I'm trying to model a combined sinusoidally pitching and plunging oscillating hydrofoil, with just the pitching as a mesh movement, while adding a velocity source term to the fluid in the opposite direction of the plunging motion to mimic the effect of plunging. The units of a general momentum source term in CFX are kg m^-2 s^-2, dimensionally equivalent to density * acceleration, so I created an expression to calculate the foil acceleration and used it to create a momentum source term with those dimensions. Using the term in a transient simulation however produces velocity fields in the fluid domain that don't make any sense, and certainly aren't consistent with the plunging motion expected. Are there any ways to formulate a general momentum source term so that it results in the addition of a purely velocity component to the fluid?
The unit is also N/m**3 which is a volume force, for instance by a plasma actuator. There will always be some time lag and smearing wrt. your goal. I think it is better to apply an unsteady inflow boundary condition directly, i.e. the angle of the (oncoming-)velocity vector alters with time.
Markus J. Kloker Hi, thank you for your reply. Since I posted my question, I have had some success using -foil velocity * density / time step for my momentum source term. I've attempted numerous variations on an inlet with a y axis velocity, opposite the foil velocity, alongside the momentum source term for the fluid domain, with the top, bottom, and outlet as openings. This was in the hope that the fluid entering and within the domain would have a uniform y axis velocity. While the flow in the vicinity of the foil was reasonable, vortices quickly formed where the inlet flow hit the fluid in the domain, and backflow through the openings was frequently an issue, with velocities of 8-9 ms-1 reached, far faster than the sum of the inlet and plunge velocities should have allowed. Evidently there are still problems with my approach.
Your method is intriguing, but as my control volume is only 40 chords to a side, necessitated by the limitations of my license, and my target pitch angle is 75 degrees, keeping the foil motionless while the inlet velocity angle provides both the pitch and plunge motion, as I believe you are suggesting, would require both the top and bottom faces to function as both inlet and outlet at alternating times in the cycle in order to provide flow on both sides of the foil situated in the centre of the domain.
I have been referencing this paper: https://www.researchgate.net/publication/258536806_Numerical_Simulation_of_Fully_Passive_Flapping_Foil_Power_Generation to inform my method, though I have had trouble with udfs in the past, hence my use of CFX’s expression driven mesh motion. The method presented is to use mesh movement to pitch the foil, and a moving reference frame, driven by Define_Zone_Motion, to drive the plunge. As of yet, I have been unable to discern how to convert this moving reference frame concept into CFX.
Ok, the problem set up is now more clear. Using a volume force can work, depending on the spatial distribution and whether the 'top' boundary condition allows inflow that is needed. Otherwise the fluid needs to come from the 'sides' resulting in strange behavior. Note that the volume force acts like a kind of a sum of small localized 'fans', and oncoming fluid must be provided. The strength of the force can be iteratively found if the BC is appropriate.
Markus J. Kloker I have attached two images of my most recent run to give an idea of the scale of the backflow, the first is 6 time steps after 1 full oscillation, starting from bottom of the cycle so as to be horizontal, the second is the final step, after 8 full oscillations. The foil is 1 m chord, pitching to a maximum of 30 deg and plunging to an amplitude of 1 m, sinusoidally at 0.255 Hz, with a 90 degree phase difference; there are 32 time steps per full oscillation, and 8 full oscillations in the transient simulation, to enable rapid changes to be made. Fluid enters the lefthand side at 2 ms-1 in the +ve x and -yd in the +ve y, where yd is the foil velocity determined by taking the first derivative of its plunge formula. The reference pressure is 1 atm, all faces are held at 283 K static temperature with low intensity 1% turbulence, turbulence model is SST, fluid is constant property water. The top and bottom faces are openings with u component velocity of 2 ms-1, the right hand side is an opening held at a static relative pressure of 0.8 atm. The volume is 1 cm and 1 cell thick, mirrored at the front and back. The momentum source term is as detailed in my previous comment. If you can think of a way to address the backflow or to otherwise fix my boundary conditions, it would be greatly appreciated.
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