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Tongkeun Chang

Publications and source records attributed to Tongkeun Chang.

At least 19 recordsLinked to original sources
arXiv2026

Global well-posedness and time-decay estimates of the Navier-Stokes equations in exterior domains for critical data

It is well known that the Navier-Stokes equations have unique global strong solutions for standard domains when initial data are small in $L^n_σ$. Global well-posedness has been extended to rough initial data in larger critical spaces. This paper explores the global strong solvability of the smooth exterior domain problem for initial data that is small in some critical spaces larger than $L^n_σ$

math.AP↗
arXiv2026

On the Existence of Boundary Layer Separation for Incompressible Fluid Flow in the Half-Space

We consider the Stokes system in the half-space with localized boundary data. We prove that a boundary layer separation point exists provided that a certain singular integral determined by the boundary data is negative. On the other hand, if this integral is strictly positive, then boundary layer separation does not occur. When boundary layer separation occurs, we also investigate the dynamics of the separation point and the sign of the pressure gradient. Furthermore, by a perturbation argument, we construct solutions to the Navier--Stokes equations in the half-space that exhibit the same qualitative behavior as in the Stokes case.

math.AP↗
arXiv2026

Flow reversal of the Stokes system with localized boundary data in the half space

We consider the unsteady Stokes system in the half-space with zero initial data and nonzero, space-time localized boundary data. We show that there exist boundary influxes for which the induced flow exhibits flow reversal, in the sense that at least one component of the velocity field changes its sign in the half-space. This phenomenon is demonstrated by a careful analysis of the representation formula for the Stokes system in the half-space, including pointwise estimates, based on the Green tensor with nonzero boundary data. We construct solutions of the Stokes system such that the tangential components of the velocity field exhibit at least one sign change, while the normal component exhibits at least two sign changes. Moreover, the normal component of the constructed velocity field has the opposite sign to the tangential components near the boundary, whereas it has the same sign as the tangential components sufficiently far from the boundary.

math.AP↗
arXiv2025

Global existence of solutions of the stochastic incompressible non-Newtonian fluid models

In this paper, we study the existence of solutions of stochastic incompressible non-Newtonian fluid models in $\mathbb{R}$. For the existence of solutions, we assume that the extra stress tensor $S$ is represented by $S({\mathbb A}) = {\mathbb F} ( {\mathbb A}) {\mathbb A}$ for $ n \times n$ matrix ${\mathbb G}$. We assume that ${\mathbb F}(0) $ is uniformly elliptic matrix and \begin{align*} |{\mathbb F}({\mathbb G})|, \,\, | D {\mathbb F} ({\mathbb G})|, \,\, | D^2({\mathbb F} ({\mathbb G}) ){\mathbb G}| \leq c \quad \mbox{for all} \quad 0 < |{\mathbb G}| \leq r_0 \end{align*} for some $r_0 > 0$. Note that ${\mathbb F}_1$ and ${\mathbb F}_2$ for $ d \in {\mathbb R}$, and ${\mathbb F}_3$ for $d \geq 3$ introduced in (1.2) satisfy our assumption.

math.AP↗
arXiv2024

Asymptotic properties of the Stokes flow in an exterior domain with slowly decaying initial data and its application to the Navier-Stokes equations

In this paper, we study the decay rate of the Stokes flow in an exterior domain with a slowly decaying initial data ${\bf u}_0(x)=O(|x|^{-\al}), 0<\al\leq n$. %which is not $L^1$ integrable. As an application we find the unique strong solution of the Navier-Stokes equations corresponding to a slowly decaying initial data. We also derive the pointwise decay estimate of the Navier-Stokes flow. Our decay rates will be optimal compared with the decay rates of the heat flow.

math.AP↗
arXiv2023

Singular weak solutions near boundaries in a half space away from localized force for the Stokes and Navier-Stokes equations

We prove that there exists a weak solution of the Stokes system with a non-zero external force and no-slip boundary conditions in a half space of dimensions three and higher so that its normal derivatives are unbounded near boundary. A localized and divergence free singular force causes, via non-local effect, singular behaviors of normal derivatives for the solution near boundary, although such boundary is away from the support of the external force. The constructed one is a weak solution that has finite energy globally, and it can be comparable to the one in \cite{Seregin-Sverak10} as a form of a shear flow that is of only locally finite energy. Similar construction is performed for the Navier-Stokes equations as well.

math.AP↗
arXiv2022

Local regularity near boundary for the Stokes and Navier-Stokes equations

We are concerned with local regularity of the solutions for the Stokes and Navier-Stokes equations near boundary. Firstly, we construct a bounded solution but its normal derivatives are singular in any $L^p$ with $1<p$ locally near boundary. On the other hand, we present criteria of solutions of the Stokes equations near boundary to imply that the gradients of solutions are bounded (in fact, even further Hölder continuous). Finally, we provide examples of solutions whose local regularity near boundary is optimal.

math.AP↗
arXiv2020

Global well-posedness of the half space problem of the Navier-Stokes equations in critical function spaces of limiting case

In this paper, we study the initial-boundary value problem of the Navier-Stokes equations in half-space. Let a solenoidal initial velocity be given in the function space $ \dot{B}_{p\infty,0}^{ -1 + n/p}({\mathbb R}^n_+)$ for $ \frac{n}3< p < n$. We prove the global in time existence of weak solution $u\in L^\infty(0,\infty; \dot B^{-1 +n/p}_{p\infty}({\mathbb R}^n_+))$, when the given initial velocity has small norm in function space $ \dot{B}_{p\infty,0}^{-1 + n/p} ({\mathbb R}^n_+)$, where $ \frac{n}3< p< n$.

math.AP↗
arXiv2019

Global in time solvability of the Navier-Stokes equations in the half-space

In this paper, we study the initial value problem of the Navier-Stokes equations in the half-space. Let a solenoidal initial velocity be given in the function space $ \dot{B}_{pq,0}^{α-\frac{2}{2}}({\mathbb R}^n_+)$ for $α+1 = \frac{n}p + \frac2q$ and $0<α<2$. We prove the global in time existence of weak solution $u\in L^q(0,\infty; \dot B^α_{pq}({\mathbb R}^n_+))\cap L^{q_0}(0, \infty; L^{p_0}({\mathbb R}^n_+)) $ for some $ 1<p_0, q_0<\infty$ with $\frac{n}{p_0} +\frac2{q_0} =1$, when the given initial velocity has small norm in function space $ \dot{B}_{p_0q_0,0}^{-\frac{2}{q_0}}({\mathbb R}^n_+)$. The solution is unique in the class $L^{q_0}(0, \infty; L^{p_0}({\mathbb R}^n_+))$. Pressure estimates are also given.

math.AP↗
arXiv2018

Initial-Boundary value problem of the Navier-Stokes equations in the half space with nonhomogeneous data

This paper discusses the solvability (global in time) of the initial-boundary value problem of the Navier-stokes equations in the half space when the initial data $ h\in \dot{ B}_{q σ}^{α-\frac{2}{q}}(\R_+)$ and the boundary data $ g\in \dot{ B}_q^{α-\frac{1}{q},\frac{\al}{2}-\frac{1}{2q}}({\mathbb R}^{n-1}\times {\mathbb R}_+) $ with $g_n\in \dot B^{\frac12 α}_q ({\mathbb R}_+; \dot B^{-\frac1q}_q ({\mathbb R}^{n-1}))\cap L^q({\mathbb R}_+;\dot{B}^{α-\frac{1}{q}}(\Rn))$, for any $0<α<2$ and $q =\frac{n+2}{α+1}$. Compatibility condition is required for $h$ and $g$.

math.AP↗
arXiv2015

Solvability for Stokes system in Hölder spaces in bounded domains and its applications

We consider Stokes system in bounded domains and we present conditions of given data, in particular, boundary data, which ensure Hölder continuity of solutions. For Hölder continuous solutions for the Stokes system the normal component of boundary data requires a bit more regular than boundary data of Hölder continuous solutions for the heat equation. We also construct an example, which shows that Hölder continuity is no longer valid, unless the proposed condition of boundary data is fulfilled. As an application, we consider a certain general types of nonlinear systems coupled to fluid equations and local well-posedness is established in Hölder spaces.

math.AP↗
arXiv2015

Solvability of the Initial-Boundary value problem of the Navier-Stokes equations with rough data

In this paper, we study the initial and boundary value problem of the Navier-Stokes equations in the half space. We prove the unique existence of weak solution $u\in L^q(\R_+\times (0,T))$ with $\nabla u\in L^{\frac{q}{2}}_{loc}(\R_+\times (0,T))$ for a short time interval when the initial data $h\in {B}_q^{-\frac{2}{q}}(\R_+)$ and the boundary data $ g\in L^q(0,T;B^{-\frac{1}{q}}_q(\Rn))+L^q(\Rn;B^{-\frac{1}{2q}}_q(0,T)) $ with normal component $g_n\in L^q(0,T;\dot{B}^{-\frac{1}{q}}_q(\Rn))$, $n+2<q<\infty$ are given.

math.AP↗
arXiv2015

Transition densities of one-dimensional Levy processes

In this paper, we study the existence of the transition densities of one-dimensional Lévy processes. Compared with past results, our results contain the Lévy processes whose Lévy symbols have logarithm behavior at infinity. Our results contain the Lévy symbol induced by the following Laplace exponent $ψ(ξ) := (\ln(1 + \ln(1 + \ln(\cdots \ln(1 + |ξ|)))))^ε$ ($n$ times), $0 < ε< 1$, $2 \le n$. We also show that $ψ(ξ)$ is a Lévy symbol with transition density.

math.PR↗
arXiv2014

Initial-boundary value problem of the Navier-Stokes system in the half space

In this paper, we study the initial-boundary value problem of the Navier-Stokes system in the half space. We prove the unique solvability of the weak solution on some short time interval (0, T) with the velocity in $C^{α, \frac12 α} ({\mathbb R}^n_+ \times (0, T)), 0 < α< 1$, when the given initial data is in $C^α({\mathbb R}^n_+)$ and the given boundary data is in $C^{α, \frac12 α} ({\mathbb R}^{n-1} \times (0, T))$. Our result generalizes the result in [30] considering nonhomogeneous Dirichlet boundary data.

math.AP↗
arXiv2013

Estimates of anisotropic Sobolev spaces with mixed norms for the Stokes system in a half-space

We are concerned with the non-stationary Stokes system with non-homogeneous external force and non-zero initial data in ${\mathbb R}^n_+ \times (0,T)$. We obtain new estimates of solutions including pressure in terms of mixed anisotropic Sobolev spaces. As an application, some anisotropic Sobolev estimates are presented for weak solutions of the Navier-Stokes equations in a half-space in dimension three.

math.AP↗
arXiv2013

Quasi-maximum modulus principle for the Stokes equations

In this paper, we extend the maximum modulus estimate of the solutions of the nonstationary Stokes equations in the bounded $C^2$ cylinders for the space variables in \cite{CC} to time estimate. We show that if the boundary data is $L^\infty$ and the normal part of the boundary data has log-Dini continuity with respect to time, then the velocity is bounded. We emphasize that there is no continuity assumption on space variables in the new maximum modulus estimate. This completes the maximum modulus estimate.

math.AP↗