Questions tagged [pde]

Partial differential equations (PDEs) are equations that relate the partial derivatives of a function of more than one variable. This tag is intended for questions on modeling phenomena with PDEs, solving PDEs, and other related aspects.

Filter by
Sorted by
Tagged with
0 votes
0 answers
45 views

Reference request for finite elements theory

Consider a domain $\Omega \subseteq \mathbb{R}^{2,3}$ which is non convex and with $C^2$ boundary. Could you recommend a good reference where it is explained how: without needing isoparametric ...
user avatar
  • 101
4 votes
3 answers
407 views

What condition ensures the global continuity of the solution in the FEM?

I know this is trivial but I don't seem to understand it. In which step of the FE formulation do we enforce the global continuity of the solution? Or in other words, how the construction of the local ...
user avatar
-2 votes
0 answers
37 views

Setting up boundary conditions to solve PDEs using method of lines

Objective: To add boundary/initial conditions (BCs/ICs) to a system of ODEs I have used the method of lines to convert a system of PDEs into a system of ODEs. The ODEs themselves involve a lot of ...
user avatar
0 votes
0 answers
62 views

How to solve spatially discretised PDEs (method of lines) in solve_ivp or ODEint?

I can discretise the spatial domain of a system of PDEs using the method of lines, converting the system of PDEs to a system of ODEs (with a time derivative only). These equations (for context they ...
user avatar
-2 votes
0 answers
49 views

ADI method for a 2D advection-diffusion equation

I have discretized energy equation (2D advection-diffusion equation) with ADI (Alternating Direction Implicit) method, like: $$\frac{\partial\theta}{\partial t}=\frac{\partial^2\theta}{\partial x^2}+\...
user avatar
2 votes
0 answers
63 views

Numerical scheme for the heat equation on the icosahedral hexagonal grid

I have a predefined grid(like this) that is spawned from a regular icosahedradron. It consists of many hexagons and 12 pentagons (corresponding to icosahedradron vertexes). I can tweak the granularity ...
user avatar
4 votes
3 answers
475 views

Why not use the convolution theorem for explicit timestepping?

Consider the advection equation \begin{equation} \frac{\partial C}{\partial t} + u\frac{\partial C}{\partial x} + v\frac{\partial C}{\partial y} = 0 \end{equation} I want to do a forward time, center ...
user avatar
  • 141
3 votes
0 answers
126 views

Numerically solving a 6th order non-linear differential equation in Matlab

I've posted yesterday a question about solving a non linear equation : it was not clear so I am reformulating my question. I am trying to solve a high-order non linear differential equation presented ...
user avatar
  • 33
0 votes
2 answers
58 views

Solving a generalized eigenvalue problem with Chebyshev spectral method: How to impose boundary conditions into the matrices?

I'm solving a local instability problem for a pipe Poiseuille flow. The coordinate system is columnar, i.e., ($r,\theta,x$) (radial, tangential and axial). The basic flow is $\bar{u_r}=0, \bar{u_\...
user avatar
  • 1
1 vote
0 answers
58 views

Preserving conservation properties across time-integration schemes

I am interested in deriving an accurate, implicit discretization of a nonlinear advection-diffusion equation $$ \partial_t f = \frac{1}{v^2} \partial_v J(f,v) \equiv F(f,v) \tag{$\star$} $$ with flux ...
user avatar
  • 695
6 votes
3 answers
163 views

Why in scientific papers convergence of finite difference and finite volume schemes is tested using multiple norms ($l_1$, $l_2$ and $l_{\infty}$)?

In peer reviewed numerical papers, the order of accuracy of finite difference and finite volume for PDEs is computed in multiple norms, usually $l_1$, $l_2$ and $l_{\infty}$, and other times $l_{\...
user avatar
  • 425
2 votes
1 answer
60 views

Shock Capturing Methods for Shallow Water Equations

I am looking for some help finding a numerical solution to the shallow water equations: $\partial_tu+\partial_x(u^2/2+g\eta)=0$ $\partial_t \eta+\partial_x(u\eta)=0$. where $u$ is the depth averaged ...
user avatar
2 votes
0 answers
67 views

solve Ax=b for outrigger A matrix python

I implement Crank-Nicolson 2D finite-difference method. I get a matrix A which is banded with 1 band above and below the main diagonal, but also contains 2 additional bands , further apart from the ...
user avatar
  • 131
2 votes
3 answers
89 views

Simple to program method for elliptic PDE with curved boundary?

I know very little about numerical PDE, so I apologize if this equation is ill-formed (and I appreciate any corrections). I am currently learning about Brownian motion. A classic result is that we can ...
user avatar
0 votes
0 answers
44 views

Why is the solution in FEM bounded by zero?

Consider the following problem: $$ -\nabla^2 u = f,$$ Referencing to this post: FEM, we write the problem in variational form I'm assuming Dirichlet boundary conditions here): Find $u\in H^1_0(\Omega)$...
user avatar
0 votes
0 answers
18 views

Transparent Boundary Conditions relationship with intermediate BCs in ADI-PR method

I have read some materials about ADI - PR method with the aim to understand how to put boundary conditions in my 2D scheme which solves the Time-Dependent Schrodinger Equation. All the theory I read ...
user avatar
  • 131
0 votes
0 answers
57 views

Transparent Boundary Conditions for Finite Difference ADI PR 2D TDSE solution

I want to put (non-dirichlet) boundary conditions inside the code I wrote to solve the 2dim TDSE using the alternating direction implicit Peaceman - Rachford method. $$ (1 + iB\Delta t/2 ) \psi^{n+1/2}...
user avatar
  • 131
0 votes
1 answer
75 views

Assembly of the Isoparametric Quadratic load vector in Matlab [duplicate]

I work to solve PDE using FEM in the case P2 on Matlab. I try to correctly assemble load vector using quadratic Lagrange shape functions $$b_i =(f,\phi_i)=\sum_{q=1}^{nq}f(r_q,s_q)*\phi_{i}(r_q,s_q)*...
user avatar
0 votes
2 answers
61 views

Solvers for odd order PDE finite difference discretisation

I am used to solving elliptic PDEs of even order. I was wondering what would one do for odd order PDEs. Notably the discretisations of those results in unsymmetric matrices. I tried solving the ...
user avatar
3 votes
0 answers
56 views

Change in Variables applied to biharmonic equation

Background I want to solve the following biharmonic equation: $$\frac{ \partial^4 s }{ {\partial \xi}^4 }+\frac{ \partial^4 s }{ {\partial \xi}^2{\partial t}^2 }+\frac{ \partial^4 s }{ {\partial t}^4 }...
user avatar
1 vote
1 answer
69 views

Direct integration of 2D Euler Equations with Runge Kutta shows oscillating Courant-Friedrichs-Lewy coefficient. Stiff or Bug?

By writing the direct integration of the 2D Euler Equations in a wide and short box where the fluid enters and exits through the horizontal faces using the Runge Kutta O(4) method I have found that ...
user avatar
  • 113
1 vote
1 answer
88 views

Prolongation and restriction operators in multigrid for high order PDEs

If I have the Poisson equation $\Delta u = f$ a standard transfer operator (for a regular grid) is the full weighting/bilinear interpolation scheme: $$K = \frac{1}{4}\begin{bmatrix}\frac{1}{4} & \...
user avatar
1 vote
1 answer
69 views

How to Impose nonhomogeneous Neumann Boundary Condition in the DG Formulation

Consider the following partial differential equation \begin{align} \frac{\partial u}{\partial t}+\frac{\partial f}{\partial x} &= g(x,t), \ \ x\in \Omega = [x_{L},x_{R}] \\ u(x,0) &= u_{0}(x) ...
user avatar
0 votes
0 answers
31 views

Solving a time-independent 2D Schrödinger equation using PDE Toolbox in MATLAB

I want to solve the following PDE eigenvalue problem, $$-\left(\frac{\partial^2}{\partial x^2}+\frac{\partial^2}{\partial y^2}\right)\psi_n(x,y)+x^2y^2\psi_n(x,y)=E_n\psi_n(x,y)$$ inside a square with ...
user avatar
  • 129
0 votes
0 answers
57 views

Overflow in Solving numerically the 1D Kuramoto-Sivashinsky Equation using spectral methods

I'm having trouble with the following implementation of the KS model (see below) found on Solving numerically the 1D Kuramoto-Sivashinsky Equation using spectral methods When tN > 300 an overflow ...
user avatar
  • 1
0 votes
1 answer
142 views

Best method to solve this system of PDEs?

I have a system of PDEs constituting an initial value problem (IVP) consisting of three coupled PDEs: \begin{align} \partial_t \rho + \partial_x(\rho v) &= \left(k_A (1-\phi) + k_B \phi \right)...
user avatar
3 votes
0 answers
43 views

Appropiate Artificial Boundary Conditions for the radial part of the Klein Gordon equation?

I am trying to simulate the following equation using FDTD $ \left(- \partial^2_t + \partial^2_x + V(x) \right) \psi(x,t) =0 $ subjected to the initial conditions $\psi(x,0) = f(x),~ \partial_t \psi(x,...
user avatar
  • 131
0 votes
1 answer
197 views

1D wave equation using Finite difference method MATLAB

I have the wave equation $$u_{tt} = 4 u_{xx}$$ with the boundary conditions $$u(0,t) = u(L,t) = 0\,,\quad x \leq 0 \leq 2\pi \,,\quad t\geq 0$$ and initial conditions $$\begin{align} &u(x,0)=\...
user avatar
6 votes
0 answers
130 views

FEM : energy minimization VS PDE solving

Engineering FEM When I studied engineering, I learned the traditional approach for finite elements for elasticity. The point was to solve the PDE $-div(\sigma)=f$ as: Multiply your PDE with a test ...
user avatar
  • 61
3 votes
1 answer
116 views

Wave equation, wave is bouncing off Neuman boundary

Wave equation. Mixed BC. Applied Neuman boundary condition ($\frac{\partial u}{\partial x}\big|_N=0$) to the RHS of the domain You may observe the sharp edge in the middle of the string in the image ...
user avatar
  • 316
2 votes
1 answer
69 views

Finite element modelling of thermal expansion of 3D solid bodies

I want to solve the thermal expansion of solid by using FEM approach. When I developed the model based on the the principle the minimum potential energy, the solutions for thermal expansion are not ...
user avatar
1 vote
1 answer
146 views

Non-reflective boundary condition

I'm currently solving incompressible Navier-Stokes system of equations with periodic flow and high viscosity. Is there any outlet boundary types that avoids the reflection of flow from the outlet back ...
user avatar
  • 316
2 votes
2 answers
305 views

Semi-infinite domain transformation

Question is mostly related to literature or suggestions. Given a semi infinite domain: $x=[0; +\infty);y=[0; +\infty)$. Willing to transform it to computational domain of: $[0,1]\times[0,1]$. I did ...
user avatar
  • 316
1 vote
2 answers
95 views

What is known about C0 triangular finite elements with nonstandard mesh point placement?

I'm curious about the general case, but for ease of explaining lets just take the case of a $P^2(\Omega)$ approximation. For simplicity, let's also just consider the reference element $(0,0), (0,1), (...
user avatar
3 votes
1 answer
82 views

"Optimal" domain partitioning in domain decomposition algorithms

When solving a PDE numerically by domain decomposition methods, what is the "optimal way" to split the domain? Are there any results stating that a particular partition of the domain yields &...
user avatar
2 votes
1 answer
219 views

Solving stiff ODEs: Dealing with Jacobian terms which take too long to compute with finite differences

I have a system of PDEs describing atmospheric chemistry and transport. I use finite-differences to make my system of PDEs into a system of ~10,000 ODEs. I then integrate the ODEs forward in time with ...
user avatar
3 votes
1 answer
98 views

Stability analysis simplification for PDE

I have the nonlinear PDE $$\frac{\partial U(z,t)}{\partial t} + A(U)\frac{\partial U(z,t)}{\partial z} + B(U)U(z,t) + C(z,t) = 0,$$ where $A(U)$ and $B(U)$ are guaranteed to be real and positive. I ...
user avatar
  • 33
0 votes
0 answers
53 views

What is the most common loss function used with collocation methods for differential equations

I was looking at the Cheney and Kincaid book (6th edition) on numerical methods, with respect to collocation method for differential equations. Now for linear systems of ODES, collocation is just a ...
user avatar
  • 245
0 votes
1 answer
37 views

How to decrease error in (FTCS) forward time centered space method?

I am using the FTCS method for solving differential equations. I know that the condition for stable output is $$ \frac{\alpha \Delta t}{\Delta x ^2} < \frac{1}{2} $$ But when I use the distance ...
user avatar
1 vote
0 answers
52 views

How to numerically solve PDE that governs the free vortex wake model?

Crossposted at Math SE I am reading a paper on the free vortex wake model for a helicopter rotor blade, which is described by the following PDE: $$\frac{\partial \vec{r}}{\partial \psi} (\psi, \zeta) ...
user avatar
0 votes
0 answers
44 views

introduction to non-overlapping domain decomposition in 1D

I am new to domain decomposition and have been searching in google for an introduction in 1D which goes over the complete procedure from the continuous to the discrete problem as well as the algorithm ...
user avatar
0 votes
0 answers
55 views

Discretizing Multi-species Ion Exchange Equations by Finite Volume Method

I'm solving a system of multispecies ion exchange equations (diffusion+drift fluxes) in 1-d spherical domain using finite volume method to obtain the ion concentrations at the next time step. After ...
user avatar
  • 1
1 vote
2 answers
181 views

Continuation of solution to $-\nabla\cdot (k(x,y)\nabla u)=f$

I'm trying to solve the following problem, I had previously opened another discussion for the implementation and well, it seems that it has turned out well, it can be found here. I need to calculate ...
user avatar
  • 123
3 votes
0 answers
88 views

Strange Picard iteration

I am interested in solving the equation $$ \begin{aligned} \nabla \cdot\left(\nabla \phi-\frac{\nabla \phi}{|\nabla \phi|}\right) &=0 & & \text { in } \Omega \\ \phi &=0 & & \...
user avatar
  • 591
3 votes
0 answers
82 views

Solving PDEs: What is the best way to deal with non-banded/dense jacobians?

I have a system of PDEs describing atmospheric chemistry and transport. I use finite-differences to make my system of PDEs into a system of ~10,000 ODEs. I then integrate the ODEs forward in time with ...
user avatar
0 votes
2 answers
372 views

Implementing routine for $-\nabla\cdot (k(x,y) \nabla u)=f$ in Matlab

I am solving the Poisson Equation for 2D given by the following expression: $$-\nabla\cdot (k(x,y) \nabla u)=f$$ in a rectangle with Dirichlet conditions on the boundary using Matlab. In principle I ...
user avatar
  • 123
0 votes
1 answer
159 views

Help me choose a book on the numerical integration of PDEs

For ODEs I have these books: Griffiths, David, Higham, Desmond J., Numerical Methods for Ordinary Differential Equations, 2010 Alfio Quarteroni, Riccardo Sacco, Fausto Saleri, Numerical Mathematics, ...
user avatar
  • 119
5 votes
1 answer
378 views

What is the weak form of a vector type Laplace equation?

For a scalar variable $u$, the weak form of the following Laplacian: $$\nabla^2 u =0 $$ with the assumption that $v$ vanishes at the boundary is $$\int \nabla v . \nabla u \, d\Omega = 0 $$ which is ...
user avatar
0 votes
2 answers
94 views

Stability of the Forward-Time Central Space method, section 9.3 in LeVeque

I am reading section 9.3 in Leveque about stability. The explanations are very short and brief so I am asking for some help to understand and elaborations. The result of the section is that for FTCS ...
user avatar
2 votes
1 answer
157 views

How to discretize a non-linear PDE with boundary conditions and intial value

Consider this non linear PDE: $$u_t + c(u^2)_x =\alpha u_{xx} \text{ , } -1<x<1 , t>0 $$ with $$u(-1,t) = g_L(t) \text{ , } u(1,t) = g_R(t) \text{ and } u(x,0)=f(x) $$ where the 3 functions(...
user avatar

1
2 3 4 5
17