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Project Number : PS 7.1 Rotorcraft Fuselage Drag Study using OVERFLOW-D2 on a Linux Cluster PI: Associate Professor EPN Duque tel : 928-523-5842 www.cet.nau.edu/~end2 Northern Arizona University Graduate Assistant/Research Engineer: Nathan Scott

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Project Number : PS 7.1

Rotorcraft Fuselage Drag Study using

OVERFLOW-D2 on a Linux Cluster

PI:

Associate Professor EPN Duque

tel : 928-523-5842

www.cet.nau.edu/~end2

Northern Arizona University

Graduate Assistant/Research Engineer:

Nathan Scott

2004 RCOE Program Review

May 4, 2004


Background problem statement
Background/ Problem Statement:

  • Evaluate fuselage force and moment prediction capability of the OVERFLOW2 and OVERFLOW-D

  • Utilize cost effective computer systems


Technical barriers or physical mechanisms to solve
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
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
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-w

  • Mentor-SST

  • include Detached Eddy Simulation (DES)


Overview
Overview

  • Explain S-A and SST Detached Eddy simulation

  • Discuss DES Implementation in OVERFLOW

  • Circular Cylinder results

  • 6:1 Prolate Spheroid results


Experimental data
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
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
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
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 w sst des formulation
k-w-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+F1Ck-ω

    • Ck-ε=0.61, Ck-ω=0.78

  • At equilibrium reduces to an algebraic mixing-length Smagorinski type model.


Implementation in overflow
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
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
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
Iso-surface visualization comparison Circular Cylinder

OVERFLOW S-A-DES

Travin-DES

OVERFLOW URANS (Not Unsteady Yet)

OVERFLOW k-w-SST-DES




Conclusions from circular cylinder
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-w-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
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
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
Surface Skin Friction and vorticty contour comparison for 6:1 Spheroid

S-A DES

S-A

SST

SST DES


Comparison of lift and pitching moment for 6 1 spheroid
Comparison Of Lift and Pitching Moment for 6:1 Spheroid 6:1 Spheroid

  • All of the models fall with error for Pitching Moment

  • All of the models under predict lift



Velocity profile at x l 0 77 and 150 from windward side
Velocity Profile at x/L=0.77 and 150 6:1 Spheroidº from Windward side




6 1 spheroid conclusions
6:1 Spheroid Conclusions 6:1 Spheroid

  • 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
Accomplishments 6:1 Spheroid

  • Summer work with Roger Strawn and Mark Potsdam at Ames

  • Presented at AIAA 43rd Aerospace Sciences Meetings.


Future work
Future Work 6:1 Spheroid

  • Grid Refinement Study on 6:1 Prolate spheroid and DES

  • New research engineer, explore new LES

  • Apply DES and LES to helicopter fuselage


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