Strategy to Determine Dimensionless Products (Pi Terms)

 

 

Key Concept:  There are two methods commonly used to form dimensionless products.  One

uses repeating variables and the other uses exponents.  Used properly both methods should

yield identical dimensionless products called Pi Terms.

 

 

Method of Repeating Variables  (Preferred Method)

 

1.

 

Select variables governing the problem.  (Critical and essential first step)  Set up a

table listing each of the variables along with the dimensions for each variable.

 

 

2.

 

Determine the number of Pi terms by subtracting the number of dimensions involved in

the variables from the total number of variables.  The collection of variables may be

sorted into three types - the dependent variable, the repeating variables, and the non-

repeating variables.

 

 

3.

 

Select the number of repeating variables equal to the number of basic dimensions in

the application.  Each  repeating variable will be used to remove one dimension from

the non-repeating variables and from the dependent variable in the problem to form a dimensionless product.

 

 

4.

 

Form each Pi term by multiplying the dependent variable and each of the non-repeating variables by the proper product of repeating variables so as to form a dimensionless

product.

 

5.

 

Check all the resulting Pi terms to verify they are dimensionless and independent. 

Note:  Products and quotients of Pi terms are also dimensionless.

 

 

 

 

6.

 

Express the final relationship among the Pi terms as

 

                     Pi  =   f( Pi|1,  Pi|2, etc)

 

where the Pi term on the left hand side represents the dimensionless form of the dependent variable and the Pi terms on the right hand side represent the dimensionless forms of the

non-repeating variables.

 

Click here for a discussion of the method of exponents.

    


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