A Novel and Economical Kite Anemometric Analysis for Velocity

Measurements in a Convective Boundary Layer along a

Reinforced coastline in Pondicherry

R. Jha1, R. Raj

1, J. Ekhlas

1, A. Siravuru

1, S. Sarkar

1, R. Bardhan

1 and *S.Ghosh

1, 2

1 School of Mechanical and Building Sciences, Vellore Institute of Technology, Vellore,

Tamilnadu, India 2 School of Earth and Environment, University of Leeds, Leeds, UK.

Abstract. In a hot, sunny country like India, the boundary layer is convective for most part of the day. This

suggests that the dispersion characteristics of such a boundary layer can only be assessed by an accurate

measurement of the turbulence characteristics of the atmosphere in question.

The aim of this paper is to devise an anemometer that can yield real time measurements of the

fluctuating velocity components over a convective boundary layer along those parts of the Tamil Nadu coastline

which were reinforced by coastal defences(in the form of rocks and boulders) after the 2005 Boxing Day

Tsunami. The coastal defences dissipated much of the wave energy in addition to inducing considerable

aerodynamic drag on the surface winds. Unfortunately, meteorological observations were not available along

many parts of coastal Tamil Nadu, let alone real time wind speeds measurements.

We show results from a sophisticated yet robust kite anemometric analysis of the boundary layer over

reinforced parts of Pondicherry. The first set of measurements with a mechanical device using a system

involving stretched springs yielded a realistic velocity profile after suitable calibration. Since a pressure

measurement is more accurate than a height measurement, we sought to use an alternative method involving the

use of pressure sensors in the subsequent sets of measurements. This only marginally increased the cost of the

entire apparatus but it streamlined the procedure in the sense that it completely automated the velocity

measurements.

Finally, the aforementioned kite anemometer will be equipped with non inertial streamers meant to

follow flow lines. Image processing is proposed as an efficient and economical method for accurately

determining the velocity profile across the coastline. Streamers attached across regular intervals along the string

attached to the kite are used to determine the relative velocity with respect to the topmost point on the trajectory

(viz., the kite). A force sensor attached to the kite for an absolute force reading shall be used to yield the

absolute velocity. A pressure sensor will also be attached to obtain the height across different points. A picture

of the trajectory and image processing will reveal in-situ velocity measurements accurately.

Keywords: Convective Boundary Layer, kite anemometer, pressure sensor

1. Introduction India’s pace of industrialization and urbanisation is spiralling exponentially as

the sub-continent has embarked on massive infrastructure related projects. During the recently

concluded Copenhagen summit, it was quite clear that India’s energy demands with its concomitant

atmospheric fallouts have to be reckoned with seriously. This entails a quantitative estimate of

atmospheric inputs and their subsequent dilution within the planetary boundary layer. Most diffusion

processes happen in the lowest part of the atmospheric boundary layer i.e., the surface layer. In order

ISBN 978-1-84626-xxx-x

Proceedings of 2009 International Conference on Environmental Science and Technology

Bangkok, Thailand, 23-25 April, 2010, pp. xxx-xxx

*Corresponding author. Tel.: +91-0416-2202207, 2243091; fax: +91-0416-2243092, 2240411. E-mail address: satyajitg@vit.ac.in

to ascertain the turbulence characterisations of such a boundary layer along coastal Tamilnadu, it is

essential to measure wind speeds from the surface upto a height of 100 meters above. This can only be

possible with either a radiosonde or hot air balloon fitted with sensors-both being costly operations.

The coastal city of Pondicherry, is being currently reinforced and a proposed port is about to be

commissioned. An environmental impact analysis of the proposed civil engineering projects require a

quick and inexpensive means to measure the turbulence characteristics of the coastal boundary layer.

It should be so inexpensive that such measurements can be taken anytime, anywhere with the least

amount of infrastructural requirements. With the above motivation in mind, we show results from a

sophisticated yet robust kite anemometric analysis of the boundary layer over reinforced parts of

Pondicherry.

Figure 1: Location of Pondicherry coastline Figure 2: Coastal defences along Pondicherry

2. Kite Anemometry As far as the history is concerned, this has been done before, but the

instrument, which is indigenously made, uses a new technique. As the name suggests, there is a kite

which is connected to a system of springs (in series) via a thread. The springs are connected and

placed in a calibrated guide tube in such a way that the raw strength generated due to the tension in

the air-borne string duly attached to the kite does not disturb the spring system as a whole. Thus,

whenever the kite flies to a desired and achievable height there is an appreciable tension in the string

which in turn pulls the system of the springs and by finding out the extension in the spring system we

can determine the wind velocity and thus get the required wind data for a particular height above sea

level. The diagram below illustrates the instrument (see Figures 3 and 4).

Figure 3: Basic design of the kite anemometer Figure 4: Profile followed by the kite’s

string.

3. Measurement After the construction of the anemometer, it was taken to Pondicherry, for a

trial run and later actual velocity measurements were made. The experiment was performed at the

coast of Pondicherry at 10:00am with clear skies and along a rough sea. The kite was flown at various

heights and the spring extensions corresponding to its elevation were noted. The value of the spring

extensions was put into the derived equation (1) to get the wind velocity. By performing an energy

balance we obtain an empirical equation relating the wind velocity to the stretch of the spring in the

following way:

0.5CdρA (U (z)) 3 t - 2μAU (z) = 0.5Kx2 (1)

Where,

Cd is the coefficient of drag, ρ the density of air (kgm-3

), A the area of the kite (m2), U (z) the velocity

of wind (msec-1

), t the time taken for the spring to stretch (sec), μ the viscosity of air (N-sm-2

), K the

spring constant (Nm-1

) and x the spring extension (m).

In equation (1) the value of μ is very small and hence we ignore the term 2μAU (z). So, the equation reduces to:

Cd ρA (U (z)) 3 t = Kx2 (2)

Yielding,

U (z) = (K x2/CdρAt)0.33 (3)

The coefficient of drag is related to the height of the kite (Stull 1994). Cd = (k/log (z /z0))

2 (4)

where,

k is the von Karman Constant (approx 0.4), z the vertical height of the kite (m) and z0 the roughness

length (m). The density of air, spring constant, von Karman constant and the area of the kite are assumed to be

constant.

Parameter Value

Density of air (ρ) 1.2 kgm-3

Area of kite (A) 0.23m2

Spring stretch time(t) 0.02 s

Spring Constant (K) 100 Nm-1

Roughness length (z0) 2.0, 0.125 m

von Karman constant 0.4

Table 1: Inputs for the kite anemometric analysis of the wind velocity

Equation (3), is evaluated using constants from Table 1. The wind velocities thus obtained at

different heights for the coastline with defences and without defences are tabulated below.

z (m) Spring

Extension (m)

Velocity

U(z) (ms-1)

16.53 0.003 0.286

43.5 0.006 2.43

45.38 0.004 1.11

48.86 0.007 3.56

52.78 0.008 4.88

z (m) Spring

Extension(m)

) (m)

Velocity U(z)

(ms-1)

40.0 0.001 0.892

43.83 0.002 1.4314

55.31 0.004 2.332

62.57 0.005 2.744

74.45 0.007 3.134

Table 2: Heights and corresponding

wind velocities for the beach with

defences

Table 3: Heights and corresponding wind

velocities for the beach without defences

(z0 = 0.125)

From the values of wind velocity tabulated above, the following graph was plotted. We can see from

Figure 5 that a wind speed reduction of about 54% takes place at the beach with coastal defences.

Figure 5: A comparision between the wind velocity profiles with and without coastal defences . Note the

decrease of 54% in wind velocity.

It is clear from Figure 5 the wind velocity along the coastline is halved when coastal defences are in

place.The figure also reveals the kite anemometric measurements(represented by *) are approximately

uniformly distributed along a standard logarithmic wind profile (Stull 1994). In the subsequent phase

of our analysis we require that such velocity measurements are repeated under stable neutral and

unstable conditions with a wider spatial and temporal coverage including measurements along a point

deep within the sea where the affects of defences are at their minimun.In the light of the above

requirements it was decided that we develop the kite anemometer fabricated thus far to an altogether

higher level of sophistication enabling us to obtain in-situ measurements automatically. This we

describe in the next section.

4. Automated Kite Anemometry using pressure sensors The setup consists of a

kite fixed to a strain gauge, sensors and a data logger. The strain gauge is placed at the holding end of

the kite. The string used to fly the kite is provided with streamers at every meter. The kite is flown for

over 100 meters height and along a stable direction of flow. By obtaining the strain developed in the

string and the angle of the string with the horizontal, the wind velocity can be calculated. The

streamers flow in accordance to the wind profile. A photograph of the kite with its string is taken

from an appropriate position. The configuration of the streamers helps us calculate the variation of the

velocity of wind with height. A temperature and pressure sensor is included for the corresponding

measures at every point. See Figure 6 for a schematic outline.

Figure 6: The schematic of the apparatus, the block diagram of the instrumentation and the snapshot of the

AVR Butterfly Board.

The Strain gauge gives the forces acting at three different points. The strain gauge on the string at the

kite end gives us the cumulative of the force with which we are pulling the string (Fp), the drag force

on the string (Fds), and the weight of the string (Fws). i.e F= Fp + Fds + Fws . (In which Fds may be

negligible). By calculating F, considering the geometry of the kite, the angle of the kite, we can

calculate the velocity of wind.

The pressure sensor consists of a diaphragm construction involving strain gauges either bonded to, or diffused into it, acting as resistive elements. Under the pressure-induced strain, the resistive values

change.

Figure 7: Wheatstone bridge based pressure sensor circuit.

5. Concluding Remarks Coastal defences in the forms of rocks and boulders are being put in

place along coastlines in many countries affected by the last tsunami. These added roughness

elements have a significant impact on the turbulence characteristics of the boundary layer. This has

not been investigated quantitatively. This first study over a convective boundary layer along the

coastline of Pondicherry shows the slowing down of wind velocity to be as much as 50%. This was

achieved by a simple kite anemometric device that measured the wind velocity accurately. In the next

part of the analysis, prompted by the success of the first, the process of procuring real time air

velocity data automatically was consolidated by the use of pressure sensors, streamers and an

electronic data logger. To our knowledge this has never been done before-the cost effectiveness and

the portability makes this apparatus stand apart. We hope that we can make available this device to

meteorologists, pollution control board scientists, fluid dynamists as well as researchers who are

engaged in diffusion experiments in the atmosphere and wind flow over complex terrain.

6. References

[1] R.Stull. An Introduction to Boundary Layer Meteorology. Kluwer Academic Publisher.1994

[2] S.Sarkar, R.Jha, R.Bardhan, R.Raj and S.Ghosh. Post-Tsunami coastal defences along Pondicherry: Impact on cloud microphysics. Geophysical Research Abstracts, Vol. 10, EGU 2008 -A- 00274, 2008.

[3] AVR Butterfly Evaluation Kit, User Guide. Atmel Corporation , 2005

The resistors (Figure 7) have a value of approx. 3.5 kΩ. Pressure

induced strain increases the value of the radial resistors (r), and

decreases the value of the resistors (t) transverse to the radius. This resistance change can be high as 30%. The resistors are

connected as a Wheatstone Bridge, the output of which is directly

proportional to the pressure.