Switching tenses from past to present within a paragraph in a research paper I am confused about which tense to use in presenting my results in a paper. 

"In this study, we created a mathematical model of the herding behavior using the A-algorithm. The goal of this mathematical model is to allow the ..." 


Am I correct in using the past tense "created" in the first sentence? 
Notice that in the second sentence I shifted to "is" instead of "was". 


When do we use past or present tense in the results portion of the paper? Your insights will be of great help.
In a paper, actions taken are typically in the past, while goals, interpretations, and conclusions are still valid in the present.

Thus, the juxtaposition of tenses in your example is entirely reasonable: the creation took place in the past, while the goal remains the same in the present.  You could also, if you wished, place the goal in past by tying it to the creation, e.g.


  Our goal in creating this model was to allow the ...