Hi,

I've investigating the thrust & efficiency of oscillating airfoils. They undergo periodic oscillation at a constant frequency (0.2-0.6) with a fixed maximum heave,pitch amplitude. Most of the results I obtain give a periodic thrust oscillation.

However, 1 or 2 particular configurations give an unsteady, non-periodic variation in the thrust/lift. The simulation has run for about 10 non-dimensional time. I wonder if I should continue until I can a periodic result or that this configuration 'll never give one in the 1st place? It's simply an unsteady non periodic case.

Can someone comment on this? Thanks

The short answer is "no, periodic structural oscillation does not guarantee a periodic flow solution".

The interaction between flow and structure can be very complex in nonlinear cases, with the flow corresponding at a variety of frequencies (not just sub-harmonics). For example, the flow itself could exhibit oscillations in form of vortex shedding or unsteady separation, possibly combined with motion of strong shocks, not synchronized with the prescribed airfoil motion. An animation of the unsteady flow field should reveal what's going on.

Bad convergence is another possibility. How well is the flow converged in each time step, how does the solution change with increased grid-resolution, how does the solution change with decreased time-steps, what is the influence of turbulence modeling (if any)... those are all questions you need to answer before you can claim that this is likely a physical phenomenon and not a numerical artifact.

Here is where it gets interesting and the real skills of a CFD user come in. This is not just about running some code to produce numbers, but about diagnostics, i.e. trying to make sense of unexpected results, separating physical from numerical behavior, and hopefully learning something along the way.

Non-linear!

Putting in a single frequency does not mean that you'll get a single frequency out. Or even a stationary signal.

Yeah, but not just any nonlinearity will do. "Nonlinear" doesn't mean aperiodic. If you get a number of harmonics in your output, it's a non-linear case, but could still be periodic, even at the forcing frequency (if all additional frequencies are multiples). You can see this behavior with strong shock motion, for example.

To be non-periodic, something else must be going on. As I said earlier, you could have a flow instability (vortex shedding, turbulence) appear with its own frequency(-spectrum).

http://www.amazon.com/Theory-Periodi.../dp/3540707239

Yuri B. Zudin Theory of Periodic Conjugate Heat Transfer

A new calculation method is presented for heat transfer in coupled convective-conductive fluid-wall systems under periodical intensity oscillations in fluid flow. The true steady state mean value of the heat transfer coefficient has to be multiplied by a newly defined coupling factor. This correction factor is always smaller than one and depends on the coupling parameters Biot number, Fourier number as well as dimensionless geometry and oscillation parameters. For characteristic periodic heat transfer problems analytical solutions are given for the coupling factor. To facilitate engineering application the analytical results are also presented in form of tables and diagrams.

Yuri B. Zudin. Theory of Periodic Conjugate Heat Transfer

A new calculation method is presented for heat transfer in coupled convective-conductive fluid-wall systems under periodical intensity oscillations in fluid flow. The true steady state mean value of the heat transfer coefficient has to be multiplied by a newly defined coupling factor. This correction factor is always smaller than one and depends on the coupling parameters Biot number, Fourier number as well as dimensionless geometry and oscillation parameters. For characteristic periodic heat transfer problems analytical solutions are given for the coupling factor. To facilitate engineering application the analytical results are also presented in form of tables and diagrams.