Numerical Research Of Ship Airwake Characteristic

As a complicated bluff-body,the superstructures and deck of large ship produce turbulent flow with large separated regions and reattachment as well as shed eddies over the ship.The interaction of flight deck moving with six-degree-of-freedom(6DOF)and the sea state creates the operational environment,different for every ship type,forcing the aircraft to land in noideal conditions and hazarding the safty of both shipboard helicopters and pilots.In addition,the turbulence in ship airwake will increase pilot workload and shipboard aircraft fatigue,which may reduce the working time of the shipboard aircraft.Now,the ship airwake is a key asset for adaptation performance between aircraft and naval ship,and the characteristic parameters of which are the important basis to develop aircraft operating envelopes.The accurate prediction of airwake can ensure safe shipboard aircraft launch and recovery operaions.At early stages,the safe operating envelops are determined by extensive dynamic interface flight tests at sea,a time-consuming,laborious and expensive process.In the past years,there has been a dramatic growth in computer and numerical simlation,and the wide application of the Computational Fluid Dynamics(CFD)in prediction of ship airwake.Further investigation into the numerical methods application in ship airwake will improve the prediction precision,optimize ship design and ensure the safety of the shipboard aircrafts,ships and pilots.The present work is primarily based on CFD theory for the computation of full-scale mounted cube,the typical simplified frigate shape SFS,USS Tarawa class amphibious assault ship LHA-1 and USS Georage Washington CVN-73 ship airwake flow fields using ANSYS FLUENT.The purpose of the current research is to validate the turbulence model and the mesh type for ship airwake and assess the impact of wind-of-deck(WOD)speeds,angles and atmospheric boundary layer(ABL)as well as the ship structure parameters on ship airwake.Firstly,computations for three dimensional mounted cube are carried out using different turbulence models based on W-F and L-Re meshes,and it is demonstrated that SKE model combined with W-F mesh system can predict the high fidelity ship airwake datasets with high efficiency.Then the SKE model combinated with W-F mesh is applied in the prediction of SFS ship airwake,and the results are compared to wind tunnel experimental data.From both the surface and off-body mean flow comparisons,it is shown that the method captures the correct:flow topology.In addition,the results indicate that the time-accurate ship airwake simulation based on USKE and DES-SA especially the latter can provide a further improvement result,while it is time-consuming,and it is not applicable to the intensive simulation in limit resource.Then,this paper compares the SFS ship airwake with structured,unstructured and hybrid structured/unstructured mesh,the comparison illustrates that the mesh type impose little impact on the ship airwake.The workload of generating hybrid mesh made up of boundary layer mesh,pyramid mesh,tetrahedral mesh and prismatic mesh is far less than that of generating structured mesh,and the number of hybrid mesh is about 80%of the unstructured one.It is a good compromise between mesh number and prediction accuracy with the best function-price ration among three different meshes.The validition simulation of LHA-1 and CVN-73 based on hybrid mesh certificates the numerical results matching the wind tunnel measurements very well and it is shown that hybrid mesh solutions provide satisfactory results.In addition,it assesses the impact of WOD speeds,angles and ABL as well as the ship structure parameters on ship airwake.The study appears that the flowfield does not change when varying the WOD speed,which means that the solution is Re independence,and will reduce the workload of ship airwake simulation effectively.While the ship airwake changes with changing WOD angles dramatically,which implie that all solutions need be run for each wind angle of interest.The research concludes that the WOD angles from 0° to±15° are the ideal range for shipboard aircrafts launch and recovery operations.It also demonstrates that the ABL exerts an unneglected effect on the ship airwake.In which the U(z)decrease the kinetic energy of entrance,and the turbulence intensity decrease the velocity gradient of flow field.Finally,it researches the ship airwake with different ship structures.For the ship with hangar and postpositive flight deck,it compares the SFS1 ship airwake with different open degree of hangar door,different hangar heights,different degrees of circular hangar transition,different dips of hangar top and different flight deck lengths,and proposes the specific parameters of hangar and flight deck which will improve flow field.For the ship with full flight deck,it compares the flow field of LHA-1 with and without bow flap.The validation simulation of 0° and 15° WOD angles demonstrates a more uniform ship airwake both for the modified SFS1 and for LHA-1.And the improvement effect is more obvious than that only modify single structure.The presented results do demonstrate that a SKE combined with hybrid mesh can accurately predict the ship airwake.It settles the trouble of meshing techniques for complex ship geometries,and studies the impact of wind conditions and structure parameters on ship airwake.In particular,it analyses the impact of ABL for the first time.In addition,it provides an economical and efficient numerical method for prediction of ship airwake,and the ability to predict airwake could be used as a design tool to address air operations early in a ship design.Such a simulation tool could be used to predict operating envelopes for shipboard helicopter and find optimal approach paths for fixed wing aircraft.