1

Application of STAR-CCM+ to

Helicopter Rotors in Hover

Lakshmi N. Sankar and Chong Zhou

School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA

Ritu Marpu Eschol

CD-Adapco, Inc., Orlando, FL

2

Background

▪ Over the past several decades, engineers have used a variety of

tools for modeling and improving rotor design.

▪ An engineering model called a “Lifting Line Model” with a look up of

2-D airfoil load characteristics look-up table is often used during

early stages of design, with empirical corrections for compressibility,

and sweep.

▪ During a second “preliminary” stage of design, hybrid CFD + Lifting

Line methods would be used.

▪ During later “detailed design” stages, a more accurate CFD based

model is used to refine the design.

▪ STAR-CCM+ is a valuable tool for this final detailed design.

▪ In this work, we show selected examples of three types of rotors, to

see how a preliminary and detailed analysis may be done.

3

Overview

▪ Introduction

▪ Detailed Analysis/Design Tool (STAR-CCM+)

▪ Hybrid CFD Methodology (GT-Hybrid)

➢ Wake Model (Single Tip Vortex)

➢ Vortex Core Modeling

➢ Full Span Wake Model

▪ Performance Predictions and Validation

➢ Sikorsky S-76 Helicopter Rotor with swept Tip

➢ Helicopter Rotor with Swept Anhedral Tip

➢ Coaxial Rotor

▪ Concluding Remarks

4

Objectives

▪ Apply STAR-CCM+ Analysis Wake-capturing Models To S76

Rotors In Hover

▪ Validate The Analysis For S76 In Hover

▪ Compare Predictions With GT-Hybrid (Preliminary Design/Analysis

Tool)

▪ Analyze The Effects Of The Anhedral Tip On The Inflow Distribution

▪ Explain Why The “efficiency” is Improved By The Anhedral Platform

▪ Analyze a Coaxial Rotor

5

STAR-CCM+ Computational Domain

6

Parameters for the S-76 Model Simulations

Mach Number at the Tip 0.65

Reynolds Number 1.20E+06

Ds 1.60E-06

y+ of first point off the wall ~1

Rotor Radius 56 inches

C 3.1inch

▪ Wake capturing model

▪ Unsteady simulations (7 to 20 revolutions)

➢ Time step : 1 Deg. Azimuth

➢ Sub-iterations: 10 to 15 per time step

▪ Physics Model

➢ Coupled Energy

➢ Ideal Gas

➢ K-Omega SST, fully turbulent flow

7

Mesh

▪ Overset Mesh Topology

The mesh for the rotor region Background

MeshThe mesh around the rotor blade

The cut plane of the rotor regionBlade surface grid

8

Refined Mesh Region

9

GT-Hybrid CFD Methodology (Preliminary

Design Tool)

▪ Hybrid Methodology

─ Reynolds Averaged Navier-Stokes (RANS)

methodology for flow over blades.

─ Lagrangian free wake to model far wake.

─ The near wake is captured inherently in the

Navier-Stokes analysis.

─ The far wake and effect of other blades

accounted for using wake model.

─ Wake induced velocities are applied as

boundary condition on Navier-Stokes domain.

Schematic View of the Hybrid Method

10

Wake Model (Single Tip Vortex)

▪ Lagrangian Free Wake model

─ Single concentrated tip vortexassumption

─ Collection of piece-wise linearbound and trailed vorticities

─ Strength of the vortex elementsis set to be equal to the peakbound circulation

─ Vortex shedding point basedon centroid of trailed circulationbetween the tip and location ofpeak bound circulation

─ Vortex trailed at discreteazimuthal intervals.

─ Vortex elements convectedthrough freestream velocitiesand wake induced velocities

11

▪ Vortex core growth using the Bhagwat – Leishman (2002) core

growth model

Vortex Core Modeling

▪ Wake behind any lifting surface must be considered as a viscous

phenomenon

▪ Velocity induced by a vortex with Vatistas (1991) core (n = 2)

2

2

1

10/1

2

0

2

21

21

4 r

r

r

rr

rrrr

rrV

nn

c

n

v

c

a

V

zzr

Re1

4

1

0

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Full Span Wake Model (FSWM)

▪ The baseline wake model

assumes a single concentrated

tip vortex trailing from a region

near the blade tip.

▪ This assumption would be

physically less accurate for

rotors in low speed forward

flight.

▪ Single tip vortex is replaced by

user specified multiple vortex

segments trailed from all the

blades.

▪ FSWM is based on vorticity

conservation laws.

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Grid Used for Numerical Studies

▪ A C-H grid topology is used

➢ Allowing flexibility with griddensity near surface

➢ Better orthogonality andsmoothness of grid lines nearblunt leading edge of typicalrotor blade airfoils

▪ Baseline Grid Size

➢ 131 x 70 x 45 ~ 0.4 million gridpoints per blade

➢ Wall spacing 1*10-5 chords

▪ The far field boundary is located

at 9 chords from surface

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Baseline S-76 Rotor Characteristics

Number of blades 4

Radius 56.04”

Nominal Chord 3.1”

Equivalent Chord 3.035”

Tip Taper 60% c

Root Cutout 19% R

Sweep (leading-edge) 35 degrees at 95% R

Solidity 0.07043

AirfoilSC1013R8,

SC1095R8, SC1095

Scale 1/4.71

Twist -10° linear twist

Baseline Model Rotor Blade Baseline Blade Planform

Twist distribution Thickness distribution

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Vorticity and Q-criterion Distribution

swept-tapered S-76 Planform, at CT/σ=0.09

▪ Near wake is well captured, including the inner wake.

▪ Far field wake is smeared due to numerical diffusion because

of the coarser grid

▪ “Starting vortex” is also seen

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Results for the Baseline Tip Case

▪ CT: Wake Capture Model Matches Well with the Experimental Data

▪ CQ: Works Better at High Collective Pitch Angle

17

Results for the Baseline Tip Case

Collective (Deg)

Th

rus

tC

oe

ffic

ien

t

0 2 4 6 8 10 120

0.002

0.004

0.006

0.008Measured

GT-Hybrid

OVERFLOW

Helios (Boeing)

OVERTURNS

STAR CCM+

Power Coefficient

Th

rus

tC

oe

ffic

ien

t

0 0.0002 0.0004 0.0006 0.00080

0.002

0.004

0.006

0.008

Measured

GT-Hybrid

OVERFLOW

Helios (Boeing)

OVERTURNS

STAR CCM+

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Parametric Studies

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Results for the Anhedral Tip Case

Thrust Coefficient

Fig

ure

of

Me

rit

0 0.002 0.004 0.006 0.0080

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Baseline

Rectangular

Anhedral

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Wake Vortex Trajectory (CT/σ = 0.09)

Radial location

Vertical location ▪ No test data available

▪ Vertical location

➢ Matches Well with OVERFLOW

➢ Good correlation could only be

achieved for the first revolution

(360 degrees of vortex age

➢ At higher vortex age, factors such

as numerical diffusion, grid density,

etc begin to cause deviations among

the various methods.

▪ Radial location

➢ Over Predict the tip vortex

contraction rate compare with other

solvers

➢ Acceptable: Matches well with

OVERFLOW

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S-76 Baseline Rotor

(Inflow is non-uniform)

Induced Velocity (at the Rotor Disk), at 9.5 Degrees Pitch Angle

r/R

vi/

RT

IP

0.4 0.6 0.8

-0.02

0

0.02

0.04

0.06

0.08

0.1

Azimuth (30 Deg.)_ST

Azimuth (60 Deg.)_ST

Azimuth (120 Deg.)_ST

Azimuth (150 Deg.)_ST

Azimuth (210 Deg.)_ST

Azimuth (240 Deg.)_ST

Azimuth (300 Deg.)_ST

Azimuth (330 Deg.)_ST

30 degrees upstream and downstream of the blade

At other stations

S-76 Baseline Rotor

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Rotor with Anhedral Tip has a more uniform inflow

r/R

vi/

RT

IP

0.4 0.6 0.8 1

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

Azimuth (30 Deg.)

Azimuth (60 Deg.)

Azimuth (120 Deg.)

Azimuth (150 Deg.)

Azimuth (210 Deg.)

Azimuth (240 Deg.)

Azimuth (300 Deg.)

Azimuth (330 Deg.)

30 degrees upstream and downstream of the blade

At other stations

Anhedral Tip

Induced Velocity (at the Rotor Disk), at 9.5 Degrees Pitch Angle

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RESULTS AND DISCUSSION

FOR THE COAXIAL ROTOR

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Harrington Rotor Characteristics

Blade Planform

Rotor Characteristics

Harrington “Rotor 1” Harrington “Rotor 2”

• Tested inside a full-scale wind tunnel at NACA Langley Research Center

• Reference: Harrington, R.D., “Full-Scale-Tunnel Investigation of the Static Thrust

Performance of a Coaxial Helicopter Rotor,” NACA TN 2318, Mar. 1951

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Hover Performance

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Contributions: Upper & Lower Rotor to Thrust

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Upper vs Lower Rotor Figure of Merit

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Effect of Number of Wake Filaments

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Tip Vortex Structures (STAR-CCM+)

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Comparison of Tip Vortex Descent Rate

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Comparison of Tip Vortex Contraction Rate

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• The aerodynamic behavior of a conventional rotors,

anhedral rotors, and coaxial rotors has been studied

using two approaches – a hybrid Navier-Stokes-free

wake solver, and a full wake-capturing approach

• Comparisons with test data have been done.

• Anhedral tips produce a more uniform induced velocity.

• This leads to a more efficient rotor.

• Coaxial rotors are compact, have reduced swirl losses,

and eliminate the need for tail rotor.

• The performance of upper and lower rotors, for equal and

opposite torque, was examined

• Comparisons of the predicted vortex descent rate and radial

contraction rate were also examined

Summary

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Conclusions

• At lower thrust settings, both methods give good

agreement with test data

• As the thrust level increases, the hybrid method tends to

underestimate the power required, and overestimate the

figure of merit

• We are in the process of improving the hybrid results using

vortex particle methods, improved tip cap grids, and

improved treatment of root regions

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Conclusions (Continued)

• In terms of computational time, the hybrid method is very

efficient, requiring 4 to 6 hours of CPU time on a Linux

cluster with 72 cores of CPU

− The wake capturing method is considerably more expensive.

• For this reason, the hybrid method is well suited for initial

design studies where the rotor geometry is

parametrically varied, and quick reasonably accurate

solutions are essential

• Once a few promising configurations have been

identified, more accurate (but computationally

expensive) wake capturing simulations may be done to

refine the design.

35

Related Prior Work

❖ Hariharan, N., Egolf, T. A., and Sankar, L. N., “Simulation of Rotor in Hover:

Current State, Challenges and Standardized Evaluation,” AIAA 2014-0041.

❖ Lorber, P.F., et al., “A Comprehensive Hover Test of the Airloads and

Airflow of an Extensively Instrumented Model Helicopter Rotor,”

Proceedings of the 45th Annual Forum, American Helicopter Society, May

1989, pp 281-295.

❖ Balch, D. T., “Experimental Study of Main Rotor Tip Geometry and Tail

Rotor Interactions in Hover, Volume 2, Run Log and Tabulated Data,”

NASA CR 177336, 1985.

❖ Marpu, R., Sankar, L. N., Egolf, T. A., and Hariharan, N., “Simulation of S-

76 Rotor in Hover Using a Hybrid Methodology,” AIAA-2014-0210, SciTech

2014, January 2014.

❖ Baeder, J., Medida, S., “OVERTURNS Simulation of S-76 Rotor in Hover,”

AIAA-2014-0045, SciTech 2014, National Harbor, MD, January 2014.