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Department of Mechanical Engineering
University of Utah
Abstract-
Owing to its relevance regarding a number of engineering applications, over
the past decade the high Reynolds number turbulent boundary layer has been the
subject of increasingly intense study. Experimental access to the detailed
nature of the high Reynolds number boundary layer turbulence, however,
presents a formidable challenge. The reasons for this are that for flows that
achieve their high Re_theta (Re_theta = U_inf theta / nu, where U_inf is the free
stream velocity, theta the
momentum deficit thickness and nu the kinematic viscosity) primarily through
increasing U_inf and/or decreasing nu, the dynamics of the smallest eddies are
driven to such small lengths and high frequencies that they are generally well
beyond the capabilities of existing sensors. Continuing studies seek to
overcome these spatial and temporal resolution challenges by exploring the
turbulence in the boundary layer that flows over the salt playa of Utah's west
desert. Owing to its low speeds and large length scales, this flow
simultaneously has a very large Re_theta and resolvable small scales. An inherent
property of high Reynolds number flows is the wide disparity between the
largest and smallest turbulent motions. In the turbulent boundary layer the
magnitude of this scale separation is reflected in the so-called Karman number,
delta+ = delta u_tau / nu , which is the ratio of the boundary layer thickness,
delta
(a measure of the largest motions), to the viscous scale, nu/u_tau (a measure of
the smallest motions). At typical laboratory scale Reynolds numbers, delta+ =
O(10^3 ), while for the flow over transport aircraft, submarines and the salt
playa, delta+ = O(10^6 ). In the present talk, the implications of this orders of
magnitude increase in scale disparity are discussed relative to the structure
of the high Re_theta wall layer and the underlying processes of momentum transport.
In doing so, velocity, vorticity and wall pressure data are examined. The data
are used to motivate a physical picture of turbulent boundary layer
development with increasing Reynolds number. The talk will conclude with a
brief discussion regarding the significance of the present results relative to
predicting and/or modeling high Reynolds number boundary layer phenomena.
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