Timothy A. Brungart

Polymer drag reduction with surface roughness in flat-plate turbulent boundary layer flow

Experimental results from a study of surface roughness effects on polymer drag reduction in a zero-pressure gradient flat-plate turbulent boundary layer are presented. Both slot-injected polymer and homogeneous polymer ocean cases were considered over a range of flow conditions and surface roughness. Balance measurements of skin friction drag reduction are presented. Drag reductions over 60% were measured for both the injected and homogeneous polymer cases even with fully rough surfaces. As the roughness increased, higher polymer concentration was required to achieve a given level of drag reduction for the homogeneous case. With polymer injection, increasing surface roughness caused the drag reduction to decrease to low levels more quickly when the polymer expenditure was decreased or the freestream velocity was increased. However, the percent drag reductions on the rough surfaces with polymer injection were often substantially larger than on the smooth surface. Remarkably, in some cases, the skin friction drag force on a rough surface with polymer injection was less than the drag force observed on a smooth surface at comparable conditions.

Velocity profile statistics in a turbulent boundary layer with slot-injected polymer

The modification of a flat-plate turbulent boundary layer resulting from the injection of drag-reducing polymer solutions through a narrow inclined slot into the near-wall region of the flow has been studied. Two-component coincident laser-Doppler velocity profile measurements were taken with a free-stream velocity of 4.5 m/s with polymer injection, water injection, and no injection. Polyethylene oxide solutions at concentrations of 500 and 1025 w.p.p.m. were injected. These data are complemented by polymer concentration profile measurements that were taken using a laser-induced-fluorescence technique. Also, integrated skin friction measurements were made with a drag balance for a range of polymer injection conditions and free-stream velocities. The immediate effects of polymer injection are a deceleration of the flow near the wall, a dramatic decrease of the vertical r.m.s. velocit}’ fluctuation levels and the Reynolds shear stress levels, and a mean velocity profile approaching Virks asymptotic condition. These effects relax substantially with increasing stream wise distance from the injection slot and become similar to the effects observed for dilute homogeneous polymer flows.

Velocity Spectrum Model for Turbulence Ingestion Noise from Computational-Fluid-Dynamics Calculations

TECHNICALNOTESareshortmanuscriptsdescribingnewdevelopmentsorimportantresultsofapreliminarynature.TheseNotescannotexceedsixmanuscriptpages and three ” gures; a page of text may be substituted for a ” gure and vice versa. After informal review by the editors, they may be published within a fewmonths of the date of receipt. Style requirements are the same as for regular contributions

A fluorescence technique for measurement of slot injected fluid concentration profiles in a turbulent boundary layer

A technique for measuring near instantaneous concentration profiles of a fluid injected through a narrow inclined slot at the wall into a high unit Reynolds number flat plate turbulent boundary layer is discussed. The concentration profiles are determined by measuring the light intensity emitted from a fluorescent dye, premixed into the injectant flow, as the injectant convects through an excitation laser beam. The fluorescence intensity is quantified by an electronically shuttered single stage microchannel plate image intensifier coupled to a linear photodiode array. This instrumentation provided the high spatial and temporal resolution required for these boundary layer concentration profile measurements. The laser induced fluorescence technique is being used to study the diffusion of injected polymer solutions away from the near wall region of the boundary layer where these solutions are effective in reducing drag. The diffusion of slot injected water has also been examined and the present results are in excellent agreement with previous studies.

Outer-flow effects on turbulent boundary layer wall pressure fluctuations

The outer-flow contribution to the pressure fluctuations occurring at the wall beneath a turbulent boundary layer was studied experimentally. A moving wall wind-tunnel facility was developed for the work. A flat test plate was suspended at various heights over the movable tunnel wall such that interacting and noninteracting turbulent boundary layers were developed in the resultant channel. Mean and fluctuating velocity components were measured for cases with and without wall motion. Pressure fluctuations were measured, with pinhole microphones on the surface of the test plate forming the upper-channel boundary, at corresponding test conditions. The data show that the wall-pressure fluctuations are relatively insensitive to the details of the outer flow, even over the range of frequencies dominated by outer-flow turbulence structures.

Feasibility of a high-powered carbon nanotube thin-film loudspeaker

The thermophone, conceived in 1917 by Arnold and Crandall, was a unique thermoacoustic loudspeaker. The high heat capacity per unit area (HCPUA) of thin-film materials at that time limited the usefulness of thermophones. Recently, researchers of carbon nanotubes (CNTs) have developed techniques to create a super-aligned thin-film of multi-walled CNTs, possessing extremely low HCPUA. This paper will discuss CNT thin-film loudspeaker theory as well as some initial investigations into the feasibility of a high-powered audio CNT speaker. The advantages of such a loudspeaker include: Ultra-lightweight, compact, no moving parts, low cost, and independence from expensive rare-earth materials.

Effect of a downstream ventilated gas cavity on turbulent boundary layer wall pressure fluctuation spectraa)

An analytical and experimental investigation is made of the effect of a 2-D ventilated gas cavity on the spectrum of turbulent boundary layer wall pressure fluctuations upstream of a gas cavity on a plane rigid surface. The analytical model predicts the ratio of the wall pressure spectrum in the presence of the cavity to the blocked wall pressure spectrum that would exist if the cavity were absent. The ratio is found to oscillate in amplitude with upstream distance (−x) from the edge of the cavity. It approaches unity as −ωx∕Uc→∞, where ω is the radian frequency and Uc is the upstream turbulence convection velocity. To validate these predictions an experiment was performed in a water tunnel over a range of mean flow velocities. Dynamic wall pressure sensors were flush mounted to a flat plate at various distances upstream from a backward facing step. The cavity was formed downstream of the step by injecting carbon dioxide gas. The water tunnel measurements confirm the predicted oscillatory behavior of the ...

Effect of a Downstream Ventilated Gas Cavity on the Spectrum of Turbulent Boundary Layer Wall Pressure Fluctuations

This paper theoretically and experimentally examines the effect of a downstream ventilated gas cavity on the spectrum of turbulent boundary layer wall pressure fluctuations. The theoretical model predicts that the ratio of the point spectrum of the turbulent boundary layer wall pressure fluctuations upstream of a ventilated gas cavity to the blocked point pressure spectrum decays rapidly to zero as the cavity origin is approached and undergoes oscillations in amplitude that relax to unity as the quantity ωx/Uc goes to infinity upstream of the cavity. Here ω is the radian frequency, x is the distance upstream from the cavity origin and Uc is the convection velocity. A water tunnel experiment was performed to investigate the theoretical predictions. Dynamic wall pressure sensors were mounted flush to the surface of a flat plate at various distances upstream from a rearward facing step. Carbon dioxide gas was injected into the separated flow region downstream of the step to form a ventilated cavity. The water tunnel measurements were unable to verify the reduction in the amplitude of the turbulent boundary layer wall pressure fluctuations as the step and cavity were approached but did verify the fundamental oscillation predicted by the theoretical model and its relaxation to unity as ωx/Uc went to infinity upstream of the step and cavity.© 2004 ASME

Installation effects on fan acoustic and aerodynamic performance

Sound power measurements were performed on an upright vacuum cleaner in order to identify predominant noise generating mechanisms and to recommend corrective noise control modifications that would cause no increase in production costs. These modifications were then implemented and their effectiveness quantified. In general, the noise radiated by the vacuum cleaner is dominated by aerodynamic sources. Aerodynamic interactions occur between the electric motor cooling fan and nearby motor mounting fixtures resulting in intense tones at blade rate and harmonic frequencies. These tones are reduced by as much as 25 dB in level, to the broadband sound power level of the unit, by eliminating certain flow obstructions, and mounting the motor in an alternative fashion. Motor cooling performance is also dramatically improved by the new mounting arrangement resulting in a 26 °C reduction in steady-state operating temperature.

Sound Generated by Gas-Jet Impingement on the Interface of a Supercavity

Predictions are made of the sound produced by the impingement of ventilating air jets on the gas-water interface of a ventilated supercavity. The interface is rippled by the jets, and the unsteady surface forces exerted on the water constitute acoustic sources of dipole type. Typical gas speeds on impact do not normally exceed about 60 m/s. In these circumstances the very large difference in the densities of the gas and water imply that the dynamics of the jet-wall interaction are, in a first approximation, similar to those occurring when a jet impinges on a rigid wall. An independent, canonical experiment is performed to determine the frequency spectrum of the surface force distribution produced by a single turbulent jet impinging on a wall, for several different injection flow rates and jet lengths, and thence to estimate the sound produced in the water when the same jet impinges on the cavity interface. The result is used to predict the contribution produced by this mechanism to the overall water borne noise generated by a specially constructed supercavitating body used at the Applied Research Laboratory at Penn State University, for which the ventilating gas enters the cavity axisymmetrically through a series of radially orientated nozzles equally spaced around a gas injector ring. A comparison is made with predictions based on force measurements of Strong, et al. [1].© 2008 ASME