> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ntop.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Aircraft Flow Analysis

Fluids \[Beta]/Analysis/aircraft\_flow\_analysis

## About this Block

**What it does:** The **Aircraft Analysis** block calculates time-averaged and instantaneous pressure, velocity, and boundary force fields around aircraft geometries using the Lattice Boltzmann Method (LBM). It uses a solver setup optimized for external aerodynamics, producing results specialized for lifting bodies and surfaces that differ from the standard **Flow Analysis** block

*Note: This block requires an NVIDIA GPU*

**Common uses:**

* Predicting lift and drag coefficients across a range of angles of attack.
* Analyzing surface pressure distributions and identifying flow separation zones.
* Comparing aerodynamic performance between geometry variants or configurations.
* Capturing unsteady wake structures and vortex shedding behind lifting surfaces.

**Setup Requirements:**

Ensure the following conditions are met before running the block:

* The geometry must be a closed, watertight surface representing the aircraft.
* No manual fluid domain or far-field boundary conditions are required; the block generates these automatically.

**Automated Simulation Setup:**

The block automatically constructs a simulation domain by extending the geometry's bounding box. The resulting domain length can be calculated using the formula L\_i(pad) ≈ L\_i × (1 + p\_i⁻ + p\_i⁺), using the following padding factors p\_i:

| Direction       | Factor |
| :-------------- | :----- |
| -x (Upstream)   | 0.5    |
| +x (Downstream) | 1.5    |
| -y / +y (Sides) | 0.5    |
| -z (Below)      | 1.5    |
| +z (Above)      | 1.0    |

### Simulation Domain Padding Example

The padded domain length is estimated as:

L\_i(pad) ≈ L\_i × (1 + p\_i⁻ + p\_i⁺)

where L\_i is the bounding box span in direction i, and p\_i⁻, p\_i⁺ are the upstream/downstream padding factors.

### Reference Geometry: [Aircraft Flow Analysis](https://storage.googleapis.com/files-learn/static/ExtendedBlockDocs/aircraft_flow_analysis.ntop)

| Property  | Value (mm)                              |
| --------- | --------------------------------------- |
| Max point | \[10,437.0213, 16,113.2812, 1,869.2176] |
| Min point | \[-8,178.7195, -488.2812, -1,869.2181]  |

**Bounding box spans L\_i:**

| Direction | Span (mm) |
| --------- | --------- |
| X         | 18,615.74 |
| Y         | 16,601.56 |
| Z         | 3,738.44  |

**Example — X axis:**

L\_x(pad) ≈ 18,615.74 × (1 + 0.5 + 1.5) = 18,615.74 × 3.0 = 55,847.22 mm

**Example — Z axis:**

L\_z(pad) ≈ 3,738.44 × (1 + 1.5 + 1.0) = 3,738.44 × 3.5 = 13,084.54 mm

* Boundary Conditions: The upstream face applies a **velocity inlet**, while the downstream face applies a **pressure outlet**. Lateral far-field faces use a **slip boundary condition** to minimize wall interference.
* Surface Modeling: A **wall model** is applied automatically to the aircraft surface.
* Compressibility: For higher Mach numbers, a **Prandtl-Glauert compressibility correction** is applied. Note that as the Mach number approaches 0.7, this correction noticeably reduces effective resolution in the Y and Z directions; plan your Cell Size accordingly.

**Tips on Cell Size:**

The simulation uses a uniform Cartesian mesh defined by the **Cell Size** parameter.

* **Quantitative Results:** To achieve accurate force coefficients, resolve the smallest relevant features (e.g., gaps, control surface edges, or slat tracks) with at least **7 cells**.
* **Qualitative Results:** For flow visualization and identifying separation patterns, a minimum of **3 cells** across the smallest feature is sufficient.
* **Memory Estimation:** Ten million cells require approximately **1.3 GB** of dedicated GPU memory. This is roughly 30% higher than the **Flow Analysis** block due to force field computations.

### Memory Estimate

Given a uniform cell size of Δ = 330 mm, the approximate cell count per axis is:

N\_i = ⌈ L\_i(pad) / Δ ⌉

| Axis | Padded Span (mm) | Cells N\_i |
| ---- | ---------------- | ---------- |
| X    | 55,847.22        | 169        |
| Y    | 33,203.12        | 101        |
| Z    | 13,084.54        | 40         |

**Total cell count:**

N\_total = N\_x × N\_y × N\_z = 169 × 101 × 40 ≈ 682,360 cells

**Estimated GPU memory** using the reference rate of 1.3 GB per 10 million cells:

M ≈ (682,360 / 10,000,000) × 1.3 GB ≈ 88.7 MB

**Notes on Transient Simulations:**

* **Iterative Process:** LBM is inherently transient and advances the solution through discrete time steps until results are statistically steady-state.
* **Instantaneous vs. Time-Averaged:** The block exposes two result fields. The instantaneous field (first) captures unsteady fluctuations like turbulence and vortex dynamics. The time-averaged field (second) reveals the mean flow structure suitable for coefficient extraction.
* **Force Coefficients:** Lift, drag, and moment coefficients are derived from the time-averaged boundary force field as the simulation converges.

**How to Use and Analyze Results:**

1. Run the Simulation: Run the block. The simulation advances until it reaches a steady state and outputs a CFD Analysis Result, which is used for visualization and data extraction.
2. View Streamlines: To visualize the flow field, select the result in the notebook and update its Display Mode to Streamlines in the View Settings.
3. Extract Near-Body Data (Advanced): To extract data points from a specific area (e.g., near the aircraft surface):
   * Find Region ID: Select the block and switch the Heads-Up Display (HUD) to Cell Region. Identify the region ID you want to analyze (e.g., Region 3 is typically the area near the body).
   * Filter Points: Add a **Filter Cell Region** block. Input the CFD Analysis Result and the Region ID to generate a Point List.
   * Evaluate Data: Use the **Evaluate Field** block with the Point List for detailed analysis of pressure or velocity near the surface.

**Tips for Resolving Errors:**

* **Solver Diverged:** Verify that the Mach number and angle of attack are within supported ranges. Ensure the **Cell Size** is not too coarse; poorly resolved gaps or excessively large time steps are common causes.
* **Geometry Issues:** Ensure the surface is watertight and free of self-intersections or open edges. Non-manifold geometry causes incorrect wall detection.
* **Insufficient GPU Memory:** Incrementally increase the **Cell Size** or simplify the geometry to remove small features that drive up cell count without impacting the primary aerodynamic result.
* **Incorrect far-field domain:** The simulation domain is derived from the geometry's bounding box, which can sometimes be computed incorrectly for certain complex implicits. If the domain looks wrong, use a **Voxel Grid From Implicit** block to voxelize the geometry, convert it back to an implicit, and set the bounding box of the original geometry to that explicitly.

## Example File

Download Example: [Aircraft Flow Analysis](https://storage.googleapis.com/files-learn/static/ExtendedBlockDocs/aircraft_flow_analysis.ntop)
![Example Screenshot](https://storage.googleapis.com/files-learn/static/ExtendedBlockDocs/aircraft_flow_analysis.png)

***

Calculates the pressure, velocity, and surface forces on an implicit body. The implicit body must be oriented in the default aircraft position with flow direction aligned to the positive X-axis, Y-axis the starboard/right wing direction and Z-axis pointing upward (lift direction). The solution uses a Lattice Boltzmann Method. A transient simulation is initiated and continues until the flow reaches a statistically steady state, then returns time-averaged pressure, velocity fields, along with the force on the implicit body surfaces. Streamlines will be calculated on the time-averaged velocity results. This block requires an NVIDIA GPU.

### Inputs

| Name            | Type                                                                 | Description                                                          |
| --------------- | -------------------------------------------------------------------- | -------------------------------------------------------------------- |
| Aircraft body   | [Implicit Body](../../../../block-documentation/types/implicit-body) | The implicit body used for aerodynamic evaluation.                   |
| Mach number     | [Scalar](../../../../block-documentation/types/scalar)               | The Mach number of the freestream.                                   |
| Temperature     | [Scalar](../../../../block-documentation/types/scalar)               | Defines the temperature of the freestream                            |
| Pressure        | [Scalar](../../../../block-documentation/types/scalar)               | Defines the pressure of the freestream                               |
| Angle of attack | [Scalar](../../../../block-documentation/types/scalar)               | The angle between the body reference line and the airflow direction. |
| Cell size       | [Scalar](../../../../block-documentation/types/scalar)               | The size of the cells used in the analysis.                          |

### Outputs

| Type                                                                             |
| -------------------------------------------------------------------------------- |
| [CFD Analysis Result](../../../../block-documentation/types/cfd-analysis-result) |