Linné FLOW Centre

Research on Ekmanat the Linné Flow Center, KTH Mechanics

Dan Henningson, Director

Linné FLOW Centre

http://www.flow.kth.se

● Funded by VR as an one of the 20 original Linné centers of excellence

Linné FLOW Centre

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”

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

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?

Linné FLOW Centre

Turbulent flows are everywhere, and they can be described by …

From www.efluids.com

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 Mflops1 processor

Ekman Dell cluster (2008) 100 Tflops12000 processors

Year

MFl

ops

1970 1980 1990 2000 201010

2

104

106

108

1010

Cray X-MP

NEC SX-5

Earth Simulator

Roadrunner

Neolith

Ekman

Cray-1

Linné FLOW Centre

Efficient simulations on many processors

1

10

100

1000

10000

1 10 100 1000 100001

10

100

1000

10000

1 10 100 1000 10000

# processors # processors

speed-u

pSIMSONNek5000

How much faster does the simulation run on many processors? linear scaling measurements (IBM BG/L)

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

Linné FLOW Centre

Direct Numerical Simulations of all scales in the turbulent flow (no models)

Turbulence close to the surface Friction Drag Fuel consumption

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

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

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

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?

Linné FLOW Centre

The ocean is turbulent!

Simulation: ECCO code using MITgcm.

JPL. NASA Ames

Linné FLOW Centre

The global circulation is very sensitive to the turbulent diffusivity

(Nilsson et al., MISU)

There is even turbulence atcentimeter scale!

Smaller scales determine turbulent diffusivity, how fast is the cold water cooling the warmer water above ?

Linné FLOW Centre

What are we doing now?

Direct Numerical Simulation of ocean turbulence2048 x 2048 x 384 = 1.6 billion grid points

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

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