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Experiments were performed that (i) document the effect of the steady spanwise buffer layer blowing on the mean characteristics of the turbulent boundary layer for a range of momentum thickness Reynolds numbers from 4760 to 10 386, and (ii) document the effect of the buffer layer blowing on the unsteady characteristics and coherent vorticity in a boundary layer designed to provide sufficiently high spatial resolution. The spanwise buffer layer blowing of the order of $u_{\tau }$ is produced by a surface array of pulsating direct current (pulsed-DC) plasma actuators. This was found to substantially reduce the wall shear stress that was directly measured with a floating element coupled with a force sensor. The direct wall shear measurements agreed with values derived using the Clauser method to within $\pm 0.85$ %. The degree to which the buffer layer blowing affected $\tau _w$ was found to primarily depend on the inner variable spanwise spacing between the pulsed-DC actuator electrodes, i.e. ‘blowing sites’. Utilizing pairs of $[u,v]$ and $[u,w]$ hot-wire sensors, the latter experiments correlated significant reductions in the $\omega _y$ and $\omega _x$ vorticity components that resulted from the buffer layer blowing and translated into lower Reynolds stresses and turbulence production. The time scale to which these observed changes in the boundary layer characteristics would return to the baseline condition was subsequently documented. This revealed a recovery length of $x^+ \approx 86\,000$ that translated to a streamwise fetch of $x \approx 66\delta$. Finally, a comparison with the recent work by Cheng et al. (2021, J. Fluid Mech. vol. 918, A24) and Wei & Zhou (2024 in TSFP13, June 25–28, 2024) that followed our experimental approach to achieve comparable wall shear stress (drag) reductions has led to a new scaling based on the baseline boundary layer $\textit{Re}_{\tau }$ and buffer layer blowing velocity.
This Digest is intended to provide lawyers with a key to those questions and Ministerial Statements in the Commonwealth Parliament in which they are most likely to be interested, and it is, of course, selective. It covers the period 21 February to 8 November 1967, and is compiled from the published debates of the Parliament of the Commonwealth of Australia.
The results of an experimental investigation of smooth-body adverse pressure gradient (APG) turbulent boundary layer flow separation and reattachment over a two-dimensional ramp are presented. These results are part of a larger archival smooth-body flow separation data set acquired in partnership with NASA Langley Research Center and archived on the NASA Turbulence Modeling Resource website. The experimental geometry provides initial canonical turbulent boundary layer growth under nominally zero pressure gradient conditions prior to encountering a smooth, two-dimensional, backward facing ramp geometry onto which a streamwise APG that is fully adjustable is imposed. Detailed surface and off-surface flow field measurements are used to fully characterize the smooth-body APG turbulent boundary layer separation and reattachment at multiple spanwise locations over the ramp geometry. Unsteady aspects of the flow separation are characterized. It is shown that the first and second spatial derivatives of the streamwise static surface pressure profile are sufficient to determine key detachment and reattachment locations. The imposed streamwise APG gives rise to inflectional mean velocity profiles and the associated formation of an embedded shear layer, which is shown to play a dominant role in the subsequent flow development. Similarity scaling is developed for both the mean velocity and turbulent stresses that is found to provide self-similar collapse of profiles for different regions of the ramp flow. Despite the highly non-equilibrium flow environment, a new similarity scaling proved capable of providing self-similar turbulent stress profiles over the full streamwise extent of flow separation and downstream reattachment.
Chapter 9 involves shock–boundary-layer interactions that are intrinsic to supersonic engine intakes, transonic gas turbine blade tip gaps and blade passages, scramjet isolator ducts, transonic and supersonic flight vehicle surfaces, and surfaces of rockets, missiles, and reentry vehicles. It is of particular interest because it can result in large temporal and spatial pressure variations, and greatly affect boundary development including causing flow separation that feeds into the flow unsteadiness, and subsequently has a large impact on aerodynamic performance. The outcome of shock–boundary-layer interactions strongly depends on steady and unsteady initial conditions that can be factored into flow control approaches. Methods for these are presented.
Chapter 5 focuses on free shear layers and jets that involve the merging of two flow streams. Away from a boundary, the mean flow that results is inviscidly unstable and rapidly leads to the formation of coherent vortical structures that drive strong fluid mixing. In jets, it can result in large acoustic levels. Free shear layers are also highly sensitive to sound excitation that can lead to resonant growth of the instability, or a means of control. With this understanding, both passive and active methods of free shear layer control are presented.
Chapter 6 deals with 2-D laminar boundary-layer instabilities and their control. It covers the full range of Mach numbers from incompressible to hypersonic. Boundary-layer instabilities leading to turbulence onset is of great practical importance. This chapter reviews methods of analysis of boundary-layer stability and illustrates several linear and nonlinear mechanisms that can play a role in the breakdown to turbulence. Such understanding is intrinsic to the methods of boundary-layer instability control that are presented in the chapter. Both passive and active flow control approaches are presented.
Chapter 1 provides background and motivation for flow control that is used to achieve a positive outcome, such as drag reduction, enhanced mixing, reduced acoustic levels, or other performance metrics. It emphasizes exploiting fluid instabilities as a means of amplifying small flow actuator inputs in both passive and active approaches. Examples are introduced for a variety of flow fields. These are later detailed in subsequent chapters.
Chapter 3 focuses on the control of bluff-body wakes, where a bluff body is generally categorized as one whose length in the flow direction is approximately the same as its height. Such shapes exhibit a wide wake on the scale of the body height, with aerodynamic drag that is dominated by a low-pressure region that forms in the near wake of the body. Bluff body wakes are complex and highly unsteady, involving boundary layer flow separation and multiple shear layer interactions. The control of bluff body aerodynamics has practical implications to airfoils at high angles of attack, aircraft landing gear, ground vehicles, and buildings and structures. Methods of control that key on the wake instabilities are presented.
Chapter 4 focuses on separated flows that occur in a variety of applications involving external flows, particularly related to aircraft, and internal flows, such as within turbomachines. Flow separation results when the flow does not have sufficient momentum to overcome an adverse pressure gradient, or when viscous dissipation occurs along the flow path. It is almost always associated with some form of aerodynamic penalty, including a loss of lift, an increase in drag, a loss of pressure recovery, and an increase in entropy. This chapter presents both passive and active methods to control these adverse effects.
Chapter 7 deals with 3-D laminar boundary-layer instabilities and their control. It covers the full range of Mach numbers from incompressible to hypersonic. A practical example of a 3-D boundary layer is the flow over a swept wing, which is susceptible to four types of instabilities that can lead to turbulence onset. Of these, cross-flow instability is the most dominant and therefore the most studied 3-D boundary-layer instability mechanism. A fundamental understanding of the instability has led to methods of control that have been successfully demonstrated at incompressible to hypersonic Mach numbers. These and other methods of control are presented.
Chapter 2 provides background on the types of flow sensors and actuators that are frequently used in fluid dynamics. The sensors are used to measure the mean (basic) flow that determines the relevant fluid instabilities. In addition, the sensors are used to document the flow conditions before and after flow control. Both passive and active flow control actuators are presented. These are demonstrated for different flow fields in subsequent chapters.
Chapter 10 considers a broad approach in which the application geometry that dictates the flow field is designed from the beginning, to enhance flow control. Examples include airfoil lift control without moving surfaces. This chapter presents a number of approaches. These range from a simple modification of a geometry to rigorous approaches that utilize an adjoint formulation of the Navier–Stokes equations that identifies sensitivity to changes in geometry and seeks those that maximize flow control authority.