[Nek5000-users] skin friction on air foil
nek5000-users at lists.mcs.anl.gov
nek5000-users at lists.mcs.anl.gov
Wed Mar 28 06:29:43 CDT 2018
Hello Neks,
Here I have a problem whie caluclating the skin friction on airfoil. Could you please help me with this? I used a piece of your code to extract the viscous the stress applied on the object by using subroutine drgtrq ( dgtq ,xm0,ym0,zm0, sij ,pm1, visc ,f,e) .
Here is what I have done,
else ! 2D
i = 0
a = 0
do j2=js2,jf2,jskip2
do j1=js1,jf1,jskip1
i = i+1
n1 = unx(i,1,f,e) ! I removed area to avoid getting Force since I need stress
n2 = uny(i,1,f,e) ! I removed area to avoid getting Force since I need stress
a = a + area(i,1,f,e)
v = visc(j1,j2,1,e)
s11 = sij(j1,j2,1,1,e)
s12 = sij(j1,j2,1,3,e)
s21 = sij(j1,j2,1,3,e)
s22 = sij(j1,j2,1,2,e)
dg(1,1) = pm1(j1,j2,1,e)*n1 ! pressure drag
dg(2,1) = pm1(j1,j2,1,e)*n2
dg(3,1) = 0
dg(1,2) = -v*(s11*n1 + s12*n2) ! viscous drag
dg(2,2) = -v*(s21*n1 + s22*n2)
dg(3,2) = 0.
r1 = xm0(j1,j2,1,e)
r2 = ym0(j1,j2,1,e)
r3 = 0.
do l=1,2
do k=1,3
dgtq(k,l) = dgtq(k,l) + dg(k,l)
enddo
enddo
enddo
enddo
endif
Then I add dgtq(1,2) + dgtq(2,2) to get the total skin friction stress acting on the wall at each faces.
Since I know the face number and element number I extract the x cordinates of the corrrespoinding wall elements.
Do you think Am i doing right?
I have some questions
1 )Since the equation for skin frcition is :
tau_w = -mu*du/dy ( at wall) C_f = 2 * nu * du/dy
In our code it is written like :
dg(1,2) = -v*(s11*n1 + s12*n2) That is , stress is again muliplied with a normal velocity, and there are two components of stress in each dg. could you please tell me why?
2) The loopshown below muliplies my answer by 4 at the end. (Its getting added each time, I didnt understand the concept behind it,
I thought its just way to store all the values into a single array) Could you please tell me why?
do l=1,2
do k=1,3
dgtq(k,l) = dgtq(k,l) + dg(k,l)
enddo
enddo
Thanks in advance
Sijo GEORGE
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