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WATER JET CONFERENCE - Waterjet Technology Association

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system. Examples of the component matrices for a pipe section and a pipe junction will<br />

be presented.<br />

The equations of motion for a section of pipe are solved by assuming that the<br />

instantaneous pressure, H, and discharge, Q, are mean quantities (H o, Q o ) plus deviations<br />

from the mean (h, q). The deviations are assumed to be sinusoidal with respect to time.<br />

The flow rate and pressure on the left side of the (i+l)th section are obtained in terms of<br />

these quantities on the right side of the ith section as follows.<br />

and<br />

L Li qi +1 = cos<br />

ai L<br />

hi+1 = − jCi sin Li ai R j<br />

qi − sin<br />

Ci Li ai R Li qi + cos<br />

ai 5<br />

R<br />

hi (1)<br />

R<br />

hi (2)<br />

Ci = ai (3)<br />

gAi where ai is the water hammer wave velocity in the ith pipe, g is the acceleration due to<br />

gravity, AI is the area of the pipe, Li is the length of the pipe and ω is the driving<br />

R<br />

frequency. The equation matrix for a length of pipe is composed of the coefficients of qi R<br />

and hi in Equations (1) and (2) above.<br />

Similar equations are obtained for each component in the modulator piping system.<br />

For example, at a series pipe junction where only the pipe size changes, the flow rate and<br />

the pressure are normally assumed to be constant. Therefore, the equations relating<br />

upstream quantities to those downstream are given by:<br />

L<br />

qi +1 = qi<br />

h i+1<br />

L = O qi<br />

L + O hi<br />

L L ( ) + hi L ( ) (4)<br />

The equation matrix for a series pipe junction is the identity matrix since the<br />

coefficient in the equations above are either zero or one. When equation matrices for each<br />

component in the modulator are obtained they are multiplied in the proper order to<br />

produce two equations relating pressure and flow rate at the upstream boundary of the<br />

system to these quantities at the downstream boundary. At the system boundaries<br />

normally two of these quantities are known and the two equations can be used to solve<br />

for the remaining two unknowns.<br />

When the method is applied to the branch piping system shown in Figure 1, the<br />

following equations result<br />

(5)

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