<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Hi Walter,<div><br></div><div>I have a number of questions regarding the Gale stress tensor and pressure output values in the .dat and vtk files.</div><div><br></div><div>Specifically, I've been doing tests with side stress boundary conditions and in doing so I noticed that there seems to be</div><div>a discrepancy between the stress tensor values and the magnitude/value of the pressure.</div><div><br></div><div>My interpretation of the pressure is (sigma_xx + sigma_yy)/2 in 2D or (sigma_xx + sigma_yy + sigma_zz)/3 in 3D.</div><div><br></div><div>However, I've found that when I calculate the pressure from the stress tensor values it does not match the pressure value</div><div>output in the vtk or .dat files.</div><div><br></div><div>As an example, I attached the output from running the cookbook model multi_material.xml using version 1.4.1. The .dat and vtk files for time step 9 are attached. </div><div><br></div><div>Based on the tensor output format described in the manual, sigma_xx and sigma_yy from the first line (x=0, y=0) in StressField.00009.dat are, respectively, 1.04375459900302 -1.05894084102394. The predicted pressure would then be ~ -0.007593. The pressure value (again, first line, x=0 and y=0 node) in PressureField.00009.dat is <span class="Apple-style-span" style="font-family: Menlo; font-size: 11px; ">-</span>0.715401855544872.</div><div><br></div><div>In the VTK file (fields.0.00009.vts) sigma_xx and sigma_yy at x=0 and y=0 (first nodal point value listed for each variable) are, respectively, 1.04399321765544 and -1.04778331139934. The predicted pressure ([sigma_xx+sigma_yy]/2 is -0.001895 while the listed pressure field value is -0.714266837882578.</div><div><br></div><div>I am completely misinterpreting what the pressure output represents or is there something else I'm not taking into account in terms of the stress tensor/pressure representation or output format? Is the pressure derived from the principal stresses (although this really shouldn't make a difference as the shear stresses are small in this case)?</div><div><br></div><div>I looked at the VTK output source code (VTKOutput.c) and the pressure is calculated (lines 652-685) from a different variable (<i>stressValues[]</i>) than the variable used to output (lines 286-295 in VTKOutput.c) the stress tensor (<i>stress[]</i>). I'm not sure what the difference is, if any, between these two variables.</div><div><br></div><div>I have some additional questions, but it will probably be easiest just to start with what I have so far, as the answers to these questions may clear up everything else.</div><div><br></div><div>Thanks in advance for the input and apologies in advance if this is just something very basic I've overlooked.</div><div><br></div><div>Cheers,</div><div>John</div><div><br><div>
<div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>John Naliboff</div><div>Postdoctoral Scholar, Department of Geology</div><div>One Shields Avenue, University of California, Davis, CA 95616</div><div><font class="Apple-style-span" face="'Myriad Pro'" size="4"><span class="Apple-style-span" style="font-size: 14px; "><br></span></font></div></div></div></div></div></div></body></html>