# Strong Interaction Phenomena

In Sub-Section 7.1.4 we have noted that if the displacement properties of a boundary layer are of 0(1 / ^/Reref), it influences the pressure field, i. e., the inviscid flow field, only weakly. We call this “weak interaction” between the attached viscous flow and the inviscid flow.[152] If, however, the boundary layer separates, the inviscid flow is changed and we observe a “strong interaction”. This phenomenon is present in all Mach number regimes, in particular also in the subsonic regime [1].

“Shock/boundary-layer interaction”, present in the transonic, supersonic, and hypersonic regimes, also leads through thickening or even separation of the boundary layer to strong interaction. “Shock/shock interaction” with the associated interaction with the boundary layer is a strong interaction phenomenon, too.

In the high supersonic and the hypersonic regime strong interaction happens also if the attached boundary layer becomes very thick, which is the case with large Mach numbers and small Reynolds numbers at the boundary-layer edge. This is the “hypersonic viscous interaction”. Associated with hypersonic viscous interaction are “rarefaction effects” which appear in the continuum flow regime with slip effects, Section 2.3. They are directly related to large Mach numbers and small Reynolds numbers at the boundary-layer edge, too.

We consider the strong-interaction phenomena in general in their twodimensional appearance. In should be noted that for instance shock/boun – dary-layer interaction usually is less severe in three-dimensional cases compared to strictly two-dimensional cases. The computation of turbulent three-dimensional interactions in general is also less problematic concerning turbulence models. This also holds for ordinary (turbulent) flow separation. On a (two-dimensional) airfoil separation at angles of attack of, say, above approximately 15°, is characterized by vortex shedding. The flow is highly unsteady. This is in contrast to three-dimensional separation at delta wings or fuselage-like bodies. Here the leeward side separation, beginning at angles of attack of, say, approximately 5°, is macroscopically steady up to, say, approximately 50°. This shows that an extended classification of separated flows, and strong-interaction flows in general, is desirable in view of turbulence modeling [2].

Unsteady pressure loads (dynamic pressure loads), due to separation phenomena with vortex shedding, further the intersection of vortex wakes with configuration components (leading to, for instance, fin vibration), and especially also due to unsteadiness of shock/boundary-layer interaction, are of large concern in flight-vehicle design. Like noise they can lead to material fatigue and thus endanger structural integrity. In hypersonic flows they are usually combined with large thermal loads, which make them the more critical.

All the strong interaction phenomena, which we treat in the following sections, can have large, even dramatic influence on the surface pressure field and hence on the aerodynamic forces and moments acting on the flight vehicle, as well as on the thermal state of the surface, and hence on thermal surface effects and on thermal loads.

We aim for a basic understanding of these phenomena and also of the related computational and ground-facility simulation problems.

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