Dimpled Cylinder Flow Control CFD Simulation (Validation)

Paper Description

The present problem simulates the waterflow inside a rectangular computing space around a hollow cylindrical body with a protrusion or dimple on its inner surface. The results of this simulation have been compared and validated with the results in the article “Control of flow past a dimpled circular cylinder“.

The fluid used in the model is water. The Reynolds number used for the fluid flow is 17980, which according to the formula for the Reynolds number, the inlet flow velocity is 0.45 m.s-1. The cylindrical body is vertical and it is located in the direction of fluid flow within the computational area where the incoming water flow hits the body of its lateral surface horizontally. As mentioned, several dimples are placed on the inner surface of this hollow cylinder; These dimples create a bulge on the inner surface and a depression on the outer surface of the cylinder.

Paper Description

In the reference paper, several patterns have been considered for the placement of dimples on the cylindrical surface. In the present work, the DF (dimple full) type pattern has been used; This means that the dimples completely fill the surfaces.

The main purpose of this work is to investigate the aerodynamic forces including drag and lift force on the outer surface of the cylinder and also to calculate drag and lift coefficients. To calculate the drag coefficient and lift coefficient, the following equations are used, where ρ represents the fluid density, U represents the free flow velocity, and A represents the surface area that face the fluid flow (0.4 * 0.04 = 0.016):

The simulation was performed as both steady-state and transient, and the amount of aerodynamic forces was obtained. In the transient mode, for a period of 60 s with a time step size equal to 1 s. The results showed that the changes of these aerodynamic forces in the unsteady state also become stable after a short time.

Dimpled Cylinder Geometry & Mesh

The geometry of the present model is drawn in three dimensions using SOLIDWORKS software. The geometry of the model consists of a channel with a rectangular cross-section in which a hollow cylindrical body is located vertically. Dimples or bumps are also created on the inner surface of the cylinder. These dimples are placed all over the entire body of the cylindrical body. The computational space of a rectangular cube is 6 m * 0.3 m * 0.4 m and the cylinder has a length of 0.4 m and a diameter of 0.04 m.

The dimples are also modeled so that the ratio of the diameter of each dimple to the diameter of the cylinder is equal to 0.1 and the ratio of the depth of each dimple to the diameter of the cylinder is equal to 0.05. The following figure shows a view of the geometry.

The meshing of the model has been done using ANSYS Meshing software and the mesh type is structured. The element number is 1966490. The following figure shows the mesh.

Flow Control on Dimpled Cylinder CFD Simulation

To simulate the present model, we consider several assumptions:

• We perform a pressure-based solver.
• The simulation is both steady and transient.
• We ignore the gravity effect on the fluid.

We give a summary of the defining steps of the problem and its solution in the following table:

Models
k-epsilon	Viscous model
standard	k-epsilon model
standard wall function	near-wall treatment
Boundary conditions
Velocity-Inlet		Inlet
0.45 m.s-1	velocity magnitude
Pressure outlet		Outlet
0 Pascal	gauge pressure
Wall		Wall of Cylinder with Dimples
stationary wall	wall motion
Wall		Side Walls
stationary wall	wall motion
Solution Methods
SIMPLE		Pressure-velocity coupling
standard	pressure	Spatial discretization
second order upwind	momentum
second order upwind	turbulent kinetic energy
second order upwind	specific dissipation rate
Initialization
Standard	Initialization method
0.45 m.s-1	x-velocity
0 m.s-1	y-velocity , z-velocity
0 pascal	gauge pressure
0 m.s-1	y-velocity, z-velocity

Paper Validation

At the end of the solution process, we obtain the diagram of drag and lift coefficient in both steady and transient modes in terms of time over a period of 60 s by taking time step size equal to 1 s. As the results show in the transient state, these coefficients reach an almost constant value after a period of time. We compare and validate these two graphs with their related graphs in the article. For this purpose, we use the DF mode of Figure 4 of the article. the following figures show the comparison and validation of the graphs.

At the end of the simulation, we obtain two-dimensional contours related to pressure and velocity, as well as velocity vectors. We draw two-dimensional contours in the X-Y section on a plane perpendicular to the body of cylindrical shapes passing through the center of this cylinder.

There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.