Hyung Suk Kang

In Situ Velocity Measurements in the Near-Wake of a Ship Superstructure

Velocity measurements in a ship airwake are obtained in situ aboard a 108 ft naval training vessel. The measurements and analyses aremotivated by the need for validation data for airwake computational fluid dynamics simulations. Three-component anemometers are placed above the bow of the ship and at numerous locations above a flight deck at the stern of the ship. Data are presented for a direct headwind (nominally 0 deg wind-over-deck). The mean velocity field shows a clear structure to the flow, dominated by a recirculation region in the near-wake of a hangar-like backward-facing step. The location of this primary vortex and the reattachment point on the flight deck are estimated. Reynolds stresses are presented to quantify the turbulent fluctuations, which are required for the prediction of unsteady loading on rotorcraft operating in this environment. Significant anisotropy is measured in the wake, both within the primary vortex and in the far field. The peak Reynolds shear stress is located in the recirculation region, while the streamwise normal stress is found to increase with height throughout the measurement domain. Finally, autoand two-point velocity correlations from the flight deck provide an estimate of flow scales, showing the potential influence of turbulence on piloted helicopter operations.

Validation of Computational Ship Air Wakes for a Naval Research Vessel

This paper provides current results of a multi-year research project that involves the systematic investigation of ship air wakes using an instrumented United States Naval Academy (USNA) YP (Patrol Craft, Training). The objective is to validate and improve Computational Fluid Dynamics (CFD) tools that will be useful in determining ship air wake impact on naval rotary wing vehicles. This project is funded by the Office of Naval Research and includes extensive coordination with Naval Air Systems Command. Currently, ship launch and recovery wind limits and envelopes for helicopters are primarily determined through at-sea in situ flight testing that is expensive and frequently difficult to schedule and complete. The time consuming and potentially risky flight testing is required, in part, because computational tools are not mature enough to adequately predict air flow and wake data in the lee of a ship with a complex superstructure. The top-side configuration of USNA YPs is similar to that of a destroyer or cruiser, and their size (length of 108 ft and above waterline height of 24 ft) allows for collection of air wake data with a Reynolds number that is the same order of magnitude as that of modern naval warships, an important consideration in aerodynamic modeling. A dedicated YP has been modified to add a flight deck and hangar-like structure to produce an air wake similar to that from a modern destroyer. Three-axis acoustic anemometers have been installed at various locations, including a large vertical array on the ship’s bow to measure atmospheric boundary layers. Repeated testing on the modified YP is being conducted in the Chesapeake Bay, which allows for the collection of data over a wide range of wind conditions. Additionally, a 4% scale model of the modified YP has been constructed and tested in the USNA recirculating wind tunnel. Comparison of YP in situ data with similar data from wind tunnel testing and CFD simulations shows reasonable agreement for a headwind condition and for relative winds 15° and 30° off the starboard bow. Analysis also indicates that CFD simulations require modeling the velocity profile in the atmospheric boundary layer to improve simulation accuracy. Finally, off-ship turbulence data collected using an instrumented 4.5 ft rotor diameter radio controlled helicopter show that the detected off-ship air wake is present where predicted by CFD simulations.

Comparison of Experimental and Computational Ship Air Wakes for YP Class Patrol Craft

This paper provides current results of a multi-year research project that involves the systematic investigation of ship air wakes using an instrumented United States Naval Academy (USNA) YP (Patrol Craft, Training). The objective is to validate and improve Computational Fluid Dynamics (CFD) tools that will be useful in determining ship air wake impact on naval rotary wing vehicles. This project is funded by the Office of Naval Research and includes extensive coordination with Naval Air Systems Command. Currently, ship launch and recovery wind limits and envelopes for helicopters are primarily determined through at-sea in situ flight testing that is expensive and frequently difficult to schedule and complete. The time consuming and potentially risky flight testing is required, in part, because computational tools are not mature enough to adequately predict air flow and wake data in the lee of a ship with a complex superstructure. The top-side configuration of USNA YPs is similar to that of a destroyer or cruiser, and their size (length of 108 ft and above waterline height of 24 ft) allows for collection of air wake data with a Reynolds number that is the same order of magnitude as that of modern naval warships, an important consideration in aerodynamic modeling. A dedicated YP has been modified to add a flight deck and hangar-like structure to produce an air wake similar to that on a modern destroyer. Three-axis acoustic anemometers, fog generators and an inertial measurement unit have been installed. Repeated testing on the modified YP is being conducted in the Chesapeake Bay, which allows for the collection of data over a wide range of wind conditions. Additionally, a 4% scale model of the modified YP has been constructed and tested in the 42×60×120 inch USNA wind tunnel. Comparison of YP in situ data with similar data from wind tunnel testing and CFD simulations shows reasonable agreement for a headwind condition and for a relative wind 15° off the starboard bow. Analysis of in situ data and wind tunnel data for a 30° relative wind also show reasonable agreement, though with a greater deviation than in the 15° relative wind condition. Furthermore, analysis indicates that CFD simulations require modeling the velocity profile in the atmospheric boundary layer to improve simulation accuracy.

Ship Air Wake Wind Tunnel Test Results (Invited)

As part of a larger program to develop analytic and computational tools to predict the air wake characteristics of naval vessels, an experimental effort has been undertaken to map the air wake of a scaled patrol craft modified with a representative hangar structure and stern flight deck. A 4% scaled model of a United States Naval Academy YP (Patrol Craft, Training) was fabricated for wind tunnel testing at 0, 15, and 30 degrees of yaw at a Reynolds number of 7.6 million. The topside configuration of the vessel simulates that of larger destroyers and cruisers, which service rotorcraft from stern hangars and flight decks. Flow measurements were made in station planes from 0.45 to 5.14 hangar-heights downstream of the aft hangar face using an 18-hole Omniprobe. The 3D velocity measurements indicated that a recirculation zone occurred over the flight deck and extended from 1.5-3 hangar-heights downstream. The effect of yaw angle on the air wake was to consolidate and shift the rotationality of the flow into a large, leeward vortex with a size on the order of the superstructure height. Furthermore, the unsteadiness of the flow was independent of the survey plane position at zero yaw, but increased by 57% at 30 deg yaw, with a large band of high vorticity where the shear layer separated and rolled up. The velocity distributions, flow angles, and vorticity will be used to compare with computational fluid dynamics (CFD) and full-scale, in-situ measurements to portray a complete, Reynoldsscaled picture of the air wake.

USNA Ship Air Wake Program Overview (Invited)

This paper provides an overview of a multi-year research project that involves the systematic investigation of ship air wakes using an instrumented United States Naval Academy (USNA) YP (Patrol Craft, Training). The objective is to validate and improve Computational Fluid Dynamics (CFD) tools that will be useful in determining ship air wake impact on naval rotary wing vehicles. This project is funded by the Office of Naval Research and includes extensive coordination with Naval Air Systems Command. Currently, ship launch and recovery wind limits and envelopes for helicopters are primarily determined through at-sea in situ flight testing that is expensive and frequently difficult to schedule and complete. The time consuming and potentially risky flight testing is required, in part, because computational tools are not mature enough to adequately predict air flow and wake data in the lee of a ship with a complex superstructure. The top-side configuration of USNA YPs is similar to that of a destroyer or cruiser, and their size (length of 108 ft and above waterline height of 24 ft) allows for collection of air wake data with a Reynolds number that is the same order of magnitude as that of modern naval warships, an important consideration in aerodynamic modeling. A dedicated YP has been modified to add a flight deck and hangar structure to produce an air wake similar to that on a modern destroyer. Three axis acoustic anemometers, fog generators and an inertial measurement unit have been installed. Repeated testing on the modified YP is being conducted in the Chesapeake Bay, which allows for the collection of data over a wide range of wind conditions. Additionally, a 4% scale model of the modified YP has been constructed and tested in the 42×60×120 inch USNA wind tunnel. The project involves USNA midshipmen who are participating in test planning, collecting and analyzing data, and in CFD modeling, providing the midshipmen with valuable professional and research experience. Comparison of YP in situ data with similar data from wind tunnel testing and CFD simulations shows reasonable agreement for a headwind condition and for wind 15° off the starboard bow.

Influence of the Atmospheric Surface Layer on a Turbulent Flow Downstream of a Ship Superstructure

AbstractThis paper describes a set of turbulence measurements at sea in the area of high flow distortion in the near-wake and recirculation zone behind a ships superstructure that is similar in geometry to a helicopter hangar/flight deck arrangement found on many modern U.S. Navy ships. The instrumented ship is a 32-m-long training vessel operated by the United States Naval Academy that has been modified by adding a representative flight deck and hangar structure. The flight deck is instrumented with up to seven sonic anemometers/thermometers that are used to obtain simultaneous velocity measurements at various spatial locations on the flight deck, and one sonic anemometer at bow mast is used to characterize inflow atmospheric boundary conditions. Data characterizing wind over the deck at an incoming angle of 0° (head winds) and wind speeds from 2 to 10 m s−1 obtained in the Chesapeake Bay are presented and discussed. Turbulent statistics of inflow conditions are analyzed using the Kaimal universal turbu...

Ship Air Wake CFD Comparisons to Wind Tunnel and YP Ship Results (Invited)

As part of a project to determine ship air wake impact on naval rotary wing vehicles, this paper compares wind tunnel and computational fluid dynamics (CFD) simulations with in situ data that has been collected on an instrumented United States Naval Academy (USNA) YP (Patrol Craft, Training). The in situ data has been collected underway on a vessel which is 108 ft long and has a 24 ft above water line height. The wind tunnel data is from a 4% scale model tested in the USNA 42 × 60 × 120 inch wind tunnel. CFD simulations were performed on a 15.5 million tetrahedral unstructured grid. Data from the three sources is compared over 11 vertical and horizontal planes above the flight deck on the stern section of the YP. Data is compared for a head wind condition and for relative wind 15° off the starboard bow. Good comparison in velocity direction, typically less than 10-15°, is observed in the vertical and horizontal planes. Normalized velocity magnitudes do not compare as well, likely arising from the lack of an atmospheric boundary layer modeled in CFD simulations and wind tunnel testing.