Solving Pipe Flow: Understanding Why $\tau =\mu(-dV/dR)$

In summary, the negative sign is included in the shear stress formula because the velocity is considered as a function of the distance from the center line of the pipe, not from the wall. When considering the velocity as a function of the distance from the wall, it is typically positive. It is also possible to consider the distance from the center line of the pipe to the wall as positive.
  • #1
foo9008
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Homework Statement


why the shaer stress ( tau) formula is given by μ(dv/ dy ) = μ(-dV / dR) , why there is a negative sign there ?

Homework Equations

The Attempt at a Solution

 

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  • #2
The negative sign is included because you are considering the velocity as a function of the distance from the center line of the pipe, not from the wall.
 
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  • #3
mfig said:
The negative sign is included because you are considering the velocity as a function of the distance from the center line of the pipe, not from the wall.
why when we are considering the velocity as a function of the distance from the the wall , it's positive ? or we can also consider the distance from the wall as positive ?
 
  • #4
mfig said:
The negative sign is included because you are considering the velocity as a function of the distance from the center line of the pipe, not from the wall.
can we consider the distance from the centerline of pipe to wall as positive ?
 

Related to Solving Pipe Flow: Understanding Why $\tau =\mu(-dV/dR)$

1. What is pipe flow and why is it important to understand?

Pipe flow refers to the movement of fluid through a pipe or conduit. It is important to understand because it is a common occurrence in many industrial and engineering applications, such as in plumbing, irrigation, and transportation of liquids and gases. Understanding pipe flow helps us to design and optimize systems for efficient and safe fluid transport.

2. What is the significance of the equation $\tau =\mu(-dV/dR)$ in solving pipe flow?

This equation, also known as the Navier-Stokes equation, relates the shear stress ($\tau$) at the pipe wall to the viscosity of the fluid ($\mu$) and the velocity gradient ($-dV/dR$). This equation is crucial in solving pipe flow because it allows us to calculate the shear stress, which is a key factor in determining the resistance of the fluid to flow through the pipe.

3. How does the viscosity of the fluid affect pipe flow?

The viscosity of a fluid is a measure of its resistance to flow. In pipe flow, a higher viscosity means that the fluid will have a higher resistance to flow, resulting in a lower velocity and higher shear stress. This is important to consider when designing systems for fluid transport, as a high viscosity fluid will require more energy to move through the pipe.

4. What factors can affect the velocity gradient in pipe flow?

The velocity gradient, which is represented by $-dV/dR$ in the Navier-Stokes equation, can be affected by several factors. These include the diameter and length of the pipe, the flow rate, and the properties of the fluid (such as viscosity and density). Additionally, any obstructions or changes in the pipe's geometry can also affect the velocity gradient.

5. How can we use the Navier-Stokes equation to solve for pipe flow?

The Navier-Stokes equation is a fundamental equation in fluid mechanics that describes the motion of fluids. To solve for pipe flow, we can use this equation in combination with other equations, such as the continuity equation and the energy equation, to form a system of equations that can be solved using various numerical methods. These solutions can then be used to predict the behavior of fluids in pipe flow and optimize systems for various applications.

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