# Fluid dynamics and sound waves

• K41
In summary, sound waves can be neglected in simple fluid dynamics applications if their effects are not significant to the problem being analyzed. This is determined by comparing the results with and without sound waves included. There is no general rule for when to neglect sound waves, so they may be explicitly included in more complex problems.
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?

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?
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.

vanhees71
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.

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?

K41 said:
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.

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?
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.

vanhees71
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.

## 1. What is fluid dynamics?

Fluid dynamics is the study of how fluids, such as liquids and gases, move and interact with their surroundings. It involves the study of forces, pressure, and flow patterns within fluids.

## 2. How does fluid dynamics apply to sound waves?

Sound waves are a type of mechanical wave that travels through a medium, such as air or water. Fluid dynamics helps us understand how sound waves propagate through different mediums and how they can be affected by factors such as temperature and pressure.

## 3. What is the relationship between fluid dynamics and turbulence?

Turbulence is a chaotic and unpredictable flow pattern that can occur in fluids. Fluid dynamics helps us understand the causes and effects of turbulence, which is important in many fields such as aviation and weather forecasting.

## 4. How do sound waves interact with objects in a fluid?

When a sound wave encounters an object in a fluid, it can be reflected, transmitted, or absorbed. The amount of each of these interactions depends on factors such as the size and shape of the object, as well as the properties of the fluid.

## 5. What are some practical applications of fluid dynamics and sound waves?

Fluid dynamics and sound waves have many practical applications, including in the design of airplanes, ships, and cars. They are also used in the development of medical imaging techniques, such as ultrasounds, and in the study of ocean currents and weather patterns.

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