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

1 files changed, 5 insertions, 1 deletions

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;