ns2d

A simple conditionally stable 2D incompressible Navier-Stokes solver on a regular Cartesian quadrilateral grid, utilizing the Chorin projection/decomposition method.

The repository contains two implementations:

• JavaScript version with browser-based solver.
• C++ console version with MSVC project files, including enhanced ASCII rendering.

In the C++ renderer, each grid cell can display not only velocity magnitude, but also normalized velocity-vector direction markers.

How to solve the incompressible Navier-Stokes equations

The governing equations are:

$$ \frac{\partial \vec{u}}{\partial t} = -(\vec{u} \cdot \nabla) \vec{u} - \frac{1}{\rho} \nabla p + \nu \nabla^2 \vec{u} $$

$$ \nabla \cdot \vec{u} = 0 $$

We use the splitting method based on the Helmholtz-Hodge decomposition. For any arbitrary vector field $\vec{w}$, we decompose it as:

$$ \vec{w} = \vec{u} + \nabla \phi $$

with:

$$ \nabla \cdot \vec{u} = 0, \quad \nabla \times \vec{u} \neq 0 $$

$$ \nabla \cdot (\nabla \phi) \neq 0, \quad \nabla \times (\nabla \phi) = 0 $$

Thus, the velocity field becomes:

$$ \vec{w} = \vec{u} + \frac{\tau}{\rho} \nabla p $$

Steps of the time-explicit method:

1. Velocity diffusion step:

$$ \frac{\partial \vec{u}}{\partial t} = \nu \nabla^2 \vec{u} $$

Finite Difference (FD) form:

$$ \vec{u} - \vec{u}_{\text{prev}} = \tau \cdot \nu \nabla^2 \vec{u}_{prev} $$

2. Velocity self-advection step:

$$ \frac{\partial \vec{u}}{\partial t} = - (\vec{u} \cdot \nabla) \vec{u} $$

FD form:

$$ \vec{u} - \vec{u}_{prev} = \tau \cdot \left( - (\vec{u} \cdot \nabla) \vec{u} \right) $$

3. Poisson equation:

$$ \nabla^2 p = \frac{\rho}{\tau} (\nabla \cdot \vec{u}) $$

FD form:

$$ -4p[i_y,i_x] + p[i_y+1,i_x] + p[i_y-1,i_x] + p[i_y,i_x+1] + p[i_y,i_x-1] = h^2 \frac{\rho}{\tau} \nabla \cdot \vec{u} = S $$

Rewritten expression for the iterative Gauss-Seidel solver:

$$ p[i_y,i_x] = \frac{1}{4} \left( p[i_y+1,i_x] + p[i_y-1,i_x] + p[i_y,i_x+1] + p[i_y,i_x-1] - S \right) $$

4. Apply pressure to correct velocity:

$$ \vec{u} = \vec{u}_{prev} - \tau \frac{1}{\rho} \nabla p $$

Demo

A test case involves lid-driven flow in a box cavity. All walls have zero Dirichlet velocity conditions, except for the top side, where a sliding velocity is defined. This velocity periodically changes its direction from left to right and vice versa.

The JavaScript solution visualizes velocity magnitude using ASCII art.

The C++ solution (MSVC project) additionally visualizes normalized velocity-vector directions for each cell. The built executable is available in the cpp/ns_2d_console/build/ directory.

Run

• JavaScript version: open js/index.html in a web browser.
• Windows compiled C++ version: run the executable from cpp/ns_2d_console/build/Release/ (compiled for ARM64 architecture).
• Manual compilation: requires Microsoft Visual Studio (MSVC toolchain) using the project file cpp/ns_2d_console/ns_2d_console.slnx.