diff --git a/theory/kernel/kernel_definitions.tex b/theory/kernel/kernel_definitions.tex index 91847a9faccabad4e2b278dc33f96adf62e4bec4..8999636109ffadcbf148ce3c1fbccdc44feafe65 100644 --- a/theory/kernel/kernel_definitions.tex +++ b/theory/kernel/kernel_definitions.tex @@ -36,16 +36,16 @@ hence be written (in 3D) as W(\vec{x},h) \equiv \frac{1}{H^3}f\left(\frac{|\vec{x}|}{H}\right), \end{equation} -where $H(h)$ is defined below and $f(u)$ is a dimensionless function, -usually a low-order polynomial, such that $f(u \geq 1) = 0$ and -normalised such that +where $H=\gamma h$ is defined below and $f(u)$ is a dimensionless +function, usually a low-order polynomial, such that $f(u \geq 1) = 0$ +and normalised such that \begin{equation} \int f(|\vec{u}|){\rm d}^3u = 1. \end{equation} $H$ is the kernel's support radius and is used as the ``smoothing -length'' in the Gadget code( {i.e.} $H(h)=h$). This definition is, +length'' in the Gadget code( {i.e.} $H=h$). This definition is, however, not very physical and makes comparison of kernels at a \emph{fixed resolution} difficult. A more sensible definition of the smoothing length, related to the Taylor expansion of the @@ -64,8 +64,8 @@ The smoothing length is then: \end{equation} Each kernel, {\it i.e.} defintion of $f(u)$, will have a different -ratio $h/H$. So for a \emph{fixed resolution} $h$, one will have -different kernel support sizes, $H$, and different number of +ratio $\gamma = H/h$. So for a \emph{fixed resolution} $h$, one will +have different kernel support sizes, $H$, and a different number of neighbours, $N_{\rm ngb}$ to interact with. One would typically choose $h$ for a simulation as a multiple $\eta$ of the mean-interparticle separation: @@ -87,9 +87,10 @@ useful quantity to use in implementations of SPH. It is defined as (in Once the fixed ratio $\gamma= H/h$ is known (via equations \ref{eq:sph:sigma} and \ref{eq:sph:h}) for a given kernel, the number -of neighbours only depends on the resolution $\eta$. For the usual -cubic spline kernel (see below), setting the simulation resolution to -$\eta=1.2348$ yields the commonly used value $N_{\rm ngb} = 48$. +of neighbours only depends on the resolution parameter $\eta$. For +the usual cubic spline kernel (see below), setting the simulation +resolution to $\eta=1.2348$ yields the commonly used value $N_{\rm + ngb} = 48$. \section{Kernels available in \swift} @@ -130,35 +131,21 @@ The kernel function $f(u)$ reads: \end{equation} -\subsubsection{Quartic spline ($M_5$) kernel} - -In 3D, we have $C=\frac{15625}{512\pi}$ and $\gamma=H/h = 1.825742$.\\ -The kernel function $f(u)$ reads: - -\begin{equation} - M_4(u) = \left\lbrace\begin{array}{rcl} - 3u^3 - 3u^2 + \frac{1}{2} & \mbox{if} & u<\frac{1}{2}\\ - -u^3 + 3u^2 - 3u + 1 & \mbox{if} & u \geq \frac{1}{2} - \end{array} - \right. - \nonumber -\end{equation} - - \subsubsection{Quartic spline ($M_5$) kernel} In 3D, we have $C=\frac{15625}{512\pi}$ and $\gamma=H/h = 2.018932$.\\ The kernel function $f(u)$ reads: -\begin{equation} - M_5(u) = \left\lbrace\begin{array}{rcl} +\begin{align} + M_5(u) &= \nonumber\\ + &\left\lbrace\begin{array}{rcl} 6u^4 - \frac{12}{5}u^2 + \frac{46}{125} & \mbox{if} & u < \frac{1}{5} \\ -4u^4 + 8u^3 - \frac{24}{5}u^2 + \frac{8}{25}u + \frac{44}{125} & \mbox{if} & \frac{1}{5} \leq u < \frac{3}{5}\\ u^4 - 4u^3 + 6u^2 - 4u + 1 & \mbox{if} & \frac{3}{5} \leq u \\ \end{array} \right. \nonumber -\end{equation} +\end{align} \subsubsection{Quintic spline ($M_6$) kernel} @@ -166,15 +153,16 @@ The kernel function $f(u)$ reads: In 3D, we have $C=\frac{2187}{40\pi}$ and $\gamma=H/h = 2.195775$.\\ The kernel function $f(u)$ reads: -\begin{equation} - M_6(u) = \left\lbrace\begin{array}{rcl} +\begin{align} + M_6(u) &= \nonumber\\ + &\left\lbrace\begin{array}{rcl} -10u^5 + 10u^4 - \frac{20}{9}u^2 + \frac{22}{81} & \mbox{if} & u < \frac{1}{3} \\ 5u^5 - 15u^4 + \frac{50}{3}u^3 - \frac{70}{9}u^2 + \frac{25}{27}u + \frac{17}{81} & \mbox{if} & \frac{1}{3} \leq u < \frac{2}{3}\\ -1u^5 + 5u^4 - 10u^3 + 10u^2 - 5u + 1. & \mbox{if} & u \geq \frac{2}{3} \end{array} \right. - \nonumber -\end{equation} + \nonumber +\end{align} \subsubsection{Wendland C2 kernel} @@ -194,7 +182,7 @@ In 3D, we have $C=\frac{495}{32\pi}$ and $\gamma=H/h = 2.207940$.\\ The kernel function $f(u)$ reads: \begin{align} - \Psi_{i,j}(u) &= \frac{35}{3}u^8 - 64u^7 + 140u^6\\ + \Psi_{i,j}(u) &= \frac{35}{3}u^8 - 64u^7 + 140u^6 \nonumber\\ & - \frac{448}{3}u^5 + 70u^4 - \frac{28}{3}u^2 + 1 \nonumber \end{align} @@ -206,7 +194,7 @@ In 3D, we have $C=\frac{1365}{64\pi}$ and $\gamma=H/h = 2.449490$.\\ The kernel function $f(u)$ reads: \begin{align} - \Psi_{i,j}(u) &= 32u^{11} - 231u^{10} + 704u^9 - 1155u^8\\ + \Psi_{i,j}(u) &= 32u^{11} - 231u^{10} + 704u^9 - 1155u^8 \nonumber\\ & + 1056u^7 - 462u^6 + 66u^4 - 11u^2 + 1 \nonumber \end{align}