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Quantitavely Seeding the Flow

John Wilson

John Wilson

Posted Jan 23, 2010
0 Comments

I think that we can all agree that one feature of CFD over test is the ability to visualize where the flow is going.  It is quite easy in CFD analysis tools, at least in our tools, to seed the flow any where within the solution and follow particles upstream and/or downstream.  The limitation of this is that it is the results are qualitative.  CFD has the capability, without an extreme amount of extra effort, to quantify where the flow goes.

As an example I have constructed a partial model of a somewhat reasonable depiction of a 1U server in FloEFD.  Each fan is fixed at 15 cfm where 2% (arbitrarily chosen) of the fluid that it delivers it tagged with a fluid marker.  For instance,  the fluid that Fan 3 supplies is 98% air and 2% “Air-Fan 3″.  “Air-Fan 3″ has the same fluid properties as air, not arbitrarily chosen.  There is also geometry that represents the power supply, which also delivers 15 cfm.  The heat sinks are a porous media of reasonable flow impedance.  The optional “perf” will be considered in another post to see if I can use it to create a more robust design in the event of a fan failure.

fan_location

layoutThe image below shows a qualitative depiction of the area of influence from Fan 1.  Nice, but we can get more information from the marking of the flow.

flow_trajectories

Below is a cross section through the center of the fans that quantifies the area of influence from Fan 1 and Fan 3.  I have clipped out the area of less than 10% influence.

air-fan-1air-fan-3To drive home the quantification I have tabulated how much air each heat sink is receiving from each fan.

table_no_perf

You can see that most of the flow that the heat sinks receive comes from Fan 2 and Fan 3.  Ideally my heat sinks wouldn’t be so reliant on flow from a particular fan.  My plan is to see if I can help the situation by adding a perforated plate between the fans and the downstream heat sinks.  Or maybe I will look at what happens when Fan 3 fails.  I do think that having this type of output would be useful in maximizing the airflow management of the design.

There is a similar, but more evolved, approach available in FloVENT via the “capture index” which is very useful in Data Center design.  Google it and check it out.  Also, I’d love to here any thoughts about the usefulness of this approach.

Flow Visualization, CFD, Concentration, Capture Index, FloEFD, FloVENT, Data Center, Electronics Cooling

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About John Wilson

John WilsonJohn Wilson joined Mentor Graphics Corporation, Mechanical Analysis Division (formerly Flomerics Ltd) in 1999. John has worked on or managed more than 100 thermal and airflow design projects. His modeling and design knowledge range from Electronics Cooling IC packaging level to Data Centers and Clean Rooms. He has extensive experience in IC package level test and analysis correlation through his work at Mentor Graphics' San Jose based Thermal Test Facility. He is currently the Consulting Engineering Manager, WRO in the Mechanical Analysis Division. Visit John R Wilson's Blog

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