I was unable to locate this on a "real life" picture of the aircraft, so have attached an image from my simulator. What is the function/purpose of this object on the exterior of the 738? It seems like it would decrease efficiency so it must surely serve an important role? Thanks.
11$\begingroup$ On the real aircraft, and while we are here, let's see inside. $\endgroup$– minsDec 11, 2017 at 17:52
28$\begingroup$ That's the right turn arrow. I fly for miles with those blinking sometimes... $\endgroup$– Brock AdamsDec 12, 2017 at 1:27
$\begingroup$ @mins the downward angle of that image is a little unsettling... $\endgroup$– Ken WilliamsDec 12, 2017 at 15:47
2$\begingroup$ Those are the top secret Chemtrail generation port. BTW its about the size of a womp rat hole... $\endgroup$– ChadDec 12, 2017 at 19:46
It's the air inlet for the Auxiliary Power Unit (APU).
Slide 18 here has an image labelled "APU Air Inlet door with Vortex Generator". The inset image on the slide looks almost exactly like your posted image.
I found a much better image here, captioned "Boeing 737-800 with APU inlet".
There is some discussion about why the 737 has vortex generators for the APU on this thread on PPRuNe.