We present the dependence of subsurface trajectory of a slender axisymmetric body on the angle of impact relative to the free surface (θ measured from vertical) and the axisymmetry of wettability (γ). These effects are demonstrated in Figures 1 and 2(a)–2(c) which show the entry of slender (length/diameter = 10) axisymmetric bodies made of acrylic (ρ/ρwater = 1.24, Figure 1) and Delrin® (ρ/ρwater = 1.42, Figures 2(a)–2(c)) with ogive nose shapes into quiescent water at velocities of 3.1 m/s. When the body enters the water normal to the free surface, it descends without planar rotation or lateral translation (Figures 1(a) and 2(a)). Dissimilar wetting angles (hydrophobic γleft = 119°, hydrophilic γright = 73°) create an asymmetric cavity which rotates the nose toward the hydrophilic side despite normal water entry (Figures 1(b) and 2(b)). When the hydrophilic axisymmetric body impacts oblique to the free surface (θ = 5°), a moment couple between the center of pressure and the center of gravity causes planar rotation, resulting in the largest trajectory deviation of the three (Figures 1(c) and 2(c)). We witness that as the impact angle increases, the rotation rate increases producing more lateral translation (not shown). Additionally, as the center of mass (CM) is placed farther from the nose (12.63 cm, Figure 2(d)), the body rotates more and experiences more lateral displacement than the cases where the CM is closer to the nose (8.63 cm, Figure 2(e)).

FIG. 1.

Three slender body objects with ogive nose shapes made of acrylic (ρ/ρwater = 1.24) dropped from 50 cm. An IMU is visible in the back half of each body. (a) Hydrophilic (γ = 73°) dropped at θ = 0°. (b) Half hydrophobic (γleft = 119°), half hydrophilic (γright = 73°) dropped at θ = 0°. (c) Hydrophilic (γ = 73°) dropped at θ = 5°.

FIG. 1.

Three slender body objects with ogive nose shapes made of acrylic (ρ/ρwater = 1.24) dropped from 50 cm. An IMU is visible in the back half of each body. (a) Hydrophilic (γ = 73°) dropped at θ = 0°. (b) Half hydrophobic (γleft = 119°), half hydrophilic (γright = 73°) dropped at θ = 0°. (c) Hydrophilic (γ = 73°) dropped at θ = 5°.

Close modal
FIG. 2.

Image sequences of slender bodies with ogive noses made of Delrin® (ρ/ρwater = 1.42) dropped from 50 cm, dt = 21 ms between each image. Sequences (a)–(c) are time series similar to Figures 1(a)–1(c). Sequences (d) and (e) illustrate the effect CM has on slender body rotation.

FIG. 2.

Image sequences of slender bodies with ogive noses made of Delrin® (ρ/ρwater = 1.42) dropped from 50 cm, dt = 21 ms between each image. Sequences (a)–(c) are time series similar to Figures 1(a)–1(c). Sequences (d) and (e) illustrate the effect CM has on slender body rotation.

Close modal

Accelerations were investigated using a custom inertial measurement unit (IMU) embedded in the tail of the body (visible in Figure 1). Post-impact decelerations reveal that the non-cavity-forming cases (hydrophilic) have decreased drag compared to the cavity-forming cases. This finding is opposite from findings for cavity-forming sphere water entry cases found by Truscott et al.1 and is likely due to the mitigation of vortex shedding caused by the slender body when wetted.

We thank Maria Medeiros and ONR ULI Grant No. N000141110872 for funding this research.

1.
T. T.
Truscott
,
B. P.
Epps
, and
A. H.
Techet
, “
Unsteady forces on spheres during free-surface water entry
,”
J. Fluid Mech.
704
,
173
(
2012
).