# Problem with understanding locale coodinates,kelvin wake

Hello,
it seems that I don’t understand the locale coordinate system.
I try to simulate a kelvin wake with particles.

The first problem is that the particle’s location attribute uses the global coordinate system.
So if the moving emitter follows a curve the particles do not turns with him.

I tried to solve the problem by converting the modified particles position into the emitter’s locale position:

`````` mat_inv = object.matrix_world.inverted() #object is the emitter

#Formulars are taken from "A VFX ocen toolkit with realtime preview " by Björn Rydahl page 16
#https://www.diva-portal.org/smash/record.jsf?pid=diva2%3A272268&dswid=1yVqr7fD

xp = old_xp+ (-phase * (2 * math.cos(t) - math.pow(math.sin(t),3)))
yp = old_yp + (-phase * (math.cos(t) * math.pow(math.sin(t),2) ))
zp = old_zp

# converting the particle's global coordinates to local coordinates
mat_loc = mathutils.Matrix.Translation((xp,yp , zp))
mat_loc = mat_inv @ mat_loc

#getting the vectors for location , rotation and scale from matrix. Rotation and scale are not used
locale_location, rot,sca = mat_loc.decompose()
``````

First question:
Is this the correct way ? Or are there any logical errors ?

If I don’t convert the particles into locale coordination system I get this:

If I convert the coordinates into the local system I get this:

later in frame 671:

end at last in frame 2498 out of 2500

If I move the emitters along the global y-axis and start the animation from I get this:

The particle flow is constant during the whole animation but the particle-curve is smaller like this in the first picture with global coordinates. All particle-curves has the same parameters (phase,t).

I expected that the flow behaves like in the first picture. Can me someone explaine why it doesn’t work this way after converting the particles global coordinates in locale coordinates ?

Explanation what you are seeing in the pictures:
The first cube(in front of the picture) is an emitter which uses this formulars for the particles location:

xp = old_xp+ (-phase * (2 * math.cos(t) - math.pow(math.sin(t),3)))
yp = old_yp + (-phase * (math.cos(t) * math.pow(math.sin(t),2) ))

The second cube uses:
xp_tr = old_xp_tr - (1/4)* phase * (5 * math.cos(t) - math.sin(t3))
yp_tr = old_yp_tr - (1/4)
phase * (5 * math.sin(t) + math.sin(t*3))

Both are moving along a path which is a straight line along the y-axis heading to the observer (you).