Objective
This article explains the difference between flow velocity and wall shear stress and clarifies how each parameter is used within OLI’s corrosion modeling framework. Readers will learn when to apply flow velocity for single-phase systems, when wall shear stress provides a more realistic representation of multiphase flow effects, and how these inputs influence mass transfer and corrosion predictions. The goal is to help users select the appropriate flow-related inputs when configuring corrosion analyses in OLI Studio: Corrosion Analyzer.
1. Flow Velocity
Definition:
Flow velocity (often written as v or V) is simply how fast the fluid is moving through the pipe — i.e., the average linear speed of the fluid.
In corrosion modeling:
- In OLI, flow velocity is an input for single-phase flow calculations.
It helps calculate the Reynolds number (Re) and, from there, the mass transfer coefficient (k) through correlations such as:
Sh = 0.0165 (Re)0.86 (Sc)0.33.
The Re and Schmidt (Sc) numbers are defined as:
Re = (V)(d) / ν , Sc = ν / D
where ν is the kinematic viscosity, V the flow velocity, d the pipe diameter, and D the diffusivity.
- The mass transfer coefficient determines how fast species (like O₂, CO₂, or H₂S) move to or from the metal surface. This influences the limiting current for each species, which in turn affects the overall corrosion rate.
When to use flow velocity:
- When you have a single-phase (liquid-only) system (e.g., water with dissolved gases).
- When you want to understand general flow effects on corrosion but don’t have multiphase flow data.
- It’s suitable for simplified corrosion rate estimation or lab-scale testing conditions.
2. Wall Shear Stress
Definition:
Wall shear stress (τ, tau) is the force per unit area that the flowing fluid exerts on the pipe wall.
It represents how much mechanical stress the flow applies to the metal surface, caused by viscous drag and turbulence.
In simple terms:
- Velocity = how fast the fluid moves.
- Shear stress = how hard that moving fluid rubs against the wall.
In OLI:
- Shear stress is crucial in oil and gas pipeline environments where a water film covers the pipe wall (even when oil or gas phases are present).
- Corrosion models use shear stress to predict:
- Corrosion rates
- Corrosion potentials
- Repassivation potentials
- Shear stress can be calculated (for single-phase flow) or imported from tools like OLGA (for multiphase flow).
The single-phase relation is:
τ = (f ρ V2) / 2
where:
- τ = wall shear stress
- f = friction factor (from the Blasius correlation for smooth pipes)
- ρ = density
- V = flow velocity
When to use shear stress:
- When you have multiphase flow (e.g., oil, gas, and water moving together).
- When you want to capture hydrodynamic effects — especially where turbulence and wall impact control corrosion and film stability.
- When using OLI with external flow models (e.g., OLGA) that provide realistic shear stress data.
3. How They Relate
- Flow velocity is the input to calculate Reynolds number → friction factor → shear stress (in single-phase cases).
- Shear stress is more physically meaningful for corrosion because it represents what the wall “feels.”
- However, in multiphase or complex systems, you cannot directly calculate τ from velocity in OLI, since oil/gas phases distort the hydrodynamics — that’s why OLI recommends importing τ from other flow simulators.
4. Key Takeaways
- Flow velocity: good for basic or early-phase modeling; drives mass transfer through empirical correlations.
- Wall shear stress: essential for realistic, multiphase, high-turbulence conditions; directly impacts corrosion rate and film stability.
- If you don’t have OLGA or other fluid dynamic simulator, OLI provides an approximate NORSOK-based wall shear stress estimation — but expect 20–30% uncertainty.
- Ultimately, shear stress is the better descriptor of flow impact on corrosion, especially for pipelines and multiphase systems.