# What is the most accurate interpolation method for a 3D-flowfield on a structured grid?

I solve multi-species, compressible Navier-Stokes equations on a 3D structured grid. I have obtained a solution on a given grid (let's say a relatively coarse one). I want now to refine my grid and interpolate my previous solution on my new grid before restarting my simulation. Currently, we have an interpolation tool that builds a k-d tree of the 2 grids and then can use 2 different methods to compute the values on the new grid:

• simple averaging
• inverse-distance-weighted (IDW)
• moving least squares (MLS)

I want to focus on accuracy because since I deal with large gradients, not capturing them correctly will generate waves when I restart my computation. I first tried simple averaging but the accuracy was not good enough.

I thought MLS method with polynomials of order 2 would give me reasonable results since it is supposed to be non-oscillatory. However, when I look at my interpolated field, I see local minima/maxima that overshoot values of my initial field. Does this mean the implementation of MLS in this program is not correct? Should I be careful with the size of my stencil and the order of the polynomials? Which other method would you recommend?

Thanks in advance !

## 1 Answer

You could use monotone cubic splines:

http://en.wikipedia.org/wiki/Monotone_cubic_interpolation

An explanation of how to do it in multi-D is here:

http://dl.acm.org/citation.cfm?id=1285766

Another option would be weighted essentially non-oscillatory interpolation; there is a recent review paper on the topic by Chi-Wang Shu.

• I checked the multi-dimensional Monotone cubic interpolation paper and there is a strong precondition for the method to be applicable: > the nodes providing the interpolation data are equally spaced or > follow a strictly monotone, continuous one-to-one mapping from > [0, n] to the interpolation interval. Clearly, this won't be true for my general 3D flowfield. I will dig the other reference though, thanks. Nov 30, 2011 at 8:55
• Here is the article I think David was refering to. Nov 30, 2011 at 14:36
• Yes Matt, that is the one. Nov 30, 2011 at 15:04