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. y+ of first point off the wall
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 Mesh
The mesh around the rotor blade
Blade surface grid
The cut plane of the rotor region

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 vortex assumption
Collection of piece-wise linear bound and trailed vorticities
Strength of the vortex elements is set to be equal to the peak bound circulation
Vortex shedding point based on centroid of trailed circulation between the tip and location of peak bound circulation
Vortex trailed at discrete azimuthal intervals.
Vortex elements convected through freestream velocities and wake induced velocities

11. 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)
Vortex core growth using the Bhagwat – Leishman (2002) core growth model

12. 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.

13. Grid Used for Numerical Studies
A C-H grid topology is used
Allowing flexibility with grid density near surface
Better orthogonality and smoothness of grid lines near blunt leading edge of typical rotor blade airfoils
Baseline Grid Size
131 x 70 x 45 ~ 0.4 million grid points per blade
Wall spacing 1*10-5 chords
The far field boundary is located at 9 chords from surface

14. Baseline S-76 Rotor Characteristics
Baseline Model Rotor Blade
Baseline Blade Planform
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
Airfoil
SC1013R8, SC1095R8, SC1095
Scale
1/4.71
Twist
-10° linear twist
Twist distribution
Thickness distribution

15. 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

16. 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

18. Parametric Studies

19. Results for the Anhedral Tip Case

20. Wake Vortex Trajectory (CT/σ = 0.09)
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
Radial location

21. S-76 Baseline Rotor (Inflow is non-uniform)
Induced Velocity (at the Rotor Disk), at 9.5 Degrees Pitch Angle

22. Rotor with Anhedral Tip has a more uniform inflow
Induced Velocity (at the Rotor Disk), at 9.5 Degrees Pitch Angle

23. Results and Discussion for the coaxial rotor

24. Harrington Rotor Characteristics
Blade Planform
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

25. Hover Performance

26. Contributions: Upper & Lower Rotor to Thrust

27. Upper vs Lower Rotor Figure of Merit

28. Effect of Number of Wake Filaments

29. Tip Vortex Structures (STAR-CCM+)

30. Comparison of Tip Vortex Descent Rate

31. Comparison of Tip Vortex Contraction Rate

32. Summary
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

33. 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

34. 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
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 , pp
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 , 1985.
Marpu, R., Sankar, L. N., Egolf, T. A., and Hariharan, N., “Simulation of S- 76 Rotor in Hover Using a Hybrid Methodology,” AIAA , SciTech , January 2014.
Baeder, J., Medida, S., “OVERTURNS Simulation of S-76 Rotor in Hover,” AIAA , SciTech 2014, National Harbor, MD, January 2014.