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SUMMARY:MEchanics GAthering -MEGA- Seminar: Physical mechanism and flow co
 ntrol of tip vortex cavitation
DTSTART:20191107T161500
DTEND:20191107T173000
DTSTAMP:20260511T204902Z
UID:b832208788b54b1223adef4f80a35f337f3847ccce42d3574c161be6
CATEGORIES:Conferences - Seminars
DESCRIPTION:Ali Amini\, LMH\, EPFL\nOccurrence of cavitation in hydraulic 
 machines is a challenging issue\, because cavitation is often associated w
 ith loss of efficiency\, noise emissions\, vibrations\, and erosion damage
 s. Tip vortices\, which are an inherent characteristic of finite-span lift
 ing surfaces\, form an ideal site for the inception of cavitation as the s
 tatic pressure at their cores usually drops much below the freestream pres
 sure due to the rotational motion. In the first part of this talk\, we foc
 us on the effect of dissolved gas content on Tip Vortex Cavitation (TVC). 
 The inception and desinence thresholds of TVC measured at different flow c
 onditions for various gas contents reveal that TVC often disappears at cav
 itation indices much higher than the inception thresholds. Our measurement
 s show that TVC desinence pressure increases with the gas content and\, un
 der specific flow conditions\, may reach to atmospheric pressure. When the
  pressure of the cavitating core is below the initial saturation pressure 
 of the dissolved gases\, water flowing adjacent to the interface becomes s
 upersaturated and diffuses the air molecules into TVC. The extent of the d
 elay in desinence due to outgassing is\, however\, dictated by the bulk fl
 ow parameters. Owing to flow visualizations\, we assert that the formation
  of a laminar separation bubble at the hydrofoil tip is a necessary condit
 ion for a delayed desinence. The separation bubble acts like a shelter and
  creates a relatively calm area at the vortex core. We show that the hyste
 resis is suppressed once the laminar separation bubble is destroyed.\nIn t
 he second part of the talk\, we present our recent results on the effectiv
 eness of winglets in suppressing TVC. In this study\, an elliptical hydrof
 oil is selected as the baseline geometry and various winglets are realized
  by bending the last 5 or 10% of the span at ±45° and ±90° dihedral an
 gles. Lift-and-drag force measurements demonstrate that the hydrodynamic p
 erformances of the winglet-equipped hydrofoils are not different from the 
 baseline. Nevertheless\, cavitation inception-desinence tests reveal that 
 undeniable advantages are achieved by the winglets. It is found that the 1
 0%-bent 90° winglets are more effective than the 45° cases\, with -90° 
 (bent down towards the pressure side) performing superior to +90°. For in
 stance\, the 90°-bent-downward winglet reduces the TVC inception index fr
 om 2.5 for the baseline down to 0.8 (a reduction of 68%) at 15 m/s freestr
 eam velocity and 14° incidence angle. Stereo-PIV measurements show that f
 or the most effective winglet (10%-bent 90°-downward)\, the maximum tange
 ntial velocity of the tip vortex falls to almost half of the baseline and 
 the vortex core size increases significantly (by almost 70%). These effect
 s are accompanied by a tangible reduction in the axial velocity at the vor
 tex core leading to further mitigation of TVC.
LOCATION:MED 2 2423 https://plan.epfl.ch/?room==MED%202%202423
STATUS:CONFIRMED
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