Book Review: The Maverick Effect

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Reading biographies is the best way to draw inspiration and learn lessons from other's experiences. The biography of an individual can give us an idea of the incidents that shaped the person into what they have become. Mahatma Gandhi's book "My Experiments with Truth" is one such example. For leadership and entrepreneurial enthusiasts, reading the biography of an institution or company helps. The story of "Made in Japan" by Akio Morita of Sony is the best example of this. How often have we come across the biography of an association such as NASSCOM?

I recently read "The Maverick Effect" written by Harish Mehta. The book narrates the growth of NASSCOM as a multilateral body whose sole aim is to tap the huge economic potential the IT industry offered, convincing and negotiating with other companies/organizations to represent their challenge in an unified voice, and lobby with the government to keep the economy rolling. I strongly recommend this book to all young and fresh bright minds of India.

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Nocturnal cooling of earth and atmosphere

Sun is the primary source of energy for all living things on earth. Sunlight is needed for photosynthesis by the leaves of the plants, which in turn produces food in the presence of few other ingredients. The heat from the sun evaporates water, forming clouds and rain, creating a water cycle. The land surface gets heated up due to sunlight, causing the atmosphere to become warmer and more liveable for most creatures. A fraction of the energy gained by various parts of the earth’s surface is transferred to the atmosphere.

Air in the atmosphere is a mixture of gases, predominantly oxygen and nitrogen, with other trace gases. Air is often loaded with a dynamic distribution of dust or solid suspended particles and water in liquid, vapour, and solid phases. Air which does not contain a significant amount of solid and liquid particles is called dry air. Certain gases in dry air can potentially absorb the energy transferred back by the earth’s surface, making the atmosphere we live in habitable. If the earth and its atmosphere retain the heat from the sun continuously, the temperature will rise till it becomes unbearable. Certain gases in the atmosphere, especially carbon dioxide, have this property. We call this greenhouse gas.

But what happens in the absence of sunlight? The darker parts of the earth no longer get heated up. The heat from the earth’s interior (a few meters below the earth’s surface) will reach the earth’s surface. The surface, in turn, will try to transfer the heat accumulated during the daytime to colder surroundings, which are the surrounding atmosphere and the deep dark space. The heat reaching from the interior to the surface is called conduction. The surface, in turn, transfers heat to the surrounding air in the form of convection, and a part of the energy will be radiated by the earth’s surface.

The radiated component is quite complex. Part of the energy released by the surface gets absorbed by greenhouse gases in the atmosphere. These are carbon dioxide, water vapour, clouds, methane, etc. The remaining part will escape the atmosphere to reach the colder space. Under clear dry sky conditions, heat convection to the surroundings becomes negligible. The energy stored from the deeper soil layers will conduct to the surface, which in turn gets radiated to the cold space. The balance between this conduction and radiation helps us to understand how quickly the earth’s surface gets colder at night, also known as nocturnal cooling.

Under suitable environmental conditions, people worldwide have used nocturnal cooling for the mass production of ice, even when the surrounding atmosphere is warmer! Tetsu Tamura, a Japanese meteorologist, in his published work [1], describes how soil conduction and clear sky radiation contribute to the faster rate of surface cooling. This work was published more than a century ago and is considered a classic example of a conduction–radiation problem in heat transfer.

[1] S. Tetsu Tamura, (1905), Mathematical Theory of the Nocturnal Cooling of the Atmosphere, Monthly Weather Review, Vol. 33, pp. 138-147.

Thermal Protection System

Rub your palms together, and you will feel the heat generated between them. This frictional heat often keeps us warm during cold winter nights. So what happens if you rub your palms faster? You will feel more heat. Imagine that you could rub your palm at 7500 m/s! Your palm can catch fire at this speed. Rockets which are used to put satellites in orbit or other heavenly objects like Moon, Jupiter, etc., are typically launched or re-entered through our atmosphere travelling at incredible speeds. The outer surface of the rocket and the surrounding air from the atmosphere act as two palms.

The extreme heat generated by friction is called aerodynamic heating. The generated heat is so high that aluminium (often used for building the space vehicle) melts and gets removed from the main body. A thermal protection system (TPS) is needed to protect the space vehicle, which can withstand extreme heat and yet does not allow the heat to penetrate the interior of the vehicle.

Heat penetrates through a solid material through conduction. If the conductivity of solid material is very high (true for aluminium and other metals), they can quickly transfer the heat to the other side. A good TPS should have low thermal conductivity to prevent the heat from penetrating inside. At a high heat rate, the vehicle’s temperature can also be very high (exceeding 1500 deg C), at which most materials melt. A good TPS should also be made from a material having a very high melting point.

However, in space, mass is a premium. Every mass added to the vehicle’s design will reduce the mass of scientific payloads such as sensors and instruments. As a trade-off, during the re-entry, a small part (layer) of the TPS material is allowed to melt. This serves two purposes: Firstly, melting involves latent heat (the amount of heat absorbed by the material to change its phase from solid to liquid). As latent heat is usually very high for most substances, part of TPS gets removed upon melting, carrying significant heat away from the vehicle. This ensures that less heat remains for conduction. The removed TPS material now allows a fresh layer of TPS to absorb the heat, and the process continues till the entire TPS material is removed by this process. The other purpose is that since melted TPS material gets detached from the remaining solid part, the melt layer gets removed from the vehicle, thereby reducing its overall weight.

To summarize, more TPS material results in an increased non-scientific payload of the vehicle. While lesser TPS material could result in the entire TPS getting melted off, exposing the base material (an alloy of aluminium) to the harsh environment. So the thickness of TPS should be designed considering the above constraints.

In a recent paper co-authored with Prof. Katte [1], we presented a one-dimensional transient heat conduction model involving phase change at the boundary to simulate the performance of TPS as a function of time. The model considers the outer surface of TPS exposed to extreme heat, while the other surface shielding the base material is insulated. The simulations are done until the TPS base reaches a predefined temperature in non-dimensional form.

Reference:

S. R. Kannan and S. S. Katte, (2018), Numerical Investigation and Correlations for Heat Diffusion through Planar Ablative Thermal Protection Systems, Thermal Science & Engineering Progress, 7, 279-287.

Intercomparison between IMD ground radar and TRMM PR observations using alignment methodology and artificial neural network

Echolocation refers to locating the size and distance of objects in the surroundings using echo. Thousands of species use echolocation to navigate the world. Drawing inspiration from this nature-driven technology, humans have designed and built radar systems to detect and track objects remotely.

During the Second World War (when applied sciences flourished), radar technology was used to detect and target enemy's warplanes. A series of false-alarm whenever it rained offered an opportunity for developing radar to observe weather systems.

There has been no turning back since then. Radar is considered an irreplaceable modern technology that can provide accurate rainfall measurement over a large area. Meteorological organizations from around the world soon integrated weather radar into their observation systems. The polar plot (aka radar chart) helped us detect and track organized precipitation systems and provided more insight into the microphysics of rainfall.

Today, more than 4000 radar stations are built worldwide that continuously monitor the weather on a large scale. When ground stations are not sufficient due to their restricted mobility, radars are also carried on specially designed aircraft for conducting field observations such as the Indian summer monsoon.

In 1997, NASA of the U.S and JAXA of Japan jointly launched the Tropical Rainfall Measuring Mission (TRMM) satellite. The satellite was the first to carry a radar onboard to monitor precipitation systems on a global scale within the tropics. This allowed researchers worldwide to tune in to their surface observations whenever the satellite made an overpass.

However, cross-comparison of radar observations from satellite and ground-based poses several challenges. Even though technically, both the instruments work on the same principle, there could be a mismatch between them due to the difference in their viewing geometry, radar frequency, and other issues such as clutter.

Concept diagram to illustrate intercomparison of ground and space radar

In a recent paper co-authored by my research scholar [1], we had compared the ground radar observations maintained by the Indian Meteorological Department (IMD) with TRMM's Precipitation Radar using alignment methodology. The comparison study showed that the ground radar overestimates rainfall during the Indian summer monsoon period of 2013. We demonstrated that the positive bias of the ground radar measurement could be "corrected" to match with TRMM PR observations using an artificial neural network.

[1] Alok Sharma and Srinivasa Ramanujam Kannan, 2021, Intercomparison between IMD ground radar and TRMM PR observations using alignment methodology and artificial neural network, Journal of Earth System Science, Vol. 130, Article ID 0020.

Effect of humidity on the performance of rooftop solar chimney

When the air gets heated up, it expands and hence, becomes lighter. The less-dense warm air rises up against the gravity, allowing cold, heavy air to sink. This is the fundamental mechanism by which heat gets redistributed with the fluid medium following the natural convection mode.

Natural convection, well, happens naturally. It means no external source of power is used to push the air around. Take a flat black plate on a rooftop and expose it to sunlight. You will notice that after a while, the plate gets hotter. Natural convection heat transfer will then take place between the hot plate and the surrounding air. If we expose the surface to solar radiation for a longer duration, its temperature will further increase. But air can only gain a certain amount of heat. So, in long exposures, the plate loses heat by the radiation mode of heat transfer as well.

Concept diagram of a solar chimney

In solar thermal applications, heat transfer by natural convection is almost always coupled with the radiation mode. Take a solar chimney, which is a device that consists of an absorber plate (to absorb sunlight) and a glass cover. The daylight first passes through the glass cover before it strikes the absorber plate. The glass cover is added to trap the long-wavelength radiation emitted by the absorber plate as it gets heated up. The chimney, with its bottom end open, is usually mounted on the rooftop of a room. 

The air present in the space between the absorber plate and glass cover will get lighter and rise upward. This creates a vacuum inside the room. Fresh cool air through windows or open ventilation inside the room will move towards the chimney, inducing air circulation.

While the air inside the chimney predominantly picks up the heat by convective mode of heat transfer, the fundamental mechanism changes when air carries some moisture with it. The thermal properties of water vapour are different from dry air and can potentially affect the natural convection inside the chimney. Also, water vapour can absorb long-wavelength emitted by the absorber plate.

Graduate students under my guidance had investigated the effect of humidity (amount of water vapour present in air) on the performance of a rooftop solar chimney. Our study [1] shows that water vapour present in the air can improve the overall performance of the solar chimney (measured in terms of air change per hour inside the room) by 10%.

[1] Himanshu Dahire, Srinivasa Ramanujam Kannan, and Sunil Kumar Saw, 2021, Effect of humidity on the performance of rooftop solar chimney, Thermal Science and Engineering Progress, (available online).

The artefact that paved way for accurate rainfall detection

A weather radar is one of the technologies that changed the way we observe rainy systems. In simple words, radar is an instrument that transmits energy in the form of a wave. The wave travels through some distance until it comes in contact with an object which reflects the wave back towards the source. By measuring the time taken by the wave to hit the object and return to its source, one can infer the distance of the object from the detector. In the early days, this technology was widely used to detect and destroy enemy planes during war-time.

It is impossible to make anything fool-proof. There will always be some glitches in the technology that limits its applications. Radar technology for the detection of enemy's planes is no exception to this. Whenever it rained, the radar sent an alert suggesting that some objects are detected within the scan range. This lead to a series of false-alarms to an extent that the army stopped altogether to rely on radar signal during rainy conditions.

Marshall was the first to recognise that the artefacts that obscure the detection of war-planes and ships comes from rainfall and snow and the same can be used to quantify rainfall. In other words, the radar which was being used for the detection of warplanes, ships, and other objects has great potential to monitor and observe the weather.

Arthur Bent, a researcher working from the Massachusetts Institute of Technology conducted a detailed study on the possibility of using radar for the detection of rainfall. Bent in his published work [1] explained how radar echoes that are observed can be used in the detection of localised and general precipitation. This was perhaps one of the earliest work that explains the fundamental operation of weather radar.

[1] Arthur E Bent, 1946, Radar Detection of Precipitation, Journal of Meteorology, Vol. 3, pp. 78-84.

How many drops make the rain?

"Little drops of water... make the mighty ocean", wrote Julia Carney. We have a name for those "little drops of water". It is called rain or raindrops. The question that intrigued many great minds for ages is what is the size of the raindrop? And when it rains, how many drops are there in it?

In 1948, John Stewart Marshall and his doctoral student Walter Palmer performed a fundamental experiment to make the measurement of raindrops [1] for correlation with radar echoes. There wasn't any sophistication involved in their work. The duo collected raindrops on dyed filter papers which they then used to measure the size and get the number distribution of rain. 

Marshall and Palmer published their findings as very short communication. The paper, which was only one and a quarter page in length, became a game-changer. More than 70 years have passed. We have developed many new and innovative technologies to measure the raindrop size more accurately. But the distribution developed by Marshall and Palmer in 1948 with only the basic tools from their time is still being used for the estimation of rainfall with remarkable accuracy!

[1] J.S. Marshall and W. McK. Palmer, 1948, The distribution of raindrops with size, Journal of Meteorology, Vol. 5, pp.165-166.