CFD, Simulation, and Survivability

In a military aircraft designed to operate in hostile surroundings, survivability is a key word. Think of it as the next step beyond reliability. All along the design continuum for military systems, mechanical engineers are responsible for survivability. It is a challenge that calls for rigorous design validation and thorough evaluation of aircraft elements ranging in scale from individual fasteners to system-level elements and their interactions. Designers of combat aircraft need tools that produce reliable results and guide the design of practical fail-safe features.

A battlefield helicopter includes defensive systems designed to protect the vehicle and its occupants in the most adverse situations. Survivability would seem to be a cost-is-no-object ideal, and indeed the manufacturers of these craft are unsparing in their commitment to protect life and limb. But that doesn’t mean developers can afford to waste money on over-engineering that serves no purpose.  

Where to draw the line? Let’s look to a real-world application for an answer. In a recent helicopter upgrade project, mechanical engineers were tasked with reviewing and refining a system that injects nitrogen gas into the craft’s fuel tank to displace oxygen as the fuel is consumed. The intent is to make the tank less likely to ignite if it is hit by an incendiary projectile. The incoming nitrogen must fill the tank’s recesses quickly and very uniformly, since any lingering concentrations of oxygen put the craft and its occupants at great risk.

Historically, proof-testing a concept for this kind of application has involved running physical tests with actual hardware components—pumps, valves, and the tank itself. This method of testing is very expensive and time-consuming, and costs are compounded by the need to bring personnel and equipment from distant subcontractors to the test site. But in the case of our application example, the Mentor Graphics FloEFD™ Concurrent CFD analysis tool was chosen to examine the internal flow characteristics of the gases in the helicopter’s fuel tank.

Concurrent CFD embeds in leading MCAD environments and enables staff mechanical engineers who are not computational fluid dynamics specialists to perform sophisticated CFD analysis. Esoteric number-crunching processes are automated and results are delivered in a host of formats including 3D views of solids, cavities, and flows; analytical charts and spreadsheets, and more.  

The project’s lead engineer first reviewed the work procedures and best practices that went into the design of the existing tank’s geometry. The tank’s initial conditions served as boundary conditions for the solid model. Extra attention went into designing the mesh and performing a detailed sensitivity analysis to ensure that the analysis solution would not be mesh-dependent. A mesh must be built with resolution sufficient to reveal small imperfections in the fluid flow. The Concurrent CFD tool automates mesh design and optimizes mesh resolution while allowing the designer to intervene if needed.

Solving the simulation yielded the visualization shown in the illustration.  The color coding summarizes the oxygen volume concentration, with the greens and blues representing the lower values. The tool also produced a chart that shows oxygen concentration in the tank over time.

 

The isosurfaces of oxygen concentration resulting from the simulation revealed that several areas in the tank were not being adequately vented. Among other changes, a second redistribution nozzle was added to the tank while some proposed changes were judged redundant. All this was determined at very low cost compared to physical testing. Had these issues remained undetected until the prototype phase, it would have been far more expensive to “go another round” and moreover, any redesign would have delayed the project.

The timely and accurate Concurrent CFD analysis saved the cost of late-cycle hardware revisions. But most importantly, it guided engineers in making design improvements that will save lives in the most threatening combat situations.

 

This Quick Technical Quide has been provided courtesy of Mentor Graphics (Mechanical)

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