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Computational Fluid Dynamics Simulations of Ship Airwake SPECIAL ISSUE PAPER 369 Computational fluid dynamics simulations of ship airwake N Sezer-Uzol, A Sharma, and L N Longà Department of Aerospace Engineering, Pennsylvania State University, Pennsylvania, USA The manuscript was received on 17 March 2005 and was accepted after revision for publication on 26 August 2005. DOI: 10.1243/095441005X30306 Abstract: Computational fluid dynamics (CFD) simulations of ship airwakes are discussed in this article. CFD is used to simulate the airwakes of landing helicopter assault (LHA) and landing platform dock-17 (LPD-17) classes of ships. The focus is on capturing the massively separated flow from sharp edges of blunt bodies, while ignoring the viscous effects. A parallel, finite- volume flow solver is used with unstructured grids on full-scale ship models for the CFD calcu- lations. Both steady-state and time-accurate results are presented for a wind speed of 15.43 m/s (30 knot) and for six different wind-over-deck angles. The article also reviews other compu- tational and experimental ship airwake research. Keywords: ship airwake, computational fluid dynamics, landing helicopter assault (LHA), landing platform dock-17 (LPD-17), dynamic interface, blade sailing 1 INTRODUCTION helicopters from a ship is to find the safe helicopter operating limits (SHOLs) for a ship/helicopter Helicopter shipboard operations present a multitude combination. These are usually defined in terms of of challenges. One of the primary factors that make it allowable wind conditions (direction and speed) extremely difficult, and potentially dangerous, to over the deck and are called wind-over-the deck perform helicopter flights to and from a ship deck (WOD) envelopes. is the unsteady ship airwake. Other factors that are It is difficult to obtain these WOD envelopes using also important are the possibly adverse weather real, full-scale experiments for a variety of reasons. and sea conditions, ship translation and rotation A series of DI flight tests for all possible combi- (oscillating landing spot), small flight decks (for nations of wind speed and azimuth (typically in some ships), and poor visibility conditions. 5 knot, 158 increments) must be performed to estab- The most challenging rotorcraft/ship dynamic lish the envelope. However, these tests are costly, interface (DI) problems occur during: often limited (due to the absence of certain wind conditions and the availability of fleet assets), and (a) engagement and disengagement (run-up and unsafe. This process also depends on subjective run-down) of the rotor system while the aircraft pilot ratings. In contrast, scale model wind tunnel is on the flight deck (i.e. the blade-sailing tests allow control over the wind conditions and phenomena); can supply detailed information but are still costly (b) takeoff and landing operations (e.g. launch, and time consuming. In wind tunnel testing, small departure, approach and recovery); size ships (with lower Reynolds number) are used (c) hovering over the moving flight deck (i.e. and the full-scale (high) Reynolds number cannot station-keeping). be obtained. In addition, the complexity of the flow field of a rotorcraft during takeoff and landing It requires tremendous practice and skill for the is impossible to reproduce in a wind tunnel. Fur- pilots to learn to perform these operations. thermore, it is both difficult and costly to obtain A common practice to ensure safe operation of high quality and complete sets of coupled ship air- wake/rotorcraft flow field measurements using ÃCorresponding author: Department of Aerospace Engineering, both full-scale and scaled-model tests for all of the Pennsylvania State University, University Park, PA 16802, USA. different WOD conditions for any ship/helicopter Downloaded from pig.sagepub.com at IOWA STATE UNIV on February 27, 2015 G01105 # IMechE 2005 Proc. IMechE Vol. 219 Part G: J. Aerospace Engineering 370 N Sezer-Uzol, A Sharma, and L N Long combination that could be used to obtain the coupled with large grids associated with such geome- SHOLs. tries, require enormous computational power. Full- A numerical procedure to analyse shipboard oper- scale or wind tunnel experiments, in contrast, can ations and to obtain the WOD envelope would be easily generate long-time records of data, but the desirable because it is controlled, safer, and cheaper. associated initial set-up cost and time can be huge. It could also be used to train pilots by coupling with a In this article, the use of CFD to simulate the ship flight simulator (as a piloted training tool). It can also airwake of two different classes of ships: the landing be valuable as a non-real-time simulation tool for use helicopter assault (LHA) and the landing platform in engineering and design by helping in the develop- dock (LPD)-17 is discussed. A comprehensive study ment of advanced flight control systems and in the of the effect of WOD angle on the airwake is per- design processes of future ships. Such numerical formed. Time accurate runs are performed for simulation tools could include: WOD angles of 08,308,458,908, 2708, and 3158. The time-averaged solutions are analysed to investigate (a) CFD simulations of the unsteady ship airwake; the characteristics of the wake. The simulations are (b) a high-fidelity flight dynamics model for the performed using a uniform steady freestream with helicopter; a wind speed of 15.43 m/s (30 knot). Although the (c) a model for the ship motion; effects of atmospheric boundary layer and atmos- (d) a model of a human pilot for analysing the pilot pheric turbulence are important, they are not workload and response. included in this article due to the computational Understanding and modelling the airwake of a time available, but these effects will be included in complex ship geometry present a number of techni- future simulations. Long-time histories (for a cal challenges. The modelling requires an accurate period of 40 s) are obtained for the WOD angles representation of the complex ship geometry with of 08 and 308 to determine dominant shedding fre- superstructures and sharp edges. The flow around quencies for both ships. The unsteady nature of the complex ship superstructures, including towers, wake is highlighted and a quantitative measure of antennae, radar dishes, exhaust stacks, and so on, the unsteadiness is presented. Because the simu- is very difficult to predict. In addition, modelling of lations are performed without using any traditional a moving flight deck because of ship translational turbulence model, the smallest turbulent scales speed and random sea motion must be taken into that are captured in these simulations are of the consideration. The effects of the atmospheric envi- order of the grid size. The flow is assumed to be ronment (atmospheric boundary layer and turbu- inviscid in these simulations and hence the simu- lence) and high winds are important. The ship lations do not capture boundary layer effects. It is airwake is also largely affected by the helicopter prohibitively expensive to do time-accurate simu- rotor and fuselage wakes and vice versa, resulting lations for a grid that can resolve the boundary in complex interactions. layer. The sharp edges of the ships’ boundaries and The ship airwake flow is highly three-dimensional, the superstructures fix the separation points of the unsteady, separated, vortical, and turbulent. There flow making it reasonably independent of the are massive regions of flow separation and there is Reynolds number. Therefore, the inviscid flow shedding of strong turbulent coherent structures approximation can be used for these simulations. from the ship’s superstructure and sharp edges. As Some results from these simulations have previously a result, the ship airwake contains a wide range of been successfully interfaced [1–3] with flight spatial and temporal scales and large, unsteady dynamics simulations to estimate pilot workload vortical regions around and behind the super- due to complex unsteady airwake. structure and over the flight deck, which affect the In the following sections, the computational and rotor response, fuselage loads, and pilot workload. experimental ship airwake research is reviewed in Therefore, turbulence modelling is also important detail. A short review of the literature on the blade- for such simulations. It is a low Mach number flow; sailing problem and DI simulations is also presented. however, there are complex interactions with the In section 2, the ship airwake CFD simulations of the complex helicopter rotor flow during shipboard LHA and LPD-17 are described. The results of these operations. Hence, accurate modelling and predic- simulations and the conclusions are presented in tion of the highly unsteady airwake of a ship is critical sections 3 and 4, respectively. for shipboard operations of rotorcrafts. To accurately resolve all the flow features: turbulence, boundary layer, flow separation, and interaction of helicopter 1.1 Previous ship airwake studies flow-field with the ship airwake, in a time-accurate simulation is a big challenge. The magnitude of Understanding and modelling the unsteady, separ- the unsteadiness demands long-time records which, ated, and turbulent airwakes of different classes of Downloaded from pig.sagepub.com at IOWA STATE UNIV on February 27, 2015 Proc. IMechE Vol. 219 Part G: J. Aerospace Engineering G01105 # IMechE 2005 CFD simulations of ship airwake 371 ships have been the focus of numerous experimental addition, the unsteady airwake and exhaust gas and computational studies [4]. trajectories over highly complex ship superstructures Experimental investigations include full-scale tests were computed over DDG-51 Flt-IIA by Landsberg aboard ships such as LHA [5–7], as well as wind tunnel et al. [36] using FAST3D flow solver and over LPD-17 scaled-model tests, e.g. LHA [6–8], DD-963 [9, 10], a by Camelli et al. [37, 38] using large eddy simulations non-aviation ship model [11–13], LPD-17 [14, 15], a (LES).
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