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**Time scale** may refer to:

- Time standard, a specification of either the rate at which time passes, points in time, or both
- A duration or quantity of time:
- Orders of magnitude (time) as a power of 10 in seconds;
- A specific unit of time

- Geological time scale, a scale that divides up the history of Earth into scientifically meaningful periods

In **astronomy and physics**:

- Dynamical time scale, in stellar physics, the time in which changes in one part of a body can be communicated to the rest of that body, or in celestial mechanics, a realization of a time-like argument based on a dynamical theory
- Nuclear timescale, an estimate of the lifetime of a star based solely on its rate of fuel consumption
- Thermal time scale, an estimate of the lifetime of a star once the fuel reserves at its center are used up

In **cosmology and particle physics**:

- Planck time, the time scale beneath which quantum effects are comparable in significance to gravitational effects

In **mathematics**:

- Time-scale calculus, the unification of the theory of difference equations with differential equations

In **music**:

- Rhythm, a temporal pattern of events
- Time scale (music), which divides music into sections of time

In **project management**:

- Man-hour, the time scale used in project management to account for human labor planned or utilized

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In physics and engineering, **fluid dynamics** is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics and **hydrodynamics**. Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and modelling fission weapon detonation.

A **mathematical model** is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed **mathematical modeling**. Mathematical models are used in the natural sciences and engineering disciplines, as well as in the social sciences.

**Quantum gravity** (**QG**) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored, such as in the vicinity of black holes or similar compact astrophysical objects where the effects of gravity are strong.

**Classical physics** refers to theories of physics that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major paradigm shift, then the previous theories, or new theories based on the older paradigm, will often be referred to as belonging to the realm of "classical physics".

**Causality** is the relationship between causes and effects. It is considered to be fundamental to all natural science – especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics. From the perspective of physics, causality cannot occur between an effect and an event that is not in the back (past) light cone of said effect. Similarly, a cause cannot have an effect outside its front (future) light cone.

**Time dilation** is a difference in the elapsed time measured by two clocks, either due to them having a velocity relative to each other, or by there being a gravitational potential difference between their locations. After compensating for varying signal delays due to the changing distance between an observer and a moving clock, the observer will measure the moving clock as ticking slower than a clock that is at rest in the observer's own reference frame. A clock that is close to a massive body will record less elapsed time than a clock situated further from the said massive body.

**Computer simulation** is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of or the outcome of a real-world or physical system. Since they allow to check the reliability of chosen mathematical models, computer simulations have become a useful tool for the mathematical modeling of many natural systems in physics, astrophysics, climatology, chemistry, biology and manufacturing, as well as human systems in economics, psychology, social science, health care and engineering. Simulation of a system is represented as the running of the system's model. It can be used to explore and gain new insights into new technology and to estimate the performance of systems too complex for analytical solutions.

**Stellar dynamics** is the branch of astrophysics which describes in a statistical way the collective motions of stars subject to their mutual gravity. The essential difference from celestial mechanics is that each star contributes more or less equally to the total gravitational field, whereas in celestial mechanics the pull of a massive body dominates any satellite orbits.

The **metabolic theory of ecology** (**MTE**) is an extension of Kleiber's law and posits that the metabolic rate of organisms is the fundamental biological rate that governs most observed patterns in ecology. MTE is part of a larger set of theory known as metabolic scaling theory that attempts to provide a unified theory for the importance of metabolism in driving pattern and process in biology from the level of cells all the way to the biosphere.

**Richard Ernest Bellman** was an American applied mathematician, who introduced dynamic programming in 1953, and made important contributions in other fields of mathematics.

**Allometry** is the study of the relationship of body size to shape, anatomy, physiology and finally behaviour, first outlined by Otto Snell in 1892, by D'Arcy Thompson in 1917 in *On Growth and Form* and by Julian Huxley in 1932.

**Theoretical astronomy** is the use of the analytical models of physics and chemistry to describe astronomical objects and astronomical phenomena.

The **dynamic energy budget** (**DEB**) **theory** is a formal metabolic theory which provides a single quantitative framework to dynamically describe the aspects of metabolism of all living organisms at the individual level, based on assumptions about energy uptake, storage, and utilization of various substances. The DEB theory adheres to stringent thermodynamic principles, is motivated by universally observed patterns, is non-species specific, and links different levels of biological organization as prescribed by the implications of energetics. Models based on the DEB theory have been successfully applied to over a 1000 species with real-life applications ranging from conservation, aquaculture, general ecology, and ecotoxicology. The theory is contributing to the theoretical underpinning of the emerging field of metabolic ecology.

In physics and astronomy, an ** N-body simulation** is a simulation of a dynamical system of particles, usually under the influence of physical forces, such as gravity.

In astrophysics, the **nuclear timescale** is an estimate of the lifetime of a star based solely on its rate of fuel consumption. Along with the thermal and free-fall time scales, it is used to estimate the length of time a particular star will remain in a certain phase of its life and its lifespan if hypothetical conditions are met. In reality, the lifespan of a star is greater than what is estimated by the nuclear time scale because as one fuel becomes scarce, another will generally take its place—hydrogen burning gives way to helium burning, etc. However, all the phases after hydrogen burning combined typically add up to less than 10% of the duration of hydrogen burning.

In astrophysics, the **thermal time scale** or **Kelvin-Helmholtz time scale** is the approximate time it takes for a star to radiate away its total kinetic energy content at its current luminosity rate. Along with the nuclear and free-fall time scales, it is used to estimate the length of time a particular star will remain in a certain phase of its life and its lifespan if hypothetical conditions are met. In reality, the lifespan of a star is greater than what is estimated by the thermal time scale because as one fuel becomes scarce, another will generally take its place – hydrogen burning gives way to helium burning, which is replaced by carbon burning.

Observations suggest that the expansion of the universe will continue forever. If so, then a popular theory is that the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario once popularly called "**Heat Death**" is now known as the **Big Chill** or **Big Freeze**.

Physics deals with the combination of matter and energy. It also deals with a wide variety of systems, about which theories have been developed that are used by physicists. In general, theories are experimentally tested numerous times before they are accepted as correct as a description of Nature. For instance, the theory of classical mechanics accurately describes the motion of objects, provided they are much larger than atoms and moving at much less than the speed of light. These "central theories" are important tools for research in more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.

**Combustion models for CFD** refers to combustion models for computational fluid dynamics. Combustion is defined as a chemical reaction in which a hydrocarbon fuel reacts with an oxidant to form products, accompanied with the release of energy in the form of heat. Being the integral part of various engineering applications like: internal combustion engines, aircraft engines, rocket engines, furnaces, and power station combustors, combustion manifests itself as a wide domain during the design, analysis and performance characteristics stages of the above-mentioned applications. With the added complexity of chemical kinetics and achieving reacting flow mixture environment, proper modeling physics has to be incorporated during computational fluid dynamic (CFD) simulations of combustion. Hence the following discussion presents a general outline of the various adequate models incorporated with the Computational fluid dynamic code to model the process of combustion.

**Project production management** (**PPM**) is the application of operations management to the delivery of capital projects. The PPM framework is based on a project as a production system view, in which a project transforms inputs into outputs.

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