USER_PROBLEM: Limit blim parameter.

Signed-off-by: Grzegorz Kowal <grzegorz@amuncode.org>

sources/user_problem.F90

@@ -100,6 +100,7 @@ module user_problem
 ! include external procedures and variables
 !
     use constants  , only : pi, pi2
+    use coordinates, only : ng, ady
     use parameters , only : get_parameter
     use random     , only : randomu, randomn
 
@@ -187,6 +188,10 @@ module user_problem
 !
     kvec = pi2 * kper
 
+! lower limit for blim
+!
+    blim = max(blim, ng * ady(1))
+
 ! prepare the random perturbation of velocity
 !
     if (pert == 1) then