[Nek5000-users] skin friction on air foil

[nek5000-users at lists.mcs.anl.gov]

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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