Note: These readers are not available for 64-bit Linux platforms

1. Select a surface or face in your model that you would like to make transparent, as shown in Figure 1.



Figure 1.  Surface selected to modify the transparency

2. Click the Colors button, as shown in Figure 2, to launch the Colors Dialog.



Figure 2.  Colors button in the Tool bar

3. On the Colors Dialog, select the second tab as shown in Figure 3. Under this tab, you can modify the Red, Green, and Blue settings for the surface. The last option is Alpha, which modifies the transparency of the selected surface or face.



Figure 3.  Colors dialog

4. Set the desired value for Alpha, then click Accept to apply the new settings.

Figure 4.  Transparent face in CFD-GEOM

The transparent surface is shown in Figure 4. Using transparent surfaces can help with complicated models where you need to see the inside of the model, e.g. updating the grid distribution on the outer section and looking at the effects on the inner section.

Note: You can also set the transparency on volumes. When sweeping the mesh through the volume using the Volume Grid viewer, the shaded surfaces of the mesh will be transparent (if grid shading is active.)

Note: tested with V2010.0

In the example below, a surface (Surface 3) will be projected onto two disconnected surfaces (Surfaces 1 and 2), as depicted in figure 1. The general steps involved are as follows:

Figure 1.  Surfaces used for the projection

1. Go to 'Geometry -> Trimmed Surface Creation Options' and select "Project Surface", as shown in Figure 2.

   

   Figure 2.  Project Surface option

2. Pick the surface(s) to be projected and click the middle mouse button. In this example, it would be Surface 3.
3. Pick the surfaces onto which Surface 3 will be projected and click the middle mouse button. These would be Surfaces 1 and 2.
4. Select the projection direction. In this case, Surface 3 will be projected normal to Surface 1 and 2 using the "Normal" projection option.
5. Click "Preview" to verify that the projection is correct.
6. Click the middle mouse or Apply button to complete the surface projection (see figure 3 below).

   

   Figure 3.  Surface after projection

If you have any questions about this tip or would like us to discuss other topics in the future, please let us know.

About Turbulence Intensity

The turbulence intensity, I, is a measure of the strength of the velocity fluctuations, u’, compared to the strength of the bulk velocity, U. By definition, I is equal to the ratio of the root-mean-square of the velocity fluctuations to the mean freestream velocity.

The best way to get values for inlet turbulence quantities specification is to have experimental data, from which turbulence intensity can be calculated. When experimental data is not available, an educated guess based on the type of flow may be sufficient. The following section provides some guidelines for a broad category of flows (internal/external).

Internal flows usually have high turbulence intensity. Good values for inlet turbulence intensity are 0.01 to 0.1 (i.e. velocity fluctuations are about 1% - 10% of the mean bulk velocity. For high speed flows in complex geometries such as turbomachinery, the turbulence intensity values could be much higher than 10%. For a fully developed flow inside a duct, the turbulence intensity can be estimated as:

Here, Re is the Reynolds number based on the hydraulic diameter.

External flows are unconstrained and therefore have smaller values of turbulence intensity. Inlet values can go down to 0.0005.

Once you have obtained a reasonable estimate for I, you can either use it to calculate the inlet value of turbulent kinetic energy, k, as described in later sections, or you can use it as a boundary value directly.

About Turbulent Viscosity Ratio

The turbulent viscosity ratio is the ratio of turbulent to laminar (molecular) viscosity, and is defined as:

For internal flows, β may be scaled with the Reynolds numbers. Some guidelines (determined with numerical experiments) for fully developed pipe flows are as follows:

For a Reynolds number of 100,000 or greater, a value of 100 is a reasonable estimate for β.

For external flows the freestream turbulent viscosity is of the order of laminar viscosity, so small values of β are appropriate, say β = 0.1 – 1.

About Turbulence Length Scale

Sometimes, it is easier to think in terms of turbulence length scale instead of turbulent viscosity ratio. The turbulence length scale, l, is a physical quantity that represents the size of the large eddies in turbulent flows. Empirical relationship between the physical size of the obstruction (or characteristic length), L, and the size of the eddy, l, can be used to get an approximate length scale. For a fully developed pipe flow, the turbulence length scale is given by:

l = 0.07L

For internal flows, you can choose the characteristic length (L) to be the inlet duct width, or you can choose to specify the hydraulic diameter (D_h).

• In the case of fully-developed internal flows, it is better to choose the Hydraulic Diameter specification method.
• In the case of wall-bounded flows, the Turbulent Intensity and Length Scale specification methods are preferred. Then use the boundary-layer thickness, δ, to estimate the turbulence length scale, l, as l = 0.4δ.

For external flows, it is often not possible to determine a good characteristic length. In using the formulas below, pick a value of β and a value of I and use the formulas on the left, the ones not involving the length scale. In the case of external aerodynamic flows choose smaller values of β (0.1 to 1), whereas in the case of wind-tunnel external flows, choose larger values of β (1 to 10).

Calculating Turbulence Boundary Conditions Values

All the turbulence models (except Baldwin-Lomax Algebraic Model) require the specification of certain variable values at the boundaries. There are several methods used for turbulence specification at the boundaries available in the codes. For all of the models, the user can directly specify the turbulence intensity, turbulence length scale or the hydraulic diameter. But it is important to know how these quantities are related to primary turbulence variables like k, epsilon and omega. In this note, we address only the RANS turbulence models. For full details of the turbulence models and equations, please see the User Manuals.

For the Standard k-epsilon model and variations (RNG, Kato-Launder, Two Layer)

For the Spalart-Allmaras Model

For the k-omega and Menter SST Models

NOTE: For external flows, it is very important to specify appropriate turbulence quantities at the freestream boundaries. If the values are unphysical, it can cause the solution to be unrealistic and can lead to divergence or non-convergence. For internal flows, the importance of the values is not as critical because usually there is much more turbulence generated internally in the flow field.

TIP: You can use the same methods as described above to calculate the initial condition values for the turbulence quantities. However, it can sometimes make for an easier simulation startup if the initial condition values generate higher turbulent viscosity. Hence, consider using 10x greater values for β and I for the initial conditions. You can also use the Turbulence Start Control feature for better convergence.

In CFD-VEW, there is an option that allows quick and easy clipping of the variable colormap to a given range of values. This option is “Per Pixel Lighting” and can be activated through the Edit menu -> Preferences -> Display, as shown in figure 1 below.

Figure 1.  Per Pixel Lighting display option

As an example, let’s consider the 1D unsteady conduction heating of a bar (see figure 2). The bar is at an initial temperature of 300 K. The top and bottom walls are adiabatic, and the right wall is kept at 300 K. At t = 0 s, the left wall is instantly brought to 500 K and is maintained at that temperature.

Figure 2.  Schematic of the example simulation

We are interested in the transient evolution of the temperature distribution in the bar. More specifically, we want to know when the temperature rises above 350 K, and for what parts of the bar. The Per Pixel Lighting option allows us to see this very easily. Simply activate the option, set the colormap to Static and set the minimum value to 350 K (figure 3).

Figure 3.  Colormap settings

Below are 2 animations showing the result of the simulation. The top one does not have the Per Pixel Lighting option activated, while the bottom one has.

Figure 4.  Transient temperature distribution - Per Pixel Lighting turned OFF

Figure 5.  Transient temperature distribution - Per Pixel Lighting turned ON

Notes:

- Per Pixel Lighting is available only with Smooth Surface rendering
- This option also provides better rendering of the graphical objects
- It is supported by most recent graphics cards

As one can see, the Per Pixel Lighting option can greatly improve results visualization. So next time you need to show only a specific range of values, remember that the Per Pixel Lighting option is an easy way to do it.

Note: tested with V2010.0