# Comsol-Pressure drop fluid flow in rectangular channel problem

• baranii2000
In summary, the conversation discusses an issue with the solver in COMSOL when trying to find the pressure drop in a rectangular channel with a specific inlet velocity. The solver shows an error message when the velocity is set too high, but works fine at lower velocities. The suggestion is made to ramp up the velocity gradually and change the boundary conditions for better stability. The issue is resolved by remodelling the problem as a time dependent file.
baranii2000
Dear all,

I am trying to solve a simple model with Comsol to find the pressure drop in a rectangular channel W=5mm, h=0.6mm, L=10mm. When i try to set the inlet velocity to 0.5m/s the solver shows the error message "Maximum number of Newton iterations reached". The solver works fine if i set the input velocity to lower velocity(eg., 0.005m/s). when i increase the velocity to 0.05 m/s the solver ends up with the error message. I even tried to increase the height of the channel, still get the same error sequence.

I am using a Stationary, Direct (Pardiso)solver.

Any comments or help is much appreciated.

Arun

I'm not familiar with COMSOL, but here are a couple things you can try. First, try ramping the velocity up. Solvers hate suddenly seeing a flow.

Also, you can try changing the boundary condition to total pressure at the inlet, and static at the outlet for a first cut. These tend to be more well posed than others and can have better stability.

Thanks Minger,

Actually your comment on the solver crash on seeing sudden high velocity at inlet was very useful. I remodelled as a time dependent file and now it works fine...

Many thanks

Arun

Right, just to clarify a little more on what happens. Unless you normalize your mean flow at t=0 to some value, at that time 0, you suddenly impose a velocity at the inlet. One point inwards your trying to either take a derivative or calculate an integral with a discontinous flow.

You will calculate an artificially high flux at that point, which will then give you an artificially high time derivative. This will just continue until your solution blows up.

## 1. What is Comsol and how does it work?

Comsol is a software used for multiphysics simulation and analysis. It works by creating a mathematical model of a physical system and using numerical methods to solve the equations and simulate the behavior of the system.

## 2. What is pressure drop and why is it important in fluid flow?

Pressure drop is the decrease in pressure that occurs as a fluid flows through a channel or pipe. It is important because it affects the flow rate and can indicate the presence of obstructions or restrictions in the flow path.

## 3. What factors affect pressure drop in a rectangular channel?

The factors that affect pressure drop in a rectangular channel include the dimensions of the channel, the fluid properties such as viscosity and density, the velocity of the fluid, and the presence of any obstructions or bends in the channel.

## 4. How can Comsol be used to analyze pressure drop in a rectangular channel?

Comsol can be used to create a model of the rectangular channel and set up the appropriate equations and boundary conditions to simulate the flow and calculate the pressure drop. It allows for the manipulation of different parameters to analyze their effects on the pressure drop.

## 5. What are some potential applications of studying pressure drop in fluid flow in a rectangular channel?

Studying pressure drop in fluid flow in a rectangular channel has many applications, such as optimizing the design of pipes and channels in plumbing and HVAC systems, analyzing the performance of heat exchangers and filters, and understanding the flow behavior in microfluidic devices and biomedical systems.

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