Wednesday, June 19, 2013

Hurricane Induced Radome Load Cycles

Infinite Technologies, Inc. RCS is committed to achieving the highest levels of technical excellence and quality in all we do.  This is especially true in the design and fabrication of our composite radomes.  One of the greatest challenges faced by engineers designing radomes is accurately predicting the wind loads these structures can expect to see, given the uncertainty of storm patterns in any particular area.  Despite this, it is possible to obtain a rough, but conservative, estimate of the frequency with which radomes will experience high wind loads. 

As an example, ITI engineers were asked to design a radome that could withstand multiple category IV hurricanes over a 20 year period.  Several steps were taken to determine the number of loading cycles that the radome could reasonably be expected to encounter.  These steps are listed below.

Step 1) Define the duration of a hurricane
Hurricanes travel at about 30 mph.  The slower, more dangerous hurricanes travel as slowly as 13 mph (Hurricane Andrew, 1992).  Winds rotate counter-clockwise around the eye with the highest wind speeds recorded in the right-hand side.  Hurricane force winds can occur as far as 300 miles from the storms eye.  The stronger winds are generally contained to a much smaller corridor.  Hurricane Katrina, for example, was a category IV hurricane at landfall and had category III wind speeds recorded over an area approximately 80 miles wide.  At 13 mph it would take an 80 mile wide circle 6.15 hours to completely pass over a single point.

Figure 1: Wind speed map of Hurricane Katrina at landfall

Step 2) Determine how often wind loads a structure during a hurricane
Hurricane wind speed is measured as the average speed in a one minute interval.  Wind gusts are the highest measurement taken during a one minute period.  Hurricanes are classified based on wind speed, not gust speed.  This means that Category III hurricanes can achieve gusting wind speeds at category IV levels for brief periods.  Assuming 4 gusts per minute, a radome would experience 1476 gusts during a 6.15 hour storm.

Step 3) Check Historical Hurricane Data
The National Oceanic and Atmospheric Administration has record of every hurricane from the past 150+ years.  With this data it’s possible to determine how many category III+ hurricanes have passed through an area.  Some regions in the Gulf coast experience less than one category III+ hurricane every 20 years.  The highest number of occurrences was in Florida City, FL which experienced 15 category III+ in the last 150 years; 7 during a 20 year period from 1931 to 1951.  The absolute worst case scenario of 7 slow-moving, category III+ hurricanes hitting one spot over a 20 year period translates to 10332 loading cycles.  Realistically, since category III hurricanes only produce category IV gusts infrequently, a more accurate assumption would be 5608 cycles (assuming three category IV+ hurricanes plus 20% of the gusts from category III hurricanes).

Figure 2: Category III+ hurricanes passing near Florida City from 1931 to 1951

Given the above assumptions and weather data, a conservative estimate for the number of loading cycles a radome near Florida City would need to sustain during the worst 20 year hurricane season ever recorded is 5608.


Friday, May 17, 2013

Radome Induced Vortex Shedding

Infinite Technologies, Inc. RCS uses an analysis-driven approach to design that focuses on gaining an in-depth understanding of the physics behind our products and the products of our customers.   An example of this is the simulation of wind loads on large structures, such as radomes.
ITI-RCS radomes are designed to withstand wind loads in excess of 100 mph (typically 155 mph).  At such speeds, pressure loads can vary widely from one side of the dome to the other, becoming a significant factor in its structural requirements.  Thus, accurate prediction of the wind load is very important to the success of a radome design.

Simulating wind flow around a radome is not as simple as it might sound.  Due to the high Reynolds numbers associated with these bluff bodies (between 105 and 107), the nature of the flow can be chaotic in the wake.  High fidelity models must be three-dimensional and unsteady since the flow separates behind the domes and forms vortices that shed in an undulating fashion.  As a result, pressure on the leeward side of the radome oscillates in both magnitude and position.

To illustrate this, we have simulated the conditions of one of the wind tunnel tests conducted in Reference 1.  In this test, D'Amato and Fanning subjected a sphere-cone radome to a uniform horizontal 100 mph wind.  The resulting Reynolds number was 2*106.  We modeled the test using a Detached Eddy Simulation (DES) run with a seven million cell unstructured mesh (see Figure 1).  Time steps were on the order of 10-3 seconds.

Figure 1: Unstructured mesh tailored for vortex shedding.

Video 1 shows the chaotic nature of the flow in the radome’s wake, over roughly 0.5 seconds.  The vorticity of the flow is shown along seven planes slicing parallel to the direction of the wind.  The undulating vortices roiling downstream can easily be seen. 

The true structure of the vortices is easier to discern in Video 2.  In this case the 150 Hz iso-surface of the vorticity is tracked through time (the surface is colored by the velocity magnitude).  Vortices of several sizes and shapes can be seen in the video.  Some are small and short lived.  However, the majority are large and persistent.  These horn-shaped vortices form just aft of the dome and are shed semi-periodically.  They are tight swirls near the centerline of the radome but then arch up and outwards until connecting with the ground at the edge of the wake zone and they grow in size as they travel.

Figure 2 shows the pressure distribution on the leeward side of the radome at two different points in time.  The unsteady nature of the vortex shedding just discussed causes high and low pressure zones to alternate side to side with time.  This variation causes lateral forces that are as high as 23% of the lift and 28% of the drag.  Obviously, this variation complicates any subsequent structural analysis.

In addition to unsteady flow around bluff bodies, ITI-RCS has the ability to simulate many other fluid flow conditions.  These include porous media, pipe flows, aerodynamics, micro components, and fluid-thermal-structural interactions, to name a few.  For more information on our modeling capabilities and how we can help meet your project needs please visit our website.

Reference 1:  D’Amato, R., Fanning, W. R., “Pressure Distributions on Sphere-Cone Radomes in Uniform and Gradient Flows,” Techincal Note, ESD-TR-68-84, Lincoln Library, MIT, 1968.

Thursday, May 9, 2013

Hail Impact Testing Sets ITI-RCS Apart

Infinite Technologies, Inc. RCS is committed to achieving the highest levels of technical excellence and quality in all we do.  This is especially true in the design and fabrication of our composite structures. 
Radomes, and other such structures, must withstand a wide range of extreme environmental conditions.  This includes ultraviolet (UV) exposure, solar radiation, hurricane force winds, salt spray, rime ice accretion, and large hail impacts.  Because of our attention to detail and focus on quality, our composite structures have been proven to outperform the competition in these conditions.
As an example, ITI-RCS recently designed a structure intended to replace a radome manufactured by a leading competitor. Verification of the product included simulated hail impact testing of the panels used to assemble the radome, as can be seen below.  Rectangular specimens were cut out of both the legacy and replacement panels and impacted by a 3 inch diameter steel ball with a kinetic energy equivalent to hail traveling at 41 mph.  Prior to impact, the specimens were conditioned to both extreme low and high temperatures. 

Figure 2:  Results of large hail impacting test on ITI's panels.
Figure 2 shows the effect of the impacts on ITI’s design.  Two of the specimens showed minor cracking of the composite face sheets with shallow dimpling of the panel’s foam core.  The third showed minimal visual damage.  In all three cases the face sheets remained fully bonded to the core material, thus maintaining overall structural integrity.

As can be seen in Figure 3, the other manufacturer’s specimens did not fare so well.  In all three cases the composite face sheets completely disbonded from the panel core.  In addition, the foam cores were incapable of absorbing the impact energy, resulting in severe crushing, cracking, and loss of structural integrity.

The results of the hail impact testing typify the way ITI’s composite structures outperform the competition.  For more information about what ITI-RCS can do to ensure the success of your engineering project contact us, we would be more than happy to answer any questions you may have.

Figure 3: Results of  hail impact testing on competitor panels.