1. Linné FLOW Centre Research on Ekman at the Linné Flow Center, KTH Mechanics Dan Henningson, Director

2. Linné FLOW Centre http://www.flow.kth.se ● Funded by VR as an one of the 20 original Linné centers of excellence

3. Linné FLOW Centre ● Vision “FLOW as an outstanding environment for fundamental research in fluid mechanics, where innovative research is born and future research leaders are fostered”

4. Linné FLOW Centre Activities and infrastructure ● Research projects ● Graduate school ● Summer programs ● Workshops/conferences ● Seminars and visiting scientists ● Infrastructure − World class wind-tunnels and acoustic measurement facilities − Climate and Turbulence computer Ekman

5. Linné FLOW Centre Research areas ● Why do flows become turbulent? ● How does turbulence behave in the ocean and atmosphere? ● Why does turbulence generate noise? ● How can we make flows behave the way we want to? ● Why do fibers bundle?

6. Linné FLOW Centre Turbulent flows are everywhere, and they can be described by … From www.efluids.com

7. Linné FLOW Centre How do we perform numerical experiments? ● Solve the Navier-Stokes equations for the velocity on grid points using super computers Cray-1 (1976) 100 Mflops 1 processor Ekman Dell cluster (2008) 100 Tflops 12000 processors

8. Linné FLOW Centre Efficient simulations on many processors # processors speed-up SIMSON Nek5000 How much faster does the simulation run on many processors? linear scaling measurements (IBM BG/L)

9. Linné FLOW Centre How can simulations help make modern aircraft more environmentally friendly? ● Suppressing turbulence on the wings (laminar flow control) improve fuel efficiency ● Better models of turbulence on wing surfaces improve engineering design ● Example: Airbus green aircraft concept EU NACRE project: Concept for quiet, light fuel efficient aircraft

10. Linné FLOW Centre Direct Numerical Simulations of all scales in the turbulent flow (no models) Turbulence close to the surface  Friction  Drag  Fuel consumption

11. Linné FLOW Centre Simulations: streamwise velocity in wall-parallel plane 20 cm x 0.8 cm, simulation on BlueGene at PDC Direct Numerical Simulations of turbulent flow ● 6144 x 385 x 576 = 1.4 billion grid points Large range of scales require huge number of gridpoints

12. Linné FLOW Centre What can we learn from even larger simulations? ● 12288 x 721 x 1152 = 11 billion grid points ● Overlap with large scale experiments Large scale experiments Re Domain size Ekman Previous simulations Ciclope project: large pipe flow facility in 130 m Italian tunnel

13. Linné FLOW Centre What can we learn from even larger simulations? ● 12288 x 721 x 1152 = 11 billion grid points ● Overlap with large scale experiments − Dynamics of large scale turbulent structures and their interaction with small near-wall structures − Provide quantities difficult to measure, input in engineering models of turbulence − Independent prediction of quantities like drag − How to best suppress turbulence on the wing

14. Linné FLOW Centre How will the ocean circulation respond to global warming? ● Ocean large heat regulator of climate ● Great conveyor belt transport warm surface water to north pole and cold water back along bottom ● Circulation affected by global warming?

15. Linné FLOW Centre The ocean is turbulent! Simulation: ECCO code using MITgcm. JPL. NASA Ames

16. Linné FLOW Centre The global circulation is very sensitive to the turbulent diffusivity (Nilsson et al., MISU) There is even turbulence at centimeter scale! Smaller scales determine turbulent diffusivity, how fast is the cold water cooling the warmer water above ?

17. Linné FLOW Centre What are we doing now? Direct Numerical Simulation of ocean turbulence 2048 x 2048 x 384 = 1.6 billion grid points

18. Linné FLOW Centre What will we do? Larger DNS of ocean turbulence 4096 x 4096 x 1024 = 17 billion grid points Ekman may give answer on turbulent diffusivity in ocean Simulation: Kelvin- Helmholz breakdown by Colorado Res. Ass. bridge gap between larger scales affected by density differences and smaller isotropic scales

19. Linné FLOW Centre Simulations on Ekman will help fill the gap between smallest scales accounted for in climate and engineering models and those of importance in nature Examples … ● Simulations of ocean turbulence to predict turbulent diffusivity influencing how the ocean responds to global warming ● Simulations of turbulence in boundary layers on aircraft and ground vehicles to improve engineering predictions of drag ● Simulations of the onset of turbulence on wings to develop laminar flow control and lower fuel consumption