FLUID32 - Fluid Flow Analyzer

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SIMULATION LINKS:

C++ source code generator for queuing models : http://www.raczynski.com/pn/cqm.htm

Low cost system dynamics simulation: SimBall program

Simulation Encyclopedia : http://www.raczynski.com/pn/encyk.htm

HEAT TRANSFER SIMULATION : http://www.raczynski.com/pn/heathit.htm

MOLECULAR SIMULATION and SOLIDIFICATION : http://www.raczynski.com/pn/mole.htm

To find out more about simulation and modeling consult: http://www.raczynski.com and http://www.raczynski.com/pn/pn.htm

Fluids5 solves the Navier-Stokes (N-S) equation for a gas or an incompressible liquid that flows through 3D channels with obstacles. The channel and the obstacles can be arbitrary 3D shapes. Optionally, the channel may be axisymmetric. See some examples of gas flow analysis at the end of this page.
Recall that the N-S equation is a nonlinear partial equation that describes fluid dynamics. The N-S equation is complemented by the continuity equation. The whole system represents a set of nonlinear partial differential equations with four unknown variables assigned to each point of the a 3D region: the pressure and the three components of the particle velocity.
This is a dynamic solution, rather than a search for a steady state solution. Perhaps steady solution is a nice one, shown in books on fluid dynamics and calculated by many fluid dynamics programs. However, it has rather academic and not practical meaning. The steady solution may exist if the flow is laminar (with low velocity) or if its behavior and properties are very "regular". In a real flow, particularly for gases, no steady solution is reached in the real flow, unless the velocities are low. The pressure and velocity fields constantly change and the flow is oscillatory or it becomes chaotic. Fluids5 provides such a dynamic, time-dependent solution that can be useful while simulating shock waves, oscillations in valves or nozzles or similar problems. However, the steady solution option has been added in the new version 5. It works for the liquid case only and with very low pressure differences.

It should be noted that to obtain a dynamic solution, even for few milliseconds of the transient process, you need a long computing time; in less than half hour of computer time you will get nothing interesting. A normal simulation time oscillates between on hour to several hours on a fast (more than GHz) machine. But it is necessary, no program can simulate a 3D liquid flow model with 100000 grid points in few minutes. Sometimes after one or two hours you merely reach a millisecond of the model time. Long simulations can provide something similar to steady solution, but remember that the main application of Fluids5 are shock waves. As for the "steady solution" note that in the real flow a steady state does not exist, unless you have very low pressure gradient and velocities. But such solutions are not very interesting, they are rather academic examples. In a real flow the velocity field is always changing, oscillating or produces turbulence. So, the Fluids5 program never stops, it has not "stop criterion". This is a dynamic simulation, where each iteration step provides an image of the velocity, pressure and temperature fields for a particular model time instant.
The flow is considered to occur in an axisymmetrical or irregular channel that has an inlet and outlet. However, the user can define any other configuration, like, for example, an open region where some internal points have fixed pressure, being sources of the flow. The boundary conditions are defined as sets of points with fixed pressure or fixed velocity. It is supposed that the velocity on all solid boundaries is fixed. Moving walls of the duct or obstacle are also supported. The pressure at the inlet/outlet points can be defined by the user, as an external excitation.
Fluids5 program includes a 3D duct editor. The duct shape is defined graphically. For the axisymmetric duct the user draws the duct projection. For the arbitrary duct the shape is defined by sections (layers) on the X-Y plane, that are given consecutive Z coordinate values. Then, the program creates the set of grid points to discretize the problem in space. A normal channel needs about 100,000 points. For each point the pressure, the temperature (gas case) and the three velocity components are calculated.

NEW: New version of Fluids5 (August 2005) can calculated the forces on the duct walls and/or obstacles, produced by the flow.

Fluids5 can import STL ASCII files created by CAD/CAM software or other 3D graphics programs. So, you can import a duct or obstacle shape instead of editing it with the Fluids5 shape editor. The "slicer" algorithm of Fluids5 converts an STL file in up to 100 slices that define the 3d duct shape. The "slicer" resolution is limited to those 200 layers, so small details of complicated shapes can be lost. But this option enables you to create shapes that may hardly be edited by Fluids5 itself.

The Fluids5 display screens show the 3D or 2D channel and obstacle images with variable view angles. The following result images are provided.

The above figure shows the velocity distribution in the Y-Z projection (a section of the channel with a vertical plane X=0). Remember that the model is 3-dimensional, and the above image shows only one section of the duct. The whole problem includes more than 60000 grid points. It is a liquid flow with external presure applied at the left side of the duct, the right side being open. The length of the velocity sections is proportional to the logarithm of the velocity. The velocity is also marked with different colors: blue sections represent low velocity, while the yellow means big velocity.

Below are some images taken from a simulation of air excited by a short pressure pulse at the left end of a duct with an obstacle. The first image shows the moment when the front of the shock wave bounces for the first time from the obstacle. The next picture shows the situation after some longer time interval. On the last figure we can see the pressure distribution in a vertical section of the duct in the same moment of the model time. Also the temperature of the gas is plotted in similar way.

As the gas flow simulation is dynamic, you can see the time-plots of the pressure in selected fixed points. Frequently the multiple reflections result in oscilations, like in musical instruments. In these cases it is very interesting to see the frequency response of the model. The frequency analysis option of Fluids32.3 does the job. The user can see the spectrum of the gas vibrations in the selected points inside the duct.

Some parts of duct and obstacle walls can be given a fixed, user-defines temperature. This results in slow clonvection gas movement. As the velocities of the convection related movements are low, this fenomemnon can be observed when no other strong excitations are defined. The below images illustrate the convection flow. A part of the right-hand side of a vertical duct is heated (red line on the right side indicated the heated region). The first figure shows the velocity distribution and the second shows the temperature several seconds after the temperature pulse have been applied.

Below is another example of slow convection velocity field in gas (a 2D section of a 3D model). The heated area has 420 degree Kelvin and the elliptic body has 150 Kelvin. The color indicates velocity, not temperature. The air cooled by the cool body falls down, and the heater generates the upwards flow. Anyway, the air near to the body goes down.

An example of an unstable stream of air. Made with the new version of the program, Fluids5.

An interesting fact is the oscillating character of the flow. The following plot shows the pressure oscillation below the wing.

Other example of oscillating gas flow. Pressure waves in a gas flow through a chamber. No steady solution in this case exists.

The following figure shows the air flow around a space capsule which enters the atmosphere.

In the above model the main duct was a cylinder, and the whole geometry as well as the boundary conditions have been ax-symmetrical. However, the resulting 3D velocity field in not.

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