Ph.D. (Engg): Large-scale unsteadiness in high-speed double cone flows

Shock-wave/separation-region interactions (SSRI) in high-speed flows can lead to large-scale self-sustained unsteadiness when the separation region grows to dimensions comparable to the characteristic length scales of the flow geometry. A canonical example is the high-speed double-cone flow — over a certain range of geometric parameters this configuration exhibits a large-amplitude periodic shock-wave unsteadiness, commonly referred to as “pulsation.” This effort aims to establish a physics-based framework for the understanding of flow pulsation and explore novel methodologies for possible control of the unsteady phenomenon.

Studies in literature suggest that pulsation is triggered when the adverse pressure gradient over the fore-cone surface becomes sufficiently strong to generate a large-scale reverse flow within the separation region. The subsequent evolution of this reverse flow is believed to directly lead to the periodic growth and collapse of the separation region and the accompanying large-scale shock-wave motion. The precise role of flow reversal in governing pulsation remains an open question. A direct way to address it is to systematically modify the reverse-flow dynamics and examine the resulting changes in the pulsation behavior. To this end, the reverse-flow dynamics are manipulated using passive and active microjet flow-control methodologies in the present work. The passive method exploits the pressure difference that develops naturally during the pulsation cycle to generate self-sustained microjets without external energy input, whereas the active-control method employs externally driven microjets to introduce controlled downstream-directed momentum into the separation region. In both approaches the injected momentum directly opposes the naturally occurring reverse flow, thereby altering its dynamics. The resulting changes in the flow are investigated experimentally at Mach 6 in the Roddam Narasimha Hypersonic Wind Tunnel.

Both flow manipulation methodologies demonstrate that systematic modification of the reverse-flow dynamics fundamentally alters the pulsation behavior. Passive control significantly increases the characteristic pulsation time scale while progressively reducing the temporal coherence of the canonical pulsation. Detailed analysis shows that increasing passive actuation strength transforms the canonical pulsation from a highly coherent, nearly periodic motion into a weakly coherent large-scale unsteadiness characterized by broadband spectral content. While active control is seen to suppress the canonical pulsation altogether, producing a sequence of flow states consisting of small-amplitude undulations and, at sufficiently large actuation strength, a quasi-steady shock wave configuration. Collectively, these observations provide direct experimental evidence that reverse-flow dynamics are a key component of pulsation, and that their systematic manipulation provides an effective strategy for controlling pulsation-type unsteadiness.

In addition to experimentally characterizing pulsation and demonstrating its control, a quantitative physics-based framework for predicting the characteristic pulsation time scale is developed as a part of the present effort. The double-cone geometry is parameterized by the fore-cone angle (theta_1), aft-cone angle (theta_2), and the ratio of the conical slant lengths (lambda). To obtain a tractable analytical description while retaining the essential flow physics, attention is focused on the spike-cylinder configuration (theta_1 = 0 degrees, theta_2 = 90 degrees), which represents a limiting case of the general double-cone geometry. To develop and assess the proposed framework, a dedicated series of Mach 6 experiments were conducted by systematically varying lambda while keeping theta_1 and theta_2 fixed. High-speed schlieren imaging together with time-resolved pressure measurements were employed to characterize the unsteady flow field. Guided by the experimental observations, pulsation is interpreted as a cyclic process involving the accumulation of high-pressure gas in the region ahead of the aft cone, and subsequent expansion of the gas through the low-pressure separation region over the fore cone. Based on this physical picture, an analytical model containing no empirical parameters is developed. The model successfully predicts the experimentally measured pulsation Strouhal number and provides the first quantitative framework linking the governing flow physics to the characteristic pulsation time scale.

Overall, the present work establishes a coherent physical framework for understanding pulsation-type unsteadiness in high-speed double-cone flows. The passive- and active-control studies demonstrate experimentally that systematic manipulation of the reverse-flow dynamics fundamentally alters the pulsation behavior. Complementing these observations, the analytical model quantitatively links the governing gas-dynamic processes to the characteristic pulsation time scale and provides a predictive framework for estimating the pulsation frequency from first principles. Together, these contributions advance the current understanding of pulsation-type unsteadiness and establish a physics-based foundation for control and prediction.

Speaker : Subhajit Das

Research Supervisor : Prof. Duvvuri Subrahmanyam