# Drag Force

Published: August 30, 2019 | Last updated: July 5, 2023

## What Does Drag Force Mean?

Drag force can be defined as the force acting on a particle in a direction opposite to the direction of the flow of its surrounding medium. In the annular flow pattern in a gas well, the gas moves fast in the center of the pipe while a thin film of liquid slowly moves along the pipe walls. This liquid may be present as a film or as entrained droplets. In multiphase flow, solid particles are also suspended in the fluid during the production process.

To transport solid entrained multiphase fluids through pipelines, the fluid must be capable of suspending these particles to prevent deposition.

## Trenchlesspedia Explains Drag Force

Multiphase flow involves the suspension of solid particles in the fluid during gas production. The drag force develops due to pressure and viscous stresses that act on the particle surface and resists the relative fluid velocity. To understand the drag force on a particle it is necessary to understand the relationship between the drag coefficient and the particle's Reynold number. The amount of drag is dictated by the particles Reynold number given by:

Rep = ρfνfdp / μf

where Rep – Particle's Reynold number, ρf – fluid density, νf – fluid velocity, dp – particle diameter, and μf – fluid viscosity.

The drag coefficient is important in modeling a multiphase process involving entrained solid particles. It represents a fraction of the kinetic energy of the settling velocity that overcomes the drag force on the particle.