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Discussion of example model ferry.doe

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					                                     Ferry simulation in Arena
A small ferry carries cars across a river. At each bank, cars appear via an exponential distribution at a
rate of 1 car per minute. When the ferry is at a certain bank and is empty, cars can get on it at a rate of
1 second per car on average. There are 10 places for cars at the ferry. If the ferry is full or if all cars at
the current bank got on board, the ferry crosses to the other bank in 4 minutes, where the cars get off,
also at a rate of 1 per second. The cars waiting at the other bank can then enter the ferry, starting a new
cycle.

The current ferry needs replacement; it is at the end of its life and there are complaints that its capacity
is too low. There are two candidates: one has an increased capacity of 15 places and the other has a
capacity of 8 places but crosses in 3 minutes instead of 4. Find out through simulation for the current
ferry and both candidate replacements what are the average number of cars per crossing, the average
number of waiting cars and the average throughput time of a car. Also calculate these numbers in case
traffic increases by 10%.

Discussion of example model ferry.doe

In the model there are two streams of cars; one for each bank. Cars are created at the prescribed rate
and then need to get on the ferry. In order to do this, they need a free place at the ferry (resource
place) and they need to cross the barrier (resources bra/brb). The resource bra represents an open
barrier at bank A and brb represents an open barrier at bank B. After a second the open barrier
resource is released to allow the next car to enter. The free place stays occupied. A cars stays on the
boat till it successfully seizes the other barrier resource. After a second this resource is released
together with the free place and the car leaves the simulation. Initially, there is one each of resource
bra/brb and ten of place.

The third stream represents the boat. It enters the simulation once and never leaves. Upon entering the
simulation, it seizes the brb resource (i.e. the boat is at bank A) and stays there for 3 minutes. Then it
seizes bra (it closes the barrier) and crosses, after which it releases brb. It stays at bank B until it has
seized brb and it then crosses back. Cars leaving the boat seize brb with high priority, those entering
the boat have medium priority whereas the boat itself has low priority. This ensures that cars enter the
boat only when it is empty and that the boat leaves only if no cars can enter it anymore.

The average number of cars per crossing can be found by looking at the resource place. The
throughput time of cars an be found by looking at the entities carA, carB.

				
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posted:11/20/2011
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