S. B. Verma

Performance Characteristics of an Annular Conical Aerospike Nozzle with Freestream Effect

An experimental investigation has been carried out to study the performance and base pressure characteristics of a Mach 2.0 annular conical aerospike nozzle with and without freestream flow. The effect of cowl length, plug length, and plug contour variation on the nozzle performance and base pressure characteristics is studied. It is observed that the overexpansion shock from the internal nozzle, overexpansion shock on the spike surface, and the expansion fan from the cowl lip of the internal nozzle dominate the overall flowfield development. The presence of freestream flow reduces the nozzle performance by approximately 4% relative to static conditions. Base pressure characteristics are observed to be strongly influenced by the movement of these shocks on the plug surface, and their subsequent interaction with the inner shear layer controls the base-wake closure. Relative to the conical plug conflguration, the contoured plug shows considerably enhanced base pressure characteristics. Real-time pressure measurements on the spike reveal highly unsteady flow in the intermittent region of separation.

Experimental study of flow unsteadiness in a Mach 9 compression ramp interaction using a laser schlieren system

This paper demonstrates the use of a simple laser schlieren technique to obtain flow information in a two-dimensional separated compression ramp-induced shock-wave boundary-layer interaction. Tests were made at a freestream Mach number of 9 and freestream Reynolds number per unit length of 2.078 × 105 m−1. The importance of this technique in studying hypersonic flows is unique since the run times of hypersonic wind tunnels are of very short duration. The method is based on the schlieren principle and uses a parallel sheet of a low-power (15 W) diode laser and an array of very fast response (4 ns) photodiodes. Although the arrangement detects an integral part of the signal from the fluctuating density gradients across the span of the flow, it yields significant insights into the details of the flow structure. The details of the flow field are discussed using time-dependent fluctuating density gradient profiles, the related power spectra and autocorrelation and cross-correlation functions in the interaction region.

Reynolds Number Influence on Dual-Bell Transition Phenomena

An experimental investigation was conducted to study the Reynolds number influence on dual-bell transition behavior for tests inside a high-altitude simulation chamber. For the range of nozzle supply pressures tested, the nozzle Reynolds number is seen to gradually decrease from a relatively high value (of the order of 107 for tests in sea-level atmospheric conditions) toward the transitional range (lower side of 106 for tests inside the high-altitude chamber). This influences the width of the inflection region, which is seen to decrease with an increase in nozzle Reynolds number. Because of the smaller negative pressure gradient experienced during sneak transition with a decrease in nozzle Reynolds number, the separation point is seen to move into the region of wall inflection much earlier and tends to stay in the region of wall inflection for a relatively longer time. Although the time duration of final transition remains more or less constant for different nozzle supply pressure values, the time durati...

Gas density effects on dual-bell transition behavior

An experimental investigation was conducted to study the dual-bell transition behavior inside a high-altitude test facility under different back pressure environments. The high-altitude chamber was evacuated using an ejector nozzle and the dual-bell nozzle driving pressure, P 0N, was varied from 30 to 5.3 bar in subsequent test campaigns. As P0N was decreased, the dual-bell transition was observed to get delayed, and the transition nozzle pressure ratio increased by as much as 20% for the lowest driving pressure tested. Similar results were also observed for the dual bell retransition nozzle pressure ratio. For very low values of P0N (of 3.5 bar), transition did not occur at all. The delay in the dual-bell transition process to higher nozzle pressure ratio is primarily attributed to the increase in the width of inflection region with decrease in Reynolds number, which seems to control the dual-bell transition behavior. During these tests, it was also observed that the visible intensity of condensation shock decreased gradually with decreasing P0N and finally, for P0N � 17:4 bar, no condensation shock was observed. The decrease in gas density/reduced mass flow is also accompanied by a significant decrease in the amplitude of wall pressure fluctuations in the region of separation due to decrease in shock strength.

Studies on the Control of Shock Wave-Boundary layer Interaction Using Steady Microactuators

unswept compression ramp in a Mach 2 flow to study their effect on the unsteady pressure loads due to the interaction. Flow visualization and fluctuating pressure measurements were obtained in the separated shock wave/boundary layer interaction produced by the ramp. Results show that the array of microjets issuing in the supersonic crossflow creates a shock which effectively reduces the Mach number in front of the separation shock. This causes the separation shock to weaken, and move upstream by as much as 4δo from its mean undisturbed location. The mean pressure distribution was altered due to the microjet control with a maximum reduction of 7% on the wall surface downstream of the separation shock and 25% on the ramp surface. The wall pressure fluctuations were also reduced significantly in the separation region with control where the intermittent region was also reduced by more than 40% with the applied control. The pressure spectra show that the unsteadiness on the wall and the ramp surfaces has been significantly reduced with the applied control.

Experimental study on a jet issuing from an elliptic nozzle with cylindrical tabs

An elliptic jet with mixing tabs was investigated using the digital particle-image-velocimetry and two-component hotwire anemometry. Two tabs of cylindrical configuration were placed on the minor-axis sides of a 2:1 elliptic nozzle. Because of their sideways pressure-relieving effect, a cylindrical tab relieves the constraint on the flow relative to a thin tab with flat surfaces which can have important implications on tab drag reduction but with similar mixing characteristics. Measurements were made for a jet exit velocity of 20 m s−1 and for Reynolds number based on the nozzle equivalent diameter (De ) of 5.08 × 104. Furthermore, the effect of tab height (h/d= 1.0 and 1.67) is also investigated. The study shows the evolution of a mushroom structure behind each tab that distorts the jet flow into a two-finger structure. The tab wake mainly comprises of base vortices and tip vortices that are enveloped from the sides by spanwise vortical structures. While the strong upwash from the base vortices causes an...

Flow and Acoustic Properties of Underexpanded Elliptic-Slot Jets

An experimental program was conducted to investigate the effect of aspect ratio on the e ow and acoustic properties of unheated jets issuing from elliptic slots. The aspect ratios investigated ranged from low (2:1) to moderatevalues (3:1and4:1). Further,passivecontrolorthemixingandnoisecharacteristicsisdemonstrated.The experimental conditions chosen ranged from fully expanded (Mj =1.0) to underexpanded jets with an equivalent Machnumberof2.0.Themean e owstudyrevealsthatin moderateaspect-ratiojettheazimuthaldeformationstake longer to evolve than in low aspect-ratio jet, resulting in lower bulk mixing. Notches, as passive control devices, enhance mixing in each aspect ratio. The acoustic properties of underexpanded jets show great variation with aspect ratio. Minor-axis sides of 3:1 and 4:1 plain jets radiate shock noise levels that are 4 and 10 dB, respectively, higher than those by 2:1 plain jet. The presence of notches alters the azimuthal emission characteristics of these jets. For a 2:1 jet notches bring down the noise levels along the major-axis plane by 5 dB, whereas for 3:1 and 4:1 jets notches effect the noise intensity along minor-axis sides and show a reduction of 3.5 and 7 dB, respectively, with a repeatability of 2%. Nomenclature bn = local unit vector in the direction of the binormal De = equivalent diameter of noncircular slot La = equivalent semimajor axis length of plain elliptic slot Lavg = average shock-cell length M = local e ow Mach number Mj = Mach number obtained by correctly expanding an underexpanded jet Pa = atmospheric pressure

Supersonic separation with obstructions

The complex problem of the supersonic flow along a flat plate as it interacts with the blunt obstructions has recieved considerable attention. The presence of a blunt obstruction in supersonic flow causes the formation of a bow shock wave ahead of the obstruction. The adverse pressure gradient resulting from the bow shock wave propagates upstream in the subsonic flow within the boundary layer and results in the separation of the boundary layer and formation of vortices ahead of the obstruction. These vortices scavenge the boundary layers leading to increased pressures and heat transfer loads. Our attempts are made to correlate the separation distance S, which is defined as the distance from the leading edge of the obstacle to the primary separation line, ahead of the obstacle to the primary separation line, ahead of the obstacle to the height H of the obstacle.

Transition control of Mach to regular reflection induced interaction using an array of micro ramp vane-type vortex generators

An experimental investigation has been conducted to favorably control/modify a Mach reflection induced interaction in a Mach 2.05 flow on a flat plate using an array of single row mechanical micro vane-type vortex generators (VGs). The objective was to study the variation in (i) control device configuration (trapezoidal and the split-trapezoidal or ramp vane-type), (ii) control device height (h/δ = 0.3, 0.5), and (iii) control location (X/δ = 9, 15 upstream of the interaction) in controlling the overall interaction. The primary aim was to investigate a control location and VG configuration which is able to effectively initiate a transition from Mach reflection to regular reflection with minimum changes to the separation characteristics for no control. While the trapezoidal configuration is seen to move the separation location upstream only slightly, the split-trapezoidal configurations result in a considerable upstream movement that is associated with significant reduction in separation shock strength. Th...

Shock-Wave Boundary-Layer Interaction Control on a Compression Corner Using Mechanical Vortex Generators

Shock-wave boundary-layer interactions (SWBLI) are prevalent in many supersonic applications, e.g., over deflected flaps, fore-body ramp corners, on leading edges where the bow shock from the vehicle nose interferes, along axial compression corners inside air-inlets, shock reflection and crossing-shock interactions in the inlets etc. The adverse pressure gradient across the interaction shock can cause separation of the incoming boundary-layer leading to increased aerodynamic drag, heat transfer and unsteady pressure loads. Much of the early work over forward-facing steps [1], un-swept compression ramp flows [2-4] and in interactions induced by blunt fins [5], circular cylinders and sharp fins at angle of attack [6] was focused on understanding the dynamic/unsteady behavior of these interactions. It has been observed that the flow in these interactions in unsteady if the pressure ratio across the oblique shock is such that the mass of the fluid reversed at the reattachment point does not balance the scavenged fluid from the separated region [7-8]. As a result, the separated region “breathes” and during one half of pulse, mass is injected into it while during the other half it is ejected out resulting into an unsteady mass exchange.