O​rganiser:
Jonathan Morrison,
Department of Aeronautics, Imperial College, London

The meeting will consist of a series of introductory talks followed by short presentations on the most recent results concerning three-dimensional wakes of bluff body geometries featuring wide recirculating flow from laminar to turbulent regimes. Much progress has been done on this topic since the creation of this SIG, that was primarily motivated by the new observations of steady symmetry breaking modes - already 10 years ago! A review and update of progress on this topic will be timely to strengthen the research actions about 3D wakes with the European network offered by the ERCOFTAC framework.
The final session of this kick-off workshop will be dedicated to a discussion to refine the activities and goals of the SIG47.

Background

Vehicles in air and water are subjected to a resistance force (drag) opposite to their motion requiring a significant power supply to sustain their velocity. This is all the truer as most vehicles are not streamlined-bodies but bluff-bodies (cars, trucks, ship superstructures, air taxis). Compared to a streamlined body (e.g. an elongated ellipsoid), the drag of a bluff body (e.g. a sphere) can be dozens of times larger while their projected area and velocity fields are identical. The high drag of a bluff body in incompressible flow (that is the case for all examples cited above) is caused by a low-pressure imprint at their base produced by the massive rear flow separation forming principally a recirculation region of slow fluid motion bounded by high activity turbulent shears and shedding large scale structure (see Fig. 1). The understanding of the underlying flow physics remains a fundamental challenge and it is not yet possible to accurately predict the resulting aerodynamic force on the body. The reason is the lack of a theory for the wake contribution due the intricate combination of flow separation, local and global instabilities, recirculating flow and turbulence – irrespective of the body geometry. For instance, the symmetry properties of the recirculation region and their drag consequences have only been revealed 10 years ago for bullet shape and rectangular base bodies [JFM 2013, JFM 2014].

Meeting objectives

There are many different methodologies to study flows. Whether they are theoretical, experimental, or computational, they help to bring comprehension to elaborate flow models and propose efficient control solutions. The aim of the meeting is definitely to put together researchers in order to have the best representation of all of these skills but with a specific focus on a major aerodynamics feature: the wake of 3D bodies. Discussions breaking the methodologies boundaries are expected to foster collaborations such as experimental/theoretical or computational/theoretical. In addition to the clear academic objectives targeted by the meeting, there is a long-term goal for training aerodynamicists to achieve the most energy efficient future transport designs based on up-to-date fundamental knowledge, appropriate representative mathematical tools and the most relevant decision making.

The programme of the meeting will first discuss laminar transitions (S1) operating at low Reynolds number as they benefit from a rigorous theoretical background (no turbulent modelling). The session will provide recent achievements in stability analyses and recently associated tools. A second session (S2) at larger Reynolds number will involve more the experimental and computational approaches with the modelling. A third session (S3) will investigate the flow control techniques to reduce, for instance, the drag. S4 will be used to summarise the challenges and identify future goals.The programme is given below with some proposed introductory lectures (proposed invited speaker's name is underlined). Sessions will include presentations from other attendees with a preference for younger researchers.

Programme:

S1 Laminar transitions
S2 Turbulent dynamic and stability
S3 Wake control
S4 Workshop questions

Introductory lectures:

S1: Zampogna G.A., Boujo E. "From thin plates to Ahmed bodies: Linear and weakly nonlinear stability of rectangular prisms." (2023) Journal of Fluid Mechanics, 966, art. no. A19

S2a: Callaham J.L., Rigas G., Loiseau J.-C., Brunton S.L." An empirical mean-field model of symmetry-breaking in a turbulent wake, " (2022) Science Advances, 8 (19), art. no. eabm4786

S2b:Podvin B., Pellerin S., Fraigneau Y., Bonnavion G., Cadot O. "Low-order modelling of the wake dynamics of an Ahmed body" (2021) Journal of Fluid Mechanics, 927, art. no. R6

S3: Camacho-Sánchez J.M., Lorite-Díez M., Jiménez-González J.I., Cadot O., Martínez-Bazán C. "Experimental study on the effect of adaptive flaps on the aerodynamics of an Ahmed body, " (2023) Physical Review Fluids, 8 (4), art. no. 044605

S4: What progress has been made in the past ten years?

1. What are the key outstanding questions to be answered?
2. What simulations and experiments are required to address these questions – simulations requirements, key experiments?
3. Control of coupled modes for drag reduction?
4. Relevant potential funding initiatives?
5. SIG47 relationship to other groups - Ground Vehicle Aerodynamics UKFN Special Interest Group?