Abstracts - KTH Mechanics

84
DNS ofaLong Laminar Separation Bubble
O. Marxen ∗ and D.S. Henningson ∗
A laminar separation bubble (LSB) can originate if an initially laminar boundary
layer is subject to a sufficiently strong adverse pressure gradient, and transition occurs
in the detached shear-layer. Further downstream, the turbulent flow often reattaches
in the mean, forming a closed bubble. LSBs canoccuronslenderbodysandmay
adversely affect their performance in terms of lift and drag.
Owen and Klanfer 1 were the first to distinguish between short and long laminar
separation bubbles. Tani 2 introduced a classification of long and short LSBs depending
on whether their influence on the pressure distribution is local or global. Under
certain conditions, for slight changes in the flow conditions short bubbles can break-up
into long ones. Gaster 3 carried out a detailed investigation to settle characteristics of
short and long bubbles, and to establish parameters that govern the bursting process.
While numerous studies, including DNS, exist on short LSBs 4,5 , flow dynamics
of long bubbles are not well understood on a fundamental level. Up tonow,investigations
of long LSBs were almost exclusively based on experiments. Baragona et
al. 6 carried out RANS simulations of long LSBs and bubble bursting, but concluded
that DNS is required to obtain reliable results. Past investigations of long LSBs and
bubble bursting indicate that the transition process plays a major role in the flow
dynamics.
Here, results from a DNS of a long LSB shall be reported. The underlying potential
flow field resembles the one created by a cylinder (i.e. a dipole) above a wall, resulting
in a strong acceleration of the boundary layer followed by a strong deceleration. At
the inflow a Blasius profile is prescribed (Reδ⋆=1000) and the pressure gradient is
induced via the streamwise velocity at the upper boundary. Transition is triggered
by blowing and suction at the wall upstream of separation.
DNS results show that in contrast to what is known from short LSBs, the saturated
disturbances are not able to reattach the flow soon after transition. Instead,
reattachment occurs only considerably downstream of the transition location. If the
triggering amplitude is chosen to be fairly large, a separation bubble forms that appears
to be more comparable to short LSBs. This underlines the important role of the
transition process. Furthermore, it strongly indicates that the disturbance content in
the flow (turbulence level) is an important parameter to consider in investigations of
bubble bursting. Future simulations shall investigate possible mechanisms leading to
bubble bursting.
∗ KTH Mechanics OB 18, SE-100 44 Stockholm, Sweden.
1 In: A.R.C. Technical Report CP–220 (1955).
2 Prog. Aerosp. Sci. 5, 70 (1964).
3 AGARD CP–4, 813 (1966).
4 Häggmark et al., Aerosp. Sci. Technol. 5 317 (2001)
5 Marxen, Dissertation, Universität Stuttgart (2005)
6 AIAA J. 41(7) 1230 (2003)