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TZID:Asia/Jerusalem
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UID:0-345@aerospace.technion.ac.il
DTSTART;TZID=Asia/Jerusalem:20181217T163000
DTEND;TZID=Asia/Jerusalem:20181217T173000
DTSTAMP:20230527T135005Z
URL:https://aerospace.technion.ac.il/events/transition-to-turbulence-over-
convex-surfaces-and-utilization-of-instabilities-for-optimal-laminar-separ
ation-delay/
SUMMARY:Transition to Turbulence Over Convex Surfaces and Utilization of In
stabilities for Optimal Laminar Separation Delay
DESCRIPTION:Lecturer:Dr. Michael Karp\n Faculty:Center for Turbulence Resea
rch\, Faculty of Mechanical Engineering\n Institute:Stanford University\n
Location:Classroom 165\, ground floor\, Library\, Aerospace Eng.\n Zoom: \
n Abstract: \n Details: \n Understanding flow instability mechanisms in bo
undary layers is valuable for designing more efficient aerodynamic and hyd
rodynamic surfaces. This talk consists of two parts\; the first discusses
instabilities and transition to turbulence in boundary layers over convex
surfaces\, such as a wing’s leading edge. The second part discusses the
utilization of instability mechanisms for optimal separation delay.\nContr
ary to flows over concave surfaces\, which are prone to the Görtler insta
bility\, boundary layers over convex surfaces are stabilized as a result o
f the curvature. Nevertheless\, non-modal mechanisms may enable significan
t disturbance growth which can make the flow susceptible to secondary inst
abilities. The influence of curvature on primary and secondary instabiliti
es is investigated theoretically and numerically. The predictions of local
stability theory are verified by direct numerical simulations\, which dem
onstrate that only sufficiently long and energetic streaks trigger the bre
akdown to turbulence.\nFlow separation often leads to reduced aerodynamic
performance. Passive control devices\, such as vortex generators\, success
fully delay separation by triggering turbulence. Although the turbulence i
nduces flow reattachment\, its adverse effects are increased drag and heat
transfer. Moreover\, finding the optimal parameters for the design of vor
tex generators is oftentimes based on trial and error\, especially their s
panwise spacing. Our aim is to find the optimal perturbation that one need
s to generate\, such that the separation location is delayed as downstream
as possible. It is shown that a perturbation from linear stability theory
leads to significant delay of separation and serves as a good starting po
int for the nonlinear optimization algorithm. The mechanism responsible fo
r separation delay is a mean flow distortion\, generated by nonlinear inte
ractions during the non-modal growth stage. The mean flow distortion augme
nts the velocity close to the wall\, counteracting the velocity decelerati
on in that region.
CATEGORIES:Seminars
LOCATION:Classroom 165\, ground floor\, Library\, Aerospace Eng.
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DTSTART:20181028T010000
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