Separation length in high-enthalpy shock/boundary-layer interaction

Abstract

Experiments were performed in the T5 Hypervelocity Shock Tunnel to investigate nonequilibrium real-gas effects on separation length using a double-wedge geometry and nitrogen test gas. Local external flow conditions were estimated by computing the inviscid nonequilibrium flow field. A new scaling parameter was developed to approximately account for wall temperature effects on separation length for a laminar nonreacting boundary layer and arbitrary viscosity law. A classification was introduced to divide mechanisms for real-gas effects into those acting internal and external to viscous regions of the flow. Internal mechanisms were further subdivided into those arising upstream and downstream of separation. Analysis based on the ideal dissociating gas model and a scaling law for separation length of a nonreacting boundary layer showed that external mechanisms due to dissociation may decrease separation length at low incidence but depend on the free-stream dissociation at high incidence. A limited numerical study of reacting boundary layers showed that internal mechanisms due to recombination occurring in the boundary layer upstream of separation cause a slight decrease in separation length relative to a nonreacting boundary layer with the same external conditions. Correlations were obtained of experimentally measured separation length using local external flow parameters computed for reacting flow, which scales out external mechanisms but not internal mechanisms. These showed the importance of the new scaling parameter in high-enthalpy flows, a linear relationship between separation length and reattachment pressure ratio, and a Reynolds-number effect for transitional interactions. A significant increase in scaled separation length was observed in the experimental data at high enthalpy. The increase was attributed to an internal mechanism arising from recombination in the free-shear layer downstream of separation, perhaps altering its velocity profile. This real-gas effect depends on the combined presence of free-stream dissociation and a cold wall.

Correlation for Length of Impinging Shock-Induced Large Separation Bubble at Hypersonic Speed

“…A number of similarity laws are reported in the literature, which relates separation length to the freestream conditions and other parameters such as incipient separation pressure/ plateau pressure and reattachment pressure. These include compression ramp/double wedge cases at supersonic [7] as well as hypersonic freestreams [3,8], supersonic impinging shock cases [9,10]; the separation length for a hypersonic double cone [11] was also found to follow a correlation similar to that proposed by Davis and Sturtevant [3] for double wedges. Recently, John and Kulkarni [12] analyzed the important correlations reported in the literature, by comparing their numerical data for compression corner for Mach numbers ranging from 5 to 11.63 (the correlation [10] for impinging shock case was also compared by suitably adapting the parameters); it was noted that the correlations were not universal.…”

“…However, the inverse cubic relationship with Mach number suggested by Katzer [10] holds for the present experiments too. Other scaling laws for separation length reported in the literature, for different flow regimes (from supersonic to hypersonic, including real gas effects) and different Reynolds number ranges, have also shown inverse cubic variation with Mach number [3,7,8]. However, for larger Reynolds numbers, the similarity law for supersonic separation bubble, proposed by Burggraf [7] based on asymptotic theory, showed non-linear dependence on pressure.…”

“…However, for larger Reynolds numbers, the similarity law for supersonic separation bubble, proposed by Burggraf [7] based on asymptotic theory, showed non-linear dependence on pressure. Other correlations at hypersonic speeds for compression corner/ double ramp flows have also shown non-linear dependence on pressure [3,9]. In particular, according to the similarity law for double ramp flow derived based on asymptotic theory by Davis and Sturtevant [3] with the inclusion of high temperature effects, the separation length scaled by the distance of the location of separation was proportional to the 3/2 th power of the differential pressure ratio (the ratio of the difference between the reattachment and plateau pressures to the pressure upstream of the interaction).…”

“…In order that the moderate to high total enthalpies associated with hypersonic flows are experimentally simulated, short duration impulse facilities like shock tunnels are required. There are however, few shock tunnel studies reported in the literature on hypersonic SBLI, over compression corners, generally with separation bubbles of lengths comparable with the boundary layer thicknesses [2][3][4].…”

Effect of Concave Wall Geometry on Heat Transfer in Hypersonic Boundary Layers

“…This type of thermocouple gauge is used extensively in the T5 reflected shock tunnel at GALCIT. [16][17][18] The high enthalpy test conditions result in adequate signal levels and the robust design of the gages make them highly resistant to damage caused by particulates in the test gas as well as the large heat fluxes. 16 The output signal is processed by a differential amplifier circuit mounted exterior to the test section.…”