Seunghyun Joo

Seunghyun Joo

Research Associate at Seoul National University | Looking for a PhD position

eVTOL Downwash/Outwash Analysis using LBM

🔍 Project Overview

Investigated the complex aerodynamic interference of eVTOL aircraft during hover and near-ground operations to address the limitations of current heliport/vertiport safety regulations.

While current vertiport design standards define a fixed safety area (e.g., 2.5D), recent studies and surveys indicate that these criteria may not sufficiently account for configuration-dependent outwash hazards of multi-rotor eVTOL aircraft.

This research aims to provide a high-fidelity aerodynamic assessment framework to support future regulation evolution.

⚙️ Methodology

Lattice-Boltzmann Method (LBM)

Used SIMULIA PowerFLOW, a commercial LBM-based CFD solver featuring:

  • Algebraic governing equations with lattice-level parallelization
  • Low numerical dissipation for vortex-dominant rotor flows
  • Efficient handling of moving geometries (rotors) via facet-based boundary treatment

LBM was selected to balance:

  • High-fidelity unsteady rotor wake prediction
  • Affordable computational cost for O(10²–10Âł) rotor revolutions
  • Capability to model complex fuselage and ground geometries

Solver Validation

Validation was performed at two levels:

  1. Aerodynamic Performance Validation
    • ONERA M6 wing
    • ROBIN fuselage
    • Caradonna-Tung rotor → Good Cp distribution agreement with experiments.
  2. Downwash/Outwash Validation
    • CH-47D tandem rotor helicopter
    • Compared against experimental survey data and RANS / RANS-ALM → Accurate prediction of outwash velocity profiles at 1 m AGL

Configuration-Level Physical Insights

  • QSMR: Tailboom–wake interaction produced a tail-shaped outwash hazard region.
  • Side-by-Side: Strong rotor–rotor interaction generated directional front-to-back outwash jets.
  • Quadrotor: High rotor height and low disk loading prevented any region from exceeding the 15 m/s hazard threshold.
  • Lift+Cruise (LPC): Extremely high disk loading led to hazard areas exceeding current safety regulation limits.

External Wind Effects (Lift+Cruise Case)

  • 5 m/s wind expanded hazard areas by up to 30%.
  • 10 m/s forward wind increased the hazard radius to 63.6 m, approximately 3Ă— the current safety area regulation.
  • Hazard expansion followed the order: Forward > Diagonal > Side wind.

These results indicate that even moderate wind conditions far below the 25 m/s operational limit can critically amplify ground-level outwash hazards.

🧭 Research Implications

This study highlights several open regulatory and technical questions:

  • How should diverse eVTOL configurations be treated consistently in safety assessment?
  • Which representative velocity metric should be used? (Time-averaged, RMS, 95th percentile, or absolute maximum?)
  • Can existing military-based hazard models be applied to civilian UAM operations?

The findings suggest that current vertiport regulations may require configuration-aware and wind-sensitive updates.

🛠️ Key Technical Contributions

  • Established a validated LBM-based workflow for rotorcraft downwash/outwash prediction.
  • Demonstrated configuration-dependent hazard footprint variation across NASA UAM reference vehicles.
  • Quantified wind-induced amplification of ground-level outwash, showing up to 3Ă— safety-area exceedance.
  • Provided physics-based insights into the influence of disk loading, rotor height, and rotor–rotor interaction.

👤 My Contributions

Although this work was conducted as a collaborative research project, my primary technical contributions focused on high-fidelity LBM validation and rotorcraft outwash prediction.

  • Performed Lattice-Boltzmann solver validation against canonical aerodynamic benchmark cases (ONERA M6, ROBIN fuselage, Caradonna-Tung rotor).
  • Conducted CH-47D tandem rotor downwash/outwash validation:
    • Simulated 150 rotor revolutions with final 50-rev time averaging.
    • Compared ground-level velocity profiles with experimental survey data and RANS/RANS-ALM results.
  • Executed Joby S4 eVTOL geometry-based validation simulations, verifying wake structure and near-ground velocity distribution behavior.
  • Analyzed velocity-field data to assess hazard threshold exceedance (15 m/s DCA criterion).

My role centered on establishing solver credibility and ensuring the physical reliability of the downwash/outwash prediction framework.

đź“š Publications

  • Juneyoung Lim, Seunghyun Joo, Kwanjung Yee. (2025). “Downwash/Outwash Analysis of eVTOL Configurations Using the Lattice-Boltzmann Method,” APISAT 2025.