Improve to collisions

Improvement into the collision model to better compute number of
particles collisions.

src/modules/mesh/moduleMesh.f90

@@ -662,17 +662,24 @@ MODULE moduleMesh
       CLASS(meshVol), POINTER:: vol
       INTEGER:: nPart !Number of particles inside the cell
       REAL(8):: pMax !Maximum probability of collision
-      INTEGER:: rnd !random index
+      TYPE(pointerArray), ALLOCATABLE:: partTemp(:)
+      INTEGER:: i, j !random particle indexes
       TYPE(particle), POINTER:: part_i, part_j
       INTEGER:: n !collision
       INTEGER:: ij, k
-      REAL(8):: sigmaVrelMaxNew
-      TYPE(pointerArray), ALLOCATABLE:: partTemp(:)
+      REAL(8):: vRel, eRel
+      REAL(8):: sigmaVrelTotal
+      REAL(8), ALLOCATABLE:: sigmaVrel(:), probabilityColl(:)
+      REAL(8):: rnd !Random number for collision
+      INTEGER:: realCollisions
 
       IF (MOD(t, everyColl) == 0) THEN
         !Collisions need to be performed in this iteration
         !$OMP DO SCHEDULE(DYNAMIC)
         DO e=1, self%numVols
+
+          realCollisions = 0
+
           vol => self%vols(e)%obj
           nPart = vol%listPart_in%amount
 
@@ -694,21 +701,54 @@ MODULE moduleMesh
             END IF
 
             DO n = 1, vol%nColl
-              !Select random numbers
-              rnd = random(1, nPart)
-              part_i => partTemp(rnd)%part
-              rnd = random(1, nPart)
-              part_j => partTemp(rnd)%part
-              ij = interactionIndex(part_i%species%n, part_j%species%n)
-              sigmaVrelMaxNew = 0.D0
-              DO k = 1, interactionMatrix(ij)%amount
-                CALL interactionMatrix(ij)%collisions(k)%obj%collide(vol%sigmaVrelMax, sigmaVrelMaxNew, part_i, part_j)
-
+              !Select random different particles
+              i = 0
+              j = 0
+              DO WHILE (i == j)
+                i = random(1, nPart)
+                j = random(1, nPart)
               END DO
+              part_i => partTemp(i)%part
+              part_j => partTemp(j)%part
+              !TODO: I think that from here forward it can be passed to a procedure in interactionMatrix
+              ij = interactionIndex(part_i%species%n, part_j%species%n)
 
-              !Update maximum cross section*v_rel per each collision
-              IF (sigmaVrelMaxNew > vol%sigmaVrelMax) THEN
-                vol%sigmaVrelMax = sigmaVrelMaxNew
+              IF (interactionMatrix(ij)%amount > 0) THEN
+                !Obtain the cross sections for the different processes
+                vRel = NORM2(part_i%v-part_j%v)
+                eRel = interactionMatrix(ij)%rMass*vRel**2
+                CALL interactionMatrix(ij)%getSigmaVrel(vRel, eRel, sigmaVrelTotal, sigmaVrel)
+
+                !Update maximum sigma*v_rel
+                IF (sigmaVrelTotal > vol%sigmaVrelMax) THEN
+                  vol%sigmaVrelMax = sigmaVrelTotal
+
+                END IF
+
+                ALLOCATE(probabilityColl(0:interactionMatrix(ij)%amount))
+                probabilityColl(0) = 0.0
+                probabilityColl(1:interactionMatrix(ij)%amount) = sigmaVrel/vol%sigmaVrelMax
+
+                !Selects random number between 0 and 1
+                rnd = random()
+
+                !If the random number is below the total probability of collision, do collisions
+                IF (rnd < sigmaVrelTotal / vol%sigmaVrelMax) THEN
+
+                  !Loop over collisions
+                  DO k = 1, interactionMatrix(ij)%amount
+                    IF (SUM(probabilityColl(0:k-1)) + rnd <= probabilityColl(k)) THEN
+                      CALL interactionMatrix(ij)%collisions(k)%obj%collide(part_i, part_j, vRel)
+                      realCollisions = realCollisions + 1
+
+                    END IF
+
+                  END DO
+
+                END IF
+                
+                !Deallocate arrays for next collision
+                DEALLOCATE(sigmaVrel, probabilityColl)
 
               END IF
 
@@ -716,6 +756,8 @@ MODULE moduleMesh
 
           END IF
 
+          vol%nColl = realCollisions
+
         END DO
         !$OMP END DO