## Publications

17 results found

Ruban AI, Keshari SK, Kravtsova MA, 2021, On boundary-layer receptivity to entropy waves, *Journal of Fluid Mechanics*, Vol: 929, Pages: 1-33, ISSN: 0022-1120

<jats:p>In this paper, we consider the generation of the Tollmien–Schlichting waves in the boundary layer on the surface of a wing exposed to entropy waves. It is well known that the free-stream turbulence is composed of two perturbation modes: the vorticity waves and the entropy waves. The receptivity of the boundary layer to the vorticity waves has been studied extensively by various authors. The entropy waves have not attracted such attention. We show that, in high speed subsonic flows, the entropy waves are as important for the receptivity as the vorticity waves. Methodologically, our work relies on the asymptotic analysis of the Navier–Stokes equations at large values of the Reynolds number, which results in the formulation of a suitably modified triple-deck theory. The entropy waves produce oscillations of the gas temperature and density, but the velocity and the pressure remain unperturbed to the leading order. This precludes the entropy waves from penetrating the boundary layer, as happens, for example, with the acoustic waves. Our analysis reveals that the entropy waves decay rapidly in the transition layer that forms near the outer edge of the boundary layer. We find that an entropy wave alone cannot generate the Tollmien–Schlichting waves. However, when the boundary layer encounters a wall roughness, the flow near the roughness appears to be perturbed not only inside the boundary layer but also in the inviscid region outside the boundary layer. The latter comes into the interaction with the density perturbations in the entropy wave. As a result, a localised ‘forcing’ is created that produces the Tollmien–Schlichting waves. In this paper we present the results of a linear and nonlinear receptivity analysis. We find that the nonlinearity enhances the receptivity significantly, especially when a local separation region forms on the roughness.</jats:p>

Ruban A, Djehizian A, Kirsten J,
et al., 2020, On quasi-steady boundary-layer separation in supersonic flow. Part 2. Downstream moving separation point, *Journal of Fluid Mechanics*, Vol: 900, ISSN: 0022-1120

In this paper we study the perturbations produced in the boundary layer by an impinging oblique shock wave or Prandtl–Meyer expansion fan. The flow outside the boundary layer is assumed supersonic, and we also assume that the point, where the shock wave/expansion fan impinges on the boundary layer, moves downstream. To study the flow, it is convenient to use the coordinate frame moving with the shock; in this frame, the body surface moves upstream. We first study numerically the case when the shock velocity Vsh=O(Re−1/8) . In this case the interaction of the boundary layer with the shock can be described by the classical equations of the triple-deck theory. We find that, as Vsh increases, the boundary layer proves to be more prone to separation when exposed to the expansion fan, not the compression shock. Then we assume Vsh to be in the range 1≫Vsh≫Re−1/8 . Under these conditions, the process of the interaction between the boundary layer and the shock/expansion fan can be treated as inviscid and quasi-steady if considered in the reference frame moving with the shock/expansion fan. The inviscid analysis allows us to determine the pressure distribution in the interaction region. We then turn our attention to a thin viscous sublayer that lies closer to the body surface. In this sublayer the flow is described by classical Prandtl's equations. The solution to these equations develops a singularity provided that the expansion fan is strong enough. The flow analysis in a small vicinity of the singular point shows an accelerated ‘expansion’ of the flow similar to the one reported by Neiland (Izv. Akad. Nauk SSSR, Mech. Zhidk. Gaza, vol. 5, 1969a, pp. 53–60) in his analysis of supersonic flow separation from a convex corner.

De Grazia D, Moxey D, Sherwin SJ,
et al., 2018, Direct numerical simulation of a compressible boundary-layer flow past an isolated three-dimensional hump in a high-speed subsonic regime, *Physical Review Fluids*, Vol: 3, ISSN: 2469-990X

In this paper we study the boundary-layer separation produced in a high-speed subsonic boundary layer by a small wall roughness. Specifically, we present a direct numerical simulation (DNS) of a two-dimensional boundary-layer flow over a flat plate encountering a three-dimensional Gaussian-shaped hump. This work was motivated by the lack of DNS data of boundary-layer flows past roughness elements in a similar regime which is typical of civil aviation. The Mach and Reynolds numbers are chosen to be relevant for aeronautical applications when considering small imperfections at the leading edge of wings. We analyze different heights of the hump: The smaller heights result in a weakly nonlinear regime, while the larger result in a fully nonlinear regime with an increasing laminar separation bubble arising downstream of the roughness element and the formation of a pair of streamwise counterrotating vortices which appear to support themselves.

Ruban AI, Bernots T, Kravtsova MA, 2016, Linear and nonlinear receptivity of the boundary layer in transonic flows, *Journal of Fluid Mechanics*, Vol: 786, Pages: 154-189, ISSN: 1469-7645

In this paper we analyse the process of the generation of Tollmien–Schlichting waves in a laminar boundary layer on an aircraft wing in the transonic flow regime. We assume that the boundary layer is exposed to a weak acoustic noise. As it penetrates the boundary layer, the Stokes layer forms on the wing surface. We further assume that the boundary layer encounters a local roughness on the wing surface in the form of a gap, step or hump. The interaction of the unsteady perturbations in the Stokes layer with steady perturbations produced by the wall roughness is shown to lead to the formation of the Tollmien–Schlichting wave behind the roughness. The ability of the flow in the boundary layer to convert ‘external perturbations’ into instability modes is termed the receptivity of the boundary layer. In this paper we first develop the linear receptivity theory. Assuming the Reynolds number to be large, we use the transonic version of the viscous–inviscid interaction theory that is known to describe the stability of the boundary layer on the lower branch of the neutral curve. The linear receptivity theory holds when the acoustic noise level is weak, and the roughness height is small. In this case we were able to deduce an analytic formula for the amplitude of the generated Tollmien–Schlichting wave. In the second part of the paper we lift the restriction on the roughness height, which allows us to study the flows with local separation regions. A new ‘direct’ numerical method has been developed for this purpose. We performed the calculations for different values of the Kármán–Guderley parameter, and found that the flow separation leads to a significant enhancement of the receptivity process.

Ruban A, Cimpeanu R, Papageorgiou DT, et al., 2015, How to make a splash: droplet impact and liquid film applications in aerodynamics, 68th Annual Meeting of the APS Division of Fluid Dynamics doi: 10.1103/APS.DFD.2015.GFM.P0032

Mengaldo G, Kravtsova M, Ruban A,
et al., 2015, Triple-deck and direct numerical simulation analyses high-speed subsonic flows past a roughness element, *Journal of Fluid Mechanics*, Vol: 774, Pages: 311-323, ISSN: 1469-7645

This paper is concerned with the boundary-layer separation in subsonic and transonic flows caused by a two-dimensional isolated wall roughness. The process of the separation is analysed by means of two approaches: the direct numerical simulation (DNS) of the flow using the Navier–Stokes equations, and the numerical solution of the triple-deck equations. Since the triple-deck theory relies on the assumption that the Reynolds number ( ) is large, we performed the Navier–Stokes calculations at Re = 4 x 10^5 based on the distance of the roughness element from the leading edge of the flat plate. This Re is also relevant for aeronautical applications. Two sets of calculation were conducted with the free-stream Mach number Ma_∞ = 0.5 and Ma_∞ = 0.87 . We used different roughness element heights, some of which were large enough to cause a well-developed separation region behind the roughness. We found that the two approaches generally compare well with one another in terms of wall shear stress, longitudinal pressure gradient and detachment/reattachment points of the separation bubbles (when present). The main differences were found in proximity to the centre of the roughness element, where the wall shear stress and longitudinal pressure gradient predicted by the triple-deck theory are noticeably different from those predicted by DNS. In addition, DNS predicts slightly longer separation regions.

Ruban AI, Kravtsova MA, 2013, Generation of Steady Longitudinal Vortices in Hypersonic Boundary Layer, *Journal of Fluid Machanics*, Vol: 729, Pages: 702-731, ISSN: 0022-1120

Zametaev VB, Kravtsova MA, 2010, Receptivity of a boundary layer to external sonic waves, *FLUID DYNAMICS*, Vol: 45, Pages: 196-207, ISSN: 0015-4628

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Zametaev VB, Kravtsova MA, 2007, Inviscid interaction in the thin shock layer at high mach numbers, *Fluid Dynamics*, Vol: 42, Pages: 495-506, ISSN: 0015-4628

Hypersonic perfect gas flow past the weakly curved end face of a circular cylinder is considered in the thin shock layer approximation. Regimes in which the shape of the end face is not a monotonic function of the radius but contains, for example, a central body of variable height are studied. It is found that, as the central body is extended, a break is formed in the slope of the shock. Smoothing takes place in a short zone of interaction between the main part of the thin shock layer, the shock, and the small near-wall potential jet. The solution, which depends continuously on a parameter, exists over a limited height range and bifurcates when a critical value is exceeded. © Pleiades Publishing, Ltd. 2007.

Zametaev VB, Kravtsova MA, 2007, Inviscid Interaction in the Thin Shock Layer at High Mach Numbers, *FLUID DYNAMICS*, Vol: 42, Pages: 495-506, ISSN: 0015-4628

Zametaev VB, Kravtsova MA, 2006, Numerical Solution of the Problem of the Mixing of the Boundary Layers Shed from the Trailing Edge of a Wing, *FLUID DYNAMICS*, Vol: 41, Pages: 817-829, ISSN: 0015-4628

Kravtsova MA, Zametaev VB, Ruban AI, 2005, An effective numerical method for solving viscous-inviscid interaction problems, *PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES*, Vol: 363, Pages: 1157-1167, ISSN: 1364-503X

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- Citations: 12

Zametaev VB, Kravtsova MA, 2003, Effect of a Thin Longitudinal Inviscid Vortex on a Two-Dimensional Pre-Separation Boundary Layer, *FLUID DYNAMICS*, Vol: 38, Pages: 250-264, ISSN: 0015-4628

Zametaev VB, Kravtsova MA, 2003, Influence of thin non-viscous longitudinal vortex on two-dimensional pre-separation boundary layer, *Izvestiya Akademii Nauk. Mekhanika Zhidkosti I Gaza*, Pages: 97-114, ISSN: 0568-5281

KRAVTSOVA MA, 1993, NUMERICAL-SOLUTION OF THE ASYMPTOTIC PROBLEM OF THE SEPARATION OF THE BOUNDARY-LAYER OF AN INCOMPRESSIBLE LIQUID IN FRONT OF THE CORNER POINT OF THE CONTOUR OF A SOLID, *COMPUTATIONAL MATHEMATICS AND MATHEMATICAL PHYSICS*, Vol: 33, Pages: 397-406, ISSN: 0965-5425

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- Citations: 1

KRAVTSOVA MA, RUBAN AI, 1988, DETACHMENT OF A SUPERSONIC BOUNDARY-LAYER IN FRONT OF THE BOTTOM EDGE OF A CONTOUR OF A BODY, *USSR COMPUTATIONAL MATHEMATICS AND MATHEMATICAL PHYSICS*, Vol: 28, Pages: 177-184, ISSN: 0041-5553

Kravtsova M, Ruban A, 1985, Nonstationary boundary layer on a rotationally vibrating cylinder in transverse flow, *Uch. Zap. TsAGI*

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