When simulating a voltage divider using wave digital filters I get inversed polarity outputs
The problem that you are facing is one of polarity. When the 3-port series adaptor is defined in the wave-domain certain polarities are inherently assumed. If I am not mistaken the polarity of the 3-port series adaptor you are using is the following:



The circuit that you have shown has the following explicit polarities (shown in both Kirchoff- and wave-domain).



This miss-match is not a big deal for linear circuits (just invert the output). However, if you move up to nonlinear circuits polarity is crucial to the correctness of the derived algorithm. To remedy issues related to polarity, one needs to be mindful of the polarities at play and act accordingly. For the above circuit you can for example reverse the polarities of the resistors by placing a 2-port series adaptor, also known as an inverter, to invert the polarity so that it is correct with respect to the polarities as you define them.