# Fluent Induced Drag Aerofoil Convergence Error

I am currently modelling what happens, to lift induced drag, when you apply washout to a wing (Twist). However I have a few issues

1. the drag values I calculate are into the 1000s

2. the drag values oscilate between, mesh studies (I could have +ve 10000s then -ve0.001 then +ve 100s)

3. My residules never truly converge but rather oscilate at 1e-02

and i dont know why. here is the journal file for the case. I am using Kw-SST, the X/Y velocity components are set as such that there is a total of 113m/s at a pitch attitude of 0.7degrees

; READING MESH FILE

/rc doubleup.msh

; ENERGY MODEL

; Enable energy model and viscous heating
/define/models/energy y n n n y

; TURBULENCE MODEL
; Enable kw-SST turbulence model

/define/models/viscous/kw-sst y

; MATERIALS PROPERTIES

; Define air properties
/define/materials/change-create/air/air y incompressible-ideal-gas n n n n n n

; CELL ZONE CONDITIONS

; Set operating pressure to 86884 Pa
/define/operating-conditions/operating-pressure 86884.9682 q q

; BOUNDARY CONDITIONS

; Set temperature to 275.65K for pressure outlets
/define/boundary-conditions/set/pressure-outlet/farfield outlet () t0 n 275.65 q q q

; Set the temperature to 275.65K, the velocity to 113 m/s and the corresponding angles of attack in the velocity inlet
; WARNING: be careful to the axis of the model mesh and change the velocity components (x,y,z) accordingly
/define/boundary-conditions/velocity-inlet/inlet y y n 113.17 n 0 y n 0.99993 n 0.01220 n 0 n 275.65 n n y 5 10

; MONITORS

; Remove the residuals plot
/solve/monitors/residual/plot n q

; Set convergence criteria to 1e-03
/solve/monitors/residual/convergence-criteria 1e-03 1e-03 1e-03 1e-03 1e-03 1e-03 1e-03

; CALCULATIONS
/solve/init/init
/solve/iter/500
/exit ok


this is what the mesh looks like, its got a far field size of 15metres in each direction and 30metres behind the wing, I'd argue that it is refined in all the right places (BOI around wing more refined than farfield)?

this is what the residuals look like: 500 simulations doesn't seem alot but comparing with my peers is should be more than enough to reach convergence so I am not sure if going 1000+ would actually converge or just repeat the oscilation.

to calculate induced drag I create a trefftz plane at 6m beyond the aerofoil (2x Root Chord) I then used the surface integral of my custom function of: 0.5 * density * ((Vz * Vz) + ( cos (0.7) * (Vy * Vy)) - ( sin (0.7) * (Vx * Vx))) (Where x is the direction from leading-trailing edge, z is direction from root-tip and y is vertical).i then take this value divide by the dynamic pressure 0.5 * Air Density * Air Speed^2 and then divide that by the area of the trefftz plane which is 900m (30x30)

which is how i get to = 1396.322876 as its = 9858187800/7844.562451/900 which as incredibly huge value.

Anyone able to shed some help as to where I'm going wrong .

• I think you'll have to ask the people who wrote the software for help. Commented May 4, 2023 at 21:18
• Separately, that's one wild way to define the syntax of input files! Commented May 5, 2023 at 3:07
• @WolfgangBangerth its in scheme, which I think is a variant of LISP. its effectively the commands you would type in the GUI of fluent prewritten. to start the solver i would go into the /solve menu then enter init then enter init henche /solve/init/init Commented May 8, 2023 at 14:15
• I figured something like this, from the use of ; as a comment marker. It's still wild. Nobody has written programs in LISP in seriousness since the early 1990s... Commented May 8, 2023 at 20:00
• @WolfgangBangerth I mean youre talking about an industry that still writes in fortran? one of those if it ain't broken don't fix it. type moments? Commented May 9, 2023 at 2:30