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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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