> > If you calculate the drag of the hull and crew from a stationary
> > start, and for an entire (let's say) 2000m race while they are
> > moving with 6 dof, how will you know your predictions are as
> > good as, or better, than predictions made with other methods?
> Which 6 dof are you referring to? A typical fluid mechanics simulation for this case will involve 3 velocity components, pressure, and a parameter to describe the free interface. So 5 unknowns, but multiply this with about 10^7 mesh cells.
> Maybe you also mean the dof that control the position/rotation of the boat itself?
> For the unsteadiness, there are actually 2 factors: the first is the acceleration/deceleration of the boat, as I described above. The second factor is that because the rower moves relatively in the boat, the orientation of the boat (the pitch angle) will also change, which affects both steady and unsteady drag. The latter will especially contribute to wave drag.
> An important item when one performs a calculation is the validation, consisting of several parts: first of all the assumptions that go into the model (e.g. if you use a RANS model, how steady is the boundary layer actually?). The second one concerns the numerical error one makes, which can be verified by refining the mesh around the boat and see how much the solution changes. Thirdly is to test the model on a known solution, preferably an experiment. The latter is easily doable for a steadily moving boat, or a boat in a tow-tank, as there is a lot of empirical data for this. For a moving boat, I guess we could do our own experiment: if we create a velocity curve of a rowing boat (with an impeller) and measure the force exerted on the pins as a function of time, this could serve as a good validation. I know many of the measurements of the force on the pin or oar are referred to as "power" curves. But unless I am mistaken, I think they measure force. Power is force times displacement over time, and I don't see how you can measure this just from the pin, without determining the displacement. Anyway, lot's of interesting details to work out!
Sorry, for the confusion. I only have time for a few minutes here
every so often.
By 6dof I mean surge, heave and sway, yaw, pitch and roll, for both
the boat and the crew.
You are correct that force, not power, is measured at the pins.
How the oar-blade behaves in the water will certainly fully test your
RANS code, especially when combined with the hull and the moving crew.
Quads and eights are particularly difficult because there are so
many blades in the water at the same time, and so many moving human
bodies. It is quite tedious to just get the anthropometry of all the
crew, e.g. weights, heights, lengths and weights of body segments and
their centres of mass and inertia etc.
I use a completely different approach, in that I start with the simplest
useful model and then add complexity as needed. See:
Your approach, if I am not mistaken, is to start with a very complicated,
almost complete, model of the hydrodynamics and aerodynamics of the shell and
Validation is going to be a very difficult exercise. The steady case of
a DTMB 5415 hull presented enormous logistical and other problems for
the ITTC when it had that hull tested in about 30 different towing tanks
around the world. See:
The Final Report of the Resistance Committee,
I think you should start with that hull before trying a rowing model because the entire exercise was designed to provide a validation set for CFD codes.
Prof. Frederick Stern deserves the highest praise for continuing to push for proper validation of CFD codes because there are, simply put, so many bullshit claims for the accuracy of the codes. Many practitioners tweak grids and empirical constants in order to promote better concordance with experiments. Not many are game to try "blind" validation exercises, for reasons on which we could speculate!
Here are some recent references to start you off on your quest:
Alexander Day, Ian Campbell, David Clelland, Lawrence J. Doctors,
and Jakub Cichowicz,
"Realistic evaluation of hull performance for rowing shells, canoes,
and kayaks in unsteady flow",
J. Sports Science, July 2011.
Doctors, L.J., Day, A.H. and Clelland, D.,
"Unsteady Effects During Resistance Tests on a Ship Model in a
Towing Tank", J. Ship Research, Vol 52, No. 4, Dec. 2008.
I wish you well in your research project. I'll be fascinated to hear how
long a simulation of a complete race will take using RANS.
The curves in my newsletter took about 2 secs to produce and I can simulate
an entire 2000m race in less than 1 minute on a PC. If you can get similar
accuracy for the velocity and acceleration in under an hour on a PC I will be
amazed, astounded and very impressed!
All the best,