Solving Fluid Model Question with CFD

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In summary, the goal would be to calculate the velocity and pressure at a specific point in time, based on the travel of the piston.
  • #1
SergejVictorov
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I have a specific question about fluid analysis that is bugging me. I am no expert in this field but very interested in it.

Lets's say we want to model a flow where air expands out of an air reservoir across a valve into a cylinder and applies a pressure force on an airtight piston, which moves, allowing the air to further expand. Assume we know the inertial and frictional forces which resist the movement of the piston as well as the initial pressure in the reservoir before the opening of the valve. Also assume that the valve opens instantaneously. We would like to obtain (by numerical analysis) the functions of the piston's velocity as well as the pressure just behind with respect to time.

How is such a moving (dynamic) boundary, as it shows up at the piston, usually treated? At the same time, the solid boundary at the parts of the valve, which are fixed in space, should also be included.

Does anyone know how such problems could be solved using CFD methods or some other method?
 
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  • #2
Hi SV. From your three recent posts it would seem you're devising a gun of some sort. Can you explain what you need this for? What you're asking for here is simple enough to calculate.
 
  • #3
I'm not sure if I'm understanding you properly, but it seems like a situation similar to an Otto Cycle motor, where you have a high pressure generated moving a piston.

Firstly, you would need to get your volume as a funtion of piston travel, which should be easy enough. Then use whatever method you want to calculate pressure as a function of a volume. Your force is then pressure times area of the piston, then minus any frictional forces.
 
  • #4
SergejVictorov said:
I have a specific question about fluid analysis that is bugging me. I am no expert in this field but very interested in it.

Lets's say we want to model a flow where air expands out of an air reservoir across a valve into a cylinder and applies a pressure force on an airtight piston, which moves, allowing the air to further expand. Assume we know the inertial and frictional forces which resist the movement of the piston as well as the initial pressure in the reservoir before the opening of the valve. Also assume that the valve opens instantaneously. We would like to obtain (by numerical analysis) the functions of the piston's velocity as well as the pressure just behind with respect to time.

How is such a moving (dynamic) boundary, as it shows up at the piston, usually treated? At the same time, the solid boundary at the parts of the valve, which are fixed in space, should also be included.

Does anyone know how such problems could be solved using CFD methods or some other method?

It depends on how accurate you want to be. You could code it in a CFD program, but I think it is a bit exagerated for your purposes. You don't need the details of the flow, do you?.

Therefore is appropriated an Integral Formulation. What do you know about it?. Try to employ the integral Navier-Stokes equations particularized for inviscid and compressible flow, retaining the unsteady terms of volumetric variation. These terms will represent your moving boundary. Employ mass conservation and Energy to work out the pressure P(t).
 

1. How does CFD help in solving fluid models?

CFD (Computational Fluid Dynamics) is a computational tool that uses numerical methods and algorithms to solve complex fluid models. It helps in predicting the behavior and characteristics of fluids in various scenarios, which would be difficult or impossible to achieve through physical experiments alone.

2. What types of fluid models can be solved using CFD?

CFD can be used to solve a wide range of fluid models, including single-phase and multiphase flows, compressible and incompressible fluids, laminar and turbulent flows, and even non-Newtonian fluids. It is a versatile tool that can handle various types of fluid behavior.

3. What is the process of solving a fluid model with CFD?

The process of solving a fluid model with CFD involves several steps, including geometry creation, mesh generation, setting up boundary conditions and initial conditions, selecting appropriate equations and numerical methods, and finally, running the simulation and analyzing the results. It requires knowledge of fluid mechanics, numerical methods, and computer programming.

4. How accurate are the results obtained from CFD simulations?

The accuracy of CFD results depends on various factors, such as the complexity of the fluid model, the quality of the mesh, the selection of numerical methods, and the accuracy of input data. With proper setup and validation, CFD simulations can provide highly accurate results, comparable to physical experiments.

5. Can CFD be used for real-world applications?

Yes, CFD is widely used in various industries for real-world applications, such as aerodynamics, hydrodynamics, heat transfer, and chemical reactions. It is a cost-effective and efficient tool for optimizing designs, predicting performance, and reducing the need for physical testing. However, it should always be validated and verified before making critical decisions.

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