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Attachment Line Uncertainty Examined at 12 degrees and eta = 0.65


 Satt , the attachment line location, was initially estimated based solely on the experimental pressure data.  In order to see how sensitive this estimation is to the location and number of the pressure ports, the following were used:
  1.  a preliminary CFD data set  to mimic a more dense set of pressure taps
  2.  a set of virtual taps from the CFD approximately located at the experimental positions
  3.  another virtual set of taps offset from case 2
cp vs s          In the figure on the left (click figure for larger version)  you can see the results when the attachment line location is estimated using the CFD solution as the pressure information (case 1), and the results of cases 2 and 3.     In case 1 all of the CFD pressures were used except for the value of Cpmax, following the recommendations made by Bertelrud.   In Cases 2 and 3, Cpmax was retained, because the ports were  already widely spaced. The value of Satt determined strictly from the experimental pressure data is also include in the figure.  Clearly the location of the pressure ports has a direct effect on the estimation of the attachment line location.

Next it was desired to determine the effect in Case 1 of discarding the maximum value of Cp.  Including Cpmax gave a value of Satt = -0.499 versus -0.414 for without.



Next the CFD data that was provided in a TecPlot file was examined.  TecPlot will plot the streamlines and therefore the attachment line.   Picking off the location at the appropriate height for eta = 0.65, a value of Satt = -0.59 was obtained.   There was a bit of uncertainty involved in this with translating back and forth between deployed and nested and other issues. In the end, an estimated number for that uncertainty would be about ± 0.05.  Also, since the location of Cpmax is used in 2-D to specify the attachment line, that was located from the CFD data as Satt = -0.509.

So, to summarize,

Method used
 Satt
CFD without Cpmax
 -0.41
CFD with Cpmax
 -0.50
Virtual taps , case 2
 -0.52
Virtual taps, case 3
 -0.90
Experimental data
 -0.67
Satt as determined by tecplot
 -0.59
Location of Cpmax
 -0.51


Also, since the location of the attachment line has a direct effect on the calculation of Rbar, this would reflect uncertainty there as well.