The Navier-Stokes Equation

The name we use for our little blue planet “Earth” is rather misleading. Water makes up about 71% of Earth’s surface while the other 29% consists of continents and islands. In fact, this patchwork of blue and brown, earth and water, makes our planet very unlike any other planet we know to be orbiting other stars. The word “Earth” is related to our longtime worldview based on a time when we were constrained to travelling the solid parts of our planet. Not until the earliest seaworthy vessels, which were believed to have been used to settle Australia some 45,000 years ago, did humans venture onto the water.

Not until the 19th century did humanity make a strong effort to travel through another vast sea of fluid, the atmosphere around us. Early pioneers in China invented ornamental wooden birds and primitive gliders around 500 BC, and later developed small kites to spy on enemies from the air. In Europe, the discovery of hydrogen in the 17th century inspired intrepid pioneers to ascend into the lower altitudes of the atmosphere using rather explosive balloons, and in 1783 the brothers Joseph-Michel and Jacques-Étienne Montgolfier demonstrated a much safer alternative using hot-air balloons.

The pace of progress accelerated dramatically around the late 19th century culminating in the first heavier-than-air flight by Orville and Wilbur Wright in 1903. Just 7 years later the German company DELAG invented the modern airline by offering commercial flights between Frankfurt and Düsseldorf using Zeppelins. After WWII commercial air travel shrunk the world due to the invention and proliferation of the jet engine. Until a series of catastrophic failures the DeHavilland Comet was the most widely-used aircraft but was then superseded in 1958 by one of the iconic aircrafts, the Boeing 707. Soon military aircraft began exploring the greater heights of our atmosphere with Yuri Gagarin making the first manned orbit of Earth in 1961, and Neil Armstrong and Buzz Aldrin walking on the moon in 1969, a mere 66 years after the first flight at Kittyhawk by the Wright brothers.

Air and space travel has greatly altered our view of our planet, one from the solid, earthly connotations of “Earth” to the vibrant pictures of the blue and white globe we see from space. In fact the blue of the water and the white of the air allude to the two fluids humans have used as media to travel and populate our planet to a much greater extent than travel on solid ground would have ever allowed.

Fundamental to the technological advancement of sea- and airfaring vehicles stood a physical understand of the media of travel, water and air. In water, the patterns of smooth and turbulent flow are readily visible and this first sparked the interest of scientists to characterise these flows. The fluid for flight, air, is not as easily visible and slightly more complicated to analyse. The fundamental difference between water and air is that the latter is compressible, i.e. the volume of a fixed container of air can be decreased at the expense of increasing the internal pressure, while water is not. Modifying the early equations of water to a compressible fluid initiated the scientific discipline of aerodynamics and helped to propel the “Age of Flight” off the ground.

One of the groundbreaking treatises was Daniel Bernoulli’s Hydrodynamica published in 1738, which, upon other things, contained the statement many of us learn in school that fluids travel faster in areas of lower than higher pressure. This statement is often used to incorrectly explain why modern fixed-wing aircraft induce lift. According to this explanation the curved top surface of the wing forces air to flow quicker, thereby lowering the pressure and inducing lift. Alas, the situation is slightly more complicated than this. In simple terms, lift is induced by flow curvature as the centripetal forces in these curved flow fields create pressure gradients between the differently curved flows around the airfoil. As the flow-visualisation picture below shows, the streamlines on the top surface of the airfoil are most curved and this leads to a net suction pressure on the top surface. In fact, Bernoulli’s equation is not needed to explain the phenomenon of lift. For a more detailed explanation of why this is so I highly recommend the journal article on the topic by Dr. Babinsky from Cambridge University.

Flow lines around an airfoil (Source: Wikimedia Commons https://en.wikipedia.org/wiki/File:Airfoil_with_flow.webp)

Just 20 years after Daniel Bernoulli’s treatise on incompressible fluid flow, Leonard Euler published his General Principles of the Movement of Fluids, which included the first example of a differential equation to model fluid flow. However, to derive this expression Euler had to make some simplifying assumptions about the fluid, particularly the condition of incompressibility, i.e. water-like rather than air-like properties, and zero viscosity, i.e. a fluid without any stickiness. While, this approach allowed Euler to find solutions for some idealised fluids, the equation is rather too simplistic to be of any use for most practical problems.

A more realistic equation for fluid flow was derived by the French scientist Claude-Louis Navier and the Irish mathematician George Gabriel Stokes. By revoking the condition of inviscid flow initially assumed by Euler, these two scientists were able to derive a more general system of partial differential equations to describe the motion of a viscous fluid.

$Title: \rho\left(\frac{\partial\boldsymbol{v}}{\partial t}+\boldsymbol{v}\cdot\nabla\boldsymbol{v}\right)=-\nabla p+\nabla\cdot\boldsymbol{T}+\boldsymbol{f} - Description: \rho\left(\frac{\partial\boldsymbol{v}}{\partial t}+\boldsymbol{v}\cdot\nabla\boldsymbol{v}\right)=-\nabla p+\nabla\cdot\boldsymbol{T}+\boldsymbol{f}$

The above equations are today known as the Navier-Stokes equations and are infamous in the engineering and scientific communities for being specifically difficult to solve. For example, to date it has not been shown that solutions always exist in a three-dimensional domain, and if this is the case that the solution in necessarily smooth and continuous. This problem is considered to be one of the seven most important open mathematical problems with a $1m prize for the first person to show a valid proof or counter-proof.

Fundamentally the Navier-Stokes equations express Newton’s second law for fluid motion combined with the assumption that the internal stress within the fluid is equal to diffusive (“spreading out”) viscous term and the pressure of the fluid – hence it includes viscosity. However, the Navier-Stokes equations are best understood in terms of how the fluid velocity, given by $Title: \boldsymbol{v} - Description: \boldsymbol{v}$ in the equation above, changes over time and location within the fluid flow. Thus, $Title: \boldsymbol{v} - Description: \boldsymbol{v}$ is an example of a vector field as it expresses how the speed of the fluid and its direction change over a certain line (1D), area (2D) or volume (3D) and with time .

The other terms in the Navier-Stokes equations are the density of the fluid $Title: \rho - Description: \rho$ , the pressure , the frictional shear stresses $Title: \boldsymbol{T} - Description: \boldsymbol{T}$ , and body forces $Title: \boldsymbol{f} - Description: \boldsymbol{f}$ which are forces that act throughout the entire body such as inertial and gravitational forces. The dot is the vector dot product and thenabla operator $Title: \nabla - Description: \nabla$ is an operator from vector calculus used to describe the partial differential in three dimensions,

$Title: \nabla = \left(\frac{\partial}{\partial x},\frac{\partial}{\partial y},\frac{\partial}{\partial z}\right) - Description: \nabla = \left(\frac{\partial}{\partial x},\frac{\partial}{\partial y},\frac{\partial}{\partial z}\right)$

In simple terms, the Navier-Stokes equations balance the rate of change of the velocity field in time and space multiplied by the mass density on the left hand side of the equation with pressure, frictional tractions and volumetric forces on the right hand side. As the rate of change of velocity is equal to acceleration the equations boil down to the fundamental conversation of momentum expressed by Newton’s second law.

One of the reasons why the Navier-Stokes equation is so notoriously difficult to solve is due to the presence of the non-linear $Title: \boldsymbol{v}\cdot\nabla\boldsymbol{v} - Description: \boldsymbol{v}\cdot\nabla\boldsymbol{v}$ term. Until the advent of scientific computing engineers, scientists and mathematicians could really only rely on very approximate solutions. In modern computational fluid dynamics (CFD) codes the equations are solved numerically, which would be prohibitively time-consuming if done by hand. However, in some complicated practical applications even this numerical approach can be become too complicated such that engineers have to rely on statistical methods to solve the equations.

The complexity of the solutions should not come as a surprise to anyone given the numerous wave patterns, whirlpools, eddies, ripples and other fluid structures that are often observed in water. Such intricate flow patterns are critical for accurately modelling turbulent flow behaviour which occurs in any high velocity, low density flow field (strictly speaking, high Reynolds number flow) such as around aircraft surfaces.

Nevertheless, as the above simulation shows, the Navier-Stokes equation has helped to revolutionise modern transport and also enabled many other technologies. CFD techniques that solve these equations have helped to improve flight stability and reduce drag in modern aircraft, make cars more aerodynamically efficient, and helped in the study of blood flow e.g. through the aorta. As seen in the linked video, fluid flow in the human body is especially tricky as the artery walls are elastic. Thus, such an analysis requires the coupling of fluid dynamics and elasticity theory of solids, known as aeroelasticity. Furthermore, CFD techniques are now widely used in the design of power stations and weather predictions.

In the early days of aircraft design, engineers often relied on back-of-the-envelope calculations, intuition and trial and error. However, with the increasing size of aircraft, focus on reliability and economic constraints such techniques are now only used in preliminary design stages. These initial designs are then refined using more complex CFD techniques applied to the full aircraft and locally on critical components in the detail design stage. Equally, it is infeasible to use the more detailed CFD techniques throughout the entire design process due to the lengthy computational times required by these models.

Physical wind tunnel experiments are currently indispensable for validating the results of CFD analyses. The combined effort of CFD and wind-tunnel tests was critical in the development of supersonic aircraft such as the Concorde. Sound travels via vibrations in the form of pressure waves and the longitudinal speed of these vibrations is given by the local speed of sound which is a function of the fluids density and temperature. At supersonic speeds the surrounding air molecules cannot “get out of the way” before the aircraft arrives and therefore air molecules bunch up in front of the aircraft. As a result, a high pressure shock wave forms in these areas that is characterised by an almost instantaneous change in fluid temperature, density and pressure across the shock wave. This abrupt change in fluid properties often leads to complicated turbulent flows and can induce unstable fluid/structure interactions that can adversely influence flight stability and damage the aircraft.

The problem with performing wind-tunnel tests to validate CFD models of these phenomena is that they are expensive to run, especially when many model iterations are required. CFD techniques are comparably cheaper and more rapid but are based on idealised conditions. As a result, CFD programs that solve Navier-Stokes equations for simple and more complex geometries have become an integral part of modern aircraft design, and with increasing computing power and improved numerical techniques will only increase in importance over the coming years. In any case, the story of the Navier-Stokes equation is a typical example of how our quest to understand nature has provided engineers with a powerful new tool to design improved technologies to dramatically improve our quality of life.