Vertical Lift Research Center of Excellence (VLRCOE)
Abstract
Ship airwakes are the unsteady turbulent flows that are generated by the earths atmospheric boundary layer (the wind colloquially) blowing over a ship. These flow fields are highly turbulent, not easy to predict and couple with a similar wake flow field generated by a rotorcraft operating close the the ship. This coupling as expected is extremely difficult to predict let along faithfully simulate in a flight simulator. This coupling can have catastrophic consequences for the operation or rotorcraft operating in the vicinity of Naval ships. While ship airwakes have now been studied for several decades, there remain many unanswered questions and associated challenges in understanding these unsteady, three-dimensional flows, particularly concerning their turbulence characteristics and how flow scales in the airwake can potentially couple with those of a rotorcraft, including Unoccupied Aerial Systems (UAS). Navy personnel and aircraft safety remain the primary motivating factor for understanding the airwake and the interactions so produced. In this regard, developing a versatile, high-fidelity mathematical model to represent the ship airwake in a flight simulation, such as using a reduced-order mathematical representation, remains a priority for the technical community. This goal is particularly critical for more contemporary ship shapes typical of the current Navy inventory. It is toward this end that the fluid dynamic studies of the airwake are addressed in this proposed task. Furthermore, a vast majority of ship airwake measurements have not considered the interactions between an operating rotor(craft) and the airwake, another challenge the proposed task will address. Overall, the mean flow features of the ship airwake are currently reasonably well characterized, at least for simplified ship superstructures such as the SFS2. However, much of the combined spatio-temporal behavior of the ship airwake, in general, has not been measured and so the physics are still poorly understood, particularly for contemporary Navy ship shapes. Organized turbulence structures, their distribution of energy across different scales, and their interactions with, or influence on, or criticality for, a traditional rotorcraft or less conventional UAS are not understood or sufficiently documented so far. The recent time-resolved airwake measurements of the current PIs have better established the true three-dimensional nature of the ship airwake, along with other turbulent aspects of the flow that have not been previously documented. These features include the high degree of intermittency, the bistable nature of the airwake, etc. These recent measurements have highlighted the predominance of low frequencies in the airwake, but not exclusively so. They indicate the likelihood of coupling with the response of any rotor system, large or small These new measurements have emphasized the need for spatially and temporally resolved high-frequency flow measurements that capture the true three-dimensionality of the airwake flow and its turbulent aspects, including intermittency. In addition, parsing these measurements into low-order mathematical models (such as for use in FlightLab or similar) remains a challenge, both in the context of understanding the flow physics and developing a higher-fidelity representation of the airwake for use in piloted simulations. Furthermore, the challenge of measuring, understanding, and representing the interactions between the airwake and a rotor system still remains to be studied at the fidelity needed if faithful models of the airwake are to be realized. Technical Objectives (ERAU tasks only): 1) With the focus on faithfully capturing the three-dimensionality of the flow and its turbulent aspects (such as the frequency content and intermittency), time-resolved particle image velocimetry (TR-PIV) measurements with high spatio-temporal resolution will be conducted. These measurements are proposed for a more relevant ship geometry, namely the NATO Generic Destroyer (GD) of NATO AVT-315, while also investigating the differences to the widely used SFS2. Also, a representative rotor system will be introduced into the airwake to study the interactions therein. ERAU will use their new subsonic 4x6 ft wind tunnel with a mostly glass test section and the large field of view TR-PIV system awarded under an ONR DURIP. The focus will be on carrying out dual-plane, time-resolved stereo PIV (DPTR-sPIV) measurements, which allow for spatially and temporally synchronous measurements. 2) These datasets will then be used to represent the flow field using reduced-order models (ROMs). The advantages of methods such as wavelets, spectral POD (sPOD), Multi-scale Proper Orthogonal Decomposition (mPOD), and probabilistic/statistics techniques, will be used to acquire physical insights into the complex airwake environment, while describing the flow in a manner that is more relevant to the scales of UAS. This proposed approach will also offer new quantitative metrics for comparing airwakes, sorted into frequencies, which quantitatively reflect the energy distributions, and so they are much more suitable for V&V. ROMs can then be constructed, and flow field physics and interactions can be examined at each scale, whose contours should be comparable across all frequencies.
