From 9ddfd0f54ddb95b181421c851826e0bb462c249d Mon Sep 17 00:00:00 2001
From: Adrian Kummerlaender
Date: Sat, 26 Oct 2019 13:42:11 +0200
Subject: Add BGK relaxation time argument

Still thinking how various collision models (BGK, MRT and so on) may be
nicely abstracted in this setting.
---
 boltzgen.py              | 3 ++-
 boltzgen/lbm/__init__.py | 3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/boltzgen.py b/boltzgen.py
index ad76471..24a17b2 100755
--- a/boltzgen.py
+++ b/boltzgen.py
@@ -14,6 +14,7 @@ argparser.add_argument('--lattice',   dest = 'lattice',   required = True, help
 argparser.add_argument('--layout',    dest = 'layout',    required = True, help = 'Memory layout ("AOS" or "SOA")')
 argparser.add_argument('--precision', dest = 'precision', required = True, help = 'Floating precision ("single" or "double")')
 argparser.add_argument('--geometry',  dest = 'geometry',  required = True, help = 'Size of the block geometry ("x:y(:z)")')
+argparser.add_argument('--tau',       dest = 'tau',       required = True, help = 'BGK relaxation time')
 
 args = argparser.parse_args()
 
@@ -23,7 +24,7 @@ lbm = LBM(lattice)
 generator = Generator(
     descriptor = lattice,
     moments    = lbm.moments(),
-    collision  = lbm.bgk(f_eq = lbm.equilibrium(), tau = 0.6))
+    collision  = lbm.bgk(f_eq = lbm.equilibrium(), tau = float(args.tau)))
 
 geometry = Geometry.parse(args.geometry)
 
diff --git a/boltzgen/lbm/__init__.py b/boltzgen/lbm/__init__.py
index f80feaa..bafe331 100644
--- a/boltzgen/lbm/__init__.py
+++ b/boltzgen/lbm/__init__.py
@@ -51,6 +51,9 @@ class LBM:
         return f_eq
 
     def bgk(self, tau, f_eq, optimize = True):
+        if tau <= 0.5:
+            raise Exception('Relaxation time must be larger than 0.5')
+
         exprs = [ self.f_curr[i] + 1/tau * (f_eq_i - self.f_curr[i]) for i, f_eq_i in enumerate(f_eq) ]
 
         if optimize:
-- 
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