# Pressure drop in a CFD simulation of an air flow too small

I am simulating a turbulent air flow through a rectangular channel with the goal to match measured velocity profiles and the static pressure at certain points. My problem is that the pressure drop in the channel is not as high as in the measured data. Below you can see a diagram of the static pressure at the measurement points:

The black line represents the measurements, the red line the simulated data. As you can see the pressure drop is too small. As a result the simulated velocity profiles lag behind the measured velocity profiles more and more as can bee seen in the following pictures for position 1 and 4:

One possibility to increase the pressure drop would be to increase the wall roughness but then the boundary layer gets too thick. Which boundary conditions can I change to increase the pressure drop without increasing the boundary layer thickness?

Thanks!

• Do your macro parameters (flow speed of two exits, pressures at inlet-outlet) already match the experiment? Does the experiment report contain its measurement error margins/accuracy estimates? The secondary outlet might be breaking the solution, I assume you already did grid dependency (or independency) and came up with a reliable mesh? Feb 5, 2023 at 14:08

## 1 Answer

Depending on your types of boundary conditions, you have a few options to increase the pressure drop:

1. Lower the exit static pressure

2. Increase the mass flow rate

3. Increase the inlet total pressure

All of these will increase your static pressure drop and hence your velocities. Since the thickness is inversely proportional to square root of Reynolds number (see Bounded Boundary Layers) https://en.wikipedia.org/wiki/Boundary_layer_thickness#The_Bounded_Boundary_Layer_Description, the thickness should decrease with a larger velocity.