Designing Launch Site Structures for Private Space IndustryFeb 06 2018 · 0 comments · Aerospace, NISHKIAN DEAN ·0
Unique Load – Vortex Induced Vibration
By Aerik Carlton
While working with private space industry clients on launch complex structures, we have found some notable structural challenges. This article is meant to share some of Nishkian Dean’s recent launch site design experience. Specifically, an atypical structural load consideration that was identified and requested to be evaluated by our client.
Recently, launch tower structures, designed by others, near one of our project sites had been found to present weld failures at circular structural member support connections. The design and construction team that completed the nearby tower believed the weld failures to be representative of “bad welds” during construction. This conclusion was drawn because there appeared to be random failure locations. The failed connections were rewelded, but the repaired welds were found to have failed at the same locations in a follow-up inspection.
The failure of these members was surprising, mostly because they had been designed for large pressures associated with launch activities and hurricane wind forces for these structures, which the structure had not experienced. The second weld failures also occurred in the absence of either launch activities or hurricane activity, and there were only low-wind-velocity days between weld repairs and the second failure. The determined cause was attributed to wind-induced flutter, or gallop, of the members that resulted during low-velocity-wind cyclical loading. The team’s solution was to install air foils for fluid flow disruption around the circular members that experienced weld failures.
Due to the large rocket thruster impingement loadings present during launch activities, circular structural members have been found to be advantageous. Circular cross-sections allow for the same structural section properties: moment of inertia, section modulus, etc. to be present regardless of the direction of load (i.e. as the rocket gains elevation and the angle of load from the engine thrust changes, the structural member maintains the same resistance without reduction due to the change in loading angle). Wind and rocket thrust loads can be reasonably reduced by 20-30% of the projected area due to the allowable shape factor for circular structural cross-section. Fluid shedding, due to the rounded shape, is what allows the reduction in projected area for pressure loadings. When we are talking about 8 psi (1152 psf) of thrust pressure on the structure, shape factor reduction to the projected area is significantly beneficial. The use of circular framing members presents a slightly different issue, however, when considering relatively light wind loading conditions as evident from the nearby tower weld failures our client highlighted.
Nishkian Dean was tasked with mitigating the same type of behavior for a new tower at a nearby launch site in conjunction with the structural design for the client’s new facilities; for crew to vehicle access and lightning protection. The client asked that we design these operations support structures to accommodate their current vehicles as well as allowing for larger, more powerful, and greater range vehicles planned for the future.
A circular cross-sectional shape can exhibit interesting behavior in calm, steady wind. The phenomena our neighboring tower experienced is called “Karman vortex street,” and it happens during steady state fluid flow (or steady non-gusting wind) around a cylinder. You can be forgiven for not having that term readily available for recall from your fluid dynamics course work. I was fully unaware of the term until researching a solution to our issue. Karman vortex street is when the laminar (steady, non-gusting) fluid flow (or calm steady wind) around the cross-section begins to “stick” to the back side (leeward side) of the member and results in an alternating, rhythmic pattern of pressure differences from either side of the shape. This alternating pattern of “stick” creates a turbulent flow condition behind the member, and similar to an airplane wing, induces lift on the member due to the pressure differences. Pairing the lifting force in each direction and the gravitation pull on the member’s self-weight with respect to time, we end up with a vortex-induced vibration (VIV) loading condition.
To further explain and give a visual example of Karman vortex street, let’s take a look at the photo below. In the image, we can see an atmospheric example of Karman vortex street in cloud formations (located at the top of the photo) at the leeward (downwind) side of an island in the northern Sea of Japan. The photo has a wind directionality, moving from the bottom of the photo to the top, and the island is located at the bottom of the photo. As the clouds pass around the island, they swirl into eddies which alternate from the left and right sides of the island. This phenomenon is known as Karmon vortex street and was determined to be the cause of our client’s experience with weld failures.
Rishiri-to island, Sea of Japan off the northwest coast of Hokkaido, Japan
Taken: between April 19 and May 1, 2001
Photo Credit: NASA STS-100 Shuttle Mission https://spaceflight.nasa.gov/gallery/images/shuttle/sts-100/html/sts100-710-182.html
Licensing Agreement: https://www.nasa.gov/audience/formedia/features/Advertising_Guidelines.html
Designing for Vortex-Induced Vibration
Determining which members in a structure will present VIV is the first step in designing for the issue. As our client saw with the neighboring launch tower, the structural members to exhibit VIV failures appeared to be random at first glance. To understand the nature of the failure, we needed to discover that we are dealing with a vibrational load that is time-dependent, and as such, we need to calculate the natural resonant frequency of our individual structural members. This process is very similar to what we, as structural engineers, do for seismic design when establishing our earthquake lateral loads. The load is dependent on the natural period of the structure. Turning to Roark’s Formulas for stress and strain, we obtain equations for the natural frequency of our members based upon support condition, length, material, and cross-sectional properties.
We can then establish the wind loading environment to see at what wind velocities moving around our individual structural member sizes will elicit vortex shedding frequencies in the vicinity of our structural members’ resonant frequencies. The tricky part here is establishing what makes sense for applicable wind velocities at the site and what boundary conditions can be set. Because hurricane activity was a design issue already being evaluated, the lower- to mid-velocities associated with hurricane wind were the upper boundary (assuming gusting would break up the required laminar flow required for Karman vortex street to occur long enough to gain a vibrational response). Evidence to the issues on the neighboring tower was given, as some of our tower’s members had resonant frequencies close to vortex shedding frequencies at wind velocities between 5 and 10 mph.
At this point we identify the members in the structure that are at risk of VIV. Using an equation for harmonic vortex shedding lifting force from Blevins (2001), we can establish a time-history load function. The load is then applied to a single degree of freedom model for the member, and we find the deflection versus time plot for the member during VIV that accounts for stiffness and damping. From the deflections and member properties, the reaction loads to the support connections are determined. And, from there, we evaluate our connections for cyclical loading.
In our design, we were able to simply lengthen our welds on the members that exhibited VIV to add the required weld capacity to overcome cyclical fatigue. However, there are many ways of mitigating the negative response to vortex conditions. Blevins (2001) outlines several options, including: helical strakes, shrouds, axial slats, streamlined fairings, splitters, ribbons, pivoting guiding vanes, and flat spoiler plates. The neighboring tower took advantage of the streamlined fairing option, while we implemented helical strakes on our tower topping fiber reinforced plastic (FRP) mast similar to those used on exhaust chimneys in other industrial applications.
Please contact Nishkian Dean if your project has vibration analysis considerations – we would be happy to share more of our expertise on this subject.
Aerik Carlton is an Engineering Designer with Nishkian Dean. Aerik can be reached at firstname.lastname@example.org.
AISC. American Institute of Steel Construction (2011). Steel Construction Manual. 14th Ed. AISC. Chicago, IL.
Biggs, J. M. (1964). Introduction to Structural Dynamics. McGraw-Hill. New York.
Blevins, R. D. (2001). Flow-Induced Vibration. 2nd Ed. Krieger Publishing Company. Malabar, FL.
Young, W. C., Budynas, R. G. (2002). Roark’s Formulas for Stress and Strain. 7th Ed. McGraw Hill. New York.