MINOR PROJECT REPORT
EXPERIMENTAL TECHNIQUES TO MEASURE
AIR VELOCITY IN WIND TUNNEL
Submitted by
Lt NARESH KUMAR (2017 AMX5521)
In partial fulfilment of the requirements of the award of
POST GRADUATE DIPLOMA IN NAVAL CONSTRUCTION
NAVAL CONSTRUCTION WING
DEPARTMENT OF APPLIED MECHANICS
INDIAN INSTITUTE OF TECHNOLOGY DELHI
HAUZ KHAS, NEW DELHI-110016
MAY 2017CERTIFICATE
I, Lt Naresh Kumar (2017AMX5521) hereby solemnly affirm that the project report titled “EXPERIMENTAL TESHNIQUES TO MEASURE AIR VELOCITY IN WIND TUNNEL”, being submitted by me in partial fulfilment of the requirement for the award of post graduate diploma in Naval Construction,to the Indian Institute of Technology Delhi, is a record of bonafide work carried out by me under the guidance of Lt Cdr K Vignesh Kumar. The work reported in this report in full or in part has not been submitted to any University or Institute for the award of any degree or diploma.
Thesis approved by- Thesis accepted by-
(K Vignesh Kumar ) ( M. P. Mathew)
Lt Commander Commander
Project Guide Officer-in-Charge
Naval Construction Wing Naval Construction Wing
Dept of Applied Mechanics Dept of Applied Mechanics
IIT Delhi IIT Delhi
ACKNOWLEDGEMENT
It gives me immense pleasure to express my feelings of gratitude and indebtedness to the various individuals who have been the guiding forces through their expertise, suggestions and words of encouragement throughout this project.
I are deeply indebted to my project guide Lt Cdr K Vignesh Kumar, Dy Officer- in-Charge, Naval Construction Wing, who, through his knowledge, practicality, foresight and uncompromising standards has constantly guided me to aim for the best.
I sincerely thank our staff officers Cdr M P Mathew, Cdr I S Makkar, Cdr Arun E and for their guidance, coordination and suggestions in the various design interviews and for their support and critical appreciation of the project at all stages.
My sincere gratitude would also go to all my course mates and members of the NCW staff who have helped in some way or the other. Finally, I would like to thank my family and loved ones whose prayers and blessings have guided me through the rough patches throughout this project.
( Naresh Kumar)
Lt
2017AMX5521
Place : New Delhi
Date :
Abstract
Nowadays there is a great shift in use of numerical methods for design. Software package having numerical methods as backhand can solve a lot of problem but still they are not able to replace experiments done with scaled down model. Still scientists have more faith in experimental results then results from software packages. So in this project I tried to familiarize myself with experiments done in wind tunnel. In this project familiarization with different available methods to study air velocity in wind tunnel is done. Model is prepared to get velocity and pressure distribution on surface. A full detailed work is done to get best results for data we want to obtain for our model . A detailed work is carried out for setting the model and for interpretation of the result to be obtained. First time results obtained on model are satisfactory. A CFD model is also made to get numerical results for our model.
Keywords-Wind Tunnel, CFD model
TABLE OF CONTENT
S.NO. TOPIC
1. Introduction
2. Methodology
3. Software and its application
4. ` Air velocity measurement
5. Experiment
References
Chapter 1
INTRODUCTION
1.1 HISTORY
1.1.1 The wind tunnel is the reason for this much innovation in aircraft and naval industry. Today not a single designer will take risk to make full scale object without having results of its scaled down model. Scaled down tests help to know deficiency in design and also help in understanding the behaviour of design in actual environmental conditions.
Benjamin Robins (1707-1751), an English mathematician, had employed a whirling arm first time. That machine had an arm of 4 feet length. A falling weight acting on a pulley & spindle arrangement, arm tip was able to reach velocities of few feet per second. After him Sir George Cayley (1773-1857) used a whirling arm. He used it to measure drag and lift in various airfoils. His whirling arm has a length of 5 feet and attained tip speeds between 10-20 feet per second. With the result from test data, Cayley built a glider that was the first successful flight by heavier-than-air vehicle
Wind tunnel is improved version of whirling arm. This simple device consist of an enclosed path through which air is thrown by a fan or any other suitable drive system. The main part of the wind tunnel is it’s test section, in which scaled model is put in a controlled airstream, which generates a flow of air around the model, duplicating that of a full-scale model. The characteristics of the model with its flow field are measured by appropriate balances and with test instrumentation. The wind tunnel's capacity for controlled and systematic testing quickly made the whirling arm obsolete. Unique role and capabilities of wind tunnel can be appreciated by recognizing forces and moments acting the model. A classic case of experiment is, Osborne Reynolds (1842-1912) demonstrated that airflow pattern over scaled model would be same for full-scale vehicle if certain flow parameter were same in both cases. The above mentioned factor, known as Reynolds number, is the basic parameter in description of any fluid flow situations, the shapes of flow patterns, ease of heat transfer, and onset of turbulence.
1.2 MOTIVATION
1.2.1 Indian navy’s policy to become blue water navy and to operate three aircraft carrier. So that two of them are operational at all time in eastern and western command. There were many problems encountered in operation of flights on present aircraft carrier, so an efficient design was needed for better operation of flights and also for new aircrafts Indian navy planning to acquire.
"It is easy to invent a flying machine; more difficult to build one; to make it fly is everything." —– Otto Lilienthal
Similar to above to make a good design and then construct it and then operate aircrafts from it is very difficult. So a detailed design and verification of model is required to make it into a ship. Last year when Prime minister Narendra Modi visited INS Vikramaditya, in schedule it was planned to show him operation of flights on deck but due to bad environment condition it was cancelled. So if this would have happened in in war time then what can be done, so a better design is needed to solve those problems.
So a detailed study of flow of air behind the island and at different plane in the path of landing of flights is required and how it influence the landing operation of flights. So a scaled model is prepared and for studying its properties it was needed to attach the instruments on the model to study the properties of model. So it was needed to modify the main full scale ship to be converted to a scaled down model which can be used in wind tunnel and the properties obtained model can be easily scaled to full scale. It was also done that there is minimum effect of any instrument on the results we want to obtain that mean no interference by the instrument.
1.3 OBJECTIVES
Following are the objectives of the project:-
(a) To familiarize with various experimental technique used in wind tunnel
(b) To carry out experiment to calibrate the air velocity in wind tunnel
(c) To understand the working of wind tunnel
(d) For proper setting of model in tunnel to get the optimum results
(e) To understand the pitot tube and its operation
(f) To understand how to obtain desired results in wind runnel.
Chapter 2
METHODOLOGY
2.1 WIND TUNNEL
2.1.1 Wind tunnels are tubes with air flowing in them. These are used to copy the behaviour of an object in actual condition. They are used by researchers to study the effect of fluids on the object moving in them . They are also used to test scaled model of ship, aircraft, long civil structures and spacecraft. Some of them can hold full-size versions of vehicles also. In wind tunnels air moves around the object which look like actual environment condition.
Powerful fans are used to move air in tunnel. The model to be tested is secured in the tunnel so well that it does not move due to air flow. The model can be a small version of a vehicle. Object can also be a piece of vehicle. Object can be a full-size aircraft, propeller, aerodynamic vehicle, spacecraft or ship. It can even be a normal object like a tennis ball. Air moving around still object reflects what would happen if object was moving in the air. How air movement can be studied in many different ways. Smoke and dye can be put in air and are seen as it moves. Sometime threads can also be attached to see how air is moving around the object. Special instruments are also used to measure force of air on object.
2.1.2 Types of problem frequently modelled are:-
(a) flow past a solid object moving relative to fluid or gas at rest with constant velocity along straight path.
(b) Atmospheric boundary layer
Modelling of above problems are done as
(a) Flow velocity (modelling the relative velocity) in the test volume should be theoretically constant and turbulence intensity is zero.
(b) Velocity is zero close to the lower flat boundary of the test value and increase upward according to the velocity distribution in atmosphere and the vertical distribution of turbulence is prescribed. These characterstics are influenced mainly by roughness of ground.
2.1.3 Types of wind tunnels:-
classification is done according to –
i) Wind velocity
(a) Low speed Ma<0.3
(b) High speed Ma=1
(c) Supersonic Ma>1
(d) Hypersonic Ma>5
ii) To arrangements
(a) Path of air :
– Closed circuit
– Open Circuit
(b) Pressure in test section comparison to ambient pressure
– Pressurized
– Evacuated
– Ambient pressure
(c) Relation of test section and environment
– Open test section
– Closed test section
2.1.4 Types of open circuit wind tunnels:-
(a) Evacuating arrangement
1) Settling chamber
2) Closed test section
3) Diffuser
4) Axial fan
(b) Pressurized arrangement
1) Radial fan
2) Settling chamber
3) Open or closed test section connected to discharge section of fan
4) Diffuser
At both arrangements diffuser is used to reduce power consumption.
In case of open circuit arrangement a part of flow circulation is non-controlled, so the environment influences the flow quality in test section. Vortices can enter in test section by following ways:-
(a) The settling fan
(b) The inlet of radial fan
Arrangement (a) is more sensitive in terms of returning vortices then arrangement (b). This problem can be solved by using closed circuit where the effect of environment can be reduced or exclude. (The reduction of fuel consumption is not significant) Larger wind tunnels are of closed circuit.
Main features of this type of section are
a) Easy access
b) Majority of wind tunnel corrections are necessary
c) At sides of test section shear layer, entrainment
d) Increased turbulence
e) Pressure higher than the ambient and increase in wind direction.
f) At the end of confuser deviation of velocity from axial direction.
2.1.5 Elements of closed circuit wind tunnel:-
1) Test section
2) Diffuser (cone angle is about 5 degree)
3) Corner vanes in corner chamber
4) Diffuser
5) Corner vanes
6) Axial fan
7) Diffuser
8) Corner fan
9) Diffuser
10) Corner vanes
11) Diffuser
12) Settling chamber including honeycombs, woven wire screens
13) confuser
Losses in corner vanes is large part of losses of wind tunnel and influence very much the losses of diffuser, too. Honeycomb: tubes of diameter d and of 5-6d length, it reduces the velocity component perpendicular to axis of tubes. Honeycomb can reduce the uniformity of velocity distribution if there is velocity component perpendicular to tube axis. Upstream and downstream of honeycomb screens woven wires screens improves the uniformity of velocity distribution and reduces the component parallel to the plane of screen. The screen cut the vortices and help in their dissipation. Confusers reduce effectively the change of velocity component parallel to its axis. The ratio of cross section, the rounding up radii play an important role in their effectiveness. Sufficient distance should be secured between woven wire screens and rounding up of confuser. At small distance the screen can decrease the uniformity of velocity distribution.
The cross section of test section is either circular or rectangular. 1:2 ratio of vertical and horizontal sizes. Main features of closed section are:-
a) Pressure decreases, because thickening of boundary layer velocity increases
b) Downstream (compensation increasing cross section, BL suction)
c) Area blockage ratio is much smaller than that of open test section
d) Wind tunnel corrections should be made (pressure decrease, increased velocity beside the body investigated, at the periphery of test section the wall is flat, although the stream surface would be curved. Slotted walls are frequently used)
2.2 FLOW MEASUREMENT TECHNIQUES USED IN WIND TUNNEL
2.2.1 Pressure measurement method
There are a lot of methods for measuring the flow velocity in liquid and gaseous fluids. Combination of probes, transducers and many other instrument are used for measurement of stagnation pressure and static pressure. Generally, velocity in subsonic flow is measured by two ways: –
(a) Measurement of total pressure and static pressure in tunnel
(b) Measurement of static pressure in two different cross-sections of collector.
Aerodynamic forces and moments are calculated by the mean value of velocity in wind tunnel test section. The need for attaining uniform flow field make it compulsory to measure mean flow velocity. For low speed flow, velocity is found by measuring the stagnation pressure inside wind tunnel and static pressure at the walls of tunnel. Relation between stagnation pressure, static pressure and velocity is given as follows:-
Generally pitot tube is used to measure stagnation pressure. Pitot probe is a cylindrical tube having a hole in front, which is parallel to flow. Measurement errors are very small, around 0.2% up to Ma = 1, reason for less error is because the flow stop is quick and influence of friction is ignored. Shape of tube has negligible influence on measurement accuracy. The Shapes used for nose of Pitot probes are shown below. Ranges for the angle of attack in which measurement error can reach up to 1% are also described.
Pitot-static probe is a combination of Pitot tube and a static pressure measuring tap, so it is used in simultaneous measurement of the stagnation and the static pressures. It has one open-ended hemispherical nose at centre and few number of orifices on side of head at some distances from nose. Appearance of Pitot-Prandtl probe can be seen in fig as shown:-
Sometime transducers are also used to measure dynamic pressure. Elastic element can move the mechanism with the help of indicator. Some precise instruments can be used to convert the pressure into electrical signal by electrical sensor of instrument. Different types of transducers used are Bourdon’s tube, hollow thin cylinders, membranes etc. These transducers can measure both low and high pressure.
Measuring of air stream velocity by two static pressure:-
Bernoulli’s equation and law od continuity gives velocity ‘V’ under adiabatic changes. Velocity at section “2” can be written as
Cp – Specific heat at constant pressure
Cv – Specific heat at constant volume
P – Pressure ρ – air density A – Cross section area
2.2.2 Anemometry methods
LASER DOPPLER ANEMOMETRY (LDA)
Laser-Doppler Anemometry (LDA) is a non-contact optical method used for measuring fluid velocity, distribution of velocity profile and level of turbulence in flow. Main advantages of this method are
i) Non-contact – without any flow disturbances
ii) Very small probe and changing of measured volume, so it provide high resolution.
Iii) Calibration is not required as calibration constants will be defined with respect to light wavelength and angle of intersection of the laser beams.
Disadvantages of LDA methods are:-
i) Optical visibility is required i.e. part of wind tunnel wall should be transparent.
ii) Suitable configuration of wind tunnel with test section is required.
iii) Dirty wind tunnel walls or airflow can produce a lot of noise and after the test signal reduction is registered as dummy with high turbulence level.
The homogeneity, damages and stresses in glass window, can produce light refraction with result in dislocation within measured volume, this must be considered during fluid velocity measurement. It can be used for translation and rotation testing of objects having different dimensions and shapes.
HOT WIRE AND HOT FILM ANEMOMETRY
Hot wire and film anemometers are used to measure the variables in turbulent flow for example mean velocity components, mean temperature, velocity fluctuation temperature fluctuations, etc. Sensors (thin metallic units) are kept in flow field and heated up by electrical current. Heat transfer between wire (film) with fluid depends on flow velocity and temperature.
The basic parts of hot wire (film) anemometers are the probe with a appropriate sensor and a electrical circuit. Electric circuit brings electrical current for heating up the sensor.
Electric circuits used in anemometer are – constant electrical current and constant temperature. In constant electrical current circuit, frequent response for sensor depends not only on sensor’s characteristics, but also on heat exchange between sensor and airflow, a strong limitation in this system type. So It is not feasible for measurement of rapid changes in flow parameters. Thus, constant electrical current anemometer is most suitable when velocity fluctuations are small in comparison to the mean velocity. Constant temperature anemometer overcomes all such problems. Increasing airflow velocity decreases the temp hence resistance also decreases. Smaller resistance mean smaller voltage.
PARTICLE IMAGE VELOCITY
Particle Image Velocimetry (PIV) is one of the most important experimental tool in fluid mechanics and aerodynamics. Basic principles in this involves photographical recording of motion of microscopic particles suspended in fluid and that follow fluid flow. PIV is non-intrusive method, so the measurements obtained highly accurate and free from disturbances. This technique is the ideal for measurement of unsteady aerodynamic flows.
Position of particle is either recorded either on photographic film or any other CCD digital cameras. The data processing is done either by determining average displacement of particles over a small analysing region in image or displacement of individual particle between pulses of light sheet.
Chapter 3
NUMERICAL STUDIES
3.1 ANSYS
ANSYS is a software used for general purpose. It is used for simulating interactions of all disciplines from physics, vibration, structural, fluid dynamics, electromagnetics and heat transfer for engineers.
So ANSYS enables to simulate tests in actual working conditions, enables to test in virtual environment before manufacturing prototypes of products. Furthermore, determining and improving weak points, computing life and foreseeing probable problems are possible by 3D simulations in virtual environment.
ANSYS software with its modular structure as seen in the table below gives an opportunity for taking only needed in actual ures. ANSYS can work integrated with other used engineering software on desktop by adding CAD and FEA connection modules.
ANSYS can import CAD data and also enables to build a geometry with its “preprocessing” abilities. Similarly in the same re-processor, finite element model (a.k.a. mesh) which is required for computation is generated. After defining loadings and carrying out analyses, results can be viewed as numerical and graphical.
ANSYS can carry out advanced engineering analyses quickly, safely and practically by its variety of contact algorithms, time based loading features and nonlinear material models.
ANSYS Workbench is a platform which integrate simulation technologies and parametric CAD systems with unique automation and performance. The power of ANSYS Workbench comes from ANSYS solver algorithms with years of experience. Furthermore, the object of ANSYS Workbench is verification and improving of the product in virtual environment.
ANSYS Workbench, which is written for high level compatibility with especially PC, is more than an interface and anybody who has an ANSYS license can work with ANSYS Workbench. As same as ANSYS interface, capacities of ANSYS Workbench are limited due to possessed license.
Chapter 4
AIR VELOCITY MEASUREMENT
4.1 PITOT TUBE
Air velocity measurement in wind tunnel is carried out with the help of a pitot tube. To measure the velocity in wind tunnel, with the help of pitot tube a static pressure and stagnation pressure is measured. For measuring the flow velocity density of air is also required.
Static Pressure :- It is generally called the pressure of fluid. It is generally known as the pressure of fluid increases the atmospheric pressure. It is normally measured between atmospheric pressure and a flat opening taken out in direction of flow of fluid in wind tunnel.
Stagnation Pressure :- It is also the measurement of fluid pressure that exceeds the atmospheric pressure. But in this fluid velocity is converted into pressure. In this pressure is measured from a flat opening perpendicular to direction of flow and face opening in direction opposite to fluid flow that mean facing the fluid flow.
Dynamic Pressure :- it is the measure of difference in stagnation pressure and static pressure. It is the amount by which stagnation pressure exceeds the static pressure. It can also be seen as the conversion of kinetic energy into pressure or reducing the kinetic energy value to zero.
Air Density measurement
air = Patm/R*Tatm
where
air = Density of dry air (kg/m3)
Patm = air pressure
R = Specific gas constant of dry air 287.05 J/(kg.K)
Tatn = Temperature (K)
So
air = 101325/(287.05*(32+273.15) = 1.156766 kg/m3
Flow velocity measurement
Common formulas used
Bernoulli’s eq is used to measure the velocity of fluid with the help of pressure measured in pitot tube.
In the above eq v1 is zero. Also z1 and z2 are also equal so can be dropped.
By substituting in Bernoulli’s eq.
,
By rearranging the terms.
,
So now velocity obtained in our case is
h = Change in water level in pitot tube
V2 = = 12.2454 m/s
For measuring h in an inclined manometer we multiply h by sin. so actual h will be as follows
hactual = h * sin
where = angle between manometer and the horizontal on surface.
Chapter 5
EXPERIMENT
5.1 ABOUT THE EXPERIMENT
5.1.1 The whole purpose of the minor project was to familiarize with the different technique used in wind tunnel for experiments.. Results for calibration of wind tunnel are being carried out and are tabulated below.
TIME
TEMP
FREQUENCY OF MOTOR(Hz)
READING OBTAINED(mm)
ACTUAL VELOCITY(m/s)
0800
36.6
40
12
12.24
0900
38
40
12
12.24
1000
39
40
12
12.24
1200
40
40
12
12.24
1320
41
40
12
12.24
1500
40.5
40
12
12.24
1630
39.5
40
12
12.24
The whole work carried out is the setting of model in wind tunnel to get the best results and there should be minimum error in results. To get these results a long process is carried out which is explained below:-
• First problem was to take out pressure tab from the model to measure pressure on surface of the model to measure boundary layer on surface and to find exact numerical model which will be suitable for our experiments.
• Problem with pressure tabs was that they were creating interference with the flow around the model. So a modification in model was needed. So it was decided to increase the thickness of deck and so that all pressure tabs can be installed properly.
• There are 100 pressure tabs provided on model to measure pressure at hundred locations.
• At critical places more more pressure tabs are provided to capture the pressure difference properly.
• After that all pressure tabs are connected to manometer with help of pipes and they are not visible inside the tunnel and do not influence the flow of air inside tunnel.
• Manometers are cleaned and calibrated with the known values to see possible error possible while taking the results. Fro experiment inclined type of manometer are used.
• For calibration results are obtained with the help of two hole probe to measure stagnation pressure and static pressure.
• The model is secured properly secured in the wind tunnel.
• Primary results obtained are satisfactory.
• Procedure done in chapter 4 is used to convert pressure obtained in velocity of air on surface of model.
Fig 1 :- Surface straightness checking
fig 2 :- Centerline marking in wind tunnel
Fig 3 :- 36 tube manometer
Fig 4 :- Pipes for 100 pressure tabs
Fig 5 :- Pressure points on model