Boundary Layer Feature Extraction White Paper

… analyze, visualize, communicate

Extraction of Boundary Layer Characteristics from Fluent Results

Fluent 2005 CFD Summit, Dearborn, MI

Kent P. Misegades, CEI Inc., Apex, NC


Background & Real Flow Visualization

Traditional Visualization of CFD results

Boundary Layer Characterization

Boundary Layer Characteristic Extraction

Other Techniques

Future Work

Topics


Subject: Viscous, High-Speed, Compressible Flows

Slender bodies & wings at low angle-of-attack

High Reynolds numbers, thus thin boundary layers

Shock / boundary layer interaction significant

BL thickness and flow distortion in engine inlets

Extensive laminar flow may exist, thus BL transition critical

Background


Background

Lockheed-Martin HCV

Aerion SSBJ

Advanced Aircraft with Extensive Laminar Flow

SAI SSBJ

Boeing 787


High-Fidelity Fluent CFD Aerodynamic Simulations

Accuracy now comparable to legacy lab aero codes

Dense meshes needed for turbulence models & shocks

Boundary layers for RANS+SA models are well resolved

DNS, LES, DES all produce far more detail than RANS

Traditional post-processing ignores rich data in results

We need to return to basics & real flow visualization

Background


Real Flow Visualization

Laminar BL Separation and Turbulent BL Reattachment, M. Selig, UIUC



Boundary Layer Surface Flow using Paint Dots, Flometrics



Schlieren Photographs, NASA



Shadowgraph of Spacecraft Model, NASA


Fluent Test CaseGeometry: ONERA M6 Wing

Mesh: 884,736 hexahedra

Reynolds Number: 11.34 million

Freestream Mach Number: 0.84

Angle of Attack: 3.07 degrees

Fluent 6.2 results courtesy Thomas

Scheidegger, Fluent Inc.

All images created using EnSight 8.0

from CEI, Inc.

CFD Visualization

M > 1


Simple TechniquesResults displayed on model surfaces & clip planes

• Velocity, Cp, density, Mach number, etc.

Derived variables

• total pressure, vorticity, entropy, helicity, etc.

Velocity profiles

• qualitatively depicts BL profiles, thickness, separation, etc.

Isosurfaces

• total pressure, sonic (Mach=1), vorticity, entropy, etc.

CFD Visualization


CFD Visualization

Surface Pressure Pressure on Clip Plane

Total Pressure on Clip Plane Mach Number on Clip Plane


Sophisticated TechniquesRegions of Reverse flow

• Streamwise velocity at a small offset distance from a surface

Surface-Restricted Traces

• Oil flow analog, particle traces in laminar sublayer

Lines of BL Separation and Attachment

• Singularities on surface

Shock surfaces & Dilitation Contours

Schlieren & Shadowgraph simulation

CFD Visualization


CFD Visualization

Offset Velocity Magnitude Offset Vx

Surface-Restricted Traces Surface Traces and Offset Velocity

Reverse Flow


CFD Visualization

Lines of BL Separation (green)

Lines of BL Attachment (magenta)

Shock Surface Extraction


CFD Visualization

Velocity Profiles under BL Separation Bubble


CFD Visualization

Schlieren Simulation (|∇∇∇∇·�|) Shadowgraph Simulation (∇∇∇∇2�)


BL Characterization

Boundary Layers on an Airfoil, NASA


Classical Boundary Layer Characteristics

See: Abbott & von Doenhoff, Theory of Wing Sections

Thickness (δδδδ): distance at which u = 0.99 x U

Displacement thickness (δδδδ*): how BL displaces flow

Momentum Thickness (ΘΘΘΘ): loss of momentum in BL

Shape Factor (H = δδδδ* / Θ)/ Θ)/ Θ)/ Θ): separation tendency

Skin Friction Coefficient (Cf): viscous drag, transition

BL Characterization


BL “Health” Indicator: Total Pressure Loss

• Total pressure drops due to viscosity and shocks

• Isosurface of 95% freestream Ptotal indicates BL growth

• Distance from surface to this 95% isosurface is desired

Boundary Layer Transition

• Of increasing importance for DNS, LES, DES results

• Involves anaysis of variables at BL edge

BL Characterization


Incorporated in EnSight 8.x from CEI

Classical BL Characteristics• Thickness, δ, determined by proprietary edge-finding

algorithm developed at CEI

• δ* and Θ are 1-D integrals computed on surface nodes

• Shape Factor is simply H = δ* / Θ

• Cf is computed on surface

• Numerically intensive computations for irregular grids

• All characteristics available as surface variables

BL Characteristic Extraction


BL Characteristic Extraction

BL Thickness, δδδδ BL Displacement Thickness, δδδδ*

BL Momentum Thickness, ΘΘΘΘ Shape Factor, H


Elevated surfaces of BL characteristics

“Distance to value” variable on surfaces

Variables on edge of boundary layers

BL transition indicators

Animations of all techniques for transient flows

Other Techniques


Other Techniques

Elevated Surface of δδδδ Elevated Surface of δδδδ

Contour of 95% Ptotal� Distance to 95% Ptotal�


Other Techniques

BL Thickness, δδδδ Distance to 95% Ptotal�


Further Validation against DPW-I & II DLR models

Flow separation detection & reporting

Laminar/turbulent area ratios

Drag breakdown

Interactive velocity profiling

Smart flow mapping with GPS-like decluttering

Faster methods, better defaults & filters

Future Work


DPW-I DLR-F4

Pressure


DPW-I DLR-F4

Boundary Layer Thickness


DPW-I DLR-F4

Distance to 95% Ptotal�


Thank You

Boeing Commercial Airplane Company, for supporting this work

Fluent Inc., for use of FLUENT and providing example datasets

Boeing 787

Documents

Boundary Layer Feature Extraction White Paper