diff --git a/sources/user_problem.F90 b/sources/user_problem.F90
index 5687c6a..50054b7 100644
--- a/sources/user_problem.F90
+++ b/sources/user_problem.F90
@@ -99,9 +99,10 @@ module user_problem
 
 ! include external procedures and variables
 !
-    use constants      , only : pi, pi2
-    use parameters     , only : get_parameter
-    use random         , only : randomu, randomn
+    use constants  , only : pi, pi2
+    use coordinates, only : ng, ady
+    use parameters , only : get_parameter
+    use random     , only : randomu, randomn
 
 ! local variables are not implicit by default
 !
@@ -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