sci.physics.computational.fluid-dynamics

In xfoil, use the Re corresponding to wind tunnel conditions, but
prepare the airfoil geometry with unit chord. Then the Cl you get should
be comparable with wind tunnel value (I dont know if you simulate the
wind tunnel walls in xfoil, that can have some effect)

Keta wrote:
> Yes, that's how the normal Re works. However, XFoil uses its own
> definition of Re number, not taking into account any characteristic
> length. Here is what the documentation says about units:
>
> ======================================
> Units
> =====
> Most XFOIL operations are performed on the airfoil's cartesian
> coordinates x,y , which do not necessarily have a unit chord c.
> Since the chord is ambiguous for odd shapes, the XFOIL
> force coefficients CL, CD, CM are obtained by normalizing the
> forces and moment with only the freestream dynamic pressure
> (the reference chord is assumed to be unity). Likewise, the
> XFOIL Reynolds number RE is defined with the freestream velocity
> and viscosity, and an implied unit chord:
>
> CL = L / q | V = freestream speed
> CD = D / q | v = freestream kinematic viscosity
> CM = M / q | r = freestream density
> RE = V / v | q = r V^2
>
> The conventional definitions are
>
> Cl = L / q c
> Cd = D / q c
> Cm = M / q c^2
> Rc = V c / v
>
> so that the conventional and XFOIL definitions differ only by
> the chord factor c or c^2.
>
> For example, a NACA 4412 airfoil is operated in the OPER menu at
>
> RE = 500000
> ALFA = 3
>
> first with chord= , and then with chord= (changed with SCAL
> command in the GDES menu, say). The results produced by XFOIL are:
>
> c = : CL = CD = (RE = 500000, Rc = 500000)
> c = : CL = CD = (RE = 500000, Rc = 250000)
>
> Since CL is not normalized with the chord, it is nearly proportional
> to the airfoil size. It is not exactly proportional, since the true
> chord Reynolds number Rc is different, and there is always a weak
> Reynolds number effect on lift. In contrast, the CD for the smaller
> airfoil is significantly greater than 1/2 times the larger-airfoil
> CD,
> since chord Reynolds number has a significant impact on profile
> drag.
> Repeating the c = case at RE = 1000000, produces the expected
> result that CL and CD are exactly 1/2 times their c = values.
>
> c = : CL = CD = (RE = 1000000, Rc = 500000)
>
>
> Although XFOIL performs its operations with no regard to the size
> of the airfoil, some quantities are nevertheless defined in terms
> of the chord length. Examples are the camber line shape and BL trip
> locations, which are specified in terms of the relative x/c,y/c along
> and normal to the airfoil chord line. This is done only for the
> user's
> convenience. In the input and output labeling, "x,y" always refer
> to the cartesian coordinates, while "x/c,y/c" refer to the chord-
> based coordinates which are shifted, rotated, and scaled so that
> the airfoil's leading edge is at (x/c,y/c) = (0,0), and
> the airfoil's trailing edge is at (x/c,y/c) = (1,0). The two
> systems cooincide only if the airfoil is normalized.
> ======================================
>
> Note that the usual Re is written as Rc. Now I think that I have to
> scale the Reynolds number, and perform the calculations with RE=Rc/c,
> and then I'll get Cl as Cl=CL/c, but I'm not sure.
>


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