Computational ship airwake determination to support helicopter-ship dynamic interface assessment

Abstract

A highly important aspect of safe sea-based helicopter operation is the establishment of ship-helicopter operational limits for current and future helicopter/ship combinations. The capabilities of Computational Fluid Dynamics for the determination of ship airwakes have been investigated, aiming at complementing existing experimental data acquisition techniques consisting of wind tunnel and on-board full-scale measurements. In this paper, computed airwakes based on the Reynolds-averaged Navier-Stokes (RANS) equations as well as on a hybrid RANS-Large Eddy Simulation (LES) approach are compared with experimental data. The resulting airwakes have been converted for usage in a helicopter flight simulator to enable additional feed-back on the reality level of the computational approaches from experienced pilots. It is shown that, from a practical point of view, a useful first impression of the average flow characteristics in the ship airwake can be obtained from steady RANS flow modeling, although the more computationally intensive hybrid RANS-LES approach has an inherently better potential for capturing all of the physical content of the fluctuating flow fields. Nomenclature C v = ratio of local velocity to free-stream velocity, defined as (u 2 + v 2 + w 2 ) 0.5 h = characteristic mesh cell size k = turbulent kinetic energy L = reference length of the ship t = physical time t * = non-dimensional time, in CTS units u = velocity component in x-direction, scaled with U U = reference velocity of the free-stream flow v = velocity component in y-direction, scaled with U w = velocity component in z-direction, scaled with U x, y, z = orthogonal Cartesian coordinate directions β = sideslip angle of reference flow vector β local = local sideslip angle Δt = time step Δt * = non-dimensional time step = vertical deviation of local flow vector from the horizontal plane χ = horizontal deviation of local flow vector from the oncoming flow direction, β local -β ω = specific turbulent dissipation rate CFD = Computational Fluid Dynamics CFL = Courant-Friedrichs-Lewy number, stability criterion for numerical integration CTS = Convective Time Scale, defined as L/U, average time required for a fluid particle to pass the ship

Coupled flight dynamics and CFD – demonstration for helicopters in shipborne environment

“…When it comes to experimental works, these include wind-tunnel measurements of the ship airwake (24)(25)(26)(27) and interaction between obstacles and rotor wakes (28)(29)(30)(31)(32)(33)(34)(35)(36)(37) as well as full-scale campaigns (38,39) .…”

Numerical investigation of rotor loads of a shipborne coaxial-rotor helicopter during a vertical landing based on moving overset mesh method

“…Generally, the RANS solver is indeed not as satisfactory as Detached Eddy simulation (DES) and Large Eddy Simulation (LES) solvers in simulating the unsteady characteristics of ship airwake. However, a comparative study between RANS and LES in simulating velocity distribution over the flight deck has been performed by Muijden, Boelens, Vorst, and Gooden (2013), and it is demonstrated that the simulation accuracy between these two methods is of no significant difference. The reality level of RANS for the airwake determination is regarded as acceptable.…”

“…After that, the simulation is restarted for the unsteady calculation. The time step is set to 0.002 s based on the guidelines given by Muijden et al (2013). A complete unsteady calculation consists of 7500 time-steps, with 5000 used for recording the ship airwake data.…”

An investigation of coupling ship/rotor flowfield using steady and unsteady rotor methods

“…A parametric study using landing helicopter assault (LHA) was conducted by Polsky and Bruner (2000), who showed that the flowfield structure is insensitive to the Reynolds number within a certain range of wind speed. Comparisons of numerical methods on airwake simulation using Reynolds-averaged Navier-Stokes (RANS), Detached Eddy Simulation (DES) and hybrid RANS-LES (Large Eddy Simulation) methodologies have been performed in recent studies (Forrest & Owen, 2010;Lawson, Crozon, Dehaze, Steijl, & Barakos, 2012;Muijden, Boelens, Vorst, & Gooden, 2013;Thornber, Starr, & Drikakis, 2010). The results illustrate that the RANS solver is capable of predicting the fundamental flow characteristics of ship airwake, although there are some deficiencies in detail simulations of turbulent fluctuation.…”