<p>Let <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\((\textbf{u},\textbf{B})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="bold">u</mi> <mo>,</mo> <mi mathvariant="bold">B</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> be an axisymmetric self-similar solution to the stationary MHD equations with magnetic diffusion, of the form <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\textbf{u}=u^r(r,z)\textbf{e}_{r}+u^{\theta }(r,z)\textbf{e}_{\theta }+u^z(r,z)\textbf{e}_{z}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="bold">u</mi> <mo>=</mo> <msup> <mi>u</mi> <mi>r</mi> </msup> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo>,</mo> <mi>z</mi> <mo stretchy="false">)</mo> </mrow> <msub> <mi mathvariant="bold">e</mi> <mi>r</mi> </msub> <mo>+</mo> <msup> <mi>u</mi> <mi>θ</mi> </msup> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo>,</mo> <mi>z</mi> <mo stretchy="false">)</mo> </mrow> <msub> <mi mathvariant="bold">e</mi> <mi>θ</mi> </msub> <mo>+</mo> <msup> <mi>u</mi> <mi>z</mi> </msup> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo>,</mo> <mi>z</mi> <mo stretchy="false">)</mo> </mrow> <msub> <mi mathvariant="bold">e</mi> <mi>z</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\textbf{B}=B^{\theta }(r,z)\textbf{e}_{\theta }\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="bold">B</mi> <mo>=</mo> <msup> <mi>B</mi> <mi>θ</mi> </msup> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo>,</mo> <mi>z</mi> <mo stretchy="false">)</mo> </mrow> <msub> <mi mathvariant="bold">e</mi> <mi>θ</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> in cylindrical coordinates <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\((r,\theta ,z)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo>,</mo> <mi>θ</mi> <mo>,</mo> <mi>z</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, where <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\((\textbf{e}_r,\textbf{e}_\theta ,\textbf{e}_z)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi mathvariant="bold">e</mi> <mi>r</mi> </msub> <mo>,</mo> <msub> <mi mathvariant="bold">e</mi> <mi>θ</mi> </msub> <mo>,</mo> <msub> <mi mathvariant="bold">e</mi> <mi>z</mi> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is the orthonormal basis. Under the assumption that <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(u^r &lt; \frac{1}{3r} + \frac{2r}{3}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mi>u</mi> <mi>r</mi> </msup> <mo>&lt;</mo> <mfrac> <mn>1</mn> <mrow> <mn>3</mn> <mi>r</mi> </mrow> </mfrac> <mo>+</mo> <mfrac> <mrow> <mn>2</mn> <mi>r</mi> </mrow> <mn>3</mn> </mfrac> </mrow> </math></EquationSource> </InlineEquation> on the unit sphere and on its intersection with the half-space, respectively, we prove two main results. First, for the domain <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\mathbb {R}^3\setminus \{0\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mrow> <mi mathvariant="double-struck">R</mi> </mrow> <mn>3</mn> </msup> <mo lspace="0.15em" rspace="0.15em" stretchy="false">\</mo> <mrow> <mo stretchy="false">{</mo> <mn>0</mn> <mo stretchy="false">}</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, the velocity field <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(\textbf{u}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="bold">u</mi> </math></EquationSource> </InlineEquation> must be a Landau solution and the magnetic field <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\textbf{B} \equiv 0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="bold">B</mi> <mo>≡</mo> <mn>0</mn> </mrow> </math></EquationSource> </InlineEquation>. Second, in the half-space <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\mathbb {R}^3_+\)</EquationSource> <EquationSource Format="MATHML"><math> <msubsup> <mrow> <mi mathvariant="double-struck">R</mi> </mrow> <mo>+</mo> <mn>3</mn> </msubsup> </math></EquationSource> </InlineEquation> with either the no-slip or Navier slip boundary condition, we establish that all such axisymmetric self-similar solutions are trivial, i.&#xa0;e., <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(\textbf{u}=\textbf{B}=0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="bold">u</mi> <mo>=</mo> <mi mathvariant="bold">B</mi> <mo>=</mo> <mn>0</mn> </mrow> </math></EquationSource> </InlineEquation>.</p>

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On Axisymmetric Self-Similar Solutions to the MHD System

  • Shaoheng Zhang

摘要

Let \((\textbf{u},\textbf{B})\) ( u , B ) be an axisymmetric self-similar solution to the stationary MHD equations with magnetic diffusion, of the form \(\textbf{u}=u^r(r,z)\textbf{e}_{r}+u^{\theta }(r,z)\textbf{e}_{\theta }+u^z(r,z)\textbf{e}_{z}\) u = u r ( r , z ) e r + u θ ( r , z ) e θ + u z ( r , z ) e z and \(\textbf{B}=B^{\theta }(r,z)\textbf{e}_{\theta }\) B = B θ ( r , z ) e θ in cylindrical coordinates \((r,\theta ,z)\) ( r , θ , z ) , where \((\textbf{e}_r,\textbf{e}_\theta ,\textbf{e}_z)\) ( e r , e θ , e z ) is the orthonormal basis. Under the assumption that \(u^r < \frac{1}{3r} + \frac{2r}{3}\) u r < 1 3 r + 2 r 3 on the unit sphere and on its intersection with the half-space, respectively, we prove two main results. First, for the domain \(\mathbb {R}^3\setminus \{0\}\) R 3 \ { 0 } , the velocity field \(\textbf{u}\) u must be a Landau solution and the magnetic field \(\textbf{B} \equiv 0\) B 0 . Second, in the half-space \(\mathbb {R}^3_+\) R + 3 with either the no-slip or Navier slip boundary condition, we establish that all such axisymmetric self-similar solutions are trivial, i. e., \(\textbf{u}=\textbf{B}=0\) u = B = 0 .