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Basics of Fluid Mechanics, 2014a

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11.7. FANNO FLOW 459<br />

back pressure at this stage will not “move” the shock wave downstream the nozzle. At<br />

point c or location <strong>of</strong> the shock wave, is a function entrance Mach number, M 1 and<br />

the “extra” 4 fL<br />

D<br />

. There is no analytical solution for the location <strong>of</strong> this point c. The<br />

procedure is (will be) presented in later stage.<br />

4.5<br />

4.0<br />

P 2<br />

P 1<br />

3.5<br />

3.0<br />

2.5<br />

2.0<br />

1.5<br />

shock location at:<br />

5%<br />

50%<br />

75%<br />

1.0<br />

0.0 0.05 0.1 0.15 0.2 0.25 0.3<br />

4fL<br />

D<br />

Fig. -11.31. Pressure ratios as a function <strong>of</strong> 4 fL<br />

D<br />

4 fL<br />

when the total =0.3.<br />

D<br />

The Maximum Location <strong>of</strong> the Shock<br />

The main point in this discussion however, ( is to)<br />

find the furthest shock location<br />

downstream. Figure (??) shows the possible Δ 4 fL<br />

D<br />

as a function <strong>of</strong> retreat <strong>of</strong> the<br />

location <strong>of</strong> the shock wave from the maximum location. When the entrance Mach<br />

number is infinity, M 1 = ∞, if the shock location is at the maximum length, then<br />

shock at M x =1results in M y =1.<br />

The proposed procedure is based on Figure ??.<br />

i) Calculate the extra 4 fL<br />

4 fL<br />

D<br />

and subtract the actual extra<br />

D<br />

assuming shock at<br />

the left side (at the max length).<br />

ii) Calculate the extra 4 fL<br />

4 fL<br />

D<br />

and subtract the actual extra<br />

D<br />

the right side (at the entrance).<br />

assuming shock at<br />

iii) According to the positive or negative utilizes your root finding procedure.<br />

From numerical point <strong>of</strong> view, the Mach number equal infinity when left side<br />

assumes result in infinity length <strong>of</strong> possible extra (the whole flow in the tube is subsonic).<br />

To overcome this numerical problem, it is suggested to start the calculation from ɛ<br />

distance from the right hand side.

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