Example12 Complex Flows of Forced Convection and Natural Convection

General

The flows with different temperatures in the bathtub are analyzed. The water with temperature of 50[deg] is forcibly added to the water of 25[deg].
The complex flows of the forced convection and the natural convection are analyzed with the buoyancy taken into account.
The temperature distribution and the flow velocity vectors are displayed.
Unless specified in the list below, the default conditions are applied.

Analysis Space

Item	Setting
Analysis Space	3D
Model Unit	m

Analysis Conditions

Item	Setting
Solver	Fluid analysis [Bernoulli] Thermal analysis [Watt]
Analysis Type	Fluid analysis: Transient analysis Thermal analysis: Transient analysis
Laminar Flow/Turbulent Flow	Select Turbulent Flow
Option	Select Take Buoyancy into Account (Natural Convection)
Layer Mesh Setting for Wall Surface (General Settings)	Parameters for Automatic Creation Expected Temperature Difference: 25[deg] Select Create the meshes containing pyramid elements if needed
Meshing Setup	General Mesh size: 0.05 [mm]
Detailed Setting	Relaxation Coefficient (K, ε) 0.9

Tab

Setting Item

Setting

Transient Analysis

Table

Number	Calculation steps	Output interval	Timestep [s]
1	60	1	1

Model

The water in the bathtub is a solid body.

The inlet and outlet are created. The inflow (forced inflow, flow rate specified) and the outflow (natural outflow) are set. Hot water flows in from the inlet and flows out through the outlet.

The surface of the water is a slip wall assuming it is not constrained by the air

Setting of Body Attributes, Materials, and Mesh Sizes

Body Number/Type	Body Attribute Name	Material Name	Mesh Size
0/Solid	Water	001_Water *	0.05

* Available from the material DB

Boundary Condition

Boundary Condition Name/Topology	Tab	Boundary Condition Type	Setting
Inlet	Fluid-Thermal	Inlet	Inlet Forced Inflow > Specify flow rate 4.0 X 10-4 [m3/s] Inflow temperature : Direct entry :50[deg]
Outlet	Fluid-Thermal	Outlet	Outlet Natural outflow
SlipWall	Fluid-Thermal	Slip Wall	Slip Wall Layer Mesh Setting Specify for each boundary condition Do not create layer meshes

Results

The diagram below shows the temperature distribution.

The range of color scale is between 25[deg] and 35[deg].

The hot water flows along the bottom, reaches the walls, and rises with buoyancy. You can see the high-temperature region is spreading.

The temperature distributions without buoyancy taken into account are shown for reference.

	Buoyancy taken into account (forced convection + natural convection)	Buoyancy not taken into account (forced convection only)
20[s]
40[s]
60[s]