# Suction-driven sheath-flow inlet: almost no sample flow with both pressure inlets at 0 Pa

**URL:** <https://www.simscale.com/forum/t/suction-driven-sheath-flow-inlet-almost-no-sample-flow-with-both-pressure-inlets-at-0-pa/101429>\
**Category:** Project Support\
**Created:** [November 2, 2026, 10:11am UTC](https://www.simscale.com/forum/t/suction-driven-sheath-flow-inlet-almost-no-sample-flow-with-both-pressure-inlets-at-0-pa/101429 "2026-11-02T10:11:26Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![KrakhoferM](https://www.simscale.com/forum/letter_avatar/krakhoferm/32/6_22734026db12803ebb3e059622ac3534.png) [@KrakhoferM](https://www.simscale.com/forum/u/KrakhoferM)\
**Post date:** [November 2, 2026, 10:11am UTC](https://www.simscale.com/forum/t/suction-driven-sheath-flow-inlet-almost-no-sample-flow-with-both-pressure-inlets-at-0-pa/101429/1 "2026-11-02T10:11:26Z")

</div>

Hi everyone,

I am designing an air inlet for the measurement cell of a portable fluorescence sensor and need help understanding an unexpected flow distribution in SimScale.

**Project link:** [https://www.simscale.com/workbench/?pid=8168537471373415552](https://www.simscale.com/workbench/?pid=8168537471373415552)

### Physical setup

A pump draws air out through the outlet at the bottom of the measurement cell. Air enters through two paths:

- A **sample inlet** carrying aerosol-containing air.

- A **sheath-air inlet** supplying filtered air, intended to surround the sample stream and reduce recirculation in the measurement region.

At a measured total outlet flow rate of **0.81 L/min** , the experimentally determined flow split is approximately **0.30 L/min sample flow** and **0.51 L/min sheath flow**.

I also visualise the flow using a light-scattering tracer aerosol and video recordings. These show the intended sample stream surrounded by the sheath flow.

### SimScale setup

I selected **Incompressible** because the air velocities are comparatively low and I expect compressibility effects to be negligible.

- **Fluid:** Air

- **Bottom outlet:** Velocity outlet → Flow rate → Volumetric flow rate = **0.81 L/min** , directed out of the domain

- **Sample inlet:** Pressure inlet → Total pressure = **0 Pa**

- **Sheath-air inlet:** Pressure inlet → Total pressure = **0 Pa**

Both inlet pressures are set to 0 Pa to represent air drawn from the same ambient environment. In the physical setup, the sheath air passes through a filter.

### Unexpected behaviour

With **both inlet total pressures at 0 Pa** , the velocity-magnitude results suggest almost no flow through the sample path: its velocity approaches **0 m/s**. This disagrees with the experimental observations.

If I change **only the sheath-inlet total pressure to −15 Pa** , keeping the sample inlet at **0 Pa** and the outlet flow at **0.81 L/min** , a sample stream becomes visible. The velocity pattern then looks much closer to what I observe experimentally.

I tried −15 Pa to investigate the effect of the inlet pressure; this value was not obtained from a pressure measurement.

**Are these boundary conditions appropriate for representing the suction-driven setup, and what could explain why lowering the sheath-inlet pressure is necessary to obtain a flow pattern resembling the experiment?**

I am attaching velocity-magnitude screenshots for comparison:

- **Case A:** Both inlets at **0 Pa**.

- **Case B:** Sample inlet at **0 Pa** , sheath inlet at **−15 Pa**.

Both cases use an outlet flow rate of **0.81 L/min**.

Thank you for your help,

Best regards,

Matthias

 ![image](https://www.simscale.com/forum/uploads/default/original/3X/4/e/4ecc94287d476800b1d8c834abc5e8658544c8d8.jpeg)

 ![image](https://www.simscale.com/forum/uploads/default/original/3X/4/5/45147fe0b4fb45be2939559ff1ef9ec552c310bf.jpeg)