Problem Statement:
There are two concentric cylinders. The inner cylinder is rotating with a specific angular velocity. The outer cylinder is kept stationary. The water is flowing in the annular space between the cylinders. Analyzation of the effect of the cylinder rotation on the fluid flow.
CFD:
Computational Fluid Dynamics (CFD) is the process of mathematically modelling a physical phenomenon involving fluid flow and solving it numerically using the computational process.
In a CFD software analysis, the examination of fluid flow by its physical properties such as velocity, pressure, temperature, density, and viscosity. To generate an accurate solution for a physical phenomenon associated with fluid flow, those properties must be considered simultaneously.
A mathematical model of the physical case and a numerical method are used in a CFD software tool to analyze the fluid flow. For instance, the Navier-Stokes (N-S) equations are specified as the mathematical model of the physical case. This describes changes in all those physical properties for both fluid flow and heat transfer. A mathematical model varies by the content of the problem such as heat transfer, mass transfer, phase change, chemical reaction, etc. Moreover, the reliability of a CFD analysis highly depends on the whole structure of the process.
Theory:
Many important engineering flows involve swirl or rotation and ANSYS Fluent is well-equipped to model such flows. Swirling flows are common in combustion, with swirls introduced in burners and combustors to increase residence time and stabilize the flow pattern. Rotating flows are also encountered in turbomachinery, mixing tanks, and a variety of other applications.
When you begin the analysis of a rotating or swirling flow, you must classify your problem into one of the following five categories of flow:
• axisymmetric flows with swirl or rotation
• fully three-dimensional swirling or rotating flows
• flows requiring a moving reference frame
• flows requiring multiple moving reference frames or mixing planes
• flows requiring sliding meshes
Axisymmetric Flows with Swirl or Rotation
You can solve a 2D axisymmetric problem that includes the prediction of the circumferential or swirl velocity. The assumption of axisymmetry implies that there are no circumferential gradients in the flow, but that there may be non-zero circumferential velocities. Examples of axisymmetric flows involve swirl or rotation.
It is important to note that while the
assumption of axisymmetry implies that there are no circumferential gradients in the flow, there may still be non-zero swirl velocities.
Momentum Conservation Equation for Swirl Velocity:
Physics of Swirling and Rotating Flows:
In swirling flows, conservation of angular momentum (r.w = constant) tends to create a free vortex flow, in which the circumferential velocity, w, increases sharply as the radius, r, decreases (with w finally decaying to zero near r = 0 as viscous forces begin to dominate).
It can be shown that for an ideal free vortex flow, the centrifugal forces created by the circumferential motion are in equilibrium with the radial pressure gradient:
As the distribution of angular momentum in a non-ideal vortex evolves, the form of this radial pressure gradient also changes, driving radial and axial flows in response to the highly non-uniform pressures that result. Thus, as you compute the distribution of swirl in your ANSYS Fluent model, you will also notice changes in the static pressure distribution and corresponding changes in the axial and radial flow velocities. It is this high degree of coupling between the swirl and the pressure field that makes the modelling of swirling flows complex.
In inflows that are driven by wall rotation, the motion of the wall tends to impart a forced vortex motion to the fluid, wherein w/r = constant.
An important characteristic of such flows is the tendency of fluid with high angular momentum (for example, the flow near the wall) to be flung radially outward using geometry. This is often referred to as “radial pumping” since the rotating wall is pumping the fluid radially outward.
Geometry:
Meshing:
Sweep Method:
The Sweep Method begins by meshing a particular 'source' surface using either the automatic global settings or any local sizing controls/inflation layers that have been applied by the user. It will then "sweep" the source mesh (which can represent any arbitrary cross-section) through the body, spacing it by a certain incremental dimension or by splitting the swept side faces into a desired number of divisions.
It is useful when you first open ANSYS Meshing to Right-click on "Mesh" in the left-hand model tree and choose the option to “Show" -> "Sweepable Bodies”. Upon choosing this option, any bodies within your assembly that can be swept automatically will be shown in green on your screen.
Solution:
Simulation Video:
Comments