{\rho (\omega^{2}+\eta^{2})^{\frac{3}{2}}}\partial_{\eta}^{2}\omega\varphi {\rm d}\xi {\rm d}\eta\\ &=&\sum_{i=1}^{5}D_{i}. \end{matrix}$

(1)对$D_{1}$直接分部积分并利用Young不等式有

$\begin{eqnarray*} D_{1}&=&-\int_{\Omega}\eta U \partial_{\xi}W\varphi {\rm d}\xi {\rm d}\eta\\ &=&\int_{\Omega}\eta\partial_{\xi} U W\varphi {\rm d}\xi {\rm d}\eta+\int_{\Omega}\eta U W\partial_{\xi}\varphi {\rm d}\xi {\rm d}\eta+L^{\frac{3}{2}}U(\tau,0)\int_{0}^{1}\frac{\eta}{(\omega_{1}^{2}+\eta^{2})^{\frac{1}{2}}}{\rm d}\eta\\ &=&\int_{\Omega}\eta\partial_{\xi} U W\varphi {\rm d}\xi {\rm d}\eta-\frac{3}{2}\int_{\Omega}\eta U W\varphi^{\frac{1}{3}} {\rm d}\xi {\rm d}\eta+C_{0}(\tau)\\ &\geq&\int_{\Omega}\eta\partial_{\xi} U W\varphi {\rm d}\xi {\rm d}\eta-\int_{\Omega}\eta\left[\frac{\rho U^{4}}{\partial_{\xi}P}\right]^{\frac{1}{2}} {\rm d}\xi {\rm d}\eta-\frac{1}{2}\int_{\Omega}\eta\frac{\partial_{\xi}P}{\rho U}W^{3}\varphi {\rm d}\xi {\rm d}\eta+C_{0}(\tau)\\ &\geq &\int_{\Omega}\eta\partial_{\xi} U W\varphi {\rm d}\xi {\rm d}\eta-\frac{1}{2}\int_{\Omega}\eta\frac{\partial_{\xi}P}{\rho U}W^{3}\varphi {\rm d}\xi {\rm d}\eta+ C_{0}(\tau)-C_{1}(\tau), \end{eqnarray*}$

这里

$C_{0}(\tau)=L^{\frac{3}{2}}U(\tau,0)\int_{0}^{1}\frac{\eta}{(\omega_{1}^{2}+\eta^{2})^{\frac{1}{2}}}{\rm d}\eta, \quad C_{1}(\tau)=\int_{\Omega}\eta\left[\frac{\rho U^{4}}{\partial_{\xi}P}\right]^{\frac{1}{2}} {\rm d}\xi {\rm d}\eta.$

(2)由于

$A=(1-\eta^{2})\partial_{\xi}U+(1-\eta)\frac{\partial_{\tau}U}{U},\quad B=\eta\partial_{\xi}U+\frac{\partial_{\tau}U}{U}-\left(\frac{\partial_{\tau}\rho}{\rho}+\frac{\partial_{\xi}\rho}{\rho}\eta U\right),$

则对$D_{2}$分部积分有

$\begin{eqnarray*} D_{2}&=&-\int_{\Omega}(A\partial_{\eta}W-BW)\varphi {\rm d}\xi {\rm d}\eta\\ &=&\int_{\Omega}\partial_{\eta}AW\varphi {\rm d}\xi {\rm d}\eta-\int_{0}^{L}\frac{\partial_{\xi}P}{U}W(\tau, \xi, 0)\frac{\varphi}{\rho} {\rm d}\xi+\int_{\Omega}BW\varphi {\rm d}\xi {\rm d}\eta \\ &=&-\int_{\Omega}\eta\partial_{\xi}U W\varphi {\rm d}\xi {\rm d}\eta-\int_{0}^{L}\frac{\partial_{\xi}P}{U}W(\tau, \xi, 0)\frac{\varphi}{\rho} {\rm d}\xi- \int_{\Omega}\left(\frac{\partial_{\tau}\rho}{\rho}+\frac{\partial_{\xi}\rho}{\rho}\eta U\right)W\varphi {\rm d}\xi {\rm d}\eta \\ &\geq&-\int_{\Omega}\eta\partial_{\xi}U W\varphi {\rm d}\xi {\rm d}\eta-\int_{0}^{L}\frac{\partial_{\xi}P}{U}W(\tau, \xi, 0)\frac{\varphi}{\rho} {\rm d}\xi-C_{2}(\tau){\cal G}, \end{eqnarray*}$

这里利用了(1.2)式, 其中

$C_{2}(\tau)=\Big\|\frac{\partial_{\tau}\rho}{\rho}+\frac{\partial_{\xi}\rho}{\rho}\eta U\Big\|_{L^{\infty}(\Omega)}. $

(3)对于$D_{3}$, 利用Hölder不等式有

$\begin{eqnarray*} D_{3}&=&\int_{\Omega}\eta\frac{\partial_{\xi}P}{\rho U}W^{3}\varphi {\rm d}\xi {\rm d}\eta\\ &=&\frac{1}{2}\int_{\Omega}\eta\frac{\partial_{\xi}P}{\rho U}W^{3}\varphi {\rm d}\xi {\rm d}\eta+\frac{1}{2}\int_{\Omega}\eta\frac{\partial_{\xi}P}{\rho U}W^{3}\varphi {\rm d}\xi {\rm d}\eta\\ &\geq& \frac{1}{2}\int_{\Omega}\eta\frac{\partial_{\xi}P}{\rho U}W^{3}\varphi {\rm d}\xi {\rm d}\eta+\frac{1}{2} \bigg(\int_{\Omega}\left[\eta\frac{\partial_{\xi}P}{\rho U}\varphi^{-2}\right]^{-\frac{1}{2}}{\rm d}\xi {\rm d} \eta\bigg)^{-2}\left(\int_{\Omega}W\varphi {\rm d}\xi {\rm d}\eta \right)^{3}\\ &\geq &\frac{1}{2}\int_{\Omega}\eta\frac{\partial_{\xi}P}{\rho U}W^{3}\varphi {\rm d}\xi {\rm d}\eta+C_{3}(\tau){\cal G}^{3}, \end{eqnarray*} $

其中

$C_{3}(\tau)=\frac{1}{8}\bigg(\int_{0}^{L}\left[\frac{\rho U}{\partial_{\xi}P}\right]^{\frac{1}{2}}\varphi {\rm d}\xi \bigg)^{-2}.$

(4)对于$D_{4}$, 回顾$W=(\omega^{2}+\eta^{2})^{-\frac{1}{2}}$, 直接放缩可得

$\begin{eqnarray*} D_{4}&=&\int_{\Omega}\eta^{2}\left(\frac{\partial_{\tau}\rho}{\rho}+\frac{\partial_{\xi}\rho}{\rho}\eta U\right) W^{3}\varphi {\rm d}\xi {\rm d}\eta\\ &=&\int_{\Omega}\frac{\eta^{2}}{\omega^{2}+\eta^{2}}\left(\frac{\partial_{\tau}\rho}{\rho}+\frac{\partial_{\xi}\rho}{\rho}\eta U \right) W\varphi {\rm d}\xi {\rm d}\eta\\ &\geq&-C_{2}(\tau){\cal G}. \end{eqnarray*} $

(5)利用分部积分和Young不等式有

$\begin{eqnarray*} D_{5}&=&-\int_{\Omega}\frac{\omega^{3}}{ (\omega^{2}+\eta^{2})^{\frac{3}{2}}}\partial_{\eta}^{2}\omega\frac{\varphi}{\rho} {\rm d}\xi {\rm d}\eta\\ &=&3\int_{\Omega}\frac{\omega^{2}}{(\omega^{2}+\eta^{2})^{\frac{3}{2}}}(\partial_{\eta}\omega)^{2}\frac{\varphi}{\rho}{\rm d}\xi {\rm d}\eta-3\int_{\Omega}\frac{\omega^{4}}{(\omega^{2}+\eta^{2})^{\frac{5}{2}}}(\partial_{\eta}\omega)^{2}\frac{\varphi}{\rho}{\rm d}\xi {\rm d}\eta\\ && -3\int_{\Omega}\frac{\eta\omega^{3}}{(\omega^{2}+\eta^{2})^{\frac{5}{2}}}\partial_{\eta}\omega\frac{\varphi}{\rho}{\rm d}\xi {\rm d}\eta-\int_{0}^{L}\partial_{\eta} \omega\frac{\varphi}{\rho}{\rm d}\xi\\ &=&3\int_{\Omega}\frac{\eta^{2}\omega^{2}(\partial_{\eta}\omega)^{2}-\eta\omega^{3}\partial_{\eta}\omega}{(\omega^{2}+\eta^{2})^{\frac{5}{2}}}\frac{\varphi}{\rho}{\rm d}\xi {\rm d}\eta+\int_{0}^{L}\frac{\omega\partial_{\eta}\omega}{\sqrt{\omega^{2}+\eta^{2}}}\frac{\varphi}{\rho}{\rm d}\xi|_{\eta=0}\\ &\geq&\frac{3}{2}\int_{\Omega}\frac{\eta^{2}\omega^{2}(\partial_{\eta}\omega)^{2}-\omega^{4}}{(\omega^{2}+\eta^{2})^{\frac{5}{2}}}\frac{\varphi}{\rho}{\rm d}\xi {\rm d}\eta+\int_{0}^{L}\frac{\omega\partial_{\eta}\omega}{\sqrt{\omega^{2}+\eta^{2}}}\frac{\varphi}{\rho}{\rm d}\xi|_{\eta=0} \\ &\geq &-\frac{3}{2}\int_{\Omega}\frac{\omega^{4}}{(\omega^{2}+\eta^{2})^{\frac{5}{2}}}\frac{\varphi}{\rho}d \xi {\rm d}\eta+\int_{0}^{L}\frac{\partial_{\xi}P}{U}W(\tau, \xi, 0)\frac{\varphi}{\rho}{\rm d}\xi\\ &\geq& -\frac{3}{2\rho_{0}}{\cal G}+\int_{0}^{L}\frac{\partial_{\xi}P}{U}W(\tau, \xi, 0)\frac{\varphi}{\rho}{\rm d}\xi, \end{eqnarray*}$

这里利用了边界条件$\omega\partial_{\eta}\omega|_{\eta=0}=\frac{\partial_{\xi}P}{U}$.

$D_{1}-D_{5}$的估计, 结合(3.1)式可得

$\begin{matrix}\label{fin} \frac{\rm d}{{\rm d}\tau}{\cal G}\geq C_{3}(\tau){\cal G}^{3}-(2C_{2}(\tau)+\frac{3}{2\rho_{0}}){\cal G}-C_{4}(\tau), \end{matrix}$

这里$C_{2}(\tau), C_{3}(\tau), C_{4}(\tau)=C_{1}(\tau)-C_{0}(\tau)$是在$[T]$ 关于$\tau$的函数.

由(3.3)式可知, 只要${\cal G}$的初始值合适的大, 即(1.7)成立, 那么${\cal G}(\tau)$就会在$(0,T)$内发生爆破. 这一结果表明, 存在一点$(t_{*},\xi_{*},0)\in(0,T)\times[L]$, 使得$W(t_{*},\xi_{*},0)=+\infty$, 因此我们有$\omega(t_{*},\xi_{*},0)=0$, 这就完成了主定理的第二部分的证明.

4 倒流点存在的实际例子

本节我们将给出二维可压缩Prandtl方程倒流点存在的实际例子. 这个例子将说明逆压梯度越大, 倒流点越早出现.

${\bf例4.1}$ 假设$P(t,x)=\rho(t,x)$, $a,b$是两个正常数, 并且满足$a<b$, 固定$L=a$, 设Euler外流在$x\in[a]$上由下式给出 $U(x)=\kappa(b-x), $其中$\kappa$是一个待定的常数, 则给定适当的初始值, 那么将会有一个倒流点出现.

${\bf证}$ 由(1.2)式可知$\frac{\partial_{x}P}{\rho}=\kappa^{2}(b-x), $ 正如文献[31]一样, 选取任意$\tilde{u}_{0}(x,y)$满足

$ \left\{\begin{array}{ll}\tilde{u}_{0}(x,0)=0,\\ \partial_{y}\tilde{u}_{0}(x,y)>0,\quad \forall y\in[0,+\infty),\\ \lim\limits_{y\to+\infty}\tilde{u}_{0}(x,y)=b-x, \end{array} \right. $

再取(1.1)式的初始值为$u_{0}(x,y)=\kappa\tilde{u}_{0}(x,y)$,并且假设$u_{0}$满足问题(1.1)至少二阶相容性条件, 且存在正常数$C_{0}$使得

$ \int_{0}^{+\infty}\frac{\partial_{y}u_{0}}{\sqrt{(\partial_{y}u_{0})^{2}+u_{0}^{2}}}{\rm d}y\geq C_{0}, $

经计算可知

$\begin{eqnarray*} C_{0}(\tau)&=&L^{\frac{3}{2}}U(\tau,0)\int_{0}^{1}\frac{\eta}{(\omega^{2}+\eta^{2})^{\frac{1}{2}}}{\rm d}\eta=2\int_{0}^{1}\frac{\eta}{(\omega^{2}+\eta^{2})^{\frac{1}{2}}}{\rm d}\eta>0, \\ C_{1}(\tau)&=&\int_{\Omega}\eta\left[\frac{\rho U^{4}}{\partial_{\xi}P}\right]^{\frac{1}{2}} {\rm d}\xi {\rm d}\eta=\frac{1}{2}\int_{0}^{a}(b-x)^{\frac{3}{2}}{\rm d}x=\frac{b^{\frac{5}{2}}-(b-a)^{\frac{5}{2}}}{5}\kappa,\\ C_{2}(\tau)&=&\|\frac{\partial_{\tau}\rho}{\rho}+\frac{\partial_{\xi}\rho}{\rho}\eta U\|_{L^{\infty}(\Omega)}=\|\frac{\partial_{\xi}\rho}{\rho}\eta U\|_{L^{\infty}(\Omega)}\leq b^{2}, \\ C_{3}(\tau)&=&\frac{1}{2}\left(2\int_{0}^{L}\left[\frac{\rho U}{\partial_{\xi}P}\right]^{\frac{1}{2}}\varphi {\rm d}\xi \right)^{-2}=\frac{1}{8}\left(\int_{0}^{a}(a-x)^{\frac{3}{2}}{\rm d}x\right)^{-2}=\frac{25}{32a^{5}}\kappa, \end{eqnarray*} $

所以进一步可知${\cal G}(\tau)$满足如下不等式

$\begin{matrix}\label{G2} \frac{\rm d}{{\rm d}\tau}{\cal G}\geq \frac{25}{32a^{5}}\kappa{\cal G}^{3}-(2b^{2}+\frac{3}{2\rho_{0}}){\cal G}-\frac{b^{\frac{5}{2}}-(b-a)^{\frac{5}{2}}}{5}\kappa, \end{matrix}$

由上面这个不等式可知, 存在一个正常数$C^{*}$, 只要${\cal G}(0)\geq C^{*}$, ${\cal G}(\tau)$ 就会在有限时间内爆破. 此外, 显然有

$ {\cal G}(0)=\int_{0}^{+\infty}\int_{0}^{a}\frac{(a-x)^{\frac{3}{2}}\partial_{y}u_{0}}{\sqrt{(\partial_{y}u_{0})^{2}+u_{0}^{2}}}{\rm d}x{\rm d}y\geq C_{0}\int_{0}^{a}(a-x)^{\frac{3}{2}}{\rm d}x= \frac{2}{5}C_{0}a^{\frac{5}{2}}=M_{0}, $

因此只需要取$M_{0}\geq C^{*}$, 结合定理1.1(2)可知, 此时二维非稳态可压缩Prandtl方程将会有一个倒流点出现, 并且由不等式(4.1)进一步可知$\kappa$越大${\cal G}$发生爆破的时间越早, 也就意味着逆压梯度越大时, 倒流点将会更早的出现.

参考文献

Alexandre R, Wang Y, Xu C, et al.

Well-posedness of the Prandtl equation in Sobolev spaces

J Amer Math Soc, 2015, 28(3): 745-784

DOI:10.1090/jams/2015-28-03      URL     [本文引用: 2]

Chen D, Wang Y, Zhang Z.

Well-posedness of the Prandtl equation with monotonicity in Sobolev spaces

J Differential Equations, 2018, 264(9): 5870-5893

DOI:10.1016/j.jde.2018.01.024      URL     [本文引用: 1]

Dalibard A L, Masmoudi N.

Separation for the stationary Prandtl equation

Publ Math Inst Hautes Etudes Sci, 2019, 130: 187-297

DOI:10.1007/s10240-019-00110-z      [本文引用: 1]

Ding M, Gong S.

Global existence of weak solution to the compressible Prandtl equations

J Math Fluid Mech, 2017, 19(2): 239-254

DOI:10.1007/s00021-016-0274-5      URL     [本文引用: 1]

E W N.

Boundary layer theory and the zero-viscosity limit of the Navier-Stokes equation

Acta Math Sin (Engl Ser), 2000, 16(2): 207-218

DOI:10.1007/s101140000034      URL     [本文引用: 1]

E W N, Engquist B.

Blowup of solutions of the unsteady Prandtl's equation

Comm Pure Appl Math, 1997, 50: 1287-1293

DOI:10.1002/(ISSN)1097-0312      URL     [本文引用: 1]

Fan L, Ruan L, Yang A.

Local well-posedness of solutions to the boundary layer equations for 2D compressible flow

J Math Anal Appl, 2021, 493(2): 124565

DOI:10.1016/j.jmaa.2020.124565      URL     [本文引用: 1]

Gong S, Guo Y, Wang Y.

Boundary layer problems for the two-dimensional compressible Navier-Stokes equations

Anal Appl (Singap), 2016, 14(1): 1-37

[本文引用: 1]

Gong S, Wang X.

On a global weak solution and back flow of the mixed Prandtl-Hartmann boundary layer problem

J Math Fluid Mech, 2021, 23(11): 1-16

DOI:10.1007/s00021-020-00542-2      [本文引用: 1]

Gong S, Wang X, Wang Y.

Stability and back flow of boundary layers for wind-driven oceanic current

Commun Math Sci, 2020, 18(3): 593-612

DOI:10.4310/CMS.2020.v18.n3.a1      URL     [本文引用: 1]

Kukavica I, Vicol V.

On the local existence of analytic solutions to the Prandtl boundary layer equations

Commun Math Sci, 2013, 11(1): 269-292

DOI:10.4310/CMS.2013.v11.n1.a8      URL     [本文引用: 1]

Kukavica I, Vicol V, Wang F.

The van Dommelen and Shen singularity in the Prandtl equations

Adv Math, 2017, 307: 288-311

DOI:10.1016/j.aim.2016.11.013      URL     [本文引用: 1]

Li W, Masmoudi N, Yang T.

Well-Posedness in Gevrey Function Space for 3D Prandtl Equations without structural assumption

Comm Pure Appl Math, 2022, 75(8): 1755-1797

DOI:10.1002/cpa.v75.8      URL     [本文引用: 1]

Liu C, Wang Y, Yang T.

On the ill-posedness of the Prandtl equations in three space dimensions

Arch Rational Mech Anal, 2016, 220: 83-108

DOI:10.1007/s00205-015-0927-1      URL     [本文引用: 1]

Liu C, Wang Y, Yang T.

A well-posedness theory for the Prandtl equations in three space variables

Adv Math, 2017, 308: 1074-1126

DOI:10.1016/j.aim.2016.12.025      URL     [本文引用: 1]

Liu C, Xie F, Yang T.

MHD boundary Layers theory in Sobolev spaces without monotonicity I: Well-Posedness theory

Comm Pure Appl Math, 2019, 72(1): 63-121

DOI:10.1002/cpa.v72.1      URL     [本文引用: 1]

Masmoudi N, Wong T K.

Local-in-time existence and uniqueness of solutions to the Prandtl equations by energy methods

Comm Pure Appl Math, 2015, 68(10): 1683-1741

DOI:10.1002/cpa.v68.10      URL     [本文引用: 2]

Moore F K.

Three-dimensional boundary layer theory

Adv Appl Mech, 1956, 4: 159-228

[本文引用: 2]

Oleinik O A.

On the system of Prandtl equations in boundary-layer theory

Dokl Akad Nauk SSSR, 1963, 150: 28-31

[本文引用: 1]

Oleinik O A, Samokhin V N.

Mathematical models in boundary layer theory

Routledge, 2018

[本文引用: 2]

Prandtl L.

Über Flüssigkeitsbewegung bei sehr kleiner Reibung

Verhandl. III, Internat Math-Kong, Heidelberg, Teubner, Leipzig, 1904, 452: 575-584

[本文引用: 1]

Paicu M, Zhang P.

Global existence and the decay of solutions to the Prandtl system with small analytic data

Arch Ration Mech Anal, 2021, 241(1): 403-446

DOI:10.1007/s00205-021-01654-3      [本文引用: 1]

Sammartino M, Caflisch R E.

Zero viscosity limit for analytic solutions of the Navier-Stokes equation on a half-space. I. Existence for Euler and Prandtl equations

Comm Math Phys, 1998, 192(2): 433-461

DOI:10.1007/s002200050304      URL     [本文引用: 1]

Sammartino M, Caflisch R E.

Zero viscosity limit for analytic solutions of the Navier-Stokes equation on a half-space. II. Construction of the Navier-Stokes solution

Comm Math Phys, 1998, 192(2): 463-491

DOI:10.1007/s002200050305      URL     [本文引用: 1]

Schlichting H, Gersten K. Boundary-Layer Theory, Enlarged Edition. New York: Springer-Verlag, 2000

[本文引用: 1]

Sears W R, Telionis D P.

Boundary-layer separation in unsteady flow

SIAM J Math Anal, 1975, 28(1): 215-235

[本文引用: 1]

Shen W, Wang Y, Zhang Z.

Boundary layer separation and local behavior for the steady Prandtl equation

Adv Math, 2021, 389: 107896

DOI:10.1016/j.aim.2021.107896      URL     [本文引用: 1]

Wang Y, Williams M.

The inviscid limit and stability of characteristic boundary layers for the compressible Navier-Stokes equations with Navier-friction boundary conditions

Ann Inst Fourier (Grenoble), 2013, 62(6): 2257-2314

DOI:10.5802/aif.2749      URL     [本文引用: 1]

Wang Y, Xie F, Yang T.

Local well-posedness of Prandtl equations for compressible flow in two space variables

SIAM J Math Anal, 2015, 47(1): 321-346

DOI:10.1137/140978466      URL     [本文引用: 1]

Wang Y, Zhu S.

Well-posedness of thermal boundary layer equation in two-dimensional incompressible heat conducting flow with analytic datum

Math Methods Appl Sci, 2020, 43(7): 4683-4716

[本文引用: 1]

Wang Y, Zhu S.

Back flow of the two-dimensional unsteady Prandtl boundary layer under an adverse pressure gradient

SIAM J Math Anal, 2020, 52(1): 954-966

DOI:10.1137/19M1270355      URL     [本文引用: 6]

Wang Y, Zhu S.

On back flow of boundary layers in two-dimensional unsteady incompressible heat conducting flow

J Math Phys, 2022, 63(8): 081504

DOI:10.1063/5.0088618      URL     [本文引用: 1]

In this paper, we study the back-flow problem of boundary layers in two-dimensional unsteady incompressible heat conducting flow. For a kind of monotonic initial and incoming flow, we prove that the first critical point of the tangential velocity profile with respect to the normal variable, if exists, must appear on the boundary if the pressure gradient and temperature in the data are suitable. This critical point is the back-flow point. Moreover, we give a condition on the growth rate of the initial tangential velocity such that there is a back-flow point in the boundary layer.

Xin Z, Zhang L.

On the global existence of solutions to the Prandtl's system

Adv Math, 2004, 181: 88-133

DOI:10.1016/S0001-8708(03)00046-X      URL     [本文引用: 1]

Xin Z, Zhang L, Zhao J.

Global well-posedness and regularity of weak solutions to the Prandtl's system

arXiv:2203.08988v1, 2022

[本文引用: 1]

Xu C, Zhang X.

Long time well-posedness of Prandtl equations in Sobolev space

J Differential Equations, 2017, 263(12): 8749-8803

DOI:10.1016/j.jde.2017.08.046      URL     [本文引用: 1]

Zhang P, Zhang Z.

Long time well-posedness of Prandtl system with small and analytic initial data

J Funct Anal, 2016, 270(7): 2591-2615

DOI:10.1016/j.jfa.2016.01.004      URL     [本文引用: 1]

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