In this Fluid Between Rotating Disk CFD Analysis, we investigate a classic problem in fluid dynamics that appears in many engineering systems. This phenomenon occurs in computer hard disk drives, gas turbine rotors, and automotive clutches. In these systems, a thin layer of fluid fills the gap between two circular plates. Usually, one disk spins while the other remains stationary. The rotation creates friction that drags the fluid, leading to complex swirling patterns and vortices. Understanding this behavior is critical because it determines how much heat is generated and how much power (torque) is required to spin the motor. However, measuring the flow inside such a tiny gap is difficult in real experiments. Therefore, engineers use CFD simulation to visualize the physics. We use ANSYS Fluent to calculate pressure and velocity fields. This Rotating Disks fluent simulation helps designers optimize the gap distance to save energy and improve cooling.

In this report, we perform a comprehensive study of the flow transition. We analyze how changing the distance between the disks changes the fluid behavior from smooth laminar flow to complex turbulent spirals. For more basics on flow physics, please check our Fluid mechanics tutorials.

Figure 1: Schematic representation of the Fluid Between Rotating Disk geometry showing the squeezing flow concept.

Simulation Process: Rotating Reference Frame in ANSYS Fluent

For this CFD simulation, we created a 3D cylindrical geometry to represent the fluid volume between the parallel plates. We generated a high-quality Structured Mesh containing exactly 1,314,180 hexahedral cells. We arranged the cells in regular layers from the bottom plate to the top plate. This structured approach is essential for rotating flows because the cells align perfectly with the flow direction, reducing numerical errors. We also refined the mesh density near the top and bottom walls. This is critical for resolving the thin boundary layers where the velocity changes very quickly.

In the ANSYS Fluent setup, we defined the boundary conditions to match the physical experiment. We set the bottom disk as a Stationary Wall (0 velocity). We set the top disk as a Rotating Wall with a speed of 71 rpm. This condition forces the software to calculate the tangential velocity at every point on the top surface. We applied a Symmetry boundary condition on the outer cylindrical wall. This assumes the flow allows us to simulate the domain efficiently.

Figure 2: Structured Mesh Grid (1.3 million cells) of the cylindrical fluid domain, showing fine cell layers near the disk walls to capture boundary gradients.

Post-processing: Fluid Between Rotating Disk CFD Analysis of Instabilities

This section analyzes the engineering data to understand how the gap size affects performance. We interpret the velocity curves and turbulence contours to provide actionable advice for machine designers. First, we analyze the Velocity Profiles in Figure 3. The graph compares the flow speed across the gap for four different heights. The Magenta curve represents the “Very Low Distance” (smallest gap). It shows a very low Maximum Velocity (~0.010 m/s) and a flat, linear profile. This indicates that in narrow gaps, the fluid rotates like a solid body, dominated by viscous forces. However, the Red curve represents the “High Distance” (largest gap). Here, the Maximum Velocity jumps to ~0.100 m/s. More importantly, the profile creates a distinct S-shape. Engineering Insight: This S-shape proves that the flow has separated into distinct layers. There is a boundary layer on the top rotating disk and another on the bottom stationary disk, with a core region in the middle. This separation is the first sign of instability.

Figure 3: Velocity Magnitude Profiles plotted across the gap height for four different disk spacings, showing the transition from linear profiles (narrow gap) to S-shaped profiles (wide gap).

Next, we examine the Turbulence Contours in Figure 4. We see distinct colored bands on the surfaces. These are not random errors; they are Spiral Vortices (often called Taylor-Görtler vortices). The blue areas represent smooth Laminar Flow, while the red/yellow stripes show localized high turbulence. This confirms the flow is in a “transitional” state. The instability starts at the outer edge where the speed is highest and moves inward. The Fluid Between Rotating Disk CFD Analysis confirms that gap size is a critical design parameter. A wide gap allows complex vortices to form, which wastes energy and increases the torque needed to spin the disk. A narrow gap (Magenta curve) suppresses these vortices, keeping the flow stable and linear. For a hard drive manufacturer, reducing the gap size is the best way to reduce vibration and power consumption.

Figure 4: Turbulent Kinetic Energy Contours on the disk surfaces from the Fluid Between Rotating Disk CFD Analysis, visualizing the formation of spiral instabilities.

Figure 5: Streamlines colored by Kinetic Energy, illustrating the 3D spiral flow path and secondary flow structures between the disks.