CFD-Optimized Flow Conditions for Drone Icing

To study drone icing experimentally, tests are being conducted in the RTA climate wind tunnel under realistic flow and environmental conditions. To obtain reliable measurement data, both the flow velocity and the flow quality in the measurement section must meet the highest standards.
As part of a CFD simulation project, a study was therefore conducted to investigate how a second contraction nozzle and an optimized diffuser geometry behind the measurement section affect the achievable flow velocity and pressure loss.

The objective of the study was

• to identify a diffuser geometry that minimizes the energy required to generate a flow velocity of 80 m/s,
• evaluate the second contraction nozzle from a fluid dynamics perspective,
• and keep the number of mesh elements low through appropriate remeshing.
  Objects within the measurement section were deliberately not taken into account in order to analyze the influence of the channel geometry in isolation.

The CFD simulations were based on the import and processing of the existing CAD geometries of the climate wind tunnel and the first contraction nozzle. The second contraction nozzle was defined as a fixed geometry. The diffuser, on the other hand, was parameterized to enable the automated calculation and optimization of different variants.
The simulations were performed under the following boundary conditions:

• Volume flow: 500 m³/s (resulting in ~80 m/s in the measurement section)
• Air as an ideal gas, isothermal at −10 °C
• k-ε turbulence model with scalable wall function
• steady-state calculation, non-buoyant
• outlet with 0 Pa static pressure (simplifying assumption)
• computational time: approx. 45 min per case (including remeshing) on 12 cores

By varying the diffuser, several designs were compared. A configuration with the following parameters proved to be optimal:

• Diffuser angle: 5°
• Height ratio of diffuser inlet to nozzle outlet: 1.1
• Diffuser constriction: 0.95 (ratio of minimum height to inlet height)
• Position of the minimum cross-section: 2 m after the inlet

With this geometry, the pressure loss was nearly halved compared to a duct without a diffuser:

• Pressure loss without diffuser: ~3000 Pa
• Pressure loss with optimized diffuser: ~1400 Pa

Further improvement appears possible if the sharp edge in the area of the minimum diffuser cross-section is replaced by a defined curvature.
The results clearly show that a suitable diffuser geometry has a significant influence on energy efficiency and flow quality in the climate wind tunnel. The findings form the basis for future modifications to the tunnel, which are of particular importance for investigations of drone icing under realistic conditions.

As part of a CFD simulation project, the flow pattern in the RTA climate wind tunnel was comprehensively optimized. The goal was to ensure realistic inflow velocities of 80 m/s for icing tests while maintaining high flow quality.
By analyzing a second contraction nozzle and developing an optimized diffuser, the pressure loss in the wind tunnel was reduced by approximately 50%. The results provide an important foundation for energy-efficient and precise test conditions in future icing tests.