The discussion revolves around the relationship between fluid dynamics and sound waves, particularly in applications like pipe flows, jet flows, and boundary layers. Participants explore when and why sound waves, which are caused by pressure disturbances, can be neglected in fluid dynamics analyses.
Participants do not reach a consensus on the criteria for neglecting sound waves in fluid dynamics. Multiple competing views and methods are presented, indicating ongoing debate and uncertainty.
Limitations include the lack of a universally accepted rule for when to neglect sound waves, dependence on specific flow conditions, and the need for context-specific analysis in engineering calculations.
We include them when they are significant to the problem we are analyzing, and neglect them when they are not. There are also many fluid dynamics applications in which we neglect the gravitational attraction of Jupiter's moons, even though that gravitational attraction exists.K41 said:There are many fluid dynamics applications such as pipe flows, jet flows, boundary layers where we ignore any sound waves present in the system. I don't understand this though, because all sound waves are caused by pressure disturbances so why can we ignore these pressure disturbances when we deal with simple fluid dynamics applications?
Chestermiller said:We include them when they are significant to the problem we are analyzing, and neglect them when they are not. There are also many fluid dynamics applications in which we neglect the gravitational attraction of Jupiter's moons, even though that gravitational attraction exists.
I can answer that in general for any kind of engineering calculation. Run the calcs once with the effects included, and again without them. Compare the answers. If the difference is significant, then leave them in.K41 said:How do we know when to neglect them? Is there any criteria?
It don't think there is a general rule for this. If I were concerned that sound waves were having an effect on a flow, I would first set up and solve the model equations without sound waves. Then I would revisit the equations, and add a linearized perturbation to the equations involving sound wave forcing, and solve the linearized equations for the perturbation.K41 said:How do we know when to neglect them? Is there any criteria? For example, do we only consider sound waves if the flow is compressible (so we can use Mach number > 0.3 as a criteria). I guess my question is how do you know when a sound wave (or standing wave) produces a large enough pressure disturbance which may influence the flow field?