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Different one sided derivative approximation?

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Old   January 4, 2006, 23:56
Default Different one sided derivative approximation?
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In my FVM code, I am trying to approximate the face value of pressure at the boundary wall. Since it is a cell-center code, all pressure values are at the cell's center.

For e.g. at the south/bottom wall,


the simplest approx will be p_s=p(i,j)

higher accuracy from Blazark's CFD book gives p_s=(1/8)*(15*p(i,j)-10*p(i,j+1)+3*p(i,j+2)).

however, it is also possible to use


where h=delta, p_n=(p(i,j)+p(i,j+1))/2, p_nn=(p(i,j+1)+p(i,j+2))/2

simplifying, p_s=(2/3)*p(i,j)-(1/3)*p(i,j+1)-(1/6)*p(i,j+2)

based on the no. of values used, they should be of the same accuracy, but why are their values different?

Also, does anyone has similar expression for non-uniform grids?

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Old   January 5, 2006, 00:31
Default Re: Different one sided derivative approximation?
Praveen. C
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They may be of the same order but the error can be different. Remember that the error is of the form Ch<sup>r</sup> and different schemes of the same order can have different values for C.

For non-uniforms grids you can fit a polynomial

p(y) = p<sub>s</sub> + p<sub>1</sub> y + p<sub>2</sub> y<sup>2</sup> + ...

and also use

dp/dy(y=0) = 0

to obtain the value of p<sub>s</sub>
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Old   January 11, 2006, 04:40
Default Re: Different one sided derivative approximation?
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i think there is a mistake in ur simplification, it should come as Ps = (2/3)*Pi,j + (1/2)*Pi,j+1 - (1/6)*Pi,j+2
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