Outer product of flow velocities in Navier-Stokes equation

In summary: It's basically the divergence of the "momentum flux tensor." Physically, it represents the difference between the rate of momentum flowing out of the control volume and the rate of momentum flowing into the control volume. If we move it to the other side of the equation it will have a minus sign and, including the minus sign, it would represent the net rate of momentum flowing into the control volume. The term on the left side represents physically the rate of increase of momentum within the control volume. So the rate of increase of momentum within the control volume is equal to the net rate of momentum flowing into the control volume plus the various forces on the right hand side. The overall equation is basically a differential momentum balance (
  • #1
snoopies622
840
28
Reading the Wikipedia entry about the Navier–Stokes equation, and I don't understand this second term, the one with the outer product of the flow velocities. I mean, I understand the literal mathematical meaning, but I don't have an intuitive idea of what it physically represents. When I make up velocity fields and compute its value, I get something similar to the flow velocity but not exactly, and something related to divergence, but not exactly that either.
 

Attachments

  • nav.jpg
    nav.jpg
    13.3 KB · Views: 484
Physics news on Phys.org
  • #2
snoopies622 said:
Reading the Wikipedia entry about the Navier–Stokes equation, and I don't understand this second term, the one with the outer product of the flow velocities. I mean, I understand the literal mathematical meaning, but I don't have an intuitive idea of what it physically represents. When I make up velocity fields and compute its value, I get something similar to the flow velocity but not exactly, and something related to divergence, but not exactly that either.
The usual interpretation of this term is the rate of flow of momentum into and out of a differential control volume.
 
  • #3
Thanks Chestermiller, will give this some more thought and get back to you here.
 
Last edited:
  • #4
snoopies622 said:
Thanks Chestermiller, will give this some more thought and get back to you here.
I really like your avatar. Is that your pet?
 
  • #5
Nah, just a cute doggie pick I found on line years ago. In other places I use a picture of a mug of hot cocoa.

So about the momentum flow . . Would you say that this second term is basically a kind of vector version of the divergence? Never thought about it before, but I suppose one could find not only how fast fluid is rushing away from a certain location but its overall net direction as well.
 
  • #6
snoopies622 said:
Nah, just a cute doggie pick I found on line years ago. In other places I use a picture of a mug of hot cocoa.

So about the momentum flow . . Would you say that this second term is basically a kind of vector version of the divergence?
It's basically the divergence of the "momentum flux tensor." Physically, it represents the difference between the rate of momentum flowing out of the control volume and the rate of momentum flowing into the control volume. If we move it to the other side of the equation it will have a minus sign and, including the minus sign, it would represent the net rate of momentum flowing into the control volume. The term on the left side represents physically the rate of increase of momentum within the control volume. So the rate of increase of momentum within the control volume is equal to the net rate of momentum flowing into the control volume plus the various forces on the right hand side. The overall equation is basically a differential momentum balance (i.e., force balance/Newton's 2nd law) on the flow.
 
  • Like
Likes vanhees71 and BvU
  • #7
Thank you very much!
 

1. What is the outer product of flow velocities in the Navier-Stokes equation?

The outer product of flow velocities in the Navier-Stokes equation is a mathematical operation that involves taking the cross product of two vectors, representing the velocity components in the x, y, and z directions. This operation is used to calculate the vorticity, which is a measure of the local rotation of the fluid. It is an important term in the Navier-Stokes equation, which is used to describe the motion of fluids.

2. How is the outer product of flow velocities calculated?

The outer product of flow velocities is calculated by taking the cross product of two vectors, representing the velocity components in the x, y, and z directions. This can be written as a matrix multiplication, where the resulting matrix represents the vorticity. The calculation can be done using various mathematical software or by hand using the cross product formula.

3. What does the outer product of flow velocities tell us about fluid flow?

The outer product of flow velocities provides information about the local rotation of the fluid. It can help us understand the behavior of vortices and how they affect the overall flow of the fluid. This information is important in many applications, such as aerodynamics and weather prediction, as vortices can significantly impact the behavior of a fluid.

4. Why is the outer product of flow velocities important in the Navier-Stokes equation?

The outer product of flow velocities is an important term in the Navier-Stokes equation because it represents the vorticity, which is a measure of the local rotation of the fluid. This term is essential in accurately describing the behavior of fluids, as it accounts for the effects of vortices and turbulence on the flow. Without this term, the Navier-Stokes equation would not be able to accurately model real-world fluid flows.

5. Are there any limitations to using the outer product of flow velocities in the Navier-Stokes equation?

While the outer product of flow velocities is a crucial term in the Navier-Stokes equation, it does have some limitations. It assumes that the fluid is continuous and that the flow is laminar, meaning there are no turbulent or chaotic effects. In reality, these assumptions may not always hold, and additional terms or equations may be needed to accurately model the fluid flow. Additionally, the accuracy of the vorticity calculation can be affected by errors or uncertainties in the measurement of the flow velocities.

Similar threads

  • Classical Physics
Replies
7
Views
1K
Replies
18
Views
1K
  • Classical Physics
Replies
8
Views
2K
  • Classical Physics
Replies
4
Views
1K
Replies
20
Views
5K
  • Classical Physics
Replies
1
Views
4K
Replies
9
Views
2K
Replies
17
Views
2K
  • Classical Physics
Replies
6
Views
1K
Replies
1
Views
956
Back
Top