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SCIENCE · INVESTIGATE AND EXPLAIN

Close a branching flow balance without guessing the split

Volume flow rate Q is volume transported per time, not the speed of an individual fluid element. For a section of area A and mean normal speed v, Q=Av. One litre is 0.001 m³ and one minute is 60 s, so 6 L/min is 0.0001 m³/s. Converting area requires squaring the length conversion: 1 cm² is 0.0001 m².

Learning outcome: Compare two areas at the same prescribed split, then compare two splits at one area. Retain volume and mass residuals. State explicitly why continuity verifies an imposed split but does not predict a pressure-driven split.

Starting recap

Declare density, pressure reference, flow regime and admissible force constraints. Convert area and volume units before balancing forces, mass or energy.

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Try

At first-branch flow 3 L/min and second area 1 cm², second mean speed is 0.5 m/s and mass flow 0.045 kg/s. Doubling the area at that imposed flow halves the mean speed.

Open the experiment

Predict

For steady flow with changing density, the conserved flow quantity is:

Experiment

Compare two areas at the same prescribed split, then compare two splits at one area. Retain volume and mass residuals. State explicitly why continuity verifies an imposed split but does not predict a pressure-driven split.

Close a branching volume and mass balance

Worked teaching model · synthetic values · Steady incompressible liquid density 900 kg/m³ enters a junction at 6 L/min. No storage or leakage. Selected first-branch flow is independently stipulated, not predicted by a pipe-resistance model.

Reset keeps your prediction, captures and practice history.

Second branch speed at fixed inlet flow

second branch mean speed (m/s)first branch flow (L/min)060.50

Unlike graph quantities rescale independently; read units and values, not screen angles. 1: no-storage balance (solid). Joined curves show the supplied model, not additional observations.

Equivalent numerical coordinates for Second branch speed at fixed inlet flow

no-storage balance

first branch flow (L/min)second branch mean speed (m/s)
00.5
0.150.4875
0.30.475
0.450.4625
0.60.45
0.750.4375
0.90.425
1.050.4125
1.20.4
1.350.3875
1.50.375
1.650.3625
1.80.35
1.950.3375
2.10.325
2.250.3125
2.40.3
2.550.2875
2.70.275
2.850.2625
30.25
3.150.2375
3.30.225
3.450.2125
3.60.2
3.750.1875
3.90.175
4.050.1625
4.20.15
4.350.1375
4.50.125
4.650.1125
4.80.1
4.950.0875
5.10.075
5.250.0625
5.40.05
5.550.0375
5.70.025
5.850.0125
60

At most 82 representative computed coordinates including endpoints; the plotted curve can use a finer grid. A table of a revealed generator is not independent measurement evidence.

Calculated quantities under the declared measurement model
QuantityValueUnit
Inlet volume flow0.0001m³/s
First branch volume flow0.0000333333m³/s
Second branch volume flow0.0000666667m³/s
Second branch mean speed0.333333m/s
Inlet mass flow0.09kg/s
Second branch mass flow0.06kg/s
Junction volume residual0m³/s

Continuity closes the imposed flow split but does not determine it from pipe areas alone.

Q=A v for mean section velocity; a litre is .001 m³ and a minute is 60 s. In steady incompressible flow, incoming Q equals the sum of outgoing Q. With varying density conserve rho Q instead. Pressure, resistance and boundary conditions are needed to predict an unknown branch split.

Predict before inspecting the values. Save contrasting records with assumptions and one alternative explanation. This virtual display does not certify practical measurement technique.

Continuity closes the imposed flow split but does not determine it from pipe areas alone.

Q=A v for mean section velocity; a litre is .001 m³ and a minute is 60 s. In steady incompressible flow, incoming Q equals the sum of outgoing Q. With varying density conserve rho Q instead. Pressure, resistance and boundary conditions are needed to predict an unknown branch split.

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Declare the fluid, reference and evidence

Volume flow rate Q is volume transported per time, not the speed of an individual fluid element. For a section of area A and mean normal speed v, Q=Av. One litre is 0.001 m³ and one minute is 60 s, so 6 L/min is 0.0001 m³/s. Converting area requires squaring the length conversion: 1 cm² is 0.0001 m².

Build the algebraic model

This steady incompressible packet stipulates inlet flow 6 L/min and independently selects the first outlet flow. With no storage or leakage, the second outlet is the difference. If the first branch carries 2 L/min, the second carries 4 L/min. At second-branch area 2 cm², its mean speed is (4/60000)/0.0002=1/3 m/s.

Check the original numerical packet

The liquid density is 900 kg/m³, giving inlet mass flow 0.09 kg/s. The two outlet mass flows add to this same value. At first-branch flow 6 L/min, the second is zero even though its area is nonzero. Increasing an outlet area alone does not determine how a real network redistributes flow; pressure differences, resistances and boundary conditions are missing.

Challenge the assumptions and transfer

When density varies, conserve mass flow ρAv rather than assuming equal volume flow. For an original steady gas packet with inlet density 2 kg/m³ and volume flow 0.03 m³/s, mass flow is 0.06 kg/s. If outlet density is 1.5 kg/m³ with no leakage or storage, outlet volume flow is 0.04 m³/s. Changing the outlet density to 1 kg/m³ makes it 0.06 m³/s. No incompressible Bernoulli inference is warranted from that gas balance alone.

Worked original case

At first-branch flow 3 L/min and second area 1 cm², second mean speed is 0.5 m/s and mass flow 0.045 kg/s. Doubling the area at that imposed flow halves the mean speed.

Bounded investigation

Compare two areas at the same prescribed split, then compare two splits at one area. Retain volume and mass residuals. State explicitly why continuity verifies an imposed split but does not predict a pressure-driven split.

Changed-case numerical practice

Try the changed case before feedback

A steady incompressible stream has Q=3 L/min through area 2 cm². Find mean speed in m/s.

This practice is public and unsaved. A matching number does not verify understanding. Retain your explanation, assumptions and changed-case evidence in the separate course workspace.

Sources and limits

Original content checked 30 October 2026 against OpenStax College Physics 2e. All records are supplied or generated educational evidence. No pressure rigs, liquid handling, bodily measurements or physical exposure protocols are requested. Virtual admissibility does not certify real equipment safety, practical skill or independent mastery.

Apply

For steady flow with changing density, the conserved flow quantity is:

Public practice checks a specific claim. It does not establish independent mastery.

What additional evidence would be needed to infer an unknown junction flow split?

Continue

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