desing of experiments necessary?

hello

i have a structure that its geometry depends of two different geometrical parameters L and E. this parameters are in a range (continous one), i am sorry i dont have the values for the moment... and what i would like to do is model the pressure drop of a fluid that goes throught the structure,

what i would like is to make a surface with the pressure drop in the Z axe, and the L and E in the X and Y axes to be able to say where is a minimum pressure drop for the L and E values.

what i dont know if a DoE is needed in a case like this, and if it is the case which method..... from what i read from matlab, the best DoE is the D-Optimal desing but not sure i am looking for the better not the simplest or fastest.

but i though that maybe would be only necessary a grid like with constant separation in the X and Y axes?