From f300f50f897ae891c414a829b47e77f9ddf8ef5c Mon Sep 17 00:00:00 2001
From: eschnett <eschnett@6a3ddf76-46e1-4315-99d9-bc56cac1ef84>
Date: Tue, 8 Mar 2011 17:04:33 +0000
Subject: The Kerr initial data (in
 EinsteinInitialData?/IDAnalyticBH/src/Kerr.c) has a defect along the z-axis
 (that is, the spin axis of the hole). The three-metric (for instance) is not
 smooth across this axis, leading to discontinuities in second spatial
 derivatives near there.

This appears to be due to overly crude safety measures protecting from
division by zero when calculating this data. Tweaking these measures
for more self-consistency leads to much smoother behaviour. I'm
attaching a simple patch that fixes the problem at least for
metric_type = physical.

[Patch from Bernard Kelly]


git-svn-id: http://svn.einsteintoolkit.org/cactus/EinsteinInitialData/IDAnalyticBH/trunk@179 6a3ddf76-46e1-4315-99d9-bc56cac1ef84
---
 src/Kerr.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/Kerr.c b/src/Kerr.c
index dea5ee5..93a2611 100644
--- a/src/Kerr.c
+++ b/src/Kerr.c
@@ -117,8 +117,12 @@ void KerrID(CCTK_ARGUMENTS)
     /* Define coordinate functions */
     xx=x[i];  yy=y[i];  zz=z[i];
     rho_2=xx*xx+yy*yy;
-    if (rho_2<epsilon) rho_2=epsilon;
     rho=sqrt(rho_2);
+    if (rho<epsilon) {
+      xx=epsilon;
+      rho_2=xx*xx+yy*yy;
+      rho=sqrt(rho_2);
+    }
     R_2=rho_2+zz*zz;
     R=sqrt(R_2);
     R_3=R*R_2;
-- 
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