[arieljeds]

Hi,

I'm using waves2Foam for regular waves around a vertical surface piercing cylinder and I have a function object for calculating forces and forceCoeffs set up in my case. The function objects are executed and go into their correct folder.. however, my drag coefficients are absolutely huge!! Also, the values keep oscillating over time. I am having trouble understanding where the issue is.

1) If the forceCoeffs is oscillating and changing a lot, does this imply that the solution is not converging?

2) Is it even possible to use the forces and forceCoeffs function objects in a multiphase flow without changing the code at all or separating the U and p_rgh into its phases?

Thanks!
Ariel

[arieljeds]

Hi there,

Ok I know that this question and similar have been asked in multiple threads but I'm still not understanding what pressure is being solved for and how to then use this

So, here is my understanding:

p = p_rgh + rho*g*h

Which implies that p_rgh is the dynamic pressure. However, I'm sure it isn't the dynamic pressure. I know that in Niels' paper (Jacobsen, et. al 2012), p* (which I have assumed is p_rgh) is the "pressure in excess of the hydrostatic". But what does this mean, if not the dynamic pressure??

So, what I am trying to do (still) is simply calculate forces, pressure, drag and lift coefficients for a smooth cylinder. From what I was understanding, I set up two lines of probes at point in the free stream from -x to x as well as 32 probes located around the cylinder (gravity is in the z-direction)

From there, I planned to use a p (NOT p_rgh) value from the probes in the freestream (I picked one at x = -50, y = 0; my inlet relaxation zone extends to x=-70 and the cylinder origin is at (0,0,0), so this seemed to be appropriate).

I then used this p (NOT p_rgh) value as my pInf to be used in a standard calculation of the pressure coefficient at a location on the cylinder (say, theta = 0). So, in the equation Cp = (p-pInf)/(0.5*rho*magUInf*magUInf), pInf would have p = p_rgh - rho*g*h, magUInf taken from the same location as pInf.

Is this correct?? I'm sure that I am still misunderstanding exactly what the pressures are and how I can use them to compare to analytical or experimental results. I am so confused about the pressures... can someone please help clear up what everything is??

Thank you!!

[arieljeds]

Hi there,

I've been trying to get the drag coefficient out of a simulation of flow around a circular cylinder using waveFoam (stokesFirst waves). I ran several cases using a potential current, which did reach a steady state and the drag coefficient reached a steady value. However, now that I am running exactly the same simulation using linear waves, the forces (and therefore drag) is not converging, but simply oscillating without any dampening (after an initial very small dampening).

Has anyone else experienced this problem?

Additionally, I am finding the pressures on the cylinder (found using probes and corroborated with surface sampling) is a bit higher than expected, based off of expected pressure coefficient values.

Any advice??

Thanks!!

Ariel

[ngj]

Hi Ariel,

You keep asking these questions about the converging of forces on a cylinder in waves, but how can the force converge, when the loading is oscillatory? Furthermore, why do you expect it to converge under oscillatory loading?

Please read the literature on forces on cylindrical objects, where e.g. the book by Sumer and Fredsoe (1999) (Hydrodynamics around cylindrical structures, World Scientific) is a good starting point. There are also several methods to find for force coefficients (drag and inertia coefficients), which can also be found through simple searching in e.g. Google Scholar or the referenced book.

Kind regards,

Niels

[arieljeds]

Hi Niels,

Thanks for your reply. Yes, I understand now that the forces should stay oscillatory. I have been reading many books on this (Zdravkovich, Sumer & Fredsoe, Sarpkaya & Isaacson) and my concern is that I am doubting my answers from openfoam, because I am still not able to verify even the most basic case. I'm sorry for posting so many similar questions, but I'm really unsure where I'm going wrong or even if I am going wrong.

I want to verify a case for Re = 40 with a potential current so I want to output the drag coefficient, but using function objects I am not able to get close to what I'm expecting. I have satisfied myself that the forces (using a function object), if what's output is the total force over the whole length of the cylinder, is correct (based on Morison's), but I am still not outputting the expected drag coefficient. I also attempted this using probes on the cylinder and surface sampling and integrating for the force but this is not giving me what I want either.

Again, I'm very sorry for posting so many of the similar questions but I'm afraid I haven't been able to be fully satisfied with any of my attempts and am very confused about how to verify if I can't get a drag coefficient even on the correct order of magnitude.

[ngj]

Hi Ariel,

The problem could be that the function object that you are using is simply not build for free surface flows or oscillatory flows (I do not know, since I have never used the code). Why not simply take the integrated force and do some post-processing in e.g. Matlab or Python? Alternatively, check the code for the functionObject for force coefficients and make it work for your case.

Actually, since you have drag, lift and inertia coefficients in oscillatory flow, I am almost certain that the force coefficient function object is not build for oscillatory flows.

Kind regards,

Niels

[arieljeds]

Hi Niels,

Thank you so much for clearing that up!! I started to suspect that it simply didn't work for multiphase after trying so many different ways to no avail. I've been working with the integrated force as well (actually, just to clarify, are you referring to the integrated forces using a function object, or some other way to output them?) but I was trying to verify my case in several different ways (I tend to not trust my results until I've verified in a few different methods to be sure).

Again, thank you for your patience. You've been a great help to this community first by providing waves2Foam and now answering so many questions!

Best regards,
Ariel