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On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers

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Abstract

Force measurements have been undertaken at low Reynolds number (circa 50k to 220k) on a highly cambered thin circular arc with a sharp leading edge in order to investigate the laminar to turbulent transition of modern asymmetric spinnakers. Evidence of a critical Reynolds number inducing transition has been provided, as well as the existence of a velocity dependent critical angle of attack triggering transition at Reynolds numbers lower than the critical. Moreover, the ideal angle of attack for the tested geometry has been assessed, challenging the assumed value in the literature. The results also lead to reconsider current knowledge and enable to explain discrepancies previously notice in the wind tunnel testing of yacht sails, thus allowing for a new interpretation of published research. In addition, it will allow to refine the setup of future experimental testing of model spinnakers to ensure the appropriate flow regime, representative of the full-scale sail, will be replicated.
On Highly Cambered Thin Circular Arcs at
Low Reynolds Numbers
Jean-Baptiste R. G. Souppez
Institute for Energy Systems (IES)
jean-baptiste.souppez@ed.ac.uk
Ignazio Maria Viola
Institute for Energy Systems (IES)
i.m.viola@ed.ac.uk
UK Fluids Conference Manchester 4-6 September 2018
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
CONTENTS
BACKGROUND
LITERATURE REVIEW
CIRCULAR ARC GEOMETRY
FORCE MEASUREMENTS
FLOW DIAGNOSTICS
FINDINGS
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
BACKGROUND - SPINNAKERS
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
BACKGROUND WIND TUNNEL TESTING
Wind Tunnel assumption: the flow around
the spinnaker is turbulent (at the scales
typically tested in dedicated wind tunnel
facilities).
Inconsistencies noticed in the pressure
distribution on wind tunnel tested models.
Potential evidence of transition occurring in
the literature.
Highly cambered thin circular arc as a
simplified cross section through a spinnaker.
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
At low Reynolds number, evidence of a discontinuity in the lift and drag (Lombardi, 2014).
Abrupt change in the location of the separation point at the same angle (Martin, 2015).
Hypothesis: there is a combination of critical Reynolds Number and critical Angle of
Attack that will trigger transition.
CIRCULAR ARCS IN THE LITERATURE
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
CIRCLAR ARC GEOMETRY AND MANUFACTURING
Specifications:
Highly cambered: 22.32%
Thin: 1.8mm thick
Chord: 200m
Sharp leading edge
Manufacturing:
Carbon prepreg
Lebret, 2013
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
TOWING TANK EXPERIMENTAL SETUP
Force measurements undertaken in Solent University’s Hydrodynamic Test Centre:
Reynolds numbers: 53k, 68k, 150k and 220k
Angles of Attack: 5 to 20 (5 to 25 at 53k) in 1 degree increments
Angle of attack of 11 degrees for 130k < Re < 160k
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
TOWING TANK RESULTS RE = 53K
Compared with Velychko’s (2014) wind tunnel experiment.
Good agreement between the towing tank and the wind tunnel.
Velychko, 2014
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
TRANSITION: SEPARATION AND WAKE SIZE RE = 68K
Delayed separation consistent with the values of Martin (2015) and reduced wake,
characteristic of laminar to turbulent transition
AoA = 14°
Sub-critical
Souppez & Viola (2018)
AoA = 15°
Super-critical
Souppez & Viola (2018)
x/c = 46% x/c = 55%
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
TRANSITION: TURBULENT KINETIC ENERGY RE = 68K
Further evidence of the transition can be found by computing the TKE.
Sub-critical
Souppez & Viola (2018)
Separation point
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
TRANSITION: TURBULENT KINETIC ENERGY RE = 68K
Further evidence of the transition can be found by computing the TKE.
Super-critical
Souppez & Viola (2018)
Separation point
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
IDEAL ANGLE OF ATTACK
PIV employed to locate stagnation points at 8,
9, 10,11 and 12 degrees angle of attack.
Revealed 11 degrees is the ideal angle of
attack, i.e. the minimum required to inflate a
soft membrane such as a spinnaker.
On Highly Cambered Thin Circular Arcs at Low Reynolds Numbers UK Fluids Conference Manchester 4-6 September 2018
FINDINGS
Below Re = 218k, there is a combination of
Reynolds number and angle of attack that will
induce transition, demonstrated with PIV.
Ideal angle of attack occurs a 11 degrees, with a
critical Reynolds number of 144k.
Idealised model for the lift coefficient of highly
cambered thin circular arcs.
Offers a new interpretation of the data gathered
in previously tested yacht sails.
Challenges current knowledge and practice in
Wind Tunnel Testing of downwind yacht sails.
Thank You
UK Fluids Conference Manchester 4-6 September 2018
Jean-Baptiste R. G. Souppez
Institute for Energy Systems (IES)
jean-baptiste.souppez@ed.ac.uk
Ignazio Maria Viola
Institute for Energy Systems (IES)
i.m.viola@ed.ac.uk
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