college of engineering and natural sciences mechanical engineering department 1 project number : ps...

College of Engineering and Natural SciencesMechanical Engineering Department1

Project Number : PS 7.1

Rotorcraft Fuselage Drag Study usingOVERFLOW-D2 on a Linux Cluster

PI: Associate Professor EPN Duque

tel : 928-523-5842www.cet.nau.edu/~end2

Northern Arizona University

Graduate Assistant/Research Engineer:Nathan Scott

2004 RCOE Program ReviewMay 4, 2004


Background/ Problem Statement:• Evaluate fuselage force

and moment prediction capability of the OVERFLOW2 and OVERFLOW-D

• Utilize cost effective computer systems


Technical Barriers orPhysical Mechanisms to Solve :

• Appropriate grid generation over specific aircraft

• Lift and drag forces over simplified shapes such as prolate spheroid

• Grid sensitivity studies required

• Unsteady flow capturing on bluff bodies


Task Objectives:Using the OVERFLOW code

• Evaluate drag prediction on a prolate spheroid

• Evaluate drag prediction on a helicopter fuselage

• Evaluate and document effects of grid resolution

• Evaluate turbulence models upon predictions.• 1-eqn, 2-eqn, DES

• Compare results with Penn State Methods


Approaches:

• OVERFLOW2 Code

• Grid Generation• Near body grid refinement in boundary layer• Grid adaptation in the field for vortical flow

• Turbulence models• Baldwin-Barth• Spalart-Almaras• k-• Mentor-SST• include Detached Eddy Simulation (DES)


Overview Explain S-A and SST

Detached Eddy simulation

Discuss DES Implementation in OVERFLOW

Circular Cylinder results

6:1 Prolate Spheroid results


Experimental Data

Virginia Tech Stability Wind Tunnel– Wetzel, Simpson, Ahn

1.37 m 6:1 Prolate Spheroid Free stream conditions

– α=20º, Re=4.2E6, Ma=0.16

Coefficient of Pressure (Cp), Skin Friction (Cf)from Wetzel Dissertation

U/u*, y+ from Simpson’s Website


CFD Methodology Reynolds Averaged Navier-Stokes Equations

– OVERFLOW-D code developed at NASA and Army– Uses detailed overset grids– Allows for detailed geometry definition– Captures viscous effects such as unsteady flow

separation OVERFLOW2 used for turbulence model study

and Implementation of DES– Scalar penta-diagonal scheme– 1st order difference in time – 2nd or 4th order RHS (OVERFLOW2)– 2nd and 4th order central difference dissipation terms


Detached Eddy Simulation

First Formulated by Spalart as a modification to S-A model in 1997.

Later generalized to any model by Strelets in 2001.

First step was to modify the S-A model


S-A-DES formulation

Change distance to wall in S-A model dw to

– Ĩ=min(dw,CDES∆)

– ∆ is the maximum of the grid spacing in three dimensions- ∆=max(δX, δY, δZ)

– CDES=0.65


k--SST-DES Formulation Change k-transport source term:

ρβ*kω=ρk3/2/Ĩ– Ĩ=min(lk-ω,CDES∆)– lk-ω=k1/2/(β*ω)– ∆ is the maximum of the grid spacing in three

dimensions- ∆=max(δX, δY, δZ)– CDES=(1-F1) Ck-ε+F1 Ck-ω

– Ck-ε=0.61, Ck-ω=0.78• At equilibrium reduces to an algebraic

mixing-length Smagorinski type model.


Implementation in OVERFLOW Determine grid cell edge

lengths in J,K,L directions– One sided difference at

boundaries– Central difference otherwise

Background Cartesian Grids - DES always enabled


Circular Cylinder Test Case Re=140,000, Ma=0.2 Fully Turbulent S-A, S-A-DES, SST-DES turbulence

models 7.6 million grid points

– Near body 181 by 60 by 99– Background 426 by 61 by 252– Off Body grid resolution 0.05 the

diameter– H type block grid extends 10 diameters– 2 total grids

Methods– 4th central difference in space– 1st order Beam-Warming in time

Inviscid wall Boundary Conditions


Other DES work with Cylinder

Travin, A, Shur, M, Strelets, M, Spalart, P– Re = 50,000 and 140,000– Laminar Separation

» Laminar Separation» LES in Background

– Turbulent Separation» Run Fully Turbulent» Compares to higher Re


Iso-surface visualization comparison Circular Cylinder

Travin-DES OVERFLOW S-A-DES

OVERFLOW URANS (Not Unsteady Yet)

OVERFLOW k--SST-DES


0 20 40 60 80 100 120 140 160 180-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

angle from windward side

Cp

Average 1 drag cycleExp-Nunen-Re=7.6 E6Exp-Roshko-8.6 E6Scatter 1 drag cycle

Unsteady Pressure coefficient for 1 drag cycle


0 20 40 60 80 100 120 140 160 180-2

-1.5

-1

-0.5

0

0.5

1

1.5


Cp

SA-DESSA-RANSDES-TravinExp-Nunen-Re=7.6 E6Exp-Roshko-8.6 E6

Average Pressure coefficient for 1 drag cycle


Conclusions from Circular Cylinder S-A DES in OVERFLOW looks promising

– More fine scale resolution– Cross Flow on “2-D” cases– Comparable comparisons to Experimental Data

k--SST DES in OVERFLOW also looks promising– SST has been shown to approximate separation

better so more desirable in shear layer– More verification needs to be done


6:1 Prolate Spheroid Test Case Re=4,200,000, Ma=0.16 Trip to Turbulence at x/L=0.2 S-A, S-A-DES, SST-DES turbulence

models 7 million grid points

– Near body 361 by 310 by 45– First off body Grid spacing 0.08 the length– Remaining off body grids reduce in

resolution by half– Off body grids extent to 10 times the length – 61 Total grids– Grid shown to be convergent in Previous

Study Methods

– 4th central difference in space– 1st order Beam-Warming in time


Other DES work with 6:1 Prolate Spheroid

Rhee, S. H. and Hino,T.– Re = 4,200,000 Ma=0,16– Run Steady and Unsteady– Showed under prediction of Lift


Surface Skin Friction and vorticty contour comparison for 6:1 Spheroid

S-AS-A DES

SST SST DES


Comparison Of Lift and Pitching Moment for 6:1 Spheroid

All of the models fall with error for Pitching Moment

All of the models under predict lift

LiftPitching Moment

Experiment 0.61±0.03 0.23±0.04

SA 0.45 0.24

SST 0.48 0.23

S-A-DES 0.42 0.25

SST-DES 0.45 0.24

Rhee & Hino 0.48 0.24


Axial Surface Pressure at x/L=0.77

0 20 40 60 80 100 120 140 160 180-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0


Cp

ExpS-ASSTS-A DESSST DES


Velocity Profile at x/L=0.77 and 150º from Windward side

100

101

102

103

104

1050

5

10

15

20

25

yplus

U/u

*

S-A DESSST DESS-ASSTExp


Axial Skin Friction at x/L=0.77

0 20 40 60 80 100 120 140 160 1801

2

3

4

5

6

7

8

9

10


Cf

ExpS-ASSTS-A DESSST DES


Streamlines on Leeside


6:1 Spheroid Conclusions DES shown to work with

overset grids DES did not improve

integrated forces Skin friction remained the

same Surface pressure showed

slight improvement Velocity profiles remained the

same close to surface y+<10 Velocity profiles improved

farther away from surface y+>100


Accomplishments

Summer work with Roger Strawn and Mark Potsdam at Ames

Presented at AIAA 43rd Aerospace Sciences Meetings.


Future Work

Grid Refinement Study on 6:1 Prolate spheroid and DES

New research engineer, explore new LES

Apply DES and LES to helicopter fuselage

college of engineering and natural sciences mechanical engineering department 1 project number : ps...

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