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Ventilation Design Handbook on Animal Research Facilities Using Static Microisolators 
loop solves the linearized equations for each variable in turn at each cell, assuming all other 
fields are fixed. The outer loop involves updating all variable fields with the values calculated in 
the inner loop. As this process progresses, the flow field approaches its final or converged state. 
The iterative process stops when the errors in all governing equations reach acceptably small 
values. 
5.2.6. 1 The inner iteration 
The inner iteration consists of taking each variable in turn, while assuming all the others to be 
fixed, passing the relevant equations for each cell to a Gauss Siedel equation solver. All updated 
variable values are not passed on to the other linearized equations until completion of the inner 
loop, and it is within the outer loop that this takes place. 
At some point the iterative process must be terminated so that the inner iteration can proceed 
onto the next variable. Criterion must be met before the inner iterations for a given variable 
stops. Either the total number of inner iterations is reached or the reduction in the residual error 
(the amount by which the current governing conservation equation is not satisfied) for the given 
variable reaches a tenth of the level when the inner iteration began. 
52.6.2 The outer iteration 
Once the inner loop has been completed, i.e., each variable taken in turn and iteratively solved 
until the stopping criterion has been met, the outer iteration is performed. The main aim of the 
outer loop is to update all variables in all equations by the values calculated in the inner loop. It 
is within this outer loop that the velocity-pressure-coupling algorithm, SIMPLE, is 
implemented. This predicts the correct value for cell pressure based on the current cell 
momentum (Patankar, 1980). 
The outer loop is stopped when the problem is said to have converged. This occurs when the 
residual errors for all equations are acceptably small (i.e., 0.5 percent of the inlet flux for each 
variable). 
