Operational Research (O.R.) Techniques Simulation.

OperationalResearch (O.R.)

Techniques

Simulation

Supermarket checkouts

• Imagine that you need to decide on the checkout arrangements at a supermarket:

• How many checkouts?• How many basket only – less than 5 items, less than 10?• How many self service?

• What would you do?

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You could approach this problem in two ways

• Experiment directly– This is expensive and time consuming and may not give accurate

results as the different layouts will be trialled at different times and not under the same circumstances.

• Build a model– A computer simulation model allows different layouts to be run while

all other elements remain the same. The differences between the results will then be due to the different arrangements of the checkouts.


Simulation

• Simulation is a technique used extensively in business to aid the planning of new operations and investigate better ways of working current systems.

Different types of simulation

• Discrete– The variables of interest change only at particular instants of time, e.g. train

arrives, the doors open, the train departs.– This type of simulation is known as Discrete Event Simulation.

• Continuous– The variables of interest are changing continuously with time, e.g. the speed of

a train.– This type of simulation is tackled using a methodology called System Dynamics.


Why simulate?

• Cost: simulation may be time consuming and expensive in terms of skilled man power but if a real experiment goes wrong it could be very expensive.

• Time: it may take time to produce programmes for simulation but then it is possible to simulate weeks, months even years in seconds of computer time.

• Replication: simulation allows repetition which means a whole range of policies can be implemented without affecting the real world.

• Safety: the effect of extreme conditions may be one objective of simulation, to do this in real life may be dangerous or illegal.


SIMUL8

• SIMUL8 simulation software is used to model discrete event simulations.• Petrol station model:


Petrol station model

• Model structure– Cars arrive– Wait for petrol pump to become free– Use the pump– Queue to pay– Leave



• Run base case scenario.

• What was the average queue size that was waiting for fuel?

• What was the maximum time spent queuing waiting for fuel?

• What was the maximum queue size waiting to pay?



• Parameters that can be changed in the model– Number of cars per hour– Number of petrol pumps– Number of servers at pay





• What is the effect of changing the number of cars per hour?

• What effect does changing the other parameters have?

• What is the most effective combination for minimising the queues and maximising the number of customers?


STELLA

• STELLA simulation software is used to model system dynamics or continuous simulations.

• H1N1 Flu Outbreak model:


Flu outbreak model

• The core model structure of this system is a main chain of the student population which flows through the stocks of Susceptible, Exposed, Infected and Recovered.– Students are susceptible to the virus.– They then become exposed when come into contact with an ill

student.– A few days after exposure the symptoms are visible and the student is

infected. – After a number of days the infected student gets better and is

recovered.


Flu outbreak model

• Run base case scenario• How many students become sick?

• Variables that can be changed in the model– % vaccinated– % effectiveness of vaccine– Average number of days infected students stay at

home


Flu outbreak model


Flu outbreak model

• What is the effect of changing the percentage of students vaccinated?

• What effect does changing the other parameters have?

• What is the most effective combination to minimise the outbreak of the flu in this school?


Operational Research (O.R.) Techniques Simulation.

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