Aerodynamic Modeling for Urban Air Mobilityutm3.com/wp-content/uploads/2018/05/utm3.com_ventura_eVTOL_… · Patricia Ventura Diaz and SeokkwanYoon NASAAmes Research Center From VTOL

National Aeronautics and Space Administration

Aerodynamic Modeling for Urban Air MobilityPatricia Ventura Diaz and Seokkwan YoonNASA Ames Research CenterFrom VTOL to eVTOL Workshop, San Carlos, CA, May 24, 2018


Concepts of eVTOL aircraft• Both big companies and start ups are working on new concepts

for Urban Air Mobility. NASA is also contributing to this revolution in aeronautics.

From VTOL to eVTOL Workshop, May 24, 2018 2


Motivation• Three challenges for eVTOL:• Public safety. • Acceptable range. • Level of noise



Objectives• Study small Unmanned Aerial Vehicles

(UAV) or drones and apply lessons learned to medium-sized multirotor vehicles for UAM.

• Use Computational Fluid Dynamics (CFD) to understand the complex flows of interactional aerodynamics. • The interactions between multiple rotors

and the fuselage emerge as an important factor to consider in the design of safer, more efficient and quieter vehicles

• High-order accurate solver is required in order to capture the interactions



NASA’s supercomputers Pleiades and Electra• Located in NASA Advanced Supercomputing (NAS) facility

at NASA Ames Research Center.• CFD is key to be able to visualize and understand the

complex flows. It helps us design the future flying vehicles



Numerical Approach – Flow Solver• OVERFLOW: CFD Flow Solver

vSolves the unsteady Reynolds-averaged Navier-Stokes (RANS) equations

vStructured overset gridsv5th order accurate central differences with scalar dissipation

for convective terms or 4th order with matrix dissipationv2nd order accurate dual time-stepping (Δ" = 1/4', 50

subiterations)vHybrid RANS/LES turbulence model:

vSpalart-Allmaras one equation turbulence modelvDDES length scalev() ≤ 1



Numerical Approach – Overset Grids• Chimera Grid Tools (CGT): Overset Grid Generation

vGeometry processing: Geometry obtained from CAD or 3D scan techniques

vSurface Grid Generation: featured based domain decomposition, grid point distribution, mesh fill. Elliptic and hyperbolic marching.

vVolume Grid Generation: vHyperbolic Near-Body (NB)vCartesian Off-Body (OB)

vDomain Connectivity: grid points blanking, donor stencil search



Numerical Approach – Near-Body and Off-Body Grids

NB Surface Grids


NB and OB Volume Grids


Overset Grids – Quadcopter

• Complete Quadcopter:• Rotors: Original rotor blades

• Simplified Quadcopter:• Rotors: Original Quadcopter rotor blades or Floureon’s carbon fiber (CF) replica

sold as more rigid bladesFrom VTOL to eVTOL Workshop, May 24, 2018 9

Complete QuadcopterSimplified Quadcopter airframe


Overset Grids - Quadcopter


Replica Carbon Fiber blade

Original blade


Overset Grids – The SUI Endurance

• SUI Endurance: • Rotors: Original T-Motor p15x5 CF blades

SUI Original T-Motor p15x5 CF blades

SUI hybrid configuration: Fore rotors undermounted and aft rotors overmounted

SUI standard configuration: Fore and aft rotors overmounted

SUI undermount configuration: Fore and aft rotors undermounted

11From VTOL to eVTOL Workshop, May 24, 2018


Experimental validation – Single RotorA comparison of present Overflow results for the Quadcopter and SUIrotors with the experimental data shows an excellent agreement.Yoon et al., “Computational Aerodynamic Modeling of Small Quadcopter Vehicles”, AHS Forum, May 2017


Quadcopter CF blade

SUI CF blade

3600 RPM 5400 RPM 7200 RPM

3500 RPM


CFD Results – Quadcopter in Hover


Quadcopter original rotors Simplified Quadcopter with original rotors Simplified Quadcopter with CF replica rotors

Velocity

Q-criterion vorticity iso-surfaces colored by vorticity magnitude


• Complete quadcopter reduces the interactions leading to a higher rotor thrust and less noisy configuration,

• However the fuselage download is more important and the total thrust is slightly smaller than the simplified configuration

• All components and their correct placement are important for performances and acoustics

Ventura et al, “High-Fidelity Computational Aerodynamics of Multi-Rotor Unmanned Aerial Vehicles”, AIAA SciTech 2018

14

Complete Simplified

CFD Results – Quadcopter in Hover


Overmount vs. Undermount Rotors• The high pressure below the

overmount rotors pushes down the fuselage.

• The low pressure above the undermount rotors pulls down the fuselage.

• The fuselage of the off-body undermount configuration experiences much less download. However, four rotors experience strong interactions between themselves.

• Note smaller red zones below the inboard blades.


Overmount

Undermount

Off-body Undermount

Configuration Rotor Thrust Fuselage Download Total Thrust Overmount 1.025 -0.076 0.949 Undermount 1.016 -0.080 0.936 Off-body Undermount 0.954 -0.038 0.916


Overmount vs. Undermount Rotors

• The high pressure below the overmount rotors pushes down the fuselage.• The low pressure above the undermount rotors pulls down the fuselage.• Note surface pressure fluctuations for the undermount.• Undermount configuration is noisier and generates 1% less thrust.


Overmount Undermount




Effects of Gusts ! = #∘

• Rotor 1 produces the highest thrust, generating a difference in the moments. When the wind velocity increases, the difference increases too. Rotor 2 is completely immersed in the wakes, producing the smallest thrust.

• Thrust coefficient nondimensionalized by single rotor thrust

• Moment coefficients nondimensionalized by single rotor moment

18

Rotor 4

Rotor 1

Rotor 2

Rotor 3

Vwind = 17kts

%&'()

Rotor 2

Rotor 4

Rotor 1

Vwind = 10kts

%&'()

Rotor 3*

+Rotor 4

Rotor 1

Rotor 2

Rotor 3

Vwind = 20kts

*

+

%&'()

From VTOL to eVTOL Workshop, May 24, 2018


Effects of Gusts ! = #$%∘

• Rotor 1 and 2, and rotor 3 and 4 produce similar thrust. For 10kts and 17kts the thrust produced by 3 and 4 is lower than without gusts.

• The wind gusts of 25kts (29mph) highly destabilizes the quadcopter. In the Beaufort scale, this velocity corresponds to a strong breeze, with which umbrellas are used with difficulty.

19

Rotor 4

Rotor 1

Vwind = 10kts '()*+

Rotor 4

Rotor 1

Rotor 2

Rotor 3

Vwind = 17kts'()*+

Rotor 2

Rotor 3

Rotor 4

Rotor 1

Rotor 2

Rotor 3

Vwind = 25kts

,

-

'()*+



CFD Results - The SUI Endurance in FF

• Rotor-rotor interactions in hover are smaller than those for the quadcopter. However, as the forward velocity increases, rotor-rotor interactions are stronger as the wakes of the fore rotors begin to affect those of the aft rotors.

• Significant blade-vortex interactions in the fore rotors

• Trimmed conditions measured in FF for standard configuration

20

Blade vortex interaction

Outboard supertip vortex







SUI Standard, Hybrid and Undermount

• In the SUI standard the aft rotors performances are reduced by the influence of the fore rotors wake

• With the SUI hybrid configuration, the interactions between the fore rotors wake with the aft rotors and the front arms are greatly reduced. The forward thrust in the hybrid configuration is higher.

• The fully undermount configuration reduces the interactions with the arms but the wakes from the fore rotors affect the aft rotors. Previous studies also showed that undermount rotors are noisier and generate less lift.

23

Standard configuration Hybrid configuration Undermount configuration

Mach number contours



Overset Grids – The Elytron 4S UAV

• In 1924, Prandtl introduced the concept of “best wing system” and showed that a box-wing presents the lowest induced drag among wing systems that have the same wingspan, total weight and lift.

24

Elytron Propeller

Elytron 4S UAV: box wing, tilting wing UAV version of UAM concept VTOL the Elytron 4S converticopter.

Elytron 4S UAV overset grids with hole for nose fan

Elytron Nose Fan



The Elytron 4S UAV

• The Elytron 4S UAV tilt-wing can tilt 90 degrees to helicopter mode when the vehicle can take off and land vertically. It then tilts back to 0 degrees to airplane mode when the vehicle moves forward without the problems derived from the retreating blade.

• The joined-wing design reduces the wing-tip vortices for small angles of attack

• No retreating blade stall problems with the tilt-wing configuration


Helicopter mode Airplane mode


The Elytron 4S UAV in FF


!"! = $° !"! = &$°

• Wingtip vortices in the joined-wing relatively stronger for higher angles of attack.

• Blade-vortex interaction (BVI) not very important as in forward flight the vortices are carried downstream

Q-criterion vorticity iso-surfaces colored by pressure



The Elytron 4S UAV in VTOL

• Fan rotates at maximum rotational velocity producing high-frequency pressure fluctuations

Mach number contours

Q-criterion vorticity iso-surfaces colored by pressure

Q-criterion vorticity iso-surfaces colored by vorticity magnitude




Summary and Conclusions• Study of small UAVs can help us design safer, quieter

and more efficient future eVTOL vehicles.

• High-fidelity Computational Fluid Dynamics is the right tool to understand, analyze and visualize the complex flows in multi-rotor vehicles.



Work in Progress• High-fidelity CFD of NASA Ames conceptual

designs for UAM and low emissions.



Questions?


Documents

Aerodynamic Modeling for Urban Air Mobilityutm3.com/wp-content/uploads/2018/05/utm3.com_ventura_eVTOL_… · Patricia Ventura Diaz and SeokkwanYoon NASAAmes Research Center From VTOL