Radome Induced Vortex Shedding Part 2

Introduction

In a previous ITI-RCS post, we illustrated the nature of vortices that are shed from behind a sphere-cone radome in the presence of a 100 mph wind.  We did this numerically using the Detached Eddy Simulation (DES) scheme present in ANSYS CFX for simulating highly turbulent transient flows.  The simulation was a quick attempt to recreate one of the wind tunnel tests conducted by D’Amato and Fanning in Reference 1.  We used a small fluid domain and coarse mesh (relative to the anticipated turbulence scales) in order to predict the major characteristics of the flow.  This was sufficient to capture the basic response of large eddies shedding and rolling away from behind the radome, however it did not capture the full nature of the flow.

In the current post, we present the results of three different simulation methods for the same wind tunnel test, two of which are of significantly higher fidelity than the previous post.  In addition, we assess the accuracy of the methods for predicting the flow.  The first simulation is the traditional Reynolds Averaged Navier-Stokes (RANS) method.  RANS is a steady state method that averages turbulence in time and models the smallest turbulence scales so that they do not have to be resolved through mesh discretization.  The second method is Scale Adaptive Simulation (SAS).  SAS is a variant of unsteady RANS where the Von Karman length-scale has been included allowing the solver to resolve eddies similar to Large Eddy Simulation (LES).  The last method is DES, which is a combination of RANS and LES.  DES uses RANS in regions where turbulence scales are small (such as near walls) and LES in the rest of the fluid domain.    Both DES and SAS are transient methods.  All of the methods utilize the Shear Stress Transport (SST) turbulence model.

Simulation Meshes

The meshes for the three simulations are shown in Figure 1.  The upper mesh is a half domain model that assumes symmetry exists down the length of the wind tunnel.  A region approximately 15 radome diameters ahead of the radome is included in the fluid domain.  This is done in order to give the boundary layer in the entering fluid time to develop sufficiently.  Approximately 13 diameters are included behind the radome for the development of a wake.  The unstructured mesh contains 1 million cells ranging in size from 0.5 in near the dome to 20 in elsewhere.  An inflation layer is present along the radome wall with a first cell thickness of 5 mils.  A slighter courser inflation layer is present along the wind tunnel walls.

 

We use the mesh shown in Figure 1 (b) & (c) for both the DES and SAS simulations.  The dimensions of the fluid domain are the same as for the RANS model except that no symmetry is assumed.  The mesh is unstructured with 27 million cells ranging in size between 0.25 in near the radome to 20 in far from the radome.  Cells in a region anticipated to be the wake zone are no larger than 0.35 in.  An inflation layer with identical settings as the RANS model is present along the radome wall.  A similar inflation layer is present along the other walls.

(a)

(b)

(c)

Figure 1:  (a) Mesh for RANS model.  (b) Mesh for  DES and SAS models.  (c) Cross sectional view of DES and SAS mesh scoped to the wake zone around the radome.

Vortex Structures

Results of the RANS simulation are shown in Figure 2 and Figure 3.  In both figures the results are mirrored about the symmetry plane to provide a complete picture.   The pressure distribution on the radome is shown along with an iso-surface of the swirl strength around the radome.  This surface is colored based on the flow velocity.  One half is also semi-transparent to reveal the radome surface pressure contours and flow streamlines.  The iso-surface is an indication of the presence of vortices.  We see two major types of vortices in the flow.  The first is horseshoe shaped and arcs around the front and sides of the radome.  Behind the radome are a set of long wake vortices.  These are formed as the flow separates on the backside of the radome.  They are then turned and lengthened as the flow rolls them against the ground.

Figure 2:  RANS simulation results.  Close-up view.

Figure 3:  RANS simulation results.  Broad view.

The steady-state RANS model is only capable of producing a time-averaged vortex response.  One would certainly expect the true transient response to be much more dynamic.

Video 1 shows the transient response that is predicted using the SAS model.  The motion has been slowed by roughly a factor of 30.  As before, vortices around the radome are represented using swirl strength iso-surfaces, colored by the velocity magnitude.  We see that the flow is much richer than predicted using RANS.  The location of flow separation on the radome wanders with time and tightly wound vortices shed rapidly from the radome.  They then roll over one another and coalesce into larger vortices that spread outward and continue to roll downstream. As they move they change shape, spinning off and consuming smaller vortices in a chaotic fashion.  We note that the path of the vortices roughly matches that of the tubular vortices from the RANS model.

Video 2 shows the predicted response of the DES model.  As with the SAS model, time is slowed by a factor of roughly 30.  While the flow characteristics are still rich, the structure of the vortices is less chaotic.  Larger, more stable vortices are shed from the radome surface.  They combine quickly to form vortices that tend to hold their structure better as they grow and roll downstream.

Error Comparison

D’Amato and Fanning measured the time-averaged pressure at over 200 points on the radome surface during their wind tunnel test.  From this they determined the overall lift and drag forces.  Table 1 lists the measured data as well as the predicted values from three simulations.  The table also shows the errors in the maximum and minimum pressures from the simulations, the root mean squares (RMS) of their errors, as well as the lift and drag errors.  We see that the method with the lowest error is DES.  It achieves RMS and lift errors of 5.4% and 1.7%, respectively, which are roughly half that of the RANS method.  Unfortunately, none of the methods predict the drag force well.

The quality of the three methods is easier to assess in Figure 4.  The absolute error in the pressure prediction on the surface of the radome is plotted with identical color scales.  It is clear to see that the DES solution is the most accurate.

Table 1: Radome and simulation data and error.

Figure 4:  Plots of pressure error for RANS (left), SAS (middle) and DES (right) solutions

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.

ITI-RCS TO ATTEND WORLD ATM CONGRESS MARCH 10-12, 2015 IN MADRID

Infinite Technologies, Inc. (ITI) is pleased to announce its Radome and Composite Structures Group (RCS) will attend World ATM Congress in Madrid, Spain on March 10-12, 2015 Booth 119 to showcase its line of innovative high performance, cost effective radomes for terrestrial and shipboard radar protection applications. With models available from 1,4m – 25m, operating frequencies from VHF – Ka, a wide variety of configurations, and a number of accessories available, ITI-RCS can meet your individual radome needs. 

About The Show  - www.worldatmcongress.org

“In just two years, World ATM Congress has built a reputation for excellence. It has earned the Gold for Best Congress overall from the Eventoplus Awards for its inaugural 2013 edition and recognition as one of the Best Congress/Conferences in 2014 at the EuBEA (European Best Events Awards) Festival.

This year’s event builds on our past success with a large-scale Exhibition, world-class conference, and premier networking opportunities, plus the chance to learn about the latest trends and developments in the air traffic management (ATM) industry.”

About ITI-RCS

Infinite Technologies Inc. Radome and Composite Structures Group (ITI-RCS) specializes in the design, analysis and delivery of radomes for terrestrial and shipboard applications. ITI-RCS employs a talented staff experienced in all aspects of radome design including PhD in electro-magnetics, P.E. Professional Engineer, M.S. Mechanical Engineers, PMP Project Managers and composite radome / structure personnel with an extensive history in understanding what it takes to manufacture quality radome hardware. We know radomes, and our collection of experienced radome specific talent ensures ITI-RCS will provide high quality radomes to meet the unique requirements of the systems they protect.

For more information send an email request to: Info@itircs.com or visit our website at www.compositeradomes.com

Come visit us at World ATM Congress 2015 this March 10-12 in Madrid Spain, Booth 119

Infinite Technologies, Inc.
2450 E. Bidwell Street
Folsom, CA 95630

1-888-432-2533

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

Conclusion

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.

References

www.maps.csc.noaa.gov

www.nasa.gov

www.infield.com

www.esri.com

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.

Figure 2: Vortex Shedding Pressure Distribution

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.

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. 

Figure 1:  ITI radome assembled from sandwich panels 

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.