# Uniform Jump (AMI)

Source: https://help.sim-flow.com/boundary-conditions/uniform-jump-ami
Last updated: 2026-10-02

## Description

Uniform Jump (AMI) is a boundary condition that imposes a fixed difference (jump) in field values between a pair of cyclic AMI patches. Unlike the standard [Fixed Jump (AMI)](https://help.sim-flow.com/boundary-conditions/fixed-jump-ami) where the jump can vary across the surface, here the jump is **spatially uniform** but can vary in **time**.

This condition is typically applied to the owner patch of the cyclic interface and is useful for modeling global transient effects, such as a time-varying pump pressure rise or a fluctuating heat source at an interface.

\[\phi_{n} = \phi_{o} + J(t)\]

where:

- \(\phi_{n}\) - field value on the neighbor patch
- \(\phi_{o}\) - field value on the owner patch
- \(J(t)\) - specified spatially uniform jump value (time-dependent)

## Understanding Uniform Jump (AMI)

The Uniform Jump (AMI) is applied to patches that are already coupled via a **cyclic** or **Arbitrary Mesh Interface (AMI)**. It maintains an offset between the mesh regions that changes over time. The key characteristics are:

1. **Uniformity**: The offset \(J\) is identical for every face on the patch at a given time instant.
2. **Time-dependence**: The offset \(J\) can change during the simulation based on a table or CSV file.

This allows for modeling dynamic systems where the "strength" of the discontinuity evolves, for example, ramping up a fan pressure during startup.

## Application & Physical Interpretation

The Uniform Jump (AMI) is useful in transient simulations involving interface discontinuities.

### Transient Turbomachinery

**Context**: Simulating the startup or shutdown phase of a fan or pump modeled via an AMI interface.

**Physical meaning**: The pressure rise (jump) across the fan increases or decreases over time as the rotational speed changes.

### Dynamic Heat Sources

**Context**: Modeling a heating element at an interface where power output fluctuates.

**Physical meaning**: The temperature jump across the thin layer varies with time according to the power input.

Example Boundary Conditions set for Time-Varying Pressure Jump

| Variable | Owner Patch | Neighbor Patch |
| --- | --- | --- |
| Pressure | **Uniform Jump (AMI)** | cyclicAMI |
| Velocity | cyclicAMI | cyclicAMI |

## Uniform Jump (AMI) in SimFlow

The Uniform Jump (AMI) is available when an Arbitrary Mesh Interface (AMI) has been created between two patches in the **Mesh** panel. To apply it, select the AMI patch in the **Boundary Conditions** panel and choose Uniform Jump (AMI).

![Uniform Jump (AMI) definition in SimFlow](https://help.sim-flow.com/boundary-conditions/images/uniform-jump-ami/uniform-jump-ami-bc-01.png)

Figure 1. Uniform Jump (AMI) configuration in SimFlow

Jump Table - Defines the time-dependent offset value \(J(t)\). Can be defined using a table or function.

## Uniform Jump (AMI) - Alternatives

In this section, we propose boundary conditions that are alternative to Uniform Jump (AMI). While they may fulfill similar purposes, they might be better suited for a specific application and provide a better approximation of physical world conditions.

| Boundary Condition | Description |
| --- | --- |
| **[Fixed Jump (AMI)](https://help.sim-flow.com/boundary-conditions/fixed-jump-ami)** | Similar to Uniform Jump (AMI), but the jump can be spatially varying (a field) and is typically constant in time. |
| **[Cyclic (AMI/ACMI)](https://help.sim-flow.com/boundary-conditions/cyclic-ami-acmi)** | Standard cyclic coupling with no jump (\(J=0\)). Ensures continuity of the field across the interface. |