A problem about transient-wave boundary

Lee

Hi, everyone I created one model which has one straight round pipe and a cubic volume, the pipe's termination enter the center of the volume, the model is symmetrical and regular,so I only models quarter of it. I trigered one sine compression wave at the firt end for 7ms then I closed that end, and I define the boundaries at the volume(outflow region) the transient-wave transmissive boundaries, except the symmetry plane boundaries.

There are two problems:

1. When I extracted some pressure data at some points by two ways, one way is that I used subroutine posdat.f to get data; the other way is that I use the "load transient post data" function by star-cd to get the pressure data at the same points. But I found the results by the two ways have some difference, it looks that the data extracted by "load transient post data" correct, otherwise the data by postdat.f have some defects which don't obey the physics law, for example, at some time the pressure at the points drops suddenly together, although the points have a far distance. so did anybody met the same problem? What is the reason to cause that?

2. When I changed the volume size but keep the other things unchanged(one size is 0.1x0.1x0.1m3, one is 1x1x1m3), I found the compression wave in different volume has different wave velocity at the same time in the same region according to the pipe termination when the compression wave enter the volume, that is to say,the compression wave propagates fast in volume 0.1x0.1x0.1m3, but propagates slightly slowly in volume 1x1x1m3. So what is the reason to cause the problem? Because I think the wave velocity should be the same even it propagates in different volume after the moment it enter the volume ( I used the transient -wave tranmissive boundary).

Can anybody give some advice on the problems or some answers? Thanks you very much for your regarding my problem!

Lee