if you come from maya fx field the first thing you will miss is a kind “volume axisfield” from maya.in houdini there always more then one way to get to the desired result. the simplest way to use metaballs.
i’ve use a cycle primtive to copy metaballs (copy-node) to get torus shape. no just need add force node to copy sop and use the attractor inside the pop network. with correct settings it will create nice swirling motions like in this example.
this methods is nore flexible then volume filed in maya. the force can any shape build form metaballs. its not fastest methods but most situations, its good enough.
another example of the attractor node: