# Smoothed particle hydrodynamics code basics of investing

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Smoothed particle hydrodynamics code basics of investing | Table 4. This is in contrast with tools that only seek to solve specific problems. For example, in Fig. Figure 6. We then discuss the design of the PySPH framework by showing how one may solve a simple problem with it. |

Fic network crypto | While in this context the IRA for SNIa in general cannot reproduce certain observed trends, for SNII it appears to be a simple and sufficiently accurate simplification, at least for these simple tests that do not include energy feedback. The plot is based on data adapted from SD We found no significant differences between the two tests, either in the SFR or in the iron abundance, as can be seen from Figs 4 and 5. If the latter is neglected, as we have done here, metals are not mixed widely in gas that has yet to cool. Consequently, we concentrate our analysis on elements such as O or Fe where restricting to SN production may be a good approximation. Open in new tab Download slide Metallicity profiles for the stellar left panels and gaseous right panels components of the idealized disc tests performed with different resolution: R1 dotted line, particlesR2 solid line, 10 particles and Read article dashed line, 40 particles. |

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By combining the intensity of the light projected on the curve, the discrete laser reconstructs an approximated value of the height at its center. Mesh-based methods are typically Eulerian: they use a fixed grid to simulate fluid flows. On the contrary, SPH is a Lagrangian method. This simply means that particles are not fixed in space, but rather they move following the flow. To understand the difference between the two approaches, imagine that we want to deliver a package from point A to point B.

Eulerian methods do that by placing fixed delivery men along the whole path from A to B. The package is passed by one person to the next until it reaches the destination. This means that the mass of the fluid is transported from one element of the mesh to its neighbor.

In SPH instead the package is transported by the same person from the initial to the final point. In this way there is no mass transport from one particle to another, which makes the SPH method intrinsically mass conservative. The same is not necessarily guaranteed for Eulerian methods. Figure 7: Comparison of the Eulerian top and Lagrangian approaches bottom Figure 7 Comparison between an Eulerian top and a Lagrangian approach bottom A natural question at this point is: What causes particles to move?

What causes particles to move? The movement of a fluid is physically determined by three main factors: the presence of external forces, for example gravity or surface tension the fluid viscosity, which describes the friction between particles how the pressure field changes from one point of the fluid to another Let us see how these three situations concretely affect the velocity of a particle.

In the figure below you see the case of a discretized two-dimensional fluid. In a SPH simulation, the green particle at the center would interact with all its neighbors enclosed by the kernel radius. However, since this would make our example a bit complicated to understand from a visual perspective, let us assume that this particle interacts only with the neighbors on its row.

The three terms that modify the velocity of the green particle are shown in separate pictures. Figure 8: Three factors that cause particles to move Figure 8 Three factors that cause particles to move In the first case, gravity simply acts on the green particle and accelerates it downwards. Just as simple as that.

In the second example, all the central particles move upwards and their velocities are higher when the distance from the green particle is bigger, which results in the parabolic velocity profile represented in the figure. The green particle is dragged by the motion of the neighbor particles and is therefore accelerated in the same direction. The factor that describes the resistance to this transport is called viscosity. If the fluid has a high viscosity, the green particle will still move upwards but with a smaller acceleration.

In fact, a higher viscosity corresponds to a higher internal resistance or, in other words, to a higher friction for example honey has a higher viscosity than water. In the third figure, the straight line represents the graph of the fluid pressure. It decreases from left to right and determines an acceleration of the green particle towards the area of lower pressure regime. This term is for example what causes wind, moving the air from regions with higher to region with lower pressure.

Until now, we explained how we can approximate the pressure of a particle based on the pressure values of its neighbors, using our discrete laser. The issue here is that what causes the green particle to move is not the value of its pressure, but how the pressure varies from one neighbor particle to another. Lucky us, our discrete laser can reconstruct not only the value of any fluid field at a particle, but it can also measure its local variation.

This is what Mathematics call differential operators, that are nothing more than a mean of describing the local changes of these quantities. Therefore, the discrete laser can compute those terms that modify the particle velocity in the second and third example.

The first case is much easier to treat since the gravity modifies the particle velocity in a direct way and does not require any neighbor information. Anyway, going deeper into this aspect is not the scope of this article… but it will be maybe of a future one. For the moment, it is enough to know that SPH can approximate those terms that are responsible for the actual dynamics.

This is the reason why we can use SPH to reproduce the fluid motion. Is SPH better than other methods? Well, there is no universal answer to this question. It all depends on the case you want to simulate.

In general, the meshless nature of SPH makes it well suited to simulate highly dynamic and violent flows, for example the oil flow in gearboxes , tank sloshing, nozzles, or jet impingement. Furthermore, SPH performs better in dealing with free surface flows, so in general all those cases where the fluid is not completely enclosed within the boundaries, e. Mesh-based method have quite a hard time in treating the free surface, especially when it comes to modelling surface forces like surface tension.

Figure 9: Difference between a mesh and a particle discretization of a continuous free-surface flow Figure 9 Difference between a mesh and a particle discretization of a continuous free-surface flow The same is true for multiphase flows: cases that require modelling more than one fluid with drastically different densities such as a gas and a liquid. In fact, for the same reason as above, mesh-based methods struggle in treating the interface between the two phases, while no added effort is required on the SPH side.

Another strength of SPH is the handling of complex geometries. In fact, mesh-based approaches require a preliminary work to generate the volume mesh, that can be particularly tricky when the boundary shapes are more sophisticated. It is even more difficult to simulate cases with moving geometries, since the spatial grid must continuously adapt to the boundary movement and, therefore, adaptive algorithms must be provided to modify and in some cases even recreate the grid.

On the contrary, the Lagrangian nature of SPH can automatically handle the interaction between particles and boundaries, thus eliminating quite a laborious preliminary work. Last but not least, SPH is characterized by a remarkable numerical robustness, that is in general a nice-to-have property for a computational method and it also allows for the simulation of complex flows characterized by strong pressure deviations.

On the other hand, SPH is less efficient than comparable mesh-based approaches. Therefore, it is less convenient to simulate those use cases that can be accurately reproduced using a mesh-based approach, for example, singlephase internal flows e. The reason for its reduced efficiency relies in the costly neighbor search process. In fact, every time a particle moves its neighbors change and, therefore, the list of neighbors must be newly determined.

On the contrary, Eulerian mesh-based methods have the advantage of having fixed grid elements - the delivery men are standing still and pass the package one to the next. Furthermore, an element of the grid generally communicates with a smaller number of neighbors compared to a particle. This drastically reduces the amount of computations performed and thus the total computational cost.

Therefore, some aspects of the method are not yet fully understood and are currently the focus of dedicated research. In the discrete world of SPH, the curve of figure 1 and 2 would look a bit different. It would no longer be continuous, but rather it would appear as a piece-wise constant curve, like a staircase made of steps having the same depth.

This device is made of a finite number of sensors, each one lighting a single piece of the curve. Like for the continuous laser, the intensity of the sensors is higher at the center and lower at the extremities, but it now varies with a discrete gradient. By combining the intensity of the light projected on the curve, the discrete laser reconstructs an approximated value of the height at its center.

Mesh-based methods are typically Eulerian: they use a fixed grid to simulate fluid flows. On the contrary, SPH is a Lagrangian method. This simply means that particles are not fixed in space, but rather they move following the flow. To understand the difference between the two approaches, imagine that we want to deliver a package from point A to point B. Eulerian methods do that by placing fixed delivery men along the whole path from A to B.

The package is passed by one person to the next until it reaches the destination. This means that the mass of the fluid is transported from one element of the mesh to its neighbor. In SPH instead the package is transported by the same person from the initial to the final point.

In this way there is no mass transport from one particle to another, which makes the SPH method intrinsically mass conservative. The same is not necessarily guaranteed for Eulerian methods. Figure 7: Comparison of the Eulerian top and Lagrangian approaches bottom Figure 7 Comparison between an Eulerian top and a Lagrangian approach bottom A natural question at this point is: What causes particles to move? What causes particles to move? The movement of a fluid is physically determined by three main factors: the presence of external forces, for example gravity or surface tension the fluid viscosity, which describes the friction between particles how the pressure field changes from one point of the fluid to another Let us see how these three situations concretely affect the velocity of a particle.

In the figure below you see the case of a discretized two-dimensional fluid. In a SPH simulation, the green particle at the center would interact with all its neighbors enclosed by the kernel radius. However, since this would make our example a bit complicated to understand from a visual perspective, let us assume that this particle interacts only with the neighbors on its row.

The three terms that modify the velocity of the green particle are shown in separate pictures. Figure 8: Three factors that cause particles to move Figure 8 Three factors that cause particles to move In the first case, gravity simply acts on the green particle and accelerates it downwards. Just as simple as that. In the second example, all the central particles move upwards and their velocities are higher when the distance from the green particle is bigger, which results in the parabolic velocity profile represented in the figure.

The green particle is dragged by the motion of the neighbor particles and is therefore accelerated in the same direction. The factor that describes the resistance to this transport is called viscosity. If the fluid has a high viscosity, the green particle will still move upwards but with a smaller acceleration.

In fact, a higher viscosity corresponds to a higher internal resistance or, in other words, to a higher friction for example honey has a higher viscosity than water. In the third figure, the straight line represents the graph of the fluid pressure. It decreases from left to right and determines an acceleration of the green particle towards the area of lower pressure regime. This term is for example what causes wind, moving the air from regions with higher to region with lower pressure.

Until now, we explained how we can approximate the pressure of a particle based on the pressure values of its neighbors, using our discrete laser. The issue here is that what causes the green particle to move is not the value of its pressure, but how the pressure varies from one neighbor particle to another. Lucky us, our discrete laser can reconstruct not only the value of any fluid field at a particle, but it can also measure its local variation.

This is what Mathematics call differential operators, that are nothing more than a mean of describing the local changes of these quantities. Therefore, the discrete laser can compute those terms that modify the particle velocity in the second and third example. The first case is much easier to treat since the gravity modifies the particle velocity in a direct way and does not require any neighbor information.

Anyway, going deeper into this aspect is not the scope of this article… but it will be maybe of a future one. For the moment, it is enough to know that SPH can approximate those terms that are responsible for the actual dynamics. This is the reason why we can use SPH to reproduce the fluid motion. Is SPH better than other methods? Well, there is no universal answer to this question. It all depends on the case you want to simulate.

In general, the meshless nature of SPH makes it well suited to simulate highly dynamic and violent flows, for example the oil flow in gearboxes , tank sloshing, nozzles, or jet impingement. Furthermore, SPH performs better in dealing with free surface flows, so in general all those cases where the fluid is not completely enclosed within the boundaries, e.

Mesh-based method have quite a hard time in treating the free surface, especially when it comes to modelling surface forces like surface tension. Figure 9: Difference between a mesh and a particle discretization of a continuous free-surface flow Figure 9 Difference between a mesh and a particle discretization of a continuous free-surface flow The same is true for multiphase flows: cases that require modelling more than one fluid with drastically different densities such as a gas and a liquid.

In fact, for the same reason as above, mesh-based methods struggle in treating the interface between the two phases, while no added effort is required on the SPH side. Another strength of SPH is the handling of complex geometries. In fact, mesh-based approaches require a preliminary work to generate the volume mesh, that can be particularly tricky when the boundary shapes are more sophisticated.

It is even more difficult to simulate cases with moving geometries, since the spatial grid must continuously adapt to the boundary movement and, therefore, adaptive algorithms must be provided to modify and in some cases even recreate the grid.

On the contrary, the Lagrangian nature of SPH can automatically handle the interaction between particles and boundaries, thus eliminating quite a laborious preliminary work. Last but not least, SPH is characterized by a remarkable numerical robustness, that is in general a nice-to-have property for a computational method and it also allows for the simulation of complex flows characterized by strong pressure deviations.

On the other hand, SPH is less efficient than comparable mesh-based approaches. Therefore, it is less convenient to simulate those use cases that can be accurately reproduced using a mesh-based approach, for example, singlephase internal flows e. The reason for its reduced efficiency relies in the costly neighbor search process. In fact, every time a particle moves its neighbors change and, therefore, the list of neighbors must be newly determined.

### Smoothed particle hydrodynamics code basics of investing forex pro lot size calculator

FRIDAYS FOR FEATURES // 01: Basics of SPH### INVESTING GUIDES L FISHER INVESTMENTS

This extended laser does not return the exact height of the curve at its center, but an approximated value, and the measurement error increases when the laser width is bigger. You might be thinking, why can we not just take the thinnest laser that exists? Well, this would of course give a more precise measurement, but it comes with a drawback: it is more expensive. Indeed, the smaller the laser is, the more expensive it gets. Therefore, we must settle for a device that is, let us say, good enough, meaning that the value it returns is not so far from the actual curve height.

But how does this laser work for fluids? Well, imagine the curve represented above to be the plot of a physical quantity of a one-dimensional fluid, for example its pressure. Simply, the fluid pressure at a point is approximated by combining the values of the fluid pressure in its proximity. This is done through so-called Kernel functions, that give a weight to the neighboring information depending on its distance from the point of interest. Closer points have higher weights and provide a larger contribution to the information at the point, while those further away have a smaller weight and provide a smaller contribution, exactly how the laser intensity is stronger in the middle than far from it.

These weights are distributed as a bell-shaped curve of Gaussian type and their values depend on the kernel radius. When the kernel radius is bigger the weights become smaller a bigger laser is less intense than a smaller one. Conversely, when the kernel radius becomes smaller, the kernel functions get closer to a Dirac delta the thin laser and our approximation of the fluid field becomes more precise. Figure 4: Two examples of kernel functions with different radii Figure 4 Two examples of kernel functions with different radius The discretization Now we are finally ready to put the SPH glasses on!

When we wear them, the way we see the reality changes. We switch from the continuous world, where we can see things with a perfect resolution, to the discrete world, where everything looks as if it is made up of bigger pixels. These pixels are what we call particles. At this stage, the physical information of fluids is no longer available at each single point but only at the level of particles. Therefore, the question now is: How can we reconstruct the value of a particle field based on the neighboring particles?

Let us go back to our laser example. In the discrete world of SPH, the curve of figure 1 and 2 would look a bit different. It would no longer be continuous, but rather it would appear as a piece-wise constant curve, like a staircase made of steps having the same depth. This device is made of a finite number of sensors, each one lighting a single piece of the curve.

Like for the continuous laser, the intensity of the sensors is higher at the center and lower at the extremities, but it now varies with a discrete gradient. By combining the intensity of the light projected on the curve, the discrete laser reconstructs an approximated value of the height at its center.

Mesh-based methods are typically Eulerian: they use a fixed grid to simulate fluid flows. On the contrary, SPH is a Lagrangian method. This simply means that particles are not fixed in space, but rather they move following the flow. To understand the difference between the two approaches, imagine that we want to deliver a package from point A to point B.

Eulerian methods do that by placing fixed delivery men along the whole path from A to B. The package is passed by one person to the next until it reaches the destination. This means that the mass of the fluid is transported from one element of the mesh to its neighbor.

In SPH instead the package is transported by the same person from the initial to the final point. In this way there is no mass transport from one particle to another, which makes the SPH method intrinsically mass conservative. The same is not necessarily guaranteed for Eulerian methods. Figure 7: Comparison of the Eulerian top and Lagrangian approaches bottom Figure 7 Comparison between an Eulerian top and a Lagrangian approach bottom A natural question at this point is: What causes particles to move?

What causes particles to move? The movement of a fluid is physically determined by three main factors: the presence of external forces, for example gravity or surface tension the fluid viscosity, which describes the friction between particles how the pressure field changes from one point of the fluid to another Let us see how these three situations concretely affect the velocity of a particle. In the figure below you see the case of a discretized two-dimensional fluid.

In a SPH simulation, the green particle at the center would interact with all its neighbors enclosed by the kernel radius. However, since this would make our example a bit complicated to understand from a visual perspective, let us assume that this particle interacts only with the neighbors on its row.

The three terms that modify the velocity of the green particle are shown in separate pictures. Figure 8: Three factors that cause particles to move Figure 8 Three factors that cause particles to move In the first case, gravity simply acts on the green particle and accelerates it downwards. Just as simple as that. In the second example, all the central particles move upwards and their velocities are higher when the distance from the green particle is bigger, which results in the parabolic velocity profile represented in the figure.

The green particle is dragged by the motion of the neighbor particles and is therefore accelerated in the same direction. The factor that describes the resistance to this transport is called viscosity. If the fluid has a high viscosity, the green particle will still move upwards but with a smaller acceleration. In fact, a higher viscosity corresponds to a higher internal resistance or, in other words, to a higher friction for example honey has a higher viscosity than water. In the third figure, the straight line represents the graph of the fluid pressure.

It decreases from left to right and determines an acceleration of the green particle towards the area of lower pressure regime. This term is for example what causes wind, moving the air from regions with higher to region with lower pressure. Until now, we explained how we can approximate the pressure of a particle based on the pressure values of its neighbors, using our discrete laser. The issue here is that what causes the green particle to move is not the value of its pressure, but how the pressure varies from one neighbor particle to another.

Lucky us, our discrete laser can reconstruct not only the value of any fluid field at a particle, but it can also measure its local variation. This is what Mathematics call differential operators, that are nothing more than a mean of describing the local changes of these quantities. Therefore, the discrete laser can compute those terms that modify the particle velocity in the second and third example. The first case is much easier to treat since the gravity modifies the particle velocity in a direct way and does not require any neighbor information.

This laser computes the intensity of the light projected on the curve and, based on these measurements, returns an approximated value of the curve height at its center. Figure 2: The laser measuring device used to approximate the curve height at its center Figure 2 The laser measuring device used to approximate the curve height at its center Now imagine an extremely thin laser placed at a certain point of the horizontal line.

This measuring device computes the height of the curve above with exact precision because it concentrates all its intensity on a single point. Figure 3: The thin laser that measures the height of the curve at a point with exact precision Figure 3 The thin laser that measure the height of the curve at a point with exact precision This is what Mathematics call a Dirac delta: an extremely powerful tool centered at a single point that can be used to return the exact value of the curve above.

Well, unfortunately such a thin and precise device does not exist in the real world! What the real world offers instead is a larger device: an extended laser with a finite width, like the one presented first. This extended laser does not return the exact height of the curve at its center, but an approximated value, and the measurement error increases when the laser width is bigger.

You might be thinking, why can we not just take the thinnest laser that exists? Well, this would of course give a more precise measurement, but it comes with a drawback: it is more expensive. Indeed, the smaller the laser is, the more expensive it gets. Therefore, we must settle for a device that is, let us say, good enough, meaning that the value it returns is not so far from the actual curve height. But how does this laser work for fluids? Well, imagine the curve represented above to be the plot of a physical quantity of a one-dimensional fluid, for example its pressure.

Simply, the fluid pressure at a point is approximated by combining the values of the fluid pressure in its proximity. This is done through so-called Kernel functions, that give a weight to the neighboring information depending on its distance from the point of interest. Closer points have higher weights and provide a larger contribution to the information at the point, while those further away have a smaller weight and provide a smaller contribution, exactly how the laser intensity is stronger in the middle than far from it.

These weights are distributed as a bell-shaped curve of Gaussian type and their values depend on the kernel radius. When the kernel radius is bigger the weights become smaller a bigger laser is less intense than a smaller one. Conversely, when the kernel radius becomes smaller, the kernel functions get closer to a Dirac delta the thin laser and our approximation of the fluid field becomes more precise.

Figure 4: Two examples of kernel functions with different radii Figure 4 Two examples of kernel functions with different radius The discretization Now we are finally ready to put the SPH glasses on! When we wear them, the way we see the reality changes.

We switch from the continuous world, where we can see things with a perfect resolution, to the discrete world, where everything looks as if it is made up of bigger pixels. These pixels are what we call particles. At this stage, the physical information of fluids is no longer available at each single point but only at the level of particles. Therefore, the question now is: How can we reconstruct the value of a particle field based on the neighboring particles?

Let us go back to our laser example. In the discrete world of SPH, the curve of figure 1 and 2 would look a bit different. It would no longer be continuous, but rather it would appear as a piece-wise constant curve, like a staircase made of steps having the same depth. This device is made of a finite number of sensors, each one lighting a single piece of the curve.

Like for the continuous laser, the intensity of the sensors is higher at the center and lower at the extremities, but it now varies with a discrete gradient. By combining the intensity of the light projected on the curve, the discrete laser reconstructs an approximated value of the height at its center. Mesh-based methods are typically Eulerian: they use a fixed grid to simulate fluid flows.

On the contrary, SPH is a Lagrangian method. This simply means that particles are not fixed in space, but rather they move following the flow. To understand the difference between the two approaches, imagine that we want to deliver a package from point A to point B. Eulerian methods do that by placing fixed delivery men along the whole path from A to B. The package is passed by one person to the next until it reaches the destination. This means that the mass of the fluid is transported from one element of the mesh to its neighbor.

In SPH instead the package is transported by the same person from the initial to the final point. In this way there is no mass transport from one particle to another, which makes the SPH method intrinsically mass conservative. The same is not necessarily guaranteed for Eulerian methods. Figure 7: Comparison of the Eulerian top and Lagrangian approaches bottom Figure 7 Comparison between an Eulerian top and a Lagrangian approach bottom A natural question at this point is: What causes particles to move?

What causes particles to move? The movement of a fluid is physically determined by three main factors: the presence of external forces, for example gravity or surface tension the fluid viscosity, which describes the friction between particles how the pressure field changes from one point of the fluid to another Let us see how these three situations concretely affect the velocity of a particle.

In the figure below you see the case of a discretized two-dimensional fluid. In a SPH simulation, the green particle at the center would interact with all its neighbors enclosed by the kernel radius. However, since this would make our example a bit complicated to understand from a visual perspective, let us assume that this particle interacts only with the neighbors on its row. The three terms that modify the velocity of the green particle are shown in separate pictures. Figure 8: Three factors that cause particles to move Figure 8 Three factors that cause particles to move In the first case, gravity simply acts on the green particle and accelerates it downwards.

Just as simple as that. In the second example, all the central particles move upwards and their velocities are higher when the distance from the green particle is bigger, which results in the parabolic velocity profile represented in the figure. The green particle is dragged by the motion of the neighbor particles and is therefore accelerated in the same direction. The factor that describes the resistance to this transport is called viscosity.

If the fluid has a high viscosity, the green particle will still move upwards but with a smaller acceleration. In fact, a higher viscosity corresponds to a higher internal resistance or, in other words, to a higher friction for example honey has a higher viscosity than water. In the third figure, the straight line represents the graph of the fluid pressure.

It decreases from left to right and determines an acceleration of the green particle towards the area of lower pressure regime. This term is for example what causes wind, moving the air from regions with higher to region with lower pressure.

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