求教一个用ansys做随机振动分析的问题
<P>我用ansys做随机振动分析，请问如何在post 26中提取协方差，就是covariance, <BR>我每次提取时他都说：“store, psd command has previously been issued, cvar command can not be used in the same session, Please reset, redifine variables and then reissue CVAR"<BR>可是我reset了也redefine了，可是还是弹出同样的错误，请问如何解决？</P>ANSYS使用手则是这样的
<P 0cm 0cm 0pt">在<FONT face="Times New Roman">ANSYS 5.7</FONT>中用<FONT face="Times New Roman">post 26</FONT>后处理器来计算协方差：</P><P 0cm 0cm 0pt"><FONT face="Times New Roman">1 </FONT>选择菜单途径<FONT face="Times New Roman">Main Menu>TimeHist Postpro>Define Variable</FONT>，弹出<FONT face="Times New Roman">Define Time-History Variables</FONT>对话框。单击<FONT face="Times New Roman">Add</FONT>，弹出<FONT face="Times New Roman">Add time-History Variable</FONT>对话框。<p></p></P>
<P 0cm 0cm 0pt"><FONT face="Times New Roman">2 </FONT>选择<FONT face="Times New Roman">Nodal DOF result</FONT>，然后单击<FONT face="Times New Roman">OK</FONT>，弹出<FONT face="Times New Roman">Define Nodal Data</FONT>对话框。<p></p></P>
<P 0cm 0cm 0pt"><FONT face="Times New Roman">3 </FONT>指定<FONT face="Times New Roman">Ref. Number of variable</FONT>为<FONT face="Times New Roman">2</FONT>，<FONT face="Times New Roman">Nodal number</FONT>为<FONT face="Times New Roman">245</FONT>，<FONT face="Times New Roman">User-specified label</FONT>为<FONT face="Times New Roman">UZ245</FONT>，在<FONT face="Times New Roman">Data item</FONT>的右边窗口选择<FONT face="Times New Roman"> Translation UZ</FONT>。单击<FONT face="Times New Roman">OK</FONT>，关闭<FONT face="Times New Roman">define Nodal Data</FONT>对话框。<p></p></P>
<P 0cm 0cm 0pt"><FONT face="Times New Roman">4 </FONT>单击<FONT face="Times New Roman">Add</FONT>，弹出<FONT face="Times New Roman">Add Time-history Variable</FONT>对话框。<p></p></P>
<P 0cm 0cm 0pt"><FONT face="Times New Roman">5 Ref. Number of variable</FONT>为<FONT face="Times New Roman">3</FONT>，<FONT face="Times New Roman">Nodal number</FONT>为<FONT face="Times New Roman">240</FONT>，<FONT face="Times New Roman">User-specified label</FONT>为<FONT face="Times New Roman">UY240</FONT>，在<FONT face="Times New Roman">Data item</FONT>的右边窗口选择<FONT face="Times New Roman">Translation UZ</FONT>。单击<FONT face="Times New Roman">OK</FONT>，关闭<FONT face="Times New Roman">Define Nodal Data</FONT>对话框。<p></p></P>
<P 0cm 0cm 0pt"><FONT face="Times New Roman">6 </FONT>单击<FONT face="Times New Roman">Close</FONT>，关闭<FONT face="Times New Roman">Define Time-History Variables</FONT>对话框。<p></p></P>
<P 0cm 0cm 0pt"><FONT face="Times New Roman">7 </FONT>选择菜单途径<FONT face="Times New Roman">Main Menu>TimeHist Postpro>Calc Covariance</FONT>，弹出<FONT face="Times New Roman">Calculate Covariance</FONT>对话框，给<FONT face="Times New Roman">Reference number of resulting variables</FONT>输入<FONT face="Times New Roman">4</FONT>，给<FONT face="Times New Roman">Ref. no. of variables to be operated on</FONT>分别输入<FONT face="Times New Roman">2</FONT>和<FONT face="Times New Roman">3</FONT>。<p></p></P>
<P 0cm 0cm 0pt"><FONT face="Times New Roman">8 </FONT>单击<FONT face="Times New Roman">OK</FONT>，协方差计算结果输出到<FONT face="Times New Roman">Output window</FONT>窗口中。<p></p></P>
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