Morgan Large Cavitation Channel Research Articles

Using flow-induced vibrations for structural health monitoring of high-speed naval ships

It has been demonstrated theoretically and experimentally that an estimate of the impulse (or structural) response between two receivers can be obtained from the long-time average of the cross-correlation of diffuse vibrations (or ambient noise) recorded at these two receivers in various environments and frequency ranges of interest: ultrasonics, underwater acoustics, seismology, and structural health monitoring. Indeed, those estimated impulse responses result from the cumulated contributions over time of random vibrations (e.g., as created by flow-induced vibrations) traveling along the test structure and being recorded by both. Hence, this technique provides a means for structural health monitoring using only the ambient structure-borne noise (e.g., generated by flow-induce vibrations) only, without the use of active sources. We will review work conducted using (1) high-frequency random vibration (100 Hz–5 kHz) data induced by turbulent boundary layer pressure fluctuations and measured on a hydrofoil and a plate at the Navy’s William B. Morgan Large Cavitation Channel. (2) Low frequency random vibration data (1 Hz–50 Hz) collected on high-speed naval ships during at-sea operations were strong wave impact loading took place. [Work sponsored by ONR.]

Turbulence profiles from a smooth flat-plate turbulent boundary layer at high Reynolds number

Much is known about smooth-flat-plate turbulent boundary layers (TBLs) at laboratory-scale Reynolds numbers because of a wealth of experimental data. However, smooth-flat-plate TBL data are much less common at the high Reynolds numbers typical of aerodynamic and hydrodynamic applications (Rex∼108–1010), and at the even higher Reynolds numbers of many geophysical flows. This paper presents new LDV-measured profiles of the stream-wise velocity variance, the wall-normal velocity variance, and the Reynolds shear stress from the TBL that formed on a smooth flat plate at Karman numbers from 15,000 to 60,000 (Rex from 75 million to 220 million). The experiments were conducted in the William B. Morgan Large Cavitation Channel on a polished (k+<0.2) flat-plate test model 12.9m long and 3.05m wide at water flow speeds up to 20ms−1. The TBL on the model developed in a mild favorable pressure gradient having an acceleration parameter K∼10−10. When plotted with the usual inner and outer scalings, the stream-wise velocity variance profiles display a Reynolds number dependence that is consistent with prior lower Reynolds-number zero-pressure-gradient TBL measurements. However, using the same normalizations, the profiles of wall-normal velocity variance and Reynolds shear stress are found to be Reynolds number independent, or nearly so, when experimental uncertainties are considered.

The mean velocity profile of a smooth-flat-plate turbulent boundary layer at high Reynolds number

Smooth flat-plate turbulent boundary layers (TBLs) have been studied for nearly a century. However, there is a relative dearth of measurements at Reynolds numbers typical of full-scale marine and aerospace transportation systems (Reθ = Ueθ/ν &gt; 105, where Ue = free-stream speed, θ = TBL momentum thickness and ν = kinematic viscosity). This paper presents new experimental results for the TBL that forms on a smooth flat plate at nominal Reθ values of 0.5 × 105, 1.0 × 105 and 1.5 × 105. Nominal boundary layer thicknesses (δ) were 80–90mm, and Karman numbers (δ+) were 17000, 32000 and 47000, respectively. The experiments were conducted in the William B. Morgan Large Cavitation Channel on a polished (k+ &lt; 0.2) flat-plate test model 12.9m long and 3.05m wide at water flow speeds up to 20ms−1. Direct measurements of static pressure and mean wall shear stress were obtained with pressure taps and floating-plate skin friction force balances. The TBL developed a mild favourable pressure gradient that led to a streamwise flow speed increase of ~2.5% over the 11m long test surface, and was consistent with test section sidewall and model surface boundary-layer growth. At each Reθ, mean streamwise velocity profile pairs, separated by 24cm, were measured more than 10m from the model's leading edge using conventional laser Doppler velocimetry. Between these profile pairs, a unique near-wall implementation of particle tracking velocimetry was used to measure the near-wall velocity profile. The composite profile measurements span the wall-normal coordinate range from y+ &lt; 1 to y &gt; 2δ. To within experimental uncertainty, the measured mean velocity profiles can be fit using traditional zero-pressure-gradient (ZPG) TBL asymptotics with some modifications for the mild favourable pressure gradient. The fitted profile pairs satisfy the von-Kármán momentum integral equation to within 1%. However, the profiles reported here show distinct differences from equivalent ZPG profiles. The near-wall indicator function has more prominent extrema, the log-law constants differ slightly, and the profiles' wake component is less pronounced.

Partial Cavity Drag Reduction at High Reynolds Numbers

Air lubrication to reduce hull skin friction is an idea that originated more than a century ago. There are few implementations of this concept, and there are even fewer systematic investigations at high Reynolds numbers. To address this, an experimental investigation was performed at the W. B. Morgan Large Cavitation Channel that examined the drag-reducing effects of a ventilated partial cavity at high Reynolds numbers. The design was accomplished using both linear gravity wave theory and a two-dimensional inviscid numerical model via Fluent. The physical model was a 12 m long flat plate with a plenum on the bottom. The plenum was formed by an abrupt step near the nose and a long sloping reattachment region toward the rear. Air was injected from the aft face of the step to create a cavity approximately 17.8 cm deep. Friction loads, air flow, and cavity pressure were measured over a range of air fluxes and speeds near the cavity design speed of 3.4 m/s. Cavities were shown to be stable with respect to large changes in air flux and slow perturbations in tunnel speed and pressure. Stable cavities were produced that reduced the skin drag by more than 95% over the extent of the cavity, including the cavity closure.

High-Reynolds-number turbulent boundary layer friction drag reduction from wall-injected polymer solutions

A set of controlled high-Reynolds-number experiments has been conducted at the William B. Morgan Large Cavitation Channel (LCC) in Memphis, Tennessee to investigate the friction drag reduction achieved by injecting aqueous poly(ethylene oxide) (PEO) solutions at three different mean molecular weights into the near-zero-pressure-gradient turbulent boundary layer that forms on a smooth flat test surface having a length of nearly 11m. The test model spanned the 3.05m width of the LCC test section and had an overall length of 12.9m. Skin-friction drag was measured with six floating-plate force balances at downstream-distance-based Reynolds numbers as high as 220 million and free stream speeds up to 20ms−1. For a given polymer type, the level of drag reduction was measured for a range of free stream speeds, polymer injection rates and concentrations of the injected solution. Polymer concentration fields in the near-wall region (0 &lt;y+&lt; ~103) were examined at three locations downstream of the injector using near-wall planar laser-induced-fluorescence imaging. The development and extent of drag reduction and polymer mixing are compared to previously reported results using the traditionalK-factor scaling. Unlike smaller scale and lower speed experiments, speed dependence is observed in theK-scaled results for the higher molecular weight polymers and it is postulated that this dependence is caused by molecular aggregation and/or flow-induced polymer degradation (chain scission). The evolution of near-wall polymer concentration is divided into three regimes: (i) the development region near the injector where drag reduction increases with downstream distance and the polymer is highly inhomogeneous forming filaments near the wall, (ii) the transitional mixing region where drag reduction starts to decrease as the polymer mixes across the boundary layer and where filaments are less pronounced and (iii) the final region where the polymer mixing and dilution is set by the rate of boundary layer growth. Unlike pipe-flow friction-drag reduction, the asymptotic maximum drag reduction (MDR) either was not reached or did not persist in these experiments. Instead, the nearest approach to MDR was transitory and occurred between the development and transitional regions. The length of the development region was observed to increase monotonically with increasing polymer molecular weight, injection rate, concentration and decreasing free stream speed. And finally, the near-wall polymer concentration is correlated to the measured drag reduction for the three polymer molecular weights in the form of a proposed empirical drag-reduction curve.

Bubble-induced skin-friction drag reduction and the abrupt transition to air-layer drag reduction

To investigate the phenomena of skin-friction drag reduction in a turbulent boundary layer (TBL) at large scales and high Reynolds numbers, a set of experiments has been conducted at the US Navy's William B. Morgan Large Cavitation Channel (LCC). Drag reduction was achieved by injecting gas (air) from a line source through the wall of a nearly zero-pressure-gradient TBL that formed on a flat-plate test model that was either hydraulically smooth or fully rough. Two distinct drag-reduction phenomena were investigated; bubble drag reduction (BDR) and air-layer drag reduction (ALDR).The streamwise distribution of skin-friction drag reduction was monitored with six skin-friction balances at downstream-distance-based Reynolds numbers to 220 million and at test speeds to 20.0ms−1. Near-wall bulk void fraction was measured at twelve streamwise locations with impedance probes, and near-wall (0 &lt;Y&lt; 5mm) bubble populations were estimated with a bubble imaging system. The instrument suite was used to investigate the scaling of BDR and the requirements necessary to achieve ALDR.Results from the BDR experiments indicate that: significant drag reduction (&gt;25%) is limited to the first few metres downstream of injection; marginal improvement was possible with a porous-plate versus an open-slot injector design; BDR has negligible sensitivity to surface tension; bubble size is independent of surface tension downstream of injection; BDR is insensitive to boundary-layer thickness at the injection location; and no synergetic effect is observed with compound injection. Using these data, previous BDR scaling methods are investigated, but data collapse is observed only with the ‘initial zone’ scaling, which provides little information on downstream persistence of BDR.ALDR was investigated with a series of experiments that included a slow increase in the volumetric flux of air injected at free-stream speeds to 15.3ms−1. These results indicated that there are three distinct regions associated with drag reduction with air injection: Region I, BDR; Region II, transition between BDR and ALDR; and Region III, ALDR. In addition, once ALDR was established: friction drag reduction in excess of 80% was observed over the entire smooth model for speeds to 15.3ms−1; the critical volumetric flux of air required to achieve ALDR was observed to be approximately proportional to the square of the free-stream speed; slightly higher injection rates were required for ALDR if the surface tension was decreased; stable air layers were formed at free-stream speeds to 12.5ms−1with the surface fully roughened (though approximately 50% greater volumetric air flux was required); and ALDR was sensitive to the inflow conditions. The sensitivity to the inflow conditions can be mitigated by employing a small faired step (10mm height in the experiment) that helps to create a fixed separation line.

Experimental Investigation of the Submarine Crashback Maneuver

In order to decelerate a forward-moving submarine rapidly, often the propeller of the submarine is placed abruptly into reverse rotation, causing the propeller to generate a thrust force in the direction opposite to the submarine’s motion. This maneuver is known as the “crashback” maneuver. During crashback, the relative flow velocities in the vicinity of the propeller lead to the creation of a ring vortex around the propeller. This vortex has an unsteady asymmetry, which produces off-axis forces and moments on the propeller that are transmitted to the submarine. Tests were conducted in the William B. Morgan Large Cavitation Channel using an existing submarine model and propeller. A range of steady crashback conditions with fixed tunnel and propeller speeds was investigated. The dimensionless force and moment data were found to collapse well when plotted against the parameter η, which is defined as the ratio of the actual propeller speed to the propeller speed required for self-propulsion in forward motion. Unsteady crashback maneuvers were also investigated with two different types of simulations in which propeller and tunnel speeds were allowed to vary. It was noted during these simulations that the peak out-of-plane force and moment coefficient magnitudes in some cases exceeded those observed during the steady crashback measurements. Flow visualization and LDV studies showed that the ring vortex structure varied from an elongated vortex structure centered downstream of the propeller to a more compact structure that was located nearer the propeller as η became more negative, up to η=−0.8. For more negative values of η, the vortex core appeared to move out toward the propeller tip.

Using cross correlations of turbulent flow-induced ambient vibrations to estimate the structural impulse response. Application to structural health monitoring

It has been demonstrated theoretically and experimentally that an estimate of the impulse response (or Green's function) between two receivers can be obtained from the cross correlation of diffuse wave fields at these two receivers in various environments and frequency ranges: ultrasonics, civil engineering, underwater acoustics, and seismology. This result provides a means for structural monitoring using ambient structure-borne noise only, without the use of active sources. This paper presents experimental results obtained from flow-induced random vibration data recorded by pairs of accelerometers mounted within a flat plate or hydrofoil in the test section of the U.S. Navy's William B. Morgan Large Cavitation Channel. The experiments were conducted at high Reynolds number (Re > 50 million) with the primary excitation source being turbulent boundary layer pressure fluctuations on the upper and lower surfaces of the plate or foil. Identical deterministic time signatures emerge from the noise cross-correlation function computed via robust and simple processing of noise measured on different days by a pair of passive sensors. These time signatures are used to determine and/or monitor the structural response of the test models from a few hundred to a few thousand Hertz.

Structural monitoring from noise cross correlation

It has been demonstrated theoretically and experimentally that an estimate of the Green’s function between two receivers can be obtained from the long-time average of the cross correlation of ambient noise at these two receivers in various environments and frequency ranges of interest: ultrasonics, underwater acoustics, and seismology. This result provides a means for structural monitoring using the ambient structure-borne noise only, without the use of active sources. The coherent wave-fronts emerge from a correlation process that accumulates contributions over time from noise sources whose propagation path passes through both receivers. We present experimental results using random vibration data collected on eight accelerometers mounted on a hydrofoil or plate at the Navy’s William B. Morgan Large Cavitation Channel. The tests were conducted at high Reynolds number, with the primary excitation source being turbulent boundary-layer pressure fluctuations on the upper and lower surfaces of the plate or foil. Identical deterministic time signatures emerge from the noise cross-correlation function based on robust and simple signal processing between passive sensors. These time signatures can be used to estimate and monitor the structural response of the test models.

High Reynolds number experimentation in the US Navy's William B Morgan Large Cavitation Channel

The William B Morgan Large Cavitation Channel (LCC) is a large variable-pressure closed-loop water tunnel that has been operated by the US Navy in Memphis, TN, USA, since 1991. This facility is well designed for a wide variety of hydrodynamic and hydroacoustic tests. Its overall size and capabilities allow test-model Reynolds numbers to approach, or even achieve, those of full-scale air- or water-borne transportation systems. This paper describes the facility along with some novel implementations of measurement techniques that have been successfully utilized there. In addition, highlights are presented from past test programmes involving (i) cavitation, (ii) near-zero pressure-gradient turbulent boundary layers, (iii) the near-wake flow characteristics of a two-dimensional hydrofoil and (iv) a full-scale research torpedo.

Experimental Methods for Hydrodynamic Characterization of a Very Large Water Tunnel

The methodology for hydrodynamic characterization of a very large water tunnel is described. Results are presented for the U. S. Navy William B. Morgan Large Cavitation Channel in Memphis, Tennessee, the world’s largest water tunnel. Three key characteristics of tunnel velocity were measured: temporal stability̱, spatial uniformity̱, and turbulence̱. The velocity stability at a single point for run times greater than 2 h was measured as ±0.15% at the 95% confidence level for velocities from 0.5 to 18m∕s(1.6–59ft∕s). The spatial nonuniformity for the axial velocity component was ±0.34 to ±0.60% for velocities from 3 to 16m∕s(9.8–52ft∕s). The relative turbulence intensity was measured as 0.2–0.5% depending on tunnel velocity.

Experimental Investigation of the Flow past a Submarine at Angle of Drift

When a submarine executes a high-speed turn, the sail experiences a crossflow velocity component, creating a vortex at the sail tip. The flowfield induced by the vortex creates higher pressures on the hull deck and lower pressures on the keel, leading to a nose-up pitching moment. Tests to examine such flows were conducted in the U.S. Navy's William B. Morgan Large Cavitation Channel in Memphis, Tennessee. A submarine model of 6.92-m length was tested at drift angles of 0, 5, and 9.5 deg, with the sail on and off, and at speeds up to 14.9 m/s. Pressure distributions and laser Doppler velocimeter surveys were obtained at two axial locations. Forces and moments were measured using an internal force balance. The pressure distributions were integrated to obtain section force coefficients, and the velocity data were used to estimate the vortex circulation. The pressure distributions were consistent with the hypothesized mechanism by which the vortex created the nose-up pitching moment. The pitching moment and sectional force coefficients demonstrated the nonlinear variations with drift angle predicted by theory. The circulation values were consistently somewhat higher than those obtained by other researchers

Hydrofoil near-wake sound sources at high Reynolds number

An important hydroacoustic noise source from a fully submerged noncavitating hydrofoil is often the unsteady separated turbulent flow near its trailing edge. Here, hydroacoustic noise may be produced by boundary layer turbulence swept past and scattered from the foils trailing edge, and by coherent vortices formed in the foils near-wake. Such vortices may generate an energetic tonal component that rises above the broadband trailing-edge hydroacoustic noise. This presentation describes results of an experimental effort to identify and measure vortical flow features in the near-wake of a two-dimensional hydrofoil at chord-based Reynolds numbers ranging from 0.5 to 60 million. The experiments were conducted at the U.S. Navy’s William B. Morgan Large Cavitation Channel with a test-section-spanning hydrofoil (2.1 m chord, 3.0 m span) at flow speeds from 0.25 to 18.3 m/s. Two trailing-edge shapes were investigated, and foil-internal accelerometers were used to monitor structural vibration. Velocity fluctuation spectra were measured in the foils near-wake with a two-component LDV system, and dynamic surface pressures were measured near the foils trailing edge with flush-mounted transducer arrays. Both indicate Reynolds number and trailing-edge shape-dependent vortex shedding. [Significant assistance provided by personnel from NSWC-CD. Work sponsored by Code 333 of ONR.]

Turbulent boundary layer pressure fluctuations at large scales

The pressure fluctuations underneath a turbulent boundary are an important excitation source for noise and vibration for both aircraft and ships. This presentation describes experimental results from a new study of flat-plate turbulent boundary layer pressure fluctuations in water at large scales and high Reynolds number. The experiments were performed at the U.S. Navy’s William B. Morgan Large Cavitation Channel in Memphis, TN on a polished flat plate 3.05 m wide, 12.8 m long, and 0.18 m thick. Flow velocity, skin friction, surface pressure, and plate acceleration measurements were made at multiple downstream locations at flow speeds ranging from 0.5 m/s to 19 m/s for a Reynolds number (based on downstream distance) range of several million to 200 million. Dynamic surface pressures were recorded with 16 flush mounted pressure transducers forming an L-array with streamwise dimension of 0.264 m and cross-stream dimension of 0.391 m. Measured 99% boundary-layer thicknesses were typically of order 0.10 m. Results for spatial and temporal correlation functions, as well as auto- and cross-spectra are presented and compared with prior lower-Reynolds-number results using either inner or outer variable scaling. [Work sponsored by the Defense Advanced Research Projects Agency and ONR Code 333.]