Fluid dynamics and sound waves

[K41]

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?

 

[anorlunda]

ping @Chestermiller

 

[Chestermiller]

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?

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]

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.

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?

 

[anorlunda]

How do we know when to neglect them? Is there any criteria?

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.

It is a question of how accurate you need to be.

 

[Chestermiller]

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?

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.

 

[boneh3ad]

Whether you explicitly "allow" them or not, any calculation/simulation that allows for spatiotemporal variations in pressure will capture sound waves. In most simply problems, however, we just assume the flow is steady and does not vary in time, which would indicate that the only sort of waves admissible are standing waves.