The spectral method is solving certain differential equation by some "basis function", typically sinusoids with Fourier method With the Navier-Stokes equation, it can remove presssure term in N-S equation and solve viscous term analytically. This is huge because solving Poisson equation (pressure term) usually takes very long time compared to other terms. Moreover, I can expect more accuracy than simple numericla discretization. However, I can apply spectral method only to periodic domain, but it doesn't matter when solving isotropic turbulence.
Original Navier-Stokes equation in convection form is
\begin{align} \dfrac{\partial u_i}{\partial t} &= -\dfrac{\nabla p}{\rho} - (u \cdot \nabla) u + \nu \nabla^2 u \\ \nabla \cdot u &= 0 \end{align}Using following vector identical solution,
\begin{align} \dfrac{1}{2} \nabla (A \cdot A) = (A \cdot \nabla) A + A \times (\nabla \times A) \end{align}I can convert original equation to rotational form.
\begin{align} \dfrac{\partial u_i}{\partial t} &= -\dfrac{\partial P}{\partial x_i} + H_i + \nu \nabla^2 u \\ \dfrac{\partial u_i}{\partial x_i} &= 0 \end{align}where
\begin{align} P &= \dfrac{p}{\rho} + \dfrac{1}{2} u_j u_j \\ H_i &= \epsilon_{i,j,k} u_j \omega_k = u \times (\nabla \times u) \end{align}If I take divergence from Navier-Stokes equation in rotational form,
\begin{align} \nabla^2 P = \dfrac{\partial H_j}{\partial x_j} \end{align}By expanding both equation (N-S equation & Poisson equation) to Fourier space, I got following equation
\begin{align} \dfrac{d \hat{u}_i }{d t} &= -i \kappa_i \hat{P} + \hat{H}_i - \nu \kappa^2 \hat{u}_i \\\ -\kappa^2 \hat{P} &= i \kappa_j \hat{H}_j \end{align}Combining two equation and then..
\begin{align} \dfrac{d \hat{u}_i }{d t} &= -i \kappa_i \left( -i \dfrac{\kappa_j}{\kappa^2} \hat{H}_j \right ) + \hat{H}_i - \nu \kappa^2 \hat{u}_i \\\ \dfrac{d \hat{u}_i }{d t} &= -\dfrac{\kappa_i \kappa_j}{\kappa^2} \hat{H}_j + \hat{H}_i - \nu \kappa^2 \hat{u}_i \end{align}where $\kappa$ is a wavenumber. Now I have new N-S equation without pressure term
\begin{align} \dfrac{d \hat{u}_i }{d t} &= -\dfrac{\kappa_i \kappa_j}{\kappa^2} \hat{H}_j + \hat{H}_i - \nu \kappa^2 \hat{u}_i \end{align}To treat a viscous terms analytically, I just multiply following formula to N-S equation without pressure form
\begin{equation} f(t) = e^{\nu \kappa^2 t} \end{equation}Then the equation changed like this
\begin{align} \left[ \dfrac{d \hat{u}}{dt} + \nu \kappa^2 \hat{u}_j \right] \times f(t) &= \left[ - \dfrac{\kappa_i \kappa_j}{\kappa^2} \hat{H}_j + \hat{H}_i \right ] \times f(t) \\\ f(t) \dfrac{d \hat{u}}{dt} + (\nu \kappa^2 f(t))\hat{u}_j &= \left[ - \dfrac{\kappa_i \kappa_j}{\kappa^2} \hat{H}_j + \hat{H}_i \right ] f(t) \\\ f(t) \dfrac{d \hat{u}}{dt} + (\nu \kappa^2 e^{\nu \kappa^2 t})\hat{u}_j &= \left[ - \dfrac{\kappa_i \kappa_j}{\kappa^2} \hat{H}_j + \hat{H}_i \right ] f(t) \\\ f(t) \dfrac{d \hat{u}}{dt} + (\dfrac{d e^{\nu \kappa^2 t}}{dt})\hat{u}_j &= \left[ - \dfrac{\kappa_i \kappa_j}{\kappa^2} \hat{H}_j + \hat{H}_i \right ] f(t) \\\ \dfrac{d \hat{u}_i f(t)}{dt} &= \left[ - \dfrac{\kappa_i \kappa_j}{\kappa^2} \hat{H}_j + \hat{H}_i \right ] f(t) \end{align}this can be more simpler by introducing new term $\widehat{NL}$
\begin{equation} \dfrac{d \hat{u}_i e^{\nu \kappa^2 t}}{dt} = \widehat{NL} e^{\nu \kappa^2 t} \end{equation}For low-storage RK3 method (2-register, 3-stage, 3rd order), the coefficients are following table
| order | $a_n$ | $b_n$ | $c_n$ |
|---|---|---|---|
| 1st | 8/15 | 0 | 0 |
| 2nd | 5/12 | -17/60 | 8/15 |
| 3rd | 3/4 | -5/12 | 2/3 |
If I assume to solve following equation,
\begin{equation} \dfrac{\partial Q}{\partial t} = R(Q) \end{equation}The low-storage RK3 method applied to the equation using above coefficients.
\begin{align} Q^1 &= Q^n + \Delta t \left( \dfrac{8}{15} R^n \right) \\\ Q^2 &= Q^1 + \Delta t \left( \dfrac{5}{12} R^n - \dfrac{17}{60} R^1\right) \\\ Q^{n+1} &= Q^2 + \Delta t \left( \dfrac{3}{4} R^n - \dfrac{5}{12} R^2\right) \end{align}Before Navier-Stokes equation, I can apply low-storage RK3 method to reaction-diffusion equation
\begin{align} \dfrac{\partial \psi}{\partial t} &= G + L \psi \\\ \psi^{n+1} &= \psi^{n} + a_n \Delta t G^n + b_n \Delta t G^{n-1} + (a_n + b_n)\Delta t\left(\dfrac{L \psi^{n+1} + L \psi^n}{2} \right) \end{align}Then the N-S equation should be..
\begin{align} \dfrac{d \hat{u} e^{\nu \kappa^2 t}}{dt} &= \widehat{NL} e^{\nu \kappa^2 t} \\\ \dfrac{\hat{u}^{n+1}_i e^{\nu \kappa^2 (t+a_n \Delta t + b_n \Delta t)} - \hat{u}^{n}_i e^{\nu \kappa^2 t}}{\Delta t} &= a_n\widehat{NL}^n e^{\nu \kappa^2 t} + b_n\widehat{NL}^{n - 1} e^{\nu \kappa^2 (t - a_{n-1} \Delta t - b_{n-1} \Delta t)} \\\ \hat{u}^{n+1}_i e^{\nu \kappa^2 (a_n \Delta t + b_n \Delta t)} - \hat{u}^{n}_i &= a_n \Delta t \widehat{NL}^n + b_n \Delta t \widehat{NL}^{n - 1} e^{\nu \kappa^2 (- a_{n-1} \Delta t - b_{n-1} \Delta t)} \\\ \hat{u}^{n+1}_i &= \left[a_n \Delta t \widehat{NL}^n + \hat{u}^{n}_i \right ] e^{-\nu \kappa^2 (a_n + b_n) \Delta t} + b_n \Delta t \widehat{NL}^{n - 1} e^{\nu \kappa^2 -(a_n + b_n + a_{n-1}+ b_{n-1}) \Delta t} \\\ \end{align}