Friction Correlations

The friction in the Flow Simulator element can be calculated by the given equation, where:


stands for the calculated friction based on the user-selected friction mode (Abauf, Swamee), friction type (Fanning or Darcy), and Re (to determine whether it is in a turbulent region, or in a laminar region).(2)

If ReDh < ReTurb, laminar friction calculations take place, otherwise, the turbulent friction calculation routine is used.

Nomenclature Subscripts
f: friction F: fanning
Re: Reynolds number D: darcy
FMULT: friction multiplier turb: turbulent flow
ε: sand grain roughness lam: laminar flow
A: Cross sectional area Abuaf: Abuaf friction relation
L+: Inlet station + 1/9 of 2nd station Smooth: smooth surface
XMU: dynamic viscosity Rough: rough surface
W: mass flow rate SJ: Swamee-Jain approximation
X: station length Dh: hydraulic diameter
L: equivalent diameter

Laminar Friction

Calculates the friction coefficient for laminar flow in shaped ducts based on the references Yunus A. Cengel, 2006 and Bruce Munson, 2005.

Friction coefficient for hydrodynamically fully developed flow can be calculated as:(3)

For a Tube Element, Laminar Friction Inlets effects can be accounted. Friction coefficient for hydrodynamically developing flow with “Muzychka Yovanovich Laminar Inlet Effects” can be calculated as:


The friction coefficient for combining developing flow and fully developed flow can be calculated as:


Darcy type friction is calculated as


Fanning type friction is calculated as:


Turbulent Friction

Calculates the turbulent friction for smooth or rough walls.

Abuaf Friction Relation
The Abuaf friction relation should generally be used for smooth walled tubes.

In Flow Simulator, you have the option to use the Abuaf friction relation together with wall roughness. The following adjustment equation is used:

Swamee-Jain Approximation of the Colebrook-White Equation (Moody Diagram)
The Darcy and Fanning type frictions are calculated as:(11)
User-specified Friction Factor


Surface roughness values can be entered in four different measurement types. The roughness values are converted to sand grain roughness equivalents using the following equations from table 1 of reference 63.
ε=5.863∗Ra, Ra=Average Absolute Roughness
ε=3.100∗Rrms, Rrms=Root Mean Square Roughness
ε=0.978∗Rzd, Rzd=Peak to Valley Roughness

Non-Circular Shapes in Flow Simulator Tubes

The friction factor and heat transfer coefficient (HTC) correlations were developed for circular pipes. The traditional method to use these correlations on non-circular shapes is to calculate a hydraulic diameter based on the shape area and perimeter.

Dh=4 *AreaPerimeter

The errors associated with this method can be +/-40% for laminar flow but less for turbulent flow, +/-15% (see White, ref 3).

A more accurate option is to adjust the hydraulic diameter with a friction factor ratio (see White, ref 3). The effective hydraulic diameter can then be used in the friction factor and HTC correlations.

The following table summarizes the relationship between the Dh based on 4*A/P and the effective hydraulic diameter.

Shape Effective Dh Equation Aspect Ratio (AR)
Circle DheffDh=1 AR=1

DheffDh=23+1124*AR*(2-AR) AR=b/a

D h e f f D h = 1 - . 2109 * ( 1 - A R ) 2 AR based on area and perimeter of the ellipse.

Isosceles Triangle

DheffDh=6448+11.442*AR-6.0026*AR2 AR based on area and perimeter of the triangle.


DheffDh=1+AR2+1-AR2ln(AR)1-AR2 AR=b/a

(Arbitrary- Shape)

DheffDh=AR*1+AR*1-192π5*AR*tanhπ2*AR.75*DhArea AR based on area and perimeter of freeform shape using a rectangle equation.

A R = ( P - P 2 - 16 * A ) ( P + P 2 - 16 * A )

See Blevins (ref 15) and Muzychka et al. (ref 50) for additional information.

If the compressible tube, advanced orifice, and incompressible tube have a cross sectional shape that is not circular, the equations in this table are used for the effective hydraulic diameter equation.