Specific analysis data in the results file can often be processed to yield additional variable sets that provide valuable information. For example, by defining a displacement variable in a transient analysis, you can calculate the velocity and acceleration by taking derivatives with respect to time. Doing so yields an entirely new variable that you may wish to analyze along with other analysis data.
Some commands identify the variable and the format for the output, while others identify the variable data to be used to create the new variable. The actual calculations are performed by specific commands.
Example 8.3: Finding the Derivative of a Variable UYBLOCK with Respect to Variable TIME
NSOL,2,100,u,y,UYBLOCK ! Variable 2 is UY of node 100
DERIV, 3,2,1,,VYBLOCK ! Variable 3 is named VYBLOCK It is the
! derivative of variable 2 with respect
! to variable 1 (time) Example 8.4: Finding the Amplitude of Complex Time-History Variable PRESMID
NSOL,2,123,PRES,,PRESMID ! Variable 2 is the pressure at node 123
ABS, 3,2,,,AMPL_MID ! Absolute value of a complex variable
! is its amplitude.Example 8.5: Finding the Phase Angle (in Degrees) of a Complex Time-History Variable UYFANTIP
Pi = acos(-1)
ATAN,4,2,,,PHAS_MID,,,180/pi ! ATAN function of a complex
! variable (a + ib) gives atan (b/a)Example 8.6: Multiplying a Complex POST 26 Variable PRESMID with a Factor (2+3i)
CFACT,2,3 !Scale factor of 2+3i
ADD,5,2,,,SCAL_MID ! Use ADD command to store variable 2 into
! variable 5 with the scale factor of (2+3i)Example 8.7: Filling a Variable with Ramped Data
FILLDATA,6,,,.25,.05,ramp_func !Fill a variable with
!ramp function data.
The following commands are available to process variables, develop calculated relationships, and store the data:
ABS -- Forms the absolute value of a variable.
ADD -- Adds (sums) variables.
ATAN -- Forms the arctangent of a complex variable.
CLOG -- Forms the common log of a variable.
CONJUG -- Forms the complex conjugate of a variable.
DERIV -- Differentiates a variable.
EXP -- Forms the exponential of a variable.
IMAGIN -- Forms an imaginary variable from a complex variable.
INT1 -- Integrates a variable.
LARGE -- Finds the largest (the envelope) of three variables.
NLOG -- Forms the natural log of a variable.
PROD -- Multiplies variables.
QUOT -- Divides two variables.
REALVAR -- Forms a variable using only the real part of a complex variable.
SMALL -- Finds the smallest of three variables.
SQRT -- Forms the square root of a variable.
RPSD -- Calculates response power spectral density (PSD).
CVAR -- Calculates covariance between two quantities.
RESP -- Generates a response spectrum.