This article summarizes our present understanding of the physical behavior of two-dimensional turbulent separated flows, which occur due to adverse pressure gradients around streamlined and bluff bodies. The physical behavior of turbulence is flow dependent, so detailed experimental infor­ mation is needed for understanding such flows and modeling their physics for calculation methods. An earlier review (Simpson 1 985) discussed in much detail prior experimental and computational work, and this was followed by an updated review of calculation methods only (Simpson 1 987). Here additional recent references are added to those cited in the two other works. By separation, we mean the entire process of departure or breakaway, or the breakdown of boundary-layer flow. An abrupt thickening of the rotational-flow region next to a wall and significant values of the normal­ to-wall velocity component must accompany breakaway, or otherwise this region would not have any significant interaction with the free-stream flow. This unwanted interaction causes a reduction in the performance of the flow device of interest (e.g. a loss of lift on an airfoil or a loss of pressure rise in a diffuser). It is too narrow a view to use vanishing surface shearing stress or flow reversal as the criterion for separation. Only in steady two-dimensional flow do these conditions usually accompany separation. In unsteady two­ dimensional flow the surface shear stress can change sign with flow reversal without the occurrence of breakaway_ Conversely, the breakdown of the boundary-layer concept can occur before any flow reversal is encountered. In three-dimensional flow the rotational layer can depart without the

/pdf/turbulent-boundary-layer-separation-u84v5eocej.pdf

Turbulent Boundary-Layer Separation

An empirical model of the surface pressure spectrum beneath a two-dimensional, zero-pressure-gradient boundary layer is presented that is based on the experimental surface pressure spectra measured by seven research groups. The measurements cover a large range of Reynolds number, 1.4 × 10 3 < Reθ < 2.34 × × 10 4 . The model is a simple function of the ratio of the timescales of the outer to inner boundary layer. It incorporates the effect of Reynolds number through the timescale ratio and compares well to experimental data. It is proposed that the effect of Reynolds number is more aptly described as the effect of the range of relevant scales. Spectral features of the experimental data and the scaling behavior of the surface pressure spectrum are also discussed.

Empirical Spectral Model of Surface Pressure Fluctuations

Aspects of turbulent boundary-layer separation

Wall pressure and shear stress spectra from direct numerical simulations of turbulent plane channel flow are presented in this paper. Simulations have been carried out at a series of Reynolds numbers up to Re? = 1440, which corresponds to Re = 6:92 x 10(4) based on channel width and centerline velocity. Single-point and two-point statistics for velocity, pressure, and their derivatives have been collected, including velocity moments up to fourth order.§ The results have been used to study the Reynolds number dependence of wall pressure and shear stress spectra. It is found that the point spectrum of wall pressure collapses for Re? ? 360 under a mixed scaling for frequencies lower than the peak frequency of the frequency-weighted spectrum, and under viscous scaling for frequencies higher than the peak. Point spectra of wall shear stress components are found to collapse for Re? ? 360 under viscous scaling. The normalized mean square wall pressure increases linearly with the logarithm of Reynolds number. The rms wall shear stresses also increase with Reynolds number over the present range, but suggest some leveling off at high Reynolds number.

Wall Pressure and Shear Stress Spectra from Direct Simulations of Channel Flow

4, in both internal (channel) and external (airfoil) flows. A review of the boundary-layer parameters characterizing the pressure gradient effects is provided, and the more relevant ones are introduced as new variables in the model. The method is then compared to the zero pressure gradient model it is derived from. The influence of the pressure gradient on the wall-pressure spectrum is discussed.Finally,themethodisappliedtoprovideinputdataofaradiatedtrailing-edge noisemodelbymeansofan aeroacoustic analogy, namely Amiet’s theory of turbulent boundary layer past a trailing edge. The results are compared to experimental data obtained in an open-jet anechoic wind tunnel.

/pdf/wall-pressure-spectral-model-including-the-adverse-pressure-1f7ezy6fw7.pdf

Wall-Pressure Spectral Model Including the Adverse Pressure Gradient Effects

A novel, microphone-employing technique for the measurement of pressure fluctuations through pinhole apertures in the flow surface is used in the present study of separating boundary layer pressure fluctuation spectra and wavespeeds. The rms of the surface pressure fluctuation increases monotonically through the adverse-pressure gradient attached flow region and the detached flow zone. A correlation of the detached flow data is given which is derived from velocity and length scales of the separated flow. The celerity of the surface pressure fluctuations for the attached flow increases with frequency to a maximum, and the streamwise coherence decreases drastically after the begining of flow reversal.

Surface pressure fluctuations in a separating turbulent boundary layer

Measurements of surface pressure fluctuation spectra, coherence and convective wave speeds from zero and favorable pressure gradient turbulent boundary layers are reported for momentum Reynolds numbers from 3000 to 18,800. The acceleration parameter K is near 2 x 10 to the -7 power for the favorable pressure gradient flow. The outer variables, U sub e, tau sub w and delta sub 1 non-dimensionalize and collapse the spectra for the low to middle range of frequencies for most test cases. The grouping using the inner variable, U sub tau and gamma, collapse the spectra for the middle to high range of frequencies for all test cases. The value of p'/tau sub w was near 3.8 and 2.8 for the smallest values of d+ in the zero and favorable pressure gradient flows, respectively. The coherence exhibits a decay that is not exponential in some cases, but the Corcos similarity parameters omega Delta x/U sub c and omega Delta z/U sub c collapse the data for all test cases. The ratio of U sub c/U sub e increases with omega delta sub 1/U sub e up to omega delta sub 1/U sub e on the order of unity, where U sub c/U sub e becomes nearly constant. This was observed in the present results for both streamwise pressure gradient flows. The experimental results presented show good agreement with previous research.

Some Features of Surface Pressure Fluctuations in Turbulent Boundary Layers With Zero and Favorable Pressure Gradients

Boundary Layer Transition (BOLT) Flight Experiment Overview

To improve scientific knowledge of boundary-layer transition physics on increasingly complex geometries, the Air Force Research Laboratory/Air Force Office of Scientific Research has sponsored the ...

Final Design of the Boundary Layer Transition (BOLT) Flight Experiment

A novel set of serrated-planform lifting surfaces produce unexpectedly high lift coefficients at moderate to high angles-of-attack. Each serration, or tooth, is designed to shed a vortex. The interaction of the vortices greatly enhances the lifting capability over an extremely large operating range. Variations of the invention use serrated-planform lifting surfaces in planes different than that of a primary lifting surface. In an alternate embodiment, the individual teeth are controllably retractable and deployable to provide for active control of the vortex system and hence lift coefficient. Differential lift on multiple serrated-planform lifting surfaces provides an means for vehicle control. The important aerodynamic advantages of the serrated-planform lifting surfaces are not limited to aircraft applications but can be used to establish desirable performance characteristics for missiles, land vehicles, and/or watercraft.

https://ntrs.nasa.gov/api/citations/20000025500/downloads/20000025500.pdf

Brian E. McGrath

Papers

Surface pressure fluctuations in a separating turbulent boundary layer

Some Features of Surface Pressure Fluctuations in Turbulent Boundary Layers With Zero and Favorable Pressure Gradients

Boundary Layer Transition (BOLT) Flight Experiment Overview

Final Design of the Boundary Layer Transition (BOLT) Flight Experiment

Serrated-planform lifting-surfaces