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12 Sep 06. By bringing CFD upfront into the design process Sechan Electronics eliminated the costs associated with two iterations of prototypes of a rugged onboard computer for the upgraded Paladin Self Propelled Howitzer, and achieved its objective four months faster than if it had relied on traditional prototyping and testing methods.

An upgrade of the Paladin Self Propelled Howitzer, prompted by a decision in Congress, involved a redesign of the Paladin Digital Computer Unit (PDCU), the onboard computer that directs the projectiles. Sechan Electronics, a small Pennsylvania-based business specialising in the design and manufacture of military electronics systems and subsystems for the US Department of Defense, was tasked with developing a new digital fire control system for the Howitzer. Soldiers count on the reliability of this unit and Sechan Electronics backed it with a protracted warranty, so maximum reliability and performance was crucial.

In a combat situation, sensitive electronic weapons have to be able to withstand harsh conditions. Most electronic failures that occur can be attributed to problems with temperature control. In fact the US Air Force has determined that there is reliability degradation of approximately 10 percent for every two degrees Celsius rise in temperature in silicon-based devices. This is why Sechan Electronics uses CFdesign upfront CFD software, a high-powered, multi-purpose fluid flow, heat transfer and electronics cooling simulation software developed specifically for design engineers.

In the firing line

Sechan Electronics determined that the PDCU required a single chassis to house three single board computers and associated hardware. Maintaining safe operating temperatures for all of the devices within the chassis was essential. Ventilation was the obvious solution for heat dissipation but was not an option in this case. Electromagnetic emissions would cause interference with other electronic devices and the computer needed to work in a sealed space to be able to withstand hot water wash-downs (to counteract chemical exposure), salt corrosion, shock generated by gunfire and the intense vibration produced by a tracked vehicle. Due to the sealed nature of the chassis and the shock-mounting configuration, cooling of the chassis needed to depend solely upon free convection and radiation from the exterior of the chassis. Heat from the internal electronic devices was transferred to the chassis essentially by conduction.

Critical thermal management required an accurate prediction of the buoyancy-driven convective flow and radiation heat transfer. Chief mechanical engineer at Sechan Electronics, Jim Smith, needed a CFD package that would work directly with his Pro/ENGINEER models, eliminating the need for re-modelling, and would provide a high level of analysis with reliable accuracy within a couple of degrees Celsius. Smith turned to Blue Ridge Numerics’ CFdesign upfront CFD software.

Upfront simulation

CFdesign turns a standard MCAD workstation into a fully interactive flow bench and thermal test rig. Upfront CFD is defined as flow and thermal simulation integrated within an MCAD environment and used during early stage product development, by multi-tasking engineers, before products enter physical testing. Smith used CFdesign to systematically evaluate and interactively visualise conduction, convection and radiation heat transfer modes simultaneously.

“I ran a number of simulations, starting with simplified geometry and more encompassing assumptions, and progressing to more detailed and realistic models,” explained Smith. “For example, initially radiation was ignored and the Pro/E model was extensively de-featured. Gradually, the model became a more authentic representation of the actual hardware and environment.”

Smith used the parametric relationship between CFdesign and his native Pro/ENGINEER assembly to evaluate several design alternatives. Additionally, simulation

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