Knowledgebase: Input
Using Table Objects and reporting performance curves
Posted by Richard Raustad on 28 October 2010 12:28 PM
Question: I'm trying to understand how Performance Curves and Lookup Table objects

The Engineering reference (pg 1093 pdf) states:
A regression analysis can be performed on performance tables when the
simulation is instructed to use the regression analysis during the
simulation (i.e., ride the curve).

I check out at In/Out reference if there is any input field which define
ride the curve during simulation, but without result.

Is there any input field which have to be input at idf, in order that
regression analysis can be performed during simulation ?

Answer: You need to use the diagnostics flag to report the performance curve object to the eio file. It appears the V6 example file and the documentation are a little behind the final product.

Output:Diagnostics, DisplayAdvancedReportVariables;

Notice in the example file that all table objects using "EvaluateCurveToLimits" for the interpolation method create a curve object in the eio file (at least they will if the correct diagnostics flag is used). Since this is a table object, you can't "ride the curve" unless you create the curve object coefficients using regression analysis. Also, since these coefficients were calculated for use during the simulation, the associated performance curve object is written to the eio file for later use.

First table from LookupTables.idf

! Create a performance curve and use it throughout the simulation.
! Since the DisplayExtraWarnings flag is used, write the performance cuve to eio file.
HPACCoolCapFFF, !- Name
Quadratic, !- Curve Type
EvaluateCurveToLimits, !- Interpolation Type

Notice just above this table object in the V6 example file is the object: Output:Diagnostics, DisplayExtraWarnings;, this is incorrect and needs to be corrected to read Output:Diagnostics, DisplayAdvancedReportVariables;

From the Input Output Reference document for the field Interpolation Method:

Field: Interpolation Method

The method used to evaluate the tabular data. Choices are LinearInterpolationOfTable, LagrangeInterpolationLinearExtrapolation and EvaluateCurveToLimits. When LinearInterpolationOfTable is selected, the tabular data is interpolated within the limits of the data set and the following fields are used only within the boundary of the table limits (i.e., are only used to limit interpolation). Extrapolation of the data set is not allowed. When LagrangeInterpolationLinearExtrapolation is selected, a polynomial interpolation routine is used within the table boundaries along with linear extrapolation. Given the valid one independent variable equations above, the polynomial order is up to 5th order. When EvaluateCurveToLimits is selected, the coefficients (i.e., C in the equations above) of the polynomial equation are calculated and used to determine the table output within the limits specified in the following fields. If these fields are not entered, the limits of the tabular data are used. The minimum number of data pairs required for this choice are 2, 3, 4, 5, and 4 data pairs for linear, quadratic, cubic, quartic, and exponent curves, respectively. If insufficient data is provided, the regression analysis is not performed and the simulation reverts to interpolating the tabular data. The performance curve is written to the eio file when the output diagnotics flag DisplayAdvancedVariables is used (ref. Output:Diagnostics, DisplayAdvancedVariables).
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