Experimental Investigation of the Submarine Crashback Maneuver

Abstract

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.

Predicting Unsteady Loads in Marine Propulsor Crashback Using Large Eddy Simulation

“…LES is able to reproduce the experimentally observed increase of side force for an open propulsor with hull below an advance ratio of J = −0.7. The mean, RMS, and spectra of unsteady loads are in good agreement with the experiments of Jessup et al [4] and Bridges et al [5]. For a ducted propulsor, most of the thrust is still due to the blades, but the blade-duct interaction causes side force to be generated from the duct surface.…”

“…A low frequency, high amplitude modulation of the side-force has important ramifications for the maneuverability of the vehicle. For the K H spectra, there is a peak at a lower frequency of f = 0.12 rev −1 with the hull which agrees very well with that observed by Bridges et al [5] at f = 0.11 rev −1 . The comparison with experiments is summarized in Table 3.…”

“…Bridges et al's experiment [5] on an open propulsor with an upstream submarine hull in the Large Cavitation Channel (LCC) noted that the side force increased dramatically below an advance ratio of J = −0.7 compared to Jessup's experiment [8] geometry. Details of the hull geometry are given in [5]. In the simulations, half of the hull body is used and stabilizing fins are ignored.…”

“…Note that the vortex ring is much closer to the tip of the blade when the hull is present. Table 4 shows good agreement for the distance of the center of the vortex ring relative to the propulsor center between LES and experiment [5]. In order to compare side-force distributions with and without hull, side-force magnitudes are computed in ten radial sections in Figure 11.…”

“…Jessup et al [4] presented more detailed measurements of flow around the same propeller using PIV and laser Doppler velocimetry (LDV). Bridges et al's experiment [5] documented the effect of hull on an open propulsor. The effect of a duct was studied experimentally by Jessup et al [8] and Donnelly et al [9].…”