Constant Loss Coefficient (K-Factor)
Losses through a media can be expressed in terms of an additional pressure gradient:
where K*i is the loss coefficient in the global *i coordinate direction.
Each global coordinate direction can have its own unique loss coefficient.
Assigning Loss Coefficients
Loss coefficients in Autodesk® CFD are expressed without units, and are independent of the length of the resistance in the model. The equation describing these losses is written in terms of a pressure drop instead of a pressure gradient:
To input a constant loss coefficient
- Select the desired direction and choose Constant as the Variation Method.
- Enter the appropriate value of
(as described below) as the Through-Flow K Value.
Through-Flow Loss Coefficients
Values for loss coefficients are found in many fluids texts and the hydraulic resistance reference, Handbook of Hydraulic Resistance, 3rd Edition by I.E. Idelchik, published by CRC Press, 1994 (ISBN 0-8493-9908-4).
Alternatively, if measured data for pressure drop versus flow rate is available. Solve the equation, , for 
, using the known values of pressure and velocity to determine the value of .
Enter this value for the Through-Flow K.
Normal Direction Loss Coefficients
In many situations, such as in a flow straightener, the loss in the normal directions will be significantly higher than the loss in the through-flow direction.
To represent this, enter Normal Direction K values four or five orders of magnitude higher than the through-flow value.
This will allow the flow to go in the desired direction, and impede it in the normal directions.
Permeability
The Permeability value can be specified in conjunction with the Constant resistance method as well as the Friction Factor method. This allows a resistance to be specified in the form:
Where 
is the viscous resistance term, which is the reciprocal of permeability.
To Specify a Permeability with a constant loss coefficient
Enter the value of permeability in the resistance Material Editor. It is used in the pressure drop equation in the following manner:
where 
is the value of permeability. The unit of permeability is the Darcy, and is expressed in terms of length squared.
The combined pressure drop equation is then:
Where:
- is the permeability, in units of length squared.
- V is the velocity
- L is the length over which the resistance acts
is the viscosity- is the loss coefficient
is the fluid density
To Specify a constant loss coefficient or friction factor without a permeability
To enter a constant resistance or a friction factor without the contribution of a permeability, leave the Permeability value 0.
To Specify a Permeability without a constant loss coefficient
To apply a permeability value without a constant loss coefficient or friction factor, change the Variation Method to Permeability, and enter the appropriate value of permeability.