• AEROELASTICITY

• Royal Aeronautica SocietyMonday 14th March 2016.

•

Aerodynamic Control of Long Span Bridges

Michael Graham, David Limebeer*, Kevin Gouder and Xiaowei Zhao**

Department of Aeronautics, Imperial College London.[m.graham@imperial.ac.uk]

*Department of Engineering Science, University of Oxford..

• **Department of Engineering, University of Warwick

Acknowledgements: Konstantinos Bakis*, Yasuaki Ito, Matteo Massaro*, Martin Williams*.

• A

Active aerodynamic control of bridge deck flutter.

Kobayashi and Nagaoka 1992

Fujino, Iwamoto, Ito and Hikami 1992

Wilde and Fujino 1996

Miyata 1992, 1999

Omenzetter, Wilde and Fujino 2002

…

Kinematic Model of (2-D section of) Bridge Deckfitted with Leading- and Trailing-Edge Flaps.

Thin aerofoil (flat plate) potential flow theory.Theodorsen’s results for a wing section + flap + tab adapted to a

thin bridge section with LE and TE flaps.

Effect of a 15% TE flap with gain of 2.0 and 90deg phase relative to pitch (Theodorsen theory).

(a) Open loop (no control) (b) Closed loop (controlled)

Humber Bridge

Great Belt Bridge (Denmark)

Contents of Presentation

Wind Tunnel Tests of a bridge section in smooth flow and turbulence: Pitch / Heave Flutter Instability of flexibly mounted deck, Buffet forces induced by incident turbulence.

Control System, Open-Loop and Closed-Loop characteristics of deck dynamics.

Wind Tunnel tests of aerodynamic control effects on critical flutter speed and on turbulence buffeting.

A possible ‘passive’ mechanical system to implement the control system.

Bridge Deck Section (2nd Forth Crossing Candidate modified with Flaps) in Imperial College Honda Wind Tunnel

Schematics of Bridge section

Bridge Deck Section in No. 5 Wind Tunnel at BMT Fluid Mechanics Ltd.

Deck Section and Turbulence Grid in BMT No.5 Tunnel

Spectrum of Vertical Velocity for Grid Turbulence compared with von Karman ‘Theoretical’ Spectra.

Effect of turning on LE and TE flaps under 2nd order control with feedback from heave displacement.

Pitch feedback to LE Flap and Heave feedback to TE Flap

Application of Strip Theory for Buffet Loading and Response.(Massaro and Graham, J Fluids & Structures 2015)

Theoretical Admittances of Lift for Rigid Bridge Section in Grid Turbulence.

Theoretical and Measured Lift Spectra for ‘Rigid’ Bridge Deck in Grid Turbulence.

Theoretical and Measured Heave and Pitch Spectra, Flexibly Mounted Bridge Deck in Turbulence (Subcritical wind speed).

Block Diagram of Control System (s = Laplace transform variable)

Feed Back

PlantGain

White Noise

Von Karman FilterAerodynamics

Theodorsen Circulation Function

Heave and Pitch

Response

Root Loci of Open Loop System(wind speed swept from 5m/s[blue] to 25m/s[red]

Open Loop Damping Ratio measured for Supercritical Free Stream Speeds (without Turbulence).

Root Loci using 3rd Flutter Controller

Damping ratio versus free stream velocity for different controller gains, TE flap only with pitch feedback.

Measured Damping Ratios with 3rd Flutter Controller,LE and TE flaps

LES computed flow over bridge section.(Y. Ito)

Measured (Static) Aero Derivatives of Lift and Moment for Deck

dCL/da = -5.2

dCM/da = 2.6

Comparison of Measured and Theoretical Unsteady Lift and Pitching Moment Derivatives

dL/dh˙ dL/da˙dL/da

dM/dh˙ dM/da˙ dM/da

Theoretical Mean Square Pitch and Heave Responses in Turbulence (Open Loop and Closed Loop Flutter Controllers)

(a) Pitch (b) Heave

Open Loop

Closed Loop

Open Loop

Closed Loop

Buffet Suppression by TE flap (Buffet optimised controller).

Measured RMS Reductions in Pitch and Heave when closed-loop flap buffet control is applied.

Part Span lengths of deck during erection.

Schematic of Mechanical (‘Passive’) Feedback Control

Concluding Remarks

• Substantial increases in flutter speed and significant suppression of buffet loads on bridge decks are possible using active, distributed flap control.

• Both ‘leading edge’ and ‘trailing edge’ flaps can contribute, but in practice there are important separation issues associated with sharp edged leading edge flaps and

rounding trailing edges of flaps reduces effectiveness, (a

problem for bridges if extreme winds from either side).

• A purely mechanical system (under evaluation) can replace electronic control and input power actuation.