Influence of roughness on attachment line boundary layer transition in hypersonic flow

 Attachment line boundary layer transition on swept cylinders is studied in a low enthalpy hypersonic wind tunnel at M _∞=7.14. Sweep angles of 60° and 70° are used and transition is detected by means of heat flux measurements. The influence on attachment line transition of single 2D-roughness elements, in the form of tripwires or slots, as well as 3D obstacles is determined and the results are analyzed with respect to Poll’s criterion. Received: 16 January 1996 / Accepted: 12 July 1996     

The boundary layer on a slightly yawed cylinder

The velocity in a turbulent boundary layer on a long cylinder at a slight yaw to the free stream was measured using hot wire anemometry for yaw angles of ?0.55°?α?0.55°. The mean velocity profile retains a log region regardless of yaw with a slope that is slightly dependent upon the yaw angle. The boundary layer thickness increases nonlinearly with yaw angle, but the dimensionless distance from the wall of the maximum turbulence intensity is independent of yaw angle.     

Laminar boundary layer on a yawed infinite cylinder

Using mixed momentum and energy integral equations, a simple quadrature method is developed to compute incompressible laminar boundary layer on a yawed infinite cylinder. As an illustration, the results — including various boundary layer thicknesses, form parameters and potential and surface streamlines — are obtained for a circular cylinder and compared with a known solution.     

Evolution of vortex structure in boundary layer transition induced by roughness elements

The particle image velocimetry (PIV) and hydrogen-bubble visualization technique are used to investigate the flat-plate boundary layer transition induced by an array of roughness elements. The streamwise evolutions of the mean and fluctuation velocity are analyzed, and the critical Reynolds number Re _k,c is determined between 339 and 443 under current experimental setup. The hairpin vortices shed from supercritical roughness elements are visualized by swirling strength, in which two pairs of counter-rotating vortices can be observed: one vortex pair is the manifestation of the neck of the hairpin vortices shed from the top of the roughness; the other vortex pair, which originates from the lower part of the roughness, comes from the streamwise vortices that are constantly perturbed by the hairpin vortex shedding.     

Effect of pressure gradients on the different stages of roughness induced boundary layer transition

The physical mechanisms of roughness-induced transition (RIT) in pressure gradient boundary layers are studied using direct numerical simulations. Recent investigations have examined RIT processes in zero-pressure-gradient boundary layers (Suryanarayanan et al., 2019). The present study uses a vorticity dynamics point of view to examine how these processes are altered by a locally accelerating or decelerating flow that strains the vorticity field and creates a net vorticity flux at the wall. Flow acceleration is imposed on specific streamwise extents of the flow. This provides an understanding about how the fundamental mechanisms in different stages of RIT are affected by pressure gradients. The present results suggest that both lift-up and subsequent amplification of the unsteady perturbations are mitigated by flow acceleration. The effect on lift-up is explained by the compression (i.e. large negative value of the stretching term) of the wall-normal vorticity by negative dv/dy. Consistent with earlier experimental observations on spots and wedges, favorable pressure gradients reduce turbulent wedge spreading and nearly arrest the spreading when sufficiently strong. This result is also explained in terms of vorticity dynamics.     

Traveling disturbance appearing in boundary layer transition in a Yawed cylinder

Boundary layers that develop over a body in fluid flow are in most cases three-dimensional owing to the spin, yaw, or surface curvature of the body. Therefore, the study of three-dimensional (3D) boundary-layer transition is essential to work in practical aerodynamics. The present investigation is concerned with the problem of 3D boundary layers over a yawed body. A yawed cylinder model that represents the leading edge portion of a swept wing and the mechanism of crossflow instability are investigated in detail using hot-wire velocimetry and a flow visualization technique. As a result, traveling disturbances having frequencies f₁ and f₂, which differ by about one order of magnitude, are detected in the transition region. The phase velocities and directions of travel of those disturbances are measured. Results for the low-frequency disturbance f₁ show qualitative coincidence with results numerically predicted for a crossflow unsteady disturbance. Nameley, F₁ travels nearly spanwise to the yawed cylinder and very close to the cylinder wall. The results for the high-frequency disturbance f₂ good agreement with the existing experimental results. The ₂ disturbance is found to be the high-frequency inflectional secondary instability that appears in 3D boundary layer transition in general. A two-stage transition process, where stationary crossflow vortices appear as the primary instability and a traveling inflectional disturbance is generated as a secondary instability, was observed. Secondary instability seems to play a major role in turbulent transition.     

Aerodynamic heating in the region of shock and turbulent boundary layer interaction induced by a cylinder

Detailed distributions of heat flux in the region of shock wave and turbulent boundary layer interaction induced by a cylinder were measured in the shock tunnel. Oil flow patterns and Schlieren photographs were taken. Empirical relations were given for determining separation shock angle, peaks of heat flux and their locations on both cylinder leading edge and flat plate surface, and other characteristic parameters of the interaction region.     

Effect of surface temperature on the stability of the swept attachment line boundary layer

At fairly high Reynolds numbers instability may develop on the line of attachment of the potential flow to the leading edge of a swept wing and lead to a transition to boundary layer turbulence directly at the leading edge [1, 2]. Although the first results relating to the stability and transition of laminar flow at the leading edge of swept wings were obtained almost 30 years ago, the problem remains topical. The stability of the swept attachment line boundary layer was recently investigated numerically with allowance for compressibility effects [3, 4]. Below we examine the effect of surface temperature on the stability characteristics of the laminar viscous heat-conducting gas flow at the leading edge of a side slipping wing.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 78–82, November–December, 1990.     

Disturbances of a three-dimensional boundary layer generated by surface roughness

Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 129–134, September–October, 1989.     

The boundary layer on a rapidly rotating cylinder

The study considers plane steady flow of an incompressible fluid around a circular cylinder rotating in a homogeneous free stream. On the basis of an asymptotic analysis of the Navier-Stokes equations for high Reynolds numbers, it is shown that at a certain value of the angular velocity of the cylinder an interaction arises between the flow in the boundary layer and the external potential flow. A solution is obtained numerically which describes the flow in the region of interaction.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 36–45, September–November, 1987.     

Joint permeability and roughness effect on the supersonic flat-plate boundary layer stability and transition

The joint effect of the permeability and the roughness of the flat plate surface on the boundary layer stability and laminar-turbulent transition is experimentally and theoretically investigated at the freestream Mach number M_∞ = 2. It is shown that, as a certain roughness value is reached, and with increase in the porous coating thickness (on a certain range), the boundary layer stability against natural disturbances diminishes and laminar-turbulent transition is displaced toward the leading edge of the model.     

Experimental investigation of laminar-turbulent transition behind a three-dimensional roughness element in the boundary layer on a sharp cone

The qualitative characteristics of laminar-turbulent transition behind a three-dimensional roughness element in a zero-gradient boundary layer are investigated for high supersonic free-stream velocities. Quantitative data on the distribution of the heat transfer coefficient in the neighborhood of the roughness element and in the wake behind it are obtained. Another object of the investigation was to compare the data obtained for the position of the laminar-turbulent transition zone with well-known correlations used in practice.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 60–66, July–August, 1990.     

Large-scale spanwise periodicity in a turbulent boundary layer induced by highly ordered and directional surface roughness

The effect of converging–diverging riblet-type surface roughness (riblets arranged in a ‘herringbone’ pattern) are investigated experimentally in a zero pressure gradient turbulent boundary layer. For this initial parametric investigation three different parameters of the surface roughness are analysed in detail; the converging–diverging riblet yaw angle α, the streamwise fetch or development length over the rough surface F_x and the viscous-scaled riblet height h⁺. It is observed that this highly directional surface roughness pattern induces a large-scale spanwise periodicity onto the boundary layer, resulting in a pronounced spanwise modification of the boundary layer thickness. Hot-wire measurements reveal that above the diverging region, the local mean velocity increases while the turbulent intensity decreases, resulting in a thinner overall boundary layer thickness in these locations. The opposite situation occurs over the converging region, where the local mean velocity is decreased and the turbulent intensity increases, producing a locally thicker boundary layer. Increasing the converging–diverging angle or the viscous-scaled riblet height results in stronger spanwise perturbations. For the strongest convergent–divergent angle, the spanwise variation of the boundary layer thickness between the diverging and converging region is almost a factor of two. Such a large variation is remarkable considering that the riblet height is only 1% of the unperturbed boundary layer thickness. Increasing the fetch seems to cause the perturbations to grow further from the surface, while the overall strength of the induced high and low speed regions remain relatively unaltered. Further analysis of the pre-multiplied energy spectra suggests that the surface roughness has modified or redistributed the largest scale energetic structures.     

Numerical study of boundary layer transition in flowing film evaporation on horizontal elliptical cylinder

A numerical study of the onset of longitudinal transition between turbulent and laminar regimes during the evaporation of a water film is presented. These water film streams along a horizontal elliptical tube under the simultaneous effects of gravity, pressure gradients, caused by the vapor flow and curvature, and viscous forces. At the interface of water vapor, the shear stress is supposed to be negligible. Outside the boundary layer, the vapor phase velocity is obtained from potential flow. In the analysis Von Karmans turbulence model is used and the inertia and convection terms are retained. Transfers equations are discretised by using the implicit Keller method. The effects of an initial liquid flow rate per unit of length, Froude number, temperature difference between the wall and the liquid–vapor interface and ellipticity on the transition position have been evaluated. The transition criterion has been given in term of the critical film Reynolds number (Re)_C.     

Effect of roughness on the velocity profile of a laminar boundary layer

Flow past a rough wall is examined. Calculations are made to find the roughness-induced mean velocity which is expressed in an integral form in terms of the spectral density of the roughness and an influence function. Values of the influence function are tabulated using the known values of the modified Hankel functions of order 1/3 and their integrals. The first order change in lower critical Reynolds number due to the roughness-induced change in profile is calculated; the stability of the profile is increased due to the presence of roughness. This work was supported by U.S. Naval Ordnance Systems Command under Contract NOw 65-0123-d through the Garfield Thomas Water Tunnel, Ordnance Research Laboratory.     

Supersonic boundary layer on a plate. Comparison of calculation and experiment

For verifying the method of calculating the boundary layer in liquid rocket engine (LRE) nozzles developed by the authors on the basis of a differential three-parameter turbulence model, the boundary layer on a plate in a zero-gradient flow is calculated. Over a wide range of variation of the free-stream Mach number, the temperature factor, and the Reynolds number, based on the momentum thickness of the boundary layer, the calculation agrees satisfactorily with the known experimental data, with respect to both integral and local flow and heat transfer characteristics. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 6, pp. 64–78, November–December, 1998. The work received financial support from the Russian Foundation for Basic Research (project No.96-012-00260).     

Non-axisymmetric boundary layer induced by a swirling downdraft

Most mathematical models of conical swirling flows are exact axisymmetrical solutions of the steady and incompressible Navier–Stokes equations. Some of these models take into account the coupling mechanisms (Chaskalovic and Chauvière 1999, Shtern et al. 1998), but very few mathematical studies try to model non-axisymmetrical conical flows. We propose here a generalisation of the formulation used by Aristov (1998) to study a swirling vertical downdraft limited by a nearly horizontal plane. The use of asymptotic analysis for high Reynolds numbers allows to find non-axisymmetrical analytical solutions for the whole flow.     

Unsteady boundary layer on a circular cylinder embedded to a wedge

The boundary layer growth on a circular cylinder embedded to a wedge, when the motion is started impulsively from rest, is discussed using the method of inner and outer expansions. The equation for the time of separation involving the Reynolds number and the wedge angle is obtained. A uniformly valid solution is also found for the entire flow field. It is found that separation first occurs at the points where the cylinder meets the wedge. Also, the time of earliest separation decreases with increase in the Reynolds number and with increase in the wedge angle.