Earlier this summer, when we were walking the South West Coastal Path [see ‘The Salt Path‘ on August 14th, 2019], we frequently saw kestrels hovering above the path ahead of us. It is an enthralling sight watching them use the air currents around the cliffs to soar, hang and dive for prey. Their mastery of the air looks effortless. What you cannot see from the ground is the complex motion of their wing feathers changing the shape and texture of their wing to optimise lift and drag. The base of their flight feathers are covered by small flexible feathers called ‘coverts’ or ‘tectrix’, which in flight reduce drag by providing a smooth surface for airflow. However, at low speed, such as when hovering or landing, the coverts lift up and the change the shape and texture of the wing to prevent aerodynamic stalling. In other words, the coverts help the airflow to follow the contour of the wing, or to remain attached to the wing, and thus to generate lift. Aircraft use wing flaps on their trailing edges to achieve the same effect, i.e. to generate sufficient lift at slow speeds, but birds use a more elegant and lighter solution: coverts. Coverts are deployed passively to mitigate stalls in lower speed flight, as in the picture. When I was in the US last month [see ‘When upgrading is downgrading‘ on August 21st, 2019], one of the research reports was by Professor Aimy Wissa of the Department of Mechanical Science & Engineering at the University of Illinois Urbana-Champaign, who is working on ‘Spatially distributed passively deployable structures for stall mitigation‘ in her Bio-inspired Adaptive Morphology laboratory. She is exploring how flaps could be placed over the surface of aircraft wings to deploy in a similar way to a bird’s covert feathers and provide enhanced lift at low speeds. This would be useful for drones and other unmanned air vehicles (UAVs) that need to manoeuvre in confined spaces, for instance in cityscapes.
I must admit that I had occasionally noticed the waves of fluttering small feathers across the back of a bird’s wing but, until I listened to Aimy’s presentation, I had not realised their purpose; perhaps that lack of insight is why I specialised in structural mechanics rather than fluid mechanics with the result that I was worrying about the fatigue life of the wing flaps during her talk.
A few weeks ago we went to the Tate Liverpool with some friends who were visiting from out of town. It was my second visit to the gallery in as many months and I was reminded that on the previous visit I had thought about writing a post on a painting called ‘Bottle and Fishes’ by the French artist, Georges Braque. It’s an early cubist painting – the style was developed by Picasso and Braque at the beginning of the last century. The art critic, Louis Vauxcelles coined the term ‘cubism’ on seeing some of Braque’s paintings in 1908 and describing them as reducing everything to ‘geometric outlines, to cubes’. It set me thinking about how long it took the engineering world to catch on to the idea of reducing objects, or components and structures, to geometric outlines and then into cubes. This is the basis of finite element analysis, which was not invented until about fifty years after cubism, but is now ubiquitous in engineering design as the principal method of calculating deformation and stresses in components and structures. An engineer can calculate the stresses in a simple cube with a pencil and paper, so dividing a structure into a myriad of cubes renders its analysis relatively straightforward but very tedious. Of course, a computer removes the tedium and allows us to analyse complex structures relatively quickly and reliably.
So, why did it take engineers fifty years to apply cubism? Well, we needed computers sufficiently powerful to make it worthwhile and they only became available after the Second War World due to the efforts of Turing and his peers. At least, that’s our excuse! Nowadays the application of finite element analysis extends beyond stress fields to many field variables, including heat, fluid flow and magnetic fields.
Not a decision to give up caffeine until the festive season but a remark by my Italian research student as he finished his cup of coffee on the flight back to England. He doesn’t consider what we serve in the UK to be coffee and he won’t be back in Italy until the Christmas vacation. We were in Italy visiting the laboratory with which we are collaborating on his research project. He is right, the coffee gets much better as you move south and east from the US and UK.
Next time you are enjoying a cup of coffee watch the swirls created as you or a friend stirs in some cream. You can see streak lines that show the path of the cream in the coffee and reveal the fluid flow in your cup. It is even better if you have a clear glass. You can use this as an Everyday Engineering Example to capture students’ attention and to illustrate the kinematics of fluids as in the 5E lesson plan below.