
[Sponsors] 
October 14, 2005, 06:07 
Information for onset of unsteadiness

#1 
Guest
Posts: n/a

Hi all,
I am looking for test data, pictures, frequency measurments etc for various cases of the onset of unsteadiness in lowspeed flow regimes. Certain cases like flow over a cylinder & blunt bodies  how was the onset of unsteadiness measured  visually, frequency etc. Are there decent ways to predict this phenomenon  other than the 'rough' Re,crit ~ 40... I would really value good links on this issue. I have tried a number of avenues but solid information seems to be very sparse  most is at highspeed. What kind of results & correlations have been emerging from the CFD community? Thanks all for your kind assistance. diaw... 

October 14, 2005, 07:39 
Re: Information for onset of unsteadiness

#2 
Guest
Posts: n/a

Hi Diaw, As a first refernce I suggest you Schlichting, 2000 or recent edition, but I am not sure particularly your flow group is discussed there, another book "Stability of time dependent and spatially varying flows , by D.L. Dwoyer and M.Y. Hussaini, . Both discuss the basics esp. Tollmeinshilichting type waves, and at the early stage to transition they are responsible and models the phenomenon, onset of transition. but if you already referred them I also want to hear more on transition and its related area. thanks


October 14, 2005, 08:46 
Re: Information for onset of unsteadiness

#3 
Guest
Posts: n/a

Hi Taw,
Thanks for the references. They will be a great help. I have on hand a book by Cebeci, & another by Drazin & Reid  but these are a little thin on physical examples. I think that further deep investigation of the onset of the TollmeinSchlichting phenomenon will be a good start on my end. I had a few concerns that in Cebeci these seemed to be observation of a local sparkdriven event (external disturbance), rather than the observation of a natural progression of the onset of turbulence. You've got me thinking... thanks... diaw... 

October 14, 2005, 10:20 
Re: Information for onset of unsteadiness

#4 
Guest
Posts: n/a

I think the one way to find out the onset is observing the lift coefficient (Cl). If the flow is steady, it will be zero and in case of unsteadyness it will be nonzero. You have to run the code for time enough to make sure about the steady/unsteadyness. The time to reach the decision abt steady/unsteadyness will vary from one problem (or condition) to other.
The best way is to give some possitive disturbance in the flow for say 5 or 10 time steps and then negative disturbence for same time steps, so that the flow is disturbed and then stop the distrubence and obtain the lift at each time step and observe the behavior. If the flow is steady at that Re then the Cl will go to zero after some time, otherwise it will be fluctuating.... Hope I m right... Best regards, ramp 

October 14, 2005, 11:12 
Re: Information for onset of unsteadiness

#5 
Guest
Posts: n/a

Thanks very much for your comments, Ramp...
What would be the theoretical reason for the lift coefficient being zero for steady flow & nonzero for unsteady? Due to temporallyinduced velocity gradients? Very interesting... Thanks, diaw... 

October 14, 2005, 20:36 
Re: Information for onset of unsteadiness

#6 
Guest
Posts: n/a

I think ramp was referring to vortex shedding over a cylinder. The steady solution (below Re=47) gives zero lift, the unsteady solution shows lift oscillating about zero. You can monitor the lift over time to see the unsteadiness develop from the initial condition.
The onset of turbulence by TollmienSchlichting waves is a different animal. If you're interested in general onset of unsteadiness (in other words: stability of steadystate flows) I would rather study laminar vortex shedding first (e.g. over the cylinder). To me it seems less complex than transition to turbulence, although that may not necessarily be true. 

October 14, 2005, 21:18 
Re: Information for onset of unsteadiness

#7 
Guest
Posts: n/a

Thanks Mani  excellent...
Mani wrote: The onset of turbulence by TollmienSchlichting waves is a different animal. If you're interested in general onset of unsteadiness (in other words: stability of steadystate flows) I would rather study laminar vortex shedding first (e.g. over the cylinder). To me it seems less complex than transition to turbulence, although that may not necessarily be true.  Laminar vortexshedding... These are my thoughts precisely. What I am trying to get to is a solid benchmark to set the NS against. What I 'feel' is that either Schlieren pictures of a gradual rampup in velocity for flow over a cylinder, or rearfacing step, will provide some valuable insights. I also feel that the onset of unsteadiness ?should? produce some disturbance which can be measured  say in the frequency range. I would value any solid references in these areas  my literaturesearch attempts, so far, have all stumbled. If NS are indeed correct, in their current formulation, then, they should predict this phenomenon. The issues  in my mind at least  are 'bulk viscosity', mechanical vs thermodynamic pressure, & the 'Stokes approximations' for the stress terms. diaw... 

October 20, 2005, 09:33 
Re: Information for onset of unsteadiness

#8 
Guest
Posts: n/a

If you decide to use laminar 2Dplanar flow over a circular cylinder with Re < 180 (based on cylinder diameter), you should be able to run a NavierStokes code from very small Re (Re < 4) showing no vortices forming behind (in the wake of) the cylinder, attached vortices forming behind cylinder for Re < 49 (this number may vary slightly for various researchers/experiments), and shedding vortices for Re > 49.
I put the restriction Re < 180, since above this Re, the threedimensional nature of the flow can begin to become important (see reference below). The following references may be helpful: "Effect of threedimensionality on the lift and drag of nominally twodimensional cylinders", R. Mittal and S. Balachandar, Phys. Fluids, Vol 7, No. 8, p.1841, 1995. "Perspectives on Bluff Body Wakes", A. Roshko, J. Wind Eng. Indus. Aerodyn, Vol 49, p.79, 1993. 

October 21, 2005, 00:05 
Re: Information for onset of unsteadiness

#9 
Guest
Posts: n/a

Hi Jeff,
Thanks so much for your wonderful contribution. It will be very, very useful. Could you provide additional information on the 'exact model geometry' used in your simulation? Bounding dimensions, position of test sample, types of boundaries used in the simulation (all)  pressure, velocity imposition (sudden, or gradual)... This information is vital to placing the results in the correct context. I am currently deeply inside the mathematics of this phenomenon  from a purephysics approach  a 'birdseye view' approach. It appears that we are dealing with a nonlinear event for the onset of instability, with definite crosslinking into the other spatial directions. It is fascinating. Thanks so much for your input. Regards, diaw...  Jeff Moder wrote: If you decide to use laminar 2Dplanar flow over a circular cylinder with Re < 180 (based on cylinder diameter), you should be able to run a NavierStokes code from very small Re (Re < 4) showing no vortices forming behind (in the wake of) the cylinder, attached vortices forming behind cylinder for Re < 49 (this number may vary slightly for various researchers/experiments), and shedding vortices for Re > 49. I put the restriction Re < 180, since above this Re, the threedimensional nature of the flow can begin to become important (see reference below). The following references may be helpful: "Effect of threedimensionality on the lift and drag of nominally twodimensional cylinders", R. Mittal and S. Balachandar, Phys. Fluids, Vol 7, No. 8, p.1841, 1995. "Perspectives on Bluff Body Wakes", A. Roshko, J. Wind Eng. Indus. Aerodyn, Vol 49, p.79, 1993. 

October 22, 2005, 13:23 
Re: Information for onset of unsteadiness

#10 
Guest
Posts: n/a

I have only run the 2dplanar flow over cylinder at Re=100 as one validation case to check that an unsteady code was working correctly for laminar viscous flow.
For my Re=100 case, a cellcentered compressible finitevolume code with lowMach number preconditioning and dualtime stepping was used. A structured quad grid with 12600 vertices and 12474 cells was used. The grid used radial lines starting at the cylinder surface and extending out 20x the cylinder diameter. The other grid lines were circles concentric to the cylinder. The grid near the cylinder is fine (first cell radial length about 0.008xD, D=cylinder diameter). The number of cells in the circumferential direction is about 130 with 45 cells used in the "upstream" halfcircle of the grid and 85 cells used the wake ("downstream") halfcircle of the grid. There are only 3 boundary conditions: (1) cylinder surface is a noslip wall; (2) "upstream" halfcircle is treated as inflow with uniform velocity; (3) "downstream" halfcircle is treated as outflow with uniform static pressure condition. The flow was intialized by running the case as steady for enough iterations to get down to about 1% difference in the ratio (mass out  mass in)/(mass in). Since the experimental results for lift Strouhal number were known (0.16 to 0.17) in this case, the value of St=0.165 was used to compute a constant physical time step such that a complete lift period was represented computationally by at least 600 time steps per lift period. (In my case, freq = u/D * St = 0.1/0.1 * 0.165 = 0.165 1/sec, period of lift = 1/0.165 = 6.06 sec, dt = 1.0e02 sec, 6.06/1.0e02 = 606 time steps per lift period IF THE CODE actually compute a St of 0.165 (or close to this). ) The code predicts St = 0.165 to 0.166 depending on the pseudotime iteration convergence criterion and the number of complete lift cycles used to compute the above average. 

October 24, 2005, 01:55 
Re: Information for onset of unsteadiness

#11 
Guest
Posts: n/a

Thanks very much Jeff for providing your experimental details, in detail. They will be of much use to my research. I'll feed back to the forum as things progress on my end.
diaw... 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
using face information in a cellbased macro  ximik  Main CFD Forum  2  May 10, 2011 17:42 
Help needed for defining an source term utilizing face information  ximik  FLUENT  2  May 10, 2011 17:40 
interchanging face information with cell information/ radiation flux UDF  ximik  FLUENT  1  May 4, 2011 16:34 
PHOENICS Information  Peter Spalding  Phoenics  0  September 24, 2004 05:30 
Terrible Mistake In Fluid Dynamics History  Abhi  Main CFD Forum  12  July 8, 2002 09:11 