COMSOL Fluid Mechanics Problems

Non-Isothermal Flow Around a Cooling Device = = = = = = = = = = = = = = = = = = = = = = = = = = = = = This model shows the application of COMSOL Multiphysics in the modeling of non-isothermal laminar flow of fluids (in this case a gas). The model assumes that the expansion work done by the gas is negligible, that the variations in temperature are obtained through external heating, and that the fluid is an ideal gas.

The model treats the steady flow of a gas over a fin, which is heated by an external heat source. The heat balance includes the domain of the fin and of the gas.

The model exemplifies several very useful COMSOL Multiphysics features. This includes the ability to use Equation-based modeling to make density, and viscosity, a function of temperature.

Rising Bubble = = = = = = = = = The level set method is well suited for problems with moving boundaries in which the geometry’s topology changes with time. A bubble of oil that travels up through water and finally merges with oil at the top causes this kind of topology change. For problems where the topology is unchanged as a function of time, as in free surface movement in a tank (no splashing) and impeller stirring, it is also possible to use the ALE (arbitrary Lagrangian-Eulerian) method.

In this model, three different regions exist initially: the oil bubble, the oil at the top of the container, and the water surrounding the bubble. The Level Set Two-Phase Flow application mode makes it easy to set up the model.

Micro Mixer = = = = = = = = The development of mixers does often not only have to account for effectiveness, but also other factors must be involved, such as cost and complexity for manufacturing.

This example studies a laminar static micro mixer with two parallel sets of split-reshape-recombine mixing elements.

The mixer works through lamination of the streams without any moving parts and the mixing is obtained through diffusion, which makes it cheap and easy to manufacture.

Jet Pipe = = = = = This example models the radiation of fan noise from the annular duct of a turbofan aeroengine. When the jet stream exits the duct, a vortex sheet appears along the extension of the duct wall due to the surrounding air moving at a lower speed. The near field on both sides of the vortex sheet is calculated.

The Aeroacoustics modeling interface in the Acoustics Module describes acoustic waves in a moving fluid. However, the field equation is only valid when the velocity field is irrotational, a condition that is not satisfied across a vortex sheet. As a consequence, the velocity potential is discontinuous across this sheet and to model this discontinuity, you use assemblies that are connected through coupling pairs.

Convective Cooling of a Potcore Inductor = = = = = = = = = = = = = = = = = = = = = = = = = = The inductor is a common component in a variety of electrical devices. Its applications include power transformation and measurements, and it can also be used together with capacitors to create oscillators. In small devices with many components, such as in laptops, heat generation can be a problem and must be accounted for in the design.

This axi-symmetric model studies the heat transfer in such a potcore inductor. It solves for the heat transfer inside the inductor and in the surrounding air, taking into consideration the air flow around the inductor.

The model accounts for both convective heat transfer as well as surface to surface radiation.

Fluid-Structure Interaction with Two Fluid Phases = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = The following example demonstrates techniques for modeling a fluid-structure interaction containing two fluid phases in COMSOL Multiphysics. It illustrates how a heavier fluid can induce movement in an obstacle using the arbitrary Lagrangian-Eulerian (ALE) technique along with the Two-Phase Flow, Phase Field application mode in the MEMS Module.

The model geometry consists of a small container, in the middle of which is an obstacle. Initially a heavier fluid (water) is present in the left domain and air is present everywhere else. A feedback loop allows the displaced air to move from the right domain back to the left domain.

Flow Past a Cylinder = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = The following model examines unsteady, incompressible flow past a long cylinder placed in a channel at right angle to the oncoming fluid. The cylinder is offset somewhat from the center of the flow to make the steady-state symmetrical flow unstable.

The simulation time necessary for a periodic flow pattern to appear is difficult to predict. A key predictor is the Reynolds number, which is based on cylinder diameter. For low values—below 100—the flow is steady.

In this simulation the Reynolds number equals 100, which gives a developed Karman vortex street; but the flow is still not fully turbulent.

Optimization of a Catalytic Microreactor = = = = = = = = = = = = = = = = = = = = = = = = = = In this model, a solution is pumped through a catalytic bed where a solute species reacts as it gets in contact with the catalyst.

The model calculates the maximize total reaction rate of the solute for a given total pressure difference across the bed by finding an optimal catalyst distribution.

Displacement Ventilation of Air in a Room = = = = = = = = = = = = = = = = = = = = = = = = = = = In general, there are two classes of ventilation: mixing ventilation and displacement ventilation. In displacement ventilation, air enters a room at the floor level and displaces warmer air to achieve the desired temperature. Heating sources in the room can include

running electronic devices, or inlet jets of warm air. A potential issue with the displacement ventilation approach is that significant temperature variation and strong stratification may arise.

The present model investigates the performance of a displacement ventilation system. In the model, you set up the coupled heat transfer and turbulent flow equations describing the system using the predefined multiphysics coupling Turbulent Non-Isothermal Flow, k-ε. Given measured values for inlet velocity, inlet temperature, and heat flux, this model yields field configurations of air temperature and velocity that are consistent with experimental measurements and analytic global models.