Acoustic solver with air background, but water outlining an acoustic path.
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I'd like to create an acoustic solution for a SEFT, mouse and water filled cone. The outside of the mouse should not be the default background of water but changed to air, as I'd like to see the reflections the body/air interface creates. How do I change the background to air, as this option appears greyed out when I try to change it?
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Hi Jean, not sure how you're going about this, but I'd recommend then to make a block around the cone and then add this to the simulation as well (make sure you assign it lower priority than the cone in the voxeler - always check the voxeler to see that it was voxeled correctly) and then you can assign that to a different material property (air) in the simulation.
Keep in mind though that for correct simulation results you'll need at least 10 ppw, notice that the wavelength for air with speed of sound = 300 m/s is very different to the one in water and tissue ~ 1500 m/s so you might end up with a very large grid size. You can manually set the grid to get around this but you should be very careful about validity of simulation results in the air region.
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Thank you for the tip - I have done this, but set the air block as a reflector to avoid it calculating in the air block as I don't care about the results in that region as much(I suppose that is what setting to reflector setting means in the materials menu?). I only care about the results inside the mouse head. Unfortunately, my results seem very counter-intuitive. The water surrounded head simulation has a much higher resultant intensity than the simulation where the acoustic wave is confined to the mouse head and surrounded by air. I would have expected the intensity to be larger when reflections constructively interfere with each other inside the mouse head with air around it.
Both simulations go for 50 periods, with 1MPa max pressure and all other parameters apart from the air block are the same. Any idea why the water surrounding the mouse head simulation would be more intense?
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Ohhh I see, the reason you can't change the speed of sound of a reflector is because a reflector means that everything bounces off it, nothing goes inside. If I remember correctly, the solver just sets the speed of sound to 0 in that region such that it acts as a reflector - which is why you can't change it. It's essentially acting as a hard wall which is why you also probably get unexpected results. While a lot of the energy will reflect back into the domain at the interface of water and air, it won't be a perfect reflection which is what you get with your settings.
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Thanks Haza. I have just set another simulation running again where air is not a reflector but has the original material properties - so will update you with what happens. My airblock is small, just around the mouse head. The voxelor priorities are (0) - SEFT, (1) - air block, (2) - Ultrasound water cone, (3) mouse. Does this ordering sound right to you?
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Yup sounds right! You can also check the voxeler to make sure it looks right as well. Just another note, always be careful with your grid when simulating with air in the simulation. Since the recommended grid step is related to the smallest speed of sound available in the simulation (dx ~ cmin / f / 10) and since there's a big difference between cwater = 1500m/s and cair = 300m/s, you might have to be careful with your grid in case you set it up yourself manually.