Major project Máté József VARGA

BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS

FACULTY OF MECHANICAL ENGINEERING

DEPARTMENT OF FLUID MECHANICS

Dispersion of air pollutants in urban environ-

ment

by

Máté József VARGA /FO0YNX/

Submitted to the Department of Fluid Mechanics of the

Budapest University of Technology and Economics in partial fulfilment of the requirements for the degree of Master of Science in Mechanical Engineering Modelling

May, 2011

Project Report in Major Project /BMEGEÁTMWD1/

Supervisor: Márton BALCZÓ, assistant research fellow

Evaluation Team Members, advisors: Éva BERBEKÁR, PhD student

Balázs ISTÓK, assistant lecturer Tamás LAJOS, PhD professor Anikó RÁKAI, PhD student

Department of Fluid Mechanics Faculty of Mechanical Engineering

Budapest University of Technology and Economics

MSc MAJOR PROJECT (BMEGEÁTMWD1)

Dispersion of air pollutants in urban environ-

ment Supervisor: Márton BALCZÓ, assistant research fellow / Éva BERBEKÁR, PhD

student

1. Becoming acquainted with the concentration measurement system using lit-erature and existing documentation, checking and testing of the system, and performing necessary modifications.

2. Measurement of simplified dispersion test cases (line source in crosswind

flow, street canyon flow etc.) in the NPL wind tunnel using the concentration measurement system.

3. Discussion of measurement results, comparison to literature data.

The project can be continued as a Final Project focusing on detailed wind tunnel tests of a specific urban site in autumn 2011.

v

DECLARATION

Full Name (as in ID): Máté József VARGA Neptun Code: FO0YNX University: Budapest University of Technology and Economics Faculty: Faculty of Mechanical Engineering Department: Department of Fluid Mechanics Major/Minor: MSc in Fluid Mechanics Spec. in Solid Mechanics Project Report Title: Dispersion of air pollutants in urban environment Academic year of submission: 2010 / 2011 - II. I, the undersigned, hereby declare that the Project Report submitted for assessment and defence, exclusively contains the results of my own work assisted by my super-visor. Further to it, it is also stated that all other results taken from the technical lit-erature or other sources are clearly identified and referred to according to copyright (footnotes/references are chapter and verse, and placed appropriately). I accept that the scientific results presented in my Project Report can be utilised by the Department of the supervisor for further research or teaching purposes.

Budapest, 13 May, 2011

__________________________________ (Signature)

FOR YOUR INFORMATION

The submitted Project Report in written and in electronic format can be found in the Library of the Department of Fluid Mechanics at the Budapest University of Tech-nology and Economics. Address: H-1111 Budapest, Bertalan L. 4-6. „Ae” building of the BME.

ACKNOWLEDGEMENT

I would like to express thanks to my supervisor for the useful, practical hints he gave during the model constructions and the measurements.

vi

CONTENTS

DECLARATION ..................................................................................................................... v

ACKNOLEDGEMENT .......................................................................................................... v

1. Introduction .......................................................................................................................... 1

1.1. Abstract ........................................................................................................................ 1

1.2. Kivonat ......................................................................................................................... 1

2. The measurement system ................................................................................................... 4

2.1. The sampling [1] .......................................................................................................... 4

2.2. The detector unit and the sampling valve [1] ......................................................... 5

2.3. The control [1] .............................................................................................................. 7

2.4. Optimization ................................................................................................................ 7

2.5. Checking and setting of the system.......................................................................... 8

3. Simplified measurement case .......................................................................................... 10

3.1. Creating a wind tunnel model ................................................................................ 10

3.2. The NPL wind tunnel ............................................................................................... 12

3.2.1. Generating an urban like boundary layer .................................................... 13

3.3. The measurements .................................................................................................... 14

3.3.1. The results ......................................................................................................... 14

3.3.2. Comparison to the standard ........................................................................... 17

4. The problem of the coupling ............................................................................................ 18

5. Measurement of an urban square .................................................................................... 19

5.1. The model ................................................................................................................... 19

5.2. The measurement settings ....................................................................................... 21

5.3. Results ......................................................................................................................... 22

6. Summary ............................................................................................................................. 26

6.1. Suggestions/Conclusions/Further plans ................................................................ 27

7. Bibliography ....................................................................................................................... 28

1

1. INTRODUCTION

1.1. Abstract

Because of the augmentation of urban traffic arising from the increasing number of peoples living in cities, the investigation and reduction of dispersion of air pollutants became an important topic in today’s environmental engineering researches. The presence of hybrid and fully electronic vehicles is a good initiative to move into the direction of environment friendliness, but the majority of cars are still equipped with internal combustion engines. Although, the innovations of the car manufacturers forced by ordinances regulating the pollutant emission of vehicles reduced the pollu-tion of them, the most important task in the case of protection of air clearness is the reduction of contaminations originated from urban traffic.

The pollutants could be gases (e.g. carbon-monoxide, nitrogen-oxide NOx and its compounds) or aerosols, fluid droplets, solid particles (e.g. flying soot, PM10). The concentration, dispersion, and propagation of these materials came to the air is de-termined by the motions in the lower region of the atmosphere called atmospheric boundary layer. The flows in this 200m -2000m thick layer is directly influenced by the subjacent earth surface, vegetation and buildings. In case of settlements the most significant role in the transmission of the pollutants emitted close to the earth is played by the buildings.

The generally used mathematical pollutant dispersion models have to be validated by measurements. Beside the field tests, where appears uncertainties caused by fluc-tuating wind velocity and uncontrollable background concentrations, the next ap-proach can be the physical modelling. In case of wind tunnel modelling these pa-rameters, and the tracer release can kept constant or can be measured.

1.2. Kivonat

A félév során a rendelkezésre álló szakirodalom és dokumentációk alapján egy szennyezőanyag-eloszlás mérésére alkalmas mérőrendszer került megismerésre. A berendezés egyszerre huszonnégy pontban képes mintavételezésre lineáris léptető motorok segítségével mozgatott mintavevő hengerek által. A megfelelő számú tisztí-tási periódus – ami a minta beszívásából és annak környezetbe történő kifúvásából áll, ezáltal elkerülve az előző mérésből származó maradványokat és biztosítva, hogy a cilindert kizárólag a mérendő minta tölti ki – után a gázminta egy mechanikus, több utas útválasztó szelepen áramlik át, amely összekapcsolja az aktuális hengert a

2

detektor egységgel. Itt a gáz először a készenléti üzemben lévő mintavevő szerel-vényhez érkezik, amely az ismert térfogatú próbakörön vezeti át, amíg teljesen ki nem tölti azt. Ezután a szerelvény üzemel módba vált és sűrített levegő segítségével az elemző egységhez juttatja a mintát. Az elemzés láng ionizációs detektorral (FID) történik, melyben széntartalmú gáz hatására a hidrogénláng töltöttsége megváltozik, ami a láng körül elhelyezett elektródákhoz kapcsolt elektrométer jelén történő ugrás-szerű változásban mutatkozik meg. A csúcsok alatti terület arányos a gáz-levegő koncentrációjával, melynek középértéke három csúcs átlagolásával érhető el. Az esz-köz kalibrációja ismert koncentrációjú, nyomgázként használt metánnal történik.

Az első próbamérések nagy relatív hibát eredményeztek, ami a FID melegedési idejének nem pontos kivárásával magyarázható. A próbamérések során hat szüksé-ges tisztítási periódust állapítottam meg.

Később némi optimalizálás lett végrehajtva a rendszeren. Az útválasztó áthelyezé-sével és az őt a hengerekkel összekötő csövek hosszának lerövidítésével a pontosság növelése volt a cél.

Ezek után szélcsatorna mérés következett, amely során egy vonalforrás mögött tör-ténő koncentráció csökkenés meghatározása volt a cél a forrástól mért távolság függ-vényében. A modell rétegelt lemez alapra épült, a mérési elrendezés és a határréteg beállítása a VDI 3738 [4] szabványnak megfelelően történt. Többszöri mérés során az eredmények ismétlési hibát mutattak, de beleillettek a szabvány által előírt sávba, így a mérési technikát elfogadhatónak ítéltem.

A mérések során néhány mérőhenger nem működött megfelelően, ami a léptető-motor és a cilinder közötti merev kapcsolattal volt magyarázható. A probléma a kap-csolás átalakításával megoldódott.

Végül egy valós helyszín, a budapesti József nádor tér modellezése és vizsgálata következett. A szélcsatorna modell alapja itt is rétegelt lemez volt, amelyre szintén ebből az anyagból készült, a teret körülvevő épületeket szimulálandó kockákat és téglatesteket ragasztottam. A mérési beállítások és a modell arányai itt is a VDI szab-ványnak megfelelően lettek meghatározva. Ilyen elrendezésben, északi szélirányt alkalmazva kettő mérést hajtottam végre a következő eredményekkel:

• a várakozásoknak megfelelő közel zérus koncentráció volt mérhető a hátutcák-ban;

• a vonalforrásoktól számított első sorban, a téren nagy ismétlési hiba volt tapasz-talható, tehát itt további mérések szükségesek;

• a téren középütt és hátrább lévő sorokban homogén eloszlás volt megfigyelhető az áramlásra merőleges irányban, jobb ismétlési pontossággal;

• a téren, középen az első pontokban kisebb koncentráció volt mérhető, mint a házak tövében, ami azzal magyarázható, hogy a hátutcákból akadálymentesen fújó szél a tér szélén több nyomgázt képes a mérőpontokra fújni, valamint a tér közepén a kis ház mögött kialakuló leválási buborékban nagyfokú vertikális keveredés, és ezáltal hígulás tapasztalható;

3

• a második mérés során többé-kevésbé szimmetrikus eloszlást tapasztaltam, ami elvárható egy szimmetrikus modell szimmetriatengely mentén történő megfúvása esetén.

4

2. THE MEASUREMENT SYSTEM

To model the pollutant transport in wind tunnel a tracer gas can be used as pollut-ant. Observance of modelling and similarity laws provide that the tracer gas concen-tration obtained from wind tunnel measurements is proportional to the real pollution concentration. Simultaneous sampling in different points of the modelled area and thereafter storage of the samples are necessary, since the samples pass the gas detec-tor serially, where the concentration of each one is determined. In the followings a multi-channel automatic system developed at the Department of Fluid Mechanics will be used, which is appropriate for simultaneous sampling

2.1. The sampling [1]

The multi channel sampler equipment has been built of pneumatic elements and lets at most 24 simultaneous sampling. Each sample channel consists of a 100ml pneumatic cylinder actuated by a PC controlled linear stepper motor. The sampling velocity is proportional to the velocity of the pistons. Since each cylinder has an own step motor and every actuator can be individually controlled, the velocity of the pis-ton thereby the sampling velocity can be varied for each sampling. Up the cylinders two electromagnetic valves have been coupled. All channels are connected to a me-chanical multiplexer, a 48-port scanning valve. Figure 2.1.1 shows the sketch of the system.

Figure 2.1.1 The sampling system [1]

5

Based on the valve position each piston can:

• Suck sample from the wind tunnel model into the cylinder

• Blow samples into the surroundings (exhaust) • Put samples from the cylinder through the mechanical multiplexer to-

wards the detector unit

The sampling process is the following:

Table 2.1.1 The sampling process [1]

2.2. The detector unit and the sampling valve [1]

The detector unit contains a gas chromatograph equipped with a flame ionization detector (FID) and a 6-port, two-position sampling valve (see Figure 2.2.1). The FID, which is the most widely used detector of the chromatographs, can detect technically all kind of hydrocarbons above 100 ppb in a hydrogen flame. The carbon ions in the flame are exposed to an electric field between two electrodes and cause an electric current between the electrodes that is proportional to the amount of carbon in the flame.

The sampling valve is provides the connection between the detector and the scan-ning valve. It can also control the amount of sample gas pushed into the direction of the FID. The main functions of that are:

• Collecting a known volume of sample gas into the sampling loop(1st posi-tion)

• putting that known volume of sample into the FID (2nd position)

In the 1st (standby) position the sample comes through and fills a 50µl ample loop and is exhausted. Through the other ports air - can be called also carrier gas - goes to

6

the FID. After switching into the 2nd (operate) position the carrier gas input is con-nected to the sample loop thereby the known volume of sample determined by the volume of sample loop is pushed by the carrier gas into the detector. At this position of the valve the sample gas is exhausted directly to the surroundings. After a pre-scribed time the valve is switched again into standby position and the cycle is fin-ished.

Figure 2.2.1 The sampling valve and the detector unit in a) standby and b) operate position [1]

After the known volume of mixture of air and tracer gas arrives to the detector unit, the signal of the electrometer connected to the FID fast increases. The area un-der the peak is proportional to the concentration of the tracer in the carrier gas. The mean concentration has been determined as the average of three peaks. This means, that for the accurate result the sample loop has to be filled and inject three times from the same cylinder. This requirement and the dead volume of the cylinders and the tubes determine the required 100ml sampling volume.

Before each wind tunnel measurement the FID has to be calibrated. The calibration can be done with the help of two calibration gases of known concentration (4950ppm, 50ppm) and these known concentrations are correlated to the area of the correspond-ing measured electrometer voltage peak. A third point is given by the zero concentra-tion of the carrier gas and the zero peak belonging to it. Thus, the linear calibration

7

equation can be determined based on the three points. A typical electrometer signal and the calibration curve can be seen in Figure 2.2.1

Figure 2.2.1 a) the peaks of the electrometer signal and b) the calibration line []

2.3. The control [1]

The whole measurement process can be controlled by computer (see Figure 2.3.1). The tracer gas, methane (CH4) which substitutes the real pollutant is released by digi-tal mass flow controllers before it leads to the sources. The distribution of tracer gas over the modelled area has to be similar to the pollution distribution over the real terrain, so special point, line or area sources built in the model. The controllers are connected via RS-485 cables and HART protocol to the computer. Functions (com-mands and actual flow rate, temperature data) of the mass flow controllers can be reached through DDE protocol using a Labview program. The multi channel sampler is controlled by an USB digital I/O device, of which digital lines can set the valves and the multiplexer. All the linear stepper motors have own microcontroller chips which can start and stop, change direction of rotation and speed of the actuator, and also manages the two limit switches. To amplify the TTL digital I/O signals to the voltage and current required by the specific device, both valves and steppers have amplifier circuit. The analogue output voltage of the FID is red by a National Instru-ments PCI 6036E 16-bit data acquisition card, which can also steer the electromag-netic actuator of the 6-port sampling valve.

The sampling process is controlled automatically by a program written in National Instruments Labview.

2.4. Optimization

Before any measurements some improvements on the measurement system were executed to make it reasonable and optimal.

8

Firstly the scanning valve was repositioned into the centre of the apparatus. It was fixed on a wooden frame with four screws (see Figure 2.4.1).

Figure 2.4.1 The new position of the scanning valve

The length of the pipes between the sampling cylinders and the scanning valve was diminished to reduce measurement errors owing to the remaining samples in the tubes from previous measurements. All the pipes were reduced to the same length which was calculated by the minimum needed length between the scanning valve and the most distant piston.

Figure 2.4.1 The reduced pipes

2.5. Checking and setting of the system

It was checked, if all the equipments are working properly, because the system was unused for a longer time. Therefore 4950ppm calibration gas were sucked by each of the cylinders and pushed through the FID. In the first cases relatively large meas-urement error, around 14% was experienced. The reason for that was that possibly the measurements were carried out in the heating period of the FID.

During the tests it was found that at least six purging period of the cylinders are needed to have acceptable result by the detector. One purging period was regarded as the piston sucks in the sample and blows it into the surroundings to ensure that

9

only the sample of the current measurement fills in the cylinders and get to the detec-tor unit. To do that after the 4950ppm methane calibration gas, clean air was meas-ured and the output signal of the FID was checked if any peaks are to be observed.

10

3. SIMPLIFIED MEASUREMENT CASE

To check the suitability of the system in measurement conditions a standardized test arrangement was built up according to the VDI 3738 PART 12 [4] standard. The decrease of the tracer gas concentration in function of the downwind distance from a line source was investigated in 18 points. In accordance to the standard an urban like atmospheric boundary layer was generated in the NPL wind tunnel, and the results were compared at different wind speeds to literature data.

3.1. Creating a wind tunnel model

The model was built up onto a 500 x 800 x 5 mm pulpwood board. 100 mm from the inlet end a 2 x 400 mm gap was cut for the line sources and 18 holes were drilled for the sample points in 2 lines. The first line was in the symmetry axis and 10 holes were placed there, the second line with 8 holes was 100 mm far from the first one to check lateral homogeneity of the results. The picture of the basic arrangement can be seen in Figure 3.1.1.

Figure 3.1.1 The basic arrangement

Into the holes plastic cylindrical elements were fixed by insulating glue called Bluetech. Into the plastic elements sampling pipes were connected using Bosch fast couplings (see Figure 3.1.2).

The line source was modelled by 4 pieces of 20 x 60 x 100 mm hollow glass bricks. To ensure the uniform pressure distribution within the brick and the uniform out-flow from it 20 pieces of 40 mm long and 0.2 inside diameter needles were glued within all sources in uniform distribution. The tracer gas supply entered the sources through 2 outer joints at the lower side of the source.

Before building those into the model sources were tested if they work properly. Thus each one was dip into water and air was blown through them with controlled mass flow rate controlled by BROOKS DMFC controller. It was investigated if all the

11

needles work, if there are bubbles released from them. The check showed that above 30 normal litre per hour mass flow rate the needles work properly, so there are bub-bles above all of them. One can see a picture about the line sources and the checking process in Figure 3.1.3.

Figure 3.1.2 The sampling elements and built into the model from below

Figure 3.1.3 A model line source and the test of it

Finally the parts were put together and a 1 x 8 x 500 aluminium coversheet was

glued 1mm above the line sources to eliminate the vertical momentum of the out flowing gas. A picture about the assembled model from above and from below in the wind tunnel can be seen in Figure 3.1.4.

12

Figure 3.1.4 The assembled wind tunnel model from above and from below

3.2. The NPL wind tunnel

The tests were carried out in the NPL (National Physical Laboratory) wind tunnel, which is an open-loop, closed-test-section, suction type channel made fully of wood. In front of the test chamber - which is 2 meters long and has 0.5 x 0.5 meter cross sec-tion –a confusor type inlet can be found equipped with honeycomb structure, which can make the flow straight and decrease turbulence and with sieve at its end. Chang-ing the sieve to a rough one and fixing plates with different shapes on it the flow can be made turbulent. After the test chamber there is a diffuser and the fan which is driven by a direct current motor. The revolution number of the motor can be changed practically continuously from zero to 1500 rev/min with the help of a poten-tiometer. Thus the stream velocity can be varied from zero to approximately 15 m/s continuously.

Figure 3.2.1 Schematic picture of the NPL wind tunnel

13

Figure 3.2.2 Pictures of the NPL wind tunnel [2]

3.2.1. GENERATING AN URBAN LIKE BOUNDARY LAYER

In the wind tunnel to simulate real town circumstances an urban like boundary layer had to be generated with roughness elements and turbulence generators. The velocity profile to be produced was according to the VDI [4] standard:

( )

α

−−

=0

0

dz

dz

u

zu

refref

(3.1)

, where:

• ( )zu is the expected velocity profile; in m/s • α is the profile exponent; - • z is the height above the ground; in m • refz is the reference height; in m

• refu is the mean wind velocity at the reference height refz ; in m/s

• 0

d is the displacement height; in m

The boundary layer is of rough category with 360.=α and Hd 7500

.= correspond-

ing to urban surface roughness. This is a remarkable difference compared to the sub-urban boundary layer of the standard. Turbulence intensity, which mostly influences dispersion in this test case, has also values of urban roughness (higher turbulence).

In consequence we expect more rapid dispersion and lower concentrations, than in the VDI experiment.

The curves in Figure 3.2.1.1 prove that, measured ones (measured by LDA by Manninger [3]) run close to the lower limit given by the VDI guideline.

14

Figure 3.2.1.1Boundary layer generation and the generated velocity and turbulence profiles

3.3. The measurements

The measurements were carried out with 120 nl/h mass flow rate methane as tracer gas from the line sources at different wind speeds. The concentrations were meas-ured in the sampling points and they were recalculated into non-dimensional ones based on equation (3.2) according to the VDI [4] standard. This way the results could be compared to the standard:

LQ

zucc refref

/*

⋅⋅= (3.2)

, where:

• *c is the non dimensional concentration to be found; - • c is the measured concentration • refu is the mean wind velocity at the reference height refz ; in m/s

• refz is the reference height; in m

• n

nn T

T

p

pQQ ⋅⋅= is the emission measure, e.g. volume flow rate; in l/h

, where: o nQ is the preset volume flow of the emissions in normal circum-

stances; in nl/h o p is the measured environmental pressure; in mbar o T is the measured environmental temperature; in K o mbarpn 1013= , KTn 273= characteristics of the normal state

• L is the length of the line source

3.3.1. THE RESULTS

Three measurements were carried out with the same settings but the first one 4.37 m/s, the second and third one with 5.91 m/s reference wind speed. The results can be seen in table form in Table 3.3.1.1 and Table 3.3.1.2, and the diagram of them in Fig-ure 3.3.1.1, Figure 3.3.1.2 and Figure 3.3.1.3.

15

dis-

tance

from

the

source

dimen-

sionless

distance

1st measurement

(u = 4.37 m/s) 2

nd measurement (u = 5.91 m/s)

concentration dimensionless

concentration concentration

dimensionless concentra-

tion

at the

symmetry axis

at the

symmetry axis

at the

symmetry

axis

at side line

at the

symmetry

axis

at side line

x [mm] x/zref [-] c [ppm] c* [-] c [ppm] c [ppm] c* [-] c* [-]

30 0.6 14358 37.96 8915 10304 31.83 36.80

60 1.2 5995 15.84 3568 3988 12.70 14.20

90 1.8 3364 8.88 2065 2330 7.32 8.27

120 2.4 2272 5.99 0 1542 0 5.45

150 3 1640 4.32 978 1047 3.43 3.68

200 4 1102 2.89 738 748 2.57 2.61

250 5 803 2.10 547 567 1.89 1.96

320 6.4 0 0 0 433 0 1.48

390 7.8 455 1.18 335 - 1.13 -

500 10 354 0.92 274 - 0.91 -

Table 3.3.1.1 The results of the first and second measurements

dis-

tance

from

the

source

dimen-

sionless

distance

3rd

measurement (u = 5.91 m/s)

concentration dimensionless concentra-

tion

at the

symmetry

axis

at side line

at the

symmetry

axis

at side line

x [mm] x/zref [-] c [ppm] c [ppm] c* [-] c* [-]

30 0.6 12993 14156 44.30 48.27

60 1.2 5579 5698 18.98 19.39

90 1.8 3153 3230 10.70 10.97

120 2.4 0 2173 0 7.36

150 3 1457 1464 4.91 4.94

200 4 1063 1017 3.57 3.41

250 5 790 775 2.64 2.58

320 6.4 0 580 0 1.92

390 7.8 464 - 1.52 -

500 10 370 - 1.2 -

Table 3.3.1.2 The results of the third measurement

16

Figure 3.3.1.1 The results of the first measurement case

Figure 3.3.1.2 The results of the second measurement case

Figure 3.3.1 The results of the third measurement case

17

On the figures can be seen - according to the expectations - that the concentration is decreasing with the increase of the distance from the source. But there are distances in the first one or two points between the quantities measured along the symmetry axis and the sideline. The reason for that is probably the inconsistency of the outflow, because of the turbulence the velocity is not perfectly uniform along the line source. Another reason can be the unevenness of the coversheet, since it is a long aluminium sheet and cannot be perfectly flat.

3.3.2. COMPARISON TO THE STANDARD

Recalculating the measured concentrations into dimensionless ones gives a possi-bility to compare them to the standard.

Figure 3.3.2.1 Comparison of the results to the VDI 3738 [4] standard

In Figure 3.3.2.1 can be seen that the results get into the region what the standard prescribed, thus the modelling technique proved to be accurate enough.

But it is visible that there is a repetition error between the second and third meas-urements, e.g. in the third case the measured values are higher than in the second case, however the reference velocity was just the same.

18

4. THE PROBLEM OF THE COUPLING

During the measurements in chapter 3 some of the pistons didn’t work properly. Some of them didn’t make the prescribed purging period or didn’t move at all (see in Table 3.3.1.1 and 3.3.1.2 the zero values). The reason for that was probably that the coupling between the cylinders and the actuators were closed type, which does not allow lateral movement to the shafts (see Figure 4.1, and Figure 4.2). In this case the threaded worm of the stepper motor could get stuck.

To avoid this error the early coupling was redesigned into a new one, which allows lateral displacement. You can see the illustration of it in Figure 4.2. The barrel of the actuators shaft was widened and grooving was made at the upper part of the barrel of the cylinders shafts. The thicker worm was connected to the coupling close with a counter bolt nut and the thinner one with a threaded washer and a counter screw nut. To eliminate the rotation of the actuators shaft a plate fixed to it with two counter nuts was bended onto the coupling.

Figure 4.1 A sampler with the original coupling [1]

Figure 4.2 a) The original close coupling and b) the redesigned one which allows lateral displacement

19

5. MEASUREMENT OF AN URBAN SQUARE

The goal was to create a measurement layout similar to real urban squares to re-ceive a general idea about flow patterns inside the square. In real life it is important to see how a square ventilates according to ambient wind directions and wind speeds and how the concentration of any pollutants changes.

There are many densely inhabited areas with blocks of houses in the inner city of Budapest, where often the square itself is a missing block. An example to this ar-rangement of block houses can be seen in József nádor square, in the fifth district of Budapest.

Thus a general and simplified model was created, which is at the same time similar to the upper introduced urban square. The most important parameters of such square are the length - width ratio of the blocks, the block height - street width ratio, and the block height - block thickness ratio. Defining these parameters of the intro-duced urban square the following model was created. [3]

5.1. The model

The basis of the wind tunnel model was a 500 mm diameter and 10 mm height pulpwood board, into which a 20 x 300 mm hole was cut for the line sources, and 41 pieces of 6 diameter holes was bored for the sampling elements (see Figure 5.1.1).

Figure 5.1.1 The picture and the solid model of the basis

31 measurement points were placed on the square in increasing distance and de-creasing number from the line sources, and five – five elements were planned in the back streets with the same distribution method (see Figure 5.1.2). With such number of measurement data an appropriate concentration dispersion map can be taken down.

20

Figure 5.1.2 The basic dimensions of the model and the distribution of the measurement points

Six pieces of 77 x 77 x 46 mm wooden cube with a 31 x 31 mm hole in it were used as small buildings and 77 x 231 x 46 mm cuboids as large building (see Figure 5.1.3) and glued onto the pulpwood basis.

Figure 5.1.3 Picture of the small and large buildings

Into the final model 3 line source elements and 41 sampling elements with fast connection – just the same as in case of the measurement in chapter 3 – were built in. The sources were covered same way as in the previous measurement assemblage with an aluminium cover sheet. You can see the final model in Figure 5.1.4.

21

Figure 5.1.4 The picture and the solid model of the final wind tunnel model

The final model was placed into the wind tunnel with the help of a wooden frame (see Figure 5.1.4).

5.2. The measurement settings

During the first measurements north wind direction was prescribed and the wind velocity was set into the maximum available of the tunnel. 30 nl/h mass flow rate tracer gas was blown through each line sources.

Figure 5.2.1 The first measurement arrangement

The inlet velocity profile had to be reconstructed to be more similar to the bound-ary layer around the simulated square. Thus double size roughness elements in sparser distribution and other turbulence generators were used. The developed boundary layer can be seen in Figure 5.2.2.

22

Figure 5.2.2 The generated boundary layer profiles along the spanwise direction and the profile distri-

bution in vertical and horizontal directions [3]

5.3. Results

Two measurements were carried out with the same settings set before. The results can be seen in table form in Table 5.3.1, and the distribution of the sampling point with the numbering in Figure 5.3.1.

Figure 5.3.1Sampling point distribution with numbering

23

Point no. x y z 1

st measurement 2

nd measurement

[mm] [mm] [mm] C [ppm] C [ppm]

1 -97 50 0 9779 10185

2 -97 25 0 5835 9018

3 -97 0 0 6703 6389

4 -97 -25 0 9841 9948

5 -97 -50 0 10637 10729

6 -65 50 0 3902 4054

7 -65 25 0 3791 3997

8 -65 0 0 3416 3448

9 -65 -25 0 3741 3688

10 -65 -50 0 3850 3921

11 -31.5 50 0 2501 1781

12 -31.5 25 0 2273 0

13 -31.5 0 0 2253 2337

14 -31.5 -25 0 2369 2376

15 -31.5 -50 0 2260 2305

16 4 50 0 1759 0

17 4 25 0 1676 1723

18 4 0 0 0 1641

19 4 -25 0 1602 1632

20 4 -50 0 1716 1722

21 42 50 0 1400 1377

22 42 16.65 0 1324 1272

23 42 -16.65 0 1302 1238

24 42 -50 0 1487 1409

25 82 50 0 1166 1119

26 82 16.65 0 1049 992

27 82 -16.65 0 1048 879

28 82 -50 0 1150 0

29 127 50 0 707 676

30 127 0 0 633 608

31 127 -50 0 748 707

32 -157 56.5 0 36 0

33 -157 43.5 0 36 41

34 -181 56.5 0 35 40

35 -181 43.5 0 36 38

36 -208.5 50 0 36 32

37 -157 -43.5 0 39 38

38 -157 -56.5 0 38 41

39 -181 -43.5 0 37 41

41 -208.5 -50 0 30 31

24

Figure 5.3.2 and Figure 5.3.3 show the measured concentration values in function of the x and y coordinates, respectively. The points are coloured by the other non-zero coordinate.

Figure 5.3.2 The concentration distribution along the stream wise (x) axis in the first and second meas-urements, respectively. The wind blows from left, and the line source was placed at x = -127 mm. The

points are coloured by the perpendicular stream wise (y) coordinate.

Figure 5.3.3 The concentration distribution along the perpendicular stream wise (y) axis in the first and second measurements, respectively. The points are coloured by the stream wise (x) coordinate.

25

Figure 5.3.4 The concentration distribution along in the first and second measurements, respectively.

The wind blows from left, and the line source was placed at x = -127 mm, from y = -150 to y = 150. The points are coloured by the measured concentrations in ppm.

According to the previous expectations near zero concentrations were measured in the back streets, since because of the north flow direction no pollution can propagate in the upstream direction, the direction of them.

As it can be seen in the figures, large repetition error was found in the first row from the line sources on the square, thus here further confirming measurements are needed.

In the middle and back downstream region (from the third fourth row from the sources on the square) better repetition accuracy was established and the dispersion in the perpendicular streamwise direction (y direction) was more homogeneous.

It is visible in Figure 5.3.3 and 5.3.4 that in the middle of the front downstream re-gion (in the first three rows from the pollution sources on the square) the measured concentrations are smaller than in the side points. The reason for that is the free path of the wind through the back streets, and the developed separation bubble in the trace of the middle small building. To be more precise the wind is directly blown from the back streets onto the side points of the square, thus more tracer gas can reach that area which means increase in the concentration. And because the flow cannot follow the sharp corners of the middle small building, it separates establish-ing a separation bubble in the trace of that block. In this zone higher vertical mixing can be experienced, and concentration dilution is observable.

As you can see in Figure 5.3.3 and 5.3.4 in the second case, the dispersion is more or less symmetric to the y = 0 axis, which is in accordance to the expectations con-necting to a symmetric model blown along a symmetry axis.

26

6. SUMMARY

During the semester, after a short literature review in the topic of propagation and measurements of air pollutants in urban environment, an earlier built pollutant con-centration measurement system was checked and tested, which can carry out sam-pling in 24 points simultaneously.

The first test measurements resulted relative large error, which can be explained with the not accurate wait of the heating time of the FID. The test showed that 6 purging period is needed.

Later some optimization was carried out on the system. The aim of the reposition-ing of the scanning valve and the shortening of the tubes connecting it with the cyl-inders was to grow the accuracy.

After that a wind tunnel measurement was carried out, during which the decrease of concentration as the function of the distance from a line source was determined. The model was based on a pulpwood board, and the arrangement and boundary layer settings was according to the VDI 3738 [4] standard. After some measurements the results showed repetition error, but they fit into the region prescribed by the standard, thus the measurement technique said to be acceptable.

During the measurements some of the cylinders didn’t work properly. The reason for that was the close coupling between the piston and the stepper motor, and it was solved by the modification of the connection.

Finally the investigation of an urban square, namely the József nádor square was accomplished. The basis of that model was also a pulpwood board, onto which to simulate the buildings around the square cubes and cuboids made from the same material were glued. The dimension of the model and the settings were according to the VDI standard. In this arrangement, applying north wind direction two measure-ments were carried out with the following results:

• according to the expectations near zero concentrations were measured in the back streets;

• in the first row from the sources on the square large repetition error were found, thus here more measurements need to be accomplished;

• in middle of the rear rows of the square perpendicular to the stream direction homogeneous dispersion could be observed with better repetition accuracy;

• in middle of the front rows less concentration could be measured then in the support of the buildings, which is because of the wind from the back streets and the separation bubble in the trace of the small building

• During the second measurement symmetric dispersion were experienced which is expected by a fully symmetric arrangement.

27

6.1. Suggestions/Conclusions/Further plans

In the next semester further measurements and investigations are planned:

• Influence of wind speed / tracer flow rate on the normalized concentration dispersion

• Measuring with more wind directions • Estimation of measurement error from repeated measurements

• Simulating active pollution control of the square using the ventilation sys-tem of the underground car par

One can see in the table in chapter 5 that some of the results are still missing. But this is not a mechanical problem, then the error of the electronics or the control, be-cause the actuators didn’t get current.

28

7. BIBLIOGRAPHY

1. M. BALCZÓ, Z. SZUCSÁN, G. KALMÁR, I. GORICSÁN: Development of Sam-pling System for Investigations on Pollutant Transport. Gépészet, 2006.

2. Home page of the Dept. of Fluid Mechanics: NPL típusú szélcsatorna http://www.ara.bme.hu/cms/index.php?option=com_content&task=view&id=22&Itemid=32

3. P. Manninger: Major project report, 2011

4. VDI 3738 PART 12, Environmental meteorology, Physical modelling of flow and dispersion processes in the atmospheric boundary layer, Application of wind tunnels, 2004.