### 5.6.3. Using Domain Interfaces in Turbomachinery Applications

The setup of domain interfaces is an important consideration when defining a turbomachinery problem. The following section outlines some approved practices for use in turbomachinery applications.

- Domain Interfaces should typically be placed midway between the rotor and stator for turbomachinery cases.

- Where circular domain interfaces exist, they must be axisymmetric in shape.

- The aspect ratio of elements on the domain interface should not be greater than 10:1. Figure 5.10: Element Aspect Ratio gives a sketch of the requirement for an element aspect ratio of less than 10 on the domain interface. The element aspect ratio (x/y) should lie in the range 0.1 to 10 to avoid numerical errors.

  Figure 5.10: Element Aspect Ratio

#### 5.6.3.1. Case 1: Impeller/Volute

A basic impeller/volute sketch is shown in Figure 5.11: Basic Impeller/Volute. The edge of the inner circle shows the maximum extent of the impeller blades. A good practice here is to create the domain interface halfway across the most narrow gap between the blade and volute wall. This usually occurs around the cut-off or tongue illustrated in the diagram.

Figure 5.11: Basic Impeller/Volute

#### 5.6.3.2. Case 2: Step change between rotor and stator

For the case shown in Figure 5.12: Step change, there is a step change between the rotor and stator. A common choice for placement of the interface would be choice 1. However, take care with this set-up because the non-overlap regions above and below the interface should be specified as walls. A better alternative may be to use a domain interface upstream or downstream of the step change, at position 2 or position 3.

Figure 5.12: Step change

#### 5.6.3.3. Case 3: Blade Passage at or close to the edge of a domain

Figure 5.13: Blade extending to the edge of the rotating domain. shows a blade that extends to the edge of the rotating domain. Although it is convenient to place a domain interface at the blade edge (1), this can result in convergence difficulties. A better arrangement is to extend the rotating domain away from the blade edge. Domain Interfaces can then be created at (2), (3), and (4).

Figure 5.13: Blade extending to the edge of the rotating domain.

#### 5.6.3.4. Case 4: Blade Passage at or close to the edge of a domain

Figure 5.14: Blade passage shows a close-up view of part of Figure 5.13: Blade extending to the edge of the rotating domain., which models flow leaking from a volute back into the impeller region. To model the feature, you can use two domain interfaces (at position 1), or a single domain interface downstream of the leak (position 2).

Figure 5.14: Blade passage

#### 5.6.3.5. Case 5: Blade with thick trailing edge

As Figure 5.15: Thick trailing edge shows, when a blade with a thick trailing edge passes close to the domain interface, recirculating flow that crosses the interface can occur. Running a case like this with a Stage interface can produce unphysical results. It is recommended that you either move the domain interface away from possible recirculation zones, or run with the Frozen Rotor interface.

Figure 5.15: Thick trailing edge

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