Roger L. Simpson

Bio: Roger L. Simpson is an academic researcher from Southern Methodist University. The author has contributed to research in topics: Boundary layer & Flow separation. The author has an hindex of 16, co-authored 29 publications receiving 1554 citations.

The structure of a separating turbulent boundary layer. Part 1. Mean flow and Reynolds stresses

Abstract: The problem of turbulent-boundary-layer separation due to an adverse pressure gradient is an old but still important problem in many fluid flow devices. Until recent years little quantitative experimental information was available on the flow structure downstream of separation because of the lack of proper instrumentation. The directionally sensitive laser anemometer provides the ability to measure the instantaneous flow direction and magnitude accurately. The experimental results described here are concerned with a nominally two-dimensional, separating turbulent boundary layer for an airfoil-type flow in which the flow was accelerated and then decelerated until separation. Upstream of separation single and cross-wire hot-wire anemometer measurements are also presented. Measurements in the separated zone with a directionally sensitive laser-anemometer system were obtained for U, V , $\overline{u^2}, \overline{v^2}, - \overline{uv}$ , the fraction of time that the flow moves downstream, and the fraction of time that the flow moves away from the wall. In addition to confirming the earlier conclusions of Simpson, Strickland & Barr (1977) regarding a separating airfoil-type turbulent boundary layer, much new information about the separated region has been gathered. (1) The backflow mean velocity profile scales on the maximum negative mean velocity U N and its distance from the wall N . A U + vs. y + law-of-the-wall velocity profile is not consistent with this result. (2) The turbulent velocities are comparable with the mean velocity in the backflow, although low turbulent shearing stresses are present. (3) Mixing length and eddy viscosity models are physically meaningless in the backflow and have reduced values in the outer region of the separated flow. Downstream of fully developed separation, the mean backflow appears to be divided into three layers: a viscous layer nearest the wall that is dominated by the turbulent flow unsteadiness but with little Reynolds shearing stress effects; a rather flat intermediate layer that seems to act as an overlap region between the viscous wall and outer regions; and the outer backflow region that is really part of the large-scaled outer region flow. The Reynolds shearing stress must be modelled by relating it to the turbulence structure and not to local mean velocity gradients. The mean velocities in the backflow are the results of time averaging the large turbulent fluctuations and are not related to the source of the turbulence.

Characteristics of turbulent boundary layers at low Reynolds numbers with and without transpiration

Abstract: An extension to Coles's (1956) ‘law of the wall–law of the wake’ formulation for incompressible unblown boundary layers with momentum thickness Reynolds number Reθ > 6000. is made for Reθ 6000, is replaced by Ω = 0·40 (Reθ/6000)⅛ for Reθ 0) flows in ‘law of the wall’ and ‘velocity-defect’ representations. This law of the wall for the logarithmic turbulent region and Reichardt's sublayer variation of eM/ν are used to obtain a continuous expression for eM/ν as a function of U+, V+w, and Reθ for the wall region. This expression is in reasonable agreement with the generated eM/ν blowing results and in less agreement with the unblown and suction results. Eddy viscosity and mixing length results confirm that eM/δ*U∞ ∝ Ω2 and l/δ ∝ Ω for the outer region and that eM/δ*U∞ and l/δ are substantially independent of blowing and moderate suction, as also reflected by the velocity defect representation for injection and suction.

Features of a separating turbulent boundary layer in the vicinity of separation

Abstract: Laser anemometer measurements using a directionally sensitive system were obtained for a nominally two-dimensional separating turbulent boundary layer produced by an adverse pressure gradient. An airfoil-type flow was generated in which the flow was accelerated and then decelerated until separation. The results include the skin friction, mean velocity profiles, turbulent shear stresses and intensities, spectra, dissipation rate, turbulent/non-turbulent interfacial intermittency, and eddy speeds.

The structure of a separating turbulent boundary layer. Part 2. Higher-order turbulence results

Abstract: The velocity-probability-distribution flatness and skewness factors for u and v are reported for the separating turbulent boundary layer described by Simpson, Chew & Shivaprasad (1981). Downstream of separation the skewness factor for u is negative near the wall, whereas it is positive upstream of separation. The flatness factor for u downstream of separation differs from the upstream behaviour in that it has a local maximum of about 4 at the minimum mean velocity location in the backflow. Both upstream and downstream of separation the skewness factor for v has a profile shape and magnitudes that are approximately the mirror image or negative of the skewness factor for u. The flatness factor for v seems to be affected little by separation.Examination of the momentum and turbulence-energy equations reveals that the effects of normal stresses are important in a separating boundary layer. Negligible turbulence-energy production occurs in the backflow. Turbulence-energy diffusion is increasingly significant as separation is approached and is the mechanism for supplying turbulence energy to the backflow.The backflow appears to be controlled by the large-scale eddies in the outer region flow, which provides the mechanism for turbulence-energy diffusion. The backflow behaviour does not appear to be significantly dependent on the far downstream near-wall conditions when the thickness of the backflow region is small compared with the turbulent shear layer thickness.

Direct simulation of a turbulent boundary layer up to R sub theta = 1410

Abstract: The turbulent boundary layer on a flat plate, with zero pressure gradient, is simulated numerically at four stations between R sub theta = 225 and R sub theta = 1410. The three-dimensional time-dependent Navier-Stokes equations are solved using a spectra method with up to about 10 to the 7th power grid points. Periodic spanwise and stream-wise conditions are applied, and a multiple-scale procedure is applied to approximate the slow streamwise growth of the boundary layer. The flow is studied, primarily, from a statistical point of view. The solutions are compared with experimental results. The scaling of the mean and turbulent quantities with Reynolds number is examined and compared with accepted laws, and the significant deviations are documented. The turbulence at the highest Reynolds number is studied in detail. The spectra are compared with various theoretical models. Reynolds-stress budget data are provided for turbulence-model testing.

Turbulent flows over rough walls

Abstract: ▪ AbstractWe review the experimental evidence on turbulent flows over rough walls. Two parameters are important: the roughness Reynolds number ks+, which measures the effect of the roughness on the buffer layer, and the ratio of the boundary layer thickness to the roughness height, which determines whether a logarithmic layer survives. The behavior of transitionally rough surfaces with low ks+ depends a lot on their geometry. Riblets and other drag-reducing cases belong to this regime. In flows with δ/k ≲ 50, the effect of the roughness extends across the boundary layer, and is also variable. There is little left of the original wall-flow dynamics in these flows, which can perhaps be better described as flows over obstacles. We also review the evidence for the phenomenon of d-roughness. The theoretical arguments are sound, but the experimental evidence is inconclusive. Finally, we discuss some ideas on how rough walls can be modeled without the detailed computation of the flow around the roughness element...

Turbulent Boundary-Layer Separation

Abstract: 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

Experimental and numerical study of a turbulent boundary layer with pressure gradients

Abstract: Results are presented of an experimental and numerical study of a turbulent boundary layer with pressure gradients conducted using the recent 'fringe method' with its numerical advantages and good inflow quality. After an inflow transient good agreement is observed; the differences, of up to 13 percent, are discussed. Moderate deviations from the law of the wall are found in the velocity profiles of the simulation. They are fully correlated with the pressure gradient, are in fair quantitative agreement with the experimental results of Nagano et al. (1992), and are roughly the opposite of uncorrected mixing-length-model predictions. Large deviations from the wall scaling are observed for other quantities, notably for the turbulence dissipation rate. The a(1) structure parameter drops mildly in the upper layer with adverse pressure gradient.