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 Dianhong November 6, 2002 15:11

Dear all,

I am quite new to CFD, some of the data formats always confuses me when I read some paper or do some simulation. Maybe it is quite simple (obvious) to you, so, all experts here, please give me some help.

1. What is the purpose of IBLANK in Plot3D?

From some document, it says, 'The value is defined as 0 in all no-flow regions. At an interface it is defined as the negative of the zone number it interfaces with. It is two at solid walls and everywhere else (interior flow regions) it is 1'.

But I am just wondering when (what condition) it is used for each value? (0, negative, 1, 2). Any simple example?

2. It seems IBLANK is corresponding to overset or chimera grids, am I right? But IBLANK can also be used for hole data, right?

3. 'overset' is just exactly the same as 'chimera' (overset == chimera)? I also saw in some paper, which refer the 'fringe' boundary, so what, on earth, this b.c is used for? Same as Overset?

 greg November 6, 2002 17:18

Chimera is the same as overset. The IBLANK variable is used to denote points in grids that are removed from the flow field because they would lay inside a solid body. Remember that with overset grids the grids overlap and can cross into regions where grid points should not be. In order to set these values of IBLANK appropriately an auxiliary code such as PEGSUS is used to assemble a set of grids and produce the holes. Around each hole a boundary of fringe points is created that require values of flow field variables. This boundary is the fringe. For a hole point, IBLANK = 0, for a fringe the value of IBLANK is either -1 or the negative of the grid number from which the flow field values are obtained. This is just a bookkeeping notation. Setting it to 2 for all solid walls provides a way of determining which fringe points can be treated using a solid wall BC. For field points (points not blanked or fringe) IBLANK is set to 1. The IBLANK variable is just a tag for differentiating amongst the types of points that arise. It is used to provide a simple modification to the block equations in a structured CFD solver so that for hole points the delta values always remain zero.

 Dianhong November 6, 2002 19:37

Greg, thank you for your help.

Below are some of my understandings based on your notes:

1. Hole is only defined for the overlapped regions, the boundary of this hole is also known as fringe.

2. If there is a *real* (not overlapped) hole inside the flow field, we can still set the IBLANK to 0, but at this boundary of this hole, what kind of IBLANK value do we set, -1 (or negative value), or 2? I think it should be 2 since it is actually a wall b.c for the boundary of a really hole.

3. If this is true, so we only set the IBLANK to 1 (or negative value) for the boundary points of a "hole" when the hole is "oversetted", i.e., it needs to define connectivity information with other zones. And no matter which kind of hole, IBLANK is always 0 inside the hole.

Do I understand the IBLANK correctly? If not, please be kind to point them out.

I am still not very clear about the words below:

"The IBLANK ... is used to provide a simple modification to the block equations in a structured CFD solver so that for hole points the delta values always remain zero."

I mean, how the delta values always remain zero when numerically solving governing equations - we just manually set all the delta values related with hole points to 0, no matter what kind of IBLANK (except IBLANK = 1) it is, right?

Again, any help from you is much appreciated!

 greg November 6, 2002 23:37

The exact values of the fringe really depend on the flow solver. In the original work by Steger and Benek, the fringe points all had values of -1. Later modifications were introduced for diagnostic purposes, and for the ability to apply physical boundary conditions when available, i.e. on a solid wall. The way most overset CFD codes work is to update the fringe points (often within a Newton iteration in a sort of "quasi-implicit" fashion), and then apply all physical boundary conditions, some of which may overwrite the fringe values. For example, my code may update all fringe points including those on a solid boundary. Then in a boundary condition pass I could use a solid wall boundary condition to replace those fringe points that are part of a solid surface. The reason for this is to better represent the values at those points - such as ensuring flow tangency or wall adherence exactly rather than hoping an interpolation will serve.

The basic modification to a CFD code using IBLANKs is to rewrite basic linear system

A*dQ = RHS as

(1 - IBLANK)*A * dQ = IBLANK * RHS

Then, when IBLANK = 0, the equation reads A*dQ = 0, or dQ = 0. Otherwise, for IBLANK = 1 the equations are unchanged. This prevents any inadvertent updating of hole points.

For more information, try looking for papers by J. A. Benek, Norm Suhs, Peter Buning, or Joe Steger, among others. you can also try looking for references for the Overflow code put out by NASA.

Hope this helps.

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