Computational Fluid Dynamics

Wind Tunnels

Large tubes with air moving inside. They are used to copy the actions of an object in flight.

Francis Herbert “Frank” Wenham

A British marine engineer, was credited to be the first to introduce a proper wind tunnel.

Frank Wenham and Jim Browning

Both members of the Aeronautical Society of Great Britain, designed and built the “first” ever tunnel at Messrs.

Cross-section area = 18 in²
Length = 120 in
Drive = fan blower driven by a steam engine
Max. air velocity = 60 ft/s

The specifications of Wenham & Browning’s Wind Tunnel

Horatio Frederick Phillips

An Englishman, began experimenting with curved lifting surfaces in the 1880s to quantitatively demonstrate George Cayley’s theory related to cambered airfoils using, in part, a steam injection wing tunnel he constructed in 1844. This is by the means of a stream jet, hoping to avoid the fluctuations of the wind.

Cambered airfoils

Horatio’s experiments on ______________ were the first systematic study of such shapes.

Cross-section area = 17 in²
Length = 72 in
Drive = stream jet
Max. air velocity = 60 ft/s

The specifications of Phillip’s Wind Tunnel

Henrick Christian Vogt

A Danish mathematician who started developing his theory of lift on a wing as soon as he got back to Denmark from his job in England.

Johannes O. V. Irminger

Vogt’s friend whom he sought assistance when he ultimately made the decision to conduct experiments to validate his idea’s. The director of the Eastern Gas Works, which is situated in Copenhagen, the Danish capital.

Vogt’s and Irminger’s Wind Tunnel

Their wind tunnel became the first in the world to measure the pressure distribution on an airfoil.

Cross-section area = 9 × 4.5 in
Length = 40 in
Drive = draft from smoke stack
Max. air velocity = 24-48 ft/s

The specifications of Vogt’s and Irminger’s Wind Tunnel

Sir Hiram Maxim

In the mid-1890s, he returned to aviation after his experiences with the whirling arm, building his enormous aircraft. His wind tunnel, which was operational in 1896, was one of the testing apparatuses.

Cross-section area = 36 in²
Length = 144 in
Drive = drive box (48 in²) with twin blower fan run by a 100 hp stream engine
Max. air velocity = 72 ft/s

The specifications of Sir Hiram Maxim’s Wind Tunnel

Paul la Cour

A scientist, inventor, and teacher from Denmark. Pursued his study on wind mills using wind tunnel technology. His experimental studies utilizing wind tunnels, full scale measurements, control, and storage were his primary contributions to aerodynamics.

Albert Betz

The German physicist and pioneer of turbine technology who also acknowledged Poul la Cour's work in the foreword of his textbook Wind-Energie.

Length (for a and b) = 86 in
Cross-section for a = 39 in diameter
Cross-section for b = 19 in
Air velocity = 33 ft/s

The specifications of Paul la Cour’s Wind Tunnel

Dr Etienne-Jules Marey

A physiologist, chrono photographer, and scientist from France. Recognized for his chronophotographic research on animal movement, he began focusing on taking pictures of moving air in 1899.

Cross-section = 7.87 × 19.69 in
Air velocity = 11.81 in/s

The specifications of Dr Marey’s Wind Tunnel

Dr Albert Francis Zahm

The first comprehensive aeronautical laboratory was created by a math and engineering professor who also served as the head of the US Library of Congress's Aeronautical Division.

Hugo Mattullath

The creator of a gigantic flying boat who provided the funding for the construction of the wind tunnel, and in exchange, Dr. Zahm agreed to work as a consulting engineer for his company in the winter of 1901—albeit only during his free time.

Cross-section = 72 in²
Length = 480 in
Drive = 5 ft suction fan, driven by a 12 hp electric motor
Air velocity = 39 ft/s

The specifications of Dr. Zahm’s wind tunnel

Wilbur and Orville Wright

The two American aviation pioneers who are credited with inventing, building, and flying the world’s successful motor-operated airplane

Wright brother’s first tunnel

It is made up of a driving fan, a two-element balance positioned in the airstream, and a square tube for air channeling

A calibrated level surface and a cambered test surface inclined in the opposite direction at the same angle

Served as the balance elements in Wright brother’s wind tunnel

Vane-type balance

In Wright brother’s wind tunnel it indicated the relative lifting forces as it turned one way or the other when the wind tunnel reached its speed.

Subsonic, Supersonic, Transonic, Hypersonic

Types of wind tunnels according to speed regime

Open circuit, Closed circuit, Intermittent, Continuous

Types of wind tunnels according to geometry

Subsonic

The word "sub" is used because, as their name implies, these flows are below sonic.

Subsonic flows with low speed and subsonic flows with high speed

Two types of subsonic flows

Low-speed subsonic flows

For thin aircraft layouts, air can be considered as a perfect incompressible gas (𝑀<0.3)

High-speed subsonic flows

Flows with a regime defined as 0.3≤𝑀<1.0

Transonic

This happens when the regions of the object’s surface are mixed flow in which the local Mach number is either less or more than one and thus called as sonic pockets. (0.8<M<1.2)

Supersonic

Opposite of subsonic, are flows whose Mach number, defined as the speed of the fluid relative to the sonic speed (M=1) in the same medium, is more than unity, hence the use of prefix “super”. (𝟏.𝟎−𝟏.𝟐<𝑴<𝟓)

Shockwaves

Very small regions in the gas (about 10−5 𝑖𝑛𝑐ℎ𝑒𝑠) where the gas properties change.

Hypersonic

When the free stream Mach number is increased to higher supersonic speeds, the oblique shock shift closer to the object’s surface. At the same time, the pressure, temperature and density across the shock increase explosively at a point which the molecules of the air that surround the aircraft start to change by breaking apart. So, the flow field between the shock and object becomes hot enough to ionize the gas. (M>5.0)

Open Circuit Wind Tunnel

Using this arrangement, air is collected upstream and accelerated in a contraction using a convergent section conduit. The air then passes through the test section before being released straight into the atmosphere.

Open return or Open circuit wind tunnel

Also known by the names N.P.L. (after the National Physical Laboratory in England), which is where the tunnel was initially utilized, and Eiffel tunnel (after the French engineer).

• Low construction cost

• Superior design for propulsion and smoke visualization. There is no accumulation of exhaust products in an open tunnel.

Advantages of open circuit wind tunnel

• Poor flow quality possible in the test section.

• High operating costs. The fan must continually accelerate flow through the tunnel.

• Noisy operation.

Disadvantages of open circuit wind tunnel

Closed return wind tunnel

A type of wind tunnel geometry where, contrary to open circuits, the air circulates continuously within the wind tunnel. Also called as Prandtl tunnel, after the German engineer, or Göttingen tunnel after the research laboratory in Germany where this type of tunnel was first used.

• Superior Flow Quality

• Low Operating Costs

• Quiet Operation

Advantages of closed return wind tunnel

• Higher construction costs due to additional vanes and ducts

• Inferior design for propulsion and smoke visualization

• Hotter running conditions

Disadvantages of closed return wind tunnels

Continuous wind tunnel

The compressor continuously adds energy to the flow to allow continuous airflow through the tunnel. As a result, the air is continuously heated.

Compressors

In continuous tunnels, usually not equipped with after coolers for removing the compression heat; hence, a special cooler is required to avoid a continuous increase of the air temperature in the test section.

Intermittent wind tunnel

Induction-type supersonic wind tunnel for investigations into subsonic and supersonic flow. This includes tests on the flow around two-dimensional models at subsonic and supersonic air speeds.

Intermittent wind tunnels

Make use of surging air flow at a particular set of intervals.

• Intermittent blow-down tunnels

• Intermittent in-draft tunnels

• Intermittent pressure-vacuum tunnels

Sub-types of Intermittent wind tunnels

Test section assembly

The most delicate part of the tunnel because it houses the model/object tested, hence the reason for its name, as well as a portion of the balances that measures lift and drag.

Contraction cone

The part of the wind tunnel where the airflow accelerates. A section of the tunnel where its area converges from a larger cross-sectional area to a smaller one, directly attached to the wind tunnel’s test section.

Continuity Equation

Using this we can reason out that as the cross-sectional area of the tunnel decreases, the velocity of the air moving along the flow tends to increase.

Diffuser

A divergent section in between the test section and the suction fan located downstream of the tunnel. The utilization of this drastically lowers the power required for the operation of the installation.

Bernoulli’s Law

The concept behind the diffuser innovation where pressure and velocity are inversely proportional. Therefore, by reducing the velocity 𝑉2 of air in the diffuser section, in turn has the effect of compressing the air, increasing pressure 𝑃2.

Drive section

Drives the flow of air through the wind tunnel by producing an increase in pressure in the flow. It determines how the working fluid is moved through the test section.

Test section, Contraction cone, Diffuser, Drive section

Wind tunnel elements

Finite Element Analysis (FEA)

A numerical method for finding approximate solutions to boundary value problems for partial differential equations. It is widely used in engineering for simulating physical phenomena such as heat transfer, fluid flow, and stress analysis.

Inviscid (non-viscous) flow and Viscous flow

Types of flow

Viscosity

The ability of a fluid to withstand shearing pressures and its propensity to stick to solid surfaces

Dynamic viscosity and Kinematic viscosity

Two (2) related Measures of Fluid Viscosity

Dynamic viscosity

One way to measure a fluid’s resistance to flow when an external force is applied;

Kinematic viscosity

Another way to measure the resistive flow of a fluid when no external force is applied except the influence of weight.

Skin friction drag

The shear stress has dimensions of force/area and acts in a direction tangential to the surface 𝜏𝜔 gives rise to a drag force

1. The smoothness of the flow approaching the body,

2. the shape of the body,

3. surface roughness,

4. the pressure gradient,

5. Reynolds number, and

6. heating of the fluid by the surface.

Some factors that affect the type of flow in the boundary layer

Laminar flow and Turbulent flow

Two basic types of Viscous flows

Laminar flow

Flow in which the streamlines are smooth and regular and the fluid element moves smoothly along the streamline.

Turbulent flow

Flow in which the streamlines break up and a fluid element moves in a random, irregular and tortuous fashion.

Reynolds number

A measure of the ratio of inertia forces to viscous forces. Can be used to assess how comparable the aerodynamic flows are in a body and its scaled counterpart. This value can also be used to determine whether the boundary layer is turbulent or laminar in whole or in part.

Laminar flow aerofoil

Laminar flow can be obtained in the design of the shape of an aerofoil. The special type of aerofoil designed to encourage laminar flow

Standard shape aerofoils

More prone to have turbulent boundary layers and hence greater skin friction drag.

• Low skin friction drag.

• Has excellent high-speed properties, postponing to a higher flight Mach number the large drag rise due to shock waves and flow separation encountered near Mach 1.

Advantages of laminar aerofoil

Readily gets unstable and tries to change to turbulent flow.

Disadvantage of laminar aerofoil

Transition point

A point in the flow where it changes or “transitions” from laminar flow to turbulent flow. Its location in the chord is indicated by the critical chord 𝑥𝑐𝑟 derived from the critical Reynolds number.

Flow of fluids

Can be analysed by theory, numerical computation and experiment.

Visualization

An important tool in experimental fluid mechanics, which can provide an overall picture of the flow field.

Oil film visualization

Has become a standard wind tunnel technique since several decades of wind tunnel practice

Pressure Sensitive Paint

Essentially a luminescent dye dispersed in an oxygen permeable binder, also known as oxygen quenching.

• Smoke visualization

• Visualization by dye in water

Common techniques in tracer particles flow visualization

• Shadowgraph

• Schlieren

Common techniques in optical methods flow visualization

Shadowgraph

Scientifically explored first by Dvorak, is the simplest of the optical visualization methods. It does not require any optical element except a light source, and the shadow effect produced by refractive index fields can be observed, therefore, outside a laboratory in the open air where the sun serves as the light source.

Schlieren system

Similar to the shadowgraph technique, but the primary difference is that while shadowgraphs are sensitive to changes in the second derivative in density. This is much more sensitive with respect to density changes

Schlieren head

The parallel light beam is made convergent by a lens or a spherical (parabolic) mirror called

Mesh Generation and Stress Analysis

Basic concepts and principles of FEA

Mesh Generation

FEA involves dividing a complex geometry into a collection of simple elements connected at points called nodes.

Stress Analysis

It enables the approximation of stress and strain variations within an object under various loading conditions.

Engineering Design, Cost Savings, Risk Reduction

Applications and benefits of FEA

Engineering Design

FEA helps to optimize the design of products by simulating how they will perform under various conditions.

Cost Savings

It reduces the need for physical prototypes, saving time and resources during product development.

Risk Reduction

Identifying potential failures early in the design process helps reduce risks and improves product reliability.

Fluid Flow Analysis, Simulation Capabilities, Industry Applications

Overview of Computational Fluid Dynamics

Fluid Flow Analysis

CFD is a branch of fluid mechanics that uses numerical methods and algorithms to analyze and solve problems that involve fluid flows.

Simulation Capabilities

It can model complex flow behaviors, such as turbulence, heat transfer, and multiphase flows.

Industry Applications

CFD is widely applicable in aerospace, automotive, energy, and environmental engineering.

1. Problem Set-up

2. Numerical solution

3. Post-processing

Introduction to fluid flow simulations

Problem set-up

Defining the geometry, boundary conditions, and fluid properties for the simulation.

Numerical solution

Using discretization methods to solve the governing equations and obtain the flow field.

Post-processing

Visualizing and analyzing the simulation results to gain insights into the fluid behavior.

ANSYS Fluent

A popular CFD software with robust simulation capabilities for a wide range of fluid flow problems.

OpenFOAM

An open-source CFD toolbox offering powerful solvers and utilities for computational fluid dynamics.

Siemens Simcenter STAR-CCM+

Providing comprehensive multiphysics simulation tools to model fluid flow, heat transfer, and solid mechanics.

1. Engineering Design

2. Powerful Tools

3. Integration

Overview of Solidworks Software

Engineering Design

Solidworks is widely used in engineering for 3D modeling and simulation of mechanical components and systems.