Effect of NCR On VLM and LLT in XFLR5

 

Effect of Ncr on VLM and LLT in XFLR5 An S5010 airfoil was solved twice by setting Ncr = 9 and Ncr = 4. The effect of Ncr on CL and CD in 3D is observed. Wing: AR = 10, Re = 70,000

Alpha-CL  1.2 1 0.8      L

     C0.6

LLT N4 LLT N9

0.4

VLM N4 VLM N9

0.2 0 0

5

10

15

20

Alpha

CL Effect •

  Ncr  has no effect on VLM, VLM calculates inviscid C L  but LLT does take 2D viscous CL into account.

•

  Note that VLM doesn’t calculate stall but it stops when it reaches a local CL  in the solution that doesn’t have a corresponding one in 2D to interpolate CD  from.

 

CD-CL  1.2 1 0.8      L

     C0.6

LLT N4 LLT N9

0.4

VLM N4 VLM N9

0.2 0 0

0.05

0.1

0.15

0.2

CD

CD Effect • •

  Both VLM and LLT are affected by N cr and by 2D viscous results in general.   CD is the same in both methods vs CL, for the same Ncr but this doesn’t mean that it is the same vs Alpha since Alpha-CL curves are different

 

Alpha vs CL3/2/CD 

20 18

LLT N4

16

LLT N9 VLM N4

14

VLM N9

12      D

     C      /

10

     2      /      3      L

     C 8

6 4 2 0 0

5

10

15

20

Alpha

•

  Although both VLM curves have same CL but they have different CD since Ncr  affects CD, so the power coefficient curves are different.

 

Alpha-Cm  0.1 0.05

0

0

5

10

15

20

-0.05 LLT N4     m

     C -0.1

LLT N9 VLM N4

-0.15

VLM N9

-0.2 -0.25 -0.3

•

Alpha

  Cm in VLM is inviscid and is not affected by Ncr as in CL 

Conclusion VLM: • •

  CL is inviscid, CD is viscous from 2D solution   Stall is not estimated. The solution just stops when local C L reaches max in any span position.

LLT: • •

  CL and CD are both viscous.   Stall is approximated in some way using 2D solution.

So, in a Low Re case with separation bubble present, using the VLM is equivalent of ignoring separation separa tion bubble effect on CL and treating the problem as a s a high Re. Thus, I believe that the error of LLT not taking sweep effect into account is much smaller than treating the CL as inviscid in VLM