About TIME STEPAbout TIME STEP

• In solver option, we must define TIME STEP in flow solver

Physical Time Step(Default)Physical Time Step(Default)

• Choosing a Physical Time Step

• for the Flow Solver, it is generally recommended that you calculate a reasonable physical time step

for your model and use the physical time step method.

- Always use a physical time step for simulating natural convection (fluid buoyancy).

- The best time step value is typically some fraction of a physical time scale for the model.

• It is important to choose a time step value that is sufficiently small in order to resolve the non-lineari

ties. That is, the solution should be linear over the time step interval.

• If you choose a time step value that is too large the solution may not converge; values which are to

o small ensure convergence but at severe computational cost.

• For Natural Convection L: mean flow distance V: average flow velocity

• For Forced Convection - Gr, Grashoff Number

-

- g = Gravitational acceleration

• β= Coefficient of thermal expansion, for air =3.4x10-3 T

- ΔT= Change in air temperature from outlet to inlet, typical is 10°C to 25°C

- h = Chimney height as shown above

- ν= Kinematic viscosity, for air at 27°C =25.90x106 m2/s

VLt /2/1

2

3

2

hTgGr

Gr

ht

Local Time StepLocal Time Step

• Choosing a Local Time Scale

• Using Local Time Scale

- the flow solver computes the local time step based on local velocity and element length scale at each node (control volume) in the domain

- and then multiplies this by the local time scale factor provided..

• Using the local time scale method, the solver resolves this automatically by computing small time

steps near the inlet and larger time steps at the stagnation points.

- A converged solution is obtained with a minimum computational cost.

- The disadvantage is that the optimal local time scale factor does not depend on physical characteristics but on the number of fluid nodes.

- In practice, near optimal convergence exists for a broad range of values; 20 is a reasonable value for a medium mesh size of 20000 nodes. Use a value of 5 to 10 for a mesh size less than 5000 nodes and a value of 30 to 50 for larger models of 50000 nodes or more.

• This method is particularly useful for models which have varying time scales. That is, there are short

time scales near an inlet fan and long time scales at a stagnation point. A physical time step value

for this model would have to be sufficiently small to converge the solution in the inlet fan

region which would be excessively small in the stagnation region

RecommendRecommend

• Watch Your System

- Is there any expected stagnation(or low velocity) area?

- Can you ignore some gaps or small paths using modeling tool?• See Abstracted Skill document on APIC’s FTP(under construction)

• Use Physical Time STEP- Consider your system

• Default value(2) is large as a volume of a general PC

- Divided when there is any small paths in the whole volume