GOOD NEWS: The new version of Fluids6 supports multiprocessing (up to 8 processors), so it is several times faster. It also supports models with better resolution, up to 400 layers along the duct.
This is the new version of the Fluids package. The old Fluids5 program is no longer distributed. Model files of both Fluids5 and Fluids6.17 have the same format, so you can import your old models to Fluids6.
Multiprocessing: New PCs are now equipped with more powerful , normally dual or quad processors. Consequently, new software should be designed to run on multiple processors. Fluids program detects automatically the processors, and runs its simulation procedure on all available processors concurrently. This means that you can achieve a considerable speedup, approximately proportional to the number of processors. On the other hand, if you use full resolution of Fluids, then the number of grid points may be rather big, which means that the simulation will slow down. While creating a model, you always should think about a reasonable compromise between the model resolution and the computing time.
Resolution: Fluids uses the uniform 3D grid inside the duct. Near the duct or obstacle borders the resolution is two times greater. The program permits up to 400 layers in the main duct direction (Z-axis). The resolution in X and Y direction can be greater. The reasonable total number of grid points shoud not exceed 2 million ponts.
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The upper figure shows the resolution of the older Fluids5 version. The lower image was taken from a screen of Fluids6.17
A model with 390 layers. Red objects are obstacles, blue objects are appendices
Velocity field for the 390 layers model. Vertical section. Total grid points 867088.
To be able to use the new features of Fluids you must have a fast machine and the multiple-processor PC is recommended. The good results can be obtained on a double- or quad-core 2GHz processor or faster, at least 2GB of memory. You will also need hard disc space enough to store the projects. A typical low-resolution Fluids6-like project may need 20MB, a normal Fluids6 project will need 100MB or more. The minimal screen resolution is 1024x768.
Fluids 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 Fluids shape editor. The "slicer" algorithm of Fluids converts an STL file in up to 400 slices that define the 3d duct shape. The "slicer" resolution is limited to those 400 layers. Thus, small details of complicated shapes can be lost. But this option enables you to create shapes that may hardly be edited by Fluids itself.
The examples shown below have mostly been obtained using Fluids5. They are shown here, because the lower resolution flow images look better on a Web pages. Exactly the same sesults can be generated by Fluids6, the only difference is two-times better resolution of the model grid. As stated before, the other main advantage of FLuids6 is the computing speed. Compared to Fluids5, Fluids6 may be up to 9 times faster while running the same model on an 8-processor machine.
This image shows an axisymmetric channel with two obstacles.
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 pressure 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 there 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 displayed in similar way.
WAVES IN GAS AND FREQUENCY ANALYSIS
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 Fluids does the job. The user can see the frequency 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 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 you can see 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. However, the air near to the body goes down.
An example of an unstable stream of air.
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 is reached.
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.
Other example: A tube with an appendix (figure below).
Water enters at the left and passes through the duct. It is interesting that a vortex forms not only in the appendix, but also in the tube. The amazing flow lines are shown below. White arrows show the parts where a counterflow occurs.
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3D Fluid flow simulation program. Includes 3D duct/obstacle editor. Accepts STL files, shows fluid movements, shock waves and oscillations as function of time.
Fluids6.17 (shortly Fluids) solves the Navier-Stokes (N-S) equation for a gas or liquid that flows through 3D channels (ducts) with obstacles. The duct and the obstacles can be arbitrary 3D shapes. Optionally, the duct 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 whole model is described by 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.
Fluids provides a dynamic, rather than a steady state solution. In a real flow, particularly for gases, the steady solution may not be reached, unless the velocities are low. The pressure and velocity fields constantly change and the flow is oscillatory or it becomes turbulent. Fluids provides such a dynamic, time-dependent solution that can be useful while simulating shock waves, oscillations in valves or nozzles or similar problems.
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