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![Schematic of simplified frigate ship (SFS) geometry; SFS2 (from Zan [14]).](profile/S-Shukla-3/publication/331535793/figure/fig8/AS:905939813167120@1593004291673/Schematic-of-simplified-frigate-ship-SFS-geometry-SFS2-from-Zan-14.png)
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... known as 'SFS1' was developed in early 1985. The SFS1 was made up of rectangular shape blocks without a nose. Subsequently, a more realistic version of the simplified ship with a triangular bow was attached to SFS1 and was named as SFS2. This was developed in late 1998. A schematic diagram of both the TTCP variants; SFS1 and SFS2 are shown in Fig. 8 (dimensions are in feet). The principal objective of this program was to promote distribution of full and model-scale experimental database of helicopter-ship DI research in the public domain. For a detailed summary of the TTCP collaborative program, see [15]. Our review of experimental modelling starts with an overview of relevant ...
Context 2
... known as 'SFS1' was developed in early 1985. The SFS1 was made up of rectangular shape blocks without a nose. Subsequently, a more realistic version of the simplified ship with a triangular bow was attached to SFS1 and was named as SFS2. This was developed in late 1998. A schematic diagram of both the TTCP variants; SFS1 and SFS2 are shown in Fig. 8 (dimensions are in feet). The principal objective of this program was to promote distribution of full and model-scale experimental database of helicopter-ship DI research in the public domain. For a detailed summary of the TTCP collaborative program, see Ref. ...
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... Flight simulation has long been recognized as a valuable tool for initial establishment of shiphelicopter operating limits (SHOLs), development and pilot training in DI environments (Refs. [1][2][3][4][5]. A fundamental hurdle associated with the application of flight simulations is the selection of model fidelity levels for the constituent rotorcraft and airwake models. ...
Rotorcraft responses to idealized disturbances are examined to gain insights into model fidelity requirements for flight simulations of the ship-rotorcraft dynamic interface. Two disturbance fields are considered: an isolated straight vortex that represents the canonical vortex that results from the corners of flat top ships in oblique wind-over-deck conditions and a horseshoe vortex derived from a nondimensional characterization of the time-averaged flow observed aft of a simplified ship superstructure. Rotorcraft models considered include: an analytical blade element theory-based rotor model, where the disturbance velocities are integrated over the rotor, and a coupled blade element / free wake flight dynamic model of the full UH-60 aircraft, which is used to perform time-marching simulations with the disturbances modeled as a frozen field that is fixed in space and not interacting with the aircraft (one-way coupling), and as a distorting field (two-way coupling). Analytical thrust responses of four idealized rotors to the disturbance associated with the horseshoe vortex suggest the existence of a scaling relationship between nondimensional blade loading and the product of nondimensional vortex width and strength. Time-averaged rotor thrust responses to the disturbance induced by the isolated straight vortex indicate significant distortion of the rotor wake and disturbance field at low forward speeds, with the frozen field assumption indicating greater disk tilt responses. Modeling the horseshoe vortex disturbance as a frozen field generates a greater rotor response than in the distorting case. Inclusion of full aircraft dynamics attenuates rotor responses to the disturbances when compared to isolated rotor responses.
... These high turbulence levels can induce highly unsteady loads on piloted and unpiloted rotorcraft operating near the flight deck. Consequently, the ship airwake has been widely investigated using wind tunnel measurements ( [1][2][3][4][5][6][7][8][9][10][11][12][13]), computational fluid dynamics ( [14][15][16][17][18][19][20][21][22][23][24]), and at-sea measurements ( [24][25][26][27]). ...
... One example is the overarching goal of the present work. Turbulent ship airwakes are three-dimensional flow fields with complex temporal and spatial characteristics (Zan 2005;Shukla et al. 2019;Zhu 2021;Palm 2022), including various shear layers (Mazzilli 2022;Sydney et al. 2017), several vortical structures (Tinney and Ukeiley 2009;Zhu et al. 2022), an intermittent bimodal behavior (Herry et al. 2011), etc. As a result, airwakes induce significant unsteady airloads that elevate pilot workload (Wilkinson et al. 1998;Zan 2005), making the ship airwake/rotorcraft Dynamic Interface (DI) a severe safetyof-flight issue. ...
A high-speed, dual-plane, stereoscopic particle image velocimetry technique was complemented by a flow estimation method to construct three-dimensional Simple Frigate Ship 2 airwakes. In particular, three pairs of mutually-orthogonal streamwise and spanwise planes were measured, with the ultimate goal of simultaneously estimating various spanwise planes using an invariant streamwise plane. First, the dual-plane measurements were divided into training and test sets. Then, for each pair of planes in the training set, the large-scale structures spanning both (streamwise and spanwise) planes were learned using the proper orthogonal decomposition (POD) spatial modes. Next, in the test set, only the streamwise plane was assumed to be known, and the POD temporal modes (corresponding to the learned spatial modes) were estimated to construct low-order models of each spanwise plane. This approach found excellent agreement between the measured and estimated low-order spanwise planes, as evaluated by three metrics. Ultimately, the velocities across three spanwise planes were simultaneously calculated, demonstrating the experimental framework’s ability to perform time-dependent, three-dimensional, low-order reconstruction of complex turbulent flows.
... Such flows are dominated by the separated boundary layers from the rear edges of the superstructure, corner vortices, downwash as well as lateral turbulent fluctuations. These effects combined with helicopter rotor dynamics result in restrictions being imposed on Ship-Helicopter Operating Limits (SHOL) for safe flight operations (Shukla et al., 2019). ...
Turbulent air wake of a Simplified Frigate Ship (SFS2) model is simulated using wall-resolved Large Eddy Simulation. The wake behind the superstructure presents an asymmetric configuration for an otherwise symmetric free stream condition. Bi-stable characteristics of the wake have also been simulated as evidenced by asymmetric state switching events. The wake behind the stern also exhibits bi-stable switching and adapts an opposite asymmetric orientation to the wake behind the superstructure, as also observed experimentally. The resulting time-averaged flow fields for each bi-stable state reveal a differential flux formation around the upstream sides of the superstructure. The instantaneous wake fields also suggest a possible influence of flow structures developed around the upstream end of the superstructure to generate a particular asymmetric wake state downstream.
... where L re f = L SFS2 of the ship model and V re f is the nominal wind velocity, 30.48 m/s or 100 ft/s. A Q-criterion value of 0.5 was selected as the value that provided an optimum visual comparison. ...
Safe flight with maneuvers of rotorcraft systems in the near vicinity of Naval ships poses a number of challenges due to the rapidly changing flow conditions and unsteady wake shedding from the ship surface. To prepare pilots to operate in these conditions, it is desired to ensure accurate ship airwake simulation capabilities that provide an understanding of the flow field and that are sufficiently cost-effective for quick turnaround and so that they can be applied in flight simulators to enhance pilot training and flight safety. While the simple frigate shape (SFS2) geometry has been extensively studied, assessments of the flow environment with respect to practical usage in flight simulations have experienced fewer analyses. Flow field results from a high-fidelity, conventional unsteady unstructured Reynolds-Averaged Navier-Stokes CFD solver (FUN3D) and results from a Lattice-Boltzmann Method (LBM) solver are compared to each other and to time-resolved particle image velocimetry (PIV) wind tunnel measurements on the SFS2. Overall, the LBM predictions are very similar to FUN3D and PIV, particularly for the headwind condition, albeit with some loss in magnitude and flow structural detail. Ship orientations with yaw experiencing winds have more differences between LBM and FUN3D, and both to the experiment, when compared to the headwind condition. The LBM simulations provide computational time savings of several orders of magnitude over unstructured CFD.
... (Refs. [3][4][5]. At sea, aircraft operating to and from the aircraft carrier will encounter the unsteady airwake, generated by the prevailing wind shedding off the flight deck edges and cascading over and around the twin-island superstructure, particularly when the wind is approaching from the starboard side of the ship. ...
This paper describes an investigation of the air flow over the flight deck of a twin-island aircraft carrier with the ship’s lifts in a raised and lowered position, and the subsequent effect on the helicopter and on pilot workload. Computational Fluid Dynamics was used to model the unsteady flow over the flight deck in a 40 kt wind approaching from 60° starboard. The turbulence intensity and velocity flow field produced over the flight deck for each lift configuration was analyzed and compared. The unsteady air flow computed for each lift position was combined with a flight dynamics model of a helicopter configured to represent a SH-60B Seahawk an integrated with a full-motion flight simulator. To analyze the effect of the two airwakes on the helicopter and on pilot workload, a series of simulated flight trials were conducted in which the pilot performed landings to the flight deck of the aircraft carrier. The results show that while the lift configuration does affect the air flow over the flight deck, the effect on pilot workload is dependent on the location on the flight deck the pilot is landing to.
... The integration of aircraft operation to the shipboard of naval ships has always been both an expensive and challenging task that comes with unavoidable risk to the pilots due to the high unsteady nature of the ship airwake (Watson et al., 2019;Owen et al., 2021;Nisham et al., 2021;Shukla et al., 2019;Terziev et al., 2022;Li et al., 2020). In addition to the mentioned high unsteady turbulent flow in the ship wake, researchers have confirmed the presence of flow bi-stability at the wake of the ship (Mallat and Pastur, 2021;Rao et al., 2019;Zhang et al., 2018Zhang et al., , 2021. ...
... The complex nature of the flow is due to the ship superstructure and the presence of various military appendages. Consequently, a lot of studies have been carried out to investigate methods to reduce the effect of wake structure above flight deck (Dooley et al., 2020;Herry, 2011;Reddy et al., 2000;Shukla et al., 2019Shukla et al., , 2020Singh, 2015;Thornber et al., 2010). ...
The study of the ship airwake is critical as it explores the effect of unsteady wake flow on helicopter operations. This paper studied the near-wake flow topology of a generic ship at Re = 8×10^4 to assess the capability of a hybrid RANS/LES (Reynolds-averaged Navier-Stokes/large-eddy simulation) approach, known as IDDES (improved delayed detached-eddy simulation). The impact of computational parameters, including the mesh grid, residual level, time-step size, momentum discretization scheme and transient formulation, on the wake flow was investigated. The numerical results were validated by using the previous experimental data and LES results. The results show that except of the mesh resolution all other computational parameters varied in the current study do not have significant effect on the global drag forces, but showing large differences on the prediction of the local wake flow structures. This point has not been evidently reported in the previous work. The finer grid resolution is sufficient to produce an accurate qualitative and quantitative prediction of the flow structures, while using a poor grid resolution (coarse mesh) leads to inaccurate prediction of the flow topology. The recommended parameters for the time-step size (7.5×10^(−5) s) and residual level (1×10^(−4)) provide sufficient accuracy of wake predictions, showing good agreement with reference studies. For the convective term of the momentum equation in IDDES, the bounded central differencing scheme is proposed for its discretization, while the bounded second-order implicit is proposed to be adopted as the transient formulation.
... With this setup, Δz = 0.03 mm, meaning for Δz = 3 mm a number of n planes = 100 was possible for the computation of the spatiotemporal averages. Thus, the averaging period for this case was t avg = 13.3 s, whereas the expected shedding frequency based on the ship height is 0.16 < St < 0.25 , or 46 < f < 72 Hz (Shukla et al. 2019). ...
In this study, a variation of the stacked stereoscopic PIV technique is proposed to perform fully volumetric (3-dimensions, 3-components) measurements of average flow fields within a single experiment through the usage of an automated traversing system that continuously scans the SPIV light sheet over a linear path. The simultaneous measurement of the traverse location and the laser Q-switch pulse enables the automated assignment of instantaneous PIV fields to known physical coordinates, enabling spatiotemporal averaging in post-processing to obtain volumetric measurements of a flow field. This method provides a trade-off between spatial resolution of the volume measurements and statistical convergence of the spatiotemporal averages, enabling volumetric measurements under challenging experimental conditions where only stereoscopic PIV is viable. A comparison with the more traditional temporal averaging method and planar PIV is presented to demonstrate the capabilities and limitations of this technique in realistic, challenging experimental setups. It is found that the spatiotemporal averaging convergence behavior differs slightly from the traditional temporal averaging for the wake of a bluff body model, however relative errors lower than two standard deviations can still be attained. Thus, this technique presents a viable alternative for rapid 3D reconstruction of averaged flow fields that can provide invaluable insight of various flow topologies.
... The airwake is a result of the interaction between the atmospheric boundary layer and the bluff, unstreamlined shapes of ships [1]. Embedded within airwakes are large-scale energetic shear layers and vortical systems among other flow structures [2][3][4][5]. Consequently, the turbulent ship airwake creates unsteady loads on rotorcraft, thereby elevating pilot workload during shipborne operations. Particularly, of interest to rotorcraft operating in the vicinity of ships are extreme flow events. ...
... Previous studies have measured or simulated airwakes [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] as well as airwake/rotorcraft (one-way or two-way) coupled scenarios [21][22][23][24][25][26][27][28][29][30][31][32][33][34]; Shukla et al. [35] provided a comprehensive review. However, the aerodynamic interactions that dictate the forces and moments experienced by the shipborne rotorcraft are generally not studied in detail. ...
The turbulent airwakes produced by a generic navy frigate (Simple Frigate Ship 2) was investigated in a low-speed wind tunnel. The frigate model was subjected to a headwind and immersed in a simulated atmospheric boundary layer. High-speed stereoscopic particle image velocimetry (PIV) was performed at six different spanwise planes over the flight deck. A spatio-temporal analysis of the PIV airwake measurements was conducted by applying the proper orthogonal decomposition (POD) to individual frequency bands of the airwake. Physical interpretations of the leading POD modes were substantiated by other complementary analyses. In particular, it was discovered that turbulent structures were shed from the funnel and superstructure edges. The flow recirculation region behind the superstructure exhibited a strongly three-dimensional bistable behavior. A pair of vortices near the stern also demonstrated low-frequency bistable dynamics. Notably, the bistable flow recirculation region appeared to influence or interact with the low-frequency behavior of the other major flow features.
