
Re: Defining a nonzero starting velocity for naturalconvection models
Chris Michalski Sep 18, 2012 11:46 AM (in response to David Maxham)David 
when you first define the simulation model, there is the initial conditions on the last tab of the general settings. If you insert a vertical initial velocity here it will set all of the Zvelocity vectors to this value to start.

Re: Defining a nonzero starting velocity for naturalconvection models
Bill McEachern Sep 18, 2012 11:51 AM (in response to Chris Michalski)I am pretty sure this setting would be persistent  ie and inlet V of the set value would be applied to the study  not what he is looking for.

Re: Defining a nonzero starting velocity for naturalconvection models
Chris Michalski Sep 18, 2012 12:13 PM (in response to Bill McEachern)in this case define the fluid volume of the model and use a local initial condition and disable the solid that defines this volume
(because I think you're right, this would set that the boundary of an external simulation was always moving at this velocity)



Re: Defining a nonzero starting velocity for naturalconvection models
Bill McEachern Sep 18, 2012 11:49 AM (in response to David Maxham)Well, this is not what you really want to hear and there but you could run a simulation and solve for the 0.2 m/s and use it as an initial condition in a subsequent study. Not exactly elegent but it would work. You would use the take previous in when you invoke the solver.

Re: Defining a nonzero starting velocity for naturalconvection models
David Maxham Sep 18, 2012 1:41 PM (in response to Bill McEachern)Chris, Bill,
Thanks for your help. I should have pointed out that this is a steadystate external flow. I initially thought that the initial condition would be persistent as Bill mentions above, but I'm not sure based on what I found in the help (see below). So maybe when I do set the initial vertical velocity to 0.2 m/s, it is actually just a starting or initial condition and not persistent. I'm going to run the model both ways, first as I'm doing now with no initial velocity set, and secondly with an initial condition equal to 0.2 m/s. I'll report back with the difference, if any, in results.
Here's what the help section says. They are clear that if you want to solve an internal flow faster, use the initial condtions, as Chris suggested above. Although they don't specifically say so for the external flow, it seems to be implied. We'll see.
Dave
Initial and Ambient Conditions
Specifying Initial (for an Internal analysis) or Initial and Ambient (for an External analysis) Conditions means specifying values of Thermodynamic parameters, Velocity parameters, Turbulence parameters, Solid parameters (to solve "Heat conduction in solids") and Concentration (for more than one fluid).
 If you want to solve a steady Internal problem in a shorter time, we recommend that you use Initial conditions (initial values of the flow parameters) that are closer to the assumed solution than the default initial conditions.
If you solve a steady External problem, specifying Ambient conditions means specifying initial conditions within the Computational Domain and boundary conditions at the Computational Domain boundaries. The specified thermodynamic and velocity parameters are considered as parameters of the undisturbed external flow and used to define the initial flow state

Re: Defining a nonzero starting velocity for naturalconvection models
Chris Michalski Sep 18, 2012 1:45 PM (in response to David Maxham)yes, because you have an external problem setting an "initial" condition actually creates a boundary condition with these values. So the bottom of your bounding volume will always have an incoming flow at 0.2m/s.
You would be better to use the "check geometry" and "create fluid body assembly". Then save that part and insert it into your model and use it to define the initial velocity (then disable it). This will eliminate the continual boundary condition that Flow creates for an external simulation. This is how you define wind or some other artificial velocity for an external simulation.

Re: Defining a nonzero starting velocity for naturalconvection models
David Maxham Sep 18, 2012 2:08 PM (in response to Chris Michalski)Thanks Chris for clarifying that with an external flow, the initial condition becomes a continual condition (or using Bill's terminology, a persistent condition). I follow what you've written about using check geometry and a fluid body assembly, and will try that instead.
Just curious, when you mentioned that this same approach is used for wind or some other artificial velocity, is that wind, etc, continual or does it just die out over the iterations?

Re: Defining a nonzero starting velocity for naturalconvection models
Chris Michalski Sep 18, 2012 2:18 PM (in response to David Maxham)That's one way to set a continual wind (instead of defining it on a plane) in an external flow. I have used this in the past to define various cross winds blowing against the outside of a transformer enclosure mounted outdoors to determine how various wind angles affect natural drafting for wind assisted cooling.


