While using TurboGrid Meshing, after loading the attached curves, it may be noticed by the casual observer that the low periodic boundary is rather close to the blade in one spot.
When attempting to set up the topology, the message below appears:
"The O-grid distance is too large, or, the inlet and/or outlet points are too close to the blade".
Upon further review, reducing the distance factor with the H/J grid can make this message go away, but the resultant grid is a mess.
Is there a way to resolve this problem?
The reason for this error message is because the o-grid effectively exceeds the limits of the domain. In this case, the low periodic is too close to the blade. The reason that the low periodic is too close is a function of this blade design. The thickness, stagger angle, solidity, and asymmetry of the blade thickness are causing this low periodic to sit almost on the blade when TurboGrid calculates the periodics. To generate the periodics, TG takes the leading and trailing edge lines and makes a surface between them mid way between the upper and lower blade surfaces. It then rotates those surfaces by 360/(# of blades/2) to make the periodics. By moving the leading edge curve around a bit, the low periodic side will move away from the blade. Of course, this could cause other problems like node distribution at the leading edge, or if the curve is moved to far the high periodic will hit the blade.
The steps required to fix this problem are:
1) move leading edge points on blade
2) move inlet at the hub point away from the blade (similar distance as at the shroud)
3) choose h/j-grid topology with o-grid distance factor of 0.15
4) move the control points on the o-grid and the inlet surface near the leading edge of the blade on both the hub and shroud.