The unsteady interaction of a moving shock wave with nearly homogeneous and isotropic decaying compressible turbulence has been studied experimentally in a large-scale shock tube facility. Rectangular grids of various mesh sizes were used to generate turbulence with Reynolds numbers based on Taylor's microscale ranging from 260 to 1300. The interaction has been investigated by measuring the three-dimensional velocity and vorticity vectors, the full velocity gradient and rate-of-strain tensors with instrumentation of high temporal and spatial resolution. This allowed estimates of dilatation, compressible dissipation and dilatational stretching to be obtained. The time-dependent signals of enstrophy, vortex stretching/tilting vector and dilatational stretching vector were found to exhibit a rather strong intermittent behaviour which is characterized by high-amplitude bursts with values up to 8 times their r.m.s. within periods of less violent and longer lived events. Several of these bursts are evident in all the signals, suggesting the existence of a dynamical flow phenomenon as a common cause. Fluctuations of all velocity gradients in the longitudinal direction are amplified significantly downstream of the interaction. Fluctuations of the velocity gradients in the lateral directions show no change or a minor reduction through the interaction. Root mean square values of the lateral vorticity components indicate a 25% amplification on average, which appears to be very weakly dependent on the shock strength. The transmission of the longitudinal vorticity fluctuations through the shock appears to be less affected by the interaction than the fluctuations of the lateral components. Non-dissipative vortex tubes and irrotational dissipative motions are more intense in the region downstream of the shock. There is also a significant increase in the number of events with intense rotational and dissipative motions. Integral length scales and Taylor's microscales were reduced after the interaction with the shock in all investigated flow cases. The integral length scales in the lateral direction increase at low Mach numbers and decrease during strong interactions. It appears that in the weakest of the present interactions, turbulent eddies are compressed drastically in the longitudinal direction while their extent in the normal direction remains relatively the same. As the shock strength increases the lateral integral length scales increase while the longitudinal ones decrease. At the strongest interaction of the present flow cases turbulent eddies are compressed in both directions. However, even at the highest Mach number the issue is more complicated since amplification of the lateral scales has been observed in flows with fine grids. Thus the outcome of the interaction strongly depends on the initial conditions.

Studies of interactions of a propagating shock wave with decaying grid turbulence: velocity and vorticity fields

A decaying compressible nearly homogeneous and nearly isotropic grid-generated turbulent flow has been set up in a large scale shock tube research facility. Experiments have been performed using instrumentation with spatial resolution of the order of 7 to 26 Kolmogorov viscous length scales. A variety of turbulence-generating grids provided a wide range of turbulence scales with bulk flow Mach numbers ranging from 0.3 to 0.6 and turbulent Reynolds numbers up to 700. The decay of Mach number fluctuations was found to follow a power law similar to that describing the decay of incompressible isotropic turbulence. It was also found that the decay coefficient and the decay exponent decrease with increasing Mach number while the virtual origin increases with increasing Mach number. A possible mechanism responsible for these effects appears to be the inherently low growth rate of compressible shear layers emanating from the cylindrical rods of the grid. Measurements of the time-dependent, three dimensional vorticity vectors were attempted for the first time with a 12-wire miniature probe. This also allowed estimates of dilatation, compressible dissipation and dilatational stretching to be obtained. It was found that the fluctuations of these quantities increase with increasing mean Mach number of the flow. The time-dependent signals of enstrophy, vortex stretching/tilting vector and dilatational stretching vector were found to exhibit a rather strong intermittent behaviour which is characterized by high-amplitude bursts with values up to 8 times their r.m.s. within periods of less violent and longer lived events. Several of these bursts are evident in all the signals, suggesting the existence of a dynamical flow phenomenon as a common cause.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112000003402

The structure of weakly compressible grid-generated turbulence

In this paper, a new double diaphragm shock tube facility for studying the structural response of a circular plate resting on soil, when subjected to a shock wave, is described. The present shock tube has been designed in the framework of a more extensive research program aimed at the investigation of underground tunnel lining under blast and fire conditions. The innovative features of the facility are an end-chamber conceived to investigate soil-structure interaction and a burner equipment to heat the specimen in order to study to what extent thermal damage can affect the transmitted and reflected pressure wave as well as the structural response. Details of the shock tube design, construction and test procedure operations are discussed in the paper. Particular emphasis is placed on the principles that have driven the experimental equipment design choices. Numerical simulations have been performed to assess the ideal shock tube performance in terms of reflected pressure and test time duration as well as to evaluate how far the fire testing situation actually is from that normally used in tunnel design.

A New Shock Tube Facility for Tunnel Safety

Numerical simulations and laboratory measurements have been used to illuminate the interaction of a moving shock wave impacting on metallic grids at various shock strengths and grid solidities. The experimental work was carried out in a large scale shock tube facility while computational work simulated the flow field with a time-dependent inviscid and a time-dependent viscous model. The pressure drop measured across the grids is a result of two phenomena which are associated with the impact of the shock on the metallic grids. First are the reflection and refraction of the incoming shock on the grid itself. This appears to be the main inviscid mechanism associated with the reduction of the strength of the transmitted shock. Second, viscous phenomena are present during the reflection and refraction of the wave as well as during the passage of the induced flow of the air through the grid. The experimental data of pressure drop across the grid obtained in the present investigation are compared with those obtained from computations. The numerical results slightly overpredict the experimental data of relative pressure drop which increases substantially with grid solidity at fixed flow Mach numbers. The processes of shock reflection and refraction are continuous and they can be extended in duration by using thicker grids that will result in lower compression rates of the structural loading and increase the viscous losses associated with these phenomena which will further attenuate the impacting shock. Preliminary theoretical analysis suggests that the use of a graded porosity/solidity material will result in higher pressure drop than a constant porosity/solidity material and thus provide effective blast mitigation.

Moving shocks through metallic grids: their interaction and potential for blast wave mitigation

Shock wave impact on monolithic and composite material plates: The preferential aeroelastic response

A shock tube research facility for high-resolution measurements of compressible turbulence

A nearly homogeneous nearly isotropic compressible turbulent flow interacting with a normal shock wave has been studied experimentally in a large shock tube facility. Spatial resolution of the order of 8 Kolmogorov viscous length scales was achieved in the measurements of turbulence. A variety of turbulence generating grids provide a wide range of turbulence scales. Integral length scales were found to substantially decrease through the interaction with the shock wave in all investigated cases with flow Mach numbers ranging from 0.3 to 0.7 and shock Mach numbers from 1.2 to 1.6. The outcome of the interaction depends strongly on the state of compressibility of the incoming turbulence. The length scales in the lateral direction are amplified at small Mach numbers and attenuated at large Mach numbers. Even at large Mach numbers amplification of lateral length scales has been observed in the case of fine grids. In addition to the interaction with the shock the present work has documented substantial compressibility effects in the incoming homogeneous and isotropic turbulent flow. The decay of Mach number fluctuations was found to follow a power law similar to that describing the decay of incompressible isotropic turbulence. It was found that the decay coefficient and the decay exponent decrease with increasing Mach number while the virtual origin increases with increasing Mach number. A mechanism possibly responsible for these effects appears to be the inherently low growth rate of compressible shear layers emanating from the cylindrical rods of the grid.

/pdf/studies-of-shock-wave-interactions-with-homogeneous-and-9xiua9gq3k.pdf

J.H. Agui

Papers

A shock tube research facility for high-resolution measurements of compressible turbulence

Studies of Shock Wave Interactions with Homogeneous and Isotropic Turbulence