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Juan C.
Aerospace Engineer offering tutoring in math and physics (English or Spanish)
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MATLAB
TutorMe
Question:

You are given a task to create a MATLAB code that can process large amounts of data. Which strategies can you use to ensure your code is optimized to process this data as quickly as possible?

Juan C.

There are a number of strategies that can be used to optimize the code speed: *Whenever possible, implement the code using matrix operations rather than single operations within loops. Especially avoid creating loops within loops, as these can slow down the execution significantly. *Identify all locations in the code where the same computation is being done more than once, and try to save this value to a variable rather than re-computing it multiple times. *MATLAB's 'profiler' tool is a very useful and easy to use tool that helps you see which parts of the code consume the most time, and need more optimization. You can also simply use the 'tic' and 'toc' functions to measure the time elapsed between two lines, and help you determine which changes improve performance. *Avoid constantly displaying data or plotting values, as these will slow down execution. *In some cases it may be worthwhile implementing parallelization to take maximum advantage of CPUs with multiple cores, or even leverage GPU computing.

Mechanical Engineering
TutorMe
Question:

Explain the meaning and importance of Second Law of Thermodynamics and the concept of entropy.

Juan C.

The Second Law of Thermodynamics states that the total entropy of an isolated system (one which matter nor energy flows in or out of it) can only increase over time. Entropy is a measure of how dispersed energy is within a system. Therefore, the Second Law simply states that the energy contained within an isolated system will always tend to spontaneously disperse itself rather than concentrate itself. Let's take as an example a microwave heating a cup of coffee. In this case we are seeing the energy concentrate itself in the coffee in the form of heat, so the entropy is decreasing. However, this system is not violating the Second Law because it is not an isolated system; energy is flowing into the microwave through its power cord. If we look at the microwave and the energy generation plant that supplies its electricity as a single isolated system, the net entropy of this entire system must be increasing. Let's say the power plant is powered by coal. The entropy increase being caused by burning the coal and releasing this highly-concentrated energy is much greater than the local entropy decrease of the coffee being heated, thus the total system entropy is increasing. The principle of the Second Law of Thermodynamics is so important and fundamental because it describes 'the arrow of time'; why things spontaneously change, and in which direction they change over time. It says that things change in the direction that produce energy dispersal, and the property that makes nature change over time are not energy itself, but differentials in energy (concentrated energy sources) that are dispersed over time.

Aerospace Engineering
TutorMe
Question:

Explain the significance and importance of the Reynolds number of a flow.

Juan C.

The Reynolds number is a dimensionless value that can be used to describe a flow, and is defined as Re = V*L/K, where V is the speed of the flow, L is a characteristic linear dimension of the body immersed in the flow, and K is the kinematic viscosity of the fluid. It is a measure of the ratio of inertial forces (V*L) to viscous forces (K) in the fluid. You can get an intuition of what the Reynolds number means when you think of it as a measure of how important fluid viscosity is to the dynamics of this flow. For example, when the Reynolds number is low (V*L is small relative to K), the fluid behaves more as a viscous substance, so you see a flow that is smooth and continuous. When the Reynolds number is high, (V*L is large relative to K), the speed and energy of the flow is so high that the viscosity is not dominant, and you see a flow that displays turbulent flow instabilities. The Reynolds number thus is very useful as a measure of how likely the flow is to be turbulent as opposed to laminar. For example, if you look at the flow of a liquid over a flat surface, you would expect the boundary layer to start laminar, and turn turbulent at a distance L downstream where Re reaches a value of about 5x10^5.

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