stella and calculus
DESCRIPTION
STELLA and Calculus. STELLA numerically simulates the solutions to systems of differential equations. STELLA INTEGRATES!. Calculus Example 1 STELLA Diagram. Flow is constant. STELLA Equations: Stock_X(t) = Stock_X(t - dt) + (Flow_1) * dt INIT Stock_X = 100 Flow_1 = Constant_a - PowerPoint PPT PresentationTRANSCRIPT
STELLA and Calculus
STELLA numerically simulates the solutions to systems of differential
equations.
STELLA INTEGRATES!
Calculus Example 1STELLA Diagram
Stock X
Flow 1
Constant a
STELLA Equations: Stock_X(t) = Stock_X(t - dt) + (Flow_1) * dt
INIT Stock_X = 100Flow_1 = Constant_aConstant_a = 10
Flow is constant.
Example 1 Graph
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Graph 1 (Untitled)
Question?
• Does the graph have an equation?
• “Obviously”X = 10t + 100
Differential Equation 1
• Stock_X(t) = Stock_X(t - dt) + (Flow_1) * dt
• (Stock_X(t) - Stock_X(t - dt))/dt = Flow_1• Flow_1 = Constant_a
• (X(t) - X(t - dt))/dt = a• let dt 0
• dX/dt = a (a differential equation)
• Solution to DE: X = at + X(0)
Calculus Example 2STELLA Diagram
Stock X
Flow 1
Constant a
STELLA Equations:Stock_X(t) = Stock_X(t - dt) + (Flow_1) * dt
INIT Stock_X = 100Flow_1 = Constant_a*Stock_XConstant_a = .1
Flow is proportional to X.
Example 2 Graph
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Graph 1 (Untitled)
Differential Equation 2
• Stock_X(t) = Stock_X(t - dt) + (Flow_1) * dt
• (Stock_X(t) - Stock_X(t - dt))/dt = Flow_1• Flow_1 = Constant_a*Stock_X
• (X(t) - X(t - dt))/dt = aX(t-dt)
• dX/dt = aX (differential equation)
• Solution to DE:
X = X(0) exp(at)
Calculus Example 3STELLA Diagram
Two flows, an inflow and an outflow.
Population
Births
Birth Fraction
Deaths
STELLA Equations 3
• Stock_X(t) = Stock_X(t - dt) + (Flow_1 - Flow_2) * dt• INIT Stock_X = 1000
• Flow_1 = Constant_a*Stock_X(t-dt)• Flow_2 = Constant_b
• Constant_a = .11
• Constant_b = 100
Example 3 Graph
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Differential Equation 3
• Stock_X(t) = Stock_X(t - dt) + (Flow_1 - Flow_2) * dt• (Stock_X(t) - Stock_X(t - dt))/dt = Flow_1 - Flow_2
• Flow_1 = Constant_a*Stock_X(t-dt)
• Flow_2 = Constant_b
• dX/dt = aX - b
Solution to DE:?
Calculus Example 4STELLA Diagram
Two flows, an inflow and an outflow.
Population
Births Deaths
Death Fraction
Example 4 Graph
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Differential Equation 4
• Stock_X(t) = Stock_X(t - dt) + (Flow_1 - Flow_2) * dt• (Stock_X(t) - Stock_X(t - dt))/dt = Flow_1 - Flow_2
• Flow_1 = Constant_a
• Flow_2 = Constant_b*
• dX/dt = a - bX
Solution to DE:?
Calculus Example 5STELLA Diagram
Stock X
Flow 1
Constant a
Stock Y
Flow 2
Constant b Constant c
Flow 3
Two stocks, X and Y, and three flows.
The outflow from Stock X is the inflow to Stock Y.
STELLA Equations 5
• Stock_X(t) = Stock_X(t - dt) + (Flow_1 - Flow_2) * dt• INIT Stock_X = 100
• Flow_1 = Constant_a*Stock_X
• Flow_2 = Constant_b*Stock_X*Stock_Y
• Stock_Y(t) = Stock_Y(t - dt) + (Flow_2 - Flow_3) * dt• INIT Stock_Y = 100
• Flow_2 = Constant_b*Stock_X*Stock_Y
• Flow_3 = Constant_c*Stock_Y
• Constant_a = .2
• Constant_b = .001
• Constant_c = .01
Example 5 Graph
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Graph 1 (Untitled)
Differential Equations 5
• (Stock_X(t) - Stock_X(t - dt))/dt = Flow_1 - Flow_2
• (Stock_Y(t) - Stock_Y(t - dt))/dt = Flow_2 - Flow_3
• dX/dt = aX - bXY
• dY/dt = bXY - cY
(A pair of differential equations)
How many differential equations do we need?
Differential Equations 6
• dX/dt = -aXY
• dY/dt = aXY- bY
• dZ/dt = bY
STELLA Diagram 6
Stock X Stock Y Stock Z
Flow 1 Flow 2
Constant a Constant b
STELLA Equations 6
• Stock_X(t) = Stock_X(t - dt) + (- Flow_1) * dt• Flow_1 = Constant_a*Stock_X*Stock_Y /dt
• Stock_Y(t) = Stock_Y(t - dt) + (Flow_1 - Flow_2) * dt• Flow_1 = Constant_a*Stock_X*Stock_Y
• Flow_2 = Constant_b*Stock_Y
• Stock_Z(t) = Stock_Z(t - dt) + (Flow_2) * dt• Flow_2 = Constant_b*Stock_Y