### 6.8.2. Case 2: Flow Boundary Disturbance

A flow boundary disturbance case involves modeling the effect of an external component (a virtual blade row upstream or downstream of the fluid mesh) via an inlet or outlet boundary condition that varies in time. Such cases are supported by the Time Transformation and Fourier Transformation models.

This type of simulation can be used as a lower-order approximation to a stage simulation. In inlet disturbance simulations the upstream component is modeled by prescribing a profile that is at the inlet and that models the wake of the upstream component.

You can prescribe wake profiles via CEL expressions or via external `*.csv` files. CEL-based profiles need not be formulated to produce rotational motion in time. If a profile does not cover the whole 360° wheel, then it should consist of a periodic portion that can be replicated to produce a 360° profile. You may optionally produce a 360° profile in a `*.csv` file based on a smaller profile file by using the Expand Profile Data dialog box in CFX-Pre (see Expand Profile Data in the CFX-Pre User's Guide).

Whether using a CEL expression or a `*.csv` file, you can cause the profile data to move (typically rotationally, about the machine axis) by associating the boundary with a moving coordinate frame. For details, see:

- Local Coordinate Frames

- Frame Motion Settings in the CFX-Pre User's Guide

- Coordinate Frame in the CFX-Pre User's Guide

- Time Transformation Disturbance Settings in the CFX-Pre User's Guide

- Fourier Transformation Disturbance Settings in the CFX-Pre User's Guide

For a rotational-flow disturbance case, the required External Passage Definition input parameters are aimed at specifying the pitch of the profile (or signal) at the inlet/outlet. These are needed to help compute the phase shift ( of Figure 6.3: Inlet Disturbance Case). These parameters are important to guarantee that the resultant signal has periodic behavior.

Figure 6.3: Inlet Disturbance Case

#### 6.8.2.1. Multiple Disturbances

A multi-disturbance gust analysis can be modeled using the Fourier Transformation method. In this analysis, the blade is subjected to two simultaneous disturbances; one on the inlet and the other on the outlet of the blade passage. Typically, the inlet and outlet profiles are initially extracted from a steady-state stage simulation of a 1.5 stage configuration. The profiles are then imposed on the inlet and outlet of the rotor passage in a transient gust simulation. This modeling strategy can be used as an alternative for modeling a 1.5 stage compressor or turbine. It accounts for blade passing frequencies from the upstream and downstream rows on the modeled blades. The use of the Fourier Transformation method also allows for very large pitch variations such as with a fan subjected to inflow distortion.

In general, the procedure to set up a multi-disturbance gust analysis with the Fourier Transformation method closely follows the setup of an inlet disturbance problem with the Fourier Transformation method (see Fourier Transformation Disturbance Settings in the CFX-Pre User's Guide). Note the following important steps:

- After setting up the first disturbance, you must add a second disturbance from the same Transient Blade Row Models details view.

- Specify the desired time step in the Transient Blade Row Models details view under Transient Method > Time Period.

- Ensure that all the boundary disturbances are set up correctly. For example, in a two-disturbance case (inlet and outlet) both inlet and outlet boundary conditions must now be specified with a disturbance either through CEL or specified profiles.

- For stationary disturbances, ensure that you create and refer to a stationary coordinate frame in the Boundary details view as well as the Transient Blade Row Models details view.

Note:

- It is not ideal to use this modeling technique if strong outlet recirculation is present at the boundary.

- If the case is not periodic by nature then this modeling method will either artificially make the solution periodic or the solution may fail to converge.

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