One of these scenarios occurs at early times considering that at the beginning of the reactor operation, the moderator contacts the fuel particles for the first time. In general, it was found that the coefficient a v κ has larger values for those scenarios where fewer neutrons are traveling through the moderator. It was observed that using 3D cells also decreases the estimation of the neutronic interfacial coefficient, indicating that using nuclear rods in nuclear reactors favors the escaping of neutrons. The maximum (initial) and steady-state values of κ increase when Φ increases and for small moderator volume fractions. The general trend indicates that the steady state is achieved at shorter times for increasing values of Φ. The time to reach the steady state depends on the neutron absorption rate in the moderator. 8.10, the neutronic interfacial coefficient has the maximum value at early times and eventually reaches the steady state. At the buckling load, the stiffness has reached 0.As shown in Fig. With a linear assumption, the beam becomes unstable at an 800 K temperature increase. You will then find that the critical load factor is 80. You can do so by adding a Linear Buckling study to the model and using the thermal expansion caused by ΔT = 10 K as a unit load. However, compressive stresses soften the structure.Īnother way of looking at this is by performing a linear buckling analysis. This stress causes a significant reduction in the stiffness of the beam - an effect often called stress stiffening, since it typically occurs in structures with tensile stresses. With the given data, the stress is -10 MPa (computed as Eα xΔT). In the case of the doubly clamped beam, the thermal expansion causes a compressive axial stress. The reason for this behavior is discussed in the following sections. If you make the beam thinner, this difference would be even more pronounced.
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For all other modes, the relative shift in frequency is significantly smaller. The first thing to note is that the bending eigenmodes for the doubly clamped beam stand out and have a strong temperature dependence. The following table shows the cantilever beam results. The doubly clamped beam results are shown below.
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Orthotropic thermal expansion coefficients are used to highlight some properties of the solution.
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To better separate the various effects, Poisson’s ratio is set to zero, but this assumption does not change the results in any fundamental way. The material parameters have values that are of the same order of magnitude as those for many other engineering materials. The beam geometry and mesh used in the example. Rectangular Beam ExampleĬonsider a rectangular beam with the following data: PropertyĬoefficient of thermal expansion, x directionĬoefficient of thermal expansion, y directionĬoefficient of thermal expansion, z direction Setting up simulations that accurately capture such small effects can be a challenging task, since several phenomena can interact. In very high precision applications, the frequency stability requirements might specify a precision at the ppb (parts-per-billion, 10 -9) level. The most common parameter is temperature, but the same type of phenomena could, for example, be caused by hygroscopic swelling due to changes in humidity. Some devices require a very high degree of frequency stability with respect to changes in the environment. Studying Temperature-Dependent Eigenfrequencies We also explore effects like stress softening, geometric changes, and the temperature dependence of material properties.
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In this blog post, we show how to do this using COMSOL Multiphysics® version 5.3. In some applications, particularly within the MEMS field, it is important to study the sensitivity of a device’s eigenfrequencies with respect to a variation in temperature.