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1. Parameter Identification of Wind-induced Buffeting Loads and Onset Criteria for Dry-cable Galloping of Yawed/inclined Cables

   https://doi.org/10.1016/j.engstruct.2018.11.049  

   Jafari, M.; Sarkar, P.P. (January 2019, Engineering structures)

   null (Ed.)

   Cables of suspension, cable-stayed and tied-arch bridges, suspended roofs, and power transmission lines are prone to moderate to large-amplitude vibrations in wind because of their low inherent damping. Structural or fatigue failure of a cable, due to these vibrations, pose a significant threat to the safety and serviceability of these structures. Over the past few decades, many studies have investigated the mechanisms that cause different types of flow-induced vibrations in cables such as rain-wind induced vibration (RWIV), vortex-induced vibration (VIV), iced cable galloping, wake galloping, and dry-cable galloping that have resulted in an improved understanding of the cause of these vibrations. In this study, the parameters governing the turbulence-induced (buffeting) and motion-induced wind loads (self-excited) for inclined and yawed dry cables have been identified. These parameters facilitate the prediction of their response in turbulent wind and estimate the incipient condition for onset of dry-cable galloping. Wind tunnel experiments were performed to measure the parameters governing the aerodynamic and aeroelastic forces on a yawed dry cable. This study mainly focuses on the prediction of critical reduced velocity 〖(RV〗_cr) as a function of equivalent yaw angle (*) and Scruton number (Sc) through measurement of aerodynamic- damping and stiffness. Wind tunnel tests using a section model of a smooth cable were performed under uniform and smooth/gusty flow conditions in the AABL Wind and Gust Tunnel located at Iowa State University. Static model tests for equivalent yaw angles of 0º to 45º indicate that the mean drag coefficient 〖(C〗_D) and Strouhal number (St) of a yawed cable decreases with the yaw angle, while the mean lift coefficient 〖(C〗_L) remains zero in the subcritical Reynolds number (Re) regime. Dynamic one degree-of-freedom model tests in across-wind and along-wind directions resulted in the identification of buffeting indicial derivative functions and flutter derivatives of a yawed cable for a range of equivalent yaw angles. Empirical equations for mean drag coefficient, Strouhal number, buffeting indicial derivative functions and critical reduced velocity for dry-cable galloping are proposed for yawed cables. The results indicate a critical equivalent yaw angle of 45° for dry-cable galloping. A simplified design procedure is introduced to estimate the minimum damping required to arrest dry-cable galloping from occurring below the design wind speed of the cable structure. Furthermore, the results from this study can be applied to predict the wind load and response of a dry cable at a specified wind speed for a given yaw angle. 

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2. Wind Tunnel Study of Wake-induced Aerodynamics of Parallel Stay-Cables and Power Conductor Cables in a Yawed Flow

   https://doi.org/https://doi.org/10.12989/was.2020.30.6.617  

   Jafari, M.; Sarkar, P.P. (January 2020, Wind and Structures)

   null (Ed.)

   Wake-induced aerodynamics of yawed circular cylinders with smooth and grooved surfaces in a tandem arrangement was studied. This pair of cylinders represent sections of stay-cables with smooth surfaces and high-voltage power conductors with grooved surfaces that are vulnerable to flow-induced structural failure. The study provides some insight for a better understanding of wake-induced loads and galloping problem of bundled cables. All experiments in this study were conducted using a pair of stationary section models of circular cylinders in a wind tunnel subjected to uniform and smooth flow. The aerodynamic force coefficients and vortex-shedding frequency of the downstream model were extracted from the surface pressure distribution. For measurement, polished aluminum tubes were used as smooth cables; and hollow tubes with a helically grooved surface were used as power conductors. The aerodynamic properties of the downstream model were captured at wind speeds of about 6-23 m/s (Reynolds number of 5×10^4 to 2.67×10^5 for smooth cable and 2×10^4 to 1.01×10^5 for grooved cable) and yaw angles ranging from 0º to 45º while the upstream model was fixed at various spacing between the two model cylinders. The results showed that the Strouhal number of yawed cable is less than the non-yawed case at a given Reynolds number, and its value is smaller than the Strouhal number of a single cable. Additionally, compared to the single smooth cable, it was observed that there was a reduction of drag coefficient of the downstream model, but no change in a drag coefficient of the downstream grooved case in the range of Reynolds number in this study. 

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3. Nonlinear coupling of closely spaced modes in atomically thin MoS2 nanoelectromechanical resonators

   https://doi.org/10.1038/s41378-024-00844-9  

   Yousuf, S_M_Enamul_Hoque; Shaw, Steven_W; Feng, Philip_X-L (December 2024, Microsystems & Nanoengineering)

   Abstract Nanoelectromechanical systems (NEMS) incorporating atomic or molecular layer van der Waals materials can support multimode resonances and exotic nonlinear dynamics. Here we investigate nonlinear coupling of closely spaced modes in a bilayer (2L) molybdenum disulfide (MoS₂) nanoelectromechanical resonator. We model the response from a drumhead resonator using equations of two resonant modes with a dispersive coupling term to describe the vibration induced frequency shifts that result from the induced change in tension. We employ method of averaging to solve the equations of coupled modes and extract an expression for the nonlinear coupling coefficient (λ) in closed form. Undriven thermomechanical noise spectral measurements are used to calibrate the vibration amplitude of mode 2 (a₂) in the displacement domain. We drive mode 2 near its natural frequency and measure the shifted resonance frequency of mode 1 (f_1s) resulting from the dispersive coupling. Our model yieldsλ = 0.027 ± 0.005 pm⁻² · μs⁻²from thermomechanical noise measurement of mode 1. Our model also captures an anomalous frequency shift of the undriven mode 1 due to nonlinear coupling to the driven mode 2 mediated by large dynamic tension. This study provides a direct means to quantifyingλby measuring the thermomechanical noise in NEMS and will be valuable for understanding nonlinear mode coupling in emerging resonant systems. 

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4. Unsteadiness in Shock/Boundary-Layer Interaction Over a Compliant Panel at Mach 2

   https://doi.org/10.2514/6.2022-4136  

   Musta, Mustafa N.; Ahn, Yoo-Jin; Eitner, Marc A.; Sirohi, Jayant; Clemens, Noel (June 2022, AIAA Aviation Meeting)

   This work investigates surface pressure unsteadiness on a compliant panel under a shockwave/boundary-layer interaction (SBLI) induced by a 2D compression ramp with an angle of 20o in a Mach 2 wind tunnel. High-speed digital image correlation (DIC) and fast-response pressure-sensitive paint (PSP) measurements are used to measure the panel displacement and panel and ramp-face surface pressure fluctuations at 5kHz and 20kHz, respectively. The data reduction technique of POD (proper orthogonal decomposition) was employed both for pressure and displacement fields. POD mode distribution for the pressure fields reveals that the first six modes have 60% of the total energy and exhibit low-frequency content for both rigid and compliant panels. The vibration of the compliant panel was seen to alter the energy distribution of the high energy modes as compared to the rigid panel case. The cross-correlations between the displacement and pressure modes were made using the time coefficients. This analysis shows significant correlations were present among the lower modes. The highest correlation was between displacement mode 1 and the pressure mode 4, which stemmed from the upstream of the intermittent region. The analysis was also made for the surrogate shock foot and reattachment lines. The correlation shows that panel vibration lowers the correlation between the shock foot and reattachment line when compared with the rigid panel case. 

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5. Distributed Acoustic Sensing for Whale Vocalization Monitoring: A Vertical Deployment Field Test

   https://doi.org/10.1785/0220240389  

   Saw, Jaewon; Luo, Linqing; Chu, Kristy; Ryan, John; Soga, Kenichi; Wu, Yuxin (February 2025, Seismological Research Letters)

   Abstract There is growing interest in floating offshore wind turbine (FOWT) technology, where turbines are installed on floating structures anchored to the seabed, allowing wind energy development in areas unsuitable for traditional fixed-platform turbines. Responsible development requires monitoring the impact of FOWTs on marine wildlife, such as whales, throughout the operational lifecycle of the turbines. Distributed acoustic sensing (DAS)—a technology that transforms fiber-optic cables into vibration sensor arrays—has been demonstrated for acoustic monitoring of whales using seafloor telecommunications cables. However, no studies have yet evaluated DAS performance in dynamic, engineered environments, such as floating platforms or moving vessels with complex, dynamic strain loads, despite their relevance to FOWT settings. This study addresses that gap by deploying DAS aboard a boat in Monterey Bay, California, where a fiber-optic cable was lowered using a weighted and suspended mooring line, enabling vertical deployment. Humpback whale vocalizations were captured and identified in the DAS data, noise sources were identified, and DAS data were compared to audio captured by a standalone hydrophone attached to the mooring line and a nearby hydrophone on a cabled observatory. This study is unique in: (1) deploying DAS in a vertical deployment mode, where noise from turbulence, cable vibrations, and other sources posed additional challenges compared to seafloor DAS applications; (2) demonstrating DAS in a dynamic, nonstationary setup, which is uncommon for DAS interrogators typically used in more stable environments; and (3) leveraging looped sections of the cable to reduce the noise floor and mitigate the effects of excessive cable vibrations and strain. This research demonstrates DAS’s ability to capture whale vocalizations in challenging environments, highlighting its potential to enhance underwater acoustic monitoring, particularly in the context of renewable energy development in offshore environments. 

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