How to approximate the heightmap of processed wood?
A sine wave remapped to [0, 1] and raised to a power will give you periodic ridges: (Desmos graph)



That could be a good place to start. It will make perfectly straight, even ridges; but you could then perturb the X position where the sine is evaluated using low-frequency Perlin noise, which will make the ridges bend and waver while still going mostly along the original axis. You could use noise to fluctuate the power that the sine is raised to as well, making the ridges smoother and sharper in different places.