# What is the substantial derivative fluid mechanics?

Jul 10, 2022

Reading Time: 3 Min

In fluid mechanics, the substantial derivative is a time derivative that takes into account the motion of the fluid itself. It is used to describe the time-varying behavior of a fluid flow field.

The substantial derivative is also known as the Lagrangian derivative, after Joseph-Louis Lagrange. It is related to the Eulerian derivative, which is the time derivative that is commonly used in fluid mechanics. The Eulerian derivative is taken with respect to a fixed point in space, while the Lagrangian derivative is taken with respect to a particle of the fluid.

The Lagrangian derivative is often used in Lagrangian methods of fluid mechanics, such as Lagrangian averaging. These methods are used to track individual fluid particles as they move through the flow field. The Lagrangian derivative is also used in some numerical methods, such as smoothed particle hydrodynamics.

In general, the Lagrangian derivative is given by:

D/Dt = (partial/partial t) + u * (partial/partial x) + v * (partial/partial y) + w * (partial/partial z)

where u, v, and w are the velocity components of the fluid flow, and D/Dt is the substantial derivative.

The Lagrangian derivative can be rewritten in terms of the Eulerian derivative as:

D/Dt = (partial/partial t) + (u * partial/partial x) + (v * partial/partial y) + (w * partial/partial z) – ((partial u/partial x) + (partial v/partial y) + (partial w/partial z))

The Lagrangian derivative is often used in Lagrangian methods of fluid mechanics, such as Lagrangian averaging. These methods are used to track individual fluid particles as they move through the flow field. The Lagrangian derivative is also used in some numerical methods, such as smoothed particle hydrodynamics.

## Other related questions:

### Q: What terms does the substantial derivative include in the momentum equations?

A: The substantial derivative includes the terms velocity, acceleration, and any other forces acting on the object.

### Q: What is convective derivative in fluid mechanics?

A: In fluid mechanics, the convective derivative is a derivative that accounts for the fact that fluid elements move with the flow. It is defined as the derivative of a quantity Q with respect to time t, where t is the time measured in the moving frame of reference:

$$\frac{\mathrm{D} Q}{\mathrm{D} t} = \frac{\partial Q}{\partial t} + \mathbf{u} \cdot \nabla Q$$

where $\mathbf{u}$ is the velocity field of the fluid.

### Q: How is the substantial derivative of velocity vector denoted?

A: The substantial derivative of the velocity vector is denoted by Dv/Dt.

### Q: What does material derivative represent?

A: The material derivative is a way of measuring how a quantity changes as it flows along a path. It is used in many fields, including fluid dynamics and electromagnetism.