Rotor simulation with Reynolds number effects at variable RPM

[MarcoPic99]

Hi!

I was reading your eVTOL example, where you simulate a maneuver and i noticed that, for all the propellers, you read just one polar at a fixed Reynolds number. This means that the aerodynamics coefficients of the propellers are assumed constant during the maneuver, right? So, if i would take into account the aero coefficients variation i need to run xfoil or provide a different polar at each time step ? Or there is a different way to do that?

Thank you in advance!

[EdoAlvarezR]

Hi Marco! That is correct. The Reynolds number is assumed to be constant throughout the maneuver.

Currently, there isn't a clean way of providing different polars for different time steps, but it can certainly be done if you are brave enough to manually modify the polars at each time step inside a custom-defined runtime function.

[MarcoPic99]

Thank you very much !

Marco

[EdoAlvarezR]

Related to #114

[MarcoPic99]

Hi ! I tried to implement this feature and seems to work. In the figure, i tested a small drone propeller during a maneuver and the Reynolds effect seems to be taken into account in the CT evaluation.
I think this feature could be useful for the simulation of small-scaled propellers which operates at low-Re regimes, where the effect on the aerodynamic coefficients could be not so neglegible.

Maybe after i finish to test the function i wrote, i could upload the code here, if someone is interested.

Thank you

Marco

[EdoAlvarezR]

It would be great if you could share the code here!

[MarcoPic99]

Hi! sorry for the late response. Here is the function. I'm not a programmer and i'm not experienced in Julia, so I hope someone could improve it! This function is then passed to "run_simulation" as ad additional input. I tested the function on several maneuvers and it works. Let me now if someone finds mistakes or have suggestions to improve it!

I hope this can be helpful to someone!

`function polaRe(sim, T)

rotor = sim.vehicle.rotor_systems[1][1] # Rotor 1

# spline chord
    spline_k=5
    spline_s=0.001
    splines_s=nothing
    spline_bc="extrapolate"
    _spl_chord = uns.Dierckx.Spline1D(rotor.r, rotor.chord;
                                    k= spline_k,
                                    s=splines_s!=nothing ? splines_s[1] : spline_s, bc=spline_bc)
    spl_chord(x) = uns.Dierckx.evaluate(_spl_chord, x)

alpha_vec = collect(-20:1:20) # Polar alpha range
Re_vec=LinRange(257, 51252, 1991) # Polar Reynolds range

# Read polar map from .csv file (it can be done outside the function)
data_CL = Matrix(uns.CSV.read("polar3/CLmap.csv", uns.DataFrames.DataFrame, header=false))
data_CD = Matrix(uns.CSV.read("polar3/CDmap.csv", uns.DataFrames.DataFrame, header=false)) 
data_Cm = Matrix(uns.CSV.read("polar3/Cmmap.csv", uns.DataFrames.DataFrame, header=false)) 

# Compute rotational velocity of the rotor
omega = uns.get_RPM(sim.maneuver,1,T)*sim.RPMref


airfoils = []
for i in 1:size(sim.vehicle.rotor_systems[1][1].airfoils,1)

	# Compute Reynolds number for each radial station in airfoil .csv file
	pos = sim.vehicle.rotor_systems[1][1].airfoils[i][1] 
	x_airfoil = sim.vehicle.rotor_systems[1][1].airfoils[i][2].x
	y_airfoil = sim.vehicle.rotor_systems[1][1].airfoils[i][2].y
	roR = (rotor.hubR + pos*(rotor.rotorR-rotor.hubR))/rotor.rotorR
	ReD = (2*rotor.rotorR)/(1.54e-5)*omega/60*sqrt((2*pi*roR*rotor.rotorR)^2+(0.001*2*rotor.rotorR)^2)
	Re_c = ceil(Int, ReD*spl_chord(roR*rotor.rotorR)/(2*rotor.rotorR))
	
	#Interpolate the aerodynamic coefficients
	interp_CL = Interpolations.interpolate((alpha_vec, Re_vec), data_CL, Gridded(Linear()))
	interp_CD = Interpolations.interpolate((alpha_vec, Re_vec), data_CD, Gridded(Linear()))
	interp_Cm = Interpolations.interpolate((alpha_vec, Re_vec), data_Cm, Gridded(Linear()))

	CL_i = interp_CL(alpha_vec,Re_c)
	CD_i = interp_CD(alpha_vec,Re_c)
	Cm_i = interp_Cm(alpha_vec,Re_c)

	#Create new polar
	polar = vlm.ap.Polar(-1, alpha_vec, CL_i, CD_i, Cm_i; x=x_airfoil,y=y_airfoil)
	push!(airfoils, (pos, polar))

end

#update rotor with new polars
rotor.airfoils = airfoils
vlm._calc_airfoils(rotor, rotor.m, 1/5, false, []; rediscretize=false,
                    rfl_n_lower=15, rfl_n_upper=15,
                    rfl_r=14.0, rfl_central=true)

sim.vehicle.rotor_systems[1][1] = rotor
sim.vehicle.system.wings[1] = rotor
sim.vehicle.wake_system.wings[1] = rotor

return sim

end`

[EdoAlvarezR]

Awesome, thanks for sharing! =]