National Aeronautics and Space Administration

Wind-Induced Oscillations

Parametric Wind Tunnel Test

Sam Yunis, Donald Keller, Thomas Ivanco, Jennifer Pinkerton

NASA Langley

Presented at Spacecraft and Launch Vehicle Workshop

June 20-22, 2017

2017

Issue

2

NASA is attempting to address persistent questions on Wind Induced Oscillations (WIO) on Launch Vehicles

• What are realistic design conditions for WIO?• Is lock-in a real-world event or a contrived wind tunnel artifact?

– Full-scale events seem to indicate that WIO does exist in the wild• What wind tunnel conditions are necessary to match full scale

observations?

Different organizations are designing to different conditions• NASA has been designing vehicles to lock-in based on clean uniform

flow in wind tunnel tests (WTTs)– Often a significant burden during design, especially rollout (Ares, SLS)

• Some others have been designing to turbulent flow and not to lock-in• No complete/comprehensive data exists to quantify the best design

condition• Question dates back to 1960’s

NASA is running a parametric WTT to address these questions• Tunnel entry May 15-July 31, 2017• Time still exists to address questions

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WIO 101

3

Flow across the launch vehicle creates lateral forces due to flow separation

• Driving forcing function• Comes in many forms from laminar to turbulent to

vortex shedding

Flight vehicle is flexible which results in aeroelasticresponses

• Aeroelastic coupling can be >10x factor over rigid magnitudes

• Second mode can be more critical than the first mode because it occurs at higher velocities/energies and because of the different load distribution

Magnitude of aeroelastic response is determined by several factors:

• Type of unsteady flow forces (turbulent, vortices, laminar, etc)

• Proximity of the vortex frequencies to structural frequencies

• Damping

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Definitions

4

For purposes of this presentation:

• WIO– Generic term for the response of a vehicle to unsteady flow

– Unsteady flow includes gust, turbulence, and vortex shedding

• Vortex shedding – Quasi-periodic flow off the vehicle at certain velocities

• Lock-in– An aeroelastic event where the oscillations of the vortex shedding are forced

to coincide with the vehicle frequencies, resulting in aeroelastic amplification

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Parameters of Investigation

5

Reynolds number

Flow turbulence

Boundary layer effects, including pad height

Surrounding structure

Protuberances

Damping

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Reynolds Number

6

Full scale vehicle Re usually >106

Vortex shedding at Re = 104 looks similar to vortex shedding at 106

Concerns with Re mis-match• Scaling for Re is very difficult

– Need to match correct magnitudes and velocities

– Protuberances impossible to scale because of boundary layer differences

• Tip shedding cannot be mimicked at the wrong Re

WTT will address the importance of Re matching in WTTs

• Approach: Use air and heavy gas to span Re range

2017

Turbulence

7

Real-world flow has turbulence• Ground, buildings, butterflies• Turbulence-induced vortex shedding and turbulent response is a design condition for

fatigue and RMS response• Turbulence may prevent large resonant WIO and lock-in

Concerns with turbulence• If lock-in can be achieved, turbulent response is not a peak-load design condition• Resonant WIO/Lock-in remain

the design condition(s) for peak loads until otherwise shown– Example: Damping too high for

aeroelastic response

WTT will investigate impact of turbulence on lock-in

• Design: blocks on floor (based somewhat on method used at theCermak, Peterka, Petersen (CPP) tunnel (spires, low wall, floor blocks)

• Goal is to generate typical turbulence levels (≈ 5% - 10%)

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Earth Boundary Layer

8

The boundary layer (wind profile) creates a non-uniform velocity profile along the length of the vehicle

• Thought 1: The boundary layer reduces the correlation length of lock-in across the vehicle, thereby reducing the aeroelastic responses

• Thought 2: The larger diameter of a fairing may match the boundary layer velocities, thereby actually increasing correlation length of lock-in

Wind tunnel testing is not necessarily required to understand the impact of the boundary layer

• Improved/verified analytical methods may be sufficient to ID potential critical conditions (wind speed and direction)

WTT will investigate the impact of the boundary layer by creating a wind profile

• Design: Blocking spires, multiple height pads

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Surrounding Structure

9

Surrounding structures, both upstream and downstream, can

definitely influence the response of a vehicle• The variety of these is too big for a complete study

WTT will use a solid tower/building to demonstrate the effect

in conjunction with the other parameters• Design: Block building bigger than vehicle

• Location: Near the vehicle (2-5D away)

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Protuberances

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Protuberances often instigate separation and vortex shedding,

and have been shown to magnify results• Protuberances often act within the flow boundary layer

• Protuberances are particularly susceptible to Re mis-match

WTT will address this across Re numbers• Design: Includes raceway/feedline and block protuberances

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Damping

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WTT may attempt to vary damping• Effect of damping on smooth-flow WIO is well established

• Effect of damping coupled with other parameters is not as well

established

WTT will measure structural damping for all configurations

tested

WTT will use a variable damper • Design: In-work and unclear if this can be incorporated in time

• Trying to modify a previously-used design (Ares I-X)

• Targeting values C/Cc <1% to 2%

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WTT Logistics

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Testing will take place at the Langley

Transonic Dynamic Tunnel (TDT)

• Quick facts– Mach: up to 1.2, Re: up to 10 x106 /ft,

Medium: Air or R-134a

– Total pressure: near vacuum up to 1.0

atm.

– Test Section: 16’ x 16’ with cropped

corners, Length: 12’ – 30’

– GWL Testing: Large remotely

controlled floor turntable, data system

designed for dynamic testing

Ares I-X Model in TDT - 2008

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Wind Tunnel Test: LV Design

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Dynamic models

• Single stick vehicles

1. Constant diameter

2. Two diameters: Single diameter first/lower stage with large upper

stage and fairing

• Modes: 1st Bending, 2nd Bending (4 modes in total)

– Ability to vary mass to achieve target frequencies

• May grit surface to validate simulation of higher Re on a 3D model

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Wind Tunnel Test: Baselining

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Prior to parametric testing, the tunnel must show that it can predict

full scale responses

NASA has data from 2 full scale vehicles that experienced severe

WIO, possibly lock-in:• Wind speed and direction

• Launch vehicle response – dynamic motions and loads

Plan of attack is to figure out what parameters are needed to match

the full scale events• Baseline – Uniform flow (normal TDT turbulence and B-L profile)

• Parameters will be varied to match the response– Wind profile

– Turbulence

– Boundary Layer

– Damping

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Wind Tunnel Test: Parametric Testing

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Parametric testing on a generic vehicle• Lot of possible parameters and variations• Limited by non-infinite time

Parameters – Vehicle• Constant diameter stick and stick w/fairing at top• Protuberances• Damping – may not vary, depends baseline testing

Parameters - Flowa. Velocity: Target first and second modesb. Reynolds Number: 104 – 106

c. Turbulence: None to as large as we can getd. Boundary Layer (incl. Pad Height): None to theoretical curvee. Surrounding Structure: None to block tower

Data• Steady/unsteady pressures, acceleration response, base bending moment

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Schedule

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August 2016:

• ATP

November 2016:

• Briefing to general LV community for feedback

May 15:

• Wind-on for turbulence and boundary layer checkout

June 15:

• Baseline runs begin

July 15:

• Parametric testing begins

July 31:

• End?

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Data Release

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The parametric data will be released to the community• Assume it will be subject to ITAR rules

Timeframe for release is unclear, but as soon as possible

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Summary

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A parametric wind tunnel test for understanding WIO is

underway

Any input to the test matrix is welcome• Very soon or it will be too late