Hi Wei,

The domain defined by the SelectEVCalcDomain0 is {(x,y) | C0>0 && C1>0 && C2>0 && ... }. In your case, it should be

<FUNCTION NAME="SelectEVCalcDomain0"> 

  <E VAR="C0" VALUE="x+5" /> 

  <E VAR="C1" VALUE="y+10." /> 

  <E VAR="C2" VALUE=" 5-x" /> 

  <E VAR="C3" VALUE="10-y" /> 

</FUNCTION>

You can also define more than 1 domains, the final domain is the union of them, SelectEVCalcDomain0 + SelectEVCalcDomain1 + SelectEVCalcDomain2 + ...

Hope it works in your case.

Best

Ankang

-----Original Messages-----
From: "Wei He" <weiyh.he@gmail.com>
Send time: Tuesday, 04/29/2025 16:53:15
To: "ankanggao@ustc.edu.cn" <ankanggao@ustc.edu.cn>
Cc: nektar-users <nektar-users@imperial.ac.uk>
Subject: Re: [Nektar-users] 回复: 回复: Questions Regarding Eigenvalue Computation in Stability Analysis

This email from weiyh.he@gmail.com originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list to disable email stamping for this address.

 

Dear Ankang,

Recently I encountered a  similiar issue that when I computed the stability of a naturally steady cylinder flow, the solover always returned a 0 eigenvalue (-1e-10  + i*0) and took much longer time to converge. Inspired by your advice, I tried adding the entry SelectEVCalcDomain0 to restrict the eigenvalue calculation to the region near the cylinder locating at  (0, 5). I am expecting to compute the zone from corner (-5, -5) to (10,10), does the  entry setting correct?

<FUNCTION NAME="SelectEVCalcDomain0"> 
  <E VAR="C0" VALUE="-5" /> 
  <E VAR="C1" VALUE="10." /> 
  <E VAR="C2" VALUE=" -5" /> 
  <E VAR="C3" VALUE="10" /> 
</FUNCTION> 
 

When I used ModifiedArnoldi scheme, the running gives CFL=0 and crashed. When I switched to Arpack option, the calculation ran but the setup kept dumping out zero eigenvalue. Is there any approach that could help improve the simulation? thanks!

regards,

Wei

 

Hi Xiangjun,

You can use a smooth initial disturbance to avoid eigenvalue of 1. If BaseFlow_interporder is not zero, the code uses a Lagrange interpolation 

and it can be used for non-periodic baseflow. Additionally, you can select a near body domain to compute the eigenvalue, seting the option SelectEVCalcDomain0, 

since the far-field grids are usually coarse and contains large numerical errors.

We have tested this feature in our previous paper, see appendix

Gao A K, Cantwell C D, Son O, et al. Three-dimensional transition and force characteristics of low-Reynolds-number flows past a plunging airfoil[J]. Journal of Fluid Mechanics, 2023, 973: A43.

Best Regards

Ankang Gao

---

Research Associate

Department of modern mechanics

University of Science and Technology of China

_______________________________________________
Nektar-users mailing list
Nektar-users@imperial.ac.uk
https://mailman.ic.ac.uk/mailman/listinfo/nektar-users