Real life curvy wood grains happen because the wood has been sliced at a certain angle.
In CG, the placement of the material on the object has a lot to do with it
Download this PRJ.Click to view attachment
Look at the settings of the Spherical combiner and the turbulence added to it. These aren't necessarily ideal. I just tried a few until i got something going.
fBM is NOT necessarily the best turbulence, i just picked it quick.
I made the colors black and white for clarity, not because they look like wood.
Then look at the chor.
I've animated the translation of the material so you could see how it changes the appearance of the grain. Normally you would make these settings in the model. I did it in the chor so you could see it change.Click to view attachment
When the side of the block is cutting thru the center of the material the lines are very straight. As the side is cutting thru larger and larger rings the pattern of the turbulence gets wilder. From 3:00 to 4:00 the center of the material is very far away so the side is cutting thru nearly flat cross sections of the rings.
Offsetting the material in more than one axis would get a different effect.
angling the material would get different effects.
you can use the SHIFT-Q render to see the material change semi-real-time as you change the settings.
Try one of the wood materials from the CD and test out different translations and angles for the material.