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# Day08-slides - Program 2 in C The following approach seems...

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1 Atms 502 Numerical Fluid Dynamics Tue., Sep. 19, 2006 9/29/06 Atms 502 - Fall 2006 - Jewett 2 Program 2 -- in C The following approach seems to work: #define NX 25 #define BC_WIDTH 1 #define I1 BC_WIDTH #define I2 I1+NX-1 #define NXDIM NX+2*BC_WIDTH update(u1,i1,i2,nx) float u1[ ]; int i1,i2,nx; { int i; for (i=i1; i<=i2; i++) u1[i]=u2[i]; } update(u1,nx) float u1[ ]; int nx; { int i; for (i=1; i<=nx; i++) u1[i]=u2[i]; } OLD NEW 9/29/06 Atms 502 - Fall 2006 - Jewett 3 Discrete-Dispersion Relation Physical root behavior: 2 x speed is … Lagging phase error (decelerating) how do we know this? c phys = " phys k = 1 k # t arcsin μ sin k # x ( ) c phys \$ c 1 % k 2 # x 2 6 1 % μ 2 ( ) & ( ) * + For small k x - Durran calls “in limit of good spatial resolution” 9/29/06 Atms 502 - Fall 2006 - Jewett 4 Takacs (1985) Considerations : Odd-order schemes generally dissipative (upstream) Even-order schemes generally dispersive (leapfrog) Constructs scheme with “some of improved phase characteristics associated w/third-order scheme” Second-order schemes : 4th-order amplification errors 3rd-order phase errors Extra point => 3rd-order accuracy, reducing phase error Takacs pp. 1051-1052

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2 9/29/06 Atms 502 - Fall 2006 - Jewett 5 Takacs (1985) Amplitude error (Fig. 1) θ is k x ; μ is Courant number Odd-order schemes most dissipative Even-order schemes least dissipative
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