Problem with interpolation

[MohamedMostafaShehata]

Greetings;

I have a problem with interpolation. Given the following set of data (Renold's number vs Drag coeff.). How to find the drag coefficient for ex, Re = 270000. I understood that interpolation is done in calcpad using the index of the element inside the array. However, the reuired Re is not inside of the array and usually it lies between 2 values.

Re = 11366.13396
12467.44752
724332.8937
13386.70005
14070.21458
14788.62884
15879.02721
16808.95365
18177.16969
19105.2819
20368.53695
21715.31934
23482.90398
26313.90945
29069.64665
31660.29324
34237.37936
39192.49247
44230.92844
49563.23319
55144.67792
60487.88274
65878.47895
71240.85864
77039.72557
85715.36965
94691.95418
106107.6394
120603.3644
136107.6687
156918.6497
179629.198
213072.446
256363.8875
299797.5037
360709.4909
407080.9145
482870.976
602016.8554
865322.8397
997631.4551

Cd = 1.146401985
1.124069479
1.079404467
1.099875931
1.084987593
1.066377171
1.047766749
1.023573201
1.001240695
0.982630273
0.960297767
0.937965261
0.913771712
0.883995037
0.854218362
0.839330025
0.828163772
0.815136476
0.811414392
0.811414392
0.833746898
0.859801489
0.885856079
0.902605459
0.92866005
0.949131514
0.965880893
0.986352357
1.003101737
1.016129032
1.025434243
1.036600496
1.045905707
1.055210918
1.064516129
1.068238213
1.070099256
1.070099256
1.077543424
1.081265509
1.081265509

[Proektsoftbg]

Hi @MohamedMostafaShehata! Nice to see you on GitHub!

Yes, they are discrete values, but Calcpad includes linear and spline interpolation functions: line(x; y_values) and spline(x; y_values), respectively. You have to put your data instead of y_values and it will become a continuous function.
You should do that for both Re and Cd by defining the functions:
Re_lin(x) = line(x; Re)
Cd_lin(x) = line(x; Cd)
'Then, find x for your Reinolds number Re0 = 27000' by using the following command
x0 = $Find{Re_lin(x) - Re0 @ x = 1 : len(Re)}
'And finally extract the corresponding Cd value
Cd0 = Cd_lin(x0)

Below is the complete working code you can use:

'Reinold′s values
Re = [11366.13396; _
12467.44752; _
724332.8937; _
13386.70005; _
14070.21458; _
14788.62884; _
15879.02721; _
16808.95365; _
18177.16969; _
19105.2819; _
20368.53695; _
21715.31934; _
23482.90398; _
26313.90945; _
29069.64665; _
31660.29324; _
34237.37936; _
39192.49247; _
44230.92844; _
49563.23319; _
55144.67792; _
60487.88274; _
65878.47895; _
71240.85864; _
77039.72557; _
85715.36965; _
94691.95418; _
106107.6394; _
120603.3644; _
136107.6687; _
156918.6497; _
179629.198; _
213072.446; _
256363.8875; _
299797.5037; _
360709.4909; _
407080.9145; _
482870.976; _
602016.8554; _
865322.8397; _
997631.4551]
'Drab coefficient values
Cd = [1.146401985; _
1.124069479; _
1.079404467; _
1.099875931; _
1.084987593; _
1.066377171; _
1.047766749; _
1.023573201; _
1.001240695; _
0.982630273; _
0.960297767; _
0.937965261; _
0.913771712; _
0.883995037; _
0.854218362; _
0.839330025; _
0.828163772; _
0.815136476; _
0.811414392; _
0.811414392; _
0.833746898; _
0.859801489; _
0.885856079; _
0.902605459; _
0.92866005; _
0.949131514; _
0.965880893; _
0.986352357; _
1.003101737; _
1.016129032; _
1.025434243; _
1.036600496; _
1.045905707; _
1.055210918; _
1.064516129; _
1.068238213; _
1.070099256; _
1.070099256; _
1.077543424; _
1.081265509; _
1.081265509]
'Apply linear interpolation
Re_lin(x) = line(x; Re)
Cd_lin(x) = line(x; Cd)
'Then, find x for your Reinolds number'Re0 = 27000'by using the following command
x0 = $Find{Re_lin(x) - Re0 @ x = 1 : len(Re)}
'And finally extract the corresponding Cd value
Cd0 = Cd_lin(x0)

[MohamedMostafaShehata]

Thanks Net, it works perfectly. BTW, I have installed V.7.1.8, it is amazing. Great work

[Proektsoftbg]

Thank you! I am glad it worked.