How to use occupancy grid for obstacle avoidance?

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I have a turtlebot-2 and it has a pre-built map of the environment onboard. I wish to build a basic obstacle avoidance algorithm for the robot. Since the robot has a local and a global costmap, I was wondering how to use the available occupancy grids to avoid the robot crashing onto the walls and other moving robots if I provide manual inputs for v and w? I have read about the navigation stack and all the related questions but haven't found anything useful. I'd apprecate any related helpful links.

Thank you!