Browsing by Author "Perera, P. A. A."

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An analysis on the optimization of the upper convective zone thickness to minimize the surface heat loss of a model salinity gradient pond
(Faculty of Science, University of Kelaniya, Sri Lanka, 2021) Jayatissa, N.W. K.; Perera, P. A. A.; Attalage, R. A.
Solar energy is a renewable energy source that provides enough energy to keep the natural cycles alive. Sri Lanka is a country near the equator that has abundant sunlight throughout the year. Solar ponds are a proven technology for storing thermal energy at low temperatures. In general, a solar pond consists of three layers: the upper convective zone (UCZ), the middle non-convective zone (NCZ) and the lower convective zone (LCZ). Part of the incident solar radiation passes through the UCZ and NCZ and is trapped in the LCZ. Unlike the other two layers, the top layer of the pond is directly exposed to the environment. The energy flows associated with the UCZ and their impact on the LCZ were the focus of this experimental study. The solar pond described in the present work is constructed in the premises of the University of Kelaniya, Sri Lanka (Latitude 6.97 N, Longitude 79.91 E). The pond has a surface area of 6 m2 (3 m  2 m) and a depth 1.5 m. The bottom of the pond and the walls are properly insulated to reduce energy loss through the walls and bottom. Since the UCZ is exposed to the environment, heat is lost from this area through convection, evaporation and radiation. The average UCZ temperature of the solar pond varies from 27.3 oC to 31.5 oC depending on the month of the year. This study shows that the total energy loss (radiation, convection and evaporation) of the upper surface varies between 11 Wm-2 and 57 Wm-2. The convective heat transfer coefficient is directly related to the average wind speed, and in this study the wind effect is minimal because the water surface is blocked by the perimeter walls. Therefore, the wind effect is neglected in the energy calculations. The radiant heat loss is estimated to be about 40% of the total energy, assuming an emissivity of 0.83 for water. Since the wind effect was negligible in this study, the estimated evaporative heat loss is about 10% of the total energy. Depending on the solar insolation, energy from the NCZ and the total losses associated with the UCZ determine the heat storage of the UCZ. The easily adjustable parameter of an established solar pond is the thickness of the UCZ. A thicker UCZ can hold more incident solar insolation inside and this causes high temperature values in the zone. High temperature values in the UCZ minimize the conduction of heat transfer from the LCZ to the UCZ, while decreasing surface heat loss by convection and radiation. However, this process reduces the amount of solar radiation entering the LCZ. When the thickness of the UCZ layer was increased from 5 cm to 10 cm, the energy absorption increased from 42% to 47%. Similarly, increasing the thickness to 20 cm, resulted in an additional 6% increase. Therefore, ultimately it affects the LCZ heat storage. On the other hand, reducing the thickness of the UCZ decreases its temperature and allows more insolation to reach the bottom of the pond. Therefore, it is important to optimize the thickness of the UCZ and the results suggested that a thickness in the range 2 to 8 cm is optimal for the operation of the pond.
A modified equation for Roche limit for celestial bodies with a fluid-like structure
(Faculty of Science, University of Kelaniya Sri Lanka, 2023) Thammitage, S. U. R.; Perera, P. A. A.; Hewageegana, P.
The Roche limit is a concept in astronomy that describes the minimum distance that a celestial body (say satellite) can approach another celestial body (say planet) without being torn into small pieces by tidal forces. This happens when the tidal forces generated on the satellite by the gravitational fields of the planet exceed the self-attractional forces of the satellite. If the planet and satellite are of similar chemical composition, the theoretical Roche limit is about 2.5 times the radius of the planet. Generally, two models have been used for the derivation of Roche limit. If the satellite is assumed to be a solid object, the Roche limit is given by 𝑑 = 1.22𝑅𝑝 (𝜌𝑝 𝜌𝑠) 1 3 and if the physical properties of the satellite are akin to those of a fluid, the expression turns out to be 𝑑 = 2.44𝑅𝑝 (𝜌𝑝 𝜌𝑠)13. Here 𝜌𝑝 and 𝜌𝑠 are the densities of the planet and satellite, respectively and 𝑅𝑝 is the radius of the planet. These equations have been derived in real physical space for the case where the motion of the satellite and the planet are in the same orbital plane, that is both objects must be in the same equatorial plane. But satellites or asteroids orbit different planes or can enter from another plane like Pluto’s orbit. Therefore, a new equation for Roche limit for fluid like satellites, when the equatorial plane of the satellite is tilted to the orbital plane was developed using the tidal generating potential equation and self-gravitational potential equation. This equation takes the form 𝑑 = 2.423𝑅𝑝 (𝜌𝑝 𝜌𝑠) 13 (cos2 𝛼 – 1 3)13, where 𝛼 is the angle between the orbital plane of the satellite and its equatorial plane.
A theoretical study of variable stars
(Research Symposium on Pure and Applied Sciences, 2018 Faculty of Science, University of Kelaniya, Sri Lanka, 2018) Wijethunga, A. T. M.; Perera, P. A. A.
A variable star is, quite simply, a star that changes its brightness with a certain regularity. The Energy required for pulsation stars’ continuous operation is provided by a complex mechanism of transforming the thermal energy into the mechanical energy of pulsation. Pulsating stars can be considered as heat engines and must behave according to the same thermodynamic principles which are applicable to other thermodynamic heat machines. For most of the variable stars, the pulsating procedure consists of expansion and compression of the diameter of the stars and the energy is originated from a nuclear fusion reaction which involves H and He nuclei which are the main constituents of a typical mainsequence star. Study of pulsating stars is inherently interesting, because it is now understood that various properties of these stars can be used to determine some important cosmological distances. In the present research a simple numerical model, based on the principles of physics a typical undergraduate student learns, has been developed to explain the properties of Delta Cephei which is a prototype variable star with the observed pulsation period of 5 days, 8 hr, 48 min. The model predicts distinctly non-sinusoidal oscillations and very closely reproduces the observed period. The period “hiding” in the equations can lead to the technique of linearization and a discussion of how small departures from stable equilibrium result in simple harmonic motion. Using non-linearization and linearization methods, relationships between star mass and pulsating period has been found as follows. Non-linearization, T = -0.54 Z5 + 0.42 Z4 + 0.27 Z3 + 0.57 Z2 -1.6 Z + 3.3; Z = (M-1.4×1031)/(4.8×1030) Linearization, T = -0.42 Z5 + 0.66 Z4 - 0.035 Z3 + 0.27 Z2 -1.5 Z + 3.3; Z = (M-1.4×1031)/( 4.8×1030)