Aerodynamic Simulations with AcuSolve

Innovation Intelligence®

Aerodynamic Simulations with AcuSolve

Dr. Marc Ratzel

April 2013

Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.

Agenda

1. Motivation

• Applications of aerodynamics

2. AcuSolve for external aerodynamics

• Quick overview of AcuSolve

3. Aerodynamic examples

• Automotive aerodynamics

• Fluid-Structure-Interaction for a 100m wind turbine

4. Summary


1. Motivation

• Aerospace

• No ground effects

• A380 900 km/h (Mach 0.75)

• Interest: lift/drag ratio, stall point, capacity,…

• Automotive

• Proximity to the ground

• My car 150 km/h (Mach 0.13)

• Interest: drag, lift, noise,…

• Buildings

• Several objects in close proximity

• Avg. wind speed 20 km/h (Mach 0.02)

• Interest: pressure loads, max. speed,…


Characteristics of external flows

• Numerical model

• Large models (+50Mio volume elements)

• Complex geometry (e.g. engine, underbody)

• Very fine mesh near walls (Boundary layer, BL)

• Physics

• Different scales time/length scales

(e.g. sounds travels much faster than airflow,

high/low frequency, small/large eddies)

• Turbulence

• Transient phenomena (e.g. airborne noise CAA, wake)

• Fluid-Structure-Interaction

(e.g. structural vibration of hood noise, eigenmodes)

• Discontinuities (e.g. shock wave aerospace)

• Moving boundaries (e.g. train, wheels of a car)

Boundary layer mesh

Turbulent wake, wind turbine

Shock wave


2. AcuSolve (CFD solver)

• General

• General purpose, 3-dimensional, unstructured solver

• Based on Finite Element method (GLS)

• Originated at Stanford University (T. Hughes et al.)

• Integration with other HyperWorks tools (HyperMesh, HyperView, RADIOSS,..)

• Numerics

• 2nd order accuracy in time and space for all flow variables

• Designed from day one for large scale problems

• Advanced turbulence models (SST, k-w, SA, DES, DDES, …)

• Comprehensive physics (fan/radiator component, sliding mesh, radiation,…)

• Robust fast volume mesher included (e.g. 90Mio car, engine, underbody, 80min)

• Very low requirement for element quality (e.g. tetras in boundary layer, skew>0.999)

Excellent fit for external aerodynamics


AcuSolve (Fluid-Structure-Interaction, FSI)

• Rigid body dynamics

• 6 DOF rigid body solver

• No structural displacement

• Practical FSI (P-FSI)

• No run-time coupling

• Structural displacement computed in AcuSolve based on eigenmodes

• Limited to linear structural displacement

• Directly coupled FSI (DC-FSI)

• Codes communicate during run-time

• Loads/displacement exchange

• Non-linear structural behaviour

• Supported for RADIOSS, Abaqus, MD-Nastran


3.1 Automotive aerodynamics

• Drag force

• Accounts for 75% of the car’s resistance (100km/h)

• Computation:

• Drag reduction by minimizing CD shape optimization

• Drag coefficients CD

drag side

lift

Car shape Frontal area

Modern car: ~ 0.29 Eiffel Tower: 1.8 – 2.0 Man (upright): 1.0 – 1.3


Asmo model (Daimler & Volvo)

Amed body (S.R. Ahmed)

C_p, underbody C_p, rear C_p, roof

Drag coeff.: 0.162 (exp) / 0.164 (AcuSolve)

Drag coeff.: 0.23 (exp) / 0.23 (AcuSolve)

Classical external aero benchmarks

AcuSolve

Exp. (Volvo, Daimler)


Fluid-Structure-interaction for rear wing

• P-FSI analysis

• Generic model of an automotive rear wing

• Soft plastic material, thickness of 2mm

• 20 & 100 eigenmodes computed with OptiStruct

fixed

monitor point

Displacement of monitor point

(20 & 100 eigenmodes) Down force for rigid and elastic

4%


Fluid-Structure-interaction for rear wing

• P-FSI analysis

• Generic model of an automotive rear wing

• Soft plastic material, thickness of 2mm

• 20 & 100 eigenmodes computed with OptiStruct

fixed

monitor point

Structural deformation (vel. contour) Structural deformation (vel. contour)


Altair’s Virtual Wind Tunnel (VWT)

• External automotive CFD analysis

• Advanced physics (rot. wheels, radiator, FSI,…)

• Efficient case setup


Altair’s Virtual Wind Tunnel (VWT)

• External automotive CFD analysis

• Advanced physics (rot. wheels, radiator, FSI,…)

• Efficient case setup

• AcuSolve as CFD solver

• Automatic post-processing


3.2 Wind turbine (Fluid-Structure-Interaction analysis)

• Facts

• Cooperation with Sandia National Laboratories, CA, USA

Blade length 100m

Weight 1.1t

Max. chord 7.6m

Material Fiberglass, Resin, Foam,…

Max. operation speed 7.44 RPM (tip speed 80m/s)

Power output ~ 13MW

human scale 1.8m


Numerical models

• Structural model

• OptiStruct used as solver (eigenmode analysis)

• Composite model

• 100 eigenmodels

• CFD model

• Steady state, Multiple-Reference-Frame (MRF)

• Spalart-Allmaras RANS turbulence model

• ~ 50Mio elements

inflow periodic

outflow farfield

blade

Not true to scale

Eigenmode

CFD mesh


Results (varying inflow speed & rotor RPM)

• Surface streamlines

• Power / Thrust

4.0 m/s Wind Speed 17.0 m/s Wind Speed

Larger separation bubble

Discrepancy with FAST results Some discrepancy

(due to separation bubble)

Remark: FAST and WT_Perf are commonly used tools in the wind turbine domain, containing simplifications for the CFD part


4. Summary

• Challenges for external aero

• Large complex models (+50Mio cells)

• Advanced physics (e.g. diff. scales, turbulence)

• Transient (e.g. FSI, noise)

• AcuSolve

• Accurate, scalable, robust CFD solver

• Fluid-Structure-Interaction (FSI) capabilities

• Altair’s Virtual Wind Tunnel for external automotive aero

• Aerodynamic examples

• Automotive: Classical benchmarks (ASMO, Ahmed)

& FSI rear wing

• Wind turbine: FSI of turbine blade

• Both cases very good match with exp. data

Parking turbine blade

Virtual Wind Tunnel

Date post:	12-Sep-2014
Category:	Technology
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Aerodynamic Simulations with AcuSolve

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