Steady-state conditions are laminar in the following pipes

An Information message in AFT Fathom

Full Message Text

There are several variations of this Information message:

  • Steady-state conditions are laminar (100 <= Re <= 2300) in the following pipes. Loss factors are typically from turbulent tests and may not be accurate for connected junctions or fittings in these pipes. Laminar and Non-Newtonian Corrections are not applied.

    Pipes: P1, P2, P3, ...

  • Steady-state conditions are laminar (100 <= Re <= 2300) in the following pipes. Loss factors are typically from turbulent tests and may not be accurate for connected junctions or fittings in these pipes. Laminar and Non-Newtonian Corrections are applied with default settings.

    Pipes: P1, P2, P3, ...

  • Steady-state conditions are laminar (100 <= Re <= 2300) in the following pipes. Loss factors are typically from turbulent tests and may not be accurate for connected junctions or fittings in these pipes. Laminar and Non-Newtonian Corrections are applied with user-modified settings.

    Pipes: P1, P2, P3, ...

Note: If a Zero-Length Connector (ZLC) had laminar flow, there will be an additional note appended to the message stating that ZLCs have been excluded from the list of pipes. ZLCs cannot have Fittings & Losses, so the message does not apply to them.

Explanation

This Information message is ultimately intended to communicate that pressure losses in junctions and fittings may be different under laminar flow conditions compared to turbulent conditions. If flow through a junction or fitting is laminar, then extra consideration may be warranted for how this will impact the pressure drop, and if corrections should be made. Fathom offers methods to correct losses for laminar flow, but there is also debate in literature about if corrections should be made. Specifics will be discussed below, otherwise consult the Required Action section below.

It should be noted that this message is not intended to discuss pressure drop from pipe friction. Some notes on laminar pipe friction will be provided at the end of this explanation.

Turbulent vs Laminar Loss Coefficients

It is generally believed that the pressure drop through junctions and fittings derive from the physics of bulk fluid motion, wall separation, turbulence, etc. Therefore, the losses during laminar flow may be different compared to turbulent conditions. These pressure losses are typically modeled using data from manufacturers (Resistance Curves and Valve Cv or Kv values) or loss coefficients from academic references (K Factors). The problem is that data from these sources are typically based on laboratory tests which usually occurred under turbulent flow conditions with water.

Laminar flow is often associated with high viscosity fluids, and usually the pressure drops through junctions are higher under laminar conditions. For example, a manufacturer may provide a pressure loss data point at a specified flowrate for a strainer that was tested with turbulent water in their laboratory. If this strainer is installed in a system that pumps high viscosity oil, it is very likely that the pressure drop through the strainer at the same specified flowrate will be much larger. Therefore, it is important to consider these effects when designing systems to ensure they can meet design requirements.

Correction Options

The Laminar and Non-Newtonian Corrections panel in Analysis Setup offers some capabilities that can help estimate adjusted loss coefficient values for laminar flow. AFT generally recommends to compare scenarios with and without the Adjusted Turbulent K Factor Method (ATKF) enabled to determine the magnitude of effect. Results can be easily compared using Multi-Scenario Output.

By default the laminar flow regime is defined by a Reynolds Number less than 2300, but this value can be changed on the Environmental Properties panel of Analysis Setup. The lower bound of Re = 100 exists because there is a separate Caution message if the flow is in the "deep laminar" flow regime which is defined by Reynolds Numbers less than 100. The Reynolds Number for a pipe can be checked by adding the "Reynolds Number" output parameter from the Pipes section of the Output Control. The "Laminar and Non-Newtonian Correction Strength" output parameter can be added from both the Pipes section and Junctions section of the Output Control. The correction strength parameter for pipes will refer to the K Factors for any lumped Fittings & Losses, and not the pipe friction itself. A correction strength larger than 1.0 acts as a multiplier that increases the equivalent K Factor and pressure loss for the pipe fittings or junction.

Comment on Area Restrictions Inside Junctions

When flow in a pipe is laminar, there is also an implication that flow through connected junctions would also be laminar. This also applies to components and fittings that have been lumped into the pipe Fittings & Losses. One caveat to mention is that in reality some junctions could have an area restriction such as the path of flow through a valve, orifice, etc. In an area restriction, the velocity will increase, potentially to the point where the Reynolds Number is no longer laminar, and laminar corrections might not be needed. This might be a consideration for systems that operate in the transitional flow regime. Even when accounting for area restrictions within junctions, there is a high probability that if the connected pipe experiences laminar flow that the junction will also experience laminar flow, or at least operate under a condition with different pressure loss characteristics compared to reference K Factors.

Pipe Friction Under Laminar & Non-Newtonian Conditions

For more information on how to account for laminar or Non-Newtonian effects on pipe friction, consult the Pressure Drop in Pipes Detailed Discussion and Viscosity Model panel topics. Below is a brief summary:

  • If the pipe uses the default Friction Model (Standard), then the Solver will use the Darcy-Weisbach loss model which has an explicit correlation for the laminar friction factor (f=64/Re). In this case laminar flow is automatically accounted for in pressure drop calculations. Other Friction Models may or may not account for laminar flow - consult the equations for each friction model on the Pressure Drop in Pipes Detailed Discussion topic.

  • If a Non-Newtonian viscosity model is used, then the friction factor will be calculated according to the corresponding methods - consult the equations for each viscosity model.

Required Action

First, the user should consult the help topic on the debate of if laminar corrections are required for junctions and fittings, and use their engineering judgment to decide if they believe corrections are required. If the user does not want to apply corrections, then keep the laminar corrections disabled on the Laminar and Non-Newtonian Corrections panel (this is the default selection).

If corrections are desired, and if junction loss coefficients were from turbulent conditions, then it is recommended to enable laminar corrections. The user should consider enabling the Adjusted Turbulent K Factor method and read the corresponding help topic to learn more.

Note: All of the K Factors from Fathom's internal Fittings & Losses Library are from sources that tested under turbulent flow conditions.

If loss factors were from laminar conditions and this is confirmed with the manufacturer or reference source, then keep the laminar corrections disabled so as to not double count the effect of laminar conditions. Corrections can be enabled or disabled for an entire junction type. This may be useful if you are accounting for laminar flow in bends and valves using the 3-K Darby Method, but not for General Component junctions, and therefore you only want to apply corrections to General Component K Factors.

Previous Variations of the Messages

From the initial release of Fathom 13 to an intermediate maintenance release there were alternate variations of these messages (release 2023.09.14 up to and including 2024.02.05). Changes were made because AFT changed our philosophy on how and when to alert users. Previously an Information message would communicate when significant corrections were made (>25%), and a Caution message would communicate if no corrections were made but would have been significant if applied. Whether the Information or Caution message was displayed depended on if the user enabled corrections from the Laminar and Non-Newtonian Corrections panel. Models created in Fathom 12 or previous versions had laminar corrections enabled by default, and importing them into Fathom 13 will keep the corrections enabled. New models created in Fathom 13 have laminar corrections disabled by default, but the Informational message documented on this topic is shown to notify the user. The new philosophy is to simply alert users to laminar flow, which may or may not be expected, and allow the user to decide if applying corrections is required.

Previous Information message:

Losses at one or more pipes or junctions were significantly modified (>25%) with a Laminar or Non-Newtonian correction. The strength of the correction can be seen by checking the Laminar/Non-Newtonian Correction Strength output parameter. If the loss model source was from testing under laminar conditions, applying a correction is NOT recommended.

Previous Caution message:

Losses at one or more pipes or junctions may be significantly impacted (>25%) by laminar or non-Newtonian conditions, but no correction was made. Checking the 'Laminar/Non-Newtonian Correction Strength' output parameter is recommended. If the loss model source was from testing under turbulent conditions, applying a correction is recommended.