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Atmospheric Ventilation
High-speed marine vehicles rely upon propellers and hydrofoils for thrust- and
lift-generation. Such lifting surfaces are often operated at or very near the fluid
free-surface, making them vulnerable to atmospheric ventilation, where
atmospheric air is entrained into low-pressure regions of flow. Ventilation can
occur suddenly [1,2], inducing large dynamic load changes. Reductions of lift
of up to 75% [1] and even lift-reversal [3] can occur.
Relevance:
Ventilation has implications in the safety and controllability of any vessel that
relies upon ventilation-prone systems. High speed and heavily loaded lifting
surfaces are particularly vulnerable, including propellers and hydrofoils
operating at shallow depths, high-speed hull forms, and dynamic positioning
thrusters in highly loaded conditions. With continued progress toward the use
of lightweight and/or flexible marine structures, fluid-structure interaction (FSI)
and stability of ventilated flows becomes an important consideration. Large
changes in loading may induce vibrations, instabilities or structural failure.
A greater understanding of the physics driving ventilation and its impact upon
the hydrodynamic and structural response of lifting-surfaces in ventilated flows
will enable eventual mitigation and/or control of deleterious ventilation and
flow-induced vibrations.
Objectives:
1. Use a “rigid” aluminum surface piercing hydrofoil/strut to
a.Identify the characteristic ventilation flow regimes through towing-tank
experiments
b. Identify the change in modal response of a partially immersed strut
subjected to ventilation
2. Design and instrument a flexible PVC hydrofoil/strut to measure
instantaneous 3D deformations, quantify changes in added mass &
damping with ventilation, and characterize flow-induced vibrations
Experimental Setup
• Two identical struts manufactured from aluminum and PVC (figure 1)
• Aluminum strut experiments completed using the towing-tank of the
University of Michigan Marine Hydrodynamics Laboratory (figure 2, table 1)
• Dry and wet modal response measured for aluminum strut (figure 3).
• PVC strut instrumented with shape-sensing beams
Hydrodynamic and Structural Response of Surface-Piercing Struts in Ventilated Flows
Casey Harwood, Prof. Julie Young, Prof. Steven Ceccio
University of Michigan, Ann Arbor, USA
Characteristic Flow Regimes – Al Strut
Conclusions
Chord 𝑐 11in (29.7cm)
Foil Span 𝑆 36in (91cm)
Yaw Angle 𝛼 −5∘ to 30∘
Tip Depths ℎ
5.5,11,16.5 in
(14,28,42 cm)
Aspect Ratio 𝐴𝑅ℎ = ℎ/𝑐 0.5, 1.0, 1.5
Velocities 𝑈∞
2-20 ft/s
(0.6-6 m/s)
Depth Froude # 𝐹𝑛ℎ = 𝑈/√𝑔ℎ 0.5 - 4.5
Chord Reynolds # 𝑅𝑒 𝑐 = 𝑈𝑐/𝜈
1.7 × 105 −
1.7 × 106
Table 1: Particulars of tow-tank study
Fully Wetted (FW) Regime
• Suction side remains wetted
• Flow is relatively steady and locally stable
• Base-ventilation can occur in eddying wake
Partially Ventilated (PV) Regime
• Cavity depth (0 < 𝐷 < ℎ) OR Φ > 45∘
• Flow is unsteady and potentially unstable
• Dominated by re-entrant jet
Fully Ventilated (FV) Regime
• 𝐷 = ℎ AND Φ ≤ 45∘
• Flow is relatively steady and stable
• Cavity is open to the atmosphere
• Tip vortex often ingests air from cavity
FW
PV
FV
PV→FV
FW→PV
PV→FW
FV→PV
Steady
Regimes
Transition
Boundaries
Bi-Stable
Regions
FW/FV
FW/PV
PV/FV
Strain Gauge Half-Bridges
0 0.5 1 1.5
0
20
40
60
80
100
120
140
160
180
Submerged Aspect Ratio, ARh
Frequency,Hz
X-Bend 1 (Water-Drum)
X-Bend 2 (Water-Drum)
Z-Twist 3 (Water-Drum)
FA (Carriage)
FV (Carriage)
Shaded regions indicate FEA
results for each mode.
Bounded below by FW and
above by FV frequencies.
Figure 5: Shape-sensing beam (one of two installed in flexible strut)
Effect of Immersion Depth and Ventilation on Modal Frequencies - Al Strut
Figure 4: Flow regimes and stability regions at 𝐴𝑅ℎ = 1.0. Red boundaries indicate ventilation formation (FW → PV → FV).
Blue boundaries indicate closure (FV → PV → FW). Hatched regions indicate bi-stable overlap between locally-stable regimes.
The first three modal frequencies
were measured cases with:
• Strut suspended in drum of water
• Strut being towed (FW & FV flow)
• FEA model (FW & FV flow)
Natural frequencies decrease with
increasing immersion depth and
increase with the onset of ventilation.
The effects are dependent upon
mode shape, leading to a risk of
frequency coalescence and ensuing
instability.
Shape-Sensing with Flexible PVC Strut
Figure 7: Effect of immersion depth and ventilation on
natural frequencies of aluminum strut
Two flexible aluminum spines were installed into channels
machined into the flexible PVC strut (see figure 1). Each
beam is instrumented with four half-bridges to measure
normal bending strain.
1. The bending equation: 𝜀 𝑏𝑒𝑛𝑑 ≈ −
𝑡
2
𝜕2 𝑦
𝜕𝑥2 .
2. A polynomial is fitted through the measured strains:
εbend x ≈ ai xiNgauges
i=0
3. Integration yields a polynomial :
y x ≈ −
2
t
εbend x dx dx =
ai xi+2
i+1 i+2
Ngauges
i=0
4. y1, y2 yield lateral deflection and twist at each station
Figure 2: Experimental fixture
Figure 2: Semi-ogive cross section of struts
Rigid
Flexible
• Three steady flow regimes are identified. Transition between flow regimes occurs where the
stability regions meet or overlap one another.
• Increasing depth-of-immersion decreases resonant frequencies, indicating increased added
mass. The change in added mass depends upon the mode shape. Ventilation causes a
decrease in added mass, and hence an increase in resonant frequencies, relative to the wetted
frequencies at the same conditions.
• A shape-sensing beam has been designed using strain gages, with which it is possible to
obtain a real-time 3D deformation field on a flexible strut with high precision.
Understanding how ventilation and material properties affect the hydrodynamic and structural
performance of lifting surfaces, combined with the ability to measure hydroelastic responses in
real-time may open the door for performance/stability enhancement and vibration suppression
via active control.
[1] J. Breslin and R. Skalak, “Exploratory study of ventilated flows about yawed surface-piercing struts,”
NASA, Washington DC, Tech. Rep. 2-23-59W, 1959.
[2] C. Harwood, et al., “Experimental and numerical investigation of ventilation inception and washout
mechanisms of a surface-piercing hydrofoil,” in Proc. 30th Symp. Naval Hydrodynamics, to be published.
[3] R. Rothblum, et al.,“Ventilation, cavitation, and other characteristics of a high speed surface-piercing strut,”
NSRDC Report, no. 3023, 1969
• Perform towing-tank studies with flexible strut
• Explore the effects of ventilation on fluid added mass, damping, and stiffness matrices, flow-
induced vibrations, and hydroelastic stability boundaries (flutter and divergence).
• Develop and validate instantaneous hydrodynamic load mapping algorithm through inverse
FSI analysis of embedded strain gauge data.
Continuing / Future Work
Single-Point
LDV Vibrometer
Load Cell
at Root
3-Axis
Accelerometer
h
Figure 3: Vibration test rig
Figure 6: Photos of
applied loads and real-time
reconstructions of
deformation fields.
(Clockwise from top left:
twisting, two-node bending,
and one-node bending).
Shape sensing captures
deformation distributions
with high precision and a
low noise-floor.
Deflection𝑦,inDeflection𝑦,in
FW
PV PV
FV