Added new steps to CFD tutorial

Author: tomas-soltys

src/fea/help/tutorials/en/image-CFD-Create_cut.png

@@ -62,7 +62,43 @@ It will take some time until the solver computes all iterations and finds a conv
 
 **Menu:** _Report -> Solver convergence_
 
-## 7. Problem task flow (Step 2)
+## 7. Create a cut
+
+A good way to inspect a flow is to display the velocity as a **vector field**, where every arrow shows the direction and magnitude of the flow at one point. A vector field drawn through the whole volume, however, quickly becomes a dense cloud of overlapping arrows that is hard to read. It is usually clearer to first slice the model with a **cut** - a plane through the volume - and then display the vector field only on that plane.
+
+**Menu:** _Geometry -> Cut -> Create cut_
+
+In the **Cut editor** dialog:
+
+1. Select **Volume 1** in the **Model** tree. The cut will be created through this volume.
+2. Use **Plane position** to set a point the cut plane passes through. The default values place it in the center of the model (**X** = 7.5, **Y** = 2.5, **Z** = 1.5 `[m]`).
+3. Use **Plane normal** to set the orientation of the plane. The normal (**X** = 0, **Y** = 0, **Z** = 1) gives a horizontal plane cutting through the mid-height of the channel.
+
+The cut plane is previewed in the **3D model area** while the values are being changed. Click **OK** to create the cut. A new entity **Cut 1** appears in the **Model tree** under **Cuts**.
+
+![CFD - Create cut entity](image-CFD-Create_cut.png)
+
+## 8. Create a vector field
+
+Now display the computed velocity as arrows on the cut plane.
+
+**Menu:** _Geometry -> Vector field -> Create vector field_
+
+In the **Vector field editor** dialog:
+
+1. Select **Cut 1** in the **Model** tree. The arrows will be drawn on the cut plane created in the previous step.
+2. Select **Velocity** in the **Variables** list.
+3. Leave **3D vector field** checked. The arrows then show the full velocity vector. When unchecked, each arrow is projected onto the cut plane and only the in-plane part of the flow is shown.
+
+Click **OK** to create the vector field.
+
+![CFD - Create vector field entity](image-CFD-Create_vector.png)
+
+The arrows may be too short to be clearly visible, because their length is proportional to the flow velocity. To make them longer, open the **Results** tab, select the **Velocity - Vector** variable and increase the value in the **Scale** section. The scale is the product of the **Scale** value and the power of ten set in **Factor**; in the example below **Scale** 1 and **Factor** 2 give a scale of `1 × 10^2 = 100`.
+
+![CFD - Scale vector field entity](image-CFD-Scale_vector.png)
+
+## 9. Problem task flow (Step 2)
 
 Once the solver converges, a **transient** problem including **Contaminant dispersion** can be configured.
 
@@ -76,13 +112,13 @@ Since **Incompressible viscous flow** is a nonlinear problem, it will always nee
 
 ![CFD - Problem task flow](image-CFD-Problem_task_flow.png)
 
-## 8. Time solver setup
+## 10. Time solver setup
 
-Click on **Problem** tab. Enable **Time-solver** and specify values as shown on the screenshot below (**Time-step size** 0.001, **Number of time-steps** 20000, **Output frequency** 10; **End time** is computed automatically).
+Click on **Problem** tab. Enable **Time-solver** and specify values as shown on the screenshot below (**Time-step size** 0.01, **Number of time-steps** 2000, **Output frequency** 10; **End time** is computed automatically).
 
 ![CFD - Problem time solver](image-CFD-Problem_time_solver.png)
 
-## 9. Boundary conditions
+## 11. Boundary conditions
 
 Apply **Particle concentration** boundary condition to **Inflow** model entity.
 
@@ -94,17 +130,17 @@ In the **Component editor** dialog, time-dependent values can be specified. Valu
 
 ![CFD - Particle concentration condition](image-CFD-Particle_concentration_condition.png)
 
-## 10. Solve problem (restart)
+## 12. Solve problem (restart)
 
 Once the problem is fully configured, restart the solver. This is done just like when starting a solver, but the **Restart solver / continue** check-box must be selected. A warning **Contaminant dispersion: No implicit boundary condition assigned.** is expected and can be ignored. This will cause the solver to use already computed results as a starting point and continue in the time-marching simulation.
 
 ![CFD - Solver restart](image-CFD-Solver_restart.png)
 
-## 11. Model records (results in time)
+## 13. Model records (results in time)
 
 As the solver keeps finding solutions for each time-step, model records are being written. Each record contains a solution for a given time-step. A list of these records can be seen in the **Records** tree. By double-clicking on a record, results for the given time-step will be loaded.
 
-## 12. Record video
+## 14. Record video
 
 Computing all 20000 time-steps takes a long time. The solver can be stopped at any time (**Menu:** _Solution -> Stop solver_, confirm with **Yes**); already written records are kept.