EFFECT OF SURFACE COVER ON GROUND TEMPERATURE SEASON S FLUCTUATIONS

Transcription

1 FOUNDATIONS OF CIVIL AND ENVIRONMENTAL ENGINEERING No. Czesław OLEŚKOWICZ-POPIEL, Janusz WOJTKOWIAK, Ilona PRĘTKA Poznan University of Technology Institute of Environmental Engineering EFFECT OF SURFACE COVER ON GROUND TEMPERATURE SEASON S FLUCTUATIONS In this paper, temperature distributions measured in the ground since summer of 1999 to the end of 1 are reported. The measurements were done in Poznan City region for two differently covered ground surface locations (car park and lawn). Temperature was measured with thermocouples distributed in the ground at the depth from to 7 m. It was found that during this period the ground temperature season s fluctuations are very similar and this resemblance is rising with depth. The effect of the surface cover is seen from May to October when the ground temperatures for the car park station are higher then for the lawn station. For example, in August at the depth of about m the temperature differences reaching about K are recorded. The temperature distributions calculated with the semi-empirical Buggs s formula adapted to the Northern Hemisphere and adjusted to the Poznan City region are in a good agreement with the experimental data. Key words: natural ground temperature distributions 1. INTRODUCTION For determination thermal interactions of engineering systems with the ground a precise knowledge in undisturbed ground temperature distributions are required. Besides, the knowledge of the ground temperature is important for the estimation of nitrification processes and of the biodegradation of organic substances. Publishing House of Poznan University of Technology, Poznań ISSN -933

2 15 Czesław Oleśkowicz-Popiel, Janusz Wojtkowiak, Ilona Prętka The ground temperature distribution depends on the following factors: a) physical properties of the ground, b) ground surface cover (e.g. bare ground, lawn, snow), c) climate interaction determined by air temperature, wind, solar radiation, air humidity and rainfall. From the point of view of the temperature distribution, the following zones are introduced [5]: 1. Surface zone reaching a depth of about 1 m, in which the ground temperature is sensitive to short time changes of weather conditions.. Shallow zone extending from the depth of about 1 to m (for dry and light soil) or to m (for moist heavy sandy soils). In this zone the ground temperature distributions depend mainly on the seasonal cycle weather conditions. At the lower level of this zone the ground temperature is almost constant and close to the average annual air temperature [1]. 3. Deep zone below about m, where the ground temperature is practically constant and very slowly rising with depth according to the geothermal gradient. Numerical simulations of ground temperature distributions are not reliable, mainly because of difficulties in precise determination of the surface boundary conditions (i.e. climate and weather interactions) and as well as physical properties of ground (i.e. density, specific heat and thermal conductivity). Therefore simple semi-empirical formulas, such as for example, a formula proposed by Baggs [1, ] for Australian climate conditions are more reliable and easy to use for prediction of the average ground temperature distribution. In this communication, such a formula adopted by Popiel et al. [5] for the Northern Hemisphere and the Poznan City region climate conditions is compared with the new experimental data. The results of measurements of the temperature distributions in ground collected in the Poznan University Campus since summer of 1999 to the end of 1 are reported.. EXPERIMENTAL ARRANGEMENTS As the ground cover affects the ground temperature distribution [] two experimental stations were used for the ground temperature monitoring. One station having bare ground surface, i.e. having surface clear of vegetation was located on a car park paved with bricks. The second one was located on a lawn having short grass cover. Both stations were located in the city of Poznan on the University Campus at a distance 3 (car park) and 5 m (lawn) from low buildings. Temperature were measured with the PVC insulated Cu-Konstantan thermocouples distributed in ground at a depth from to 7 m. For temperature read-

3 Effect of surface cover on ground temperature season s fluctuations 153 ings the thermocouple meter type SR (Stanford Research System, USA) having resolution ±. 1 K was used. The temperature readings were taken every 7 or days at about 1 to 3 p.m. 3. RESULTS In Fig. 1 and Fig. the annual evolutions of the measured and calculated (with the formula 1) ground temperatures at various depths for the car park and lawn stations in the year periods of 1999, and 1 are shown. The formula of Baggs [1, ] adopted by Popiel et al. (1) for the northern hemisphere has a form T ( x, t) = ( T m ± T π cos ( t t 35 o m ) 1.7k.1335* xa v A exp(.3155* xa s.5 ).5 ) (1) where: a average annual (apparent) thermal diffusivity of undisturbed ground [], m /s A s amplitude of annual average air temperature wave, K k v vegetation coefficient, t o phase of air temperature wave, days T m average annual air temperature, C T m ground temperature differential, K Semi-empirical formula (1) is based on the theoretical solution for a transient heat conduction in a semi-infinite solid [3, p. ] where the temperature of the exposed surface (x = ) is varying periodically with time Tx=, t = As cos[ π ( t to )/ 35]. Data given in Table 1 were used in the calculation of the temperature distributions shown in Appendix 1 and. Values of T m and A s are based on the last 5 years on-site meteorological records. The values of the ground temperature differentials T m for car park and lawn, respectively, were estimated experimentally (e.g. from Appendix 1g and from Appebdix g, and using T m = 9. o C). The vegetation coefficient k υ for lawn is taken k =.5, and for car park station this coefficient is a function of time t: k v e = a + b exp(.5 ( t c / d) ) ()

4 15 Czesław Oleśkowicz-Popiel, Janusz Wojtkowiak, Ilona Prętka where: a = , b =., c =.95, d = , e = = This equation was chosen to obtain higher values of the coefficient in the period of higher solar radiation (Fig. 1). Table 1 CAR PARK LAWN A s [K] a [m /s] 55x - 55x - t o [days] 7 T m [ o C] T m [K].1.5 k υ Eq. ().5 1,5 1, Kv-car p 1,3 1, 1,1 1, TIME [days] Fig. 1. Vegetation coefficient k υ versus time t for the car park station At the depth below of about 1 m the Buggs s formula adjusted to the Poznan city region shows a reasonable good agreement with the measured ground temperature distributions for the car park station (Appendix 1) and for the lawn station (Appendix ). At the depth of x = 1 m the short-term temperature fluctuations are discernible but they cease with the depth. In the case of the car park station during the late spring and summer period some departure of the experimental data from computed results was

5 Effect of surface cover on ground temperature season s fluctuations 155 partly eleminated by means of modification of the vegetation coefficient. This departure is a result of solar irradiation showing its strong effect on the bare ground surface (car park). The grass plays a role of some kind of a solar radiation screen, and the vegetation coefficient could be assumed constant (k υ = =.5. During that period at the depth of about 3 m the ground temperature under the bare surface is about degree higher then under grass cover.. CONCLUDING REMARKS (1) It has been found that the amplitude of season fluctuations of temperature in ground depends on the depth and on the kind of surface cover. Under short grass surface the temperature fluctuations are considerably lower. The effect of the surface cover is seen from May to October when the ground temperatures for the car park station are higher than for the lawn station. It is clear that the grass cover plays a role of a screen decreasing the effect of solar radiation. () In the time of the ground temperature monitoring from the summer of 1999 to the end of 1 the season s temperature fluctuations show a good repeatability at the depth below about 1 m. (3) Semiempirical formula (Eq. 1) describes the temperature distributions in the undisturbed ground with the satisfactory accuracy at the depth below about 1m. In this formula the effect of surface cover is represented by the vegetation coefficient which for the lawn station is constant (k υ =.5) and for the car park station varies from k υ = 1. to about 1.5. Nomenclature: a average annual (apparent) thermal diffusivity of undisturbed ground, m /s A s amplitude of annual average air temperature wave (based on mean average yearly maximum and minimum on a monthly basis), K k υ vegetation coefficient, t time, day t o phase of air temperature wave, days T temperature, K T m average annual air temperature, o C T m ground temperature differential, K x depth below the ground surface, m

6 15 Czesław Oleśkowicz-Popiel, Janusz Wojtkowiak, Ilona Prętka REFERENCES 1. Baggs S.A.: Remote prediction of ground temperature in Australian soils and mapping its distribution, Solar Energy, 3 (193) Baggs S.A. et al.: Australian earth-covered building, New South Wales, New South Wales University Press 1991, (Appendices). 3. Eckert E.R.G., Drake R.M.: Analysis of heat and mass transfer, New York, McGraw-Hill Book Mihalakakou G., Santamouris M., Jewis J.O., Asimakopoulos D.N.: On the application of the energy equation to predict ground temperature profiles, Solar Energy, (1997), Popiel C.O., Wojtkowiak J., Biernacka B.: Measurements of temperature distribution in ground. Experimental thermal and fluid science, Int. Journal, 5 (1) C. Oleśkowicz-Popiel, J. Wojtkowiak, I. Prętka WPŁYW RODZAJU POWIERZCHNI NA SEZONOWE FLUKTUACJE TEMPERATURY GRUNTU S t r e s z c z e n i e Praca zawiera wyniki pomiarów rozkładów temperatury gruntu w okresie od lata 1999 do końca 1 r. Pomiary były wykonane w Poznaniu na dwóch stanowiskach o różnych powierzchniach gruntu (parking i trawnik). Temperatura była mierzona za pomocą termopar rozlokowanych w gruncie na głębokości od do 7 m. Stwierdzono, że w okresach rocznych sezonowe fluktuacje temperatury gruntu są bardzo podobne i to podobieństwo rośnie z głębokością. Wpływ rodzaju pokrycia powierzchni na rozkład temperatury jest widoczny głównie od maja do października. Wówczas temperatura gruntu pod parkingiem jest wyższa niż na stanowisku po trawą. Na przykład, w sierpniu na głębokości około m różnica temperatury gruntu pod parkingiem i pod trawnikiem osiągała około stopni. Rozkład temperatury obliczony z półempirycznego wzoru Baggsa, adaptowanego do północnej półkuli i przystosowanego do rejonu Poznania, wykazuje dobrą zgodność z danymi eksperymentalnymi. Received,

7 Effect of surface cover on ground temperature season s fluctuations 157 APPENDIX 1: History of the measured and calculated with the Eq. (1) ground temperature at various depths for the lawn station in the year period of 1999, and T [oc Eq.(1) (a): x = 1 m

8 15 Czesław Oleśkowicz-Popiel, Janusz Wojtkowiak, Ilona Prętka 1 1 (b): x = m (c): x = 3 m

9 Effect of surface cover on ground temperature season s fluctuations (d): x = m (e): x = 5 m

10 Czesław Oleśkowicz-Popiel, Janusz Wojtkowiak, Ilona Prętka 1 (f): x = m (g): x =.9 m APPENDIX :

11 Effect of surface cover on ground temperature season s fluctuations 11 APPENDIX : History of the measured and calculated with the Eq. (1) ground temperature at various depths for the lawn station in the year period of 1999, and T [oc Eq.(1) 1 1 (a): x = 1 m

12 Czesław Oleśkowicz-Popiel, Janusz Wojtkowiak, Ilona Prętka 1 1 (b): x = m (c): x = 3 m

13 Effect of surface cover on ground temperature season s fluctuations (d): x = m (e): x = 5 m

14 1 Czesław Oleśkowicz-Popiel, Janusz Wojtkowiak, Ilona Prętka (f): x = m (g): x =.9 m