Direction of flow of current in secondary circuit in AC transformer/rectifier
Using Ampere's right hand rule:



Point your thumb in the direction of the current in the primary side and you can get the direction of the magnetic field from the direction of rotation of the other four fingers. The opposite is true, rotating the 4 fingers in the direction of the magnetic field will point your thumb to the direction of current. Try it on the secondary side.

Therefore*:





*Assuming the primary and secondary windings are wrapped in the same direction around the core of the transformer. Read more about the dot notation: