Dissertation
Effect of ship motion on ship airwake aerodynamics
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Summer 2020
DOI: 10.17077/etd.005517
Abstract
The effects of ship motion on the flow field over the deck of the ONR Tumblehome geometry were investigated experimentally. Characterization of the flow field is relevant in the context of safe aircraft operation in the wake region and to provide an accurate prediction of aircraft air-loads in simulators for pilots, and to provide a validation data set for CFD. A better understanding of the interactions between ship motion and the flow field is needed since, in real-life, atmospheric turbulence and the pumping action from the surface wave field cannot be separated from ship motions, in-turn, obscuring the underlying physics. In the context of reduced-order models of the dynamic flow-field, it is paramount to understand the contributions of the different mechanisms that drive the flow over a ship’s flight deck, so only the vital flow contributors are included, saving computational power and space while maintaining validity needed in simulators.
Stereo PIV and two-component LDA measurement systems were used to measure the flow field over the hangar and flight deck on a 2-m-long ship model with a uniform flow, Re = 106. A novel dynamic base-plane was developed to prevent leakage through the baseplane/ship interface during ship motions and were used in all cases to simulate a flat free-surface, e.g., removing wave effects, leaving only ship motions left to affect the flow field. Ship motion kinematics were derived from wave interactions computed from numerical simulations of the full-scale ship for four different sea state conditions, SS3-6.
Flow measurement cases with and without an imposed pitch and heave motions were taken to quantify the ship’s motion effect on the flight-decks flow-field and were: a) static ship at zero sinkage, b) static ship at mean trim for each sea state, c) dynamic ship for each seastate, and d) phase-static ship, where the ship is trimmed to match a given motion phase of the dynamic-ship. It was found that ship motion adds dominant peaks in the velocity energy spectrum at the same frequency as the wave encounter frequency, fe, and in higher sea states an additional peak is found at 2 fe, at a position a rotor-craft would hover during recovery. Additionally, a dampening effect of the flow-field is shown between a dynamic ship at the same attitude as a phase-static ship, with the latter case exhibiting larger flow variations for velocity and RMS, especially for vertical velocity. Vertical velocity and RMS components have larger variations than their respective horizontal components for ship motions.
Qualitative observations of the velocity series over the landing deck appeared to be well-correlated with ship motion, suggesting that wake dynamics were dominated by linear phenomena such as displacement of wake flow patterns with vertical movement of the ship and a ”frozen” convection of the separating shear layer in the streamwise direction. This lead to the formulation of two hypotheses; 1) The horizontal velocity over mid-flight-deck is governed by streamwise convection of the shear-layer shed by the SSTAE. For example, sometime after the SSTAE is at its highest position, the horizontal velocity of the flow at mid-flight deck will be at it’s lowest, with a velocity in between zero and the freestream.
2) Since the flow at the deck must have the same vertical velocity as the ship, the flow at mid-flight-deck is governed by the ship’s vertical velocity close to the deck, i.e., vertical pumping.
Quantitative hypotheses testing was done by using cross-correlation analysis of the flows horizontal and vertical velocities with ship position and or velocity to obtain the best correlated time lead/delay of the flow response to the ship, τ0, at streamwise position x/L = 0.888. For hypothesis two, a convective velocity was computed using the distance to the
superstructures top-aft-edge (SSTAE) from the measurement position and dividing it by τ0, obtained from the correlation analysis. Only time delays, τ0, were needed for testing hypothesis two.
Hypothesis one, i.e., convective model is best represented by SS6 below the shear layer where it’s convective velocity is nearly constant and roughly equal to the freestream. The lower sea states were only valid for the two positions closest to the deck, except SS3 which only the lowest positions were valid. The other position below the shear layer and above were invalid for a convective model since their velocities were a lot larger than the freestream, which is unrealistic. Above the shear layer, poor correlation was exhibited whereas below the shear layer strong correlation was exhibited. Hypothesis two, i.e., vertical pumping model, is valid to describe the flow above the shear layer where the time delays are small, however, for SS6 the time delays are negative, which means the velocity leads the deck. Below the shear layer, hypothesis two becomes less valid where the time response, τ0, of the flows vertical velocity to the decks z-velocity, exhibited a nonlinear relationship, but have a strong correlation indicating that the deck z-velocity and some flow phenomenon dictate the flow near the deck. Furthermore, vorticity at the flight deck was used in trying to explain the vertical flow variations at the deck, where qualitative analysis revealed growth and decay of x-vorticity was most predominate for SS6, which had the most significant variation in τ0, which the fluctuations decreased with decreasing sea state. However, it’s not clear how the breakup of vorticity would result in larger τ0 values, so further analysis will need to be done to verify that vorticity is the other vertical flow contributor. Distinctly different behavior was exhibited for the lowest reduced frequency sea state, SS6 k = 0.67, then in the higher reduced frequency sea states, SS5-3 k = 1.06, 1.13, 1.33. It was valid for hypothesis one below the shear layer with a constant convective velocity and completely invalid for hypothesis two, which suggests that when the sea states reduced frequency is small, the convective velocity is almost the same as the freestream.
Details
- Title: Subtitle
- Effect of ship motion on ship airwake aerodynamics
- Creators
- Austin Krebill
- Contributors
- James H J Buchholz (Advisor)Corey D Markfort (Committee Member)J Ezequiel Martin (Committee Member)Albert Ratner (Committee Member)Pablo Carrica (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Mechanical Engineering
- Date degree season
- Summer 2020
- DOI
- 10.17077/etd.005517
- Publisher
- University of Iowa
- Number of pages
- xvii, 103 pages
- Copyright
- Copyright 2020 Austin Krebill
- Comment
- This thesis has been optimized for improved web viewing. If you require the original version, contact the University Archives at the University of Iowa: https://www.lib.uiowa.edu/sc/contact/
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 102-103).
- Public Abstract (ETD)
The effects of ship motion on the flow field over the deck of the ONR Tumblehome geometry were investigated experimentally. Characterization of the flow field is relevant in the context of safe aircraft operation in the wake region and to provide an accurate prediction of aircraft air-loads in simulators for pilots, and to provide a validation data set for CFD. A better understanding of the interactions between ship motion and the flow field is needed since, in real-life, atmospheric turbulence and the pumping action from the surface wave field cannot be separated from ship motions, in-turn, obscuring the underlying physics. In the context of reduced-order models of the dynamic flow-field, it is paramount to understand the contributions of the different mechanisms that drive the flow over a ship’s flight deck, so only the vital flow contributors are included, saving computational power and space while maintaining validity needed in simulators.
Stereo Particle Image Velocimetry and two-component Laser Doppler Anemometry measurement systems were used to measure the flow field over the hangar and flight deck on a 2-m-long ship model with a uniform flow. Ship motion kinematics were derived from wave interactions computed from numerical simulations of the full-scale ship for four different sea state conditions, SS3-6. A motion mechanism was developed to articulated the model ship in a recirculating water channel, enabling dynamic scaling of wave-induced ship motions to a 1:77 scale, which is a first for ship wake experiments. Also, a novel dynamic base-plane was developed to prevent leakage through the baseplane/ship interface during ship motions and were used in all cases to simulate a flat free-surface, e.g., removing wave effects, leaving only ship motions left to affect the flow field. Flow measurement cases with and without an imposed pitch and heave motions were taken to quantify the ship’s motion effect on the flight-decks flow-field.
It was found that ship motions adds larger fluctuations to the flow for higher frequency ship motion cases and lower flow fluctuations for lower frequency ship motion cases, therefore, not negligible. The maximum energy in the wake of a moving ship has the same frequency as the wave encounter frequency. Additionally, a dampening effect of the flow-field is shown between a phase-static ship at the same attitude as a dynamic ship, with the first case exhibiting more significant flow variations, especially for the flow’s vertical component. Furthermore, at positions aircraft would operate, a robust linear relationship was found between the moving ship and the resulting flow-field reveling a simple linear convective model to predict the time and direction of the flow fluctuations with respect to ship motion, which would be an easy addition to add fidelity to simulators used to train pilots in launch and recovery operations. It could also help in developing flight controller algorithms for unmanned aircraft operating from ships.
- Academic Unit
- Mechanical Engineering
- Record Identifier
- 9983987796002771
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