Fluid dynamics

The Fluid Dynamics tab estimates what will happen when you push fluid through the chip. Pressure, velocity, Reynolds number, wall shear, residence time — and heat crossing between channels that run alongside each other.

Everything here is an analytical estimate from a one-dimensional network model. What that includes and excludes is on The limits of analytical estimates, and it is worth reading before you rely on a number.

Setting up a run

The right-hand panel lists every port. For each one:

Setting	Notes
Role	Inlet or Outlet. An outlet is held at ambient pressure — whatever reaches it, leaves.
Fluid	From the preset list, or one you created.
Flow	µL/min. Inlets only.
Inlet temp	°C. Overrides the fluid’s own default for this run.

A chip needs at least one inlet and one outlet to produce numbers.

Note

The flow rate, the run duration and the wash multiplier describe a run, not a chip. Two people can open the same design and plan a ten-minute test and an overnight perfusion from it. They are not saved as part of the geometry and they are not on the undo stack — nudging a duration should not eat a ⌘Z you meant for a channel.

The summary strip

Along the top: segment count, peak Reynolds number, maximum pressure, internal volume, transit time.

Peak Reynolds is the one to glance at first. If it is well under 1 — and it usually is — the flow is firmly laminar and every mixing assumption on Mixing without turbulence applies.

Maximum pressure is what your pump has to deliver. Compare it against what your syringe pump can actually push before it stalls.

The heatmap

The canvas colours the network by whichever metric you pick:

Metric	Shows
Pressure	Pressure at each point, inlet to outlet.
Reynolds	Flow regime per segment.
Temperature	Fluid temperature along the path.
Residence	Cumulative time from the inlet.
Shear	Wall shear stress.

The legend gives the scale and the units.

Note

The pale end of a sequential colour ramp sits below 3:1 contrast against the background by design — that is what makes it the pale end. The accessible answer is that every value is also readable as text in the table below. If a colour is ambiguous, read the number.

Hovering or clicking a row in the table highlights that segment on the map, and vice versa.

The per-segment table

One row per channel segment:

Column	Unit
Fluid	—
Length	mm
W × H	mm
Volume	µL
τ (residence)	s
Flow	µL/min
Velocity	mm/s
ΔP	Pa
τ_w (wall shear)	Pa
Re	—
Regime	laminar / transitional / turbulent
T in → out	°C
Heat	mW

The regime label is reported for every segment even when it is always the same, because you might be designing for a transition and hiding the number when it looks boring would hide exactly what you are hunting for.

Heat exchange

Below the segment table, a second table for pairs of channels running close enough to exchange heat: the wall between them, the contact length, the overall coefficient U, the heat flux, NTU, effectiveness, and the approach temperature.

This is what the counter-flow exchanger example is for.

The model treats each pair as a one-dimensional exchanger through the resin between them. It does not resolve a temperature field across the channel cross-section, and it assumes the rest of the chip is adiabatic.

Consumption

The panel also estimates how much fluid a run will consume, from a duration you set, and how much wash volume to flush afterwards from a multiplier of the chip’s internal volume.

Both default to something reasonable — 60 minutes and 5× — and both are yours to change. They exist so you can find out before the experiment that you need 40 mL of a reagent you have 5 mL of.

Saving the heatmap

Save heatmap writes the current view into the project as a saved image, and the presentation bundle regenerates it at your chosen DPI rather than resampling the screen.

What the model does not include

Stated in the app on every view of this tab, and worth repeating:

• Entrance effects — the developing-flow region at the start of a channel.
• Bend and junction losses beyond the straight-channel resistance.
• Secondary flows, including the Dean vortices the spiral mixer relies on.
• Any concentration field. Mixing is assumed perfect at junctions.
• Non-Newtonian behaviour of any kind.
• Compliance in the tubing, the chip, or the syringe.

See The limits of analytical estimates for when these matter enough to need CFD.

Next

• Laminar flow and the Reynolds number
• Pressure drop and Hagen–Poiseuille
• Fluid property tables