Stephen A. Huyer

Analysis of a Turbulent Propeller Inflow

The unsteady turbulent inflow into a swirl-inducing stator upstream of propeller (SISUP) propeller is presented. The upstream Stators and hull boundary layer generate a complex, three-dimensional inflow that was measured using x-wire anemometry. High resolution measurements consisting of 12 locations in the radial direction and 600 in the circumferential direction yielded mean velocity and rms turbulent quantities for a total of 7200 points. The axial, radial, and circumferential velocity fields were thus measured. This enabled the induced velocity due to the stator wakes, the induced velocity due to the propeller, and the turbulent hull boundary layer to be characterized. To assist in decoupling the effects on the velocity field due to the stator and propeller, a potential flow computation of the swirl component was used. Spectra and autocorrelation analysis of the inflow velocity field were used to estimate the integral length scale and lend further insight into the turbulent flow structure. These data can be used to validate computational fluid dynamics codes and assist in developing of turbulent inflow models

Unsteady Propulsor Force Prediction for Spatially and Temporally Varying Inflow

A method to compute unsteady propulsor forces for spatially and temporally varying inflows is presented. A propulsor flow prediction code, previously developed by the Massachusetts Institute of Technology, was modified and upgraded to account for time varying inflow and multiple blade rotations. The original code utilizes lifting surface theory and discretizes the propulsor surface as boundary elements to compute the unsteady potential flow. Experimental data characterizing the full unsteady, three-dimensional turbulent inflow to a Swirl-Induced Stator Upstream of Propulsor (SISUP) propulsor, were used as inflow boundary conditions. Experimental data recorded the periodic velocity fluctuations due to the stator wakes as well as the broadband turbulent characteristics of the inflow. Blade force, integrated shaft force, and blade pressure are computed based on the experimental inflow. The effect of periodic variations in the inflow was examined to determine the effect on unsteady blade forces. For these cases, the time mean experimental effective inflow is used and a fluctuating component is added for flow in the axial direction. This may be viewed as an effectively fluctuating freestream. Comparisons of unsteady force and radiated noise are then made with the baseline mean flow case to gauge the time-varying effects. Fluctuating velocity dramatically altered the force spectra even at frequencies different from the velocity fluctuation frequency. This modified algorithm can now be utilized to examine a wider set of time-dependent propulsor flow problems and to calculate the associated performance due to these unsteady flows.Copyright

Postswirl Maneuvering Propulsor

This research examines the novel use of a postswirl propulsor to generate side forces sufficient for undersea vehicle control. Numerical simulations using the commercial computational fluid dynamics (CFD) code Fluent® were used to predict the side forces for open and ducted, post-swirl propulsors configured with an upstream rotor and movable downstream stator row. By varying the pitch angles of the stator blade about the circumference, it is possible to generate a mean stator side force that can be used to maneuver the vehicle while generating sufficient roll to counter the torque produced by the rotor. A simple geometric configuration was used to minimize body geometry effects to better understand the flow physics with simulations conducted in a water tunnel environment. Flow computations highlighted the component forces and were used to characterize the velocity fields between the rotor and stator blade rows as well as the velocity field in the stator wake. There was significant coupling between the rotor and stator blade rows as demonstrated by the rotor wake velocity profiles. While the flow fields were coupled, there was not a significant difference in rotor axial or side forces except for the largest pitch amplitudes. Predictions showed that the maneuvering propulsor generated side forces predominantly by the stator and body that significantly exceeded those produced by conventional undersea vehicle control surfaces with side force coefficients on the order of 0.5. These forces are approximately three times larger than those generated by conventional control surfaces on 21 in. unmanned undersea vehicles (UUVs). Even for zero flow velocities, side forces were produced due to the induced flow produced by the rotor over the stator, further demonstrating the potential for this technology to be used for undersea vehicle maneuvering.

Pre-swirl Maneuvering Propulsor: Part 1 Computations

Recent concept studies have demonstrated the potential to utilize a pre-swirl propulsor configuration with adjustable upstream stators to generate propulsor side forces. These studies led to a set of experiments and corresponding computations to validate this concept. Ducted and open pre-swirl propulsors were configured with an upstream stator row and downstream rotor. During normal operation, the upstream stator blades are all situated at the same pitch angle and pre-swirl the flow into the propulsor when generating a roll moment to counter the moment produced by the rotor. By varying the pitch angles of the stator blade about the circumference, it is possible to both generate a mean stator side force and subsequently vary the axial velocity and swirl that is ingested into the inflow. The rotor then generates side forces in response to the inflow. Wind tunnel experiments were conducted to measure the steady, spatially varying stator wake flows for various stator geometric configurations using stereo particle image velocimetry. Computations utilized both potential flow and fully viscous 3-D (Reynolds Averaged Navier-Stokes, RANS) computations to predict the stator forces, velocity field and rotor response. Rotor design space investigations varied blade parameters including blade number, rake, skew and a combination of the two. RANS was used to then validate the final propulsor design with experimental data used to validate the computations. Computational data demonstrated that total side force coefficients on the order of 0.2 could be generated by the propulsor alone with results consistent with recent water tunnel measurements. This amount of control authority exceeds current control surface capabilities at 3 knots for Navy 21” unmanned undersea vehicles.

Unsteady Vortex Flows Produced by Trailing Edge Articulation

The unsteady vortex flows produced by biologically inspired tail articulation are investigated. The application is to provide active means of reducing tonal noise due to upstream wake interaction with downstream propellers on underwater vehicles. By reducing the wake velocity defect, the periodic unsteady propeller blade pressure fluctuations that are the source of the noise should be reduced. Accordingly, experiments to measure the flows produced by an upstream stator fitted with a movable trailing edge were carried out in a water tunnel for Reynolds numbers in the range 75,000 <Re <300,000. A stator model with a hinged flapping trailing edge section operated at frequencies up to 21 Hz corresponding to a range of Strouhal number 0.0<St<0.18. Velocity measurements of the articulating stator wake were carried out by laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Reduced mean and rms LDV data show that trailing edge articulation generates vortex structures with dependence on both Strouhal number and articulation amplitude. Estimates of the time mean stator drag that were obtained by integrating the mean wake profiles were used to estimate optimal Strouhal numbers in terms of wake elimination. Instantaneous phase-averaged measurements via PIV show a transition in the unsteady stator wake flow regimes as St is increased, from a deflected vortex sheet to a series of rolled up, discrete vortices. Measurements of the wake high-light the characteristics of the vortex structures and provide a means to estimate the impact on downstream propellers.

Solution of Two-Dimensional Vorticity Equation on a Lagrangian Mesh

Anovel,vorticity-basedsolutionmethodologyhasbeendevelopedtocomputeunsteadye owpastbodies.Vorticity is evolved on a set of points, and the vorticity in the remainder of the e eld is determined by linear interpolation. Interpolationisaccomplished byDelaunaytriangularizationofthepointsin the e eld. Triangulationofthevorticity e eldprovidesabasistointegratethevorticitytocomputethevelocity.Nodalconnectivityfromthetriangularization also provides a list of the neighboring points that are used to construct a second-order least-squares e t of the vorticity. First- and second-order spatial derivatives can then be computed based on this polynomial e t. Surface vorticity on the body is computed to satisfy the no-slip boundary condition and is introduced into the e ow via diffusion. A diffusion transport velocity was derived to account for spatial movementof the vorticity dueto viscous diffusion. The points are advected by the sum of the induced velocity (computed from the Biot ‐Savart integral ) and the diffusion velocity. The remaining diffusion term includes a form of the Laplacian and is computed directly. This solution scheme was found to be stable as applied to the problem of impulsively started e ow about a circular cylinder and e at plate. Comparisons with experimental and the Blasius boundary-layer solution for a e at plate were used to demonstrate the effectiveness of this method.

Application of a Maneuvering Propulsor Technology to Undersea Vehicles

Previous computational and experimental studies that have demonstrated a method to generate vehicle maneuvering forces from a propulsor alone have been applied to a generic undersea vehicle. An open, preswirl propulsor was configured with an upstream stator row and downstream rotor. During normal operation, the upstream stator blades are all situated at the same pitch angle and preswirl the flow into the propulsor while generating a roll moment to counter the torque produced by the rotor. By varying the pitch angles of the stator blade about the circumference, it is possible to generate a mean stator side force that can be used to maneuver the vehicle. The stator wake axial velocity and swirl that is ingested into the rotor produces a counter-force by the rotor. Optimal design of the rotor minimizes the unsteady force and redirects the rotor force vector in an orthogonal direction to minimize the counter force. The viscous, 3D Reynolds-averaged Navier–Stokes (RANS) commercial code FLUENT® was used to predict the stator forces, velocity fields, and rotor response. Radiated noise was computed for the rotor separately and the entire geometry utilizing the Ffowcs Williams–Hawkings module available in FLUENT. Two separate geometries were studied—the first with a maximum stator blade row diameter contained within the body diameter and a second that was allowed to exceed the body diameter. Side force coefficients were computed for the two maneuvering propulsor configurations and compared with currently used control surface forces. Computations predicted that the maneuvering propulsor generated side forces equivalent to those produced by conventional control surfaces with side force coefficients on the order of 0.3. This translates to 50% larger forces than can be generated by conventional control surfaces on 21 in. unmanned undersea vehicles. Radiated noise calculations in air demonstrated that the total sound pressure levels produced by the maneuvering propulsor were on the order of 5 dB lower than the control fin test cases.

Noise Control Due to the Stator Wake Blade Interaction via Tail Articulation

The biologically inspired method of tail articulation is investigated as a means of reducing tonal noise due to wake deficit blade interaction in underwater vehicles. Experiments are carried out in a water tunnel under typical operating conditions for underwater vehicles. Tail articulation is implemented using a life scale stator model with a hinged flapping tail operating both in free-stream velocities corresponding to Reynolds number in the range 75000 < Re < 300000 and at frequencies up to 30 Hz to investigate the range of Strouhal number 0.0 < St < 0.35. Velocity measurements of the active stator wake are carried out by laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) to investigate the effects of tail articulation on the stator wake. Time-averaged measurements of the stator wake by LDV show that of the tail articulation has a dominant effect on the time mean stator drag. Instantaneous phase-averaged measurements of the stator wake by PIV show a transition in the unsteady stator wake as is increased, from a deflected vortex sheet to a series of rolled up, discrete vortices. Measurements are made of the wake due to both sinusoidal and nonsinusoidal tail motion profiles, which show that significant wake alteration is achieved with tail articulation. A low-order model describing the creation and convection of vorticity by tail articulation is developed which describes wake phenomena observed in LDV and PIV measurements. Finally, a 3-D unsteady propeller simulation using both experimental wake velocity data by PIV and simulated wake velocity data generated with the reduced-order model are used to predict the effect of sinusoidal tail articulation on radiated noise. Results using simulated data indicate that a significant noise alteration is achieved in all cases, and noise reduction of 5-8 dB is achieved in some cases.

Blade Tonal Noise Reduction Using Stator Trailing-Edge Articulation

The biologically-inspired method of trailing-edge articulation is investigated as a means of reducing tonal noise due to the stator wake / rotor blade interaction in underwater vehicles. This work is experimental in nature and conducted in the closed channel water tunnel at Naval Undersea Warfare Center in Newport, Rhode Island. Tail articulation is carried out with a life scale stator model with hinged flapping tail designed to (i) operate in freestream velocities corresponding to Reynolds number in the range 75,000 < Re < 300,000 and (ii) operate at frequencies up to 30 Hz in order to investigate the range of Strouhal number 0.0 < St < 0.35. Velocity measurements in the active stator wake are carried out by Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) in order to investigate the effects of tail articulation on the stator wake. Time averaged measurements of the stator wake by LDV show that Strouhal number of the tail articulation has a dominant effect on the time mean stator drag. Instantaneous phase-averaged measurements of the stator wake by PIV show three regimes of the stator wake as Strouhal number is increased; quasi-steady wake spreading, vortex roll up, and strong vortex wake. Ongoing experiments with an instrumented propeller will demonstrate the efficacy of stator trailing-edge articulation on reducing unsteady blade forces.Copyright

A Post-Swirl Maneuvering Propulsor Application to Undersea Vehicles

A method to generate vehicle maneuvering forces from a propulsor alone has been applied to a generic undersea vehicle. Open and ducted post-swirl propulsors were configured with an upstream rotor and downstream stator row. During normal operation, the downstream stator blades are all situated at the same pitch angle and generate a roll moment to counter the torque produced by the rotor. By varying the pitch angles of the stator blade about the circumference, it is possible to generate a mean stator side force that can be used to maneuver the vehicle. In addition, the side force can be increased with increasing thrust producing side forces at very low vehicle velocities enabling low-speed maneuvering capability. The viscous, 3-D Reynolds-averaged Navier–Stokes (RANS) commercial code Fluent was used to predict the vehicle and propulsor component forces as well as the velocity field. Open and ducted geometric configurations were studied and force coefficients computed and compared with currently used control surface forces. Computations predicted that the maneuvering propulsor generated side forces equivalent to those produced by conventional control surfaces with side force coefficients on the order of 0.25 for the open propulsor at the self-propulsion point. This translates to 50% larger forces than can be generated by conventional control surfaces on 21